FLOOD07985

• County: Statewide
• Basin: Statewide
• Title: Flooding and Sediment Transport in a Small Alpine Drainage Basin in Colorado
• Date: 3/14/1979
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Flooding and sediment transport
In a small alpine drainage basin
In Colorado

"\BSTRACT

"\ sl\tln.duration, high-intensity rainfall on a 0.43-km' (IDS-acre)
;dpittt' drail1a~r hasin ill the n'l1lral Colorado mOlllll;lins produced a peak
di"l"hart..!.l' iIH,"ludillt!, wall'r ;Jilt! S\..'dil1ll'nI , of lOX to 134 Il\Jjs (3,S I 0 to
. ' .
..L'730 crs). This l'\rel'ds hy an order of magnitude lh(" disch:lrge of the
I OO-yr flood as predi,.ted hy the pnKcdurL's reeoll1mcmlcd by the U.S.
Soil Conscrv;ltion Service and exceeds hy a factor of two the largest dis-
chargl' l'\'l'l' reported in ::t hasin of similar size. The Manning formuht rmd
~lll)l'n.'lcvatiol1 cah:ulatiolls were two methods lIsed to compute vc~ocitics
and discharges. These two lI1ethods depend upon il1dl'pl'IHlenl scls of
a~stl1l\ptiol1s. hut they produced similar results. Calculations of the tluid-
dYll:llnic stresses required to transport boulders in th(' upper channel also
"'1I~,gCSt l100d depths and vdocitil's within the range of valut's computed
through lIse of the l\1anning formula :lnd superell'valion calculations.
In the tower channel, bed load moved :\s illNtial grain llows. Olle flow
studied in dctaillr:lllsported sediment onto a p:lrt of" the :llluvial r~1Il not
reached by (k'cp flood W:lter. The larger clasts wcre transported to the top
:lIltl front of the grain flow. This type or sorting supporls a theory of
~J';,il\.t1ow lh'forll1:ltioll first Sll~l!,l'SIl'd hy Bagnold.

INTRODUCfION

Small mountain drainage basins are
~t'ldom instrumcnted 10 rewrd the pt:ak
,!t\lhargcs resulting from short-dur~\Iion,
high-intensity rain storms. However, as
allu\'i;:li fans in mountain areas become
populaled, design for tloods is essential.
:\ !laudt)' of d~\l~\ forces engineers rcspon-
~iblc for llood design to apply indirect
Illethods to calculatc !lood peaks, bur
tht:se mt:thods usually extrapolate data
nhtained in large basins to the small basins.
This may lead to underestimates of the
dt:~ign discharges, and, as a result, lal.:k
uf r~cognition of th'c coarse.sediment-
tramport capability of the floods.
Th(' 0000 di.'ic\lSliCd in thili paper
appears 10 have been exceptionally large
fur a small drainage basin at high cleva~
lion. Ilowen'r, t1H' limited daln for Ooods
ill .small dr~ljnagcs do not provide a good
fralllL' of rL'fl'fL'Il(L' for i,:oll1rari~OIl, Ex-
perience in the Big Thompson Canyon

(~[Ol OGY. v 7, p. ::l3-:l7

approximately one-half of the Sleep upper
basin (Fig. 2). The horizontally projected
area of the basin above the 3,170-m con.
tour is 0.43 km2. The average gradient of
the. upper basin is 31 C) and tapers to a
deeply incised rock-walled chann'el at
3,170 m. The basin and channel can dis-
charge water as high. velocity sheet runoff
because of its steep gradient and low
infiltration potential.
The lower part of the basin. between
elevations of 3.170 and 2,930 m. consists
of glacial till and outwash and material
The drainage basin involved in the I transported from these deposits by
intense nooding lies on the west side of ' various Ouvial processes.
Avery Peak (Ial 39'N, long 107'W) in the ~
southern Elk Mountains of central Colo~ I
rlldo (Fig. I). The bedrock or 'he badn I, FLOOD DYNAMICS
composed of sillstone, very fine to medium;
grained sandslOne, and conglomerate units:
,
(If the Pellnsylvanian-Perll1ian Maroon
hHlllation, whkh dip 1000:ally to the wc~t-
:-;outhwcs! at 35<>. These rocks form a
smooth, impervious bedrock surface over

,
,
,
,
and its tributaries (Soule and others, 1976) ;
suggests that very large discharges in small 1
mountain basins may be more common i
t!lO\ll previously Ihought. '
The Avery Pe:.tk Oood discussed here
provides an opportunily for estimating
storm characteristics, neod dynamics,
and coarse. sediment transport in a
small alpine drainage basin.

LOCATION AND DESCRIPTION
OF THE DRAINAGE BASIN

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, J (p,^. 0-

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Arthur 1. Mears
222 East Gothic Avenue
Gunnison. ColoraQo 81230

.mml(~J?~S)
f
MAR 14 1979 I
,
I
pJ

COLO. Wfll Ch
CONSERVATION BOARD

The flood resulted from an Intense
rainstorm over the upper basin on July
24, 1977. Rain also fell on the lower basin
nnd surrounding areas but was not of an
inlensity sufficient to produce significant
overland now and contribute to the flood

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