WSPC04651

• File Number: 8062
• Description: Federal Water Rights - Colorado Litigation - National Forest ISF Claims - Division 2
• State: CO
• Basin: Statewide
• Date: 9/17/1997
• Title: Materials of Interest - Effective Discharge and the Design of Channel Maintenance Flows for Gravel-Bed Rivers
• Water Supply Pro - Doc Type: Report/Study

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BANI<8JU OISCI'iAAGE. IN
:;UeIC METERS pE~ SECOI\~

Fig. '. ComparlSCr. of effectl'e disch.arre lj1d bankfull dls~b~r!:e.

of a Ic.1:t was evaluated by varyUlg the water surface
slope by -= 10 peru:nt. This test abo is equivalc:o.t to 3. :t
10 per:ent change in the flo.... depth and a:.edian bed;-
materi.31 panlc:le size. Uncenainty in the reach bydr:u1li,
c!J.ara::teristics bas a ..ery signific.anl effect. is large 4.S :t
300 pertent 0::1 the compu:ed mean an.:I\J,lJ bed-m:a.tcnal
load. bm only a very small effcct OD the effecti"'e discharge
or n.::1ge of dominant t;.ed-rnatenal ttansponing di~h3:gc.
The con:?uted effecti'o.e discharge is 9.09 m'/sec usmg a 10
percent increase 11\ water surfacc slope 3lld 9.66 m'/sec
using a 10 pcrc.cnt de:::rease in waler Sw10iCe slope
compared to 9.94 cr/see ~ shown In Figure 3d.
As described above. the dominant bed-material cranspon-
i:lg di~arges.. i.e. those discharges that transpott the
ccntr.aJ. 80 pelU'nt portiOD of the me;m-annual bed.m.atui.al
lo3d. 0CCt0r about 15 days per year Oll average. There is.
however. substantial yeou~(o-year variuion in the dun:lCll
of bed-ma:eri.1l t~poniDg di~lwges and. colTespond~
ingly. the qu.:mtity of bed material transponed. Ye.a.n
havi.ng rel3.lin'l'y large f\lDof( t)'Pic:l1ly Yo'in have t:W'Iy
more than the average number of days with discharge Ul
the range of dom.i..c.a.ot berl.material tnnsportillg flo.....s.
Conveoely. relati"vdy sca1l rt:.Iloff ).e.us frequenrly Will
ha'lle feo;'lo. or no days of do:niD.a.ot f)ed-lm.terial transponmg
disct:arge. Tbe 3CCUDl1Jlattve distributton of days over the
period of record wben flow occulTed .....ithin the range of
dotni.n;mt bed-matenaJ U'aIlSpOnlDg dischuie:S are shown

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for the six pgi.t:g s~atio:,.s .....tth tr.e lor.gesl penods of
record ir. Fij;Cre 5a.f. The abscissa ..3..iues Wert co:r.pu:ed
~y ordenng the a."\1';ual runoff from s:r.allesl to largest and
divided by the lengtl'1 of record. The: CrdlI1ale 'IIalutS were
.:ompu:ed by sum.~ng to'le nucber o! days of dotIUn.1I1r
bed-I:IJ.tenal nanspoJu:g lils.:Orge troI:". t.ie: smallest 10 the
largest annu:ll IU:loff yeus ar.d divided b)' the total ;J'.:.mber
of days me: dOtIllf'.3L1 disorge OCC'.J1Tcd.
Appro~un.ately .so percent of all days when tbe cail)'
a:.ean :10..... .....as WIthin the range of dO::ll:l.Z.Ot bed-material
tr3.:l.5"pomng disc~arges occur l..C. abou: 20 percel:U of the
)'e.'lTs. Conversely. dOlDmant bed-tr.,ateriaJ tr31lSponing
dis,b.arges did nOI oc:cu: dunog approxHnalely one.third of
the years. Those years .....ben dorrun.ant bed.ma:erial
transponing discl-.1fges cx;::UlTed 00 a relatively large
llumbcr of days .....ere. ",i!.h rare u:ception, ).e1rS during
whicb Jnnual runoff sigoific.a.ouy e~~ded the IODg-term
mean. For ex.a.mple. Cie Ii largest n;noff years f'tcoreed
at the ~idd.le Boulder Cruk at ~ederl3.Ild ga,e. 20 percenl
of the period of record. ha~ 57 percetlt of all days with
djsdla.rges within tJ:.e ral'1ge of c!orn.i:J.ant bed-n::aterial
nansporti.og flows. Funhennore. 51 perce::1t of all bed
material tra.n5poned during the period of record. 1908-
1993. OCCl:r i.:J. the 17 l31gest IUIloff ye.:m.
Finally. an a.dditio::ul faC40r complic3.tes a simple
description of the occurn-ncc of :he dominant bed-material
transpottiDg discharges. In wose years .....ith se"'eral cl.ays of
dorninmt dise.ha:ge. the penod of significant bed.material
trmspon is freq'Jently brokeD into two. three. and.
sometirr.es, four segma:u of a few days tach and
occurring. over a period of 3040 days.

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S. CHANSEL M.o\..I:"roiESA......CE FLOWS

The preceding analysis has idc:o.tified the malJ\itude ;md
frequency of dlschuEcs ....hich uan.spon most of the bed-
ma.teria11oad over a period of years in gravel.bed rivers
common throughOUt the westnn United States. These
disChar~S b.a~rel.aJ.iVeIY o.1!TOW nnge, bttWee::l ol.a:iDd.
\...~tim...s.\..lh~.bWfllll discb3I'tC~tnr\m2!~~
15 dayS P;U~,uYl:t2&'" Furthermore. the b.anifWI
disehuge of the nvers studied lies 'oI,..ithi.o the range of
domin:wt bed-material tra..zpon discharges and is very
nearly equal to the iDterval of disebarge whieb transpOrtS
!.he largest portion of tte loog-term bed-material load.
Therdore. i: is concluded that the dimensLol1S. morphol-
ogy. and otber physieaJ characteristics of these gra\'el-bcd
rivers are priJ:rWily delerm:.ned by a well-defined.
relauvely narrow r111ge of disch.a:ges. Tbese rcsulu
establish. the b3Sl5 for formwaung a regime of streamflows

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