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<br />SECTION Ill: SUMMARY OF HYDROLOGIC ANALYSIS
<br />
<br />land Use
<br />A percent impervious area was estimated for each subbasin for use in the
<br />SWMM program by Calculating a weighted averag€ 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 />surements taken from the lOO-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 value, 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, 10D-scale
<br />orthophotographic mapping from the City of Fort Collins, the Fossil Creek Basin
<br />study, and the location ofrrajor basin featur€s such as railroads, a rterialstreets
<br />and irrigation canals. Land uses and subareas are shown for historic, existing,
<br />andfuturebasinconditionsonFiguresII-l,II-2andll-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 reach€s and for de-
<br />riving and eualuating 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,reliablerainfal1,soilsandlandusedata
<br />are available for these basins, and these data have been applied to the deriva.
<br />tlon of synthetic flood hydrographs for this study area. Hydrologic analysis was
<br />performed to determine flood hydrographs based on runoff from the 2., S-, 10-,
<br />2S-, SO-, and 100-year storms, for historic, existing and future fully developed
<br />conditions. BecaUs€thepresentdrainagepatternsonHailCreekandMcClellands
<br />are diverse and resulting routing procedures numerous, a version of the runoff
<br />block 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 Creek performed by the U.S. Army Corps of Engineers, for areas common
<br />to both basins.
<br />The SWMM program is a kinematic wave ~drologic 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 ll-l, ::-2 and 1:-3 of the drawings
<br />(located in back 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 segment parameters used in the analysis have been detennined to
<br />approximate the existing channel, culvert, and detention ponds which carry the flow.
<br />Design stonn 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 €quation 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 ma~imum and minimum permeabilities of the different types of soils
<br />within each subbasin. Outliners were not included. A decay rate of O.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 />~ge 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 ll!-2,1!!-3,IIl-4,
<br />IlI-S, 1l1.6, Il1.7, 1!1-8 and 111.9. Pea~ flood flows for the various deS1gn
<br />frequencies have been plotted in Table lII-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 />R.linf.lJJ::.lt.:.
<br />
<br />Flood History
<br />Information concerning the historiC flooding occurrences in Mail Creek and
<br />McClellands Basin is ,parce. Because of the lack of development within the boun-
<br />dar1es of the floodplain, few r€ports were filed that indicated (due to lack of)
<br />significant flood damage
<br />
<br />Designrainfal1 values for the 2., S.,lO., 25-,SO-,andlOO-yearr eturn
<br />periods have been determined from the "Precipitation Frequency A tlas of the Wes-
<br />tern United States, Volume Ill-Colorado," published by the National Weather Ser-
<br />vice (Reference 11). Rainfall values are listed in Table IIl-l. A2-hour'de-
<br />sign storm hyetograph developed according to Weather Bureau procedures was used
<br />for this study.
<br />
<br />1I1-1
<br />
<br />IIl-2
<br />
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