<br />Table 4
<br />SUW~ARY OF POTENTIAL FLOOD PEAK OI,CHARG[S
<br />
<br />d velocity 01 3 fee~ or more per second croatcs a delini~r c~n~cr of
<br />injury or orowning.
<br />
<br />Location
<br />
<br />[)rdlnaqe
<br />AeN
<br />-~
<br />(s~. "11.)
<br />
<br />Intermediate
<br />Il<XJioMI
<br />Flood
<br />Pe"k
<br />[)is.<:harqe
<br />(c.f.s;T---
<br />
<br />Standard
<br />"ro.joct
<br />FIOCX:
<br />CM"
<br />Discharco
<br />rc:r.s:)
<br />
<br />rlood~d drGJ a~d ilood Jamdges ~ The arCdS dlong Sf. Vr~in
<br />-----------------
<br />
<br />Cree~, OOth rurd I and In Longmont. rr\3t wQ>j ld be flooded "y the I nter-
<br />rlUU idte [leg i"na I arm Sta,dard Project F lood~ 3re si\0~'~ o~ rlates I
<br />through 4. '",ater 5uri<lce prof i I es for t!,ese I loodS '~ere ceter",i ree
<br />usir>g baCKwater computdtio~~ eMployi"'J h" Stdooard Slep i.'dnod. The
<br />profiles, On plates 5 a~d 6, s~ the deptMs of llooding i~ tho chan~
<br />nel and the elevation 01 the water surface5 over the adjacent tlood
<br />plain. Head losses at bridgeswereccmputed using data In "Kydr"uljcs
<br />of f3ridga '"'~rcrwa'{s" publ iShc<;: by tnl) U. S. Dan~r~menr oj Ccnvnercc.
<br />Flood bounJaries>lore.jetermined I>y Drojocting "dter s.uriaceelev a-
<br />tions to th" ~urv()yed fjood pia i n. Plates 7 "nd 8 portr~y cross
<br />sect iOlls across the channel dnd valley and sho'~ gro\<nd el evatiofis and
<br />depths of overbank floodjng. Relerence ~oints hnva bean provided to
<br />locate t 1000 elevations at interva Is,} lone; St. VrJ in Creek. T~e
<br />roterenC8 poinrs coincid~ wirh the I.ocatjon of the $urveyaJ crosS
<br />sections Jsed In tne study. ~he location of the r~l~r~~8 points
<br />Jre shown on both the "Flood!"j Ar!'a" pl;,\res and ~he "Prof I Ie" p I~tu$.
<br />Table 5 sho~~ referonco noint da~a IncludinG the olev~tions 0' the
<br />
<br />St. Yr;JinCruck
<br />below Lyons m I (),2()O 21,800
<br />~" Vraln Cr"ok iloove
<br />LeftHdnd Creek '" 11,200 34,800
<br />S>, ~ra i n Creek bel 0'"
<br />left Kan<l Creek m 15,200 39,000
<br />
<br />Froq~()ncy
<br />
<br />Floods larger than the Sta~dard Project Flood could occur.
<br />Ho,"ever. floods 5milller th"n th<l I ~term<:!d late Roq iOM I Flood arc more
<br />frequent and i~undato smaller areas thouoh they "Iso cause camaqe,
<br />hards~lp. and irlConvcnlerlCCs. Ttle historic :lcilk di:;<;harqcs ir hble
<br />~ whe~ ccrnpared with the potent io I flood ooak dl scharo<lS in toblQ 4
<br />i Ilustroto the ="llor magr+itude of tre ">ora frequ()~t II OOdS.
<br />
<br />Kazards of Large Floods
<br />The effects of the I n1"erm",diate ROlllo",,1 and StM.dard
<br />Project Floods "ere ana lyIed to deter",i"" po~ent la I f iood hazards
<br />in the study reuch flood plain. T~e analy~Is used field conditions
<br />roprescfi1"edbybrid<)ean<lvallcycroSs5octions5urveved in 197i.
<br />Factors con~idQreJ Include ux.lont of ar<ld flooded, I load depths,
<br />11000 d"m"qes, ol>struct ions to llood i lows, v"locl ti 65 01 Iloed f lo~'s,
<br />rateoiric,eofstr"Mm level. and duration 01 floodlng. Damanc5
<br />durinq fioodlng are cffectcd ov both depth aod velocity of flows.
<br />Obstructions to 110"5 can i ncrOilSC f lood dO;l~nC,. In rcc1rd -0 hu",~n
<br />h;j~,,~ds, <)e~ora Ily f I oed ""ter 2 IRet or ",ore dcep ..Jod t I owi n'l at
<br />
<br />streambed and tha I ntermed iate il.eg ional ana Sta~dard Project FlOOdS.
<br />^ be~rir3 is also ~rnvirtprt ~hnwi~ thr orirnt~tion of the cross
<br />soctjon throuqh the reierence point. Depths of flow for the Inter-
<br />med iate f,e'l ionill ~nd StandJrd Proji.!ct FloodS can bo esrl"'ated from
<br />the pref i I ~s, cr055 s{'Ctions, or reference tilble. For l!ny ,>pecii ie
<br />situlIt ion '~h>;re accuracy 01 f looJed ;'Jrp., d!!1 i ~elltlo, 15 needed it
<br />wi II be nOC<JS'i.1ry t" takE> tMe "~propr iate f loo~ elevation Ircrn one
<br />of these sourcosand loc"te by 3urvey thecorrC500Mlnq around eleva-
<br />tlonontoefloodplain.
<br />
<br />1J~~tr.'''''' fn~.' Longmont in the study reach tho Intor",ooiate
<br />fle~ion~1 FlOOd ~OJld inundat",,~ "re~ aV8rilgir.g ~,OOr, I..,..~ ~'ld~
<br />
<br />10
<br />
<br />20
<br />
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