Erosional and Depositional 

Aspects of Hurricane Camille in 

' 

Virginia, 1969 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 804 

Erosional and Depositional 

Aspects of Hurricane Camille 

Virginia, 1969 

By GARNETT P. WILLIAMS and HAROLD P. GUY 

• 

In 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 804 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1973 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600265 

For sale by the Superintendent of Documents U.S. Government Printing Office 

Washington, D.C. 20402- Price $1.80 (paper cover) 

Stock Number 2401-02442 
Abstract ----------------------------------------

Introduction ------------------------------------- General description of study area ------------------

The storm ---------------------------------------

The flood ---------------------------------------- Estimated storm-average discharges -----------

Estimated peak discharges -------------------

Geomorphological features of drainage basins ------

Slopes of drainage areas ----------------------

Stream length and drainage area --------------

Longitudinal profiles of stream channels -------

Topographic map data -------------------

Field measurements ----------------------

Erosion -----------------------------------------

Debris avalanches ----------------------------

Source material --------------------------

General description of avalanche scars ----

Geometry of scars -----------------------

ldethods of study --------------------

Longitudinal profiles -----------------

Cross profiles ------------------------

Possible causes of avalanches in Nelson 

County ---------------------------- Theoretical considerations ------------

Stream action at the base of the hillslope ------------------------------ Depressions or troughs on the hillside __ 

Orientation of hillside ----------------

Hillside gradients --------------------

Length of hillslope -------------------

Soil depth ---------------------------

Summary --------------------------- Effect of debris avalanches on streamflow __ 

Stream channels -----------------------------

Sediment yield ------------------------------- Volume of avalanche erosion --------------

Sediment yield from stream channels ------

Total sediment yield and average denudation_ 

CONTENTS 

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Erosion-Continued 

Sediment yield-Continued 

Estimated sediment-transport rates --------

Depositional features ----------------------------

Channel deposits -----------------------------

Sediment sampling, computation of sizefrequency distributions ---------------------

Definitions of size-distlibution characteristics __ 

Alluvial fans --------------------------------

Amount of deposition --------------------

Cross-fan profiles ------------------------

Grain-size characteristics -----------------

Highway deltas -----------------------------

Sedimentary features --------------------

Vertical variations in grain-size 

characteristics -------------------------

Flood-plain deposits --------------------------

Sedimentary features --------------------

Vertical changes in grain size -------------

Lateral variations in grain size -----------

Downstream changes in flood-plain depositsgeneral ------------------------------- Downstream changes in amount of deposition 

Polly Wright Cove -------------------

Rucker Run -------------------------

Downstream changes in average grain size __ 

Average particle size, local slope, and 

drainage area ------------------------- Downstream variation in size of largest 

stones -------------------------------- Downstream changes in sorting -----------

Downstream chang~?s in skewness ----------

Scarcity of coarse sand and gravel --------

Downstream changes in grain shape and 

roundness -----------------------------

Conclusions -------------------------~-----------

References --------------------------------------

ILLUSTRATIONS 

FIGURE 1. 

2. 

3. 

4. 

5-10. 

~ap of study area -------------------------------------------------------------------------- Photograph showing upstream view of Wills Cove and Dillard Creek -------------------------

~ap of rainfall distribution ------------------------------------------------------------------

Topographic maps of Ginseng Hollow, Polly Wright Cove basins, and headwaters of Wills Cove --

Graphs: 

5. Percentage of drainage-basin area steeper than a given slope, for basins of figure 4 ------

6. Variation of stream length with drainage area for three Nelson County basins -----------

7. Longitudinal profiles of stream channels ----------------------------------------------

8. Longitudinal channel profiles plotted in the form of LIF versus L ---------------------

9. Profiles of mountain channels and alluvial fans ---------------------------------------

10. LIF versus L plot of longitudinal stream profiles ---------------------------------------

ill 

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IV CONTENTS 

FIGURES 11-14. Photographs: 

TABLE 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22-26. 

27. 

28. 

29-32. 

33. 

34-36. 

37. 

38. 

39. 

40-45. 

46-57. 

11. Erosion and deposition ----------------------------------------------------------------

12. Typical avalanche scars -------------------------------------------------------------

13. Avalanche scar at Lovingston ______ ---------------------------------------------------

14. Avalanche scars on sides of intermontane valleys ---------------------------------------

Contour maps of two avalanche scars, Polly Wright Cove --------------------------------------

Profiles of hillslopes and their scars ________ ---------------------------------------------------

Graphs of local gradient versus distance from hilltop ------------------------------------------

Cross profiles of avalanche scars --------------------------------------------------------------

Graphs showing relation of hillslope length to number of scars per acre and to average distance of 

scarring per acre -------------------------------------------------------------------------- Sketch of hypothetical drainage areas showing influence of hillslope length and drainage area on 

avalanching -------------------------------------------------------------------------------

Sketch map of peak-flow study in Ginseng Hollow ----------------------------------------------

Photographs: 

22. Eroded mountain channels ------------------------------------------------------------

23. Channel erosion at S. K. Wills' home----------------------------------------------------

24. Channel erosion along highways ------------------------------------------------------

25. Stream erosion under a railway trestle ------------------------------------------------

26. Channels showing both erosion and deposition -------------------------------------------

Diagram of orientation and relative volume of avalanches ------------------------------------

Graph of volume of avalanche erosion with respect to average surface slope ----------------------

Photographs: 

29. Boulders which probably were too large to be moved by floodwaters ---------------------

30. Log jam in a mountain channel -------------------------------------------------------

31. Range of ·particle sizes in upstream deposits --------------------------------------------

32. Imbricated boulders ------------------------------------------------------------------

Graph showing typical particle-size distribution for a wide range of particle sizes ----------------

Photographs: 

34. Alluvial fans ------------------------------------------------------------------------

35. Bryant fan -------------------------------------------------------------------------- 36. Eroded zone at apex of Campbell fan __________________________________________________ _ 

Graph o.f amount of deposition with distance down fan -----------------------------------------

Cross-fan profiles ---------------------------------------------------------------------------- Graph showing change in particle-size-distribution characteristics along alluvial fans -------------

Photographs: 

40. Edes Hollow highway delta -----------------------------------------------------------

41. Dillard Creek highway delta -----------------------------------------------------------

42. Sedimentary features of a highway delta -----------------------------------------------

43. Typical flood-plain deposits in headwater area -----------------------------------------

44. Common flood-plain deposits -----------------------------------------------------------

45. Close view of sandy flood-plain deposits ------------------------------------------------

Graphs: 

46. Size-frequency characteristics of flood-plain sediments on Dillard Creek ------------------

47. Size-frequency characteristics of flood-plain sediments on Rucker Run --------------------

48. Volume of deposition per foot of channel length, Polly Wright Cove ----------------------

49. Volume of deposition versus distance downstream on Rucker Run -----------------------

50. Downstream changes in mean particle size of flood-plain deposits, for three streams _____ _ 

51. Average particle size versus local flood-plain slope -------------------------------------

52. Average particle size versus drainage area ---------------------------------------------

58. Downstream changes in size of largest stones ------------------------------------------

54. Downstream changes in sorting of flood-plain deposits -----------------------------------

55. Change in sorting with average grain size ----------------------------------------------

56. Downstream changes in skewness ------------------------------------------------------

51. Change in skewness with average grain size --------------------------------------------

TABLES 

1. Drainage area and average surface slope in tributaries and subbasins of Wills Cove, Ginseng Hollow, and Polly Wright Cove ______________ ---------------------------------------------------

2. Hillside and scar dimensions and erosion volumes for measured avalanche scars -------------------

3. Factors contributing to instability of earth slopes -----------------------------------------------

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TABLE 

a 

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CONTENTS v 

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4. Debris-avalanche scars per 1,000 feet of stream channel ---------------------------------------- 29 

30 

40 

45 

57 

66 

75 

77 

5. Effect of hillslope length and drainage area on frequency and amount of scarring -----------------

6. Estimated net sediment yields and storm-average transport rates -------------------------------

7. Computational procedure of combining a sieve analysis with a pebble count _ ----------------------

8. Vertical variations in grain-size characteristics in highway deltas --------------------------------

9. Volume and percentage of deposition by size classes in Polly Wright Cove -----------------------

10. Number of cases of deficiency in weight percent, for various particle-size classes ------------------

11. Average grain shape and roundness values in Polly Wright Cove -------------------------------

SYMBOLS 

Dimensions 

coefficient in the equation of longitudinal 

stream-profile 

cross-sectional flow area --------------------- L 1 

drainage area ------------------------------ L 2 

coefficient in various equations, representing 

slope of best-fit line 

coefficient representing portion of total rainfall 

that runs off the ground surface 

constant of integration in Hack (1957, p. 70) 

equation for stream profile 

average intermediate diameter of sediment 

particles --------------------------------- L 

grain size for which 10 percent of the 

distribution is finer ----------------------- L 

average intermediate diameter of the five largest 

rocks at a site ---------------------------- L 

base of natural logarithms 

vertical fall from drainage divide ------------ L 

acceleration due to gravity ------------------ LT-2 

rainfall intensity --------------------------- LT-1 

k 

L 

n 

Q 

s 

sk 

v 

w 

X 

a 

f3 

"' 11h 

Dimensions 

constant in Hack equation for stream profile 

horizontal distance from drainage divide ______ L 

constant in Hack equation for stream profile 

discharge of water ------------------------- ur-1 

average radius of curvature of a channel bend_ L 

inside radius of curvature of a channel bend __ L 

outside radius of curvature of a .channel bend _ L 

local slope (gradient) 

skewness of a sediment size-frequency 

distribution 

sorting of a sediment size-frequency 

distribution 

mean velocity of flow ----------------------- LT-1 

water-surface width of a stream ------------- L 

distance downstream from start of deposition __ L 

constant in equation of stream profile 

constant in equation of stream profile 

constant in equation of stream profile 

elevation difference between high-\vater and 

low-water marks around a channel bend ___ L 
ENGLISH-METRIC CONVERSION TABLE 

Length: 

Inches X 0.0254=meters 

Feet X 0.3048=meters 

Yards X 0.9144=meters 

Miles X 1.609=kilometers 

Area: 

Square inches X 0.0006452=square meters 

Square feet X 0.09290=square meters 

Square yards X 0.8361=square meters 

Acres X 4047 =square meters 

Square milesX2,590,000=square meters 

Volume: 

Cubic inches X0.01639=liters 

Cubic feet X 28.32=liters 

Cubic yardsX764.6=liters 

Pints X 0.4732=liters 

Quarts X 0.9463=liters 

GallonX3.785=liters 

Acre-feetX 1233=cubic meters 

Weight: 

Grains X 0.06480=grams 

Ounces (avoirdupois) X28.35=grams 

Pounds (avoirdupois) X453.6=grams 

Tons (short) X907.2=kilograms 

Tons (long) X1016=kilograms 

Specific combinations: 

Feet per second X 1.097 =kilometer per hour 

X0.3048=meters per second 

Miles per hourX1.609=kilometers per hour 

X0.4470=meters per second 

Pounds per square inchX70.3=grams per square 

centimeter 

Pounds per square foot X 0.4885=grams per square 

centimeter 

Tons (short) per square footX0.9765=kilograms per 

square centimeter 

Tons (short) per acreX0.2241=kilograms per square 

meter 

Tons (short) per square mileX0.0003502=kilograms 

per square meter 

Pounds per cubic footX0.01602=grams per cubic 

centimeter 

Cubic feet per secondX1.699=cubic meters per minute 

X0.02832=cubic meters per second 

Cubic feet per second for 1 dayX1.983=acre feet 

X2446=cubic meters 

Degrees Fahrenheit-32X0.556=degrees Celsius. 
EROSIONAL AND DEPOSITIONAL ASPECTS OF 

HURRICANE CAMILLE IN VIRGINIA, 1969 

By GARNETT P. WILLIAMS and HAROLD P. GuY 

ABSTRACT 

Probably the worst natural disaster in central Virginia's 

recorded history was the flood resuiting from an 8-hour deluge 

of about 28 inches (710 mm) of rain on the night of August 

19-20, 1969. This study examines some of the intensive sediment erosion and deposition that resulted from the storm 

and flood. 

Most of the 150 people whom the flood killed in this 

mountainous area died from broken bones and other bluntforce injuries, rather than by drowning. The transport of 

sediment and other debris by the water therefore was very 

significant in loss of life and in property damage. 

Erosion resulted mainly from debris avalanches down the 

mountain-sides and channel scour along streams and headwater tributaries. Total amounts of sediment yield from 

certain mountainous areas in Nelson County were about 

3.2-4.6 million cubic feet per square mile, probably the equivalent of several thousand years of normal denudation. 

Characteristics of the debris avalanches were that (1) 

they usually followed pre-existing depressions on hillsides 

and occurred on slopes greater than 35 percent, (2) the 

upslope tip of the avalanche scar tended to be located at 

the steepest part of the hillside, where the convex slope 

merged with the concave or planar zone immediately below, (3) hillsides facing north, northeast and east were 

more susceptible to avalanching than slopes facing other 

directions, and (4) debris-avalanches caused rapid and devastating surges of water and sediment in the mountainstream channels. Such surges in some instances temporarily 

blocked the channel flow upstream. 

Slightly more than half of the total sediment contributed 

to the stream system was from erosion of stream channels. 

Channel erosion was very irregularly distributed; some 

ravines 10-20 feet wide and 5-10 feet deep were s·coured 

in places which formerly had only a very small channel, 

whereas other channels only a few hundred yards away experienced little or no channel erosion. 

By the use of figures for the total amount of sediment 

removed from a drainage basin and the duration of the 

storm, estimates were made of the storm-average s·edimenttransport rate at the mouth of various basins. For drainage basins ranging up to about 1.5 square miles, the estimated storm-average sediment-transport rates varied from 

practically nothing to as much as 172,000 pounds per second (7.4 million tons per day). 

The types of sediment deposits were (1) debris-avalanche 

deposits, rather rare, at the base of hillslopes, (2) mountain-stream channel deposits, usually in scattered sediment 

patches but locally occurring as large wedge-shaped deposits 

behind debris dams, (3) alluvial fans, ( 4) delta-like deposits 

at the junction of a stream and major highway, where 

water backed up during the flood due to plugging of a culvert, and ( 5) accretion deposits on flood plains. The highway 

deltas and some downstream flood-plain sediments consisted 

mostly of sand-sized grains, but the other types of deposits 

usually contained particles ranging from silt or clay to 

boulders 5-10 feet in diameter. Changes in grain size and 

in volume of deposition with distance downstream were 

measured, and sedimentary features of the various types 

of deposits are described. 

INTRODUCTION 

On the night of August 19-20, 1969, the central 

part of Virginia was deluged with some 28 inches of 

rain from the remnants of Hurricane Camille. The 

loss of life and property due to the flash floods, landslides and other sediment damage accompanying the 

storm has been called the worst natural disaster ever 

to strike Virginia. The storm was one of the most 

severe ever recorded in the United States. 

The region most affected was Nelson County, a 

rural area midway between Charlottesville and 

Lynchburg (fig. 1). About 150 people, 125 of them 

in Nelson County, died in the storm and flood. 

Damage in Nelson County alone amounted to $116 

million worth of public and private property, including about 150 homes and other buildings, 120 miles 

of roads, 150 bridges and culverts, hundreds of cars 

and trucks and 25,000 acres of cropland, including 

orchards. About $150 million in public funds, mostly 

federal, has been spent on recovery, including $8 

million in debris clearance. Particularly heavy damage occurred in Massies Mill, Woods Mill, Roseland, 

Tyro, Lovingston, Norwood, Rockfish, and along 

Davis and Muddy Creeks (fig 1). 

On the basis of the extensive damage to life and 

property, the Commonwealth of Virginia has made 

recommendations for future land use in Nelson 

County (Commonwealth of Virginia, 1970). Kuhaida 

(1971) also offers some pertinent observations. 

This report is a study of the extraordinary erosion 

1 
2 HURRICANE CAMILLE IN VIRGINIA, 1969 

.Shipman 

4 KILOMETERS L....L_L__L_____l _ _L___J 

FIGURE 1.-Study area. 

a.nd deposition which the storm inflicted on parts of 

Nelson County. Most of our field observations and 

measurements were made within 3 months after the 

flood. Erosion and deposition, somewhat less severe, 

also occurred in regions bordering Nelson County, 

but time limitations permitted only a brief reconnaissance of these nearby areas. 

Because much of the sediment deposition from the 

flood interfered with the daily operations of man, a 

massive sediment-cleanup campaign began soon after 

the flood. Bulldozers effectively destroyed most of the 

alluvial fans and many flood-plain deposits in the 

weeks and months following the flood. Necessary 

though this work was, it did limit the scope and 

thoroughness of a study of the erosion and deposi- · 

tion. All of the data presented in this report were 

obtained before man's interference with the deposits. However, the writers would like to emphasize 

that anyone wishing to study flood deposits should 

act as soon as possible after the flood. 

We acknowledge with thanks the many helpful 

manuscript suggestions of Dwight R. Crandell, Luna 
GENERAL DESCRIPTION OF STUDY AREA 3 

B. Leopold, and John T. Hack. Jon W. Nauman provided valuable assistance with some of the field work. 

Mr. and Mrs. S. K. Wills, Robert Bryant and Guy 

Spencer, residents of Nelson County, allowed us to 

roam at will over their land. Many excellent photographs were provided by the Virginia Division of 

Mineral Resources, the Virginia Department of Highways, Ed Roseberry and others. 

GENERAL DESCRIPTION OF STUDY AREA 

The devastated area lies within the James River 

drainage basin in the Blue Ridge geomorphic province. Most of the region is mountainous (fig. 2), with 

peaks towering as much as 1,200 feet above the valleys. The climate is temperate-humid. Trees, bushes, 

small plants and dead leaves cover all of the hillslopes and ridges. Tree varieties include walnut, 

poplar, oak, ash, locust, hickory, and sycamore. 

Bedrock is mainly Precambrian gneisses, granites 

and schists of the Lovingston and Marshall Formations (Bloomer and Werner, 1955). The geology is 

very complex, with a variety of rock types. Jointing 

is present in some places and apparently absent in 

others. On hillslopes the bedrock generally lies from 

a few inches to about 20 feet below the surface. The 

soil, formed in place from the bedrock, is typically a 

reddish or brown loam and generally contains some 

rock fragments. 

THE STORM 

Hurricane Camille struck the State of Mississippi 

on the Gulf of Mexico on August 17, 1969. As it 

moved inland to the north and then curved eastward, 

the hurricane weakened considerably. Eastern Kentucky received only 1-2 inches of rainfall. 

On the night of August 19 a rare combination of 

circumstances brought about a rapid intensification 

FIGURE 2.-Upstream view of Wills Cove and Dillard Creek, a typical drainage basin studied in this report. The photograph, taken a few days after the flood, shows valley flood-plain deposits and hillside scars due to debris avalanches. 

Main stream from Fortunes Cove enters from right foreground. (Photograph courtesy of Virginia Division of Mineral Resources.) 
4 HURRICANE CAMILLE IN VIRGINIA, 1969 

so· 79 ° 

39° EXPLANATION 

--15--

Line of eqtml total rainfall, in inches 

WASHINGTON, D.C. e 

20 0 20 40 MILES 

20 0 20 40 KI LOMETERS 

38° 

ROANOKEe 

37°L-----------------~-----------------------L----------------------~--------------------~ 

FIGURE 3.-Rainfall from noon August 19 to midnight August 20, 1969. Adapted from DeAngelis and Nelson (1969). 

of the rainfall in the region just east of the Blue 

Ridge Mountains, especially around Nelson County, 

Virginia (Schwarz, 1970). The air in the region 

southeast of Nelson County was nearly saturated 

with moisture that had been accumulating for several days before the storm. This moist air extended 

up to an altitude of about 3,000 feet. The hurricane 

itself contained a large amount of moisture at higher 

elevations. As Camille arrived, the remnant circulation of the hurricane created a flow of wind in a 

northwesterly direction at the lower levels. These 

winds drew moist air from the southeast, and this 

moisture moved under and joined that of the hurricane to form a very deep layer of moist air. Thunderstorms developed which, because of the unusual 

thickness of the layer of moist air, were particularly 

intense and persistent. Figure 3 shows the rainfall 

distribution in central Virginia for the period covering the storm. 

In some places the orography may have contributed to the intense rainfall by funneling the northwest-flowing air into the mountain ridges. However, 

the effect of orography on the rainfall is difficult to 

determine. Over 21 inches of rain fell in a region 

about 15 miles southeast of Charlottesville where the 

terrain, while not flat, is devoid of any steep or high 

mountains. Thus some areas received torrential rain 

with little or no influence from nearby mountains. 

The deluge was of catastrophic cloudburst proportions. Rainfall of 12-14 inches was common in Nelson 

County and nearby areas, with reliable reports of 

27-28 inches in the central part of Nelson County 

(Camp and Miller, 1970; Schwarz, 1970). Other 

sources have reported unconfirmed amounts of as 

much as 46 inches (Simpson and Simpson, 1970, 

p. 31). 

The storm lasted about 7 or 8 hours, from around 

2030 hours August 19 until 0330 or 0400 August 20. 

Except for a brief lull at about 2330 or 2400 hours, 

the rainfall, thunder, and lightning were intense 
THE FLOOD 5 

during the entire storm. A particularly strong cloudburst struck about 0250, cutting off electricity and 

telephones for many of the inhabitants. At this time 

lightning-mainly sheet or horizontal lightningwas so severe that the sky was virtually a continuous bluish-white. 

The staggering amount of rain easily ranks among 

the largest quantities ever measured in the United 

States. DeAngelis and Nelson (1969, p. 458), for 

example, compare it to the following: 

12 inches in 42 minutes at Holt, Mo., in 1947, 

19 inches in 2 hours, 10 minutes at Rockport, 

W. Va., 1889, 

22 inches in 2 hours, 45 minutes at D'Hanis, 

Tex., in 1935, 

31 inches in 4 hours, 30 minutes at Smethport, 

Pa., in 1942, 

34 inches in 12 hours, at Smethport, Pa., in 

1942. 

Schwarz (1970, p. 853) states that Camille's rainfall in Virginia was within about 80--85 percent of 

the estimated maximum possible rainfall, for areas 

up to 1,000 sq mi over a 12-hour period. Y ameli 

(1935, p. 50-51) presents figures showing that the 

maximum 8-hour rainfall for central Virginia might 

be 3.5-4.0 inches once every 10 years and 5.5-6.0 

inches once every 100 years. After 25 years of additional data, Hershfield (1961) presented charts 

which for the eastern U. S. are nearly the same as 

Yarnell's. Hershfield's curves show that for central 

Virginia a rainfall of 7 inches in 12 hours (no 8-hour 

data given) would be expected only once in 100 years. 

Thus the Camille rainfall in Virginia was about 3-4 

times the amount expected once every 100 years, 

for an 8-hour storm. Thompson (1969) reported that 

for Virginia the amount of rainfall associated with 

this storm occurs, on the average, only once in more 

than 1,000 years ! 

THE FLOOD 

Comparisons of this flood with previous floods in 

the James River basin are difficult because of the 

different lengths of record at the gaging stations. A 

comment by Camp and Miller (1970, p. 21) indicates 

the magnitude of the 1969 flood: "The flow of the 

James River at Richmond peaked at 222,000 cubic 

feet per second and is considered to be the second 

highest discharge on the James River since Jamestown was settled in 1607." Towns along the James 

River and its main tributaries were inundated to 

depths of as much as 14 feet. 

At Buena Vista, a town about 15 miles west of 

Nelson County on the Maury River, the measured 

flood discharge of 105,000 cfs (cubic feet per second) is not likely to be equaled or exceeded, on the 

average, more than once every 130 years (Camp and 

Miller, 1970). When one considers that the flow on 

the James River at Richmond was the second highest since at least 1607, discharges as large as the 

1969 flood occur at Richmond an average of about 

once every 180 years. These estimates of 130- and 

180-year floods provide a very rough idea of the 

recurrence frequency of the Camille flood at stations 

en two major rivers, the Maury and the James, near 

Nelson County. 

There are no streamflow records for most streams 

in Nelson County, where much of the heaviest rainfall occurred. Data for the few sites where gaging 

stations have been established go back only a few 

decades. For the nine sites in the area at which a 

peak discharge measurement was obtained (six 

measurements were by indirect methods), the 

August 1969 flood exceeded the previous record by 

factors ranging from 1.5 to 8. The discharges in the 

deluged upstream basins undoubtedly are less common in those basins than the 130- and 180-year 

occurrences estimated for the Maury and James 

Rivers. 

This study unfortunately has no water-discharge 

measurements in the upstream reaches of the drainage basins, where much of the sediment movement 

occurred. Flow depths, according to eyewitness accounts, sometimes changed radically within minutes. 

One farmer described the flow in Cub Creek as increasing in depth by intermittent jumps of as much 

as 3 feet at a time. A resident of Woods Mill reported 

that the water in his back yard rose about 5 feet in 

20 minutes. Another Woods Mill resident estimated 

that the water near his house rose about 8 feet in 

less than 30 minutes. Even with allowances for 

errors in estimates, the discharge of even the larger 

streams undoubtedly changed rapidly, and in the 

mountains the discharge of streams draining only a 

few hundred acres probably changed even faster as 

a result of the impact from the debris avalanches 

(Guy, 1971). 

Even if the discharge at a given site had been 

monitored closely throughout the flood, the investigator would have the problem of what discharge to 

relate to the resulting sediment deposits. 

ESTIMATED STORM-AVERAGE DISCHARGES 

Measured rainfall amounts can provide a rather 

crude estimate of the average water discharge for 
6 HURRICANE CAMILLE IN VIRGINIA, 1969 

the duration of the storm for the smaller upstream 

drainage basins. This method involves the so-called 

rational formula 

Q==ciAd 

where Q is water discharge or surface runoff (in 

acre-inches per hour, for this formula), c is a coefficient representing the proportion of total rainfall 

that runs off the ground surface, I is average rainfall intensity in inches per hour and Ad is drainage 

area in acres. Flow rate in acre-inches per hour is 

about equal to flow rate in cubic feet per second. The 

coefficient c is usually estimated on the basis of previous experience with similar areas and reflects the 

role of infiltration losses, surface detention, valley 

and channel storage, and general physical characteristics of the drainage basin (Foster, 1948, p. 343). 

The effects of these factor~ probably were minimized 

for this heavy storm, especially for the small steep 

drainage basins in the study area. Reasonable values 

for c therefore are taken to be about 0.90 for basins 

of 100 acres or less, decreasing by 0.02 for each 

additional 100 acres of basin size up to 2,000 acres. 

In the upstream regions the flood probably lasted 

from an hour or so after the onset of heavy rainfall 

to an hour or so after the rain stopped, a period of 

about 7 hours. According to the rainfall data mentioned above, plus the figures which Camp and Miller 

(1970) listed, the average total rainfall over the general study area can be taken as about 20 inches. If 

20 inches accumulated during the 7 -hour period, the 

average intensity I was about 2.9 inches per hour. 

The mountain drainage basins (fig. 1) studied in 

some detail in this report, selected because they 

showed some of the most widespread sediment 

erosion and deposition, are Ginseng Hollow (drainage area Ad==0.667 sq mi), Polly Wright Cove 

(Ad=0.953 sq mi), Wills Cove (Ad=l.577 sq mi), 

and Freshwater Cove (Ad~2.333 sq mi). (Some 

neighboring regions, such as the Davis Creek area 

and Fortunes Cove, also experienced considerable 

erosion and deposition and might have been included 

in the investigation had time permitted.) If one 

uses the rational formula with the assumption just 

mentioned, the approximate 7 -hour average discharges were 1,500 cfs for Ginseng Hollow, 1,400 cfs 

for Polly Wright Cove, 2,100 cfs for Wills Cove and 

2,700 cfs for Freshwater Cove. 

ESTIMATED PEAK DISCHARGES 

Investigations made after the flood usually, of necessity, deal with the peak discharge. Two ways to 

estimate the peak discharge in the upstream reaches 

of interest are (a) to extrapolate a best-fit line relating drainage area to measured discharges for downstream sites or (b) to measure the channel slope and 

estimate the cross-sectional flow area for each study 

reach, for use in the Gauckler-Manning formula. 

Both of these attempts were fruitless in the present 

case. A plot of drainage area versus peak discharge 

per square mile, with data for the few downstream 

sites measured, showed too much scatter to permit a 

reliable extrapolation to small drainage areas. The 

slope-area method faltered over a reliable estimate 

of the resistance coefficient, as there was no way to 

determine this value for a stream carrying and depositing particles ranging from clay to large boulders, along with trees and other debris. 

A third possible way to estimate the peak discharge in the small upstream drainage basins is to 

resort again to the rational formula Q=clAd, in 

which I represents a peak rate of rainfall intensity 

and c and Ad have the same values used above. If 

one may judge from the data which Jennings (1950) 

compiled, a reasonable estimate of the peak rate of 

rainfall intensity in the basins studied here is about 

25 inches per hour, even though such an intense 

burst probably did not last for more than 5--10 minutes. If this peak intensity and the assumptions regarding c are valid, the rational formula yields approximate peak discharges of 9,100 cfs for Ginseng 

Hollow, 12,200 cfs for Polly 'Vright Cove, 18,200 cfs 

for Wills Cove and 23,100 cfs for Freshwater cove, 

or about 6-9 times as large as the values ·(estimated 

above) for the 7 -hour average discharge. These 

peak values approximate the Myers rating of 100, 

that is, the peak discharge per square mile is about 

equal to 10,000/ Ad0 •

50 • Some indirect measurements 

which Camp and Miller (1970, p. 59) reported for 

this storm also approach this rating; so the rationalformula assumptions have some support. 

All of the above discharge estimates are for water 

alone, exclusive of the large amounts of sediment 

and vegetative debris contributed by the hillsides and 

stream channels. 

GEOMORPHOLOGICAL FEATURES OF 

DRAINAGE BASINS 

The amount and extent of the erosion and deposition can be better understood if some background 

information is provided on the landscape and streamchannel characteristics of the study area. The following concerns selected geomorphological aspects 

and their interrelations. 
GEOMORPHOLOGICAL FEATURES OF DRAINAGE BASINS 7 

SLOPES OF DRAINAGE AREAS 

Steep hillslopes provided excellent conditions for 

mass wasting. Figures 2 and 4 indicate the general 

steepness of the regions studied. 

The areal distribution of slopes for the three drainage basins of figure 4 is shown in figure 5. These 

slopes, listed in table 1, were obtained from the contour maps (scale 1:24,000) by map measurements of 

the local slope (50 ft upslope and 50 ft downslope) 

at each of many grid points, shown as black dots on 

figure 4 and spaced 400 feet apart. Subbasin drainage areas were outlined as in figure 4, and the areas 

were measured by planimeter. The general slope 

TABLE !.-Drainage area and average surface slope in tributaries and subbasins of Wills Cove, Ginseng Hollow, and 

Polly Wright Cove 

Drainage basin 

Wills Cove 

West tributary ------- -------- Middle tributary - ------------ Falls tributary 

Subbasin 1 --------------

2 --------------

3 -------------- 4 -------------- 5 -------------- 6 -------------- 7 ------------ -- 8 ---------- ---- 9 ------------ -- 10 -------------- 11 -------------- 12 - ------------- 13 -------------- 14 -------------- 15 ------------- - 16 -------------- 17 - ------------- 18 --------------

19 -------------- 20 -------------- 21 -------------- 22 --------------

Drainage area 

(sq mi) 

0.238 

.145 

.040 

.037 

.029 

.068 

.046 

.030 

.026 

.052 

.047 

.033 

.027 

.024 

.114 

.060 

.032 

.063 

.064 

.014 

.022 

.006 

.008 

.038 

Subtotal (1-22) ____ ____ .880 

Noncontributing ________ __ .314 

Total, Falls tributary 1.194 

Average slope, basins 8-14' __ _ _ Total Wills Cove ______ 1.577 

Ginseng Hollow 

Lower ----------------------- 0.172 

North tributary -------------- .086 

South tributary - ------------- .115 

Main> 34002 - ---------------- .222 

East bowl ------------------- .082 

Total ------------------ .677 

Average slope, entire basin 

See footnotes at end of table. 

Average 

surface 

slope 

(percent) 

46.2 

46.5 

39.5 

45.0 

41.4 

48.0 

40.0 

39.1 

41.5 

41.3 

48.1 

42.7 

43.9 

38.0 

32.1 

41.5 

TABLE !.-Drainage area and average surface slope in tributaries and subbasins of Wills Cove, Ginseng Hollow, and 

Polly Wright Cove-Continued 

Drainage basin 

Drainage 

(sq mi) 

Polly Wright Cove 

Tributary------- ---- 1 ------

Total (1-13) 

Average slope, 

2- 5 ------

6 ----- 7 ------

8 ------

9 ------ 10 ------

11 ------ 12 ------ 13 ------

0.043 

.086 

.037 

.026 

.069 

.060 

.034 

.037 

.043 

.024 

.459 

Average 

surface 

slope 

(percent) 

49.3 

38.6 

39.3 

40.0 

45.8 

45.5 

53.0 

54.2 

47.5 

52.0 

tributaries 1-13 ---------------- 45.6 

Lower ----------------------- .130 37.7 

Noncontributing -------------- .364 21.8 

Total, entire basin ___ __ _ .953 

Average slope, entire basin -------- 35.4 

1 Weighted according to contributing drainage area. 2 Refers to all draii!lage area upstream from a station 3400 ft up the main channel, the measurement of this distance having begun at the mountain front (apex of alluvial fan) . 

representing each such drainage area is the average 

of all local slopes (grid points) within that area. 

The terrain in Polly Wright Cove is virtually the 

same as in the other basins, but the curve in figure 5 

shows a lesser part of the total drainage area with 

steep slopes for Polly Wright Cove because the map 

measurements included more of the flood plains of 

the basin. 

Hillslope erosion appeared to be negligible or nonexistent in tributaries or subbasins having an average surface slope of about 35 percent or less. 

STREAM LENGTH AND DRAINAGE AREA 

The time for debris eroded near the stream headwaters to arrive at a given downstream site depends 

in part on the distance along the stream channel 

which the debris must travel. Also, the amount of 

water and sediment which a stream carries depends 

partly on the stream's drainage area. Thus, stream 

length and drainage area are important factors in 

the geomorphology of a flood-damaged region. 

Various geomorphic studies have established that 

stream length L is approximately related to drainage area Ad, where L is measured from topographic 

maps (scale 1:24,000 in this case) as horizontal distance from drainage divide along the channel of the 

longest stream above the station. Hack's data (1957, 
8 HURRICANE CAMILLE IN VIRGINIA, 1969 

West 

Tributary 

37° 

-47' 

30" 

8 9 

Base from U.S. Geological Survey, 

Horseshoe Mountain and Lovingston, 1967 

.5 

14 

3r 

47'-. -

30" 

79° 55' 

I 

19 

16 17 18 

SUBBASINS IN HEADWATERS OF WILLS COVE 

0 

0 

Area of West Tributary= 0.238 square mile 

Area of Middle Tributary=0.145 square mile 

Area of Falls Tributary = 1.194 square miles 

1 MILE 

.5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET 

DATUM IS MEAN SEA LEVEL 

FIGURE 4.-Above and right. Contour maps of Ginseng Hollow, Polly Wright Cove and Wills Cove basins, showing locations of grid points (dots) used to determine average surface slope for tributaries and subbasins. Subbasins are outlined with solid lines. Arrow at mouth of each basin shows flow direction. Notable hillslope erosion lacking in patterned areas. 
GEOMORPHOLOGICAL FEATURES OF DRAINAGE BASINS 

A7 ' 30"--- 370 

GINSENG HOLLOW 

Area =0.677 square mile 

10 

37" 4'7' 30"--

2 3 4 

6 

2 

7 

8 9 

78° 52' 30" 

+ 37"47 ' 30" 

78° 52'30" 

East Bowl 

South Tributary 

POLLY WRIGHT COVE 

Area= 0.953 square mile 

9 
10 HURRICANE CAMILLE IN VIRGINIA, 1969 

100 . 

UJ 

a.. 

0 

--' (/) 

z 

UJ 

> 80 (5 

z 

<( 70 I 

Ia: 

UJ 

a.. 60 UJ 

UJ 

I-

(/) 

<( 

UJ 

a: 

<( 

z 

V5 

<( 

Ill 

u.. 

0 

UJ 

(!) 

<( 

Iz 

UJ 

(.) 

a: 

UJ 

a.. 

' '•, '• ' ' • ' ' ' • \ 

I 

\ 

\ 

\ 

~ \ \ \ \ ._, h 

' \ lit., \ 

\ \ \ \ 

EXPLANATION 

Polly --- Wright Cove 

Ginseng Hollow 

t:r---t---..t;:,. 

Wills Cove 

\ \ 

\\ 

\ '1\ 

' \ ~, \ 

c...... .... 

10 20 30 40 5.0 60 70 80 90 

SLOPE, IN PERCENT 

FIGURE 5.-Percentage of drainage-basin area steeper 

than given slope for Polly Wright Cove, Ginseng Hollow and headv. <tters of Wills Cove. 

p. 64) for some 85 measurements in nearby Virginia 

and Maryland showed that if all of the points are 

viewed collectively, then L ex: Ad0 ·

6 • Hack found this 

same general relation for 400 observations reported 

by Langbein (1947) for the northeastern United 

States. Two other areas which Hack tesb J-{me in 

Arizona and one in South Dakota-als<; had power 

relations but with an exponent of 0.7. Mueller (1972) 

studied 65 large drainage basins (ranging from 

5,000 to 5,000,000 sq mi) from various continents 

and reported the general relation L ex: Ad0 ·

466 • 

Figure 6 is a plot of stream length versus drainage 

area for three of the major streams studied here 

(see fig. 1). A general relationship in which L varies 

approximately with A d0 ·

6 probably could be fitted to 

the complete group of points. But the scatter and 

range of data are such that other exponents might 

also apply. More interesting is the fact that the 

points for any one stream show little scatter about 

an eye-fitted line for the particular stream (values 

of L from the equations in figure 6 can be determined 

to within about ± 15 percent or better). The exponents of lines drawn for individual streams show 

considerable variation, ranging approximately from 

0.4 to 0.7. Hack (1957) also found significant departures from his general relation L o:: Ad0 ·

6 • Consequently, although general relations are often useful and interesting, some attention also should be 

given to the extent to which individual streams depart from the general relation. 

A low exponent means a lesser rate at which 

stream length increases, for a given enlargement in 

drainage area. Conversely, for given increments of 

stream length, the stream with a lower exponent 

drains relatively larger areas as distance increases. 

Low exponents can reflect a lack o: meandering 

(smaller increases in stream length) for given increases in drainage area, or they might suggest a 

relatively rapid enlargement of drainage area downstreamward. The latter is the reason for Dillard 

Creek's low exponent of 0.42. 

LONGITUDINAL PROFILES OF STREAM CHANNELS 

The longitudinal profie of a stream, a plot of distance versus elevation, shows how the channel slope 

changes with distance and is one of the basic characteristics of a stream channel. 

TOPOGRAPHIC MAP DATA 

Longitudinal stream-profiles of five prominent 

channels were measured from topographic maps 

(scale 1:24,000). The profiles extend for distances of 

3-8 miles downstream from the drainage divide. 

Figure 7 shows these longitudinal profiles (fall F 

versus horizontal distance L) on arithmetic scales. 

The characteristic concave-upward profiles are 

smooth and regular for three of the streams (Campbell Hollow, Rucker Run and Polly Wright), but the 

other two (Ginseng Hollow and Wills Cove) include 

a pronounced deviation or local steepening. 

Many longitudinal profiles of streams and alluvial 

fans plot as straight lines on semilog paper. For 

some streams the elevation (ordinate) must be on 

the log scale, whereas for others the distance (abscissa) must be on the log scale. Krumbein (1937), 

Shulits (1941), Yatsu (1959), Leopold and Langbein (1962), and Fok (1971) dealt with the former 

type; their studies were concerned mainly with 

graded or well-developed rivers rather than mountain streams. Semilog plots of the profiles shown in 

figure 7, with elevation on the log scale, have a 

strong curvature, and no constant could be found 

(computed) to rectify the curves. 

Hack (1957, p. 70; also Hack and Goodlett, 1960, 

p. 11) and Leopold and Langbein (1962, p. A9) dealt 

with the second type of plot, where elevations vary 

with the logarithm of distance. A diagram of this 

sort, an exponential or depletion function, describes 

the five profiles of figure 7 in a rather approximate 

way. The plotted points, not shown here, have a 

sinuous path, and a straight line fitted by eye predicts the altitude to within about ± 17 percent. 
GEOMORPHOLOGICAL FEATURES OF DRAINAGE BASINS 11 

1-

w 

w 

u.. 

u.. 

0 

en 

0 

z 

c{ 

en 

::::> 

0 

I 

1-

~ 

....... 

I 

1-

C!l 

z 

w 

....J 

~ 

c{ 

w 

Q: 

1-

en 

10 

L = 11,800 A~- 42 

(Dillard Creek) 

L = 7800 A~- 64 

(Polly Wright Cove) 

L = 6550 A~- 67 

(Rucker Run) 

DRAINAGE AREA (A,:1), IN SQUARE MILES 

FIGURE 6.-Variation of stream length with drainage area for three Nelson County basins. 

For the range of elevations involved with these 

stream profiles, the actual values of the elevations 

are approximately proportional to the logs of the 

elevations. Consequently a graph of elevation (not 

vertical fall) versus L on log paper, not included 

here, also shows an approximate straight-line relation. The alinement of points on such a logarithmic 

graph is slightly better than on the semilog plot. A 

straight line drawn by eye predicts the altitude to 

within about +13 percent or better, with the possible 

exception of the point nearest the drainage divide. 

(Incidentally, plotting the fall F versus channel 

length L on logarithmic paper did not produce a 

straight line, nor could any constant be found to 

rectify the curve.) 

The type of plot which best expresses the elevation 

or fall along the five streams is a hyperbolic equation, like the power law, and takes the form 

F=L!(a+bL) 

where a= the extrapolated value of the ordinate at 

L = 0 and b is the slope of the best-fit line on the 

graph. Rearranging the equation into the form 

L/ F=a+bL gives the type of diagram on which the 

data plot as a straight line (fig. 8). (An alternative 

way of rearranging the equation is 1/F=a(1/L)+b, 

which indicates that a plot of 1/ F versus 1/ L, not 

included here, also gives a straight line on arithmetic 

scales. This variant of the plot is not practical because much of the stream length is squeezed into a 

small section on the abscissa.) 
12 HURRICANE CAMILLE IN VIRGINIA, 1969 

1-

UJ 

UJ 

..... 

~ 

,.....:;. 

~ 

UJ 

0 

> 

0 

UJ 

" < z 

;;: 

0:: 

0 

::!: 

0 

~ 

..... 

....1 

....1 

< 

..... 

....1 

< (.) 

i= 

0:: 

UJ 

> 

....1 

< 1-

0 

1-

~ 

• • 

0 

1 

EXPLANATION 

X Ginseng Hollow and Hat Creek 

0 Campbell Hollow and East Branch Hat Creek 

0 Rucker Run 

e Polly Wright Cove and Muddy Creek 

'Y Wills Cove and Dillard Creek 

0 5000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 

TOTAL HORIZONTAL DISTANCE FROM DRAINAGE DIVIDE (L), IN FEET 

-FIGURE 7.-Longitudinal profiles of stream channels, based on data from topographic maps. Vertical exaggeration is 

X 10. 

One distinctive feature of this diagram is the 

striking linearity of data points. Second, the local 

steepening in Ginseng Hollow is quite apparent and 

divides the profiles into two segments. This local 

steepening was much less recognizable in the semilog and log plots. Last, the first data point on the 

Campbell profile (100 ft from the drainage divide) 

is markedly offset from the best-fit line. The ratio 

L/F for this point is higher than would be expected, 

so for this particular horizontal increment the fall 

is less than would be expected. Probably the concave 

profile is not as well-developed this close to the drainage divide. 

After some algebraic manipulations, the equation 

F=L! (a+bL) assumes a form very close to one of 

the equations which Hack (1957, p. 70, eq. 11) found 

for stream profiles in nearby areas of Virginia and 

Maryland. The pertinent Hack equation is for 

streams in which the average size of the bed particles 

decreased downstream, as is the case in the streams 

studied here. Written in logarithmic form, Hack's 

equation is 

log (F-C)= log ( k I (n + 1)) + (n + 1) log L 

where C is a constant of integration and k and n are 

also constants. The present equation, rearranged and 

put in logarithmic form, is 

log (F-a ) =logy-log (L+,B) 

in which a=1/b, ,B=alb and y= (-a/b2 ). This is 

essentially Hack's equation with n=-2 except that 

the right-hand side has L + ,B, rather than L. 

The coefficients a and b both reflect such features 

as the channel steepness and do not lend themselves 

to simple descriptive labels. Values of a in this study 

ranged from 1.1 to 3.1, and b-values ranged from 

0.000600 to 0.000870. Eye-fitted lines predict the fall 

F to within about 9 percent error and usually to 

much greater accuracy. 

The appearance of the local steepening in Ginseng 

Hollow on the L/F versus L diagram might seem to 

suggest that this type of plot is sensitive to local 

channel aberrations. This, however, is true only near 

the drainage divide, where the distance L is still 

rather short and F increases significantly. The apparent sensitivity virtually disappears after L has 

increased to ·about 2 or 3 miles, by which stage F 

usually increases by relatively minor increments. 
GEOMORPHOLOGICAL FEATURES OF DRAINAGE BASINS 18 

25r---------~----------.---------~~--------~----------~--------~----------~ 

10 Ginseng Hollow and Hat Creek 

\ L - =1.7+0.00059 L 

F 5 

o o~--------~----------~----------L----------L----------~--------~L---------~ i= 

c( 

0:: 

10 \ L - = 1.2+0.00071 L 

F 

5 Campbell Hollow and East Branch Hat Creek 

~~--------~~~--------~-o~.oo_o _________ I5~.oLoo---------2-o.oooL---------2-5~.ooo----------3o~.oo-o---------3~5.ooo 

TOTAL HORIZONTAL DISTANCE FROM DRAINAGE DIVIDE (L}, IN FEET 

FIGURE B.-Longitudinal channel profiles plotted in the form of L IF versus L (topographic map data) . 

FIELD MEASUREMENTS 

On three of the channels-Ginseng, Polly Wright 

and Campbell-more detailed longitudinal profiles 

were measured in the field with a stadia rod and 

either a Zeiss level (for flatter reaches) or a hand 

level. Distances along the channel are virtually the 

same as horizontal distances and will be so treated 

in the following discussion. The distance and fall 

from the drainage divide to the uppermost channel 

station were taken from the topographic map. 

These longitudinal profiles of mountain channels 

and, where present, their· adjoining alluvial fans 

have the usual concave-upward shape when plotted 

on arithmetic scales (fig. 9). The local steepening in 

Ginseng Hollow, discussed earlier, appears in the 

upstream part of the Ginseng profile. The profiles 

tend to consist of straight-line segments, a feature 

which Bull (1964) pointed out for alluvial fans in 

California. The present field data show that this segmented characteristic applies to the channel upstream from a fan, in addition to the fan itself. 

Furthermore, the profiles show no sudden change in 

slope at a fan apex, and the location of the apex 

cannot be found from studying the profile. This is 

also a feature which Bull (1964, p. 101) noted for 

his profiles in California. And Denny ( 1965, p. 55) 

remarked that the mountain channels and alluvial 

fans which he studied in California and Nevada have 

a smooth profile with no break in slope at the fan 

apex. As Morisawa (1968, p. 94) noted, the primary 

cause of fan deposition therefore is not a sudden 

flattening of channel gradient. Instead, such deposition most likely results from the sudden spreadingout of the flow as it escapes the confining mountain 

channel. 

Profiles on semilog and log paper, not included 

here, also consist of straight-line segments. The number of segments decreases progressively from arithmetic to semilog to log plots. As with the topographic-map data, a straight line on the log diagram 

is a better approximation of the longitudinal profiles 

than a straight line on semilog paper. The L/F 

versus L relation (fig. 10) describes the data most 

accurately. 

The equations for obtaining the vertical fall F 

from the drainage divide, in feet, for a given horizontal distance L from the drainage divide, in feet, 

are: 

Ginseng-Bryant: 

Polly Wright: 

Campbell: 

F=LI (2.52+0.00050£) 

F=LI (1.36+0.00075£) 

F=LI (1.20+0.00065£) 
14 HURRICANE CAMILLE IN VIRGINIA, 1969 

1-

UJ 

UJ 

LL 

g 8] UJ .... _I 

<( 

UJ 

(/) 

UJ 

> 

0 

fD 

<( 0 

z 

0 

i= 

<( 

> 

UJ 

_I 

UJ 

/End of fan 

EXPLANATION 

X Ginseng Hollow and Bryant fan 

e Polly Wright Cove 

0 Campbell Hollow and fan 

2000 5000 6000 7000 8000 9000 10,000 11,000 

TOTAL HORIZONTAL DISTANCE FROM DRAINAGE DIVIDE (L), IN FEET 

FIGURE 9.-Longitudinal profiles of mountain channels and, where present, adjoining alluvial fans. Data from field measurements. Vertical exaggeration is X 10. 

These equations were obtained graphically from figure 10, in which the best-fit lines have been drawn 

by eye. Except for the local steepened reach in Ginseng Hollow, the equations predict the total fall from 

the drainage divide to any station on the stream 

channel to within -+-2.5 percent. 

In summary, this section has presented measured 

values of drainage areas, stream lengths and the 

steepness of the terrain, together with some relation- 1 

ships between these factors. A mathematical expres- . 

sion of the longitudinal profile has been found for 

several of the main streams in the study area. Inasmuch as some kinds of erosion described in the following section are typical of mountainous regions, 

future studies using more data may discover some 

mathematical relation between the geomorphological 

characteristics of the drainage basins, on the one 

hand, and the amount and type of erosion expected to 

result from a given quantity of rainfall. Similarly, 

when more data are available it may be possible to 

relate the geomorphological features to the downstream amount and pattern of deposition. Some tentative steps toward these goals are taken in later 

sections of this report. 

EROSION 

DEBRIS AVALANCHES 

Measurements and estimates of amounts of erosion presented later in this report suggest that nearly 

half of all the storm-eroded sediment came from 

downslope-trending strips on the hillsides. Many 

scars (figs. 11 and 12) testify to the magnitude of 

this hillslope degradation. The nature of these scars 

and their association with the heavy rainfall indicate 

that they were caused by a type of mass-movement 

which Sharpe (1938, p. 61) called a debris avalanche. A debris avalanche is a rapid down-hill flowage of soil, rock, trees and other vegetation (if 

present) and water. According to Sharpe, "the typical debris-avalanche has a long and relatively narrow track, occurs on a steep mountain slope or hillside in a humid climate, and is almost invariably 

preceded by heavy rains." In Sharpe's classification 

debris avalanches differ from landslides in that landslides involve very little water. The Nelson County 

avalanches included soil, trees and other plants, 

water, and usually rocks ranging from gravel to 

boulders up to 10 feet in intermediate diameter. 
EROSION 15 

4.5 

4.0 

3.5 

3.0 

EXPLANATION 

X Ginseng Hollow and Bryant fan 

• Polly Wright Cove 

o Campbell Hollow and fan 

11,000 

TOTAL HORIZONTAL DISTANCE FROM DRAINAGE DIVIDE (L), IN FEET 

FIGURE 10.-Longitudinal profiles of figure 9 (mountain channels and, if present, adjoining alluvial fans), plotted in 

the form of L/ F versus L. Data from field measurements. Every other point plotted. 

A previous report (Williams and Guy, 1971) analyzed various aspects of the 1969 Nelson County 

debris avalanches. 

Hack and Goodlett (1960) described in considerable detail the important role of debris avalanches 

in the overall adjustment among slope processes, 

mountain form, and vegetation for a similar centralAppalachian region in Virginia and West Virginia. 

Eyewitnesses or survivors of debris avalanches 

say that the entire event occurs very quickly. Observations during the present storm were obscured because of the darkness. Mr. S. 0. Mawyer of Woods 

Mill was almost swept away in a small avalanche 

near his home and only saved himself by grabbing a 

bush that was growing just beyond the slide border. 

He said the ground started oozing slowly downhill 

for a fraction of a second, and then the entire section 

of the hillside suddenly slid quickly down the slope, 

accompanied by a loud noise. Mr. B. R. Floyd, whose 

Lovingston, Va., home (a few hundred yards downstream from the base of a long, high mountain) was 

severely damaged by a 3,000-foot-long avalanche 

(fig. 13), estimated the duration of the event in the 

vicinity of his home to be about 3 minutes. Mr. W. E. 

Davies, a geologist for the U.S. Geological Survey, 

saw a debris avalanche in a 1949 West Virginia 

storm and testified (oral commun., 1970) that the 

whole strip of hillside started moving at about the 

same instant. These subjective descriptions give 

some indication of the short duration of a debris 

avalanche. 

Rapp (1963, p. 198-200) quoted an eyewitness 

description of debris avalanches in western Norway: 

"A mass of earth, boulders, trees and water moves 

down the slope and a new slide track is formed * * * 

The river (at the base of the hill) is filled with a 

porridge of earth which flows downstream, mixed 

with a crowd of naked birch stems, twisting and 

whirling * * * New slides are coming down. It looks 

like a wave of water that squeezes earth and trees 

out of the ground and back again. The trees fall 

down immediately (with their bases pointing downslope, according to Rapp). Then they are pushed 

together with the earth and boulders on the way 

downslope, so they reach the river naked, without 

twigs and bark. Water sprays out in small cascades 

from the moving earth." 

People living in or near the mountains in Nelson 
16 HURRICANE CAMILLE IN VIRGINIA, 1969 

FIGURE H.-Erosion and deposition caused by the torrential rainfall, Davis Creek area, Nelson County, Va. Scars on 

hillsides mark locations of debris avalanches. Stream flood plain shows both erosion and deposition. (Photograph by 

Ed Roseberry.) 

County were unanimous in emphasizing the awful 

roar they heard associated with both the debris 

avalanches and the swollen streamflow. The noise 

was variously described as sounding like a squadron 

of airplanes warming up for takeoff; or continuous 

discharges of dynamite, one charge right after the 

other, sometimes overlapping; or many cannonballs 

rolling along inside a bowling alley, with cracking, 

popping, and explosions. 

The slides occurred at intervals ranging from "one 

right after the other" or "every few minutes" to 

"every once in a while" (Simpson and Simpson, 

1970, pp. 8-9). The accounts of local residents suggest that the avalanching was more frequent toward 

the end of the storm. Some slides occurred after the 

rain had abated to a sprinkle or mist, at about 0400 

hours on August 20. 

At least the larger debris avalanches caused a very 

noticeable quivering or trembling of the earth's surface for a radius of several miles. 

There is no way to determine how many of the 125 

people who lost their lives in Nelson County were 

killed by debris avalanches. A number of missing 

houses, particularly in the Davis Creek basin, originally stood in small hollows which were scoured by 

avalanches. Other lives were lost when buildings and 

property hundreds of feet downstream from hillslopes were ravaged by avalanche water and debris. 

Dr. J. H. Gamble of Lovingston, who was in charge 

of identifying and examining the flood victims, found 

that almost none of the dead showed evidence of 

drowning. "The vast majority were just multiple 

fractures-rib ... , spine ... , large bone ... , skull 

fractures-from rocks and trees and so forth. Most 

died from massive 'blunt force' injuries." (Simpson 

and Simpson, 1970, p. 270). Such testimony shows 

that sediment damage during floods can be very important in terms of lives as well as property. 
EROSION 17 

SOURCE MATERIAL moved in a single debris avalance was about 88,000 

cubic feet or about 3500--4500 tons. The soil type on the mountainsides, according to 

the U.S. Department of Agriculture classification, is From the upslope tip, every debris avalanche conof the order inceptisol-a young soil that presumably tinued all the way down the hillside. Most scars behas formed rather quickly, mostly from alteration of come slightly deeper with distance downslope. 

parent materials (here, the underlying crystalline Widths tend to increase downslope, as do avalanche 

rocks) and which lacks horizons of marked accumu- tracks in other areas (Swanston, 1969) ; however, 

lation of clay, iron oxide, and aluminum oxide. The there are many exceptions where the width stays 

subdivisions within the inceptisol order would be about constant or decreases (fig. 12A). 

ochrept and dystrochrept. The heads of individual scars can be almost anyThe soil usually is about 1-3 ft thick on the steeper where on a hillside, except that not one is located at 

parts of the terrain. Logically, the thickness of the the crest of a hill. Some avalanches involved only the 

profile should vary inversely with the long-term ero- foot of a hillslope, while others extend nearly to the 

sion rate and (or) directly with the weathering rate crest. Most scars originate in the middle third of the 

of the rocks. Most profiles on the hillslopes contain slope between the ridge crest and the channel. 

some larger rock fragments, including a few of boul- From a top view the break at the head of the scar 

der size. can be curved, irregular, or even a virtually straight 

Two or 3 inches of dark-colored organic matter line, normal to the downslope direction. 

cap the soil profile. Below this organic layer the soil Almost all avalanches removed the entire vegetal 

color is mostly red-brown, The texture is that of a cover in their path, including large trees. In many 

soil matrix, rather uniform in size-composition with cases the bedrock was exposed along the full length 

depth, containing a few larger fragments ranging of the scar. On other scars a foot or two of soil still 

up to several feet in diameter. Soil samples from the covers the bedrock, particularly in the lower reaches. 

side walls of 10 scars-three in Polly Wright Cove, Some of the avalanches-probably those that ocfive in Wills Cove, and two in Ginseng Hollow- curred near the end of the storm-removed only the 

were analyzed for particle size. Such samples were vegetation and part of the soil, so that no bedrock 

small (1 or 2 kilograms) and did not include parti- was exposed. Many scars eroded to bedrock occur 

cles larger than about 25 mm (millimeters). There next to scars which still have a partial soil cover (fig. 

appears to be no significant difference in particle 12C) · 

size among the 10 soil samples. The average for the The break between most slide areas and the adja10 samples was 15 percent gravel, 48 percent sand, cent unaffected ground is clean and sharp, especially 

31 percent silt, and 6 percent clay-sized particles by at the top of the scar. Many roots of the vegetation 

weight. The median size ranged from 0.08 to 0.28 which had been growing in the eroded soil were 

mm. The larger fragments occur at diverse levels broken and exposed. Most of these roots point downwithin the soil profile and usually do not show any slope. 

pronounced trend of becoming larger with depth. , Mud deposits around the base of tree-trunks along 

Some of these large fragments are on the top surface the edges of some scars suggest that the top surface 

and probably move downslope by creep. No repre- of the moving avalanche was as much as 3 feet 

sentative size-frequency distribution was obtained higher than the normal ground surface. 

for these large rocks. Nearly all the avalanches in the headwater areas 

GENERAL DESCRIPTION OF AVALANCHE SCARS 

Typical avalanche scars range from about 200-800 

feet in horizontal length, 25 to 75 feet in width and 

1 to 3 feet in depth. Some scars are as short as 20 

feet or as long as 1,000 feet, as wide as 200 feet, and 

as deep as 15 or 20 feet. Average gradients are steep, 

ranging in most cases from about 0.30 ft per ft (foot 

per foot) (16 degrees) to 0.80 ft per ft (39 degrees). 

The most common average gradients were 0.50-0.60 

ft per ft. Based on 12 typical scars that were measured in detail, the average amount of sediment reentered first- or second-order streams at the base of 

the hillslope. These streams begin at the head of a 

mountain ravine which itself experienced debris avalanching or at least severe channel enlargement during the storm. The streams wind through the mountains for distances ranging from a few hundred feet 

to as much as several miles, before emerging into 

the broad intermontane valleys. 

Along such mountain-stream channels the banks 

opposite the scars rarely had deposits from the avalanches, nor was any material left in the channel itself. Thus at some time between the avalanche and 

the end of the flood, the stream removed nearly all 
18 HURRICANE CAMILLE IN VIRGINIA, 1969 

A B 

c 

FIGURE 12 (above and right) .-Typical scars left by debris avalanches on hillslopes : A-C, headwaters of Wills Cove; D, drainage basin upstream from 

Campbell fan; E, Ginseng Hollow. 
EROSION 19 

D 

the dislodged material. Few, if any, of the avalanches 

therefore occurred after the flood. 

Avalanches on the sides of the broader intermontane valleys continued beyond the base of the hillside for hundreds of feet and in most cases reached 

the flooded zone in the center of the valley. After the 

flood subsided, the path of these avalanches was 

characterized by an incised channel and by sediment 

deposition along each side of the channel (fig. 14). 

The huge volume of soil and rock eroded by debris 

avalanches is discussed in the section on "Sediment 

yield." 

GEOMETRY OF SCARS 

METHODS OF STUDY 

We measured the longitudinal profiles, cross profiles, widths, and depths of 12 scars which to the eye 

appeared to be typical and representative of the 

various types. For seven of these, the tools were a 

hand level, tape, and surveyor's rod. From the tops 

of these seven scars to the base of the hillside, we 

measured distance down the scar centerline with the 

tape and read elevations at intervals of 10- 100 feet 

(usually about 40-50 feet), depending on the steepness of the slope. We estimated or measured widths 

E 

at intervals of 20-100 feet down the scar by extending the 25-foot rod from one edge of the scar toward 

the opposite edge. At a few stations where the width 

was not determined in this manner, we simply estimated the total width by eye. We estimated erosion 

in the center of the scar at all stations either by eye 

or by holding the rod on one edge of the scar, pointing the other tip toward the opposite edge and assuming that the rod represented the original surface. 

The other five scars were mapped with an engineer's transit and surveyor's rod in sufficient detail 

FIGIJRE 13.-Scar of major ava!a!1che that penetrated Lovingston, about 0300, August 20, 1969. (Photograph by 

Ed. Roseberry.) 
20 HURRICANE CAMILLE IN VIRGINIA, 1969 

A 

FIGURE 14 (above and right) .-Debris avalanches along sides of intermontane valleys. A, East Branch of Hat Creek. Creek 

is at lower left foreground. Note deposition of logs, boulders and other particles along route from avalanche to creek. 

B, Stevens Cove. Note damage to orchard. (Photographs courtesy of Virginia Division of Mineral Resources.) 

that 10-foot contours could be drawn. With this 

method the areal spacing of measurements was about 

the same as with the hand-level method except that 

additional readings were taken on the nearby undisturbed ground surface, both above the head of the 

scar and on either side. Figure 15 presents maps of 

typical scars and locations of cross sections. No 

depth-of-erosion estimates were made in the field for 

scars mapped by transit; these depths instead were 

estimated from the plotted cross profiles. 

Elevations and distances from the drainage divide 

to the head of the avalanche scar were taken from 

the topographic map after we plotted the total horizontal scar distance, as measured in the field, on the 

map. The vertical distance from base of hillside 

(usually taken as the channel edge) to top of scar, 

as measured in the field, agreed very well with the 

distance indicated by the contour lines on the map; 

so for longitudinal-profile purposes the field and map 

measurements could be combined with no significant 

error. 

A profile measured along the -scar centerline in 
EROSION 21 

B 
22 HURRICANE CAMILLE IN VIRGINIA, 1969 

Polly Wright-C 

N 

Pol ly Wright-A l 

0 50 100 FEET 

CONTOUR INTERVAL 10 FEET 

ARBITRARY DATUM 

EXPLANATION 

0 

Control point 

I I I I I I I II I I I I I I II 

Cross profile 

Longitudinal profile 

Edge of scar 

FIGURE 15.-Contour maps of two avalanche scars, Polly Wright Cove, showing locations of cross and longitudinal 

profiles. 

most cases does not exactly represent the longitudinal profile before the avalanche because of the missing soil and rock in the scar. However, the overall 

lengths (ranging from 560 to as much as 1,600 feet) 

and vertical distances (240-599 feet) of the hillsides 

studied are large compared to the estimated depth of 

soil removed (generally less than about 5 feet). By 

adding to the measured elevations the thickness of 

soil and rock removed, as estimated in the field, we 

were able to reconstruct the approximate pre-storm 

profile for seven of the scars. These reconstructed 

profiles closely approximated those measured in the 

field. The only difference of any significance might be 

near the base of those hillslopes from which a particularly large amount of sediment was removed. 

This potential error might affect the apparent degree 

of concavity of that region of the hillside. The poststorm profiles as used here probably are quite close to 

the original profiles, in most cases. The avalanches 

certainly did not affect the profiles as far as the definition of convex and concave zones on the hillside is 

concerned. 

LONGITUDINAL PROFILES 

Figure 16 shows the 12 scar and hillside profiles, 

all of which have been standardized so that the units 

of length and elevation are from zero to 1.0. The abscissa is a sliding scale which we shifted systematically for plotting purposes, in order to put all of the 
EROSION 23 

ui 

0 

z 

~j en- Cii 

0.9 

0.8 

_Ju... 

~0 

S2·a.. 

1--0 

15 I- 0.5 >o 

_JI-

~ ~ 0.4 

Ql--

1--f--

g§ g 0.3 

a.. 

0 

a:: 

a.. 0.2 

0.1 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LO 

PROPORTIONAL HORIZONTAL DISTANCE, BOTTOM TO TOP OF HILL 

FIGURE 16.-Longitudinal profiles of hillslopes, including avalanche scars. Vertical and horizontal distances have been 

standardized to extend from 0 to 1.0. Table 2 gives actual scar and hillside dimensions. Arrow indicates position 

of head of avalanche scar. See table 2 for profile numbers and scar designations. 

profiles on one graph. An arrow on each profile 

marks the head of the avalanche scar. Table 2 gives 

the actual scar dimensions. 

The profiles in figure 16 show a variety of shapes, 

some details of which will be described in the paragraphs below. The reader should look at each profile, from base to hillside, and note (a) the general 

hillside steepness in the region of the head of the 

scar (arrow), and (b) the elevation of the head of 

the avalanche relative to the vertical distance from 

the bottom to the top of the hill. The scars generally 

tend to begin in the zone where the local gradient is 

steepest. In these profiles the steepest gradient is 

located from about 0.3 to 0.95 of the horizontal distance and from about 0.3 to 0.95 of the vertical distance from base to hilltop. The head of the scar 

TABLE 2.-Hillside and scar dimensions and erosion volumes 

for measured ava.lanche scars 

Hillside dimensions Scar dimensions Erosion 

Profile Scar volume 

number designa- Length Fall Length Fall (1,000 

tion (ft) (ft) (ft) (ft) ft3 ) 

1 ------ Ginseng-2,020 980 435 320 190 18 

2 ------ Ginseng-East Br 620 240 260 120 67 

3 ------ Ginseng-4,740 9()0 330 570 250 81 

4 ------ Ginseng-Middle 646 310 500 250 144 

5 ------ Ginseng-800 830 397 760 390 86 61 ----- Polly Wright-A 690 375 400 200 43 

7 ------ Polly Wright-B 560 300 300 170 52 81 ----- Polly Wright-C 930 420 400 200 45 

9 ------ Wills-5,850 880 505 260 170 32 

10 ------ Wills-1,300 660 385 420 270 100 

11 ------ Wills-1,100 1,200 600 870 390 162 

12 ------ Wills-7,550 1,620 565 860 380 233 

1 Figure 15 shows contour map of scars. 

occurred on the average at 0.59 of the horizontal 

distance and 0.62 of the vertical distance from base 

to hilltop. 

Hillslopes typically have a convex upper part and 

a concave lower part. In some cases a middle section 

of nearly constant gradient separates these two 

zones; otherwise, the convex and concave portions 

grade directly into one another. White (1966) has 

shown that convex, straight, and concave profiles can 

be most readily defined by plotting local hillslope 

gradient as a function of horizontal slope distance, 

on log scales. On such a diagram the convex profile 

near the crest of a hill in many instances plots as a 

straight line trending upward (positive exponent), 

because the local gradient increases with slope distance. A horizontal line on the graph indicates that 

the local gradient stays constant with distance away 

from the crest: that is, the hillslope is straight. The 

concave profile over the lower regions of a hillside 

means that the local gradient decreases with horizontal distance from the crest, and this type of profile commonly plots on log paper as a straight line 

sloping downward. All the slopes which White studied had smooth, regular profiles with prominently 

convex upper regions and concave lower regions. 

To examine the 12 profiles in more detail we plotted the data on log paper using White's method and 

drew lines of best fit by eye. Figure 17 contains four 

typical plots. Local gradient for any station was 

reckoned from the arithmetic midpoints between the 
24 HURRICANE CAMILLE IN VIRGINIA, 1969 

1-

0 

0 

u.. 

a:: 

LLJ 

c.. 

1-

0 

A. POLLY WRIGHT-C 

0.5 

2 0.2 '-----"-<----'----'---.L._----L_L___..l...._..l...._L....I... __ __J 

~ 

~ 

z 

LLJ 

0 

<( 

a:: 

0 

.....J 

<( 

(.) 

0 

.....J 

C. WILLS-1100 

B. GINSENG-4740 

• 

0 -- ---o -- --- 0 

80 100 

HORIZONTAL DISTANCE FROM DRAINAGE DIVIDE (L), IN FEET 

FIGURE 17.-Variation in local hillslope gradient with distance from hilltop, for four typical hillsides on which debris 

avalanches occurred. Arrow marks head of avalanche scar. 

station of interest and the stations immediately 

above and below. The corresponding value on the 

abscissa is cumulative horizontal distance L from the 

drainage divide to the station. The graphs presented 

a wide variety of slope-types, as might be surmised 

from figure 16. Some showed enough scatter that 

various lines, straight or curved, might be fitted to 

the points, and the difference between convex and 

straight or between straight and concave segments 

of the profiles sometimes became a subjective decision. 

What features do all the profiles have in common? 

First, all have a convex zone beginning at the top of 

the mountain. This upper convex zone varies widely 

in length-from the hilltop to about 0.1 or up to as 

much as 0.7 of the horizontal distance to the base of 

the hill. Some profiles showed two zones in this upper 

convex region, corresponding to two straight lines 

(both trending upward but at different rates) on the 

gradient-versus-length graph (fig. 17B). 

Another common feature was that, with one exception, the profiles have a concave zone at the base 

of the hillside. This zone may include as little as 

about 0.1 or as much as 0.8 of the entire slope length. 

Also, the concave zone often consists of two different 

sections rather than one, as indicated by two downward-trending straight lines on the graph (for example, fig. 17C). 

The profile pattern between the upper convex and 

lower concave zones varies. In five cases these two 

segments merged, with no intermediate zone (for 

example, fig. 17C). Two other profiles showed a 

straight section between the convex upper slope and 

the concave lower region. In the remaining five profiles the intermediate zone showed various combinations of convex, straight and concave segments, with 

no consistent pattern (fig. 17A,B,D). 

The local gradient-versus-length plots verify the 

deduction that the top of the scar is generally on the 

Rteepest part of the hillslope. The plots also show 

this point in relation to the convex and concave 

zones. The persistent relation apparent from a study 

of these gradient-length graphs is that the avalanches in at least 9 of the 12 cases began at the 

lower end of the upper convex zone on the hillside, 

that is, at the junction where the convex upper zone 
EROSION 25 

merges with the concave or straight section immediately below. This would in fact be the steepest 

point on the hillside. More scar data should be analyzed to strengthen this conclusion, but the 12 scars 

do include different slope lengths, heights, profile 

shapes and degrees of erosion down their paths. In 

spite of these and other differences, there is a pronounced tendency for the head of the scar to be at 

the steepest region on the hillside. 

For the same hillslopes a previous report (Williams and Guy, 1971) examined gradient-length 

graphs on which the origin of the profile was taken 

as the top of the avalanche scar. Because those 

graphs omitted the upper (convex) part of the hillside and began with the station several feet down 

from the start of the scar, the graphs indicated that 

the avalanches tended to form in the concave part 

of the hillside. It was therefore not evident that the 

convex upper slope of most profiles happened to terminate about at the top of the scar. Thus, more useful and accurate information is obtained by starting 

the profile plot at the crest of the hill. 

Another pertinent point in regard to the origin of 

local-gradient plots is that because the abscissa is 

horizontal distance on a log scale, the exponent or 

slope of a best-fit line will vary widely depending on 

the origin, and consequently the magnitude, of the 

horizontal measurement. Although local gradient is 

fixed for any station on the hillside, the plotted points 

can be spread horizontally to cover several log cycles 

or compressed horizontally into a very short band, 

depending on the abscissa-values being plotted. 

White (1966) discussed other major problems associated with the sensitivity of such diagrams to choice 

of origin. 

CROSS PROI<'ILES 

Transverse measurements of avalanche scars were 

made in the field; however, because the field profiles 

were not obtained in true straight lines across the 

scars and because the steep hillslopes would magnify 

the error due to any such deviations, the cross profiles presented here (fig. 18) were obtained from 

contour maps of the kind shown in figure 15. The 

section was marked with a straight line on the contour n1ap and the elevation interpolated between 

contours at 5-foot intervals, starting at the deepest 

portion of the scar and proceeding to each side in 

turn. 

The shapes and dimensions of the cross profiles 

vary considerably. Most scars are broad relative to 

the depth. Near the top of a scar, the profiles tend 

to be rather fiat. Toward the base of a hill, the profiles have a variety of shapes-flattish, for example, 

LEFT BANK RIGHT BANK 

~019 ~0.19 

Wills-1100 

~ ---- 1 0.58 

- ~ 

~ 

0.84 

I 

0.32 

0.23 

0.43 

0.73 ~ 

-

----L ~ .J--- o.94 

Wills-1300 

~0.10 

~0.25 

~0.35 

~0.50 

Polly Wright-A 

~0.09 

~0.31 

~0.47 

-----.;.1 I 0.68 

Polly Wright-8 

~0.45 

~0.66 

Polly Wright-C 

20 40 60 80 100 FEET 

60 FEET 

40 

20 

FIGURE 18.-Typical cross profiles of avalanche scars, drawn 

looking downslope. Number at edge of each profile indicates proportional distance of profile section down hillslope 

from start of scar. Vertical tick marks show edge of scar. 

Dashed line=estimated pre-storm ground surface. 
26 HURRICANE GAMIL·LE IN VIRGINIA, 1969 

where the soil depth was shallow and bedrock is 

exposed (fig. 18-Polly Wright B), or more Ushaped where the soil was relatively deep (fig. 18-

Polly Wright A). Flaccus (1958) found V-shaped 

channels to be typical of the lower parts of a valanche scars in the White Mountains of New Hampshire. 

The profiles also show typical trends of scar width, 

with distance downslope. 

The scar depths indicated by the cross profiles in 

figure 18 seem to remain about constant or to increase downslope, depending on the soil thickness. 

POSSIBLE CAUSES OF AVALANCHES IN NELSON 

COUNTY 

The torrential rainfall undoubtedly was the primary cause of the debris-avalanches. But why did 

avalanches occur down certain strips of hillside and 

not on adjacent parts of the same slope or on other 

hillsides only 50 yards or less away? One possible 

reason is an uneven distribution of rainfall. A very 

severely eroded stream channel commonly occurred 

within a few hundred yards of a channel which appeared to have carried only a small flow of water. 

The most likely conclusion is that the rainfall intensity was highly irregular, both in time at a given 

spot and in area. Schwarz (1970) in fact affirmed 

the complex nature of the rainfall for this extreme 

event. He reported that a U. S. Weather Bureau survey team, investigating the various rainfall reports, 

found that for each of four observations of over 

20 inches total rainfall, less than 6 inches was measured at a location less than 5 miles away. Huff 

(1967) states that multicellular patterns of precipitation cause highly variable intensities and quantities near the centers of areas covered by heavy 

storms. 

Other factors which authors have mentioned as 

possible influences on avalanching are steepness of 

hillslope, vegetation type and density, kind of bedrock, attitude of stratified or jointed bedrock, erosion or bombardment of the base of a slope by a 

debris-laden stream, soil texture, orientation (aspect) of hillslope, length of hills lope, soil depth, susceptibility of soil to infiltration, ability of soil to 

transmit water (hydraulic conductivity), the initial 

presence of depressions or troughs along the hillslope, bolts of lightning, the vibrations of heavy 

thunder, and the uprooting of trees due to strong 

winds. 

Although detailed measurements were not made, 

vegetation type and density appear to be essentially constant over the scarred area of the county, 

even when differences in orientation and steepness 

of slope and possibly in soil depth are considered. 

Nearly all the bedrock of the area is rather impervious. From exposures along scars and stream channels, it appears that the avalanches were more common over massive crystalline rocks than over schists, 

but the former seems to be the predominant rock 

type anyway. There is thus no evidence that the type 

of bedrock influenced the likelihood of avalanching. 

In some cases more than one rock type is exposed 

along a scar. Some exposed bedrock is jointed, 

whereas other exposures have no joints or cleavage. 

Whether jointing increases the likelihood of avalanching could not be determined in our study. 

The possible influence of soil texture on the ability of soil to conduct water and ta be susceptible to 

infiltration cannot be sufficiently evaluated in a study 

initiated after the avalanches have occurred. Soil 

analyses at various stations on a hillside, before and 

after the avalanches, would be the best way to find 

any consistent associations. The same is true of the 

internal shear strength of any layers in the soil profile. Rapp (1963, p. 203) indicated that the main 

slides at Ulvadal, western Norway, occurred along 

planes mostly at or near the bottom of the podzolic 

layer and the top of the fine-grained substratum in 

the illuvial layer (the B horizon or layer accumulating clay for hundreds of years). Such a layer was 

not readily noticeable along the avalanche walls in 

Nelson County. 

No large trees were found uprooted at the heads 

of scars. Some scars began immediately downslope 

from huge boulders, and the heads of other scars 

were many times wider than the zone which tree 

roots would disrupt. Although the role of uprooted 

trees in starting an avalanche could not be assessed, 

this factor at most was probably of minor importance. 

Some big slides were heard after the thunder and 

lightning had stopped around 0400 August 20. Also, 

the lightning during the storm was mainly the horizontal type rather than vertical bolts which might 

have struck the ground. These considerations suggest 

that thunder and lightning probably were not major 

factors in the Jnitiation of debris avalanches. 

The remaining possibilities which could be associated with the Nelson County avalanches are stream 

action at the base of the slope, depressions down the 

hillslope, orientation of the hillside, the steepness of 

the slope, the horizontal length of the hillside and 

the soil depth. These factors are discussed in the 

following pages. 
EROffiON 27 

THEORETICAL CONSIDERATIONS STREAM ACTION AT THE BASE OF THE HILLSLOPE 

Immediately prior to an avalanche, slope stability 

is in a delicate balance between shear force and shear 

strength in the underlying materials. In soil mechanics, the ratio of these two forces is used to express a 

factor of safety that estimates the danger of sudden 

slope failure : 

Safety Factor= 

sum of shear forces on critical surface 

shear strength along the surface 

A calculated factor of safety slightly greater than 1 

in some cases may mean the possibility of slow yielding or creep, instead of a slope failure. 

The mode of deformation can be planar or rotational. Except for some pockets of deep soil, the most 

common types in Nelson County was planar, involving a relatively thin layer of "loose" soils or products 

of weathering over an inclined bedrock surface. 

Where evidence of rotational deformation was found 

on deep soils, it was in some cases evident that a 

larger planar deformation had occurred immediately 

downslope. 

Table 3 summarizes the various factors contributing to instability of earth slopes. Of the factors con_, 

tributing to high shear stress, the weight of the rainwater is considered by the authors to be the most 

important in Nelson County. Bank-cutting by 

streams is discussed in the paragraphs below. The 

shear strength of the mantle probably was lowered 

mainly by changes in intergranular forces due to 

pore water. 

Streams reportedly can cause avalanching by undercutting the hillside (Wenner, 1951; Scott and 

Gravlee, 1968; Rice and others, 1969). The mantle 

along the base of a slope does provide some support 

for the material uphill from it, so a slope would become less stable if the base of the hillside were 

eroded. 

What is the field evidence for or against erosion 

of the base of the hillslope causing avalanching in 

Nelson County? The evidence in favor is that a 

stream channel trending approximately perpendicular to the direction of the avalanche had eroded its 

banks for as much as 20 feet along the base of some 

avalanche scars. However, it is possible that, instead 

of bearing a cause-and-effect relation, channel erosion and debris avalanches are both independent 

products of the torrential rainstorm. 

Against erosion at the base of the mountain triggering avalanching is, first, the important fact that 

many debris avalanches occurred on hillsides which 

were not touched by stream floodwaters (fig. 14). 

Many avalanches, in fact, travelled overland for hundreds of feet (fig. 14) before moving into the stream 

channel, as for example, in the Hat Creek basin and 

along highway U.S. 29. Second, some of the avalanches were over a thousand feet long and only 

about 25-75 feet wide. It is unlikely that a few feet 

of erosion along the bottom of the hillside could 

trigger such long, narrow slides, unless the avalanching progressed upslope bit by bit. Testimony 

TABLE 3.-Factors contributing to instability of earth slopes 

[From Varnes, 1958] 

Factors that contribute to high shear stress 

A. Removal of lateral support 

1. Erosion-bank cutting by streams and rivers 

2. Human agencies-cuts, canals, pits, and so forth 

B. Surcharge 

1. Natural agencies-weight of snow and ice and 

rainwater 

2. Human agencies-fills, buildings, and so forth 

C. Transitory earth stresses-earthquakes 

D. Regional tilting 

E. Removal of underlying support 

1. Subaerial weathering-solutioning by ground 

water 

2. Subterranean erosion-piping 

3. Human agencies-mining 

F. Lateral pressures 

1. Water in vertical cracks 

2. Freezing water in cracks 

3. Swelling 

4. Root wedging 

Factors that contribute to low shear strength 

A. Initial state 

1. Composition-inherently weak materials 

2. Texture-loose soils, metastable grain 

structures 

3. Gross structure-faults, jointing, bedding 

planes, varving, and so forth 

B. Changes due to weathering and other physicochemical reactions 

1. Frost action and thermal expansion 

2. Hydration of clay minerals 

3. Drying and cracking 

4. Leaching 

C. Changes in intergranular forces due to pore water 

1. Buoyancy in saturated state 

2. Loss in capillary tension upon saturation 

3. Seepage pressure of percolating ground 

water 

D. Changes in structure 

1. Fissuring of preconsolidated clays due to 

release of lateral restraint 

2. Grain structure collapse upon disturbance 
28 HURRICANE CAMILLE IN VIRGINIA, 1969 

presented earlier, however, indicated that the whole 

strip probably came down as a single event. These 

considerations suggest that stream erosion (or undercutting at the base of a hillside) probably was 

not a major cause of the avalanching, though it 

might locally have been a factor. 

A second theory (Kuhaida, 1971) regarding 

stream action is that avalanching can be induced by 

vibrations in the bedrock as a result of bombardment 

of the slope-base by stream-transported debris. This 

process would apply more commonly to slopes on the 

outside of meander bends, where bombardment 

would be more direct. Most streams flow at right 

angles to the hillside, so there is some question as to 

the intensity, as well as the role, of any vibrations 

produced by the sediment moving in the stream. 

Also, as mentioned, many hillsides on which avalanches occurred were hundreds of feet away from 

a stream. Thus, while bombardment of a slope base 

could conceivably produce vibrations strong enough 

to trigger an avalanche, this probably was not a 

major factor in the 1969 flood. 

The avalanches themselves seem to have produced 

the strongest vibrations during the· storm. As mentioned above, people miles from the slide areas intermittently felt tremors, occurring at the same time as 

a loud rumble, which they later attributed to avalanches (Simpson and Simpson, 1970, p. 9). However, if vibrations caused debris avalanches, the latter probably would have occurred in groups rather 

than being irregularly spaced through time. 

DEPRESSIONS OR TROUGHS ON THE HILLSIDE 

Downslope-trending depressions or grooves on a 

hillside collect water, and because they become 

saturated sooner than the rest of the slope, they are 

likely places for avalanching. Field observations and 

photographs confirmed that many debris avalanches 

did indeed take place where indentations or incipient 

channels already existed on the hillside. In addition, 

cross sections of the scar and the adjacent unaffected 

ground, drawn from the transit surveys, commonly 

showed that the projected original soil surface over 

the scar definitely occupied a lower strip (fig. 18). 

Furthermore, bedrock exposed along some scars 

showed a weathered strip down the middle of the 

scar where the color and surface texture were noticeably different from the adjacent freshly exposed 

bedrock. This suggests that such strips had served 

as small incipient hillslope channels for many years 

before the storm. Finally, moss that was about 5 or 

more years old was growing on some of the exposed 

bedrock in the moist depression along the center of 

a few scars. Moss likes moisture, and moisture tends 

to collect in a depression or trough. (A study of the 

growth rate of moss on newly-exposed rock could 

help date previous debris avalanches.) 

In general, 85 percent or more of the debris avalanches seem to have occurred along a previously 

existing depression in the hillside. 

In their study of a neighboring Appalachian region, Hack and Goodlett (1960, p. 44) observed that 

"most of the chutes (scars) occupy former depressions or groove-like areas, and the impression is inescapable that the chutes are indeed incipient hollows 

or channelways that were partly obliterated during 

the passage of time by falling blocks and mass movement. They are, at rare intervals of time, flushed out 

and deepened by heavy runoff and the avalanching of 

debris." Thus Hack and Goodlett felt that the hillside depressions where avalanches tend to occur are 

themselves the scars of former avalanches or of similar rapid erosional processes. They further mention 

(p. 56) that the avalanches represent a headwater 

extension of the drainage network, that is, an increase in the drainage density. Figure 11 shows this 

feature. 

Swanston (1969) found that debris avalanches in 

southeast Alaska in many cases occur along local 

drainage concentrations down the hillside. 

ORIENTATION OF HILLSIDE 

The aspect or orientation of the hillside could determine ( 1) the amount of sun the slope normally 

receives, which in turn influences the amount and 

type of vegetation, the initial (prestorm) moisture 

content of the soil, and consequently the depth and 

character of the soil profile; and (2) the angle of the 

attack and therefore the amount of rain on the slope, 

in view of the wind direction during the storm. For 

example, Tricart (1960) in the French Alps, Pippan 

(1969) in the Austrian Alps, Diseker and Richardson (1962) in Georgia (USA), and Rice, Corbett, 

and Bailey (1969) in southern California all found 

erosion or mass movement to be more common on 

slopes of a certain aspect. The preferred aspect, however, was not necessarily consistent among these 

studies. Flaccus ( 1958) , on the other hand, concluded 

that the frequency of debris avalanches he studied in 

New Hampshire had no relation to the direction in 

which the hillsides faced. 

In this study the amount and type of vegetation 

did not appear to vary significantly with different 

hillslope orientation. Visual examination along scar 

edges revealed no apparent differences in the soil 

profiles on slopes of different aspects. The angle of 
EROSION 29 

raindrop impact for a given rainfall intensity had 

little or no effect on sheet erosion because the ground 

surface was protected by an umbrella of trees, as 

well as by a mat of decaying leaves and low-growing 

vegetation. The angle of impact could, however, 

affect the amount of water striking the hillslope, in 

that more rain strikes a given area as the angle of 

rainfall becomes more normal to the hillside. Thus 

the chief potential importance of hillslope orientation 

probably was in regard to (a) prestorm moisture 

content of the ground, as influenced by the sun, and 

(b) the amount of water which the hillslope received 

during the storm, as influenced by the wind. The intensity and direction of winds blowing at ground 

level can vary dramatically during severe thunderstorms. 

Counts were made of the number of scars occurring on hillsides facing each of the eight major 

compass directions (N., NW., W. and so forth). This 

was done by walking along selected drainage channels and marking, on a topographic map (scale 

1:24,000), the location and extent of each scar. The 

orientation and average gradient (base to head of 

scar) were subsequently measured from the map. 

The amount of territory covered ranged from 9,000 

to 16,000 feet of distance along the stream channel 

for each of the eight categories of hillside aspect, and 

many different stream channels were involved. (A 

given channel or reach serves two hillslope orientations, simultaneously.) The results (table 4) were 

expressed in terms of average number of scars per 

1,000 feet of mountain stream channel (base of hillside), for each different category of approximately 

constant hillside (scar) gradient and aspect. A few 

scars for slopes less than 0.30 and more than 0. 79 

are not listed. Most of this work was done in the 

headwaters of Ginseng Hollow, Polly Wright Cove 

and Wills Cove. However, scars for some combinations of aspect and gradient (mainly on slopes less 

TABLE 4.-Debris avalanche scars per 1,000 feet 

[Figures refer to average number of scars per 1,000 ft of reach inspected 

along stream channel or base of hillside, for a given hillside orientation A given channel distance or reach serves two orientations simul- taneously. A few scars for slopes less than 0.30 and more than 0. 79 are not included] 

Aspect 

of hillside 

N ----------- NE ---------- E ----------- SE ---------- 8 ----------- sw ---------- w ----------- NW ---------

Without 

respect 

to 0.30-- slope 0.39 (1) (2) 

3.4 4.0 2.8 1.6 2.4 2.0 1.5 (1) 

1.1 1.4 1.3 .8 1.4 1.0 10 0 

With respect to slope (ft per ft) 

0.40- o.5o- 0.60- 0.70- 0.49 0.59 0.69 0. 79 Total (3) (4) (5) (6) (7) 

2.1 

.6 

• 7 

.5 

2.9 

1.6 

.3 

.8 

3.0 3.6 4.6 2.8 4.9 3.4 

2.3 .7 .6 .6 

1.4 2.0 .3 (1) 

.9 1.4 

5.4 18.1 ( 1 ) 9.6 

5.2 16 2 (1) 3.5 ( 1 ) 5.5 

(1) 5.8 

(1) 1.6 

1.0 4.1 

1 No avalanche scar observed for this slope and aspect. 

than 0.40 and greater than 0. 70 ft per ft) were 

sparse, and additional observations were made in 

Fortunes Cove (eight scars) and the Davis Creek 

basin (16 scars) to supplement some of the deficient classes. 

The data in table 4, while sufficiently representative for the following analysis, may be incomplete or 

faulty in several ways. Although 186 scars were recorded, involving some 103,000 feet measured along 

mountain stream channels, the number of scars for 

any one aspect and slope category ranged from 0 to 

13, averaging about 3. The total distance along the 

main stream for any one aspect and slope category 

ranged from 0 to 6,300 feet, averaging about 2,300 

feet. In a few cases the amounts examined may not 

be enough to give a true picture of the avalanche 

frequency distribution. Also, not every combination 

of aspect and gradient was present to any significant 

extent in the study area. Furthermore, in some cases 

the determination of zones of constant slope and 

hillside aspect from the topographic map involved 

some subjectivity. Finally, plotting and measuring 

avalanche scars on the map entails some error. From 

the slopes of the 12 scars measured in the field, the 

maximum error in measuring average scar slopes 

from the map is judged likely to be about ±0.10 ft 

per ft, but usually the error is less than 0.05 ft per 

ft. In spite of these possible errors, the data should 

be reliable enough to indicate general trends. 

Column 1 of table 4 shows the scar frequency for 

different compass orientations, without respect to 

the steepness of the hillside. If steepness and scar 

dimensions are not considered, then column 1 suggests that hillslopes facing north, northeast, and east 

experienced several times as many avalanches as 

slopes facing most other directions. Columns 2-6 

give the number of scars for each slope category and 

aspect. Although the data are not as conclusive as 

might be desired, slopes facing north, northeast, and 

east still tend to have more scars, for approximately 

constant hillside gradient. Hence with most other 

factors virtually constant, the susceptibility of a 

hillside to debris avalanches in the study area tended 

to be associated with the aspect of the hillside. Slopes 

facing north, northeast, and east suffered the greatest number of avalanches, probably because the absence of direct sunshine left these slopes with a 

greater pre-storm moisture content and (or) the 

wind drove a greater amount of rainfall onto these 

slopes during the storm . 

HILLSIDE GRADIENTS 

Avalanching, soil slippage and soil erosion should 

occur more readily on steeper hillslopes, as reported 
30 HURRICANE CAMILLE IN VIRGINIA, 1969 

by Rice, Corbett and Bailey (1969) and Zingg 

(1940). This is because the force required to start a 

particle moving down a slope decreases as the slope 

steepens. 

The horizontal lines of table 4 show the frequency 

of avalanching with increasing gradient, for a constant compass direction. On the basis of these data 

alone, and contrary to what might be expected, one 

cannot conclude that avalanching in Nelson County 

occurred more often on steeper slopes. Some hillside 

aspects (N., SW.) show this relation, but others 

(SE., S., W.) definitely do not. 

If it is assumed that the data in table 4 are sufficiently representative, steeper slopes, beyond a minimum of about 0.30 ft per ft (below which very few 

avalanches occurred), do not seem directly related 

to greater frequency of avalanching, for this storm. 

Unlikely as this seems, there may be some undetected 

factor or combination of factors contributing to this 

conclusion. One possibility is that the gradient at the 

head of the scar may be the pertinent slope, rather 

than the average gradient as measured from head to 

base of scar. Another possibility is that the steeper 

slopes, say greater than about 0. 70 ft per ft, do not 

occupy a large enough distance along the stream 

channel or a large enough hillside area to give a 

statistically meaningful frequency of avalanching. 

LENGTH OF HILLSLOPE 

Longer hillslopes understandably can provide 

greater quantities of sediment than short slopes 

(Zingg, 1940). In addition, longer hillsides may enhance the likelihood of avalanching because the 

downhill movement of water produces more water 

on and in the soil with distance downslope. Data on 

observed scars and topographic map measurements 

of hillslope length were used to determine if debris 

av~lanching was more common and more extensive 

on longer hillslopes. 

The fieldwork consisted of marking on the topographic map the locations of avalanche scars and 

estimating their lengths by eye. Only scars having 

north, northeast, and east aspects were considered 

because these aspects suffered more avalanching 

than others. Gradient was ignored, on the basis of 

the tentative finding that avalanching occurred with 

about equal frequency for all slope categories between 0.30 and 0.80 ft per ft. 

Topographic-map analysis consisted of measuring 

the horizontal length of the hillslope, from ridge 

crest to stream channel, at each scar location, and 

measuring the distance along the stream channel, 

for each category of hillslope length. Columns 1 and 

3 of table 5 show the categories of hillside length and 

the total channel distance inspected for each category. The distances in column 3 are converted to percentages in column 4. (Hillslope lengths less than 

200 ft represented only 2 percent of the drainage 

area and had essentially no scars.) 

In table 5 the number of observed scars for each 

category of hillslope length (col. 7) does not truly 

indicate any influence of slope length because all 

slope lengths were not equally present. For example, 

28 avalanches occurred on hillsides 400-500 feet 

long, while only seven scars were found on hillsides 

longer than 1,000 feet; however, slopes 1,000 feet 

long were scarce, relative to those in the 400-599-

foot class. The number and (or) length of observed 

slides in each category of hillslope length therefore 

should be compared to the percentage of valley distance along the mountain stream channel (col. 4) 

and also to the relative amount of drainage area 

(col. 5) for each category. Because we inspected 

nearly all channels in which the sideslopes had north, 

northeast, and east aspects, column 4 in table 5 is 

TABLE 5.-Ef!ect of killslope length and drainage area on frequency and amount of scarring 

(1) (,2) (3) (4) (5) (6) (7) (8) (9) (10) 

Percent 

of total 

sample 

Area of area Average 

Category Midpoint- represented Total REiative length 

of average Valley Percent sample = by each length number of scars hillslope length of distance of total (2) (3) category of Number of all of scars per acre length hillside inspected valley 43,560 hillslope of scars scars1 per acre = (8)/(5) 

(ft) (ft) (ft) distance (acres) length observed (ft) = (7)/(5) (ft) 

200-399 ----------- 300 27,800 28 192 14.0 20 3,540 0.104 18.4 

400-599 ----------- 500 29,900 30 344 25.1 28 8,5-60 .081 24.9 

600-799 ----------- 700 23,700 23 381 27.7 19 7,850 .050 20.6 

800-999 ----------- 900 12,400 12 256 18.7 14 7,430 .055 29.0 

~1,000 ------------ 1200 7,200 7 199 14.5 7 4,960 .035 24.9 

Total ----------------- 101,000 100 1,372 100.0 88 32,340 

1 Length of individual scars estimated during field inspections and summed for each hillslope category. 
EROSION 81 

representative of the entire drainage area of the 

basins studied. Drainage area, as used in table 5, is 

the product of the mean hillslope length (col. 2) and 

the valley distance for the particular category (col. 

3) , in units of acres. The percentage of the total 

area represented by each category (col. 6) was then 

determined from column 5. 

The number of observed scars (col. 7) is roughly 

proportional to the percentage distance along the 

mountain stream channel (col. 4), for a given category. Therefore the number of scars is essentially 

independent of the length of hillside, that is, the 

length of the hillside probably had little or no influence on the likelihood of avalanching. 

In regard to the effect of drainage area on the frequency and amount (length) of scars, column 9 in 

table 5lists the relative number of scars per acre and 

column 10 gives the average total length of scars per 

acre. Figure 19 shows how these factors vary with 

hillslope length. The relationships in figure 19 can bereduced to a straight line by using the square root 

of the hillslope length, but this would not improve 

their accuracy. (See Wischmeier and Smith, 1965, p. 

9.) While hillslope length is not an important factor 

in the number of slides reaching a unit length of 

stream valley, it is an important factor in the number per unit of drainage area and in the total length 

of scars per unit drainage area. As length of hillslope increased, the number of scars per acre decreased (fig. 19A) and the total length of scars per 

acre increased (fig. 19B). For example, about 10 

scars occurred per 100 acres on 300-foot slopes in 

contrast to only about three avalanches per 100 acres 

on 1,200-foot slopes (fig. 19A). On the other hand, 

the total length of scarring was about 1,600 feet per 

100 acres on 300-foot slopes and 2,800 feet per 100 

acres on 1,200-foot slopes. 

These general conclusions regarding slope length 

can be more readily visualized in a drawing of two 

hypothetical sections of hillslope (fig. 20). Both sections have the same drainage area, but section B is 

2.5 times the slope length of A. The unit distance 

along the stream channel in figure 20 is distance XY. 

The number of avalanche scars encountered in walking along this unit reach is the same for both sections (five for this example), so that hillslope length 

is not a factor in the frequency of avalanching per 

unit distance along the stream channel. However, on 

the basis of unit drainage area, section A has 12 

scars whereas B has only five scars. This shows how 

more avalanches occurred on the short than on long 

slopes, for a given drainage area (col. 9 of table 5 

and fig. 19A). Lastly, summing separately the scar 

0.12 

LL.I 0.10 0:: 

(.) 

<( 

0:: 

LL.I 

a.. 

(j) 

0:: 

<( 

(.) 

(j) 

LL 

0 

0:: 

LL.I 

a:l 

~ 

::::> 

z 

A 

0 

30 0 

ILL.I 

LL.I 

LL 

~ 

u.i 

0:: 

(.) 

<( 

0:: 20 

LL.I 

a.. 

(j) 

0:: 

<( 

(.) 

(j) 

LL 

0 

I 

I-

~ 10 

z 

LL.I 

_J 

LL.I 

~ 

<( 

0:: 

LL.I 

> 

<( B 

0 

0 500 1000 1500 

HILLSLOPE LENGTH, IN FEET 

FIGURE 19.-R.elation of hillslope length to number of scars 

per acre and to average distance of scarring per acre~ 

for hillsides having north, northeast, and east aspects. 

lengths in sections A and B shows that the scars in 

B amount to a greater total length. Since the acreage 

of A and B are equal, the average length or distance 

of avalanching per acre is greater on long than on 

short slopes (col. 10 of table 5; fig. 19B). The latter 

conclusion actually holds for unit distance along the 

stream channel, as well as for unit drainage area. 

SOIL DEPTH 

The influence of soil depth on avalanching could 

not be determined. Most of the avalanches began in 

soil depths of about three feet or less, although minor 

soil slips were common near the bottoms of hillslopes 
32 HURRICANE CAMILLE IN VIRGINIA, 1969 

A 

"""'<'-------Slope length -----=>~1 

B 

~-----------Slope length-----------~ 

.TAvalanche sea"\ 

- Stream channel at base of hillslopes 

- -

-

-

Avalanche scars 

Unit distance XY along stream channel 

-

FIGURE 20.-Top (plan) view of two hypothetical drainage areas, A and B, showing influence of hillslope length and 

drainage area on frequency and length of avalanching. Areas of A and B are equal, but the hillslope length of B 

is 2.5 times that of A. Though the length of scarring is greater on B, the number of scars is greater on A. 

where the soil depth was unknown and where the 

slip may have been triggered by undercutting. 

Soil depth may affect avalanching, in that water 

can filter downward for greater distances in deep 

soil, whereas a shallow soil would quickly become 

saturated. The potential for soil slippage increases 

with increasing saturation, which causes a greater 

bulk weight. Thus from this viewpoint, one might 

expect more avalanching in shallower soils, other 

factors being equal. 

On the other hand, the shear force would be proportional to the depth of the soil and water. Deeper 

soils, which also could contain more water, therefore 

would be subjected to a greater shear force. There 

would also be more possibilities for planes of weak 

shear strength with deep soils. These considerations 

suggest that avalanching might occur more readily 

on deep soils, if other factors are equal. 

If the effect of greater shear over deeper soils is 

important, deep-soil avalanches might tend to occur 

on flatter slopes than those in which shallow-soil 

avalanches occur. Such a depth effect could explain 

why we found as many avalanches on relatively flat 

slopes (0.30-0.39 ft/ft) as on steep slopes (0.70-

0.79 ft/ft). 

SUMMARY 

The head of a debris avalanche tended to be at the 

steepest point on the hillside, that is, at the junction 

between the upper convex slope and the straight or 

concave section. The only factors associated with 

greater frequency of debris avalanching for this 

storm were depressions on the hillside, aspect of 

hillside, and length of hillside per unit drainage area. 

More avalanches occurred on slopes facing north, 

northeast, and east than on slopes facing other direc-
EROSION 33 

tions. Some factors, such as soil depth and various 

soil properties, may affect the likelihood of avalanching, but their importance could not be assessed. 

Other variables (for example, average steepness of 

hillslope within the range of 30-80 percent gradient) 

did not appear to be related to the occurrence of an 

avalanche. Hillslope length did not affect the number 

of slides reaching a unit length of stream channel; 

however, on a unit-drainage-area basis more scars 

occurred on short than on long slopes, whereas the 

length of scarring was less on short slopes than on 

long hillslopes. All the above conclusions depend on 

the uncertain assumption that the rainfall intensity, 

although probably varying in time and area, did not 

affect any of the possibly relevant factors. 

EFFECT OF DEBRIS AVALANCHES ON STREAMFLOW 

The sediment deposited by a debris avalanche or 

landslide can temporarily block or severely impede 

the flow along a stream channel. Famous examples of 

this phenomenon are the 1925 slide on the Gros 

Ventre River in Wyoming and the 1959 landslide 

into the canyon of the Madison River near Yellowstone National Park. In Nelson County channel 

blockage probably occurred just upstream from a 

sharp bend in the headwaters of Ginseng Hollow 

(Guy, 1971). The evidence for channel blockage here 

is the very big difference between the maximum 

water discharge attributable to precipitation rate 

alone and the much higher discharge which actually 

flowed through the bend as measured by indirect 

methods. The large discharge through the bend 

partly reflects the release of a volume of water stored 

upstream of the avalanche as it was sliding across 

the channel and partly reflects the volume of the slide 

material that was added to the stream discharge. 

An estimate of the channel discharge which would 

result from the maximum rate of rainfall was made 

in the same manner described earlier in the report, 

using the rational formula. With 25 inches per hour 

as the maximum probable rainfall intensity, the 

measured drainage area as 20 acres, and a coefficient 

of 0.90, the maximum discharge in the channel attributable to rainfall would be about 450 cfs. 

An actual peak flow of about 8,000 cfs in the channel was determined from topographic field measurement at section B-B' (see fig. 21) . Because these 

measurements were obtained by hand level, stadia 

rod and pacing, the results are not precise and are 

only approximations. The required measurements at 

section B-B' were (1) the superelevation !::..h, that is, 

the vertical distance between the high-water marks 

on the inside and outside of the bend, (2) the crossNormal channel 

boundary 

Debris avalanche area~ 

High-water line 

Peak outflow 8000 

cubic feet per second 

N 

I 

0 100 FEET 

L.L.L.J__J 

FIGURE 21.-Sketch of debris avalanche zone, normal channel 

boundaries and high-water lines left by peak flow, northeast branch of Ginseng Hollow. 

sectional flow area A, (3) the outside and inside 

radii of the bend (ro and r;, respectively), (4) the 

average radius of curvature rc = (ro + r;) / 2 and 

(5) the water-surface width W. 

The approximate mean flow velocity V in the 

bend can be computed from an equation transformed from Chow (1959, p. 448): 

V=( !::..~rc)% 

where g=acceleration due to gravity. Inserting the 

measured values gives 

(

13.5X32.2X80 )'h V = 80 

=21 ft per sec. 

A different method of computing V is through the 

use of Grashof's theory as given by Schoklitsch 

(1937, p. 151): 

V2 ro 

!::..h=2.30-logg ri 

Rearranging the equation and inserting the field 

data yields 

( 

13.5 X32.2 )% 

V = 120 = 20 ft per sec 

2.30xlog40 

which differs by less than 5 percent from Chow's 

more approximate equation. The computed peak 

discharge Q is then Q=A V = (390) (20) = 8,000 cfs. 
34 HURRICANE CAMILLE IN VIRGINIA, 1969 

FIGURE 22.-Examples of eroded mountain channels. Upper photograph is channel about 100 yards 

upstream from apex of Campbell fan. Lower photograph shows headwaters of a tributary in 

Ginseng Hollow. 
EROSION 35 

Au error analysis of this calculated 8,000 cfs 

should consider the effects of the measuring techniques used, nonsteady flow, irregular velocity distributions, irregularly shaped channel boundaries, 

imperfect circular shape of the bend, the effect of 

sediment and other debris in the flow, and the possibility that some channel enlargement may have 

occurred after the peak flow. Also, a tributaryfrom 

the north enter ed the channel just upstream from 

the bend (fig. 19) ; however, near the junction the 

high-water marks on this north tributary were much 

lower than those on the study reach ; so the flow 

from the north tributary probably was stored behind the rushing water-sediment mixture and did 

not contribute to the peak flow in the bend. The many 

indeterminates, plus the imprecise basic data measurements, could amount to an error of about ±3,000 

cfs in the computed maximum discharge. Nevertheless, the peak discharge definitely was many times 

(about 18 times, as estimated here) the discharge 

reasonably attributable to an assumed very high 

rate of rainfall runoff. The most likely explanation 

for the high flow rate is that the 57,000 cubic-foot 

avalanche moved through the reach in only a few 

seconds, together with water which the slide had 

temporarily blocked in the stream channel. 

This case exemplifies one way in which a debris 

avalanche could cause serious damage to man and 

structures, especially in a small drainage area. By 

temporarily disrupting normal runoff, a hillside 

avalanche in the headwaters can trigger a chain 

of events culminating in a tremendous surge of 

sediment-laden water traveling at a very high 

velocity. The large difference in discharge between 

a relatively steady flood flow, though it may be at 

record heights, and a big surge of water and debris 

can mean the difference between minor damage and 

devastation. 

STREAM CHANNELS 

Mountain streams in Nelson County commonly 

range from 1 to 10 feet wide. During the flood the 

stream channels were enlarged by severe erosion 

and the removal of trees and shrubs (fig. 22). The 

exact extent of such erosion could not be precisely 

determined, as the dimensions and location of the 

channel before the flood in most reaches were uncertain. However, estimates and in some cases measurements were made of the amount of channel erosion for headwater streams, as discussed under 

"Sediment Yield." 

Figure 23 shows a channel which formerly was 

only a few inches deep and narrow enough for a man 

to step over. The magnitude of stream erosion in this 

photograph was not uncommon in the mountainous 

parts of the study area. 

In some upstream reaches of mountain channels 

the bedrock on the side slopes restricted the bank 

erosion. In other areas bank erosion was evident 

where vegetation such as trees and crops still clung 

by a few tenacious r oots to alluvium along the edges 

of stream banks. Many stret ches of old logging 

roads in the mountains and of paved highways in the 

downstream valleys were partly or complet ely 

washed away (fig. 24). Farmers along Indian Creek 

and near the mountain ravine upstream from the 

Campbell alluvial fan testified that the new channels 

are at least twice as wide as the preflood channels. 

Two factors restricted the downcutting of stream 

beds. On the steeper mountain channels the resistant 

bedrock was exposed either prior to the flood or soon 

after the flood started, due to the thin mantle. Downstream in the broader valleys the overall gradient 

was much flatter, and this limited the amount of 

downcutting which the stream could accomplish 

while still maintaining a slope steep enough to satisfy the hydraulic and sediment-transport requirements. 

Farther downstream, severe scour commonly ocFIGURE 23.-Channel carved by the flood in the yard of S. 

K. Wills, at the head of Wills Cove. Before the flood this 

channel was but a few inches deep and so narrow that a 

man could easily step over it. 
36 HURRICANE CAMILLE IN VIRGINIA, 1969 

FIGURE 24.-Channel erosion along highways. (Upper photograph courtesy of Virginia Division of Mineral Resources; lower photograph courtesy of Virginia Department of Highways.) 
EROSION 37 

FIGURE 25.-Scour of stream banks under a railroad trestle-Tye River at Norw~od, Va .. ~ames River 

in background. Bridge completely washed out in foreground. Note also fioodplam deposition. (Photograph courtesy of Virginia Department of Highways.) 

curred next to bridges and railway trestles (fig. 25). 

In some localities as much as 20 feet of the original 

stream bank was eroded, on each side of the stream. 

The many trees carried by the raging streams probably contributed to this. The trees caught on bridge 

piers and collected into huge debris jams that blocked 

parts of the normal stream channel. Blockage of 

parts of the flow cross section could cause faster 

velocities in the unobstructed sections, thus increasing the stream's erosive power. The jams also encouraged overbank flooding which in some cases 

resulted in erosion of the surface of the floodplain. 

W. N. Whitehead (Simpson and Simpson, 1970, 

p. 150) happened to observe the process by which a 

roadway was eroded next to a bridge on Route 151 

at Tye River. 

"The river was up and passing over the roadway. As it 

cascaded down the east (downriver) side of the fill, it began 

to erode this, and we watched it as it kept digging back, with 

the road collapsing bit by bit. The water ate its way back, 

from the downriver side, all the way through the fill. Once 

it got back thin enough the whole thing broke, and the water 

just charged through. Then the gap between the bridge and 

the edge of the road started getting wider and wider. This 

was occurring on each side of the bridge." 

After the water broke through, the gaps accommodated some of the flow and the level dropped. At its 

peak, the water had been "well over the bridge 

railings." 

Another type of channel erosion in some downstream areas was the caving or slumping of freshly 

deposited sediment into the stream, along the banks 

(fig. 25). This erosion probably occurred as the 

flood was receding and involved sediment laid down 

earlier by the same flood. The amount of sediment 

eroded by bank caving is difficult to estimate. 

Channel erosion, then, was certainly quite severe 

in some places; however, other channels apparently 

underwent little or no erosion. Thus the amount of 

channel erosion varied widely, even within areas as 

small as an acre. 

Bank erosion frequently exposed old alluvial 

deposits. Some of these are 30 feet thick or more, 

with particles ranging from sand to boulders in 
38 HURRICANE CAMILLE IN VIRGINIA, 1969 

size. Stratification is sometimes evident, but in other 

cases all particles are completely mixed in a vertical 

section. Occasionally some lenses of imbricated particles appear. These alluvial deposits probably resulted from ancient floods. 

Along any given reach, a stream could exhibit 

deposition as well as erosion (fig. 26), but on balance 

more erosion occurred in the headlands, whereas 

deposition predominated in the flatter and wider 

B 

A 

downstream valleys. Both erosion and deposition 

occurred, in about equal amounts, in the apex region 

of alluvial fans. 

SEDIMENT YIELD 

VOLUME OF AVALANCHE EROSION 

The volume of sediment removed by a debris 

avalanche could not be measured precisely, and all 

c 

FIGURE 26.-Channels showing both erosion and deposition. A, South Fork of Rockfish River. B, Ginseng Hollow. Note men 

for scale. C, Small tributary to Hat Creek. 
EROSION 39 

of the data presented here are based on field esti- 4 

mates of the prestorm ground configurations. 

N 

t Table 2 lists the estimated erosion volumes for the 

12 scars measured in detail. The amount of sediment 

removed from these typical scars ranged from about 

18,000 cubic feet to 233,000 cubic feet. The average 

of the 12 is 88,000 cubic feet. The 12 examples selected do not include the largest and smallest scars 

in the entire storm area. 

Figure 27 shows both the amount of erosion and 

the compass orientation for these 12 scars. If the 

orientations and volumes of the scars shown in 

figure 27 are representative of the entire drainage 

areas, the diagram strengthens the earlier conclusion that hillsides facing north through east were 

most prone to sliding, not only in frequency of avalanches but also in total amount eroded. 

Field inspection was made of all of the upper part 

of the Wills Cove drainage area (shown in figure 4), 

all of Ginseng Hollow, and about 86 percent of 

the Polly Wright drainage area. Visual estimates 

were made of the length, average width and average 

depth of each scar encountered. Column 2 of table 6 

lists the total estimated amounts of avalanche erosion for the various tributaries and subbasins. The 

avalanches in Wills Cove produced about 2.60 million cubic feet, those in Ginseng Hollow about 1.44 

million cubic feet, and those in Polly Wright Cove 

&bout 1.46 million cubic feet of sediment. The neighboring Davis Creek, Fortunes Cove and adjacent 

areas were also severely affected by avalanches; so 

the total amount of sediment eroded by this process 

was considerable. 

Column 5 of the table shows that the volume of 

avalanche erosion per square mile ranged from about 

1.5 to 2.1 million cubic feet, with the greatest 

amounts originating in Ginseng Hollow. 

A very crude relation exists (fig. 28) between 

the average slope of the ground surface, as determined from the the topographic maps (table 1), 

and the volume of sediment eroded in debris avalanehes. The scatter prevents the determination of 

a reliable regression curve, but the trend is from no 

erosion at 32-35 percent average slope to 3 or 4 

million cubic feet per square mile at 50 percent 

average slope. 

SEDl:\fE~T YIELD FROM STREAM CHANNELS 

As a part of the detailed studies made in Wills 

Cove, Ginseng Hollow and Polly Wright Cove, estimates were made of the net volume of channel erosion in many of the tributaries and subbasins. These 

estimates were based on visual field inspection of the 

12 

1 

FIGURE 27.-0rientation and relative volume of the 12 

avalanches that were measured in detail. Scale: 1 

inch= 100,000 cubic feet. 

complete lengths of most tributaries and on the 

results of 18 surveyed channel cross sections which 

compared prestorm and poststorm profiles. About 

21;2 years before the storm the U.S. Geological Survey had taken aerial photographs of the entire area 

for topographic map purposes. Using a Kelsh stereoscopic plotting instrument with these photographs, 

we measured on the photographs 18 prestorm channel cross sections at locations which, judging from 

the aerial photographs, would be easiest to locate 

in the field. We then went to the field with a transit 

and level rod and measured the post-storm cross 

section at these 18 sites. 

Although the poststorm profiles could be measured 

quite accurately, the final erosion estimates include 

several possible errors. The 1967 photographs may 

not exactly represent the condition of the channels 

just before the 1969 flood, but any channel erosion 

s I I ! I 

0 ~ 

30 

EXPLANATION 

• Polly Wright Cove 

D. Wills Cove 

X Ginseng Hollow 

X 

D.D.x 

D. D. 

• X 

I I 

X 

X 

• 

D. 

X 

• D.x 

D. D. 

r- X X 

I l 

35 40 45 50 

AVERAGE SURFACE SLOPE, IN PERCENT 

-

-

-

D. 

55 

FIGURE 28.-Volume of avalanche erosion with respect to 

average surface slope. 
40 HURRICANE CAMILLE IN VIRGINIA, 1969 

TABLE 6.-Estimated net sediment yields and storm average transport rates 

[Estimates are based on field inspection of all of the Wills Cove (upper part) and Ginseng Hollow drainage areas and about 85 percent of the 

Polly Wright Cove basin. See figure 4 for designation of drainage basins indicated in column 1] 

Drainage basin 

(1) 

Net yield (103 ft3) 

Avalanche 

(2) 

Channel 

(3) 

Total 

(4) 

Net yield per square mile 

( 106 ft3 ) 

Avalanche 

(5) 

Channel 

(6) 

To till! 

(7) 

Average 

denudation (inches) 

(S) 

Weight 

of sediment 

eroded (1()4 tons) 

(9) 

Storm-average 

sediment- transport rate 

(10~ tons per day) 

(10) 

(103 lb per sec) 

(11) 

Wills Cove, upper part 

West tributary Middle tributary Falls tributary: 

Subbasins 1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

908 873 1,781 

379 403 782 

316 158 474 52 23 75 2 4 6 1 75 76 0 68 68 28 56 84 1 -1 0 136 106 242 65 108 173 32 48 81 32 131 163 50 86 136 65 20 85 98 120 218 98 9 107 60 81 141 30 20 50 120 0 120 45 96 141 4 7 11 0 24 24 81 42 123 

Subtotal (1-22) ------- 1,316 1,282 2,598 

Noncontributing --------------

Main channel ----------------

Total, Falls tributary __ 

Average, Falls tributary_ 

Total, Wills Cove Average, Wills Cove __ _ 

======================== 0 0 0 

965 965 

1,316 2,247 3,563 

======================== 

------------------------- 2,603 3,523 6,126 

38 

2.6 

7.9 

1.4 

.07 

.01 

0 

.9 

.04 

2.6 

1.4 

1.0 

1.2 

2.1 

.6 

1.6 

3.1 

1.0 

.5 

8.6 

2.0 

.7 

0 

2.1 

1.1 

1.7 

3.7 

2.8 

3.9 

.6 

.14 

1.1 

1.5 

1.9 

-.04 

2.0 

2.3 

1.5 

4.9 

3.6 

.18 

2.0 

.3 

1.3 

.3 

0 

4.4 

1.2 

3.0 

1.1 

1.9 

2.2 

7.5 

5.4 

11.8 

2.0 

.2 

1.1 

1.5 

2.8 

0 

4.6 

3.7 

2.5 

6.1 

5.7 

.8 

3.6 

3.4 

2.3 

.8 

8.6 

64 

1.9 

3.0 

3.2 

3.0 

3.9 

3.2 

2.3 

5.1 

.9 

.1 

.5 

.6 

1.2 

0 

2.0 

1.6 

1.1 

2.6 

2.5 

.3 

1.5 

1.5 

1.0 

.3 

3.7 

2.8 

.8 

1.3 

1.7 

1.3 

1.4 

104 

46 

27 

4 

.3 

5 

4 

5 

0 

14 

10 

4 

10 

8 

5 

13 

6 

8 

3 

7 

8 

5 

2 

7 

155 

0 

60 

215 

365 

357 

158 

93 

14 

1 

17 

14 

17 

0 

47 

34 

14 

34 

27 

17 

45 

21 

27 

10 

24 

27 

17 

7 

24 

531 

206 

737 

83 

37 

22 

3.2 

0.2 

4.0 

3.2 

4.0 

0 

11.1 

7.9 

3.2 

7.9 

6 3 

4.0 

10.3 

4.8 

6.4 

2.4 

5.6 

6.4 

4.0 

1.6 

5.6 

124.1 

48 

172.1 

Ginseng Hollow 

Lower subbasin ------------------ 523 712 1,235 North tributary ------------------ 104 152 256 South tributary ------------------ 616 526 1,142 Main above sta. 3400 ------------ 197 303 500 East bowl (noncontributing) ____ 0 0 0 

------------------------- Total, Ginseng Hollow ----- 1,440 1,693 3,133 

Average, Ginseng Hollow __ _ 

3.0 

1 2 

5.4 

.9 

0 

2.1 

4.1 

1.8 

5.6 

1.4 

0 

2.5 

7.1 

3.0 

11.0 

2.3 

0 

4.6 

3.1 

1.3 

4.7 

1.0 

0 

2.0 

73 

15 

67 

30 

0 

185 

250 

51 

230 

103 0 

634 

58 

12 

53 

24 

0 

147 

Polly Wright Cove 

Subbasins 1 2-5 

6 

7 

8 

9 

10 

11 

12 

13 

Total, subbasins 1-13 ------

Lower --------------------------- Noncontributing ------------------

Total, Polly Wright Cove __ _ Average, Polly Wright Cove __ 

85 

180 

80 

40 

133 

156 

248 

204 

187 

28 

1,341 

115 0 

1,456 

114 

194 

211 

90 

268 

230 

86 

105 

128 

92 

1,518 

123 0 

1,641 

199 

374 

291 

130 

401 

787 

334 

309 

315 

120 

2,859 

238 0 

3,097 

during this 21/2-year period probably was minor. 

The plotting-instrument technique, probably one of 

the smallest sources of error, was at best accurate 

to within about ± 1 foot in elevation. Horizontal dis 

tances, measured from the plotted Kelsh elevation 

points with an engineer's sc::tle, were accurate to 

2.0 

2.1 

2.2 

1.5 

1.9 

2.6 

7.2 

5.5 

4.4 

1.2 

1.5 

0 

1.5 

2.7 

2.3 

5.7 

3.5 

3.9 

3.8 

2.5 

2.8 

3.0 

3.8 

1.6 

0 

1.7 

4.7 

4.4 

7.8 

5.0 

5.8 

6.4 

9.7 

8.3 

7.4 

5.0 

3.1 

0 

32 

2.0 

1.9 

3.4 

2.2 

2.5 

2.8 

4.2 

3.6 

3.2 

2.2 

.9 

0 

1.4 

12 

22 

18 

8 

24 

23 

19 

18 

18 

7 

169 

14 

0 

183 

41 

75 

62 

27 

82 

79 

65 

62 

62 

24 

48 

0 

9 

17 

14 

6 

19 

18 

15 

14 

14 

6 

11 

0 

within 1 percent. The major problem was locating 

in the field the specific site that had been measured 

on the photographs. (The photographic cross sections had to be measured first because of the limited 

choice of suitable locations identifiable from the 

photographs.) Numerous abandoned logging roads 
DEPOSITIONAL FEATURES 41 

along the ravines helped pinpoint the selected sites, 

but several of the sections were difficult to locate in 

the field because some of the roads, old cabins, and 

other landmarks noted on the photographs were 

washed away during the flood. The various sources 

of error could amount to about 25 to 50 percent in 

the amount of erosion attributed to any site. In addition, the visual estimates, the method used to measure poststorm profiles in most reaches, involve an 

unknown amount of error. 

While observations were being made of channel 

erosion, attention was also given to any significant 

channel deposition. Such deposition was notable and 

persistent along some parts of the main channel in 

the Falls tributary of Wills Cove and in Ginseng 

Hollow . . Deposition occurred mainly in reaches of 

flatter gradient and less confined valley walls where 

stream velocity probably decreased. The figures for 

net sediment yield from channels ( cols. 3 and 6, 

table 6) represent the estimated erosion minus any 

deposition. 

Channel erosion, as near as can be determined 

from these volume estimates, accounted for more 

than half of the total sediment erosion (col. 3, table 

6). The amounts of yield from individual tributaries 

were as much as 5.7 million cubic feet per square 

mile (col. 6, table 6). The average yield for the 

three basins was 2.2, 2.5 and 1.7 million cubic feet 

per square mile for Wills Cove, Ginseng Hollow and 

Polly Wright Cove, respectively. The lower average 

for Polly Wright reflects the effect of a rather large 

part of the basin on which little or no erosion 

occurred. 

Net channel erosion apparently was slightly 

greater than avalanche erosion, as far as volume of 

erosion per square mile is concerned ( cols. 5 and 6 

of table 6). 

TOTAL SEDIME:.JT YIELD AND AVERAGE DENUDATION 

If the amounts of yield from the separate subbasins or tributaries are added, the estimated total 

yield from the headwaters of Wills Cove is 6.1 million cu ft, from Ginseng Hollow about 3.1 million 

cu ft, and from Polly Wright Cove about 3.1 million cu ft. These volumes represent 3.9, 4.6 and 3.3 

million cu ft per sq mi for the Wills (upper part), 

Ginseng and Polly Wright basins, respectively. 

If each of these amounts were imagined to be removed uniformly from the entire area of the respective drainage basin, the denudation from Wills Cove 

averaged about 1.4 inches, from Ginseng Hollow 

about 2.0 inches, and from Polly Wright Cove about 

1.4 inches. The average denudation for an individual 

tributary or subbasin (col. 8 of table 6) ranged from 

little or nothing to about 5.1 inches. 

Judson and Ritter (1964) give estimates indicating that for central Virginia the average denudation 

due to sediment removal by suspended load and bedload is in the range of about 0.6-1.4 inches per 

thousand years. The 1.4- 2.0 inches removed in one 

storm for the drainage basins studied here show an 

extent to which individual basins and rare events 

can depart from the long-term average. 

ESTIMATED SEDIMENT-TRANSPORT RATES 

Although no measurements of sediment-transport 

rate were made during the flood, a very rough idea 

of the likely average rate d the mouths of some 

tributaries can be obtained from the estimated total 

sediment yield and the duration of the storm. As 

transport rate customarily is expressed in weight 

per unit time, the estimated volumes of sediment 

yield must be transformed into weights. This transformation (col. 9 of table 6) was made by using 

110 lb per cu ft (pounds per cubic foot) for the 

avalanche soils and 125 lb per cu ft for the coarser 

channel material. Seven hours was the approximate 

time during which the sediment moved in these 

headwater areas. The storm-average sediment transport rates (cols. 10 and 11 of table 6) ranged from 

very little at the mouths of some subbasins to as 

much as about 7,370,000 tons per day (172,000 lb 

per sec) at the mouth of the Falls tributary in Wills 

Cove. These estimated rates are for the full stream 

width. Since the stream at the mouth of the Falls 

tributary was about 100 feet wide during the storm, 

the estimated rate of sediment movement at that 

section may have averaged about 1,700 lb per sec 

per ft of width for the 7 -hour period. (By way of 

comparison, laboratory flumes rarely exceed 1 or 

2 lb per sec per ft width.) Such magnitudes reflect 

the large quantities of rock and earth unleashed by 

debris-avalanches and severe channel erosion. Because of this irregular rate of sediment introduction, the transport rate at any one place must have 

varied considerably with time. 

DEPOSITIONAL FEATURES 

The floodwaters laid down e"l\tensive deposits of 

sediment, often to the detriment of man and his 

property. In some places the sediment came to rest 

very close to its source, as at the foot of a hillslope 

or on an alluvial fan at the mouth of a mountain 

channel. At the other extreme, some of the small 

particles eroded from the mountainsides undoubt-
42 HURRICANE CAMILLE IN VIRGINIA, 1969 

edly moved all the way to Chesapeake Bay or the 

Atlantic Ocean. 

Five kinds of deposits were left after the flood: 

(1) debris avalanche deposits at the base of hillslopes; (2) mountain-channel deposits, usually in 

sediment patches scattered along the newly eroded 

channel, but occasionally as large debris piles behind a temporary dam ; (3) alluvial fans where a 

narrow mountain channel emerged into a relatively 

broad intermontane valley; ( 4) deltas at the j unction of a stream and major highway, where backwater formed during the flood due to plugging of a 

culvert; and (5) vertical accretion deposits on 

floodplains. 

Only rarely were debris avalanche deposits left 

at the base of the hillside. Some of these particles 

were probably too big to be moved by the floodwaters 

(fig. 29). In other cases the slide probably did not 

occur until after the peak flow, when the flow in 

the ravine was no longer sufficient to carry away the 

debris. Where no stream channel flowed past the 

hillside, the momentum of the avalanche nearly 

always transported most of the material away from 

the base of the slope. 

FIGURE 29.-Boulders which were dislodged in a 

debris avalanche and which probably were too 

large to be moved by the floodwaters (Ginseng 

Hollow). Man on rock shows scale. 

CHANNEL DEPOSITS 

Some eroded material was left here and there 

on the bed of the mountain channel after traveling a 

distance ranging from a few feet to nearly a mile. 

These channel deposits were either laid down by the 

recession flow (since the peak flow probably could 

have moved the particle sizes involved) or trapped 

behind dams such as huge rocks or log jams (fig. 30), 

in which case the deposition could have occurred at 

various stages of the flood. 

Deposits are sparse along the enlarged mountain 

channels and generally absent near the heads of the 

streams. As a stream approached its outlet into the 

broad valley, the scattered deposits on some channels-those which became wider and flatter-became 

more common, thicker (as much as 5 ft), arrd locally 

continuous. Downstream reaches of mountain channels which did not become appreciably wider and 

flatter usually had no sediment patches. 

Particle sizes in channel deposits range from silt 

to boulders 10 feet or more in intermediate diameter 

(figs. 31 and 26). Some of these sediments were 

measured to determine their size-frequency distribution, but this subject is discussed later in the report 

because the deposits studied for grain-size distribution were continuous with downstream alluvial fan 

and (or) floodplain deposits. 

Most of the channel deposits were unsorted. Others 

FIGURE 30.-Upstream view of typical log jam in Ginseng 

Hollow. Rocks of all sizes accumulate behind such dams. 

Normal water flow, about 3 feet wide, can be seen in 

foreground just to right of man. 
A B 

c D 

FIGURE 31.-Range of particle sizes commonly present in upstream deposits. A, Ginseng Hollow. B, small ravine near Shaeffer Hollow. C, D, Polly Wright 

Cove. Except forB, the deposits can be interpreted as either floodplain or mountain-channel deposits. Note normal water flow in left-center foreground 

t:::1 

l"'.l 

l"tj 

0 

C/l 

.... 

~ .... 

0 

z 

> t:"' 

>%j 

l"'.l 

~ c:: 

~ 

l"'.l 

C/l 

of photograph A. t 
44 HURRICANE CAMILLE IN VIRGINIA, 1969 

showed thin lenses or layers of well-sorted pebbles 

or cobbles, often in an imbricated pattern. Such 

well-sorted deposits in a few places formed minor 

ridges along the edge of the channel, extending for 

about 15-30 feet downstream parallel to the stream 

direction. The ridges may be remnants of a deposit 

that filled the channel bottom at some stage during 

the flood, with the rocks from the central zone being 

removed by recession flow. The ridges definitely 

were formed after the peak flow because they were 

on the channel bottom well within the maximum 

cross-sectional flow area, and the peak flow would 

have easily removed them. 

Boulders as well as cobblestones accumulated in 

imbricated patterns, as figure 32 shows. Such large 

particles, although definitely moved by the flood, 

probably were deposited during or soon after the 

peak flow. 

Debris piles behind dams in mountain channels 

are as much as 200 ft long and extend all or most 

of the way across the channel (up to 100 or even 

200 ft). These wedge-shaped deposits are as much 

as 20 ft deep at the downstream face. They show 

little or no stratification or preferred orientation 

of particles. 

Deposits in the normal stream channel downstream from the mountains were under water at the 

time of the investigation. These submerged sediments were not studied because they probably were 

altered by the postflood streamflow to the extent that 

they did not represent flood deposits. Valley sediments located on either side of the normal stream 

FIGURE 32.-Downstream view of imbricated boulders left in 

a stream channel in Edes Hollow. 

are here treated as flood-plain deposits, except for 

the alluvial fans and highway deltas. 

SEDIMENT SAMPLING, COMPUTATION OF 

SIZE-FREQUENCY DISTRIBUTIONS 

Except for the scattered debris piles in the mountain channels the sediment deposits were inspected 

for grain-size characteristics and other features. 

For various reasons it was usually impossible to 

define a sedimentation unit (a thickness of sediment 

deposited under virtually constant physical conditions). This fact, plus practical considerations, dictated that in most cases only the surface material 

was examined in regard to particle sizes. 

The sediments laid down more than a mile or two 

downstream from the alluvial fans or the head of tpe 

main valley were mostly sandy. These fine-grain~ 

deposits were sampled in a manner described below 

and were analyzed by sieving. 

In the upstream areas near the head of the valley 

a typical flood deposit commonly included particles 

ranging from fine silt to large boulders (fig. 31). 

These deposits were measured by the "pebble-count" 

method (Wolman, 1954). We stretched a measuring 

tape perpendicular to the stream axis across the full 

width of the deposit and recor4ed the intermediate 

axis of the surface particle lying under every twofoot tape increment. Grains smaller than about 10 

mm were listed simply as "sand" for convenience, 

and the intermediate axis of larger particles was 

measured with a meter stick. The number of individual particles counted ranged from about 30 to 

more than 300, depending on the width of the deposit. At 5-foot intervals across the deposit a "sand" 

sample was removed, except in those instances where 

no fine material happened to occur at a given 5-foot 

station. These samples represented the top inch or 

two of sediment. All such sand samples for a given 

section or reach were combined, and in the laboratory a single composite sieve analysis was made, reflecting an "average" size-frequency distribution for 

this finer material. If the sand sample contained 

about 10-15 percent less than 0.062 mm, a pipet 

analysis of this part was also performed. In these 

cases the results of the sieve and pipet analysis were 

directly combined and recomputed as one distribution, on the assumption that the sieve diameter is 

similar to the sedimentation diameter, within the 

silt and clay range. 

Ideally one would prefer a single size-frequency 

distribution representing all the particle sizes present in the deposit. Several factors had to be considered to achieve this goal. 
DEPOSITIONAL FEATURES 45 

Particles for a sieve and pipet analysis are usually 

obtained with little thought as to possible size differences between grains exposed on the top surface 

of the deposit and those beneath the surface. The 

sample usually contains both. The actual grain size 

fortunately does not influence the quantity of each 

size sampled, because the opening of the container 

ordinarily is considerably larger than the grains. 

The analysis yields percentages by weight, for each 

size class. Particles in a pebble count or stone count, 

however, represent particles lying only on the top 

surface of the deposit and are selected with a definite 

bias, namely in proportion to the exposed area of the 

stone. Moreover, the recorded data give percentages 

on a number basis rather than a weight basis. All of 

these differences had to be resolved, at least as far 

as possible, in order to combine the sieve and pebblecount data into a single overall size-frequency distribution. 

Kellerhals and Bray (1971) show that if the grain 

sizes in a deposit are randomly dispersed the number-percentages for surface grains are equal to the 

weight-percentages of a three-dimensional sample of 

the deposit, on the assumption that all particles have 

the same specific gravity. This means that for the 

sediments analyzed here no conversion factor was 

needed to transfer pebble-count (number) percentages to sieve (weight) percentages. On this basis we 

directly combined the pebble-count and sieve-analysis 

percentages into one continuous frequency distribution, performing in the process an adjustment for 

the relative proportions of large (pebble-counted) 

stones and finer (sieved) particles. 

The resulting frequency-distribution is a weightpercent distribution representing all particles within 

the three-dimensional body of the deposit. This distribution, incidentally, is approximately the same as 

the percent of surface area each size class covers on 

the top of the deposit. It is not a weight percentage 

distribution of surface grains, that is, not the percentages that would result if all grains visible from 

a top view were plucked from the surface and 

weighed. 

The assumptions involved are (a) homogenous 

dispersal of all grain sizes throughout the body of 

the deposit and (b) constant particle shape, specific 

gravity, orientation and packing. Naturally these assumptions are never completely true. Small grains 

are often less abundant on the surface, as they tend 

to hide in the voids or be winnowed away; however, 

we believe we sampled most or all of the pertinent 

flood sediments before any postflood erosion significantly altered the surface of the deposits. 

A simple example will show the computing procedure. At a sampling site (cross section over the surface) on an alluvial fan 59 entries were recorded in 

a rock count: 14 large stones and 45 particles of 

"sand." Column 3 of table 7 shows the percentages 

by number for the large stones, and column 4 lists 

the percentages on a weight basis for the sieved 

grains. Directly combining these two distributions 

would imply that each of the two groups occupies 50 

percent of the surface area of the deposits. However, 

the stone count showed 14 tallies in the large-stone 

range and 45 recordings of "sand." Large stones, in 

other words, covered only 1%9 of the surface area 

of the deposit. The percentage in each size class for 

large stones was therefore multiplied by 1%9 or 

0.237, and the percentage in each size class of the 

sieve analysis was adjusted by a factor of 4%9 or 

0.763 (col. 5). This step produced a single weightfrequency distribution for the entire deposit at the 

sampling site, and the usual cumulative size-frequency curve could then be drawn (fig. 33). 

The sampling at each pebble-count site also included a measurement of the intermediate axis of all 

of the largest stones within about 10 feet upstream 

and downstream of the tape section. The theory was 

that these boulders would be proportional to the 

stream competence, that is, to the stream's ability to 

transport sediment. They would represent a limiting 

particle size showing what range of sizes the flow 

TABLE 7.-Computational procedure of combining a sieve 

analysis with a pebble count 

[Location of sample: Bryant fan, station 1,800. Adjustment factors: 

14/59 = 0.237 for large (pebble-counted) stones, 45/59 = 0 763 for 

fine (sieved) particles] 

(1) (2) (3) (4) 

Percentages within 

subgroups 

Size class 

(mm) 

4,096-2,048 

2,048-1,024 

1,024-512 -----------

512-256 ----------- 256-128 ----------- 128-64 ------------ 64-32 ------------ 32-16 ------------

Number 

of large stones stones 

1 7.1 1 7.1 2 14.3 6 42.9 4 28.6 

14 ----wo.o 16-8 -------------·------------------

8-4 ------------------------------- 4-2 ------------------------------- 2-1 ------------------------------- 1-0.5 ------------------------------ 0 5-0.25 -----------------------------

0.25-0.125 ---------------------------- 0.125-0.062 ---------------------------- 0.062-0.031 ----------------------------

sievepipet 

analysis 

14.2 

11.4 

14.3 

19.0 

15.8 

12.2 

7.1 

3.2 

2.8 

100.0 

(5) (6) 

Cumula- Percent tive 

by percent 

weight, finer 

entire tban 

range upper 

of size 

sizes limit 

1.7 100.0 

1.7 98.3 3.4 96.6 10.2 93.2 6.8 83.0 

10 8 76.2 8.7 65.4 10.9 56.7 14.5 45.8 12.1 31.3 9.3 19.2 5.4 9.9 2.4 4.5 2.1 2.1 

100.0 
46 HURRICANE CAMILLE IN VIRGINIA, 1969 

100 

90 

w 

N 

en 

0 80 w 

1-

<( 

(.) 

0 70 ~ 

z 

<( 

I 60 1-

0::: 

w 

z 

u:::: 50 

en 

w 

_J 

(.) 

i= 40 0::: 

<( 

a.. 

LL.. 

0 30 w 

C!l 

<( 

1-

z 

w 20 (.) 

0::: 

w 

a.. 

10 

0.1 

PARTICLE DIAMETER, IN MILLIMETERS 

FIGURE 33.-Typical particle size-frequency distribution, based on percent by weight, of a deposit that has a wide 

range of particle sizes (Bryant fan, station 1800) . 

could and could not carry, in the region of the sampling site (on the assumption that all sizes were 

available for transport). Further reflection, however, suggests that the theory of competence is more 

complicated than might seem at first glance. First, 

the rate at which sediment entered the channel probably was· unsteady, responding in part to the occurrence of debris avalanches on the hillsides upstream. 

There is in fact no way of knowing ( 1) whether the 

total volume or·weight rate of sediment introduction 

was steady (it probably was not) or (2) which sizes 

of rocks entered the channel at a given stage during 

the flood. Besides these uncertainties regarding the 

sediment introduction, there are several aspects of 

the water discharge which could affect the stream's 

ability to transport sediment. The amount of water 

entering the channel, for example, varied with time 

in some unknown way because of the uneven distribution of rainfall intensity throughout the storm. 

Was the water discharge highest at the time the 

largest stone arrive~ at the head of the study reach? 

And did the discharge increase downstreamward, as 

is common, thus increasing the stream competence 

with distance? Against such a downstream increase 

in competence would be the flatter channel or floodplain gradient with distance, which would reduce 

the stream competence. In addition, the channel configuration varied with distance, at some places causing the flow to become narrow and deep and at other 

places wide and shallow. Such changes in flow cross 

section can affect the sediment transport. "Stream 

competence," then, actually involves a number of 

different factors, all of which might be important in 

determining the stream's ability to move the stones 

delivered to it. The largest stones finally deposited 

at a given section could have entered the channel at 

different times and could have been deposited and 

shifted a number of times during the flood. Thus the 

final areal distribution of large stones represents the 

combined effects of (1) some unknown rate of sediment introduction for each particle size throughout 

the flood and (2) varying degrees of stream competence at different times and locations. 

In most cases it was possible to tell whether a 

given boulder had been transported and deposited 

by this particular flood. However, as a safety factor 

against any possible misinterpretation and freak 

stone occurrence, th_e data on the following pages 
DEPOSITIONAL FEATURES 47 

represent the average size of the five largest stones 

at each site, within 10 feet of either side of the cross 

section line. 

DEFINITIONS OF SIZE-DISTRIBUTION 

CHARACTERISTICS 

Cumulative particle size-frequency curves were 

plotted on semilog paper as in figure 33, with percent finer than a given grain size on the ordinate 

(arithmetic scale). The following simple measures, 

easily and quickly obtained graphically, were 

adopted to describe the curves and are used in the 

subsequent discussion. 

The average particle size dav was estimated by 

averaging the grain sizes corresponding to the lOth, 

50th, and 90th percentiles of the distribution ( d1o, 

dso and ds0 , respectively). Since grain size is plotted 

on a log scale, dav is the antilog of the quantity (log 

dlo+log dso+log so) /3. 

The range, spread or sorting of particle sizes (S.) 

is reflected by the percentile range dso-dlO. Because 

of the log scale for grain sizes, this range is log 

dso-log d1o, or log (d9o/d,o). The dso and d10 values 

were selected because they cover the widest possible 

definable range common to all curves. This measure 

includes most of the distribution and is not affected 

by the actual sizes involved. 80=0 means "perfect 

sorting" in the range between d90 and d10 , that is, all 

particles in this range have the same size. In such 

case dso=dlo, the quotient is 1.0 and the log or sorting is zero. So values of 1.0, 2.0, 3.0, and so on mean 

that the percentile range d90--d10 covers one, two and 

three log cycles of grain size, respectively. 

The measure of asymmetry of the distribution or 

skewness sk is (log d10+log d90- 2 log dso) I (log 

deo-log d10). This is similar to the relative skewness 

listed in Croxton and Cowden (1939, p. 254) but 

deals with a distribution of the logarithms of particle diameters. Possible values of sk fall within 

-+-1.0, and in a symmetrical distribution sk=O. A distribution having a preponderance of small grains 

has a positive value of sk, whereas negative values 

indicate a preponderance of large particles. 

The average size (geometric mean) of the five 

largest stones (d1.)is the antilog of (sum of logs of 

intermediate diameters, divided by five). The geometric mean, rather than the arithmetic mean, is 

used because particle sizes customarily are treated on 

a logarithmic scale. 

ALLUVIAL FANS 

Upon escaping the confinement of the rather narrow mountain channels, several of the swollen 

streams deposited a large amount of sediment in the 

form of alluvial fans (figs. 34 and 35). Such alluvial 

fans or cones of debris seem to be common depositional features of floods in hilly areas (Eisenlohr, 

1952; Jahns, 1947; Chawner, 1935). Fans actually 

did not form on many streams, as the ground configuration often did not favor the formation of fans. 

For example, the mountain channel may have widened considerably before reaching the broad valley, 

so that no sudden spreading out of the flow occurred. 

In other cases the channel leaving the mountains was 

deep enough to contain much of the flow for some 

distance beyond the mountain front, so that only 

typical flood-plain deposition resulted. Some drainage basins had very few debris avalanches in the 

headwaters, so the streams in those basins carried 

relatively little sediment. 

Some of the more notable alluvial fans were 

formed at the home of R. L. Bryant at the mouth of 

the Ginseng Hollow (fig. 35), at the home of W. A. 

Campbell near Shaeffer Hollow, at the home of S. K. 

Wills at the head of Wills Cove, and at a downstream 

site on Cub Creek (fig. 34). Fans ranged in length 

from a few feet to nearly 0.4 mile. For example, the 

Bryant fan was about 2,000 feet long, the Campbell 

fan about 1,830 feet and the Wills partial fan about 

850 feet. (Another stream eroded the downstream 

part of the Wills fan.) 

The cascade of water, rock and rubble that formed 

an alluvial fan often caused considerable damage to 

buildings and property, because fans usually occupied areas of regular land use. At R. L. Bryant's 

home at the mouth of Ginseng Hollow, part of the 

house was torn away, including the section in which 

Mr. Bryant was sleeping at the time. (He rode 140 

ft downstream on his mattress, along the edge of 

the flow, sailed in through the open door of his barn 

and spent the rest of the night sitting in the upper 

part of his barn-a lucky survivor.) 

Alluvial fan sediments were devoid of any recognizable sedimentary features, such as bedding, lamination, crossbedding, mud cracks, imbrication, or 

other preferred orientation of particles. Except that 

many rocks seemed to be lying on their long and 

intermediate axes, the deposits looked as if they had 

been shaken in a giant mixer and dumped. 

Also dumped on fan surfaces were many trees and 

branches, isolated or in piles. Isolated large trees 

tended to be parallel to the flow direction, although 

the roots could be pointing upstream or downstream. 

(This tendency could be a useful aid in determining 

flow direction in ancient sediments, if remains of 
48 HURRICANE CAMILLE IN VIRGINIA, 1969 

A 

B c 

FIGURE 34.-Upstream views of alluvial fans which formed where the stream escaped the confinement of the mountain 

channel. A, Campbell fan. B, Wills partial fan (arrow points to man for scale). C, fan entering valley of Cub Creek 

about 2 miles down from headwaters. 

trees could be found.) Trees in log jams tended to 

be oriented at oblique angles to the flow direction. 

The flow depth over an alluvial fan should decrease proceeding downstream from the apex. There 

was very little evidence to indicate the flow depth 

over the Nelson County fans. From some high-water 

marks on a hillside along the left-bank edge of the 

Bryant fan, the peak flow depth about half way 

down the fan was judged to be about 3-4 feet. 

In all cases the stream at the fan apex, and occasionally even further downstream on the fan, cut as 

much as about 10 feet vertically into old alluvial 

deposits and soil horizons (fig. 36). In fact the general ground configurations reveal many old alluvial 

fans where mountain channels enter the broad valleys in Nelson County. For example, all the fresh fan 

deposits discussed here were formed on old alluvial 

fans; so the 196~ storm was only one in a chain of 
DEPOSITIONAL FEATURES 49 

FIGURE 35.-Aerial 1view of Bryant fan and lower part of Ginseng Hollow (Photograph courtesy of Virginia 

Division of Mineral Resources). 

similar major events in the geomorphic history of 

the area. 

For a few weeks after the flood the creeks on the 

fan tended to show a braided pattern, especially 

toward the downstream part of the fan. 

AMOUNT OF DEPOSITION 

Sediment was transported to a fan over a period 

of time during which the discharge and sediment 

transport rate undoubtedly varied considerably. The 
50 HURRICANE CAMILLE IN VIRGINIA, 1969 

FIGURE 36.-Eroded zone at apex of Campbell fan. 

deposition or growth of the fan therefore proceeded 

in stages. Due in part to the varying erosion and 

deposition with time at the apex of the fan, the flow 

arriving at the fan apex probably was diverted to 

.~ghtly different directions during the flood, and the 

successive depositional stages probably manifested 

themselves as "tongues" of deposit which varied in 

areal location on the fan. As a result, the fan thickness varies from one spot to another. 

We analyzed amounts of deposition on the Bryant 

and Campbell fans by setting up stations down the 

fan centerline. At a number of these stations the 

cross-sectional area of the deposit, in a plane normal 

to the fan centerline (that is, roughly normal to the 

flow direction) Was determined by stretching a measuring tape over the full width of the fan and recording the depth of fill along this cross section. The 

depth varied from place to place, but usually it did 

not exceed 1-2 feet. Depth was determined on the 

basis of exposure of the original surface at some 

points or by digging through the deposit. Along any 

given cross section it was possible to define increments over which the depth of fill was approximately 

constant, and the many subareas were summed to 

get the total cross-sectional area of deposition, for 

each downstream station. 

The sediment deposited on a fan should depend in 

part on the water discharge and fan slope. However, 

the problems of which type discharge (for example, 

storm-average or various instantaneous rates) and 

slope (local slope or average slope from fan apex to 

top) determine or indicate the final amount and distribution of the fan deposit are unresolved. These in 

fact would be interesting subjects for future research. As estimated earlier, the storm-average water discharge at the apex of the Bryant fan was 

roughly 1,050 cfs. For the apex of the Campbell fan 

the rational formula yields an estimate of only 134 

cfs, owing to the much smaller drainage area (0.08 

sq mi). Average fan slopes (fig. 9) were 0.045 ft 

per ft and 0.080 ft per ft for the Bryant and Campbell fans, respectively. The product of discharge 

times slope, roughly indicative of stream power, is 

thus more than about four times greater for the 

Bryant fan than for the Campbell fan. However, 

aside from the fact that the storm-average discharge 

and overall fan slope may not be the factors most 

closely related to the volume of the deposit, the 

sporadic nature of the amount and rate of sediment 

supply to the two fans makes comparisons difficult. 

Figure 37 shows how the cross-sectional area of 

deposition varied with distance down two fans. The 

actual horizontal distance has been converted to proportional distance from apex to downstream tip of 

fan, in order to compare the two fans. 

The apex or upstream region of an alluvial fan 

typically showed both erosion and deposition. On 

balance either process could predominate, depending 

on the width of the upstream mountain channel relative to the width of the fan apex. At the Bryant 

home the channel upstream was not very constricted 

(fig. 35), and some deposition occurred before the 

stream left the mountain channel. Consequently 

deposition predominated over erosion around the fan 

apex. The mountain channel upstream from the 

1- 300 

w 

<t:t:::' 

~ w 250 <t:O:C <t: -'::l 

~ 55 200 0 

t=z 

u-

~ ~ 150 -

~~ 0 e:; 100 

~0 u.. 

0 50 

/ 

/ 

x 

/ 

/ 

/ 

)!"---\ . 1 \Campbell 

1 \ fan 

I \ 

\ 

\ 

\ 

\ 

PROPORTIONAL DISTANCE FROM APEX 

TO DOWNSTREAM TIP OF FAN 

FIGURE 37.-Amount of deposition with distance down fan, 

for the Bryant and Campbell alluvial fans. 
DEPOSITIONAL FEATURES 51 

Campbell fan was deep and V-shaped (fig. 36), and 

the fan had net erosion for about the upstream 0.1 

of the length of the fan. 

On the basis of figure 37, the Bryant and Campbell 

fans each received about 300,000 cubic feet of sediment during the flood. 

A striking similarity in the two curves of figure 

37 is that the volume of deposition builds up abruptly 

from very little at the apex to a peak amount and 

then diminishes abruptly toward the downstream tip 

of the fan. On the Bryant fan the peak deposition 

occurs about 0.3-0.4 of the way down the fan, while 

on the Campbell fan the peak is about 0.5-0.7 of the 

way along the fan. 

CROSS-FAN PROFILES 

Cross-fan profiles are surface profiles measured 

normal to the long axis (centerline) of the fan, that 

is, measured approximately parallel to the mountain 

front. Such profiles normally would be convex owing 

to greater deposition in the middle part (Bull, 1964, 

p. 114). ' 

Figure 38 contains cross-fan profiles for the Campbell and Bryant fans. The Bryant profiles are convex 

except for the first profile, which is 0.2 of the ·'\\ray 

down the fan. The original stream ran along the 

south (left bank) edge of this fan; so the topography 

is generally lower along that edge. In the downstream 0.4 of the fan length, the deposit ~plits into 

two prongs (fig. 35). The apex of the Campbell fan, 

as mentioned above, is mainly an erosiQnal rather 

than a depositional surface. Significant ' deposition 

began around 0.17 of the way down the fan, but the 

profile for that station generally reflects erosional 

processes more than deposition. Cross . profiles further down the fan are only slightly convex. In the 

downstream half of the fan, the main drainage is 

along the left edge; therefore, as with the Bryant 

fan, the surface is lower along that edge .. 

GRAIN -SIZE CHARACTERISTICS . 

On the Bryant, Campbell, and Wills fans particlesize analyses were made across the full width of the 

deposit, at various stations down the fan. Figure 39 

is a graph of the various grain-size characteristics 

as a function of distance down the fan. The grainsize features at a given station down the fan centerline represent the particles over the full width of the 

fan, at that downstream station. Except for the 

large-rock measurements, the sampling stations on 

the Wills fan were too few in number (2) to warrant 

plotting. 

Average grain sizes range from about 2 to 6 mm. 

Although one might expect the average grain size to 

decrease with distance down the fan (Chawner, 

1935), such a trend is not very pronounced (fig. 39). 

The largest value of dav occurs not at the apex of the 

fan but a short distance downstream-0.2 of the way 

from the apex to the lower end of the Bryant fan and 

about 0.4 to 0.5 of the way down on the Campbell 

fan. Deposition did not begin as near the apex on 

the Campbell as on the Bryant fan, as mentioned 

above. 

The geometric mean of the five largest stones 

ranges from about 500 to 1, 700 mm. On the Bryant 

fan the size of the largest stones generally shows a 

gradual decrease with distance of travel. Large 

stones on the Campbell fan increase in size over the 

upper 0.4 of the fan length, then gradually decrease 

in size with further distance down the fan. The 

trend in fact is quite similar to that of the average 

grain size for the Campbell fan, but this is not the 

case on the Bryant fan. Large stones on the Wills 

partial fan show no particular tendency to increase 

or decrease with distance, but this truncated fan 

was less than half the length of the other two. 

Pashinskiy (1964, p. 277) noted that on debris cones 

bordering the Psezuapse River, near the Caucasian 

Mountains, USSR, the coarsest material generally is 

located in the middle part of the cone. Lustig (1965, 

p. 145), on the other hand, found that large stones 

on California fans were most abundant near the apex 

region. 

The largest stones on the Bryant fan are smaller 

than those on the Campbell and Wills fans. This is 

probably because the mountain ravines upstream 

from the latter fans were confined, and practically no 

channel deposition occurred. The Ginseng Hollow 

channel upstream from the Bryant fan, on the other 

hand, had a greater width/depth ratio and contained debris jams and channel deposits (fig. 26B, 

30, 31A). These deposits included many of the large 

boulders which otherwise would have been dumped 

on the fan. 

In general the alluvial fans are characterized by a 

wide range of particle sizes-from clay or silt up to 

boulders about 2 meters in intermediate diameter. 

The sorting, in other words, is quite poor and was 

probably affected by deposition of finer sediments at 

the end of the flood. The d9o-d10 range covers as much 

as 3. 7 log cycles of grain size along most of the 

length of the fan (fig. 39). Sorting on the Campbell 

fan stays virtually constant at a value of about 3.4 

along the entire study reach. On the Bryant fan the 

upstream 0.6 of the fan length also has virtually 

constant sorting, an So value of about 3.4 to 3.7, but 
52 HURRICANE GAMILLE IN VIRGINIA, 19·69 

BRYANT 

80 

~ 

:::l 75 f-

<( 

0 

>- 70 0:: 

<( 

0:: 

f- 65 

CD 

0:: 

<( 60 

w 

> 0.40 0 

CD 55 <( 

fw 50 w 

1..!... 

~ 45 

z 0.60 0 40 i= 

<( 

> 

w 35 _J 

w 

30 

200 100 50 0 50 100 150 200 250 309 350 

DISTANCE FROM FAN CENTERLINE, IN FEET 

CAMPBELL 

100~ 

> N 

I 

~ 95 

90 

~ 

:::l 

~ f-

<( 70 0 

>- 0.33 0:: 65 <( 

0:: 

fCD 60 0:: 

<( 

w 

> 0 

~ CD 

<( 45 

fw 

w 40 1..!... 

~ 

z 35 

0 

i= 30 <( 

> w 

_J 

w 25 

20 

15 

10 

150 100 50 0 50 100 150 200 

DISTANCE FROM FAN CENTERLINE, IN FEET 

FIGURE 38.-Cross-fan profiles, looking down the fan, for the Bryant and Campbell alluvial fans. Number next to line of profile indicates proportional distance down fan, from apex to toe. Profiles reflect all pre-1969 deposition, in addition to Camille flood deposition. 

the sorting improves with greater travel distance, so 

that at 0.9 of the way down the fan the d90-d10 

ranges encompasses only about 2.3 log cycles of the 

grain size-perhaps reflecting the fewer large stones. 

dominance of the smaller grain sizes. The weight of 

the smaller grains in the fan, in other words, is 

greater than that of the large boulders. With distance along the fan the skewness values change inAll skewness values are positive, indicating a pre- versely with the average grain size (fig. 39). The 
en 

a:: 

UJ 

IUJ 

:::;: 

::::; 

..J 

~ 

~ 

.. 

-o 

2 

2000 

en 

a:: 

UJ 

IUJ 

:::;: 1000 ::::; 

..J 

:::;: 

~ 

--o 

500 

400 

en 

I- 4.0 z 

::::> 

0 

0 

..J 

~ 

0 

z 

i= 

a:: 

0 "2.0 en 

+ 0.6 

en 

t::: 

z 

::::> 

0 

0 

..J 

~ 

en 

en 

UJ 

z 

s: 

UJ 

00:: 

en 

0 

0 

DEPOSITIONAL FEATURES 

0.3 

EXPLANATION 

0 Bryant fan 

X Campbell fan 

e Wills fan 

PROPORTIONAL DiSTANCE FROM APEX TO DOWNSTREAM TIP OF FAN 

1.0 

FIGURE 39.-Change in particle size-distribution characteristics with distance along alluvial 

fans. 

53 

reason for this is that for a given range of grain 

sizes a decrease in dav means the smaller grains are 

becoming relatively more abundant in terms of 

weight than the large boulders, so that sk takes on 

larger positive values. Conversely, given a distribution having a preponderance of smaller particles, an 
54 HURRICANE CAMILLE IN VIRGINIA, 1969 

increase in dav means that the larger stones are becoming more important is terms of weight. The distribution therefore tends to become more symmetrical, that is, the sk values tend toward zero. If the 

large stones were to become more abundant than the 

small grains, sk would ba negative. 

HIGHWAY DELTAS 

Vast bodies of sediment accumulated in deposits 

herein called highway deltas (figs. 40 and 41). Passageways under small bridges or highways became 

clogged with vegetation and other debris, probably 

early in the flood. In some cases the raging stream 

scoured a new channel by eroding the highway next 

to the bridge, but in other cases the highway became 

an effective dam. A large body of water with a mean 

flow velocity considerably slower than that of the 

unaffected flow formed just upsbeam from the dam. 

The stream filled sediment into the ponded water 

until a sediment wedge or delta had build up to the 

road level, after which time the water carried the 

rest of its load across the highway and on to some 

farther destination. 

Where present, the highway deltas are within I 

A 

' 

about 4 miles of the :stream headwaters. Culverts or 

bridges this close to the headwaters are rather small 

and became plugged fairly easily. Farther downstream the streams ' are larger and flowed · under 

major bridges that washed out completely or that did 

not become plugged with debris, so that deltas did 

npt form. 

I The dimensions of rhighway deltas are limited laterally by the valley or floodplain walls and vertically 

b¥ the height of the r!>adway. Delta lengths often are 

difficult to define because the deposit gradually 

merges upstreamward with the regular floodplain 

deposits. One of the smallest deltas is on route 151 

about a mile south of Brent Gap. The deposit is about 

200 feet wide at the highway, 150 feet long, 10 feet 

deep at the thickest place on the downstream face, 

and gradually thins laterally. The resident whose 

house is only about 50 feet outside the edge of this 

deposit testified that all of this sediment arrived 

within about a 2-hour period. Other deltas (fig. 40) 

are as much as about 700 feet wide and 10 to 12 feet 

thick at the downstream face. Lengths are always 

B 

FIGURE 40.-Delta that formed where drainage from Edes 

and Melton Hollows meets highway U.S. 29. A, Overall 

upstream view looking up Melton Hollow. Edes Hollow 

forks to the right at middle of picture. U.S. 29, a divided 

four-lane highway, in foreground. Dotted Iinework indicates boundary between grass and sand areas. (Photograph 

by Ed Roseberry.) B, Closeup aerial view of downstream 

part of delta. An inhabited house and two smaller buildings disappeared from this part of the delta during the 

flood. House in center of photograph is nearly half buried 

in sand. Construction crew at work unplugging culvert. 

(Photograph courtesy of Virginia Department of Highways.) 
DEPOSITIONAL FEATURES 55 

FIGURE 41.-Upstream view of delta deposited by Dillard 

Creek at highway U.S. 29. Flow is from upper right to 

lower left. One half of the four-lane highway is completely buried. (Photograph courtesy of Virginia Department of Highways.) 

greater than maximum widths and can be defined 

conservatively as about 1,000 feet for the examples 

shown in figures 40 and 41. 

The large delta that formed where Edes Hollow 

meets highway U. S. 29 (fig. 40) has an estimated 

cross-sectional area of 5,600 sq ft at the downstream 

face (about 700 feet wide and an average of 8 feet 

deep). If the volume is determined as approximately 

~1:! X area of downstream face X length, the delta 

contains an estimated 1.9 million cu ft of sediment. 

The drainage basin upstream is about 1.3 sq mi, of 

which about 0.9 sq mi experienced erosion that, from 

aerial photographic and field inspection, seemed as 

severe as that in Ginseng Hollow, Polly Wright Cove 

and Wills Cove. In the latter three basins the severe 

erosion contributed an average of about 4.9 million 

cu ft of sediment per square mile. If the avalanche 

and channel erosion in the Edes Hollow basin is 

similar to that in these three basins, Edes Hollow 

contributed about 4.4 million cu ft. The highway 

delta therefore contained an estimated 39 percent of 

the sediment eroded from the mountainous part of 

the basin. Of the remainder, some was deposited upstream and some was carried downstream from the 

delta. 

Because the Edes Hollow delta is only about a mile 

from the drainage divide, the flooding and deposition 

in the delta probably did not last for more than 7 or 

8 hours. A storm-average sediment-transport rate 

for the region of the delta can be estimated from the 

amount of sediment contained in the delta and the 

period of accumulation. Actually the postflood delta 

surface was filled to the lowest level of the highway 

(fig. 40B, upper left) and showed that some sediment 

definitely moved across the highway. Consequently, 

the volume of sediment left in the delta (about 1.9 

million cu ft) divided by the period of deposition will 

produce a minimum estimate of the average transport rate. If 1.9 million cu ft accumulated in 8 hours, 

the storm-average sediment-transport rate for the 

apex of the delta was about 66 cfs, or 6,600 lb per 

sec (285,000 tons per day). Field notes indicate that 

the water surface in the vicinity of the apex of the 

delta was about 200 feet wide. The approximate 

storm-average sediment-transport rate at the apex of 

the delta therefore was about 33 lbs per sec per foot 

of channel width. Instantaneous rates may well have 

been higher. 

The estimated average transport rate in any case 

represents just the delta region, rather than points 

upstream from the delta. The transport rate into the 

delta reflects the combined contributions of all upstream tributaries and debris avalanches, minus any 

deposition occurring upstream from the delta. 

The large delta that formed where Dillard Creek 

meets highway U. S. 29 (fig. 41) has a cross-sectional area at the highway of about 3,150 sq. ft. 

(about 350 feet wide by an average of 9 feet deep). 

It gradually merged with flood-plain deposits upstream. If 1,000 feet were considered the length, the 

delta contained about 1 million cu ft of sediment. 

The Dillard Creek basin has 3.3 sq. mi. of mountainous area, 1.6 sq. mi. of which is in Wills Cove, 

where the estimated yield was 4.9 million cu ft per 

sq mi. We hiked extensively over the Fortunes Cove 

area (the other part of the Dillard Creek basin) and 

found it to be eroded to a similar degree as Wills 

Cove. If the Wills Cove erosion yield of 4.9 million 

cu ft per sq mi applies also to Fortunes Cove, the 

Dillard Creek highway delta trapped an estimated 

22 percent of the sediment yield. This figure is somewhat less than the 39 percent obtained for the Edes 

Hollow delta because the Dillard delta is about 2 

miles farther downstream from the sediment source, 

and much of the eroded sediment was deposited on 

flood plains upstream from the delta. 

As the flood receded, the flow on the surface of 

highway deltas began meandering and often assumed 

a braided pattern (fig. 40B, and 41). Toward the end 

of the flood, the flow became very slow or even 
56 HURRICANE CAMILLE IN VIRGINIA, 1969 

stopped over much of the delta, as indicated by the 

nature of the deposits, described below. When workmen subsequently unplugged the culverts, the deposits in the central parts of the deltas were usually 

eroded considerably as the stream worked its way 

down to the culvert level. 

From the relative elevations of the highway and 

the floor of the delta, it appears that the water depth 

in which these deltaic deposits were formed ranged 

very approximately from 1 or 2 feet for the topmost 

sediments to 10 or 12 feet for those deposited earliest. The 10-12-foot estimate is a maximum depth; in 

all probability the water level rose as the sand laminae accumulated, so that the water depth during 

deposition was probably somewhat less than 10 feet. 

A 

FIGURE 42 Above and right.-Sedimentary features of the 

Dillard Creek highway delta. A, Sand laminae. Section 

shown here is about 5 feet thick. Units on measuring tape 

are feet and tenths of a foot. B, Sand laminae capped by 

cross-bedded layer of fine sand, with mud layer on top 

surface. This sequence probably reflects diminishing water 

flow over the delta. Flow direction uncertain. C, Ripples 

and mud cracks. 

SEDIMENTARY FEATURES 

The sand deposits which make up nearly all of the 

deltaic sediments have distinct layers, approximately 

horizontal, ranging in thickness from a fraction of 

an inch to about 1 inch (fig. 42A). These laminae are 

present from the base of the deposit to within a few 

inches of the top surface, and exist over the entire 

area of the delta. Single laminae cannot always be 

traced very far laterally, and they are often nearly 

\ 

I I . .._' • 

'. 

B 

c 
DEPOSITIONAL FEATURES 57 

inconspicuous in some areas and well defined in 

others. 

In addition to color differences, laminae in many 

cases differ from one another in grain size. For example, although most laminae are medium and fine 

sand, some are noticeably coarse in texture. Lenses 

as much as 6 inches thick, consisting of gravel or 

pebbles, are present but are not very common. Such 

lenses, where present, are usually at or near the 

bottom of the deposit. Sometimes rocks up to 6 or 8 

inches in diameter are found within a sand or gravel 

matrix. These larger particles can occur singly, on 

no common level, or they may occur in lenses which 

extend varying distances-five to several hundred 

feet in length. 

The sand laminae in the delta on Dillard Creek at 

U.S. 29 (fig. 41) are capped by a 3-6-inch layer of 

fine sand that shows well developed small-scale crossbedding (fig. 42B). These features are known to 

form only during hydraulically tranquil (subcritical) flow and slow sediment-transport rates. They 

probably are due to the braided streamflow that developed toward the end of the flood. Water depth at 

the time of deposition probably was on the order of 

one to three feet. Since the local flow direction in the 

braided network probably varied considerably from 

time to time, there is no assurance that a given exposure is cut parallel to the flow direction. The crossstratification was not preserved in the center of the 

delta, perhaps because such strata in this region 

were eroded when the culvert was cleaned. 

The top surface of most highway deltas was covered with a layer of mud (fig. 42C) that ranged in 

thickness from a fraction of an inch to about 1 foot. 

This mud layer in most cases is missing along the 

central part of the delta, so if originally present it 

probably was swept away when the culvert was unclogged. The mud probably was deposited by slowmoving or almost still waters, near the end of the 

flood. 

Ripples and mud cracks are common surface features of highway deltas. Figure 42C shows mud 

cracks in flat areas as well as superimposed on 

ripple marks. The ripples may or may not be symmetrical, and it is quite difficult to determine from 

them the current direction at the time of deposition. 

Wavelengths of these ripple marks are commonly 

about 0.25-0.5 foot, and heights are about 0.05 foot 

or less. 

In the days and weeks after the flood some settling 

of deltaic deposits, especially the small grains, occurred as water gradually escaped from the pore 

spaces. The final thickness of such deposits therefore 

is less, by some unknown factor, than the original 

thickness. 

VERTICAL VARIATIONS IN GRAIN -SIZE CHARACTERISTICS 

Sediment samples at selected vertical intervals 

were obtained at a total of four sites on the various 

deltaic deposits. Two locations were on the delta 

where Dillard Creek meets highway U. S. 29 (fig. 

41) ; a third site was on the Edes Hollow-U.S. 29 

highway delta (fig. 40) ; and the fourth was on a 

small delta (about 200 ft wide by 150 ft long by a 

maximum of 10 ft deep) in the headwaters of Hat 

Creek on State route 151. Samplings at the latter site 

and at one Dillard Creek site were on the front 

(downstream) face of the delta. The second location 

on the Dillard Creek delta was 200 feet upstream 

from the front face, about 0.2 of the way from the 

front face to the apex. On the Edes Hollow delta the 

sampling was about 50 feet upstream from the front 

face (about 0.05 of the distance from the front face 

to the apex). The sites were approximately on the 

center axis of the delta except for the spot 200 feet 

upstream on the Dillard delta. This site was about 

20 feet from the left-bank edge of the delta and was 

chosen because of the good vertical exposure. 

In all four vertical sections the deposited material 

generally became finer as the flood progressed, that 

is, the sediments became finer upward. Table 8 gives 

the pertinent size-frequency characteristics. Depths 

greater than 3.5 feet on State route 151 delta were 

not sampled, but field notes emphasize that the lower 

sediments were noticeably coarser than the overlying ones. At the Edes Hollow site the lower half of 

the deposit does not show a progressive coarsening 

with depth, but the upper half definitely does. Thus 

TABLE 8.-Vertical variations in grain-size characteristics in 

highway deltas 

Depth 

below 

Delta top 

surface dav 

(ft) (mm) So sk 

State route 151 --------------- 0.5-1.5 0.039 1.16 -0.11 

2.5-3.5 .43 2.09 -.46 

Edes Hollow ------------------ .5 .22 .55 -.12 

2.5 .36 .84 .01 

4.5 .60 97 -.13 

6.5 .48 1.05 -.24 

8.5 .40 1.01 -.32 

Dillard Creek-front face ---- Top surface .09 .58 .02 

.5 .11 .55 -.21 

2.5 .18 .68 -.38 

4.5 .19 .79 -.42 

6.5 .24 .93 -.50 

8.5 .30 .88 -.45 

Dillard Creek-200 

ft upstream -------------- Top surface .01 1.47 -.58 

.3 .07 59 -.06 

1.0 .12 .71 -.17 

2.0 .24 .84 -.32 
58 HURRICANE CAMILLE IN VIRGINIA, 1969 

during the depositional phase of the flood the coarsest material dropped out earliest, with finer and finer 

sizes being deposited as the flood progressed. The 

rate at which the average grain size decreases varies 

considerably from one sampling location to the next. 

The average grain size seems to depend primarily 

on how close the delta is to the sediment source 

(mountain hillslopes). The State route 151 delta was 

closest to the hills (about 14 mile), and this site had 

more gravels and pebbles than the other locations. 

The Edes Hollow delta is about 1j2-11;2 miles from 

the sediment sources and had smaller average grain 

sizes. Sediment in the Dillard delta probably had 

traveled 3-4 miles and thus was the smallest of the 

group. Changes in grain size with distance are discussed in more detail in the section on "Floodplain 

Deposits." 

The first sediments deposited had the widest range 

of grain sizes, and as deposition continued the sediments became better sorted (table 8) . The only exception to this general trend was the top mud layer 

at the site 200 feet upstream on the Dillard delta; 

because of the clay sizes in this sample, the size curve 

extends over a greater range. The mud layer at the 

face of the delta was washed away before it could 

be sampled, when highway crews unclogged the culvert. 

The actual values of sorting and the rate at which 

the sediments became better sorted upward seem to 

depend primarily on how close the delta is to the 

~ediment source (mountain hillslopes). The State 

route 151 delta was close to the hills (about 14 mile) 

and consequently received a wider range of particle 

sizes (more of the gravels and pebbles). The sediments in this delta are the most poorly sorted. Field 

notes suggest that sorting improves upward most 

rapidly at this site. At the Edes Hollow delta, which 

is somewhat farther from its sediment source, sorting is much better than at State route 151 and shows 

less overall improvement upward. The sediments in 

the Dillard delta had traveled the farthest, have the 

best sorting, and improve upward at the slowest rate. 

Skewness values are mostly negative, indicating a 

preponderance of the coarser grains over the fines 

(table 8). Sediments laid down first were the most 

skewed, and the size distributions generally became 

more and more symmetrical as the delta-building 

progressed. There are no noticeable trends regarding any effect of proximity of source material on 

rate of change of skewness. 

To summarize the trends in size-frequency distribution with time: as the flood progressed, the sediments transported to and deposited in highway deltas became increasingly finer in size, better sorted, 

and more symmetrical in weight-frequency distribution. Most of these trends also appeared, but often 

were less pronounced, in the depth variations of 

flood-plain deposits, as discussed below. 

FLOOD-PLAIN DEPOSITS 

Flood-plain deposits (figs. 43, 44) were laid down 

on the gently sloping ground immediately adjacent 

to the normal stream channel. As used in this report, 

the term "flood-plain deposits" excludes particles left 

in mountain channels and, where present, alluvial 

fans. Thus flood-plain deposits begin at or just below the mountain front at the head of the cove and 

continue, in some places intermittently, for many 

miles downstreamward. 

The volume and grain sizes of flood-plain deposits 

depend to some extent on local valley topography. 

Where the valley narrows, the mean flow velocity 

probably increased and less material was deposited. 

Conversely, more sediment was deposited in wider 

and flatter sections of a valley. In choosing the study 

areas we tried to avoid either of these extremes. 

Some reaches underwent both erosion and deposition. 

Very little flood-plain deposition occurred in valleys which had few debris avalanches in the headwaters. 

Flood-plain deposits are as much as 5 feet thick 

but generally are from about 0.2 foot to 3 feet. As 

one would expect, at any downstream location they 

tend to be thickest toward the center of the valley 

and thinner near the edges. Flood-plain widths range 

from a few feet to about 500 feet. The floods were 

nearly everywhere many times wider than the original channel. 

SEDIME~T AR Y FEATURES 

Flood-plain deposits within a mile or so of the 

headwaters and avalanche areas have no discernible 

sedimentary structures. Rather, they are quite similar to alluvial fan deposits in that all particle sizes, 

ranging from silt to boulders, seem to have been 

dumped all over the flood plain (figs. 43, 44A). Large 

rocks and gravel in flood-plain deposits usually did 

not accumulate in any distinctive way, such as in 

bars or splays. 

Farther downstream the deposits consist mostly 

of sand. From three sites studied in detail-one each 

on Rucker Run, Dillard Creek and Muddy Creekand from other general observations, it appears that 

such sandy flood-plain deposits in Nelson County 

have several characteristics. 

The laminations are a noticeable feature of nearly 

all sandy flood-plain deposits (fig. 45). These lami-
DEPOSITIONAL FEATURES 

FIGURE 43.-Upstream view of typical valley flood-plain deposits, headwaters of Davis Creek. Note damage to orchard. (Photograph courtesy of Virginia Division of Mineral Resources.) 

59 
60 HURRICANE CAMILLE IN VIRGINIA, 1969 

nae are generally about 0.01 foot thick or less. Sometimes they appear to extend over the whole floodplain width, but in other cases a single layer can be 

traced only for about 5 feet before it pinches out 

between other laminae. In addition to color differences, some laminae are distinguishable by a slightly 

coarser texture. Also, some layers are different in 

that they do not remain in the same plane as most of 

the others; instead, these errant laminae may deviate 

or dip by as much as 0.2 foot over a 2-foot horizontal 

distance. They seem to be isolated, however, and are 

rather rare. 

The laminated sand often contains an occasional 

1-2-inch pebble. 

B 

A 

The exposure near the center of the valley on 

Rucker Run (fig. 45) has conspicuous pockets or 

lenses of varying textures. The pockets usually include coarse sand and may have gravel and a few 

pebbles. Normal laminae, on the other hand, seemed 

to consist mainly of medium and fine sand. As figure 

45 shows, the top and bottom surfaces of the pockets 

are quite irregular and can rise or fall by as much as 

a foot or so over about a 5-foot horizontal distance in 

the approximate direction of flow. 

No crossbedding was observed at any of the three 

flood-plain sites studied in detail. 

Ripple marks are common surface features of the 

sandy flood-plain deposits and evidently formed as 

c 

FIGURE 44.-Common flood-plain deposits. A, Rucl~er Run about 0.3 mile downstream from head of valley. Circle around 

man shows scale. B, Rucker Run about 0.75 mile upstream from highway U.S. 29. C, Muddy and Davis Creeks about 0.2 

mile south of Woods Mill, looking upstream (Photograph courtesy of Virginia Department of Highways.) 
DEPOSITIONAL FEATURES 61 

FIGURE 45.-Sandy flood-plain deposits showing laminae, pockets, lenses and coarse particles 

(Rucker Run about 0.75 mi upstream from highway U.S. 29). Tape is in feet and tenths, 

and exposed thicknesses are about 2.5-3.0 feet. 
62 HURRICANE CAMILLE IN VIRGINIA, 1969 

the water receded. However, they tend to occur in 

patches rather than covering the entire deposit, so 

they may or may not be present at any given spot. 

The ripple heights are rather low-about 0.01 or 

0.02 foot. Wavelengths are only about 0.3 foot in 

most cases, the range being from about 0.25 to 0.5 

foot. Crests are oriented approximately normal to 

the valley axis. 

Mud cracks or shrinkage cracks also occur in 

patches on the surfaces of flood plains. The thickness 

of this mud layer can range from about 1 inch to a 

foot or more. The greater thicknesses tend to be 

found at longer distances downstream on the larger 

streams, for example, at the confluence of two major 

rivers or upstream from railroad embankments. 

Such deposits probably suggest considerable backwater during the flood. 

The various sedimentary features listed above 

seem to be similar to those of floods in other geomorphic regions (Jahns, 1947; Williams 1970, 1971; 

McKee and others, 1967). 

VERTICAL CHANGES IN GRAIN SIZE 

Grain-size distributions conceivably could vary 

with depth at a single location, lateral distance toward the valley wall, and distance downstream. Size 

variations in depth at a single location could not be 

studied as far as large partiCles were concerned 

because one large rock fragment in many cases occupied the full thickness of the deposit. For finer grains 

two flood-plain localities were examined in regard to 

particle size-frequency characteristics with depth. 

This of course reflects the sediment deposition with 

time during the flood, at the sampling site. The localities, chosen mostly on the basis of accessibility 

and undisturbed vertical sections, were on Rucker 

Run, about 0.75 mile upstream from highway U. S. 

29, and on Dillard Creek, about 1.3 miles downstream 

from highway U.S. 29. At the location of sampling, 

the deposit on Rucker Run is about 2 feet thick, that 

on Dillard Creek about 3 feet thick. Both locations 

are about the same distance downstream ( 4-4.5 mi) 

from the deposits at the heads of the valleys. A possibly important difference is that upstream from the 

sampling site Dillard Creek had a large highway 

delta, whereas Rucker Run hao1 only a small or partial delta. The Dillard Creek highway delta trapped 

a very large amount of sediment, and this probably 

occurred before deposition at the sampling site farther downstream. Thus the grains at the Dillard 

locality may represent material transported later in 

the flood. 

There is, unfortunately, no reliable way to determine the time of deposition relative to the peak 

water flow, nor is there any way of knowing the 

rates of sediment transport and deposition at various stages of the flood. 

Nearly all the flood-plain sediments investigated 

for vertical changes in the grain size are sand-sized 

or finer. At both the Dillard and Rucker sites the top 

surface material was noticeably finer than that below the surface (figs. 46, 47). These fine-grained topmost sediments very likely were deposited in relatively slow-moving water near the end of the flood. 

Below the top surface the deposits at the Dillard 

site are progressively coarser with depth. The average diameter at the base of the deposit was 0.37 mm, 

and this average size gradually decreases to 0.11 mm 

at the top surface. The sediments at this site also 

became better sorted upward (fig. 46). The d9o-d1o 

range encompasses one log cycle of grain diameter 

at the base of the deposit, and this range decreases 

to about two-thirds of a log cycle for the sediments 

deposited latest. The skewness shows no noticeable 

trend. The bottom and top deposits have approximately symmetrical distributions. The middle layers 

are also close to having symmetrical grain-size distribution but show a very slight tendency to include 

a greater weight-percent of coarse grains than fine. 

With the exception of the surface grains, the 

Rucker Run sediments, in contrast to those on Dillard, show no noticeable vertical trends. The average 

grain size of the Rucker Run sediments fluctuates 

between 0.15 and 0.45 mm throughout most of the 

deposit and becomes finer only in the upper part. 

Sorting stays about constant with depth. A possible 

exception is the top surface, which is slightly more 

poorly sorted than the sediments below. A more 

poorly sorted surface material would represent the 

opposite trend from the Dillard Creek site, where the 

topmost material was better sorted than the underlying sediments. Skewness values at Rucker Run, as 

with average size and sorting, did not vary widely 

for the deposits below the surface. The skewness 

range is about -0.13 to -0.24, indicating a very 

slight preponderance of coarser sizes. The surface 

material, on the other hand, has a skewness of 

+0.04, which means the size distribution is approximately symmetrical. 

To sum up the main grain-size changes of these 

flood-plain deposits with depth: at both the Dillard 

and Rucker sites the top-surface (top inch or two) 

grains are noticeably finer than the material below 

the surface. The Dillard sediments become progressively finer upward (that is, they become finer as 

time progressed during the flood). Grain sizes at 

the Rucker site, on the other hand, do not change 
100 

w 90 N 

en 

0 

w 

1- 80 <{ 

(.) 

0 

~ 70 

z 

<{ 

I 

1- 60 

a:: 

w 

z 

u:: 50 en 

w 

_J 

(.) 

i= 40 a:: 

<{ 

a.. 

LL.. 30 0 

w 

(!) 

<{ 

1- 20 z 

w 

(.) 

a:: 

w 10 a.. 

0 

0.001 0.01 

DEPOSITIONAL FEATURES 

3 feet below top surface (base of deposit) 

Grain-size characteristics 

Depth below 

top surface 

(ft) 

dav 

(mm) 

sk 

---------- ------ ----- --------

Top surface 

1 

2 

3 

0.1 1.0 

PARTICLE DIAMETER, IN MILLIMETERS 

0.11 

.15 

.20 

.37 

0.64 

.67 

.81 

1.03 

10.0 

-0.03 

.20 

.24 

- .04 

63 

100 

FIGURE 46.-Size-frequency characteristics of flood-plain sediments on Dillard Creek, about 4.5 miles downstream from 

headwaters. 

w 

N 

en 

0 1.4 feet below top surface 

w 80 ,, 1-

<{ 0;9 foot below top surface (.) 

0 I ~ 1 0.4 foot below top surface 

z 

<{ 

I ~1.9 feet below top surface 1-

a:: 

w 

z 

u:: 50 en 

w 

_J Grain-size characteristics (.) 

i= Depth below a:: dav <{ So top surface (mm) sk a.. 

LL.. (ft) 

0 --------- w Top surface 0.088 0.98 0.04 (!) 

<{ 0.4 .23 .75 -.15 1- .9 .45 .75 -.13 z 

w 1.4 .17 .74 -.23 (.) 

a:: 1.9 .22 .90 -.24 w 

a.. 10 2.3 .15 .79 -.17 

0 

0.001 0.01 0.1 1.0 100 

PARTICLE DIAMETER, IN MILLIMETERS 

FIGURE 47.-Size-frequency characteristics of flood-plain sediments on Rucker Run, about 4 miles downstream from headwaters. 
64 HURRICANE CAMILLE IN VIRGINIA, 1969 

significantly below the surface. Sorting differences 

at the Dillard site indicate that as the flood progressed, the deposits became better sorted. At Rucker 

Run, however, the sediments deposited last were a 

bit more poorly sorted than those laid down earlier. 

At both sites the distributions show a limited tendency to be skewed (minor predominance of coarser 

grains), except for the surface material which is 

distributed approximately symmetrically, on a 

weight basis. 

The measured trends of grain-size distribution 

with depth are probably a product of both the sizes 

of particles offered to the floodwaters upstream from 

the study site~ and the transporting ability of the 

stream. Thus, the deposition of finer and finer sizes 

as the flood progressed (Dillard locality) could mean 

that the larger particles were less frequently eroded 

from the upstream hillsides and channels with time 

and (or) the transporting ability of the flowing 

water gradually diminished over the period of deposition. Similarly, various possible combinations of 

particle availability and stream power could account 

for an absence of a distinct size-distribution trend 

during deposition at the Rucker Run site. 

LATERAL VARIATIONS IN GRAIN SIZE 

For flood-plain deposits that include large ranges 

in particle size the pebble-count data generally show 

an apparently random distribution across the width 

of the deposit. Sand-sized and finer grains in these 

deposits tend either to form the matrix for the whole 

deposit or to collect in scattered patches. Some of the 

fringe areas along the outer edges of the deposits 

tend to have more sand than larger stones. On the 

other hand, stones up to 1 meter diameter occur in a 

few places where no finer materials are deposited. 

The sand deposit described above on Rucker Run, 

about 0.75 mile upstream from highway U. S. 29, 

was sampled to determine any lateral changes in 

particle size of fine-grained deposits. The study area 

was the right bank flood-plain deposit, which was 

about 60 feet wide. Single laminae or strata were 

either too difficult to trace laterally or too thin to 

sample individually. Composite vertical samples representing the entire thickness of the deposit (about 

2 feet) were therefore taken at each of five sites15, 25, 35, 45, and 55 feet, respectively, away from 

the present stream bank. The cumulative size-frequency curves for these five samples very nearly 

coincide. For a composite curve derived from all five 

sieve analyses dav=0.23 mm, 80 =0.942 and sk= 

-0.153. Comparing the individual curves to the average curve, the maximum deviations were+ 13 percent for dav ( ± 0.03 mm), ± 8 percent for So and 

± 34 percent for sk. The relatively minor differences 

between curves could be due to actual differences in 

the grain-size frequencies at the sampling sites, sampling errors, or splitting of samples for sieve analysis. Thus for the flood-plain deposits sampled there 

was no significant lateral variation in grain-size 

characteristics. 

DOWNSTREAM CHANGES IN FLOOD-PLAIN DEPOSITSGENERAL 

Flood-plain deposits of three streams-Polly 

Wright, Rucker Run and Dillard Creek-were chosen to study such features as changes in amounts of 

deposition and in grain size with distance downstream. From one viewpoint the ideal stream for 

studying such changes would have no tributaries at 

· all, and all of the deposited sediment would come 

from a single upstream source. Unfortunately, Nelson County appears to have no such stream, particularly since some deposited material undoubtedly 

came from the bed and banks along the whole length 

of the creeks. 

The stream in Polly Wright Cove received sediment from three major tributaries near the middle 

of the study reach. Rucker Run received sediment 

from at least 13 debris avalanches or minor tributaries within the upstream 8 percent of the total distance studied. Toward the downstream end, five 

creeks join Rucker Run between the last two sampling stations; however, of this group only Dillard 

Creek contributed much sediment. Some minor 

creeks also contributed sediment along other parts 

of the Rucker study reach. Three tributaries joined 

Dillard Creek, at about 0.2, 0.5 and 0.8 of the way, 

respectively, along the 6.6-mile study zone. 

The streams were less than ideal for sedimentdispersion studies, also from the standpoint of time 

of introduction of sediment. The exact times when 

sediments emerged from the mountain ravine are 

unknown. Sediment eroded from the banks and beds 

of the mountain channels and higher order streams 

probably was transported more or less continuously 

during the flood, at a rate proportional to the stream 

power. However, the large quantities dislodged by 

the debris avalanches on the hillslopes entered the 

channel at various times during the storm. Therefore 

the resulting downstream deposits in small drainage 

basins would reflect several stages and rates of sediment transport and deposition. Transport and deposition would logically be more uniform with increase 

in upstream drainage area. 

The various types of source rocks on the hillsides 

contain no distinctive and plentiful minerals which 
DEPOSITIONAL FEATURES 65 

would identify points of origin for particles in the 

downstream deposits. 

In spite of these possible disadvantages from the 

viewpoint of ideal dispersion studies, the deposits 

that remained after the flood subsided are probably 

typical of those left by other catastrophic floods in 

similar environments. Lag deposits and material in 

the stream channel prior to the flood were flushed 

out or buried during the flood ; so the sediments 

sampled represent only material deposited by the 

August 19-20 flow. 

Tbe upstream sampling stations were usually chosen at approximately equal intervals of distance, although sometimes the location was shifted slightly 

to sample a deposit that seemed to be more 

typical or that was not buried under piles of trees 

and other debris. For at least the downstream half 

of tbe flood-plain deposits investigated, the sampling 

stations were selected on the basis of accessibility, 

freedom from tributaries entering near the station, 

and a subjective evaluation of how typical or representative the deposits appeared to be. Some downstre.am reaches had little or no deposition, often due 

to a local constriction in the channel. 

Tbe valley in Polly Wright Cove is only 0.8 mile 

long. After emerging from Polly Wright Cove, the 

stream merges with another sediment-bearing 

stream (Muddy Creek). In reporting changes in 

grain size with distance downstream, we will include 

the mountain-channel deposits in Polly Wright Cove. 

These deposits were continuous with the downstream 

flood-plain deposits. 

Within a week or two after the flood, the streams 

had returned to their usual flow widths and depths, 

these commonly being 1-20 feet wide and 0.1-0.6 foot 

deep. These creek widths were negligible or minor 

compared with the widths of the flood deposits. 

Nearly all the sampling was done on dry ground. 

DOWNSTREAM CHANGES IN AMOUNT OF DEPOSITION 

POLLY WRIGHT COVE 

At 11 stations along Polly Wright Cove measurements were made of the volume of the flood-plain 

deposits, using the same techniques as on alluvial 

fans. These data, expressed in volume of sediment 

per unit (foot) of channel length, are shown in figure 

48A. The downstream distance, x, was reckoned 

from the first major deposits, excluding debris piles 

and other scattered deposits in mountain ravines. 

The volume of sediment decreases downstreamward, 

although the scatter on the graph hinders determination of the precise relation. In an effort to reduce the 

scatter, we combined the data for the 11 individual 

:::c 

1-

(.!) 

z 

LLJ 

....J 

....J 

LLJ 

z 

z 

<( 

:::c 

(.) 

u.. 

0 

1-

0 

0 

u.. 

0::: 

LLJ 

a.. 

1-

LLJ 

LLJ 

u.. 

(.) 

co 

:::> 

(.) 

~ 

.,_: 

en 

0 

a.. 

LLJ 

Cl 

u.. 

0 

LLJ 

2 

:::> 

....J 

0 

> 

A. Individual stations 

1000 

500 

+ 

+ 

-~-

+ -------.:t... + 

+ -

100 + --- 50 

B. Averaged data 

500 

100 

0 5000 

DISTANCE DOWNSTREAM FROM START OF DEPOSITS, IN FEET 

FIGURE 48.-Volume o.f flood-plain deposition per foot of 

channel length along Polly Wright Cove. 

sections into four averages, consisting of the upstream three measuring stations, the second three, 

the third two and the downstream three. With the 

data combined into reaches about 1,000 feet long, the 

scatter diminished considerably. Figure 48B, a plot 

of these results, shows that the volume of deposition 

decreases exponentially with distance downstream. 

According to the eye-drawn line, the volume of deposition per foot of channel length at any downstream 

site is equal to 370e-0 •

00025x, where e is the base of 

natural logarithms. The deviations of data points 

from the regression line range from + 15 percent to 

-12 percent. 

The actual quantity deposited ranged from about 

370 cu ft per ft at the upstream end to about 120 

cu ft per ft at the downstream end of the 0.8 milelong deposit. 

The total volume of sediment dumped in Polly 

Wright Cove was about 0.97 million cu ft. An estimated 3.1 million cu ft was eroded from the headwaters (table 6). Sediment not accounted for in the 

flood-plain deposits must have been transported completely out of the cove. Extrapolating the exponential equation just derived· to a distance beyond the 
66 HURRICANE CAMILLE IN VIRGINIA, 19'69 

cove is both risky and intriguing. If the equation 

were applicable for as far as appreciable deposition 

continued, the total amount deposited would be 370/ 

0.00025, or about 1.48 million cu ft, somewhat less 

than the 3.1 million cu ft estimated to be eroded. 

Nearly all of the 1.48 million cu ft, according to the 

equation, would be deposited within three miles of 

the upper end of the deposits. If the measurements 

and the resulting equation for the 0.8-mile-long 

study reach are accurate, either the equation should 

not be extrapolated or the estimated 3.1 million cu 

ft of erosion is too high. 

Table 9 gives the volume of deposition by particlesize categories and the size distribution by categories, for the four separate reaches along the 0.8-milelong cove. If the entire flood-plain deposit along the 

length of the cove is considered, sand is by far the 

most abundant size class. The cove received more 

than twice as much sand'- (449,000 cu ft) as any 

other size group. The other outstanding feature is 

the scarcity of silt and day, this group contributing 

only about 51,000 cu ft, or 5 percent of the deposition. The silt and clay, if eroded in substantial 

amounts, could have been carried away as suspended 

load. Most particles larger than sand, if eroded from 

the headwater regions, probably would have come to 

rest somewhere within the cove rather than moving 

through the cove to some downstream site; the volumes and percentages of these larger particlesgravel, cobbles and boulders-dwindle noticeably 

with distance along the cove, as table 9 shows. It 

therefore appears that a much greater volume of 

sand was eroded in the headwater regions, as compared to larger particles. 

In the reach farthest upstream, sand (31 percent 

of the deposit at this site, or 90,000 cu ft) and boulders (29 percent or 85,000 cu ft) are the most abundant size classes. Boulders, if eroded from the headwater regions in appreciable quantities, should be 

abundant in this first reach because they would logically be deposited farther upstream than smaller 

particles. In the next reach (about one-third of the 

way along the cove's flood-plain deposit) sand and 

gravel are the most abundant groups, especially 

sand. The volume and percentage of cobbles and 

boulders are less than in the upstream reach, and 

the amount of silt-clay is about the same. Sand becomes even more plentiful in the third reach and 

amounts to 138,000 cu ft or 60 percent of the deposit. 

The quantity and percentage of silt-clay are about 

the same as in the two upstream reaches. Gravel is 

still more plentiful than cobbles and boulders, but 

these three categories, especially the latter two, are 

scarcer than in the previous reach. Finally, in the 

reach farthest downstream, sand comprises nearly 

three-fourths (111,000 cu ft) of the total deposit. 

Gravel is next in importance (18 percent), while siltclay, cobbles and boulders total only 10 percent 

(16,000 cu ft) of the deposit. 

In summary, the volumes and percentages of cobbles and boulders decrease steadily with distance 

downstream and are practically negligible at the 

end of the 0.8-mile study zone. Gravel everywhere 

makes up about 16-25 percent of the sediment and 

is more abudant about one-third of the way along 

the cove than elsewhere. Sand consistently becomes 

more important with distance, progressing from 31 

percent to 72 percent of the deposit. Curiously, the 

percentage of silt-clay remains virtually constant at 

about 5 percent along the entire cove. 

Later sections of this report examine other aspects 

of grain-size trends with distance. 

RUCKER RUN 

Figure 49 shows how the volume of flood-plain 

deposition varied with distance for the upstream 

12.5 miles of Rucker Run. The somewhat erratic 

nature of the deposit volume for the upper two or 

three miles is due mainly to the addition of sediment 

from small tributaries. The point labeled "partial 

highway delta at U.S. 29" illustrates the amount of 

sediment trapped by the highway. More sediment 

probably would have accumulated here, that is, a 

complete highway delta would have formed, if a 

section of the highway had not washed out during 

the flood. The volume of deposition at any given site 

down to highway U.S. 29, about the first 4 miles, is 

TABLE 9.-Volume and perc.entage of deposition by size classes with distance downstream in Polly Wright Cove 

Volume 

Proportional Length of deposi- Volume by size classes (1,000 ft3) Percentages .in class 

distance down- of tion In stream to mid- reach reach Silt- Siltpoint of reach (ft) (1,000 ft3) cla.y Sand Gravel Gobble Boulder clay Sand Gravel Gobble Boulder 

0.08 ------------------- 875 292 15 90 47 55 85 5 31 16 19 29 

.32 ------------------- 1,233 288 14 110 72 40 52 5 38 25 14 18 .59 ------------------- 998 231 14 138 46 12 21 6 60 20 5 9 .89 ------------------- 1,254 155 8 111 28 5 3 5 72 18 3 2 

Total ------------ 4,360 966 51 449 193 112 161 
DEPOSITIONAL FEATURES 67 

c:: 

w 

a... 

tw 

w 

u....I 

S::?0 

~t:i 100 ()....J 

~L;j _z 

zz 0<( Eo 

(f) 

Ou.... 

a...o 

w 

Cltu....o 

0~ 

w 

~ 

::::::> 

....J 

0 

> 

/Partial highway delta at U.S. 29 

----)(~ -x--x ~ 

Confluence with Dillard Creek 

X 

1~--~----~--~----~----L---~--~ 

0 10 20 30 40 50 60 70 

DISTANCE DOWNSTREAM FROM START OF DEPOSITS, 

IN THOUSANDS OF FEET 

FIGURE 49.-Changes in volume of flood-plain deposition 

with distance downstream on Rucker Run. 

in the approximate range of 150-300 cu ft per foot 

of channel length. 

Downstream from the partial trap at the highway 

the amount of deposition dwindled rapidly. At the 

confluence with Dillard Creek (approximately the 

7-mile station) the volume of deposition was about 

90 cu ft, and at 12.5 miles downstream from the 

head of the cove the flood plain contained only 1 or 

2 cu ft of sediment, a very thin layer. 

The first 7 miles of the Rucker Run flood plain 

received a total of about 7.8 million cu ft of sediment, according to figure 49. 

Some of the silt and clay, and possibly sand too, 

moved in suspension into the James River. Towns 

and flood plains along the James, both upstream and 

downstream from the junctions with tributaries 

draining Nelson County, were covered with finegrained sediment to depths up to a foot. The source 

of this material could be any place from the upper 

reaches of avalanche scars in the headwaters to the 

banks of the James River itself. Downstream from 

Richmond the greatly increased concentration of 

suspended load in the James River estuary reached 

a maximum around Aug. 25, about 3-4 days after 

the peak water discharge at Richmond (Maynard 

Nichols, written commun., 1969). 

One effect of the flood was to erode the mountain 

hillslopes and aggrade the upstream parts of the 

valleys. If rare catastrophic events produce most of 

the erosion and deposition over a long period, the 

results of this storm suggest a trend toward a peneplain, due to net hillside erosion and valley deposition. On the other hand, the storm may be a temporary deviation in a long-term trend which may in 

time be established by more moderate erosional and 

depositional processes. 

Again, if this type of catastrophic event is dominant, the Wills, Polly Wright and Freshwater Cove 

flood plains, as well as certain others, are being constructed primarily by overbank deposition. They 

would thus represent an exception to the mechanisms 

which Wolman and Leopold (1957) proposed, whereby about 80-90 percent of a flood-plain thickness is 

build by point-bar (lateral) accretion within a migrating stream channel. In the present case the crucial factor in determining the amount of overbank 

deposition seems to be the quantity of sediment introduced at the head of the valley. Where the mountains underwent extensive debris avalanching, the 

heads of the valleys received considerable overbank 

deposition. This vertical accretion ranged from an 

average of about 3 feet at the upstream ends of the 

coves to 0.1 foot or less at a downstream distance of 

1-8 miles. On the other hand, where few debrisavalanches occurred in the mountains, as in the Indian Creek basin, the downstream-valley flood plains 

received noticeably less sediment and may in fact 

have undergone a net sediment loss due to lateral 

erosion of the stream channel. 

DOWNSTREAM CHANGES IN AVERAGE GRAIN SIZE 

Most authors have found that average grain size 

generally decreases downstreamward (Pettijohn, 

1957), although exceptions have been reported 

(McPherson and Rannie, 1969). 

Figure 50 is a graph of the mean particle size dav 

versus distance down-valley for Rucker Run, Polly 

Polly Wright Cove and Dillard Creek. as before, x 

is distance downstream from start of deposition. In 

general, dav decreased with distance downstream. The 

distribution of points on at least two of the three 

plots is such that various lines can be fitted. For 

example, either the solid or dashed lines can represent the relation for the Polly Wright Cove and 

Dillard Creek sediments. The solid line for all three 

streams suggests that given enough travel distance, 

dav may become asymptotic toward some limiting 

minimum value. The Rucker Run data are for a relatively long study reach (12.5 mi) and tend to support this type of relation. The dashed lines for 

Polly Wright Cove and Dillard Creek represent negative exponential functions, a type of relation rather 

common in geomorphology and sedimentology. 

The average particle size at the upper end of the 

deposits was 1.6 mm for Rucker Run, 14.4 mm in 
68 HURRIC'ANE CAMILL.E IN VIRGINIA, 1969 

10.0 

RUCKER RUN 

1.0 

en 

0::: 0.1 LLI 

ILLI 

:2: 

::::i 

....1 0 40 50 60 70 

~ 

~ 100.0 I I I I ,...:; 

1 K POLLy WRIGHT COVE 

LLI 10.0 - N ~-/(dav= 14.4e-0.00071X) U5 

<( 

z 

0::: 1.0 r- '+~ - (.!} 

+ - LLI 

(.!} 

<( 

0::: 0.1 I I l l LLI 

> 0 2 3 4 5 <( 

1.0 

0.1 ~ 

0 20 24 

DOWNSTREAM DISTANCE (x), IN THOUSANDS OF FEET 

FIGURE 50.-Downstream changes in mean particle size 

(dav) of flood-plain deposits. 

Polly Wright Cove and 0.64 mm (extrapolated) for 

Dillard Creek. This value depends on at least two 

factors: the sizes of particles dislodged from the 

hillsides and the sizes and numbers of particles 

transported to the flood plains. Lithology and erosional history undoubtedly influence the sizes of the 

rocks dislodged from hillsides. Also, many (but not 

all) large rocks were retained in mountain ravines 

or alluvial fans and therefore were not included in 

the flood plain analysis. This, however, was not the 

case in Polly Wright Cove, where the deposits were 

virtually continuous over the entire study reach and 

where no alluvial fan formed. The relatively large 

value of dav (14.4 mm) at X=O ft in Polly Wright 

Cove reflects this condition. 

Why does dav on Polly Wright decrease nearly 10 

times as fast as on Rucker and Dillard? The answer 

seems to lie in the sizes of particles introduced at 

the head of the study reach and in the effect of 

several side tributaries which supplied more large 

rocks downstream. The upstream Polly Wright deposits were closer to the steep mountain hillslopes 

which provided the debris. This fact, plus the absence of an alluvial fan, means that the upstream 

deposits in Polly Wright Cove contained a greater 

proportion of large boulders, thus contributing to a 

larger average particle size. As mentioned elsewhere, 

large rocks were not moved very far from the mountainous areas during the flood (usually considerably 

less than a mile). Thus the average particle size in 

Polly Wright Cove decreased rather rapidly with 

distance. On Rucker Run many slides and tributaries 

furnished large rocks to the flood-plain deposits 

along the study reach, so dav did not decrease as 

rapidly as if large rocks had been introduced at a 

single point source at the head of the reach. In the 

Dillard .creek basin some deposition in mountain 

ravines and in a fan trapped most of the large rocks 

before deposition of the flood-plain sediments. Hence 

the possible range of variation in dav along the Dillard reach was much more restricted. 

A decrease in grain size can result from selective 

deposition, breakage of particles, and abrasion or 

wearing down of the grain surface. Any of these 

can be influenced by such factors as flow properties 

(discharge, mean velocity, depth, width), channel 

properties (gradient, roughness, sinuosity), particle characteristics (size, shape, specific gravity, 

mineralogy) , and erosional history or degree of 

weathering. For two streams in the Black Hills area 

of South Dakota, Plumley (1948) concluded that 

selective deposition accounted for 75 percent and 84 

percent of observed grain-size reductions, respectively, with abrasion accounting for the remaining 

small percentage. Scott and Gravlee (1968) reported 

that for a flood surge on the Rubicon River in California sele·ctive deposition caused more than 90 percent of the measured size decline and that breakage 

was not significant in the overall size reduction. 

Bradley, Fahnestock, and Rowekamp (1972) concluded that sorting processes caused 87 percent of 

an observed reduction in the size of flood-transported 

gravel along a 16-mile reach of the Knik River, 

Alaska. The remaining small percentage in size reduction was attributed to abrasion. 

Preflood weathering which could promote breaking during transport could be a minor factor with 

the present sediments ; however, on the basis of 

field observations, most :rarticles, at least those 

visible on the surface and in cuts eroded through 

the fresh deposits, seemed to be relatively fresh. 

Breakage, however, could have occurred during 

transport, especially with the larger rocks. Aside 

from the possibility that some of the measured size 

decline could be due to breakage, there is no firm 

evidence to evaluate the extent to which breakage 

during transport was important. Intuitively it would 

seem to be a minor factor. Abrasion or wearing 

down of the particle surface probably could not have 

been significant, because the rock (igneous and 
DEPOSITIONAL FEATURES 69 

metamorphic) generally was hard and fresh, and the 

distance of travel was quite short, ranging from a 

few feet to about 12.5 miles for the data obtained 

here. Thus the most likely reason for the progressive decrease in size is selective deposition, with 

breakage possibly affecting the trend to a minor 

extent. 

AVERAGE PARTICLE SIZE, LOCAL SLOPE, AND 

DRAINAGE AREA 

What factors might be most influential in determining the particle sizes that are left at a given 

site? The two which appear most reasonable are 

the local gradient and water discharge. For Rucker 

Run and Dillard Creek local gradients were measured from topographic maps (1: 24,000). These 

slopes equal the vertical distance on the maps between the contour line upstream· from the sampling 

site and that downstream from the site, divided by 

the horizontal distance along the channel between 

the two contour lines. All slopes along Polly Wright 

Creek were measured in the field with a Zeiss level 

and stadia rod. 

Average particle size shows some correlation with 

local flood-plain slope S (fig. 51). The line fitted by 

least-squares in figure 51 has the relation dav= 

2,270 81.98 • Thus the average particle size decreased 

as the slope became flatter. The deviations of data 

points from the regression line range from +600 

percent to -50 percent. 

Hack (1957, p. 57) studied Virginia and Maryland streams near this area and included a much 

greater variety of geologic environments and drainage-basin sizes. He found that with decrease in 

slope the size of the bed material could increase, 

decrease. or remain constant, depending on the surface lithology of the drainage basin. Also, the sediments Hack studied had been extensively reworked 

by normal streamflows. (In fact, he felt that the 

sizes of bed-material particles partly determined 

the local slope, whereas the reverse probably was 

true for the Hurricane Camille deposition.) Plumley 

(1948) found power relations between median grain 

size and local slope for ancient stream sediments 

along three streams in the Black Hills of South 

Dakota. Plumley's three exponents, with slope the 

independent variable, were 0.53, 0.53 and 1.25, and 

dav decreased at a lesser rate as local slope decreased, 

as compared with relations in Nelson County. 

Water discharge, as mentioned, was not measured. 

However, the drainage area usually shows a power 

relation to water discharge. Consequently, drainage 

area, as measured from topographic maps (1 :24,000) 

(/) 

a:: 

LLJ 

ILLJ 

::::?; 

::J 

_J 

~ 

~ 

~ 

~ 

LLJ 

N 

en 

w 

_J 

u 

i= 

a:: 

<( 

a_ 

w 

(? 

<( 

a:: 

w 

> <( 

100 

+ Polly Wright Cove :I "' Dillard Creek 

o Rucker Run 

-f 

0 

0.1 

Upstream ~ 

LOCAL SLOPE (S), IN FEET PER FOOT 

FIGURE 51.-Change in average particle size with local 

·flood-plain slope. The two points at steepest slopes for 

Dillard Creek are alluvial fan sediments. 

with a planimeter, was plotted against average grain 

size. The drainage area for any sampling site includes the drainage area of all upstream tributaries 

which enter the major stream. 

Figure 52 indicates that with increase in drainage 

area Ad, the average particle size decreased according to the least squares relation dav=1.9Ad-1 ·

34 • This 

equation pertains to all of the plotted points as a 

group. Deviations of measured data from the regression line range from +300 percent to -26 percent. 

Individual drainage basins may not follow the general equation. In Polly Wright Cove, for example, the 

average grain size decreases with the -2.8 power 

(approximately) of drainage area, as suggested by 

the dashed line in the diagram. 

Since average particle size increases with the 1.98 

power of local slope S and decreases with the 1.34 

power of drainage area Ad, these relations can be rearranged and combined into the form S oc (dav0

•

25 ) I 

Ad0 ·

34 ). Writing the relation this way implies that 

slope is the dependent variable, probably not the case 
70 HURRICANE CAMILL.E IN VIRGINIA, 1969 

100.0 

(f) 

0::: 

LLI 

1-

LLI 

~ 

::::i 

...J 

~ 

~ 

~ 

::i 

LLI 

N 

(i5 

LLI 

...J 

(.) 

i= 

0::: 

<( 

Q_ 

LLI 

'-' <( 

0::: 

LLI 

> 

<( 

... 

.. 

1.0 

+ Polly Wright Cove 

-. Dillard Creek 

o Rucker Run 

DRAINAGE AREA (Ad), IN SQUARE MILES 

FIGURE 52.-Variation of average particle size with drainage area. Dillard Creek points include two alluvial fan 

stations (coarsest values of dav) and do not include 

highway delta deposits. 

in the present study, but the equation in this form 

can be compared to Hack's (1957, p. 58) relation 

S oc (dav0

•

6 ) I (Ad0 ·

6 ). The differences in the exponents 

are not very big, considering the amount of scatter 

on the plots in both investigations. Hack's equation 

applies to streams in various rock types. 

It will be interesting to see if future research 

finds empirical relations of the sort discussed here 

to be applicable to other physiographic regions. 

Denny {1965) studied desert washes in California 

and Nevada and could not find any correlation at all 

between local slope, median particle size, and drainage area. 

DOWNSTREAM VARIATION IN SIZE OF LARGEST 

STONES 

The largest stones at a site, if all sizes were available for transport, provide some indication of the 

particle sizes the flow was able to move. Figure 53 is 

a plot of the average size (intermediate diameter) 

6 

L___j______~----:----------j 2 4 6 12 

DOWNSTREAM DISTANCE (x), IN THOUSANDS OF FEET 

FIGURE 53.-Downstream chal).ges in average intermediate 

diameter of the five largest stones at each station. Lines 

fitted by least squares. 

of the five largest stones (dts) as a function of distance downstream for Polly Wright and Dillard 

Creeks. The Dillard Creek data include the Wills 

partial fan at the head of the cove (five data points), 

and the downstream distances begin at the fan apex 

for this particular diagram. Only three pebble 

counts-a number insufficient for plotting-were 

taken on Rucker Run. 

A negative exponential function, fitted here by 

least squares, describes the data reasonably well. For 

Polly Wright d15=1,420e-0 •

00024111, with maximum deviations ranging from +135 percent to -75 percent 

of the regression value. For Dillard Creek d1s= 

1,330e-0 •

00013111, and maximum deviations range from 

+145 percent to -70 percent of the regression value. 

The Polly Wright exponent of -0.00024 means that 

for every 1,000 feet of distance the geometric mean 

of the largest stones decreases by 24 percent. Similarly, along Dillard Creek the size at each 1,000-foot 

station was about 13 percent less than that at the 

previous 1,000-foot station. The exponents show that 

the size of the largest stones decreases nearly twice 

as rapidly along Polly Wright Cove as on Dillard 

Creek. (Among other differences, the flood-plain 

slope decreases more rapidly in Polly Wright Cove, 

too.) 

As a general trend the largest rock fragments 

along Polly Wright Cove decreased from 142 em in 

the deposits farthest upstream to about 50 em after 

0.8 mile of travel. In Dillard Creek the average diameter of the five largest stones was 133 em at the up-
DEPOSITIONAL FEATURES 71 

stream end of the creek and about 30 em a little 

more than 2 miles downstream. 

The best-fit lines should not be interpreted as suggesting that at any given downstream location the 

average intermediate diameter of the five largest 

stones would necessarily be the diameter indicated 

by the line. At some intermediate locations no particles larger than pebbles or coarse sand appeared on 

the surface. Considerations governing the selection 

of sampling sites were mentioned earlier in the report. 

DOWNSTREAM CHANGES IN SORTING 

The flood-plain deposits generally became better 

sorted with distance down-valley (fig. 54). Again an 

asymptotic relation seems to be the best fit to the 

Rucker Run points, as is true for Dillard Creek. Most 

of the boulders came to rest within a mile or so of the 

upstream deposit so that the sorting has the largest 

values and improves most rapidly in this zone. Sorting on these creeks continues to improve with distance downstream but at a progressively lesser rate. 

The sorting can only become zero when all grains 

between d9o and d 10 are the same size, a situation 

extremely unlikely to occur. Thus the sorting probably should not continue to improve at a constant 

rate with distance down valley; instead, a more 

reasonable relation is a change by lesser amounts as 

the possible range of improvement decreases, that is, 

So should become asymptotic toward some limiting 

minim urn value. 

A straight line fits the data points for Polly 

Wright Cove, indicating a negative exponential relation for the study reach. As explained above, this 

relation probably could not continue indefinitely 

with distance. For example, extrapolating the 

straight· line suggests that about two miles down 

from the upper end of the study reach, So would 

have the unlikely value of zero, that is, all grains 

from d9o to d1o on the cumulative size frequency curve 

would be the same size. In fact, the data for the 

4,500-foot reach of Polly Wright Cove fit reasonably 

well onto the points for Rucker Run and Dillard 

Creek, as shown in the bottom part of figure 54. This 

suggests that the So relation may be steep and linear 

(exponential) within the first mile of emergence 

from the mountainous area and then takes on a curve 

approaching a limiting value some distance downstream. 

In addition to a downstream limiting minimum 

value, the sorting probably had a limiting maximum 

value in the upstream deposits, due to the finite sizes 

of boulders available for movement and movable by 

the floodwaters. This value seems to be about 4, that 

is, the range log dg0-log d1o for the poorest sorted 

deposits did not encompass more than about 4 log 

cycles of grain size. This range usually- extended 

from about 0.1 or 0.2 mm to about 1,000 or 2,000 

mm. Sorting values higher than 4 could have been 

restricted by a relative scarcity of larger boulders 

available for transport or an inability of the flow to 

move more than a few such boulders from the upstream areas to the valley flood-plain deposits. 

InS<pection of the stream channel upstream from 

depositional areas revealed only a few really huge 

boulders; so not many boulders larger than about 

two meters in intermediate diameter were available. 

A few boulders of such dimensions were moved by 

the flood, for example in Ginseng Hollow, but probably only for short distances along the mountain 

channel. 

These probable limitations on sorting show up 

more clearly in a plot of So versus dav (fig. 55). In 

the upstream reaches dav is large, in spite of the 

ubiquitous sand and silt, because of the presence of 

the large rocks. The associated large value of sorting 

tends to remain high, as dav decreases from about 

55 mm to 4 mm. With further decreases in dav, that 

is, proceeding farther downstreamward, sorting improves rapidly as the larger rocks become scarcer. 

But in the downstream reaches, while dav continues 

to decrease, So begins to level off, that is, the range 

of sizes present in the deposits tends to become 

constant. This suggests that in the downstream regions the flow could easily transport all the available 

grain sizes. 

DOWNSTREAM CHANGES IN SKEWNESS 

Along Dillard Creek and much of Rucker Run the 

skewnesses are close to zero (fig. 56), indicating 

approximately symmetrical distributions. Rucker 

Run actually may show a very slight trend over the 

full 12.5-mile study reach whereby the size distributions go from slightly positive (abundance of small 

grains) at the upstream end to slightly negative 

(preponderance of the larger grains within the distribution) at the downstream end; however, it is 

difficult to say how much of this possible trend is 

significant and how much could be due to chance 

sampling. In Polly Wright Cove the deposits in the 

upstream 1,000 feet had a preponderance of large 

rocks (negative skewnesses), whereas the remaining 

section of the study reach had slightly positive 

skewnesses. Thus in these downstream areas of Polly 

Wright Cove the smaller grains were more predominant than the large rocks in terms of weight-percent. 

The skewness varies with the average grain size, 

for the flood-plain and alluvial fan deposits, in a 
72 HURRIC'ANE CAMILLE IN VIRGINIA, 1969 

RUCKER RUN 

0 0 

o~----------ILQ----------~2oL_ ________ ~3o __________ _J~-----------~L-----------6Lo----------~7o 

0 

"15' 

tlO 

.2 

POLLY WRIGHT COVE 

3 

2 

+ 

I 1~--------~L---~----~----------~----------~----------~ J 0 

tlO 

.2 

II 

........ 0 

~ 

2 3 4 5 

~ 3,-----------,----------,,---------~----------~----------~----------~ z 

i= 

a::: 

0 

en DILLARD CREEK 

2 

o~----------L---------~----------_J----------~-----------L----------~ 

0 4 8 12 16 20 24 

RUCKER, DILLARD, AND POLLY WRIGHT DATA 

oL-----------4L---------~8----------~12-----------l~6-----------2~0----------~24 

DOWNSTREAM DISTANCE (x), IN THOUSANDS OF FEET 

FIGURE 54.-Downstream changes in sorting of flood-plain deposits. 
3.0 

0 

-.a 

tiO 

.2 

I 

1 

tiO 

.2 2.0 II 

0 

~ 

(!) 

z 

i= 

a:: 

0 

en 1.0 

+1.0 

[D 

EXPLANATION 

+ Polly Wright Cove } 

'Y Dillard Creek Floodplain 

D Rucker Run deposits 

X Campbell 

0 Bryant 

'V Wills } 

Alluvial fan 

deposits 

DEPOSITIONAL FEATURES 

0 0 

0 

0 

Upstream direction ----+ 

AVERAGE GRAIN SIZE, IN MILLIMETERS 

FIGURE 55.-Change in sorting with average grain size. 

RUCKER RUN 

I I I I I I 

0 

o~a -o - Line for symmetrical distribution ~ 

0 

-t5' 

tiO 

0 0 

~ ()-

1 tiO 0 0 

'ti"l 

~-J + tiO 0 0 -o--

tiO 

.2 

II 

-1.0 

0 

+LO 

-{]. 

I I I I I I 10 20 30 40 50 60 

POLLY WRIGHT COVE 

Preponderance of small particles 

_________________ ,_ __ ~--

+ Preponderance of large particles 

2 -l.QL-----------~-----------L-----------J------------~----------~ ~ 0 

en 

en 

UJ 

z 

~ UJ 

::.::: 

en +1.0 

0-

-1.0 0 

I 

, 

I 

4 

2 3 4 5 

DILLARD CREEK 

I I I I 

• • 

I I I I 8 12 16 20 24 

DOWNSTREAM DISTANCE (x), IN THOUSANDS OF FEET 

FIGURE 56.-Downstream changes in skewness of flood-plain deposits. 

73 

-

70 
74 HURRICANE CAMILLE IN VIRGINIA, 1969 

rather peculiar way (fig. 57). This graph should be 

interpreted as showing a relation between dav and 

skewness, rather than as an implication that a 

change in dav causes a change in skewness. Distributions that have a large average grain size are negatively skewed, meaning the large boulders predominate as far as weight frequency is concerned. 

As dav decreases to about 8 mm the distributions 

gradually become more and more symmetrical. With 

a further decrease in dav-down to about 2 or 3 mm 

-the distributions become more and more skewed 

in the opposite direction, that is, gain a preponderance of smaller grains. The reason for this skewness 

trend is the progressively smaller percentages of 

large stones contained in the deposits. Average size 

gets smaller, in other words, and the larger particles 

within each sample become prQgressively less common. As dav decreases from about 2 mm, the sorting 

improves rapidly (fig. 55), as the large stones begin 

to disappear from the deposits. This means a trend 

back toward more symmetrical distributions, as 

shown in figure 57. Approximately symmetrical distributions are reached when dav has decreased to 

about 0.5 mm. For smaller average grain sizes, the 

distributions tend to be approximately symmetrical 

or slightly negatively skewed. 

SCARCITY OF COARSE SAND AND GRAVEL 

Geologists have long speculated on the curious 

fact that many grain-size analyses show a dearth of 

particles in the coarse sand or fine gravel ranges. 

This supposed scarcity of certain sizes causes the 

histograms or size-frequency curves to show more 

than one peak or mode, as long as the range of sizes 

represented goes at least from medium sand to 

coarse gravel. Pettijohn (1957, p. 44) gives a good 

review of cases where such polymodality has been 

reported in the literature. Russell (1968) goes into 

the subject in some detail and concludes that grains 

of about 1-6 mm diameters are deficient in fluvial 

deposits because such grains are more readily entrained and more rapidly transported than larger 

or smaller particles. The "missing" grain sizes, in 

his view, are moved downstream beyond river 

mouths and tend to be concentrated on beaches. The 

following paragraphs summarize the Nelson County 

flood deposits in regard to polymodality of sizefrequency distributions. 

The first problem was how to define polymodality. 

For example, if a class had a decrease of only one or 

two percent below the percentages in the adjoining 

size classes, it seemed unrealistic to call such a slight 

decrease a deficiency, because the minor differences 

could easily be due to sampling or splitting (for sieve 

analysis) errors. Then the question was how much 

of a difference in weight percentage to require in 

order to label the distribution polymodal. 

To resolve this problem we measured the approximate error that could be attributed to laboratory 

splitting of the field sample. This was done by sieving 

the entire contents (1-4 lb) of 10 of the samples 

brought from the field (called herein the field samEXPLANATION 

+ Polly w,;ght Cove } 

~ Dillard Creek Floodplain deposits 

X Campbell} 

0 Bryant Alluvial fan deposits 

0 Rucker Run \1 Wills 

0.6 I I I I I I lo I I I I I 

0.4 -

+ /if"}~ 0 

-

0.2 r- ~(/" ~xf -

(f) 

Preponderance (f) :!-/ 0 ' t w 

~ z of small particles !!: 0.0 _rr.- ...... ' w 

~ -- - b ' t ::.::: ----- ?" Prepondera nee (f) ' ~ ' of large particles 

-0.2 -

0 ' +', -0.4 f-- ' ' \1 -

+' 'V -0.6 I I I I I I I + I ', I I 0.01 0.1 1.0 10.0 100.0 

AVERAGE GRAIN SIZE, IN MILLIMETERS 

FIGURE 57.-Change in skewness with average grain size. 
DEPOSITIONAL FEATURES 75 

pie) and comparing the results to the corresponding 

split samples. The field samples were~plft 1-3 times 

in the laboratory, to obtain the split sample. The particular size classes of interest in the test were those 

which were usually present in the field and which 

were often involved in the apparent scarcity: 32-16, 

16-8, 8-4 and 4-2 mm. For these four size classes 

both the full field sample and the split sample were 

sieved, and the percent by weight in the total field 

sample was compared to that in the split sample, for 

a given size class. The largest discrepancy or difference was 5.3 percent, that is, one of the field samples had 5.3 percent by weight in one of the size 

classes, whereas for the same class the split sample 

had zero percent by weight. General agreement was 

quite good, however, with the arithmetic average 

discrepancy between full field sample and split sample amounting to 1.5 percent. Based on the data 

from these sieve tests, we estimated the standard 

deviation of the discrepancies or differences between 

any two size classes of a given sample, using certain 

simplifying assumptions. Two times this standard 

deviation was 3.05 percent; that is, the chances were 

about 95 out of 100 that within a size-frequency 

distribution a difference of 3 percent or less between 

any two size classes could be attributed to splitting 

error. The criterion for polymodality therefore was 

that at least one size class have at least 3 percent 

by weight less than a coarser and finer class. 

There were 63 grain-size analyses, involving two 

flood-plain sites and four highway-delta sites, which 

covered the complete range of sizes at a sampling 

station. Of these analyses 27 dealt with a study of 

grain size changes with depth. All such samples 

were medi urn sand or finer and were analyzed by 

sieving,, occasionally including also a pipet treatment. None of these 27 analyses was polymodal. 

The remaining 36 analyses were surface sediments which usually involved a wide range of particle sizes, thus requiring in most cases a pebble 

count in addition to the sieve analysis. Of these 36 

cases, 24 showed a definite polymodality. 

Table 10 shows the frequency with which each 

size class was deficient, according to the 3 percent 

rule described earlier. Particles which were especially scarce were those in the size range from 4 to 

64mm. 

The polymodal samples have certain common 

characteristics which are generally absent in the 

unimodal distributions. As mentioned above, polymodality showed up markedly in surface samples 

but did not appear in sediments below the surface. 

TABLE 10.-Number of cases of dejic,iency in weight-percent, 

for the indicated particle size class 

[Classes not listed had no instances of deficiency. Deficiency is at lea,st 

3 percent by weight less than a finer and a caarser size class, within same sediment sample] 

Size class, in millimeters 

c<l 

Stream or .... 

LQ CQ 00 .., ~ LQ c<l study area c<l .... CQ c<l ~ LQ I I I ..., 00 c<l 

~ CQ 00 I I I """ c<l .... ~ ! LQ 

~ """ c<l ~ I I I LQ c<l CQ ..., 00 """ c<l 

Polly Wright 

(11 stations, 8 

polymodal). 1 2 7 5 6 6 3 2 2 Dillard Creek 

(7 stations, 2 polymodal). 2 1 2 2 Rucker Run ( 6 stations, 2 polymodal). 2 Oampbell Fan ( 5 stations, all 

polymodal). 5 5 5 4 4 4 1 

Bryant Fan (7 stations, all 

polymodal) . 2 3 2 7 5 5 2 2 1 

Sums: 3 8 17 20 19 18 9 8 4 36 stations 24, 

having polymodal distrLbutions 

Deposits such as alluvial fans which had significant 

proportions of boulders and cobblestones consistently showed polymodal size distributions. Also, the 

presence of such large rocks meant that the final 

distribution represented a combined pebble count 

and sieve ~nalysis. (All 24 cases of polymodality 

were combined pebble-count-sieve analysis distributions ; an additional five such combined analyses did 

not show polymodality.) 

The scarcity of grains in the 4-64 mm range may 

be either real or apparent. Apparent in this sense 

means the scarce grains actually existed on the 

surface of the deposit but were somehow missed 

in the analysis. The possible ways they could have 

been missed are: (1) failure to be tallied in the 

pebble count; (2) failure to be included in the 

scooping up of the "sand" sample; (3) erroneous 

loss in percent by weight due to the mathematical 

analysis of the data, that is, resulting from the 

method of combining the pebble count with the sieve 

analysis. 

The pertinent sizes might be missed during a pebble count in at least two ways. Due to the presence 

of larger rocks, they may well have been sheltered 

or overlapped by these rocks. Also, unless the investigator is particularly careful, he tends to select 

large rocks in preference to smaller grains during 

the pebble-counting. 

In a few places, especially on flood plains downstream, a surface deposit contained only a few rocks 

larger than gravel, so that the pebble count necessarily had a small number of observations. However, 
76 HURRICANE CAMILLE IN VIRGINIA, 1969· 

this possible source of sampling error would apply 

in only a few of the 24 cases of polymodality. Furthermore, such errors should apply equally to all size 

classes rather than only to certain classes. 

Failure to be scooped up with the "sand" sample 

is a possible but not very probable source of error, 

because most of the scarce sizes would not normally 

be included in a sample of fine material. The field 

sample almost always contained grains 8 mm in 

diameter, often included pebbles up to 16 mm diameter, and not uncommonly contained particles up 

to 32 mm in diameter. 

The method of combining the pebble count with 

the sieve analysis certainly depends on several questionable assumptions, as explained earlier. However, 

four size classes showed about an equally large frequency of scarcity (table 10). A fault in the mathematical treatment would more likely vary in some 

way with the grain size. 

If the scarcity of the particles in question is real, 

the possible causes are that: (1) such grains were 

never produced to any significant degree of abundance; (2) the particles were either retained up-· 

stream (mountain channel deposits) or were carried 

downstreamward beyond the deposits we inspected; 

(3) the particles were laid down in the sediments we 

inspected but were not abundant on the surface and 

so were not sampled. 

The theory that such grains were not produced in 

significant quantities cannot be disproved but does 

seem highly unlikely, inasmuch as particles ranging 

from clay to 10-foot-thick boulders were eroded by 

the floodwaters. The appearance of the polymodality 

in deposits upstream from which no mountainchannel deposits existed shows that the scarce grain 

sizes probably were not retained upstream in the 

headwater region. If particles ranging from 4 to 

64 mm are particularly susceptible to entrainment, 

they should nevertheless have been trapped in highway deltas like the one where Dillard Creek meets 

highway U.S. 29. In fact, being scarce relative to 

adjacent size classes upstream, they might even be 

expected to be more abundant in highway deltas. 

This, however, was not the case, nor could they be 

located at greater distances downstream. The distances studied amounted to as much as 12 miles, by 

which distance the deposits were strictly sand and 

finer, with average grain sizes less than 1 mm 

(fig. 50) and excellent sorting (fig. 54). These considerations, especially the absence of excessive quantities of the pertinent grains in highway deltas, cast 

doubt on the theory that the missing grains were 

moved all the way to the James River or farther. 

It is quite possible that the pertinent grain sizes 

were present in the flood deposits but were not 

abundant on the surface. This situation could result, 

for example, if the floodwaters laid down rocks of 

all sizes at one or more stages but deposited medium 

sands and finer grains in scattered patches toward 

the end of the flood. The scattered sand patches 

would tend to cover gravel and pebbles while leaving, the cobbles and especially the boulders exposed. 

A surface sampling would then record a relatively 

large number of big rocks and sand grains but 

would miss many pebbles and gravel-sized particles. 

If the deposits had not been destroyed, this possible 

cause of polymodality could be tested in the field. 

In conclusion, flood deposits consisting mainly of 

grains smaller than coarse sand were all unimodal. 

Deposits which included a wide range of particle 

sizes, however, were usually polymodal with a relatively scarcity of particles 4-64 mm in diameter. 

The most likely reason for this polymodality is that 

the scarce grains were covered up to a significant 

extent by sands deposited during the recession stage 

of the flood. Other possibilities are that many of the 

deficient sizes were hiding under the edges of the 

larger rocks and that sampling bias favored the 

selection of larger rocks during the pebble counting. 

DOWNSTREAM CHANGES IN GRAIN SHAPE AND 

ROUNDNESS 

The roundness of a grain is the degree of smoothing or rounding, ranging from very sharp to perfectly rounded, of the edges or corners of the grain. 

Particles become more rounded due to weathering 

and to abrasion and wear, that is, with longer time 

and greater distance of transport. Observations on 

grain roundness therefore might be helpful in estimating distance of transport. 

Grain shape, defined more specifically below, is a 

three-dimensional measurement of a particle's general form and is a separate concept from roundness. 

Grain shape depends on amount of abrasion, initial 

shape, cleavage and natural hardness. The mineralogy generally influences these last three items. Because it affects settling velocity, the grain shape conceivably could influence various aspects of particle 

behavior in water, such as ease of entrainment and 

mode and rate of transport. It could also provide 

information on the hydrodynamic conditions of 

deposition. 

Changes in grain shape and roundness with distance downstream are still poorly understood due 

to scarcity of data, especially for river flood conditions. Krumbein (1940) studied California flood 
CONCLUSIONS 77 

gravels over a distance of about 7 miles. He found 

that roundness increased from 0.28 at the 1-mile 

station to 0.44 at the 7 -mile location ( 0.1 being very 

sharp particle edges and 1.0 being perfectly rounded 

edges) ; grain shape, however, showed no significant 

change over this distance. Scott and Gravlee (1968) 

found that a flood surge on a stream in the Sierra 

Nevada mountains caused boulders to increase in 

average roundness from about 0.25 to 0.40 over the 

first two miles of travel. Studies of shape and roundness changes for other environments, such as 

beaches, rivers at normal flow, and in laboratory experiments, show various and even contradictory relations (Pettijohn, 1957, p. 549) . 

We examined the deposits of Polly Wright Cove 

for possible downstream changes in particle shape 

and roundness. Most of these sediments orginated 

in a reasonably localized area (about 0.6 sq mi). The 

total distance of deposition along the stream unfortunately was only about 0.8 mile, the upstream half 

of which was subject to "contamination" from two 

minor tributaries, as mentioned earlier. This 0.8-mile 

distance might be considered too short to show any 

shape and roundness changes, but laboratory experiments (Krumbein, 1941a) have suggested that 

roundness values during the first mile of travel can 

increase from about 0.15 to 0.45, that is, roundness 

increases considerably during the first mile or two 

of transport. 

The Polly Wright grains examined were in the 

0.250-0.500 mm sieve class, from each of the 11 

field stations. A colleague coded the 11 samples so 

that the operator could not know the field location 

of the grains being examined. Using a microscope, 

the operator then measured the shape and estimated 

the roundness of 50 randomly-chosen grains, for each 

sample. Shape equals the short particle axis divided 

by the square root of the intermediate times the 

long axis, where all three axes are mutually perpendicular (Schulz and others, 1954). Krumbein's 

(1941b) visual comparison chart served as the criterion for roundness. Possible values of both these 

definitions range from 0 to 1.0, where 1.0 is maximum possible shape or roundness. Owing to the complex and varied mineralogy of the parent rocks it 

was not possible to restrict the measurements to 

only one mineral. The grains measured were two 

unidentified translucent minerals that occurred the 

most abundantly and consistently throughout all 11 

samples. 

Table 11 lists the average shape and roundness 

values for each sampling station. All shape values 

fall within the range 0.63-0. 76. Grain corners and 

TABLE 11.-Average grain shape and roundness values with 

distance downstream in Polly Wright Cove 

Average Ave:mge 

Distance downstream grain-shape roundness 

from first deposits (ft) value value 

0 ----------------- 0.74 0.19 

290 ----------------- .63 .21 

740 ----------------- .74 .23 

1,007 ----------------- .76 .21 

1,413 ----------------- .71 .24 

1,803 ----------------- .73 .20 

2,413 ----------------- .72 .22 

2,773 ----------------- .74 .20 

3,439 ----------------- .71 .21 

3,834 ----------------- .68 .18 

4,358 ----------------- .71 .17 

edges were quite sharp, and the low roundness values 

range from 0.17 to 0.24. The data show no significant change in either shape or roundness with distance downstream. 

Krumbein's (1940) and Scott and Gravlee's 

(1968) roundness studies showed a definite increase 

in particle roundness over the first few miles of 

travel. The use of gravel-to-boulder-sized particles 

in those studies may be one reason why the present 

roundness results differ from the previous work. 

Also, the infusion of sediment from side tributaries 

near the middle of the study reach may have confused the present results slightly. Other possible 

causes of discrepancy might be the range of grain 

sizes of the transported and deposited materials, the 

mineralogy and the physical conditions (hydrology 

and geomorphology) of the floods. All of these discrepancies warrant future study in both field and 

laboratory. 

The absence of an increase in grain shape agrees 

with Krumbein's field study (1940). His tumblingbarrel experiments (1941a), which may not have 

simulated catastrophic flood conditions, produced a 

very slight increase in grain shape over the first 

mile of travel. 

CONCLUSIONS 

The storm and flood of August 1969 in Virginia 

was an extreme event (probably occurring no more 

than once every several hundred years on the average) which caused enormous amounts of sediment 

erosion and deposition. In the region most affected, 

Nelson County, rainfall amounted to as much as 28 

inches over the 8-hour storm. Peak streamflow was 

estimated to be about 10,000-12,000 cfs per sq mi of 

drainage area. 

The longitudinal profiles of the five mountain 

streams studied, including alluvial fans where present, are very closely described by a hyperbolic equa-
78 HURRICANE CAMILLE IN VIRGINIA, 1969 

tion of the form F=L/ (a+ bL), where F and L are 

vertical fall and horizontal distance respectively, 

from the drainage divide, and a and bare coefficients. 

Nearly half of the erosion took place in the form 

of debris avalanches on upland hillslopes. Field observations and measurements on avalanche scars 

reveal the following characteristics : ( 1) the scar 

on the hillside usually extended up the slope to the 

region where the local hillside gradient was steepest: (2) this upslope tip of the avalanche tended to 

be located at the point on the hillside where the 

convex upper zone merged with the concave or 

straight section immediately below, a point which 

could be from 0.3 to 0.95 of the horizontal distance 

from base to top of hill: (3) debris avalanches 

usually occurred down previously-existing grooves, 

minor channels or depressions on the hillside, a 

finding in agreement with other studies: ( 4) slopes 

facing north, northeast and east had more debris 

avalanching than hillslopes facing other directions; 

and (5) on a unit drainage area basis, longer hillslopes had a greater total length of avalanche scars 

but fewer scars than short hillslopes. 

Slightly more than half of the erosion that occurred was estimated to be from the enlargement 

of stream channels. 

The total volume of sediment eroded during the 

storm was estimated to be about 6.1 million cu ft 

(3.9 million cu ft per sq mi) from the headwaters 

of Wills Cove, 3.1 million cu ft ( 4.6 million cu ft 

per sq mi) from Ginseng Hollow and 3.1 million 

cu ft (3.2 million cu ft per sq mi) from Polly Wright 

Cove. These amounts correspond to average denudations of 1.4, 2.0 and 1.4 inches from Wills Cove 

(upper part), Ginseng Hollow and Polly Wright 

Cove, respectively, and are probably the equivalent 

of several thousand years of "normal" denudation. 

The types of deposits associated with the storm 

and flood were: (1) debris-avalanche deposits, quite 

rare, left at or near the base of hillslopes; (2) mountain-channel deposits, in some cases in the form of 

huge debris piles behind log jams; (3) alluvial fans 

where the narrow mountain channels entered the 

open intermontane valleys; ( 4) deltas where a swollen stream's path was temporarily blocked by a 

highway embankment; and (5) flood-plain deposits. 

The size-frequency distribution of the particles at 

any locality on such deposits is described by combining a sieve analysis with a pebble count in a 

method which apparently has not heretofore been 

used. 

Except for highway deltas and some downstream 

flood-plain sediments, the particle sizes in a deposit 

generally ranged from clay or silt to boulders. However, nearly all particles larger than about fine sand 

were deposited after 5 or 10 miles of travel from 

their source. Some of the silt and clay probably 

reached the James River and its estuary. 

Two prominent alluvial fans each contained about 

300,000 cubic feet of newly deposited sediment. The 

average grain size on alluvial fans showed no distinct trend with distance down the fan. Several other 

authors have reported that particles tend to become 

smaller with distance on a fan. Possible reasons for 

this difference are the short lengths of the present 

fans (up to about 2,000 ft) and the probably fast 

velocity of the flow at the fan apex, for this catastrophic event. 

Highway deltas vary widely in size. One of the 

largest contains an estimated 1,900,000 cubic feet of 

sediment. Such deltaic sediments consist mainly of 

sand that has distinct laminations. Cross-stratification near the surface and ripple marks and mud on 

the top surface are also characteristic. As the flood 

progressed, the particles trapped in highway deltas 

generally became finer in size, better sort~d and more 

symmetrical in weight-frequency distribution. The 

storm-average sediment transport rate at the apex 

of one major delta was estimated to be about 33 lb 

per sec per foot of channel width. 

Flood-plain deposits in the upland region have a 

wide range of particle sizes and show no distinct 

sedimentary features. The sandy flood-plain sediments farther downstream commonly have laminations, ripple marks and mud cracks. Of two sites 

chosen to study textural changes with depth (reflecting relative time of deposition during the flood), 

the particles at one site become finer and better 

sorted toward the top surface, whereas at the other 

site the grains show no noticeable trend except for 

a layer of finer material on the surface. 

The amount of deposition on flood-plains decreased 

with distance downstream, though at varying and 

irregular rates. Side tributaries, highway deltas and 

other factors influenced the measured rates at which 

volume of flood-plain deposition diminished with 

distance. Upstream deposition on Rucker Run was 

about 150-300 cu ft per foot of channel length, but at 

12.5 miles downstream the amount of deposition was 

only minor. Beginning at the head of a valley just 

beyond the mountain front, flood-plain sediments 

show the following textural changes with distance 

downstream: (1) average grain size decreases rather 

systematically, reaching about 0.1 mm in about 5-7 

miles of travel; (2) average grain size therefore 

decreases with both a decrease in local gradient and 
CONCLUSIONS 79 

an increase in drainage area; (3) the average diameter of the five largest stones, reflecting the stream 

competence, decreases exponentially ; ( 4) sorting 

improves to the extent that the d90-d1o range decreases from about 2-4 log cycles of grain size to less 

than one log cycle of grain size, over 5-7 miles of 

travel; and ( 5) most size distributions are approximately symmetrical, a notable exception being the 

deposits in Polly Wright Cove. 

Some of the deposits apparently were polymodal, 

being deficient in fine gravel, a situation common to 

many other sediments. The gravel particles in all 

probability were as available as other sizes in the 

headwaters, were not selectively retained upstream 

from the floodplain deposits, and were not selectively 

transported beyond the several miles of study reach. 

The most probable reason for the supposed scarcity 

of gravel-sized particles is the possibility that the 

"missing" grains were mostly covered by sands deposited during the recession stage of the flood. 

Particles from Polly Wright Cove in the 0.250-

0.500 mm sieve class showed no significant changes 

in either shape or roundness, over about 0.8 mile 

of travel. 

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*u.s. GOVERNMENT PRINTING OFFICE: 1973-543-579/43