42• 

METRIC CONVERSION FACTORS 

Multiply 

foot (ft) 

cubic yard (yd3 ) 

mile (mi) 

square mile (mi 2 ) 

cubic foot per second ere /sec) 

By 

0.3048 

0.7646 

1.609 

2.590 

0.02832 

To obtain 

meter (m) 

cubic meter (m3 ) 

kilometer (km) 

square kilometer (km 2 ) 

Cubic meter per ser:ond (m3 /s) 

105' 

120• 110' I 

~ 49·~ r~a ------L ------/T__j______________ 115' I ---------~ I ~-~ o I \ 

I ! I 

Spokane o ' \ I ! t ' I '-, MONTANA I 

I \ 0 \ 

• Mount St Helens ! '1{, Helena •

1 

·Seattle 

r-------~ 

~--- ------------ ' ( \ Portland ) -~ __ J 

0 

Salem 

OREGON 

I \ ---------------- / '\ --f'~ 1 "~ I 

I I 

f 0 Boise \ 

I IDAHO \ 

! \ ---- _______________ _j _________ ~----------~ 

0 100 200 300 MILES 

0 100 200 300 KILOMETERS 

COVER: North Fork Toutle River, June 30, 1980. Volcanic mud flow breccia 

and debris from the May 18, 1980, eruption of Mount St. Helens (in upper 

right) are as much as several hundred feet thick in the reach shown. 

Photograph by Austin Post, U.S. Geological Survey. 
Emergency Assessment of Mount St. Helens 

Post-eruption Flood Hazards, Toutle and 

Cowlitz Rivers, Washington 

By M. E. Jennings, V. R. Schneider, and P. E. Smith 

Hydrologic Effects of the Eruptions of 

Mount St. Helens, Washington, 1980 

GEOLOGICAL SURVEY CIRCULAR 850-1 

1981 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Doyle G. Frederick, Acting Director 

Library of Congress No. 81-600131 

Free on application to Distribution Branch, Text Products Section, 

U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
FOREWORD 

On May 18, 1980, after more than a month of earthquakes and eruptions, Mount St. 

Helens, in southwestern Washington, exploded in a volcanic eruption more violent than 

any in the conterminous .United States during the 20th century. A lateral blast of hot gas 

and rock particles devastated an area of about 150 square miles on the northern side 

of the mountain knocking down trees to a distance of 15 miles. Several minutes later, a 

giant ash cloud rose to about 60,000 feet. Winds then carried the ash cloud across the 

United States, with heavy fallout and deposition in eastern Washington and parts of 

Idaho and Montana. Earlier, smaller eruptions deposited ash in western Washington 

and parts of Oregon and Canada. 

The hydrologic effects of the May 18 eruption have been both widespread and intense. 

During the eruption, a massive debris avalanche moved down the north flank of the 

volcano depositing about 3 billion cubic yards of rock, ice, and other materials in the 

upper 17 miles of the North Folk Toutle River valley. The debris deposits are about 

600 feet thick in the upper reaches of the valley. Following the avalanche, runoff from 

the melted glaciers and snow, and possible outflow from Spirit Lake, caused an extraordinary mudflow in the North Fork Toutle River. The mudflow shattered and uprooted 

thousands of trees, destroyed most of the local bridges, and deposited an estimated 

25,000 acre-feet of sediment in the Cowlitz River channel. A considerable amount of 

additional sediment was conveyed through the lower Cowlitz into the Columbia River 

where it was deposited and formed a shoal that blocked the shipping channel. Mudflows 

also occurred in the South Fork Toutle River and in tributaries on the east flank of 

Mount St. Helens which enter Swift Reservoir. 

As part of a concerted Geological Survey effort to study the volcanic event and to 

identify potential hazards, Survey hydrologists have mounted an intensive program to 

document the hydrologic effects of the eruptions. The major initial hydrologic findings 

are reported in this circular series. Quick, useful assessment was made possible only 

because the Survey has long conducted extensive water-resources investigations in the 

affected areas of Washington, Oregon, and Idaho. Hence, there was a well-defined basis 

for identification and documentation of the typeR and magnitudes of hydrologic changes. 

The Geological Survey Circular 850, "Hydrologic Effects of the Eruptions of Mount 

St. Helens, Washington, 1980," consists of individually published short chapters that 

emphasize data collection activities, field observations, and initial comparisons of preand post-eruption conditions. The series will cover hydrologic events occurring on 

May 18 in the Toutle and Cowlitz River; physical alteration of the Toutle River system; 

the chemical and physical quality of precipitation, streams, and lakes affected by volcanic ash fall; ash-leaching studies; and Mount St. Helens glaciers. 

III 

Doyle G. Frederick 

Acting Director 

CONTENTS 

Page 

Abstract___________________________________________________________________ I1 

Introduction --------------------------------------------------------------- 1 

Dam-failure hazard computer models ------------------------------------------- 1 

Spirit Lake dam-failure hazard ------------------------------------------------- 3 

Elk Rock dam hazard ------------------------------------ ____________________ 6 

Conclusions ------------------------------------------------------------------ 7 

References ---------------------------------------------·--------------------- 7 

ILLUSTRATIONS 

Page 

FIGURE 1. The Mount St. Helens area ________ --------------------.--------------- I2 

2. Approximate pre- and post-eruption bed profiles of the North Fork Toutle and 

Toutle Rivers -------------------------------------------------- 3 

3. Selected cross sections of the North Fork Toutle, Toutle, and Toutle River 

channeis utilized in modeling studies ------------------------------- 4 

4. Profiles of peak discharge along the Toutle River for Spirit Lake dam failure 

modeling study ------------------------------------------------ 5 

5. Selected cross sections for the Cowlitz River valley utilized in model studies__ 6 

TABLES 

TABLE 1. Summary of peak discharges and elevations along the North Fork Toutle and 

Toutle Rivers for three assumed cases of the Spirit Lake debris-dam 

Page 

failure ------------------------------------------------------- I5 

2. Summary of peak discharges and elevations along the North Fork Toutle, 

Toutle, and Cowlitz Rivers for the CASE II Spirit Lake dam failure ______ 6 

3. Comparison of discharge simulated by model runs with measured or 

estimated peak discharges on the North Fork Toutle and Toutle Rivers for 

the Elk Rock dam failure---------------------------------------- 7 

v 

HYDROLOGIC EFFECTS OF THE ERUPTIONS OF 

MOUNT ST. HELENS, WASHINGTON, 1980 

EMERGENCY ASSESSMENT OF MOUNT ST. HELENS 

POST-ERUPTION FLOOD HAZARDS, TOUTLE AND 

COWLITZ RIVERS, WASHINGTON 

By M. E. jENNINGS, V. R. SCHNEIDER, and P. E. SMITH 

ABSTRACT 

Flood hazards created by natural dams on the Toutle and 

Cowlitz River systems formed from rock and debris deposits 

derived from the May 18, 1980, eruption of Mount St. Helens 

were analyzed and evaluated. The post-eruption structures investigated include the dam impounding Spirit Lake and the dam 

at Elk Rock on the North Fork Toutle River. Working under 

emergency conditions, efforts were made to adapt modeling programs to the unique drainage conditions confronted. Dam-break 

and flood-routing models utilized in the study are described. 

The Elk Rock dam failed after the assessment was conducted. 

Computed peak discharge values were approximately the same 

as those measured or estimated during and following the 

failure. 

The studies illustrate that useful dam-break and flood-hazard 

estimates are possible with existing computer models, even 

though data may be only approximate and incomplete. 

INTRODUCTION 

Mount St. Helens, in southwestern Washington, 

erupted at 8:32a.m., P.s.t., on May 18, 1980. The 

north side of the volcano's cone was blown off by a 

powerful lateral blast, and a huge volume of debris 

buried land and water features in the drainage 

area immediately north and northwest of the 

mountain. Major geomorphic and hydrologic 

changes extended downstream along the Toutle 

and Cowlitz Rivers, with mudflows and hydraulic 

effects reaching the Columbia River (fig. 1). 

Details of these post-eruption events are described 

in other reports of this volume. 

This report deals with emergency flood hazard 

situations created by the massive debris avalanche 

deposit in the North Fork Toutle River valley and 

by downstream sediment deposition. The debris 

deposit is up to 600 ft thick; it formed a dam approximately 500 ft high at the outlet of Spirit Lake, 

creating a set of conditions conducive to overtop11 

ping and subsequent downstream flooding. To date 

(March 1981), this has not occurred. However, 

Geological Survey hydrologists witnessed and 

recorded the overtopping and failure of a similar 

but smaller dam in the North Fork Toutle River 

near Elk Rock. A flood-hazard assessment made 

the day before the dam failure proved to be 

reasonably accurate and useful for the choice of advance protective measures. 

The report contains (1) a brief description of the 

computer models employed to make the damfailure and flood-threat analyses; (2) analyses of 

the dam-failure potential at Spirit Lake and at Elk 

Rock; and (3) hypothetical flood routings from both 

of these sites. 

DAM-FAILURE HAZARD COMPUTER 

MODELS 

Modeling methods were employed in an effort to 

identify and compute the flood hazards associated 

with the massive post-eruption drainage modifications. Land (1980) described and evaluated 

available dam-failure, flood-wave computer models 

and concluded that the model developed by Fread 

(1980) was the most generally applicable. Land 

prepared a modified version, named the Dam 

Break Flood Forecast (DBFF) model, which was 

utilized for the Mount St. Helens dam-break 

hazard calculations. 

The DBFF modification of Fread's model 

employs the current understanding of dam-failure 

mechanics and hydrodynamic flow theory to 

predict the formation of a dam-failure wave and its 

downstream movement. The modified model is a 

practical tool with wide applicability. It can function with input data r3.1\ging from complete and 
10 20 MILES 

FIGURE 1.-The Mount St. Helens area. 

accurate data specification to merely rough 

estimates of input parameters. The model is 

economical to run, requiring only nominal amounts 

of time on large computers. -

The DBFF model consists of three functional 

parts (Fread, 19.80; Land, 1980): (1) description of 

the mode of dam failure,. that is, the temporal and 

geometrical description of the breach; (2) computation of the hydrograph of outflow through the 

developing breach as governed by the physical and 

hydraulic features of the reservoir, mainly rates of 

inflow to the reservoir and its storage 

characteristics; and (3) routing of the outflow 

hydrograph · through the . downstream valley to 

determine discharge hydrograph changes resulting 

from downstream variations .in valley storage 

capacity and from frictional resistance to flow. 

The DBFF model permits adoption of a variety 

of breach formation characteristics to simulate 

either overtopping or piping failure. The user 

specifies the failure time interval and the final size 

and shape of the fully developed breach. During 

model operation, the breach formation commences 

when the reservoir's water-surface elevation (h) exceeds a critical input stage (hr). An overtopping 

failure is simulated when hr is set to a value such 

that a sufficient amount of water is flowing over 

the crest of the dam to cause· failure. A piping 

failure is simulated when hr is set to a value below 

the height of the dam. Overtopping failure modes 

12 

were assumed for the North Toutle River calculations. The mode of breach failure selected influences the results significantly. 

DBFF computes the outflow from the breached 

dam as the summation of breach flow and flow 

through spillway outlets. The total outflow 

hydrograph is a function of water-surface elevation 

and outlet geometry and is related to depletion of 

reservoir storage volume. The reservoir routing in 

DBFF uses either (1) a hydrologic storage routing 

technique based on the law of conservation of 

mass; or (2) a hydraulic approach. The hydrologic 

storage routing technique was used for all reservoir routings described in this report. 

After the outflow hydrograph from a breached 

dam is determined (expressed in terms of stage 

and discharge by the model), it is necessary to 

route the hydrograph through the downstream 

river valley. The DBFF model utilizes a hydraulic 

routing method based on the two differential equations governing one dimensional unsteady flow, 

the equations of continuity and momentum. A 

nonlinear implicit finite-difference algorithm was 

employed to solve the equations. The computational procedure is capable of simulating both subcritical and supercritical flows. 

According to Fread (1980), a distinguishing feature of flood waves generated by dam failure is the 

great magnitude of the peak discharge compared 

to runoff-generated flood waves. A dam failure 
flood is usually many times greater than the runoff 

flood of record. For example, the most severe flood 

estimated for a Spirit Lake debris dam failure was 

about 25 times the maximum flood observed on the 

Toutle River during a 54-year period. The timespan from the first rise to the peak of the discharge 

hydrograph generally is very short. This feature 

can cause serious computational difficulties. In the 

process of routing dam-failure flood hydrographs 

down the North Fork Toutle and the Toutle Rivers, 

considerable time was spent eliminating computational problems introduced by the rapid rise in 

stage. 

An unsteady streamflow simulation model, J879, 

documented by Land (1978) was used to route flood 

hydrographs down the North Fork Toutle River, 

and then down the Cowlitz River to its juncture 

with the Columbia River. J879 is a computer program for simulating one-dimensional subcritical, 

gradually varied unsteady flow. It does not perform dam-failure calculations. The program solves 

the differential equations for unsteady flow, using 

a linear implicit finite-difference algorithm as the 

computational method. Details of the J879 computational scheme are contained in a report by Land 

(1978). Model J879 was employed because the 

DBFF model did not include the option that would 

accommodate the correct downstream boundary 

condition at the Columbia River. 

SPIRIT LAKE DAM-FAILURE HAZARD 

Many uncertainties arose immediately following 

the eruption of Mount St. Helens concerning the 

extent of hazards, both geologic and hydrologic. 

The massive debris blockage- estimated to be 3 to 

4 billion cubic yards (Youd and Wilson, 1980)-was 

_J 4000 <{en 

z~ RIVER MILES 0 TO 26 

known to block the outlet of Spirit Lake. However, 

due to poor visibility from aircraft and the dangers 

of onsite observation, only limited informatioi1 was 

available on the size of the post-eruption lake and 

on the stability of the debris dam. 

On May 19, Geological Survey hydrologists 

visited the area and determined that the elevation 

of the lake surface had been raised about 200ft by 

the events of May 18. It was thought that the lake 

represented a possible flood hazard, and the 

Survey thus initiated work to estimate the 

downstream flow peaks that would occur if the 

debris dam failed. The estimates were to be completed within 2 days and, therefore, based largely 

on initial field observations of water and land conditions. This input information was fragmentary 

and approximate, particulary for Spirit Lake 

storage characteristics and the debris dam. 

Figure 2 compares an approximate debrismodified bed profile of the North Fork Toutle 

River compared to the originals bed profile. Figure 

3 shows representative channel cross sections constructed from topographic maps and used in the 

modeling study for the North Fork Toutle and Toutle Rivers. Note the marked variation in channel 

cross-sectional shape. Although pre-eruption 

cross-sectional configuration was used in the 

absence of post-eruption information, vertical 

elevations of cross section in the first 14.5 mi below 

the lake outlet were raised in accordance with early estimates of the debris-modified bed profile. The 

elevations adopted are somewhat lower than those 

given in the report by Y oud and Wilson (1980, fig. 

2). 

The use of approximate cross-sectional information from small-scale maps was to prove a hindrance to acceptable computational results. The 

o.- 3000 

f=L.L Post-eruption (debris modified) riverbed profile. 

<t:O 2000 z~ Elk Rock" 

Pre-eruption riverbed profile W:::::J >IO<t: 

1000 

coo 0 <t:_J 26 1-<t: 24 22 20 18 16 14 12 10 8 6 4 2 0 

wu 

w- u_l- 4000 a: Zw RIVER MILES 26 TO 53.6 -_> 3000 zu 

Qi= 2000 1-W 

<!O 

>0 1000 ww 

Lti<.:J 0 1 I I 

53.6 52 50 48 46 44 42 40 38 36 34 32 30 28 26 

RIVER MILES DOWNSTREAM FROM SPIRIT LAKE OUTLET 

FIGURE 2.-Approximate pre- and post-eruption bed profiles of the North Fork Toutle and Toutle Rivers pre-, May 18, 1980, eruption. Spirit Lake outlet is at mile 0. Adapted from Youd and Wilson (1980, :fig.2) 

I3 
~ 3390 

u.. <1l 33807 t- \ 

o 3360 -

~ 3350 

~ ;;~~ -

I 

RIVER MILE 0.0 -

-

-

-

- 5 ;;~~ t- \ - ~ 3300~--~~---~~--~~~~--~--~~ 

w 

> 

490~~--~---.---.--~.---.1---.-,--.--. 

1- 480- w 

w 

u.. 470-

460-

~ 450-

z 440 '- 0 

~ 430 '-

:; 420 r 

RIVER MILE 39.0 -

-

-

-

-

-

-

~ 410 t- I - w 400L-~~~--~--~~~--~--~--~~ 

0 500 1 000 1500 2000 2500 3000 3500 4000 4500 

TRANSVERSE DISTANCE, IN FEET 

ia5o~~---.---.--.---.---.---.--.---. 

1840 

1830 

1820 

1810 

1800 

1790 

1780 

1770 

1760L-~--~~-L--~--~--~~~~--~ 

690~~--~---.---.--.---.---.---.--. 

680 

670 

660 

650 

640 

630 

620 

610 

600L-~--~---L-L-L--~~~--~--L-~ 

140~~--~--~---.--~--.---.--.,--. 

130 

120 

110 

100 

90 

80 

70 

60 

50L-~--~---L---L~~--~--~--L-~ 

0 500 1 000 1500 2000 2500 3000 3500 4000 4500 

TRANSVERSE DISTANCE, IN FEET 

FIGURE 3.-Selected pre-, May 18, 1980, eruption cross sections of the North Fork Toutle, Toutle, and Toutle River channels 

utilized in modeling studies. 

DBFF model computations failed to provide 

reasonable answers because the actual crosssectional shapes varied markedly in places from 

the idealized inputs used for the modeling exercise. 

One of the many difficulties encountered in 

evaluation of the Spirit Lake flood hazard was 

determination of the most probable mode of failure 

of the Spirit Lake dam (Y oud and Wilson, 1980). In 

addition to the stability of the debris material 

forming the dam, the probable mode of failure was 

dependent on the height and lateral width of the 

dam, the size and volume of the lake, and the 

height of water behind the dam. Field information 

was minimal. Attempts to treat the debris as a dam 

proved only partly useful. It was finally decided to 

conceptualize the Spirit Lake hazard as an overtopping, rather than a piping failure or other possible 

form of breach. Because of the great downstream 

extent of the debris (14.5 mi), piping or other forms 

of failure were considered unlikely. 

Overtopping conceivably could result from 

mudflows entering the lake, from snow- and 

icemelt runoff, or a combination of these events. 

Water discharged by overtopping failure was 

visualized to flow over the debris and down the 

channels carved in the debris mass by the mudflow 

that followed the May 18 damming of Spirit Lake. 

14 

Working from this hypothesis of the mode of 

failure, calculations were made for overtopping 

and failure of the Spirit Lake dam throughout the 

estimated 53.6-mi reach from Spirit Lake (mi 0.0) 

to the mouth of the Toutle River (mi 53.6). A 

trapezoidal breach was adopted as the most probable geometric profile, and side slopes of 2 vertical 

to 1 horizontal were assumed. Three cases were investigated, assuming different sizes and durations 

of breach development: 

CASE I-Duration of breach development 

equals 1 hr; final breach depth equals 150 

ft; maximum width of breach equals 200 ft; 

discharge at dam equals 1,130,000 ft3/s; 

CASE II-Duration of breach development 

equals 1 hr; final breach depth equals 100 

ft; maximum breach width same as in 

CASE I; discharge at dam equals 613,000 

ft3/s; 

CASE III-Duration of breach development 

equals 3 hr; final breach depth equals 50 ft; 

maximum breach width same as in CASE 

I; discharge at dam equals 174,000 ft3/s. 

The discharges in each case were computed on the 

basis of estimated reservoir stage and the assumed 

breach dimensions. 
140 

'<t 

o. 

120 Elk Rock North Fork Toutle River~ ~Toutle River X 

0 

z 

0 

,_, ___ _ 

----........... u 

UJ 100 (/') 

a: 

UJ 

a.. 

IUJ 80 UJ 

u.. 

u 

CD 

::J 

u 60 

~ 

UJ .. (!) 

.... , ______________ _ 

a: 

<( 40 :r:: 

u 

(/') 

0 

~ 

<( Case 111-DBFF 

---------------~-------- ---------------- UJ 

a.. -------------

ooL-----~------~~----~----~------~----~~-----L~----~----~------~----~55 5 1 0 15 20 25 30 35 40 45 

DISTANCE DOWNSTREAM FROM SPIRIT LAKE, IN RIVER MILES 

FIGURE 4.-Profiles of peak discharge along the Toutle River for Spirit Lake dam failure modeling study. 

Results of calculations for the three cases are 

presented in table 1 a.nd figure 4. The computed 

range in downstream discharge was wide and convergence of profiles at the mouth was only 

moderate, indicating little attenuation of the flood 

wave. Thus, uncertainty in the prediction of potential failure characteristics resulted in corresponding uncertainty in the dam-failure flood-hazard 

estimates. 

The CASE II results shown in table 1 were reported to Geological Survey headquarters as the 

basis for emergency planning. Subsequently, 

calculations were made for CASE ll conditions in 

the Cowlitz River using the flood-routing program 

J879. A special river mileage system was adopted 

for the Cowlitz River flood-routing exercise and 

the calculations were based on the channel cross 

sections shown in figure 5. As for the North Fork 

Toutle and Toutle Rivers, no post-eruption data 

were available, necessitating the use of preeruption configurations. In addition, assumptions 

were made of a constant flow of 9,000 ft3/s from 

the upper Cowlitz River and a constant Columbia 

River stage elevation at mile 73.1 (mouth of 

Cowlitz River) of 5."5 ft above National Geodetic 

Vertical Datum (NGVD). The Columbia River at 

TABLE 1. -Summary of peak discharges and elevations along the North Fork Toutle and Toutle Rivers for three assumed cases of 

Spirit Lake debris-dam failure (computed using the DBFF model) 

River miles 

below 

Spirit Lake 

CASE I 1 CASE II 2 Case III 3 

0. 0 ---------

17.0---------

39.9---------

5 53.0 ________ _ 

Discharge 

(ft3/s) 

1,130,000 

1,010,000 

475,000 

474,000 

Elevation 4 

(ft) 

3,376 

1,214 

482 

98 

Discharge Elevation Discharge 

(ft3/s) (ft) (ft3/s) 

613,000 3,358 174,000 

546,000 1,209 158,000 

296,000 466 128,000 

295,000 89 127,000 

1 CASE I.-Duration of breach development equals 1 hr, terminal breach depth equals 150ft, and the maximum width of the breach equals 200ft. 2 CASE H.-Duration of breach development equals 1 hr, terminal breach depth equals 100ft, and the maximum width of the breach equals 200ft. 3 CASE III. -Duration of breach development equals 3 hr, terminal breach depth equals 50 ft, and the maximum width of the breach equals 200 ft. 4 Elevations are referenced to National Geodetic Vertical Datum of 1929. 6 River mile 53.6 is the mouth of the Toutle River. 

15 

Elevation 

(ft) 

3,330 

1,204 

444 

77 
TABLE 2. -Summary of peale discharges and elevations along 

the North Fork Toutle, Toutle, and Cowlitz Rivers for the 

CASE II Spirit Lalce dam failure (computed using the J879 

models) 

River miles 

below 

Spirit Lake 

17.0 -----------------

39.0 -----------------

53.0 -----------------

2 54.3 -----------------

63.4 -----------------

4 73.1 -----------------

CASE II 

Discharge 

(ft3/s) 

557,000 

176)000 

174,000 

3 183,000 

138,000 

72,500 

Elevation 1 

(ft) 

1,209 

450 

85 

65 

36 

5.5 

1 Elevations are referenced to National Geodetic Vertical Datum of 1929. 2 River mile 54.3 is first cross section on Cowlitz River (fig. 5). 3 A constant flow of 9,000 ft3 /s was assumed to come from the upper reach of the 

Cowlitz River. 

• River mile 73.1 is the mouth of the Cowlitz River. The downstream boundary condition adopted was a constant water surface elevation of 5.5 ft. 

this location is affected significantly by tides, but 

for simplification the average elevation was 

adopted. 

The results of the calculations for CASE II, 

using J879, from mile 17.0 below Spirit Lake to 

mile 73.1 are given in table 2. The discharge 

estimate of 138,000 ft3/s at mile 63.4 can be used as 

a point of context. This discharge corresponds to 

about a 500-year flood for an unsilted Cowlitz 

River channel. It is necessary to note, however, 

that sedimentation following mudflows on May 

18-19 markedly reduced the floodwater capacity of 

the lower Cowlitz River. 

A comparison of results obtained using DBFF 

and J879 was possible below mile 17 on the North 

Fork Toutle and Toutle Rivers. The flood routing 

technique in J879 gave considerably more attenuation than DBFF (fig. 4.). The explanation is not 

obvious and requires further research. It is possible that J879, at least for large, steep-fronted flood 

waves, may contain some artificial numerical attenuation, especially where reach sizes and time 

steps are large. The attenuation may be the result 

of using the linear implicit finite-difference 

algorithm. The attenuation of the flood wave 

yielded by the DBFF model was so small that it is 

somewhat questionable, especially in reaches of 

the Toutle River downstream from the junction of 

the North and South Forks. 

ELK ROCK DAM HAZARD 

The surface of the debris clogging in the North 

Fork Toutle River was highly hummocky, with 

many depressions and pits, some of which contained ponded water. The debris dammed the 

mouth of several tributaries of the North Fork, including Maratta and Coldwater Creeks. During July and early August 1980, a small lake impounded 

at the mouth of Maratta Creek (fig. l) broke 

through its dam as a result of saturation of the dam 

material and overtopping. The lake water flowed 

downstream a short distance where it became 

trapped behind an obstruction on the hummocky 

surface of the debris deposit. The obstruction soon 

failed and the water moved 4 mi farther 

downstream, where it collected behind another 

obstruction, forming a 250 acre-foot lake near Elk 

Rock. 

The new lake presented a flood hazard and the 

Geological Survey personnel, thus, made a series 

of dam-break calculations using DBFF and input 

data obtained from onsite observations. The dam 

was 112 ft high and 215 ft wide. The ponded water 

was 30 ft deep, placing the pond surface 82 ft 

above the base of the dam. 

The model computations were made on August 

26. The next day, the dam failed following a 

rainstorm. Over a period of several hours, 

Geological Survey personnel observed the 

breaching of the dam (which took about 40 min) 

and the downstream movement of the flood wave. 

120 I 

110 r- RIVER MILE 54.3 -

100 -

90 - 80 r- -

70 

~ -

60 r50 rO'l 40 r- N 30 O'l 

~ .- 80 LL. 

0 0 70 ....I RIVER MILE 63.4 w ~ 60 CD :J 50 

0 1-

<( 40 

z 0 30 <( 

w ....I 20 

<( > 10 

0 ~ 0 CD 1-

<( a: -10 w 1- > w 80 w u LL. 70 RIVER MILE 68.1 - f= 

~ w 60 -

0 50 -

z 0 

w 40 r- -

0 (.9 30 r- v -

f= ....I 20 r- - <( <( 10 - > z w 0 Or- - ....I 

w f::-10 

<( 

z 60 

50 RIVER MILE 73.1 -

40 

30 - 20 

~ - 10 -

0 

-10 

-20 r0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 LO ~ LO 0 LO 0 LO 0 LO 

N N ("') ("') "¢ "¢ 

TRANSVERSE DISTANCE, IN FEET 

FIGURE 5.-Selected pre-, May 18, 1980, eruption cross sections 

for the Cowlitz Rive;r valley utilized in model studies. 
TABLE 3.-Comparison of discharge simulated by model runs 

with measured or estimated peak discharges on the 

North Fork Toutle and Toutle Rivers for the Elk Rock 

dam failure. 

River miles 

below 

Elk Rock 

0. 0 ------------

5.6------------

3 26.4 ------------

4 41.6 ------------

Simulated 

discharge 

(ft3/s) 

1 20,000 

10,400 

1,500 

Measured or estimated 

discharge (ft3/s) 

U.S. Corps 

USGS of Engineers 

2 16,000 

2 8,000 

5 2,660 

5 1,400 

2 16,000 

2 8,500 

1 Average of discharges computed using hypothetical failure widths of 60, 80, and 

100ft. 2 Visually estimated. 

3 Toutle River at Silver Lake gage. 

• Toutle River at U.S. Highway 99 gage. 

6 Measured. 

Visual estimates of flood discharge were made by 

Geological Survey and U.S. Corps of Engineers 

personnel at several locations, and discharge was 

measured at two gage sites. Table 3 compares the 

computed peak discharge at various locations with 

the estimated and measured peak discharges. The 

computed discharge at the dam was based on the 

assumed formation of a near-vertical 30-ft slot (100 

percent breach) in the 112-ft high and 215-ft wide 

plug during a period of 0.1 hr. Calculations were 

made and results averaged using hypothetical 

failure widths of 60, 80, and 100 ft. The actual 

failure occurred progressively but erratically during 40 min, and resulted in an irregular opening 

about 52 ft high. 

Considering the hypothetical assumptions on 

which the model calculations were based, the comparison between the simulated and the 

estimated/measured discharges is considered to be 

good. 

The Elk Rock dam failure resulted in damage to 

channel-maintenance equipment in the North Fork 

Toutle River, but no loss of life or injuries were 

incurred. The success of the preevent prediction 

underscores the utility of computer-based models 

for estimating flood hazards resulting from dam 

breaks. 

CONCLUSIONS 

Flood hazards posed by potential dam failures 

were evaluated under hurried, emergency conditions in order to plan for and minimize dangers of 

flooding associated with the volcanic eruption and 

debris flow. Investigations included the debris pile 

impounding Spirit Lake and the dam formed at Elk 

Rock. The DBFF model was used to evaluate 

hypothetical failures and to route flows to the 

Cowlitz River. A second model (J879) was used to 

route flow down the Cowlitz to the Columbia River. 

For both models, uneven levels of data accuracy 

and completeness hampered computation of 

reliable hazard estimates. 

The Elk Rock dam failed the day following the 

theoretical failure analysis, providing an opportunity to compare hypothetical and actual results. 

The comparison between the simuiated and 

estimated discharge is good, indicating that computer models can play a useful role in the assessment of dam-break flood hazards, even though input data are approximate and incomplete. 

REFERENCES 

Fread, D. L .,1976,Flood routing in meandering rivers with flood 

plains: Third Annual American Society of Civil Engineers, 

Rivers, '76, Symposium of Waterways, Harbors, and 

Coastal Engineering Division, August, Proceedings, v. I, p. 

16-35. 

--1980, DAMBRK: The NWS dam-break flood forecasting 

model: Silver Springs, Maryland, Office of Hydrology, National Weather Service (NWS) report, 37 p. 

Land, L. F., 1978, Unsteady streamflow simulation using a 

linear implicit finite-difference model: U.S. Geological 

Survey Water Resources Investigations 78-59, 59 p. 

--1980, Evaluation of selected dam-break flood-wave 

models by using field data: U.S. Geological Survey Water 

Resources Investigations 80-44, 60 p. 

Y oud, T. L., and Wilson, Raymond, 1980, Stability of Toutle 

River blockage: Mount St. Helens hazard investigation: 

U.S. Geological Survey Open-File Report 80-898, 14 p. 

i:r U.S. GOVERNMENT PRINTING OFFICE: 1981- 341-614/229 

I7