THE BLACK HILLS (South Dakota)

FLOOD OF JUNE 1972:

IMPACTS AND IMPLICATIONS

by Howard K. Orr

USDA Forest Service

General Technical Report RM-2

March 1973

Rocky Mountain Forest and Range Experiment Station

Forest Service

U.S. Department of Agriculture

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
Abstract

Rains of 12 inches or more in 6 hours fell on the east slopes of

the Black Hills the night of June 9, 1972. Resulting flash floods

exacted a disastrous toll in human life and property. Rainfall and discharge so greatly exceeded previous records that recurrence intervals

have been presented in terms of multiples of the estimated 50- or 100-

year event. Quick runoff was produced in the heaviest rainfall areas

regardless of hydrologic condition. Flood sources included all major

geologic and soil types and practically all land uses in the Black Hills.

The highest measured peak runoff per unit area came from a 7-squaremile drainage, all on sedimentary formations, the upper portion of

which burned over in 1936, but which is now well vegetated, apparently stable, and in good hydrologic condition. Greatest damage

occurred where man-origin debris piled up against bridges, highways,

homes, and other improvements.

Keywords: Floods, watershed management, hydrologic data, flash

floods, storm runoff, record rainfall.
USDA Forest Service

General Technical Report RM-2

March 1973

The Black Hills (South Dakota) Flood of June 1972:

Impacts and Implications

by

Howard K. Orr, Hydrologist

Rocky Mountain Forest and Range Experiment Station

'Research reported here was conducted at Station' s Research Work Unit at

Rapid City, in cooperation with South Dakota School of Mines and Technology;

central headquarters is maintained at Fort Collins in cooperation with Colorado

State University

.
Contents

Page

The Storm 1 Hydrologic Condition 2

Antecedent Rainfall and Moisture Conditions 3

Flood Peaks 3

Experimental Watershed Response 8

Summary and Conclusions 12

Literature Cited 12
The Black Hills (South Dakota) Flood of June 1972:

Impacts and Implications

Howard K. Orr

The Black Hills storm and flash floods of

June 9, 1972 will long be remembered — not only

for the tragic loss of life and property but also

as a hydrologic event of such magnitude and

rarity that the recurrence interval is more than

ordinarily problematical. It also has provided a unique opportunity for land managers and re- searchers to study and observe environmental

and land management responses and implications under the stress of a rare storm event.

The purpose of this report is to examine

and describe physical factors and relationships

involved in the production of excessive flood

flows from a number of Black Hills drainages

on June 9, including the Sturgis Experimental

Watersheds, and to postulate supporting theory.

The Storm

The storm was concentrated in an area about

40 miles long by 20 miles wide. The area includes downstream portions of nearly all the

watersheds draining east out of the Black Hills

from Bear Butte Creek on the north to Iron

Creek (tributary to Battle Creek) on the south

(fig. 1).

According to recording gages on the Sturgis

Experimental Watersheds (in the northern part

of storm area, fig. 1), precipitation began at

about 1450 (m.s.t.), and gradually built up to a maximum intensity of nearly 6 inches per hour

about 1630. Rain continued steadily but at gradually declining intensity until between 0200

.and 0230 on June 10.

At the Black Hills Experimental Forest,

about 15 miles south and slightly west, the

storm began between 1600 and 1615. Less rain

fell (total was 5.82 inches in one gage) than

either to the northeast or southeast, but rainfall

ended at about the same time (by 0230 on June

10). At Rochford, about 5 miles further southwest, total storm precipitation was only 1.77

inches.

At Pactola Dam, about 12 airline miles east

of the Experimental Forest and slightly further

south (about halfway along the north to south

axis of the storm but still slightly west of the

approximate north-south line of heaviest precipitation cells), light rainfall started between 1400

and 1500. Maximum rain fell between 2000 and

F-iguAd 7. --Total fiouLYi^aJUL duxlng cvzyiing

June. 9 into mon.viing o{ Jam 10, 79 72 (U. S.

V^pcuvtrntvit Commence 79 72).

2100 (1.83 inches) and by midnight 6.86 inches

had accumulated. The maximum 6-hour precipitation, 6.30 inches, fell between 1700 and 2300.

The storm ended there between 0200 and 0300,

after 7.03 inches of rain fell. The best available information from locations further east, and nearer the apparent

center of heavy precipitation cells in the Rapid

Creek drainage, indicates rainfall started about

1900 and accumulated to 13 inches by 0700 on

1
June 10. Nearly 12 inches of rain fell in 6 hours

(about 1930 June 9 to 0130 June 10).

Putting these facts together, the heavy rainfall obviously started earliest in the northern

Hills. As separate cells fed in on strong east

and southeast winds, the storm built to larger

and larger proportions as it spread southward.

H. J. Thompson, Office of Hydrology, National Weather Service, described the general

meteorological circumstances in which the storm

developed (Thompson 1972, p. 163). Strong lowlevel winds from the east forced moist air

upslope on the east side of the Hills. Sustained

orographic effect helped force the air to rise

and cool as it fed in from the east. At the

same time, midlevel moisture was impinging

from the south. Another especially significant

contributing factor was the unusually light wind

at higher atmospheric levels which did not

disperse the moisture or move the thunderstorms

eastward as rapidly as usual. The result was

repeating thunderstorms in an apparently unusual combination of meteorological conditions.

Under "reverse shear" conditions (St. Amand

et al. 1972), an almost continuous line of

thunderstorms formed and remained quasistationary over the eastern Hills for as long as

6 to 8 hours, whereas the more ordinary

thunderstorm moves out in 1 to 2 hours.

Thompson (1972) asserts that the recurrence

interval of such an extraordinary event cannot

be computed with any degree of accuracy, but

adds in effect that the measured rainfall was in

an amount unlikely to occur more than once

in several thousand years. In a report from the - National Oceanic and Atmospheric Administration, (U.S. Department of Commerce 1972) it is stated that the 6-hour rainfall averaged about

four times the amount to be expected once

every 100 years.

Hydrologic Condition

In all of the most heavily flooded watersheds

examined, there was consistent evidence of

surface runoff from slope areas even where

there were no clearly visible drainage patterns,

and further upslope than in the case of more

ordinary storms. The consistency of this kind

of evidence, irrespective of site, is a key factor

in analyzing the production of the flood flow.

Surface runoff occurred from all types of areas,

including dense forest and well-grassed slopes.

In other words, classification in "good" hydrologic condition did not in this case preclude the

production of excessive surface runoff. Surface

runoff is here differentiated from overland flow

in the sense that surface runoff was evident in

shallow depressions of microtopography not

ordinarily observed, but there was little evidence

of sheet movement of water that resulted in

sheet movement of soil or its protective cover

of litter and humus. This is a generalization

that applied across forest types and densities,

other vegetation types, slopes, topography,

parent materials, geology, and soils. Thick forest

floor, where present, apparently acted as an

efficient water conveyance system, but without

displacement except where water was diverted

or concentrated by stones, roots, or other obstacles (fig. 2). The storm and runoff produced

were obviously of a magnitude that plays a major role in shaping topography as we see it today.

FiguK.2 2.--Wate^ appa/izntLy

{Lotozd tkh.ou.Qk thiA tki.ck

^oh.2J>t {Look, uxltkout

cLUtuAbance. antiL it u)<u

cLLveAtdd ok. pondzd by

i>tonz£>. Wkm thu

ha.pp2.nzd, tkz LittzK

{Loatzd and movzd

downALopz. [ Knight to Lz{t)

,

Leaving a maJLL 6 pot o{

ba/i2. minzhal 6otL. Many

6 ack i>po&> coutd be. 6 2,2.n on 6om2. 6Lop2A .

2
Antecedent Rainfall and Moisture Conditions

Precipitation had not been excessive, January through May, at any of the continuous

measuring stations in the Hills. Distributions

by months were near average, and totals for

representative stations varied from slightly

below to slightly above average (table 1). Temperatures had not been extreme. In short, there

were no apparent clues leading up to the

extreme storm and flood event of June 9-10.

Apparently it was a random event, neither preceded nor followed by distinctive extremes.

On the other hand, the topography and

location of the Black Hills in relation to the

nature and movement of major weather systems

would appear to make the area a more likely

prospect for development of the kind of storm

that occurred on June 9 than most parts of the

continental United States. In other words,

atypical behavior of meteorological elements,

such as the low-velocity winds aloft which

occurred over the Hills on June 9, might, in

combination with other factors, be expected to

evoke a more violent reaction than in other

areas.By June 9 evapotranspiration usually ex- ceeds precipitation input, and soil moisture

storage deficit is increasing. Before the end of

May, soon after the start of the growing

season, streamflow is usually declining while

precipitation increases to maximum in June.

Precipitation and flow were, in general, following the usual patterns up to June of 1972. Some

stations received practically no precipitation in

the week preceding June 9, while others (Pactola

Dam in table 1) received relatively large amounts

as late as June 4. Flooding occurred from watersheds in all of the major geologic types in the Hills.

Though the nature of flooding was clearly influenced by soils and geology, precipitation

amounts and intensities were the primary factor.

Type of vegetation and biomass per unit area

(relative evapotranspiration) no doubt also had

less effect than they would have had later in

the growing season. Effects of differential evapotranspiration would not yet have accumulated

to as great a degree.

Flood Peaks

Peak unit area discharges (as computed from

USGS measurements) 2 do not coincide with

ranking of mean area rainfall (table 2). The

highest mean area rainfall (of the watersheds

computed) occurred on Este Creek, a 6-squaremile tributary of Boxelder Creek. Parent material is primarily metamorphic. Though the unit area discharge is no doubt among record highs

for watersheds of comparable size in the entire

United States, it was not the highest measured.

0 , Data obtained from Rapid City Subdistrict

Office, Water Resources Division , U. S. Geological Survey.

Table 1

. --Preci p i tat i on at selected stations (from January 1, 1972), Drior to the June 9, 1972

Black Hills storm and flood, in comparison with average

Sturgis Experiment Forest Pactola Rapid

I nches

Janua ry 1 . 37 0 96 0 21 0 3k

Feb ruary 1 01 76 31 k]

Ma rch 1 32 1 07 67 52

Apri 1 k 1

1

2 62 2 2k 2 k5

May 5 48 k 2k 2 89 3 19

June 1-8 13 93 3 91 3k

Sum through May 1972 13- 29 9 65 6 32 6 91 Long-term average through May Ik 56 1

1

12 8 ]k 6 k8

Sum through June 8, 1972 13 k2 10 58 10 23 7 25

3
Peak unit area discharge was practically the

same from Victoria Creek (tributary to Rapid

Creek) as from Este Creek, but the recurrence

interval is much greater, although parent material is also metamorphic and computed mean

rainfall was considerably less. Reasons are not

apparent, although they may be related to the

method used by U.S. Geological Survey (USGS)

to calculate estimated 50-year discharge (Patterson 1966). The recurrence intervals in table 2 are expressed in terms of multiples of the 50-

year peak discharge as computed by USGS.

The highest per-unit-area discharge, and by

far the greatest recurrence interval, occurred

from Cleghorn Canyon (Wild Irishman Creek),

tributary to Rapid Creek at the west edge of

Rapid City (table 2). Area average rainfall was

not the highest, though as much as 13 inches

reportedly fell in the middle to lower reaches

of the watershed. One distinctively different

physical characteristic of the watershed (compared with others listed) is that it is almost

entirely on sedimentary formations, Deadwood

sandstone and Pahasapa limestone in upper

reaches to sandstones with interbedded limestones and red shales of the Minnelusa formation

at the junction with Rapid Creek. Also, the

upper portion of the watershed burned in

September 1936 (the 760-acre Johnston fire).

Much of this old burn has not come back to

pine (figs. 3, 4, 5). Aspen and birch are now

abundant on the more moist slopes and in

stream bottoms. Most of the remaining area is well vegetated with herbaceous species and

shrubs. There is considerable private land and

residential development in the lower reaches

(Cleghorn Canyon). There was evidence of

runoff from all types of areas, but slope

damage was minimal, even on the kind of steep

south-facing slope shown in figure 3. This is one of the slopes bared in the 1936 fire. Figure

6 is a closeup of the left portion of this same

slope. Pine reproduction is sparse, but the site

is well stabilized by herbaceous vegetation. One

landslide, visible in figure 3 and shown closeup

in figure 7, occurred on this slope. The slide

was not large, about 32-35 feet long and about

half as wide, and apparently resulted from a combination of water concentration from upTable 2. --Peak discharge and area average precipitation of selected Black Hills watersheds, flood

of June 9, 1972 (in order of area average rainfall as computed by Rocky. Mountain Forest

and Range Experiment Station)

Area

Drainage Peak dii scha rge

Recurrence

i nterva

1

2 ave rage

ra i nf a 1 1 Area

C.f.s. 1 C.s.m. 1 nches Sq.Mi

.

Este Creek (near Nemo) 6,620 1 ,078 6.0 10. 69 6.14

Deer Creek at campground 8 miles west of Rapid City 3,530 825 23 9. 96 4.28

Gordon Gulch (near Sheridan Lake) ( 3

) 9. 42 "3.15

Horse Creek (at Sheridan Lake) 1 ,830 181 6.7 9. 09 10.

1

Deadman Creek (at Sturgis) 4, 740 797 8. 78 5.95

Battle Creek (at Keystone) 10,800 794 8.8 8. 34 13.6

Little Elk Creek (at Elk Creek) ( 3

) 8. o4 '19.7

Cleghorn Canyon (Wild Irishman Creek) 12,600 1 ,813 61 7. 87 6.95

Prairie Creek (at Rapid Creek) ( 3

) 7..21 14.0

Grizzly Bear Creek (near Keystone) 6,230 676 6.4 7.. 1

1

9.22

V i c tor i a C reek 6,860 1 ,022 35 7., 10 12.8

1 ron C reek ( 3

) 6. 68 16.3

From USGS measurement by slope-area method.

2 Multiple of 50-year peak discharge as computed by USGS.

3 Not measured by USGS but flooding severe. Measurement from Rocky Mountain Forest and Range Experiment Station.

4
Figure 3.--A a ouuth- fading slope In the upper

K2.CLch.Qj> o{ Wild Irishman Creek [CJL2.Qh.ot1n

Canyon). The. 0x2.0. burned over 36 years ago,

and much o^ It has not come back to pine

forest. The lock outcrop Is Vahasapa timestone, with darker Veadwood sandstone outcAop [V) about 1/4 o{ the way downslope.

Vegetation Is W2.ll established and the

soils are stable. S> Indicates slide area

closeup In figure 7.

Figure 5 .--Valley bottoms In the old 1936

Johnston Vine [760 acK.es) were Invaded by

aspen and birch. These species also Invaded

on the more moist north- facing slopes, as shown In figure 4. This view Is {rom the

top o{± the ridge In figure 3, looking almost

dlxectly east toward Rapid City In the ^ar

background.

Figure 4 .--looking back southwest ^rom high on the slope In figure 3. One o& the main

tributary channels ofc ttlld Irishman shows

In the lower portion ofa the photo, and

ilood- deposited material Is visible [arrow) , together with an old road.

Figure 6 .--Looking mostly west [le&t) across the le^t portion o{ the slope In figure 3. Vegetation Is a well-es tabllshed mixture ofa grasses and fiorbs . Soils are stable &or the

most part.

5
ViguKZ 7. --To thz night 0& thz

ojiza in fiiguAZ 6 ii> tka> bmaLl

filow Alidz about 30 to 35 {zzt

long and about hal{ at> Midz.

It was thz only visible, ouzo, on thz zntOiz &lopz u)hzn.z any

appftzztablz amount oft i>oil woa cLUplaczd.

slope and liquefaction of shallow, stony soil on

bedrock (possibly upper Deadwood sandstone)

which was near but did not intersect the slope

plane. The flow slide material moved all the

way downslope to the main channel, and was

no doubt the source of some of the waterdeposited material in the lower portion of figure

4. Vegetation was not stripped from the slope

in the slide path, however.

Some additional channel cutting and deposition, associated mainly with old roads, was

evident in upper reaches. Practically all of that

debris was intercepted and filtered out by shrub

and herbaceous vegetation in open meadow

areas such as that shown in figure 4. Severe

channel cutting and erosion became increasingly

evident further downstream in the narrower,

more constricted lower canyon.

Much debris (including tree branches, limbs,

and roots) lodged on the upstream side of trees

(fig. 8). Several such deposits were examined

closely. None of the debris originated as logging

slash, as was at first thought. No axe or saw

cuts were visible. The debris obviously came

from trees close enough to the channel that

record high flow literally tore them out and

stripped them of limbs and foliage. Ring counts

of some of the trees deposited in debris piles

indicate they were 80-85 years old. In other

words not since at least 1890 had any combination of storms, fire, timber stand conditions,

grazing, or other land use produced such large

floods. In Boxelder, and undoubtedly in other

drainages, trees were downed that had been in

place more than 100 years—long before settlement by white man.

FiguAz 8.--UoocU dzbnib 8 to 10

fizzt high against thz up6tAzam

i>idz 0^ tAzzb in Wild iKi&hman

[Clzgholn Canyon), douinittizam

{h-om thz axzah t>howvi in ^iguxzh

3 to 1 . Hznz thz channzl haA

buoadznzd and thzh.z it> lz66

ghjxdiznt, but it i>tzzpznt> and

noAAom again bz&oKz joining

Rapid C^zzk at thz a)z6t zdgz

o& Rapid City.

6
This conclusive evidence negates serious

speculation that this rare flood event may indicate general environmental degradation. Severity of consequences of the flooding, though none

the less tragic, were in direct proportion to

man's encroachment on stream channels and

floodways. Also, man-origin debris no doubt

caused more environmental deterioration or damage than any a priori upstream land use or management. Similar consequences, though less

spectacular and without the great loss of life,

occurred in 1962 and earlier.

Another watershed of particular interest is

Grizzly Bear Creek, which drains from a research

natural Area on the Precambrian granite of the

Harney Range (fig. 9). This area has had little

disturbance of any kind in many years. Yet the

stream flooded inside as well as outside the

boundary of theNaturalArea, enough to drastically erode and alter some channel sections.

Litter piles and associated upslope bare spots

again attested to the production of significant

surface runoff from a microtopography that

would not ordinarily be noticed. There is evidence, however, that watersheds in this general

area have low storage capacity due to coarse- textured parent material, and an obvious history

of flooding, even during much lesser storms.

A large proportion of the most heavily

flooded watersheds was in areas of metamorphic

parent material of the general type shown in

figure 10. Material of this type characteristically

breaks down to loamy soils, often quite stony.

The kind of erosion shown in figure 11 is typical of what happened to cut slopes in residual soil. Fines were washed out, leaving stones

protruding. Slumping of steep road cuts was

not uncommon in this kind of situation.

F-iguio, 9 .--Channels iKodid 6Q.vzA.nly,

zvm In that have, k.q,cza.v<lo'

Little, tun In many yzanA . Shown

hth.0, ii Gntzzly BzaA Cti2.dk,

ciuu.nA.ng out tht Pino. C^eefe

UatuAal kn.ua In the. HaAn&y Range.

VaAunt n.ock aj> gtianitz, which

hunji wnathzXA to a coaAAe.-te.xtuA2,d

6 hallow 6 oil with low 6 tonago.

ca.pa.cJXy.
Experimental Watershed Response

The best quality overall precipitation and

flow records that we know of are from the

three Sturgis Experimental Watersheds — 217,

89, 190 acres — in the northeastern Black Hills.

These records are used to illustrate flow timing

and distribution in relation to precipitation.

The total volumes of flood flow cannot be

determined, however, because of debris accu- mulations which practically covered the stations

during recession (figs. 12, 13, 14). Accumulation

apparently started at about the same time as the peaks occurred or soon after. The stations

were not structurally damaged, however, despite

the force of debris and peak flow about twice

the design discharges computed for 1.87 inches

of rain in 1 hour (table 3). Hence, the flume

hydrographs are available from start of rise to

the approximate peaks at all three stations.

ViguJiz 10 .-- Chan.actnnAj>ttc

moXamoKpktc patent mat&nial

Jin pcuvt oh i>tonm aA.ua that

tec&ivzd i>omn oh the. huavteAt

Kainhall. Flooding ^nom a

dJiainago. no mote than 1 mile,

long cauAdd tkii, chann&l

damage, which u)a6 on tk<L

out^tdz. oh a tuJtn.

Table 3-"~Rainfall by individual gages at the

Sturgis Experimental Watersheds,

June 9, 1972

Gage

Total

prec i p i tat i on Maximum 6-hour

p rec i p i tat i on

1 nches 1 nches Hours

A-l 8.68

A-2 11. 64 10.47 1 500-2100

A-3 10.49

A-4 9-57

A-5 8.86 7.76 1450-2050

A-6 10.01

A7

10.96 '9-72 n

1 By proportion, from gages A-2 and A~5;

distribution record lost, only total depth

ava i 1 abl e.

VlguAd 11. --An old tead cut

thAough teAidual i>oiZ,

matamoApkic patent matanial,

ahtin. the June 9 ^lood.

TtnoA LOdte woAkad oat.

Platy teck wat> tkm qJMiclh.

l<iht ptetAuding on. It h<tll

out and into an accumulation

at tha ba6& oh the 6lopz.

Tkti, hOhX oh thing did not

kappm on nm&ti back-blop&d

teach

.

8
Figure I 2 .--to'aten>hed 1 gaging station nearly

cove.ie.dl over with

4 tone, and other debris,

and completely inoperable

a^tzn. the June 9 ^lood.

Figure 1 3. --Watershed 2 gaging

station afiieA. June 9 filood.

San VimaA faZume. and channeZ

it> ^ull o{ -4 tone and other

debris at le{t {upstream

farom headwall) . Weir pond

it> completely obscured.

Station loo6 intact but

inoperable.

Figure 14. --Watershed 3 gaging - station a^ter June. 9 {lood.

The. station continued in

operation despite, the. debris

pileup. Nearly 400 man-houAA

ofi labor were required to

return all three, gaging

6tation£> to temporary

standby operation.

9
Watershed 3 (WS3) flume functioned through

the entire storm, but flow figures are not reliable after debris started to pile up following

the peak, and because some water crossed over from WS2 to WS3 just upstream from the

gaging stations. At least up to the peak the

WS3 record is judged the best.

Rainfall started at almost precisely 1450

(m.s.t.) in all three recording gages at the

watershed. The main streams started to rise

almost immediately in all three watersheds, but

the rise was relatively slow until between 1600

and 1700, when the heaviest sustained rainfall

began. Flow then accelerated very rapidly.

Time at which flow started to accelerate

most rapidly in WS3 coincided almost precisely

with the start of a second period of highintensity rain (at 1630) at rain gage A-2 in the

upper reaches of the watershed. Over 3 inches

of rain had already fallen — apparently enough

to fill available soil storage capacity and cause

the flow to respond almost immediately to additional high-intensity rain. From 1630 to 1820

the discharge increased steadily from 6 to 80

c.f.s. Maximum 1-hour rainfall occurred in this

time interval — 3.25 inches at rain gage A-2

(1630 to 1730) in the head of WS2, and 2.12

inches at rain gage A-5 (1645 to 1745) on the

east boundary of WS1.

The watersheds behaved about as would be

expected. WS2 apparently received most rainfall

(fig. 15), and produced the highest unit area peak runoff (table 4). WS3 received second

largest amount of rain, produced the second

highest unit area peak runoff, and the second

largest total area depth of runoff to the hydrograph peak. WS1, the most easterly of the

watersheds, received the least rain, produced

the lowest unit area peak flow, and the least

depth to peak.

Maximum peaks (fig. 16) were less clearly

recorded in WS1 and WS2 than in WS3. Nevertheless the peaks, as well as amounts already

presented, are reasonable. WS1 hydrograph did

not start maximum acceleration until about 1 hour after WS3, which is not surprising considering the fact that by the time of peak in

WS3, 2 inches more rain had fallen there than

in WS1.

Reasons for the two recorded peaks at WS1

are not clearly apparent. It may be that landslides (several occurred in WS1 and WS2, fig.

17) temporarily blocked flow. The debris dams

then washed out, probably releasing enough

surge of water to cause another peak at the

gaging station. Several such slides may also

have been one reason why the hydrograph in

WS2 lagged behind both WS1 and WS3, particularly if the slides occurred at about the same

time. Response times were opposite what might

have been expected considering the fact that, in

calculating design discharges, the concentration

times were 12.6, 9.0, and 16.2 minutes for WS1,

WS2, and WS3, respectively.

The slides themselves are of interest. Most

of the ones adjacent to stream channels appear

F-iguAd 7 5 .--IsokyeXat map o&

tha lam 9, 79 72 filoodpK.odacA.nQ htohm on thz

StuAQ-Us EvpuHyimnvvtaZ

Wa£eA6 kddi, ••

CdwtzK., Matufvbkdd I; Zight [<KUt] watdhAkud 7.

10
Table 4. --Area rainfall and storm discharge,

Sturgis Experimental Watersheds,

June 9-10, 1972

I tern Watersheds

2 3

Average precipitation

( i nches )

:

Th i essen

I sohyeta

1

9.28

9-59

Recorded hydrograph:

Approximate start of

rise (m . s . t .

)

Time of peak (m. s. t.

)

Time to peak (hrs.& min.) 3:50

D i scha rge

Peak (c.f.s.)

Peak (c. s .m.

)

Depth to peak(area inches)

1450

1840

76

225

.28

10.83

10.86

151^

1930

4:16

49

349

.47

10.47

10. 16

1450

1 832

3:42

80

269

.44

Computed 25~year

design discharge (c.f.s.) 45 20 37

1972 multiple of 25-year 1.7 2.4 2.2

TlguJid 7 6. --June 9, 79 72 6 torn &low

hydn.ogh.aphA to about timi oh p&akb .

VohhJj* deposits cauhid lot 6 oh the.

nmalndzh. o{ the. hydh.ogh.aphi>.

1400 1600 1800

Time

2000

Ttguh.z 17. --Flow btido. hiom

channel, bank In Stuh.gl6 WS 7

.

A numbch. oh 6uch AlideA

blocked channel* until enough

watch, accumulated to o\)2.htop

and cut through the de.bhjj> . VebhlA In hono.gh.ound aj> on man. bide oh channel, which

extend* h^om flight to leht

whe.h.e. man ti> standing

.

1 1
to have occurred as the result of liquefaction of

the toe of a slope by water flowing more than

channel deep. Most of the slides were of the

flow type (versus the rotary type). None we

have seen within the watersheds were large, but

there were enough of them to no doubt contribute a substantial portion of the bedload that

choked the gaging stations and deposited in

water supply reservoirs downstream. Several

larger flow slides occurred on talus slopes along

the main access road, which appear to have

started as moisture accumulated to low tension

along a road cut. Debris flow appears to have

started at the cut face and progressed upslope — to near the ridge crest in several cases.

Several such slides blocked the main access road.

Timber was harvested from WS3 in 1970 and

1971 (132 acres, 73 percent of total area). Some

temporary haul road was constructed, but most

was well up on a slope and away from main

channels. Harvesting activity does not appear

to have accelerated either the runoff or channel

erosion. Haul roads and skid trails suffered

minor damage, and little, if any debris reached

major channels.

Summary and Conclusions

An unusual combination of atmospheric

conditions triggered thunderstorms of unprecedented depth and duration from midafternoon

on June 9, 1972 to early morning June 10, along

the east slope of the Black Hills. Flash floods

resulted in great loss of life, particularly in

heavily populated areas along Rapid Creek, and

property damages estimated at more than $100

million in Rapid City alone.

The storm was of such magnitude that re- currence interval is more than ordinarily

problematical. Six-hour amounts ranged as high

as 12 inches. Recurrence is variously estimated

as four times the amount expected once in 100

years on the average (U.S. Department of Commerce 1972) to once in several thousand years

(Thompson 1972). Flood flow peaks ranged as high as 62 times the once in -50 -year discharge

as estimated by the United States Geological

Survey.

Portions of all of the major geologic types

of the Black Hills were present in the torrent

area. All major land uses ranging from commercial forest and agriculture, to intensive urban

development were also present. All types of areas produced runoff of unprecedented

proportions. Some areas yielded channel flow

where long-time residents never before saw

surface flow. The obvious conclusion is that

runoff was produced regardless of how good the

hydrologic condition. For example, surface

runoff occurred under forest with thick protective litter, and concentrated in drainageways of

microtopography that would not ordinarily be

seen. The event was apparently so rare that the

flash flooding cannot be taken as indication of

general environmental degradation.

The land, regardless of condition or use,

obviously could not receive and transmit the

amount of rain that fell without changes in the

face of the land itself. Part of these changes

were the inevitable result of natural processes.

In this framework, classification as "damage"

is problematical.

On the other hand, damage was unprecedented where there was loss of life and the

works of man were involved. A lesson to be

relearned from this disaster is that encroachment on natural flow channels will inevitably

result in damaging floods — some time. Flood

damage in 1962 and earlier was also greatly

compounded by man-origin debris , and, although

cause and effect were both well documented,

they were soon forgotten. All the major drainages in this 1972 flood have some known

history of flooding, and they will flood again —

some time. Extent of future damages will

depend on sophistication of engineering works

designed to control and meter flood flows, thus

permitting continued occupancy of flood plain, or it will be necessary to withdraw from the

most flood-susceptible areas. The final action,

if man does not again forget too soon, will no

doubt be a combination of the two.

Literature Cited

Patterson, James L.

1966. Magnitude and frequency of floods

in the United States. Part 6-A, Missouri

River Basin above Sioux City, Iowa.

U.S. Geol. Surv.Water Supply Pap. 1679,

471 p.

St. Amand, Pierre, Ray J. Davis, and Robert D.

Elliott.

1972. Report of Rapid City flood 9 June

1972: Report to South Dakota Weather

Control Commission 27 June 1972 by

Board of Inquiry. Dec. 1972.

Thompson, Herbert J. 1972. The Black Hills flood. Weatherwise

25: 162-167, 173.

U. S. Department of Commerce.

1972. Black Hills flood of June 9, 1972. A

Report to the Administrator. USDCNOAA, Natural Disaster Surv. Rep.

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