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

I

',: ..

~
1

i

--.

jlt:~lk. Sm~ll tributaries below 3,170 m were
illac!ivc or carried little water during the
rJooJ. Water llowing from the upper basin
had a small sediment load bCGlllSC the
l'\.lcnsivc bedro.:k surface in this area
llllllimill'J ~urra(c erosion. lkc;\usc of
this the 11000 had much excess energy and
ill(Mpor:Hl'd glal'ial1;L~ dqwsils ill the
I.:ll<lnllc! below 3,170 Ill. l"he !lood W;IVC
jl1 llh:lbly bl''':;llllC ill(rl'asin~ly heavily laden
\\ ilh sl'dimcnt as it entrained the deposits
dllll tralhportcu them onto the alluvial r~ln.
The fesulting llood in the lower pari
of the basin was a l'ilscadc of gra\'t~l,
cobbles, bouldl.'rs, and organic material
transported by muddy \\'<lI<:r. Lateral
ridt'cs of (obbks and boulllo.:rs wne de-
posited as much as 3 III above the nood
..:hall11d between 3,100 .lnd 3,170 111
ek\':\lion and were frec of the matrix of
~ill~ and clay'> that ..:haradcrize llluLl and
ddni~ Ill)W tkpll"its (Johnson, IlJ70). This
suggests that the llood surge was turbulent
ratlll.:r than laminar and enables rough
3pfJroximations of Oood velocities and
di..,..:harges through application of lhe
~lallning formula and slIperclev<ltion
l'l rt'd.\ ill t:h~tnnds.
The t\lanning formula (f).mghcny and
Fralllini, 19(5) was one method used to
..:akulatc the peak "elocities and disclwrge'i
of the !loud surge. This formula, in
Illetrk llllit~, is wrillCIl

l' .1- /~c. 'Sl
"

where V is the me:w now velocity in a
gin:n cross se..:tion, R is the hydraulic
radius, S is the ..,10pe of the stream energy
lillt', ~lfld 11 j" the tmDLlknt frktion co-
cffi..:ienl (Manning's n). The reliability of
this formu!;.l for noods that arc heavily
debris laden is unknown bl'cause the con-
ditions here differ from those in which it
ha~ proVl,,'1l rt.li:thlc in lhe past. Spccific~d]y,
",ready, uniform flow conditions were
prnbably maintained for only a few sCl:onds
o.w vcry short distances. Furthcr, Man-
ning's n would be affected by both the
volume of lhc bed load and the amount
of fine SCJilllCllt _'IuspcndcJ ill thc llows.
"11H."e probk.ms rl,,'uur:c the reliability of
lhe ~Ialllling rg.~!I)Ula, but it can still pro-
\'ide an approximation of the pC.lk now.
j[ was applied in Ihe lower channel at six
,>traiglH readll,,'S with wcll-ddine.'d Ooml
limit, (Fig. 2). lhe ,,,ults, ba,ed 011 an
;IS_\Ullln.l fl of 0.075, an: showll in T:dlk I.
Dt'posits in thc stream channel at
l'rOSS sections c and d (Table.' l) appear

~)l~

109'
,,'

PARK .
. ,~M'GE ,:

."".

Avery
Pea(...,
.' ElK.
:' ^MTS..:

~
>
.
\~:
'.
-'1:1'
.J<:
z.
. ~-.
~,

"."

Gunnison

SAN
JUAN
MOUN rAINs ~

"I ",.\,1'

,,'

102"

t'\\

~.....~
-.-:0-
co
'x
~~
.:;l)
~ J<
.- Z
o
~<",'I:", -

Denver
D

N

1

Colorado Springs
o

"".
.-
0 100 200
Km
.

Figurl' I. ~bp of Color;tdo showing location of Avery )lC':Ik rclalivc to some major mountain
rangl's.

(II

to be from <I subaqueous grain flow (dis-
cllssed below) and may have led to an
ov~n:stilllatc of the flood depth at these
IOG\tions and hence the large discharge
estimates. Similar deposits were not present
or were more limited in extent tit the other
cross sections, and remains of plants still
growing in the channel suggested a mini- :
mum of erosion at the sample cross sections~
The Froude numbers indicate that the flood,
was supcrcrilical at all six cross sections.
A second method of estimating the
nood velocities and discharges was applied
at curves in the channel where centrifugal:
forccs caused the flood deposits to be
higher on the outsides of the curves than
on the insides. This flood supcrclcvation
crfCl't v..as well defined al two locations
bel ween cros~ sections band c and was
used to estimate velocity and discharge
there. Givf:l1 the radius of curvalure, r,
and vertical superelev3lion angle, <P.
measured in a plane normallo the flow
direl'tion, the average flood velocity, V,
over the cross section during the nood
pC'ak is estimated by

[" = vxr t:ln $,

where g is the gravitational acceleration.
Solution of I.;qu~\lioo 2 .\\\lids {he need
10 as\ume n friction cocrli.:icnt and was
llsed as an independent check on the results

( 2)

of the Manning formula. This method
yields velocities of 7.9 and 7.4 m/s and
,1;<;chargcs of 108 and 110 ml/s, similar
to values given in Table 1. Henceforth,
it will be assumed that the discharge peak,
including water and solids, was between
.108 and 134 m) /s (lowest and highest esti-
males, disregarding sections c and d).
These discharges are far in excess of
the IOO-yr discharge predicted for the basin
by applying the methods recommended in
the technical engineering manual II Peak
flows in Colorado" (Soil Conservation
Service, 1977). Assuming a very 'Iow infil~
tration potential because of the large
amount of exposed bedrock, the concen-
tration time for the basin was computed
to be only 10 min. A IOO-yr nood dis.
charge peak of 13 m) Is was calculated
for Ihedrainage basil1. The Oood of 1977
exceeds this discharge by an order of mag-
nitude. which suggests either a return
period far in excess of 100 yr or that the
flood potential in high mountain drainage
basins is oat amenable to standard
I engineering technique.,
Baker (1977) presented a trend line of
maximum flood discharges as a function
of drainage basin area using data of Oood
pcaks collected from the elllire Ul1ited
Slates. The peak discharge of the Avery
Peak flood exceeds by roughly a factor of
two the maximum discharge of record in

JANUAR Y 197\J.

~J-

.~ A.