Storm-Induced Slope Movements 

at East Brady, Northwestern 

Pennsylvania 

U.S. GEOLOGICAL SURVEY BULLETIN 1618 

Storm-Induced Slope Movements 

at East Brady, Northwestern 

Pennsylvania 

By John S. Pomeroy 

Description of the August1980 debris 

avalanching and other slope movements 

and a discussion of the factors involved 

U.S. GEOLOGICAL SURVEY BULLETIN 1618 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1984 

For sale by Distribution Branch 

Text Products Section 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, Virginia 22304 

Library of Congress Cataloging in Publication Data 

Pomeroy, JohnS., 1929-

Storm-induced slope movements at East Brady, northwestern 

Pennsylvania. 

cU.S. Geological Survey bulletin; 1618) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1618 

1. East Brady <Pa.)--Landslide, 1980. 

2. East Brady (Pa.>--Storm, 1980. I. Title. II. Series. 

QE75.B9 no. 1618 557.3.s [551.3'53] 84-600115 

[QE 599.U5] 
CONTENTS 

Abstract 1 

Introduction 1 

The East Brady storm 2 

Geology of the East Brady area 4 

Slope movements opposite East Brady 4 

Area A 4 

Area B 6 

Area C 7 

Area D 8 

Area E 9 

Slope movements east of East Brady 10 

Discussion 12 

References 16 

FIGURES 

1. Map of the East Brady region, Pennsylvania, showing total rainfall 

for the August 14, 1980, storm, and prestorm rainfall for 1 5-day and 

30-day periods 2 

2. Map of slope opposite East Brady, Pa., showing slope movements and 

geologic features 3 

3-9. Photographs showing slope movements opposite East Brady 

3. Large rock slump opposite East Brady, Pa., November 1976 5 

4. Slumped bedrock 5 

5. Large rock slump opposite East Brady, Pa., November 1980 6 

6. Head of longest debris avalanche 7 

7. Amphitheater-shaped head scar of debris avalanche 8 

8. Part of head of debris avalanche 9 

9.Siopemovementsin 1980 10 

10. Map showing slope movements east of East Brady 11 

11. Photographs showing slope movements east of East Brady 12 

12-13. Schematic diagrams showing: 

12. Colluvial slope movements 14 

13. Rock slump and subsequent colluvial movement 15 

Contents Ill 

Storm-Induced Slope Movements at East 

Brady, Northwestern Pennsylvania 

By JohnS. Pomeroy 

"Every trip taken into the Appalachian Plateaus provides continuing education 

about the full scale and scope of mass movement of loose material. There is so 

much to learn from observing the signs of active slumping and mass wastage 

that one cannot but wonder why such phenomena are not better known and 

more widely publicized." 

Abstract 

The East Brady, Pa., area was subjected to an 

intense rainfall during the night of August 14, 

1980, resulting in debris avalanching and other 

slope movements. Prestorm rainfall was 

substantially above normal during the 1980 

summer. 

Almost all the debris avalanching took place 

along existing hillside depressions. In comparison 

with the geomorphic effects of the 1977 Johnstown 

storm, the greater concentration of slope 

movements at East Brady is a reflection of steeper 

slopes and a higher drainage density. Colluvium is 

minimal and bedrock is exposed in almost all scars. 

The bedrock-colluvium interface served as the 

slippage surface for most of the 1980 storm slope 

movements. Head scarps revealed a concentration 

of water or seeps at contacts of permeable and 

impermeable horizons. About 90 percent of the 

slope movements originated at or below the Lower 

Kittanning coal bed, which lies below a thick 

sandstone section. 

Slope movement involving both colluvium 

and bedrock has been continual throughout 

historic and prehistoric times. One historic bedrock 

slump measures 60 m wide at the head and 137 m 

at the toe. The 1980 storm reactivated 20 to 25 

percent of the deposit, but all movement took 

place in colluvium where slippage was at or close to 

the bedrock-colluvium interface. Only one 

conspicuous hollow along the slope opposite East 

Brady showed no storm-induced movement from 

the 1980 storm; however, extensive hummocky 

material in that hollow attests to older slope 

movements. 

Stress release along valley walls has 

contributed to an increase and widening of joints 

and an opening of bedding-plane fractures. 

Byron N. Cooper (1969, p. 66) 

Movement of separated blocks and slumping of 

rock masses all contributed to the formation of 

colluvium. Subsequent buildup of weathered rock 

and soil and infrequent flushing out of that 

material has led to colluvial slope movements such 

as those that occurred in August 1980. 

Storm-induced movements in August 1980 

affected about 2.5 percent of the slope surface in 

two areas adjacent to East Brady and ranged in 

density from 56 to 85 movements per square 

kilometer. A minimum of 50,000 tons of regolith is 

estimated to have been transported along the 

slopes opposite and east of East Brady. About one 

quarter of this amount is believed to have entered 

the Allegheny River. 

INTRODUCTION 

The area adjacent to East Brady and Bradys 

Bend, Pa., was subjected to more than 13 ern of rain 

on the evening of August 14, 1980 (fig. L Pomeroy, 

1980a). Slope movements, chiefly debris 

avalanches of saturated colluvium, occurred during 

the storm along the steep sides of the Allegheny 

River valley (fig. 2). 

A slope-stability reconnaissance study of 

western Pennsylvania based on aerial photographic 

interpretation and local field investigations 

(Pomeroy, 1978) delineated an extensive rock slump 

opposite East Brady as well as deposits of other slope 

movements. These other slope deposits, mostly 

prehistoric, do not necessarily represent single 

events but resulted from successive movements of 

saturated regolith. Limited radiocarbon dating of 

similar deposits from the Upper Ohio Valley 

indicate a Wisconsinan and post-Wisconsinan age 

(Gray and others, 1979). 

Introduction 1 
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EXPLANATION 

-10(4)- lsohyet for August storm showing 

precipitation in centimeters (inches) based on rainfall data from 

NWS 

- 10(4)j_ lsohyet showing prestorm rainfall for 

15-day period. Arrow indicates direction of increasing rainfall 

- - 18(7)- lsohyet showing prestorm rainfall for 

30-day period 

10 20 MILES 

20 30 KILOMETERS 

PENNSYLVANIA 

Figure 1. East Brady region, Pennsylvania, showing total 

rainfall for the August 14, 1980, storm, and prestorm rainfall 

for 15-day and 30-day periods. 

The purpose of the current investigation was to 

describe the features of slope movements caused by 

this locally severe storm and to relate the geologic 

and other factors, including slope and rainfall, to the 

slope movement in the East Brady area. A few days 

were spent in the area on each of three occasions--

immediately after the storm in August 1980, in midNovember 1980, and in early May 1981. During the 

earlier visits, slopes to the northeast along the 

2 Storm-Induced Slope Movements at East Brady, Pennsylvania 

Allegheny River and along Sugar Creek (figs. 1, 2) 

were also examined. Aerial photographs taken in 

1938, 1972, and 197 4 provided evidence of some 

recent slope movements. 

In this report, I have adopted the term "slope 

movement" rather than "landslide" as a general, 

inclusive term. "Landslide" has been widely used as 

an all-inclusive term for almost all types of slope 

movements, including some that involve little or no 

sliding (Varnes, 1978, p. 11). The popular 

geomorphic term "mass movement" is synonymous 

with "slope movement" and is not used in this 

report. 

The term "debris avalanche," the most common 

type of slope movement feature produced by the 

storm, was originally defined by Sharpe (1938). A 

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" (Sharpe, 1938, p. 61). Varnes (1978, 

p. 18) continued to use the term and defined it as a 

variety of very rapid to extremely rapid debris flows. 

Both authors concluded that the transport 

mechanism for debris avalanches is chiefly flowage 

of the viscous soil-water matrix. 

Although many workers in the Eastern United 

States have adopted the term "debris avalanche," 

other investigators have preferred the term "debris 

slide" to describe similar movements I Pomeroy, 

1980b). The term "debris slide" is used herein to 

define only those movements that are commonly 

rapid and that involve translational or planar 

movement without flowage (Varnes, 1978). 

"Slumps" are regarded by all workers as rotational 

movement and are either rock slumps involving 

bedrock or earth slumps involving colluvium. 

THEEASTBRADYSTORM 

An intense rainfall during the evening of 

Thursday, August 14, 1980, extending with 

diminishing force into early Friday morning, 

August 15, was centered over East Brady and 

affected parts of Armstrong, Butler, and Clarion 

Counties. 

The town of Bradys Bend along Sugar Creek 

(fig. 1) was the most severely affected community 

because of the flash flooding. The thunderstorm 

began about 8 p.m. and became intense between 9 

p.m. and 10 p.m. The rainfall was the heaviest ever 

experienced by several long-term residents. 

Flooding along Sugar Creek began at about 11:30 

p.m. (August 14) and peaked 1 t to 2-! hours later. 

The creek was back within its banks by 9 a.m. 

(August 15). Nine people lost their lives in the 
25 50 MILES 

25 50 75 KILOMETERS 

EXPLANATION 

HISTORIC SLOPE MOVEMENT 

_,- August 1980 

cJ Post-1938 pre-1974. Largely coal spoil 

~ Pre-1938 

PREHISTORIC SLOPE MOVEMENT 

1"'-- c..._l Individual and coalesced deposits. Includes undifferentiated historic 

movements on slope opposite East 

Brady 

IPa IPc --- - Contact, approximate to concealed 

IPp IPa (modified from Butts, 1904) 

-F-- -LK- Coal bed, approximate to concealed 

(modified from Butts, 1904) 

___ r:n_q_ __ _ 

~ 

Acid mine drainage 

Strike and dip of beds 

Strike and dip of joints 

__.?_S inclined 

--- vertical 

A Area discussed in text 

IPc, Conemaugh Group 

IPa, Allegheny Group 

F, Freeport coal 

LK, Lower Kittanning coal 

IPp, Pottsville Group 

Figure 2. Slope opposite East Brady, Pa., showing slope movements and geologic features. 

Bradys Bend area, and an estimated $50 million 

worth of property was damaged or destroyed 

(National Weather Service, 1980). 

The heaviest recorded precipitation (14 em) 

occurred at an unofficial rain gauge at Queenstown 

(Ralph Folino, National Weather Service, oral 

commun., 1981), which lies 1 km north of Bradys 

Bend. :\1ore rain probably fell at higher elevations. 

Because rain gauges are commonly sited in valley 

communities or on farmlands, rainfall 

measurements do not give a true indication of 

precipitation falling on hillside forestland. Lee 

(1980) stated that the annual amount of 

precipitation is about 4 to 8 em greater for each 100 

m of elevation. In addition to receiving more rain, 

upland forests lose less water by evaporation 

because of lower temperatures, and hence maintain 

wetter soils than do valley communities and 

farmlands (Eschner and Patrie, 1982). 

that 

The National Weather Service (1980) reported 

The locally heavy rain occurred at the intersection of two 

instability lines. One instability line had moved into 

Pennsylvania during the afternoon and then stalled east to 

west through central Pennsylvania. A second line in a 

north to south orientation, and, with associated 

thunderstorms, moved in from Ohio late in the afternoon. 

This second line intersected with the first line during the 

evening of the event. The net effect was to increase the 

intensity and duration of rainfall at the point these two 

lines intersected. 

Rainfall data for western Pennsylvania for 

July and early August show that the ground was 

well saturated because of above-normal rainfall in 

the weeks preceding the event (fig. 1). Precipitation 

for the 2-week period ending August 14 ranged from 

9 em at Clarion to 11.5 em at Kittanning. The 4-

week prestorm rainfall ranged from 18 em at Butler 

The East Brady Storm 3 
to 24 em at Clarion. Total precipitation for the 

period June 1 to August 14 was 37 em at Butler and 

39 em at Kittanning, much above average. 

GEOLOGY OF THE EAST BRADY AREA 

The region is underlain by cyclothemic sections 

of alternating beds of shale, siltstone and sandstone, 

and coal of Pennsylvanian age. Butts' bedrock 

geologic map ( 1904) shows the slope opposite East 

Brady to consist predominantly of beds of the 

Allegheny Group, which is approximately 106 m 

thick. A maximum of 18 m of beds of the 

Conemaugh Group caps the highest parts of the 

upland surface. The poorly exposed and less steep 

lower slope is underlain by the Pottsville Group. 

Most of the rock that crops out along the slope in 

enlarged drainages is shale and lesser amounts of 

siltstone and sandstone. Two coal beds, Freeport 

and Lower Kittanning, have been mined here in the 

past. Claystone forms an underclay beneath coal 

beds. Limestone is rare and occurs only in the lower 

part of the Allegheny Group. Outcrops, mostly 

resistant sandstone, are conspicuous only above the 

Lower Kittanning coal, which occurs two-thirds of 

the way up the slope. 

The attitude of the rocks is subhorizontal with 

a regional dip of less than 1°. 

The East Brady area lies 32 km southeast and 

downstream of the maximum extent of Pleistocene 

glacial ice (Illinoian and Wisconsinan stages). The 

Allegheny Valley was a major drainage leading 

from successive ice borders. In this periglacial 

environment, the Allegheny Valley was twice filled 

with glacial outwash deposits (Butts, 1904). Since 

glacial times the Allegheny River has been eroding 

its present channel in these deposits. 

Soils along the East Brady slopes are classified 

as Weikert-Gilpin soils (USSCS, 1977) formed in 

colluvium and debris derived from interbedded 

shale, siltstone, and fine-grained sandstone. The 

soils are typically shallow and well drained, and rate 

of permeability is moderately rapid. 

Incised drainages along the lower parts of 

slopes indicate that the maximum thickness of 

colluvium is about 5 to 6 m. The colluvium is most 

extensive on c9ncave (sidewise) slopes below 

sandstone ledges. 

X-ray diffraction analysis of light-gray clay 

from head areas of slope movements along the slope 

opposite East Brady shows illite to be 70 to 81 

percent of the sample and kaolinite, 19 to 30 percent. 

Both illite and kaolinite are moderately well 

crystallized, and mixed layering (if any) is barely 

4 Storm-Induced Slope Movements at East Brady, Pennsylvania 

discernible (Virginia Gonzalez, written commun., 

1982). 

Clay from a 19-m-wide storm-induced earth 

slump at the Queenstown Road-Whiskey Run 

intersection, 2.5 km northwest of East Brady, was 

examined for its mineralogy. The dark-gray silty 

clayey colluvium from the Allegheny Group 

contained chiefly illite and subordinant kaolinite 

(Gloria Hunsberger, written commun., 1981). The 

illite is potassium-deficient, which is a condition 

prevalent elsewhere in shales and mudstones of 

Devonian to Permian age in western Pennsylvania. 

Fisher and others (1968) documented a similar 

potassium deficiency in the unstable illitic red 

shales and mudstones of Pennsylvanian and 

Permian age in southeastern Ohio. 

SLOPE MOVEMENTS OPPOSITE EAST 

BRADY 

A description of the slope movements opposite 

East Brady is keyed to figure 2 (A-E). Individual 

slope movements are discussed from north to south 

for areas A to E. 

Area A 

An inventory of slope stability features in 

western Pennsylvania revealed an apparent 

massive slope movement that had taken place 

opposite East Brady (figs. 2 and 3). Field inspection 

confirmed the presence of a rock slump. Slumped 

bedrock containing strata dipping moderately back 

toward the slope (fig. 4) could be differentiated 

among dislocated sandstone blocks on the very 

gently inclined bench (fig. 3, area A), in front of a 

bedrock-faced headwall. The width of the slump in 

the head area is approximately 60 m, increasing 

downslope to 137 m at the slump foot. Because the 

arcuate head scarp is easily recognized, the slope 

movement is probably historic . The movement 

probably occurred before 1920 because none of the 

older inhabitants of East Brady who were 

interviewed remembered the event. Furthermore, 

some trees as old as 60 years are rooted on the slump 

block. Slanted and curved tree trunks indicate that 

creep has occurred since the initial movement. 

No evidence points to any recent deep-seated 

reactivation of the slump block. However, the 1980 

storm reactivated parts of the deposit below the 

bench (fig. 5) affecting about one-fifth to one-fourth 

of the entire slump block. The most recent slope 

movement was shallow and took place in colluvium 

where slippage was at, or close to, the bedrock-
Figure 3. Large rock slump opposite East Brady, Pa., photographed in November 1976 (fig. 2, area A). Note indication of arcuate head 

scarp on pre-1938 rock slump. A is locale of figure 4. Subhorizontal bedrock adjacent to slump at B. 

colluvium interface. Although outcrop was sparse 

along the fresh scars except in the head areas, 

weathered shale, carbonaceous shale, and 

sandstone float were abundant. Steep slopes above 

the sandstone headwall have been scarred by small 

debris slides. Small hollows a few meters wide 

above the old main scarp directed storm runoff water 

from the 13° (24 percent) sloping grassland on top. 

Scarring of the steeper slopes lying above the main 

scarp resulting in removal of thin soil and 

vegetative cover has made the head scarp more 

visible than before (fig. 5). Some precariously 

perched weathered bedrock along the nearly vertical 

headwall of the slump fell as a result of the 1980 

storm. 

The slope in the head areas of the two 

conspicuous movements ranged from 34° (68 

percent) to 38° (78 percent). Each movement was 

approximately 21 m wide, and main scarps were 

1.5 m high. No loss of life resulted from the 

reactivation of part of the older deposit. However, 

several mobile homes were extensively damaged, 

and at least two cars were destroyed. 

A possible prehistoric rock slump was found 

150m northeast of this area (fig. 2, area A). 

Figure 4. Slumped bedrock that lies within large historic rock 

slump (fig. 2, area A). 

Slope Movements Opposite East Brady 5 
Area B 

The highest debris avalanche originated about 

12 m below the upland rim (fig. 2). The head was 6 

m wide and developed in thin sandy colluvium along 

the steep south side of a southeast-trending hollow. 

Seeps lie 15 m below the head at about 1,070 feet 

altitude at the contact of a 0.5-m-thick bluish-gray 

claystone and an overlying sandstone bed. Incised 

drainage below the main spring revealed a 12-m 

section largely of shale but containing minor very 

thin beds of sandstone. The path of the debris 

avalanche widened to 12m before it joined the main 

hollow but then narrowed considerably, confined to 

the preexisting drainage. The height of the 

movement was shown by dried clayey silt 2.5 m 

above the ground on the bark of an undisturbed tree 

along the avalanche path. The debris avalanche 

stopped before it reached the Allegheny River 

because of a gentle lower slope. 

About 60 m to the south and along a 41, (85 

percent) slope, an 8.0-m-wide debris avalanche head 

was located in very thin (less than 1 m) colluvium 

overlying splintery carbonaceous shale. The flow 

followed a gulch that is not reflected by the 

topographic contours. Transported debris included a 

large tree that battered a trailer. 

The head of a 6-m-wide debris avalanche was 

found along a 100-percent- grade planar slope. Less 

than 1 m of soil covered very thinly bedded shale and 

sandstone in the uppermost 2 m of the head. Above 

the head, a 15-m-wide concave-shaped (sidewise) 

slope had concentrated the water feeding into the 

disturbed area. Thin-bedded sandstone ledges flank 

the more extensive head area of the older slope 

movement. The debris avalanche cascaded over a 6-

m cliff composed of a 4.5-m-thick shale section 

capped by 1.5 m of sandstone and widened as it fell 

into a nearly 100 percent grade, 18-m-wide, bowlshaped depression. This concave-shaped area is 

itself the head area of a former larger slope 

movement. Material from the 1980 debris 

avalanche here consisted of coarse shale and 

sandstone fragments surrounded by a clayey silt 

matrix. The debris avalanche narrowed downslope 

and terminated on a more gentle slope, except for a 

highly fluid clay-rich appendage that stopped within 

a few meters of a mobile home. 

Along the lower slope to the south, two 7.5-mwide (head area) movements occurred along a 37° (77 

Figure 5. Large rock slump opposite East Brady, Pa., photographed in November 1980 (fig. 2, area A). Compare with figure 3. 

Scarring of headward margins at A. Slope movements (8} occurred during August storm. Site C (at foot of recent flow) is former 

location of trailer seen on figure 3. 

6 Storm-Induced Slope Movements at East Brady, Pennsylvania 
Figure 6. Head of longest debris avalanche (fig. 2, area C l. !'-l"ote thinness of removed regolith in head (Al and pileup of material at 

bench in foreground (8). The debris avalanche continued downslope from bench. Person (C) is at lateral margin of slope movement. 

percent) planar slope. They narrowed downslope, 

converged, and flowed into the river. The foot of the 

combined movements later became part of a 15-mwide, 4.5-m-long earth slump with a 1-m-high scarp 

at the river bank. 

Hummocky terrain, which includes mounds 2.5 

m high and 15 m wide along the lower slope, attests 

to prehistoric slope movement. 

Area C 

Two heads of debris avalanches (at 970 and 

1,060 feet altitude) contributed colluvium to a 

conspicuous V -notched drainage (fig. 2). The 3-mwide upper slope movement along the south side of 

the drainage took place in thin soil above 

interbedded sandstone and shale. Below the head 

scarp, debris was wrapped around the uphill side of a 

tree and extended 1 m above the ground surface. 

The lower head along the north side of the drainage 

showed carbonaceous shale and light-gray 

underclay. The combined detritus from the gulch 

formed an 18- to 24-m-wide fan at the edge of the 

river. A 30-m section of shale interbedded with thin 

sandstone crops out in the enlarged gulch. 

In the midst of a larger old slope movement, 

where living trees are scarce among vines and briar, 

a 9.0-m-wide movement started at 980 ft altitude 

and extended downslope for only 15 m. The head 

scar lies within a shale sequence below a sandstone 

ledge (and within a laterally concave slope or 

shallow gully not discernible on the map). 

A trio of narrow debris avalanches occurred in 

the same older slope movement area previously 

mentioned. All the movements originated in silty to 

clayey colluvium derived largely from shale. The 

debris avalanches, approximately 10m wide at their 

heads, moved down a 40° (82 percent) slope. 

The longest (180 m) debris avalanche 

originated on the south side of a wide, steep, concave 

(sidewise) slope 10 m below the rim (fig. 6). The 

debris avalanche was 4.5 m wide at the head~ sandy 

fragmental colluvium, 0.3 to 1.0 m thick, was 

exposed along its margin. The unusually steep (64°) 

bedrock-colluvium interface and a conspicuous 

north-south trending joint dipping 75° to the east 

were noted at the head of the slope movement. 

The debris avalanche broadened downhill over 

a very steep (greater than 100 percent) slope and 

reached a maximum width of 12 m in an area of 

Slope Movements Opposite East Brady 7 
abundant seeps slightly below a sandstone bench 

The slope movement followed the V-drainage, and 

the width narrowed to less than 6 m. The foot 

widened laterally to 24 m where the slope 

moderates immediately above the shoreline. A 

poststorm, 12-m-wide debris slide that formed at 980 

feet altitude on the south side of the channel choked 

the path of the debris avalanche. Similar poststorm 

tensional features have formed near Johnstown 

(Pomeroy, 1982) and can be attributed to a lack of 

support along the slope bordering the deepened 

debris avalanche channel. 

Area D 

A 15-m-wide debris avalanche along the south 

side of a laterally concave slope (fig. 2) showed thin 

colluvium overlying shale at its head. The 

sandstone-ribbed cove steepens above the head scar. 

The conspicuous drainage to the north appears 

to be the only one along the slope opposite East 

Brady without any evidence of recent storm-induced 

mass movement, probably because of the extreme 

thinness of colluvium. However, hummocky 

material from ancient slope movements covers the 

lower slope. 

About 12 m below the highland rim, a head 

scar (fig. 7) of a debris avalanche exposed 4.5 m of 

interbedded shale and sandstone with the lowermost 

half in splintery brown shale. The 8-m long by 10-m 

wide amphitheater-shaped head revealed a 6-mhigh main scarp cut at an angle greater than 50°. 

The adjacent slope measures 38° (78 percent). 

Although the debris avalanche commenced as an 

earth slump, it cascaded and flowed downslope over 

a 45°-50° (> 100 percent) surface and maintained a 

width of 7.5-9.0 m. The debris avalanche stopped on 

the conspicuous bench below 1,100 feet. 

A 13-m-wide debris avalanche started at the 

rim of the bench directly in line with the upper slope 

movement and continued downslope for about 100 

m. 

A large debris avalanche (fig. 8) originated at 

about 1,050 feet altitude at the edge of the bench 

south of the previously mentioned movement. The 

main part flowed downslope until it stopped along 

the more gentle slope about 30 m from the river. In 

its lower extremity the flow bifurcated, and a 4.5- to 

6.0-m-wide segment flowed to the river. The fact 

that the main path was cut below the level of the 

intersection of the fork indicated that the appendage 

represented an earlier event during the storm. 

The head (fig. 8), nearly 30 m at its widest 

dimension, is part of a slightly larger concave 

(sidewise) slope. The adjacent slope measures 39° 

8 Storm-Induced Slope Movements at East Brady, Pennsylvania 

Figure 7. Amphitheater-shaped head scar of debris avalanche 

dig. 2, area 0\ with outcrop <al in left center of photograph. Shirt 

hanging on tree indicates scale. 

(80 percent). Carbonaceous and noncarbonaceous 

shale and small amounts of sandstone form the face 

of the 45° (100 percent) scarp. Slippage occurred at 

the bedrock-colluvium interface, and, as a result, a 

thickness of about 1.5 m of colluvium was removed 

from the head area. Small seepages were observed 

along the head scarp where boulder colluvium 

overlies thin sandstone, coal, and claystone. This 

part of the slope has been subjected to previous 

historic slope movements as documented on aerial 

photographs. 

The 4 o to 5° westward dip of beds exposed in the 

present scarp (fig. 9A) indicates that the recent 

failure occurred in an older rock slump of probable 

prehistoric age. The carbonaceous shale and coal in 

the colluvium below the scarp are similar to the 

slope debris at locality A. The very gently sloping 

bench above the main scarp is hummocky with 

sandstone boulders to the base of the cliff and 

resembles the slope movement at locality A. Trees, 

at least 60 to 65 years old, are growing on massive 
sandstone blocks. The fact that they are not slanted 

nor do they show curved trunks indicates a lack of 

recent creep movement. One slump block dips 37° 

back toward the slope (fig. 98). 

A large prehistoric coarse rock debris deposit 

lies to the south. The bouldery surface containing 

large sandstone blocks is masked by a forest no 

younger than that on the adjacent slope to the south. 

The movement was 85 m wide at its base and caused 

the shoreline to bow out slightly in this area. 

At the head, a sandstone cliff nearly 30 m high 

with closely spaced (1.0 to 1.5 m) joints trending 

north-south and dipping 70°-85° to the east supplied 

the rock debris. 

Area E 

An adit in the Lower Kittanning coal is found 

at about 1,085 feet altitude. Acid mine drainage is 

copious (fig. 2) and shows the influence of the 

underlying impermeable underclay. Though no 

slope movement occurs at this horizon, the position 

of the underclay indicates lithologic control for 

seepage. Also, the coal has closely spaced joints, 

which makes the coal an excellent aquifer. 

The head scarp of a debris avalanche 

originating at the 1,000-ft contour was 25 m wide 

and occupied the north side of a larger older slope 

movement. Shale and coal were exposed in the 

headwall along a 36° (72 percent) slope. A few 

sandstone ledges lie immediately above the head. 

The debris avalanche continued to the river where a 

20-m-wide earth slump took place about 15 m above 

the shoreline. 

Small scars 4.5 to 6.0 m wide at the head of a 

small cove were observed just below a nearly vertical 

slope of sandstone. The highest scar shows 1. 5 m of 

Lower Kittanning coal and underclay along a 39° (80 

percent) slope. Seepage at the coal-underclay contact 

probably caused the colluvial slope movement. 

Several episodes of historic slope movement 

have originated in the major gulch east and 

southeast of a prominent knob affected by strip 

mining (fig. 2). The most recent movement, part of a 

wider concave (sidewise) slope where sliding and 

flowage have occurred in the past, began a few 

Figure 8. Part of 30-m-wide head of debris avalanche. Small seepages are common above person (center of photograph) where 

sandstone boulder colluvium and sandstone overlie thin coal and claystone. Gently sloping surface above scarp is part of head area of 

prehistoric rock slump (fig. 2, area m. Note person for scale. 

Slope Movements Opposite East Brady 9 
meters below the level of the Low·er Kittanning coal 

and underclay along a 36° (72 percent) slope. Shale, 

mostly fissile, crops out along the head scarp and 

path below. Tree growth of various ages surrounding 

the head area suggests that several historic episodes 

of slope movement have taken place. Seepage was 

noted at a sandstone-shale contact near the top of 

the head scarp. With the addition of water from the 

overlying seep at the Lower Kittanning coalunderclay contact, the pore-water pressure of the 

thin soil probably exceeded its shearing resistance, 

and the debris avalanche was triggered. Extensions, 

6 to 9 m wide, have taken place post-dating the 

storm above both sides of the head scarp . The 

terminus of the August 1980 debris avalanche lies 

on topofanold (pre-1938) flow. 

The debris avalanche at the south end of the 

area shown in figure 2 showed a head area 

containing shale along a 38° (78 percent) slope below 

a much steeper slope of sandstone and shale. The 

slope movement took place in the colluvial soil at or 

immediately below the contact of the permeable and 

impermeable horizons. 

SLOPE MOVEMENTS EAST OF EAST BRADY 

The August 1980 debris avalanches, debris 

slides, and slumps also took place elsewhere in the 

East Brady area, particularly along the north-facing 

slope 1.5 to 3.0 km east of East Brady (fig. 10). 

Lesser affected areas included the slopes 3.0 to 8.0 

km up the river from East Brady, in the Templeton 

region (16 km southeast of East Brady along the 

Allegheny River), and the Sugar Creek valley and 

adjacent tributaries west of East Brady (fig. 1). Only 

the north-facing slope east of East Brady will be 

discussed. 

In at least 10 locations, an unimproved dirt 

road at the base of the slope was blocked by soil, 

rock, and tree material from debris avalanche 

channels. Most of these toe deposits were 9 m in 

width or less, and average thickness was slightly 

more than 1.5 m. 

Most of the debris avalanches east of East 

Brady occupied well-defined drainage channels 

showing nearly continuous bedrock exposure. Few 

were more than 120m long; most involved less than 

83 m relief and were confined to the lower half of the 

slope. Debris avalanches on 100 percent grade or 

steeper slopes were uncommon because of an 

extremely thin cover of colluvium. The lower slope 

is steeper than that opposite East Brady and shows 

onlv thin W-5 m) colluvium. Much of the material 

alo~g the chutes cascaded over a sandstone cliff 

above the unimproved dirt road and flowed into the 

Figure 9. Slope movements in 1980. A. View across 1980 major slope movement <fig. 2, area min colluvium. Gently slumped bedro~k 

is part of midsection of older (prehistoric) movement. B. Slumped bedrock in foreground dipping 39o back toward slope. Bedrock lS m 

head area of same prehistoric rock slump as in 9A.. 1980 slope movement (a) in background (fig. 2, area Dl. 

10 Storm-Induced Slope Movements at East Brady, Pennsylvania 
0 1 MILE 

0 .5 1 KILOMETER 

EXPLANATION 

HISTORIC SLOPE MOVEMENT 

~ August 1980 

@ Pre-1938 

PREHISTORIC SLOPE MOVEMENT 

Individual and coalesced deposits. Includes 

undifferentiated historic slides 

A,B Slope movements referred to in text 

IPa IPc Contact, approximate (from Butts, 1904; Pp--IP;- Patterson and Van Lieu, 1972) 

-F-- -LK- Coal bed, approximate to concealed 

(modified from Butts, 1904) 

.-!3 Strike and dip of joints 

IPc, Conemaugh Group 

IPa, Allegheny Group 

F, Freeport coal 

LK, Lower Kittanning coa l 

IPp, Pottsville Group 

Figure 10. Slope movements east of East Brady. 

Allegheny River. A greater thickness (by about 

9 m) of Pottsville Group rocks is exposed here than 

in the area opposite East Brady, and the steeper 

topography reflects less shale and more sandstone. 

At locality A (figs. 10, 11A, B) the slope in the 

midsection of the debris avalanche averaged 40° (82 

percent) but lessened to 33') ( 65 percent) at the head. 

The 3-m-high main scarp was within a concave 

(sidewise) slope. Almost 2 m of colluvium was 

estimated to have been removed at the head. The 

head (fig. 11 B) was slightly wider than the chute 

below, which was 6 m wide and 2.5 to 3.0 m deep. 

The path of the debris avalanche measured 135m. A 

minimum of 1,500 m3 of colluvium was probably 

flushed out of the channel and entered the 

Allegheny River. 

As the lower slope moderates slightly eastward 

toward the river bend, a thicker cover of colluvium 

shows a few wider and more irregularly shaped 

debris avalanches (fig. 10). Heads of some scars 

were as much as 25 m wide. 

Along this north-facing slope, evidence of older 

but historic debris avalanches abounded. One old 

30-m-long linear scar, covered with 1.2- to 1.8-mhigh brush, was equal in width and continuous with 

the fresh debris avalanche immediately below it. 

Brushy slope areas, 20-25 m wide, devoid of forest 

growth represent obvious older historic movements. 

Slope Movements East of East Brady 11 
Parts of a few of these areas showed reactivation. 

An roughly equidimensional scar (B, fig. 10) is 

seen in the upper part of the slope. The width and 

length were 45 m and 48 m respectively along a 45° 

(100 percent) slope (fig. 11C). Thin soil and 

weathered rock, most not exceeding 0. 3 m in 

thickness, were stripped off this large area, which is 

underlain by shale and sandstone. The terminus of 

the detritus was along a natural rock bench at the 

base of a 7.5-m-high sandstone cliff which was 

overridden by the debris. Well-developed joints, 

striking N. 80° W. and dipping 73° NE. paralleling 

the river, account for the loose, highly weathered 

regolith that contributed to the rock debris. 

More than 90 percent of the historic slope 

movement along this side of the river has taken 

place beneath the position of the Lower Kittanning 

coal. Seepages along this horizon probably played a 

role in the initiation of slope movement. 

DISCUSSION 

Rainfall, slope, and geologic factors contributed 

to the occurrence of the debris avalanching and 

related movements in the East Brady area. 

The intensity of the 1980 storm was a major 

factor; much of the rain fell within an hour or 

slightly longer period. By comparison, the 

Johnstown storm of 1977 (Pomeroy, L980b, 1982) 

involved more rainfall (as much as 30 em) over a 

longer period of time, but with concentrated rainfall 

during short intervals. Wolman and Gerson (1978, 

Figure 11. Slope movements east of East Brady. A, View across debris avalanche <A in fig. 10). Chute is about 2.6 m deep and 6.0 m 

wide. Slope is 35° (70 percentl. Regolith is 1.5 m thick. 8, Head of debris avalanche <A in fig. 10>. Note break in slope between head 

and chute. Brush-covered linear depression (former debris avalanche site) lies above fresh scar. C, Slope movement scar (fig. 10, area 

Bl along steep slope (45°, 100 percent) east of East Brady. Note extreme thinness of removed regolith. Distance from position of 

camera across slope to other lateral margin is 45 m. 

12 Storm-Induced Slope Movements at East Brady, Pennsylvania 
p. 202) concluded that intensive rainfall is the most 

important prerequisite for extensive slope 

movements in humid montane environments. The 

author concurs with this assessment on the basis of 

both the East Brady and Johnstown investigations 

(Pomeroy, 1980b, 1982) but would add that an 

abnormally high total precipitation during the 

weeks preceding the catastrophic event is a 

significant factor in hillside failure. 

Prestorm rainfall totals were substantially 

above normal for the 15-, 30-, 45-, 60-, and 75-day 

periods during the 1980 summer for a large part of 

western Pennsylvania. Only the 15-day prestorm 

total at Johnstown in 1977 was comparable, but 

below normal precipitation characterized the other 

periods at ,Johnstown. Thus, the prestorm soil 

moisture content in 1980 was significant, especially 

when coupled with the intensity of the August 

storm. I disagree with the conclusions of some 

workers (Eschner and Patrie, 1982, p. 346), who 

suggest that about 13 em (5 inches) of rain per day is 

the approximate threshold amount necessary to 

saturate soil and initiate debris avalanching or any 

other form of mass movement. The wetter the soil at 

the onset of the storm, the less rain is needed to 

start any type of slope movement. Thus, antecedent 

moisture is the determinant factor concerning how 

much rainfall in a single storm is required to initiate 

slope movement. 

Orientation of the affected slopes was a factor 

in the density of the slope movements. East-facing 

slopes are exposed to the sun only in the morning 

when temperatures are lower. Thus, the slope 

opposite East Brady has an inherently high soil 

moisture content at all times. The north-facing 

slope east of East Brady is even slower to dry out. 

More than 90 percent of the debris avalanching 

took place along existing hillside depressions in 

contrast to the siting of debris avalanches at 

.Johnstown (Pomeroy, 1980) but similar to other 

storm-induced phenomena in Appalachia (Hack and 

Goodlett, 1960; Bogucki, 1976, 1977; Williams and 

Guy, 1973; Stringfield and Smith, 1956). The 

greater concentration of the slope movements at 

East Brady, as opposed to Johnstown, probably is a 

reflection of steeper slopes and a higher drainage 

density. Slopes in the head areas of debris 

avalanches at East Brady average 38° (79 percent) 

ranging from 32° (62 percent) to 50° (100 percent). 

Johnstown debris avalanche heads have an average 

slope of 33° (65 percent) and range from 27° (50 

percent) to 38° (78 percent). 

Most colluviilm affected by slope movement has 

accumulated within hollows or laterally concave 

slope areas (fig. 12). Hollows develop where they do 

in part possibly because of an underlying density of 

tension joints in the bedrock greater there than in 

adjacent slopes, as suggested by Gray and Gardner 

(1977) and Woodruff (1971). At East Brady, outcrop 

is abundant only in the hollows where debris 

avalanche chutes have carved their paths, and it is 

difficult to compare fracture density there with that 

of either side. Nevertheless, intensive ground-water 

movement over a period of time in the hollows (or 

coves) has produced a deeper regolith than that 

along adjacent slopes. Unusually heavy 

precipitation occurs infrequently, at which time the 

loose rock and soil is flushed out, and drainages are 

further deepened by sliding and flowage of rock 

material from higher on the slope. Some movements 

of colluvium along planar slopes have caused 

accumulation on benches (fig. 12). Many slope 

movements involved materials from both types of 

accumulation (figs. 6, 12). 

The stratigraphy, structure, and lithologic 

composition of the bedrock at East Brady are 

comparable to those at Johnstown except that 

considerably more sandstone occurs in the 30-m 

interval above the Lower Kittanning coal at East 

Brady. However, the extent and thickness of the 

colluvial cover is vastly different. At East Brady 

the colluvial thickness is minimal, and weathered 

bedrock is exposed in almost all the scars. In the 

Johnstown area, weathered bedrock seldom crops 

out in the scars; a minimum thickness of 5 m of 

colluvium exists at the heads of some debris 

avalanches near Johnstown (Pomeroy, 1982). 

During the August 1980 storm, slope 

movements in the East Brady area took place 

chiefly on the bedrock-colluvium interface; less 

common were slippage surfaces within the 

colluvium. The base of the soil mantle overlying the 

weathered rock represents a weak plane on which 

creep has taken place. Surface water percolating 

through the thin soil (commonly less than 0.6-1.0 m 

thick) tends to accumulate on top of the rock surface 

(usually shale) or the thin soil above it. 

Observations indicate that shale or claystone is 

usually exposed along the head scarp of a slope 

movement where more permeable sandstone lies 

directly above and a concentration of water is 

present at the contact. 

About 90 percent of the slope movements 

originated at or below the Lower Kittanning coal 

bed both east and west of East Brady (figs. 2, 10). 

Though seeps were seen at other horizons, the most 

pronounced springs emanated from the contact of 

the coal (densely jointed) and underclay below a 

dominantly sandstone section. In the absence of 

coal, seepages were common at the base of a 

Discussion 13 
Figure 12. Schematic diagram showing colluvial slope movements along concave slopes or hollows and above benches. 

Relative distribution of lithologies is suggestive of those along slope opposite East Brady. Symbol S denotes horizon of 

abundant seeps; Qc, Quaternary colluvial deposits; Qal, Quaternary alluvial deposits. Diagram suggested by Gray and 

Gardner, 1977. 

sandstone ledge or, where the slope was covered by 

colluvium, seeps were present in the soil beneath the 

inferred position of a sandstone ledge. High pore 

pressures probably originated at the colluviumbedrock contact in such areas. In contrast, the head 

areas of the few debris avalanches above the Lower 

Kittanning coal bed were commonly dry. 

As pore-water pressure increases and shearing 

resistance decreases in the thin colluvial cover 

because of an intense rainfall, a transformation 

process called spontaneous liquefaction (Terzaghi, 

1950, p. 110) takes place. The change of state from a 

supersaturated soil to a thick viscous fluid follows. 

The slope movement of colluvium was of two 

types. One type consisted of downhill rotational or 

planar sliding for a short distance (4.5 to 9.0 m) from 

the head scarp followed by flowage, such as that at 

the highest debris avalanche in area D (figs. 2 and 

7). More commonly, the rotational or planar sliding 

is not evident and, instead, the flowing action begins 

at the head scar as it did at the highest debris 

avalanches at areas Band C (figs. 2 and 6). 

Various amounts of large sandstone, siltstone, 

and shale clasts dominate the 1980 planar slide 

movements as at area A (fig. 2) and in the rotated 

head areas of several debris avalanches. A clay- to 

silt-rich matrix is dominant over the sandstone and 

siltstone clasts downslope along the flowage part 

(and the toe) of the debris avalanches. 

The amount of regolith removed from slope 

areas in the 1980 event is difficult to estimate 

because pre storm configurations of drainages cannot 

be determined. Scott (1972) believed that the 

average thickness of colluvium removed along 

14 Storm-Induced Slope Movements at East Brady, Pennsylvania 

several Blue Ridge debris avalanche paths was 

approximately 1.2 m during an intense storm. On 

the basis of measurements at selected debris 

avalanche paths, I conclude that the average depth 

might have been slightly less at East Brady, or 

about 1. 0 m. Assuming a density of 1 ton per cubic 

yard for the weight of the saturated regolith, a 

minimum of 50,000 tons of hillside material is 

estimated to have been removed from the slopes 

opposite and east of East Brady in August 1980. 

The east-facing slope opposite East Brady (fig. 

2) and that part of the slope 1 to 3 km east of East 

Brady (fig. 10) show a somewhat similar number of 

movements, 34 and 28 respectively, but with a 

higher density of movements opposite East Brady, 

85 versus 56 movements per square kilometer. The 

density of the 1977 movements along three slopes at 

Johnstown ranged from 65 to 93 movements per 

square kilometer. Similar data from storm-induced 

slope movement areas elsewhere in Appalachia is 

lacking or is based on maps of a smaller scale, which 

would invalidate a comparison. 

About 2.5 percent of the slope surface from the 

two East Brady areas was affected by slope 

movements during the 1980 storm. 

Origin of slope movements.--Observations 

confirmed the presence of many episodes of 

continuous slope movements involving both 

colluvium and bedrock throughout historic and 

prehistoric times. Stream undercutting has been a 

dominant geomorphic process because the slope 

opposite East Brady (fig. 2) (and that east of East 

Brady, fig. 10) is on the outside bend of a meander. 

Probably more significant to the origin of slope 
movements has been valley stress relief phenomena 

as described by Ferguson (1967), Ferguson and 

Hamel (1981), and Gray and others (1979) in several 

sites in western Pennsylvania and northern West 

Virginia. Wyrick and Borchers (1981) have applied 

the concept in a hydrologic study in southern West 

Virginia. Valley stress phenomena are independent 

of tectonic processes. Stream erosion has removed 

horizontal support from valley walls and vertical 

support from the valley floor in the subhorizontal 

cyclothemic rocks of the Appalachian Plateau 

(Ferguson and Hamel, 1981). Because of the 

unequal stress distribution in the walls and floor, 

weathering, erosion, and slope movements are 

intensified. 

Observations in the East Brady area revealed 

that nearly vertical to vertical tension joints are 

closer spaced and wider toward the valley wall. 

Blocks have separated along joints and moved 

downhill; some rock masses slumped or failed along 

subsurface planar surfaces. Seasonal thawing 

contributed to an accumulation of water in the upper 

colluvial layers and bedrock that produced a waterlogged condition and reduced shear strength of the 

soil and rock material. Slope movement then 

resulted from the additional weight of the unstable 

-.-J."· 

sh/slt -_----

-··:-· . ....:...~.-

regolith surcharged with moisture and from the 

reduction in the internal friction and cohesion of the 

regolith. 

Stress relief has intensified secondary 

permeability (through fractures or joints), which is 

believed to be more significant to ground-water 

movement than primary permeability in the 

Appalachian Plateaus (Wyrick and Borchers, 1981). 

The role of secondary permeability in the origin of 

debris avalanches has been speculated for an area 

affected by an August 1972 heavy rainfall in 

southwestern West Virginia (Everett, 1979). 

Enlargement of a joint near the top of a hill and the 

eventual buildup of water pressure in the slope 

caused a rock slump at Brilliant Cut, Pittsburgh, in 

1941 (Gray and others, 1979). The failure surface 

was concave upward with its lowest position in a 

claystone at the base of the cut as determined by 

borings. Deep-seated rock slope movements 

probably underlie extensive colluvial movements at 

several localities along I-79 north of the Ohio River 

west of Pittsburgh. Since the site was first studied 

in 1969 (Gray and others, 1979), large tension cracks 

parallel to the valley axis farther up the slope and in 

bedrock have opened and are widening. Both of the 

Pittsburgh slope movements probably originated 

Oldest shear surface 

Figure 13. Schematic diagram showing rock slump and subsequent colluvial movement along the slope 

opposite East Brady. Note that the recent shear surface is probably the site of an older colluvial 

movement. Qc, Quaternary colluvial deposits. 

Discussion 15 
from valley stress relief. 

A number of claystone layers, any one of which 

could serve as a failure surface, are found 

throughout the rock section at East Brady. The 

steeply dipping to vertical joint faces in sandstone 

above the colluvium-draped benches at several sites 

opposite East Brady are believed to have served as a 

point of origin for rock slump movement (fig. 13). 

Subsequent buildup of weathered rock and soil and 

infrequent flushing out of the material has followed 

in the form of colluvial slope movements (fig. 12) 

such as those that occurred in August 1980. 

Colluvium is thickest and most extensive on concave 

slopes called hollows or coves. Reactivation of rock 

slumps is a possibility whenever the optimum set of 

conditions is present. 

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16 Storm-Induced Slope Movements at East Brady, Pennsylvania 

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