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<br />SECTloNlll: SUMMARY OF HYDROLOGIC ANALYSIS
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<br />Land Use
<br />A percent impervious area was estimated for each subbasin for use in the
<br />SWMM program by calculating a weighted average of the different types of land uses
<br />within each subbasin. Types of land use for the City of Fort Collins were taken
<br />from the aerial mapping, current City and County zoning maps and land use plans.
<br />Typical percents of impervious area for each type of land use were ta~en from the
<br />Fort Co 11 ins Ora inage Criteria Manua 1 (Reference #2) and ~erifi ed aga inst real mea.
<br />surernents ta~en from the lOa-foot scale mapping. Percents impervious for existing
<br />basin conditions includes all subdivision proposed and approved although not neces.
<br />sarily constructed at the time of this analysis. Future basin values are derived
<br />primarily from zoning maps and land use plans for the City and Larimer County, and
<br />assume all development in the basin is complete according to the zoning.
<br />Subareas were determined from USGS 7.1/2 minute quadrangle maps, 100-scale
<br />orthophotographic mapping from the City of Fort Collins, the Fossil Cree~ Basin
<br />study, and the location ofrrajor basin features such as railroads, a rterialstreets
<br />and irrigation canals. Land uses and subareas are shown for historic, existing,
<br />andfuturebasinconditionsonFigureslI.l,II.2andlI.3,respectively(included
<br />in the rear poc~et).
<br />
<br />A detailed hydrologic analysis was performed to define the runOff quan.
<br />tities for historic, existing and future basin conditions. This analysis was
<br />done as a basis for delineating floodplains for the study reaches and for de.
<br />riving and evaluating alternative solutions to flooding problems. For hydraulic
<br />design purposes, flood flows derived from future basin conditions are used be.
<br />cause future development increases potential runoff.
<br />
<br />Derivation of Hydroqraphs
<br />There is little recorded information regarding past floods for either Mail
<br />CreekorMcClellandsBasins;however,reliablerainfall,soilsandlandusedata
<br />are available for these basins, and these data have been applied to the deriva.
<br />tion of synthetic flood hydrographs for this study area. Hydrologic analysis was
<br />performed to determine flood hydrographs based on runoff from the 2., 5-, 10.,
<br />25~, 50-, and lOa-year storms, for historic, existing and future fully developed
<br />conditions. Because the present drainage patterns on Hail Creek and McClella nds
<br />are diverse and resulting routing procedures numerous, a version of the runoff
<br />bloc~ of the Environmental Protection Agency's Stormwater Management Model, (SWMM) ,
<br />as modified by the Corps Missouri River Division, was selected to model the drainage
<br />basin runOff characteristics. The input data was calibrated against SWMM runs
<br />for Fossil Cree~ performed by the U.S. Army Corps of Engineers, for areas common
<br />to both basins.
<br />The SWMM program is a ~inematic wave hydrologic model whiCh synthesizes
<br />flood hydrographs by routing precipitation through a system of subcatchments
<br />and stream channels. Input requirements for the SWMMmodel are rainfall hyeto-
<br />graphs, watershed parameters, and Channel segment characteristics. The drainage
<br />basins, sub.area and channel configurations developed in this study for applica.
<br />tion in the S~~~ program are shown on rigures 11.1, ::-2 Jnd 1:-3 of the drawings
<br />(located in bac~ pOCket). Final calibrated values for the Mail Creek and McClel.
<br />landS Basin parameters used in the SWMM analysis are listed in the Technical Adden-
<br />dum. Channel segrnentparameters used in the analysis have been determined to
<br />approximate the existing channel, culvert, and detention ponds which carry the flow.
<br />Design storm hydrographs have been derived by routing runoff from design
<br />rainfall through the channel and pipe system of Mail Creek and McClellands drain.
<br />agcways.
<br />
<br />Losses
<br />Losses due to infiltration were estimated from USDA Soil Conservation Ser.
<br />vice Soils Survey. A variable infiltration rate using Horton's equation is avail.
<br />able in the SWMM program, and this option was utilized in the derivation of hydro.
<br />graphs. Initial and final infiltration rates were estimated by taking weighted
<br />averages of the maximum and minimum permeabilities of the different types of soils
<br />within each subbasin. Outliners were not included. A decay rate of D.0018/sec.
<br />was adopted from the Fossil Creek Study. Values of infiltration coefficients for
<br />the subbasins range from 0.40 to 3.00.
<br />Maximum depression storage values were obtained from the Urban Storm Drain.
<br />age Criteria Manual. Uniform depression storage coefficients of 0.3 for pervious
<br />dnd 0.1 for impervious areas were used in this study.
<br />
<br />Flood HydroqraphS
<br />SelectedfloodhydrographsareillustratedlnFigures II!-2,1!!-3, 11l-4,
<br />1l1.5, 1l1.6, Ill.?, !!I.8 and III-g. Pea~ flood flows for the various deS1gn
<br />frequencies have been plotted in Table 111.2. Complete hydrologic data and .com.
<br />puter output is contained in the TeChnical Addendum, available through the City
<br />of Fort Collins.
<br />
<br />Rainfall::.:.t.:.
<br />
<br />Flood History
<br />Information concerning the historiC flooding occurrences in Mail Cree~ and
<br />McClellandS Basin is sparce. Because of the lack of development within the boun.
<br />dar1es of the floodplain, few reports were filed that indicated (due to lac~ of)
<br />significant flOOd damage
<br />
<br />Designrainfal1 values for the 2., 5.,10., 25-,50.,andlOo.year return
<br />periods have been determined from the "Precipitation Frequency A tlas of the Wes.
<br />tern United States, Volume 111.Colorado," published by the National Weather Ser-
<br />vice (Reference 11). Rainfall values are listed in Table 11l-1. A2-hourde.
<br />sign storm hyetograph developed according to Weather Bureau procedures was used
<br />for this study.
<br />
<br />ITI-l
<br />
<br />II1-2
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