FLOOD06148

• County: Ouray
• Community: Ouray County
• Stream Name: Uncompahgre River
• Basin: Gunnison
• Title: Hydrology Report Floodplain Information Study Uncompahgre River
• Date: 8/1/1977
• Prepared For: Ouray County
• Prepared By: Neil Grigg
• Floodplain - Doc Type: Floodplain Report/Masterplan

~~e hydrologic condition of t:ie basir..s is ass~H:ied to be "goodll. Soils E..r-e
_ bS2!:'VE:d to be. ge'!!e:;-211y of t~e "ell 0r liD" types 1-,2.vi:1g hj.gh r"--lf'.off ilO-:2r..-
t.:.ial. Antecede.nt ,.~O::"SL:,lre condition C;'HC) II is 3.s.sUl":12d for all (:2.52S.
T!le :"2sult of these 2Ssu~_?tior:.s is a ri..lTIoff curve TIll:::ber of 75 (eX=!5).

..ne S(~S :',lethod '.::1.5 s212cted bec211se it is a toJidely accepted l'on_the_
si,clC' :',:0.thcd ','Iith t-he c3.p2bi] ity to model ph2nOt71ena such as small moun-
tainO\lS basin runoff. Other rc.s02arch :TIodels h-ould be as good Or perhaps
~etter but lack 1:31ibration and verification for this area.

In applying the SCS ",,,thod th" analyst has "ide flexibility in the selec-
tion of coefficients and estimating parameters. Two basins arE"! of inter-
"st, both tributary to the Dncompahgre and flo"ing through Ouray.

Flood Hydrology of Portland Creek and Cascade Creek

.:ortLJ.nd Creek dCL1ins t~rough the A.i'Tlphithe.ater collecting 2.82 square
-:tileS at its c(outh. Cascade Creek collects 1.29 square miles. Both
Creeks slope to the west and r~n right through Ouray. See Figure 5.

30th cdtchments are steep and rocky dnd have ch2racteristic heavy wooded
.:;lopes at elevations belmv tiT'lbc.rline. :.;;lxt;,:um elevations for both catch-
,ents are on the 8dme ridge at 2bout 21cv3tion 12,400.

The SCS method allo"s the consideration of possible rainfall-runoff events
,long "ith thE> gaged data. It is "ell knmm that small tributaries some-
times have higher runoff than larger streams. 'Je therefore expect that the
12gio~al Eqllatio~s will not hold for these smaller basins and that esti-
~3.ted fl(H\15 for t~2 srr:aller basins ,viII be relatively high.

:t is ~ot really po~sible to predict future runoff from small mountain-
':llS. basins \,;i.th great prec.ision. There are too many unknm.;rns and uncer-
~ainti,,?s. Actll2.l obe.rs,~rvatio::1s of gaged events are the best source. -r\ain-
~all-(::r10ff calc'JL.=<.ti...:ns '-.,;lSt i.1i;,ccssarily'incLlde estiIJ.ates of rJauy pG.raw-
e.t2rs ';"chil:h :lre ~ltJt ~i;O\\n. R(:sults of suc'h c2lcuLltil'iis :Tlust be use.d 'V."ith
:onsid,:::r:~b1e j"dGg~Ent..

The a?proach selected ~or the 2nalysis is based on the U. S. Soil C03serva-
~ion S2rvice techniq~2s as reported in R2fere~c2s 6 and 8. Reference 8
:~3S b~2a espetia11y dl~~pted for and aclrlre.saed to ecl_orado conditio~s. I~
:his c~se it sce2S rccso~~ble to assU~e that both Portland and Cascade C~22~S
':.s.n be rep-c,:;sl?l'.ted 2.S l,':::l)ed '.,'atersheds witho',Jt trying to siJTlulate rlJnoff
from SUb-~3t2tsheds 2nd route to the basin outlet.

~:'e "Graphical ~!etr.Jd of Jett?}Tlining P2ak Dischargell fro:1 Refer2nce 8 is
_?prc?riate si~ce a si:~~le C~ is applied for each ~atershed.

,

:0 2?ply one ~r3pllic;11 :12thod, the fol]0~ing ?2r~~2t2rs are u0,!d~d:

1. Ti~e of Concentratjon

:,:t:l bas irIS 1.2-,"2 ::i~out t'hc s,~;:e t"ti:lvel time t.Jith 1500' oVQrl.qnd