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~ City 0/ Littleton ~~'I. ~:"'- \~-'~' ~.rf~~ M.;1W'y Dravv ~nd SJCD ISourhl MajOf Dr"W1c1ge..vay Planning Update Conceptual ~!"l Report FeOnJ"yy 2006 Project Area Description HEC-RAS to HEC-GeoRAS to produce floodplain maps and to identify flooding problems, Appendix A contains tables and figures developed during the hydrologic analysis. The study area is located in southern Jefferson County, north of Colorado State Highway C.470 and west of the South Platte River, and includes the Massey Draw and SJCO (South) watersheds. The area east of South Platte Canyon Road is located in the City of Littleton. The remainder of the study area is located in unincorporated Jefferson and Arapahoe counties. Figure ES~ 1 shows the project study area. Alternative Analysis The watershed characteristics and results of the hydraulic analysis were compared to UDFCD and Jefferson County design criteria to identify problems within the study area by reach. Major observations from the analysis include the following: The Massey Draw watershed covers approximately 9.0 square miles. The study limit for Massey Draw is the hogback and for the North Tributary, it is Continental Divide Road. The Massey Draw watershed is fully developed except for a parcel located southwest of South Carr Street and West Chatfield Avenue, and some in-fill parcels located throughout the watershed. SJCD (South) begins at South Platte Park and extends west to approximately South Simms Street. It has three tributaries. The SJCD (South) watershed covers approximately 3.4 square miles and is fully developed, except for some parcels between South Carr Street and South Kipling Parkway. Peak flows and hydrographs were developed for each of the 81 subwatersheds for the 2-, 5-,10-,25-, SO-, 100-, and SOD-year return period storms using the Colorado Urban Hydrograph Procedure (CUHP2000). Subwatershed hydrographs were routed using the Urban Drainage Stormwater Management Model (UDSWM2000) to determine peak flow rates at select design points. . Steep channel slopes ranging from a.7 to 2.4 percent exist and have caused lateral and vertica instability of the streambanks. . Maximum depth and velocity criteria are exceeded in many reaches of the drainageway. . Flood hazards in the study area include structure flooding and culvert overtopping. . Several reaches flow through private property. There are no drainage easements or tracts dedicated for drainage maintenance, drainage improvements, or floodplain preservation. . Development has encroached into the floodplain in certain areas. . A flow split occurs on the Deer Creek Golf Course and results in structure flooding along S. Oak Street and W. Rockland Drive. . Massey Draw and SJCD (South) flow into Chatfield Reservoir and South Platte Park. Water quality is therefore a concern. A calibration model and a baseline model were prepared based on the existing conditions in the study areas since the watersheds are essentially fully developed. The calibration model was used to approximate the 1 DO-year flows from the Flood Insurance Study (FIS), Jefferson County and Incorporated Areas, and a 1985 UDFCD memorandum that updated flows along Massey Draw. The 1 aD-year flows from the calibration model matched reasonably well with the target flow values. Existing detention on Massey Draw, upstream of Carr Street, and on SJCD (South), upstream of Kipling Parkway, were added to create the baseline model, which was used to identify problems and evaluate alternatives. The baseline model for Massey Draw differs from than the model presented in the April 2004 Hydrology Report. Modifications to the original baseline model contained herein were made to incorporate flow patterns observed during the June 27, 2004 flash flooding event. A summary of 100-year peak flows for select locations within the watersheds is shown in Table ES-3. Improvements to alleviate the identified problems were developed and evaluated along each of the reaches, with the exceptions of Massey Draw downstream of Wadsworth, and SJCD (South) downstream of South Platte Canyon Road, where improvements have already been approved or are being designed. The alternative improvements that were evaluated included: channel stabilization (low flow grade control, drop structures, stream bank armoring or reshaping); floodplain preservation; regional stormwater detention; culvert improvements; easement and/or property acquisition; and regional stormwater quality enhancement facilities. Six alternatives that specifically address the flooding problems in the Meadow Ranch area were evaluated. Maintaining the status quo was also considered. Selected Plan I I I l00-year Flow lefs\ 3027 1,181 2,070 946 2,276 The Notice to Proceed and Selected Plan for Massey Draw and SJCD (South) were issued on April 11 , 2005, and are contained in Appendix D. The Selected Plan essentially followed the recommendations made in the Alternatives Evaluation Report for Massey Draw and SJCD (South) Major Drainageway Planning Update, dated February 2005 (Reference 14). Specific direction was given to Drainaaewav Massey Draw North Tributary Massey: Draw Massey Draw SJCD (South) North Tributary SJCD (South) Table ES - 3 10o-vear Peak Flows at Select Locations I Location I Wadsworth Wadsworth C-470 Ken Caryl S. Platte Canyon . Reexamine the need for a low flow channel downstream of the diversion levee on Massey Draw intended to divert the 1 DO-year flows to the south side of West Rockland Drive, . Comply with the minimum FEMA levee requirements for structural integrity and freeboard (Le., minimum of 3 feet), The flow rales from the baseline hydrology model were used in a hydraulic analysis that was performed to estimate the capacity of the existing drainageways, determine water surface elevations, and to identify flooding problems and culvert overtopping. The project mapping, supplemented by field investigations, a drainage structure inventory, and available construction drawings and reports were used in the analysis. The U.S. Army Corps of Engineers HEC-RAS River Analysis System program and HEC-GeoRAS were used to conduct the analysis. The 1 DO-year flood profile was exported from . Add or enlarge the low flow channel, as needed, so as to not increase the floodplain or floodway elevations, that exists without the levee, with the added flows. The legal opinion provided by UDFCD is contained in Appendix E. 0\ ES-2