FLOOD06961 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I & City of Littleton Massey Draw and SJCD (South) Major Drainageway Planning Update Conceptual Design Report February 2006 3.0 Hydrologic Analysis Jefferson County Community Plan was used to determine the future anticipated land uses and related imperviousness. The percentages of imperviousness within the study area are shown in Figures A-3 and A-4. 3.1 Overview Peak flows and hydrographs were developed for each of the 81 subwatersheds for the 2-,5-,10-, 2S-, 50-,100-, and 500-year return period storms using the Colorado Urban Hydrograph Procedure (CUHP2000), Version 1.1.0 (Reference 4), Hydrographs for the subwatersheds were routed using the Urban Drainage Stormwater Management Model (UDSWM2000), Version 1.4.6 (Reference 5) to determine peak flow rates at select design points. A calibration model and a baseline model were prepared. The calibration model included two existing detention basins on Massey Draw that are located west of the hogback. This model was used to approximate the 1 OO-year flows from the Flood Insurance Study (FIS), Jefferson County and Incorporated Areas (Reference 6) and the 1985 UDFCD memorandum (Reference 7), which updated flows along Massey Draw, In the baseline model, the existing detention on Massey Draw, upstream of Carr Street, and on SJCD (South), upstream of Kipling Parkway, was added. The baseline model was used in the alternative evaluation phase of the study. 3.4 Depression Losses Pervious depression loss of 0.4 inch and impervious depression loss of 0,10 inch were used for the area west of the hogback. These values match those used in the Final Stormwater Detention Study ''The Valley" (Muller Report, Reference B). For the remainder of the study area, recommended pervious depression loss of 0.35 inch and the impervious depression loss of O.OS inch from Table RO- 6 in the UDFCD Criteria Manual (Reference 9) were used, 3.5 Infiltration Soils in the watersheds are predominantly hydrologic soils group 8, C and D, as defined by the NRCS, These soils are generally characterized by low permeability and high runoff. Hydrologic soils group B soils are also present. Figures A-5 and A-6 show the hydrologic soil groups for the study area. West of the hogback the soil infiltration parameters from the Muller Report were used. The initial infiltration rate range was 3.9-4.5 inches per hour, the final infiltration rate range was 0,6-0,7 inches per hour, and the Horton's exponential decay rate coefficient range was 0.0016 to 0,0018. For the remainder of the study area the parameters from Table RO-7 in the UDFCD Criteria Manual for type C and D soils were used. For type C and D soils the initial infiltration rate is 3.0 inches per hour, the final infiltration rate is O.S inch per hour and the decay coefficient is 0,0018. The Horton's exponential decay rate coefficient was 0.0018. 3.2 Subwatershed Delineation Subwatersheds in the study area were initially delineated using USGS digital elevation models (DEM's). Adjustments to the subwatershed boundaries were made, based on field observations, to reflect the flow pattern changes that were the result of development. The areas of the subwatersheds delineated for this master plan update were compared to those delineated for the Original Plan, and are presented in Table 1. The area of the North Tributary of Massey Draw was 16.6 percent greater than that in the Original Plan, due to a more refined delineation of the watershed between South Carr Street and South Kipling Parkway, along the southerly boundary. The resulting delineated subwatersheds are shown in Figures A-1 and A-2 in Appendix A. 3.6 Rainfall The following 1-hour rainfall depths from Table S01 of the Jefferson County Storm Drainage Criteria (Reference 10) were used. The 25-year and SOO-year 1-hour rainfall depths were determined by interpolation and extrapolation, respectively, of a logarithmic plot of the 1-hour rainfall depths. The rainfall depths shown in Table 2 were entered into CUHP2000 to generate hydrographs for the study area subwatersheds. Tables A-1 and A-2 in Appendix A show the subwatershed characteristics in the form of the input parameters for the CUHP2000 model for Massey Draw and SJCD (South), respectively. Each storm event was run separately, and Tables A-1and A-2 show the 100-year storm model input. Table 1 - Comparison of Watershed Areas Oriainal Plan vs. Master Plan U date Original Original Original Update Watershed Plan Plan Plan Area Percent Design Subwater- Area Difference Point sheds l(sQ mil (sq mil Massev Draw 11 D1, D2, F 7.84 7.52 -4.1% North Tributarv Massev Draw 9 C1,C2 0.99 1.1S 16.6% Massev Draw and North Tributarv 8.83 8,67 -1.8% SJCD (South) 3 A, B1, B2, 3.04 3.35 10.4% E1,E2 SJCD (South\ North Tributarv 0.70 0,61 -12.9% SJCD (South\ and North Tributarvl 3.74 3.96 6.0% Table 2 - One-Hour Rainfall Depths Zone I Zone IIA Below 6000' Above 6000' 2-year 1,02 0.95 5-year 1.42 1,33 10-year 1,68 1.S7 25-year 2.06 1.93 SO-year 2.32 2.17 100-vear 2,66 2.48 500-vear 3.30 3.08 3,3 Watershed Imperviousness The study area is essentially fully developed, with vacant in-fill parcels scattered throughout the study area. The subwatershed imperviousness was determined using GIS land use maps provided by Jefferson County and aerial photographs. For those areas that are not yet developed, the South 0\ 3-1