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<br />Concrete n = ,018
<br />Grass n = ,032
<br />Natural with Debris n = 0.64
<br />
<br />720 C.F.S.
<br />400 C.F.S.
<br />200 C.F.S.
<br />
<br />'J
<br />
<br />
<br />
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<br />
<br />3) U.S. Army Corp:> of Engmeers, Missouri River Distnct, "Runoff Block," in
<br />Storm Water ~anagement Model (SWMM). U.S Environmental Protection
<br />Agency, Omaha, NE:L:J:S:'"Army Corps of Engineers. 197~1.
<br />4) U.S. Department of Agriculture, Soil Conservation ServicH. .National r;:;-..,
<br />EnQineenn-9J1andb_ook, Section 4 Hydrology. Washington, D.C., 1971 ~
<br />For ungauged rural areas, the hydroiogist should use a regional analysis
<br />approach, This approach involves developing representative regression
<br />curves for the region based on available gauge records for both rainfall and
<br />snowmelt.Acceptabl<~ procedures are:
<br />
<br />1) U.S. Water Resources Council, Guidelines for Determinina Flood Flow Fre-
<br />quency, BullE~tin # 17A. Washington, D.C., 197J.
<br />2) Colorado Water Conservation Board, Manual for Estimating Flood Charac.
<br />teristics of Natural"Flow Streams in Colorado. Technic;~1 Manual :1:1. Denver.
<br />CO 1976, - -- --
<br />3) U.S. Department of Agriculture, Soil Conservation Senlice, Procedures for
<br />Determininq Pe~/ows in Colorado. Denver, CO 191'1. .
<br />4) U.S. Department of Commerce, NOAA Atlas #2, Pr~ita!Jqn.Erequency
<br />Atlas of the Western United States, Volume !II. Colorado, Silver Spring, MD
<br />19n@ - -- '
<br />For gauged basins, both urban and rural, with at least 20 years of systematic
<br />stream gauging records the hydrologist should perform a probability distribu-
<br />tion analysis of thE! gauge data in order to assign probabilities to different
<br />flows. The anaiysis should use the log-Pearson III technique. and it should
<br />hciude a regional skew coefficient appropriate to the region being studied.
<br />I,cceptable procedures are:
<br />
<br />1) U.S. Water Resources Council, Guidelines for Determining Flood Flow
<br />Frequency.
<br />2) Beard, Leo R. ~:tati~,ticaf Methods in Hydrolo9't Sacramento, CA, U. S. Army
<br />Corps of Engineers, 1962<~
<br />
<br />l-lydraulic Detern,inati,)ns
<br />When the topographiC Shape of the stream channel and adjacent lands in
<br />the valley have been adequateiy represented by cross"sE!ctions, and when
<br />the flow rates and volumes have been determined, the next step is a hydraui-
<br />ic analysis. This analysis will describe the depth of flow along a seiected
<br />ntudy reach and present it in profile view. These depth'of-flow determinations
<br />assist in establishing regulatory flood elevations for the 10-, 50., 100-, and
<br />1;oO-year floods and allow the preparation of a 'floodplain map to show the
<br />floodplain in plan Vi'1W.
<br />The 10-,50-,100- and !500-year water surface eievations and profiles must
<br />be calculated by using the Corps of Engineers' HEC-II Computer Program or
<br />an acceptable backwat"r - step procedure,
<br />A part of the hydraulic analysis is the seiection of rouahness coefficients,
<br />"n's," for each cross"section based on the existing conditions along the
<br />ntream and floodpiain, These "n's," which represent the roughness or resis'
<br />lance to water flow created by vegetation, rocks, and other channel features.
<br />are essential for appropriately describing the flow, Past flood data, if avail-
<br />able, are used to verify the adopted roughness coefficients. taking into con.
<br />nlderation any altEHation in the channel subsequent to those floods. If there
<br />are stream gauging stations within the study reach, the computed water sur-
<br />lace profiles are chE!cked against the rating curves for those stations to verify
<br />in the n-value.
<br />Another part of hydraulic analysis is determining the effects of cuiverts and
<br />ixidges. They can affect flood flows by constricting them or obstructing them
<br />completely. All culverts and bridges are evaluated for the foilowing hydraulic
<br />characteristics:
<br />. propenSity for reduced conveyance capacity due to debris blockage of
<br />the culvert or bridge;
<br />. damming effElct due to undersized structures; and
<br />. diversion of flood flows or washout of structures due to the above
<br />characteristics.
<br />The potential for biockage of structures such as brid~les and culverts and
<br />.the subsequent roduction in conveyance is based on watershed characteris-
<br />tics such as erodibility of channel banks, amount and type of vegetation
<br />~Iong the stream, and nize and character of thE! waterway.
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