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<br />
<br />TABLE 3
<br />
<br />PEAK FLOWS
<br />101l- AND SIlIl-YEAR FLOODS
<br />
<br />Stream
<br />Colorado Riv<er
<br />Reed Wash
<br />Big Salt Wash
<br />Little Salt Wash
<br />
<br />Peak flow
<br />",
<br />100-Year SOO.Year
<br />flood Flood
<br />
<br />Locallon
<br />Gaging Station (At Fruita)
<br />At DRGWRR
<br />Below Ruby Lee Reservoir
<br />At Grand Valley Canal
<br />
<br />82.000
<br />3,500
<br />7,900
<br />4,300
<br />
<br />107,000
<br />7,600
<br />16,700
<br />8,100
<br />
<br />As noted previously,informationoncloud_
<br />burst flooding inthe Fruitaarea is practically
<br />nonexistent. However,measurementsof2A5,
<br />2.50, and 2.55 inches of rain have been
<br />recorded at nearby locations in the region
<br />during 24-hour periods (actual storm
<br />durations unknown). In another location, 2.15
<br />inches Were rceorded in a2 hour period. Duc
<br />to similar precipitation and runoff
<br />characteristics between the Indian Wa..';h
<br />watcr~h"d in the Granrl Junction area and the
<br />tributary watersheds in the Fruita area, a
<br />standard project storm'for tlwstudy area
<br />washes was developed from a depth-area
<br />curve drawn from a frequency computation of
<br />p,~ipitationrecordsfo,GrandJunction.The
<br />standard project storm, designated as 40
<br />percent of the 3-hour probable maximum
<br />storm, was then centered over the portion of
<br />Big Salt Wash drainage basin that is above
<br />Book Cliffs and O"er the entire drainage
<br />basin" of Little Salt and Heed Washes. The
<br />ratiosofthelOO-and500-yearfloodstothe
<br />standard project flood were determined from
<br />frequency cur,'es developed for other re.orional
<br />drainage basins that are similar in size to the
<br />study area tributary drainages and
<br />experience essentially the same type cloud.
<br />
<br />burst eYents as occur on those tributaries_
<br />Loss rates for the tributary drainage basins
<br />were estimated because cloudburst floods in
<br />the area could not be reproduced due to
<br />limited streamflow data. Information on
<br />runoff in the IndianWashareuindieatesalow
<br />OAOinchinitiallo$s.Howewr,inaccordance
<br />with "trcamflow and precipitation data from
<br />similar basins in other parts of the area.
<br />initial loss rates for 100. and 500-year
<br />cloudbursl floods wcrc increased to 1.00 inch
<br />on BigSaltand LittleSaltWa~hesandtoL20
<br />inches on Reed Wash,ConstantlosscgofO.13
<br />inch per hour for Big Salt and Little Salt
<br />Wa~hesandO.20inchperhourforRcedWash
<br />were adopted. Higher loss rak8 were used On
<br />Reed Wash beeaus-c that basin has more
<br />intensive agricultural u"ethan the other
<br />basins. The resulting flows arc contained in
<br />Table 3_
<br />InformationonIO-and50-yearfloodevents
<br />was also developed as part of the hydrologic
<br />studies conducted for this report. Basically,
<br />these would also be snowmelt floods on the
<br />Colorado River and cloudburst floods on the
<br />tributary washes, but. with the <exception of
<br />showing thcir water surface profiles on Plates
<br />43.55. arc not covered in the report.
<br />
<br />'Tho <torm rosulli"" /,om tho mO!!j ""'OTO rombin.Uon of n"'t",,"lo~io,1 <onditioo< th.t 'CO <"",id",,,1
<br />'.''''''ably'~.n'';t<";.tioorth.~~raphi<al.,",'a;",,,hkha'I''''mba.,ini,I",".t,,"
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<br />OBSTRUCTIONS
<br />Natural obst,uc!ions to floodflow include
<br />trees, brush, a~d other vegetation growing
<br />along "trcams. During flood., vegetation
<br />impedes floodflow and results in backwater
<br />conditions and increased flood heights. Brush
<br />or trees washed out during floods and carried
<br />downstream could collect on bridges or plug
<br />cuh'erts.thuscreatingadammingeffectand
<br />overbank flow, Asfloodflow increases, masses
<br />of debris Can destroy a bridge or cause water
<br />le,'e15 to rise higher and more rapidl~' than
<br />normal, thus eroding and damaginj!:
<br />
<br />-abutment.. approaches, and the overlying
<br />roadbed.
<br />In generaL obstructions restrictfloodflows
<br />and may cause overbank flows, unpredictable
<br />areas of flooding, possible damag" to or
<br />destruction of bridges and other stream
<br />crossings, and increased velocity of flow
<br />immediately downstl'eam. Most of the stream
<br />crossings in the study at'ea arc obstructive to
<br />floodflows. Pertinent data on all structures
<br />are shown in Table 4, page 8. The effect of
<br />obstructions may be "een On Plates 43-55.
<br />
<br />VELOCITIES OF FLOW
<br />Duringa100-yearflood,averagevelocitics
<br />of flow in main channel and overbank areas
<br />would be as shown in Table 5, page 9,
<br />In sheet flow' areas. velocities would range
<br />from 1-3 feet per second, In some localized
<br />stream reaches, dO,,"Mtream from natural or
<br />man made obstructions, for e:<ample,
<br />velocities of flow could significantly exceed
<br />those shown in Table 5. Velocity of flow
<br />durin.o: a SOO-year flood woulll be slightly
<br />higher than during a 100-ycarflood.
<br />
<br />Water flowing at a rate of7 fcet per "econd
<br />or greater will cause severe erosion of
<br />streambanks and is capable of transporting
<br />large rocks. Streambanksand the fill around
<br />bridgeabutmentsm~-beerodedandlarge
<br />amounts of gravel. sand, and silt may be
<br />transported by wawr flowing at a rate of 5-7
<br />feet per second. Wawr flowing at about 2 feet
<br />perseoond or less will deposit sand,silt, and
<br />other debris,
<br />
<br />FLOODED AREAS
<br />The areas that would be inundated by the
<br />100-and 500-year floods arc shown on Plate.,
<br />2.42_ As may be "e<.'n frem tho~c pi::!;'.,
<br />flooding during a 100-year flood on the
<br />Colorado River would be out of banks
<br />throughout the study area and closely l!aral1el
<br />the channel witbin the natural flood plain.
<br />Floodflows will inundate sand bar islands,
<br />and bottom lands and gravel pits on both sides
<br />oftbe river. Intef5tatc IIighway 70 would be
<br />overtopped for several hundred feet between
<br />21 Road (extended) and the Highway 6.50
<br />aceesstothesouthlS70serviceroad,forabout
<br />\jl mile extending west from the Big Salt Wash
<br />erossing,and forabout~milcextendingeast
<br />from the Reed Wash crossing. Scattered rural
<br />
<br />residences, principally between Interstate
<br />Highway 70 and the river, would suffer flood
<br />damage, Floodwaters would in~ndate ma"y
<br />county roads. inclullirlg most of the river
<br />aCCesS-roads. Some agricultural lands would
<br />be flooded,and,"outh of Fruita. the northern
<br />approach to the old highway bridge would be
<br />inundated,
<br />Flooding on Reed, Big Sa1t. and Little Salt
<br />""ashes would be rcstrieted t(l immediately
<br />adjacent overbank areas except in the lower
<br />reaches where floodw&terswould spread out
<br />along the north side of Interstate Highway 70,
<br />Practic&lIy all flooding on the washes would
<br />beeonfine<l to altTiculttlral lands except fora
<br />shortresch of Little Salt Wash that traycrses
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