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I I I I I I I I I I I I I I I I I I I SECTION IV Drainage Basin Lharacteristi-es Drainage basin characteristics of hydrologic significance include drainage area, slope, basin shape, land use, soils, surface depressions, and vegetative cover. Drainage areas and slopes were determined from U.S. Geological Survey Quadrangles. The major drainage basins and sub-basins are delineated on the Basin Boundary Map (Figure IV-I). Sub-basin areas, slopes and other para- meters related to basin shape are tabulated in the Phase A and FHAD Reports. SUMMARY OF HYDROLOGIC ANALYSES Genera 1 An integral part of any major drainageway planning study is estimating the magnitude and frequency of future floods. Golden's flood history dates back to the early 1860's with loss of life and numerous incidents of severe property damage havi ng been recorded. It is important to recogni ze that floods are a natural phenomenon which have occurred in the past and will occur in the future. The purpose of the hydrologic analysis is to provide estimates of peak flow rates and corresponding probability relationships. Land Use Estimates of basin imperviousness were determined for both existing and future basin conditions. The basin imperviousness is the key variable affecting the future condition hydrology used in this study. Values of percent impervious- ness were est imated from recent aeri a 1 photography (Ref. 3) and i nformati on regarding future annexation plans, Golden's water distribution system, exist- i ng 1 and use zoni ng and proposed development plans. The future 1 and use condition assumes maximum development in each sub-basin based upon current planning. Flow frequency information is generally expressed in terms of percent probabi- lity or return period (e.g. 100-year). By definition, the 100-year flood represents a flood magnitude or di scharge havi ng a one-percent chance or probability of being equalled or exceeded in anyone year. Soils, Surface Depressions and Vegetative Cover In addition to estimating peak discharge information, the hydrologic analyses were used to develop flood hydrographs whi ch provi ded i nformat i on regardi ng flood volumes and duration of flooding. The stormwater runoff timing information determined from flood hydrographs may also be useful in implemen- ting local or County flood warning and evacuation plans. Soils information was obtained from the U.S.D.A. Soil Conservation Service. The soil types generally exhibit poor to moderate drainage characteristics and are classified predominently as Type C and D soils (Refs. 11, 12). The better drained soils represent a low percentage of soils in the basins and are typically found at the base of the foothills in the floodplains. Surface depressions and vegetative cover playa part in the hydrologic cycle at the beginning of a rainfall event where initial losses occur prior to the beginning of surface water runoff. These factors are taken into account in estimating rainfall losses. The hydrologic analyses of the Tucker Gulch and Kenneys Run drainage basins were performed to determine peak discharges and flood hydrographs for the 2-, 5-, 10-, and 100-year rainfall events. The hydrology was developed assuming future basi n condit ions with and without selected dra i nageway improvements. Hydrology was also developed for the 100-year frequency storm under existing basin conditions for comparison purposes. For additional information regarding hydrology for the unimproved condition, the user should refer to the Phase A and FHAD Reports. The following paragraphs summarize the drainage basin characteristics, methodologies, and results of the hydrologic analyses. Rainfall and Loss Parameters One-hour point rainfall depths were based on information presented in the Precipitation-Frequency Atlas of the Western United States, NOAA Atlas 2, Vol. III, Colorado, 1973, for the 2-, 5-, 10-, 50- and 100-year return periods. The one-hour values were used to develop the two-hour design storms based upon a time distribution of rainfall recommended by Urban Drainage and Flood Control District. For the Tucker Gulch basin, areal reductions of rainfall IV-1