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I I I I I I I I I I I I I I I I I I I 8 City of Littleton ~ < ~ ~~ @ R:~ Massey Draw and SJCD (South) Major Orainageway Planning Update Conceptual Design Report February 2006 4.0 Hydraulic Analysis 5. Massey Draw and SJCD (South) flow into Chatfield Reservoir and South Platte Park, respectively. Design for a pilot project to correct bank erosion problems and to reduce the phosphorus loading of the reservoir was recently completed. The project design includes bank stabilization, drop structures, and wetlands and habitat improvements. The City of Littleton is concerned about the sediment load and water quality that is discharged into the lakes within South Platte Park. Although the PMF channel that will be constructed as part of the South Platte Reservoir includes a sediment basin and an extended detention basin, additional measures may be warranted to enhance the water quality in SJCD (South). 6. During a flash flood event June 27. 2004, a flow split occurred on the Deer Creek Golf Course west of West Rockland Drive that directed flow to the north side of West Rockland Drive. South Oak Street and West Rockland Drive were overtopped and several structures were flooded along 7. West Rockland Drive. Drainage studies and plans for this area of Meadow Ranch did not show the flow split. The culverts under West Rockland Drive and South Oak Street were not designed to accommodate the increased flows. 8. As part of this project, wetland mapping and threatened and endangered species habitat assessment were performed by ERO Resources Corp. The report and figures are included in Appendix F. Wetland and riparian area along the drainageway corridors include wetlands ranging from wide cattail marshes to small patches along concrete trickle channels, Both Massey Draw and SJCD (South) and their associated wetlands appear to be waters of the United States and would be regulated by the U.S. Army Corps of Engineers, Potential habitat for three federally listed species, Preble's Meadow Jumping Mouse, Ute ladies' Tresses Orchid, and Colorado Butterfly Plant, may occur within the study area. Prior to construction or other activities, appropriately timed surveys for these species must be conducted. \ 4.1 Evaluation of Existing Facilities A hydraulic analysis was performed to estimate the capacity of the existing drainageways and structures, water surface elevations and to identify flooding problems. The project mapping was used in the analysis and was supplemented by field investigations, the drainage structure inventory, and available construction drawings and reports. The U.S. Army Corps of Engineers HEC-RAS (version 3.1.3, Reference 11) River Analysis System program and HEC-GeoRAS (version 3.1.1, Reference 12) were used for the analysis. Geometric data forthe HEC-RAS model was obtained using HEC-GeoRAS and a digital elevation model (DEM) that was generated using the project mapping, This data included the cross sections, river stations, and channel and overbank lengths. The HEC-GeoRAS data was imported into the HEC-RAS model. The boundary conditions, culvert data and flows were then added, and the models were run for the necessary storm events. The flow rates from the baseline hydrology model were used in the hydraulic analysis. These flow rates are presented in Table A-5. The drainage structure inventory was used to obtain the culvert data needed for the models, The culvert at C-470 and West Ken Caryl Avenue was determined to have a slope-tapered inlet. Due to the flow in the stream and the configuration of the culvert inlet, field measurement was not possible. Construction plans were obtained from CDOT to obtain the dimensions and elevations needed for the model, The model results were used to identify culvert overtopping. The 100-year profile was exported from HEC-RAS to HEC-GeoRAS to produce floodplain maps and identify flooding problems. The results of the hydraulic analysis and field observations are summarized in Table B-1 in Appendix B. The existing culvert sizes are shown on Figure B-1, also in AppendiX B. Massev Draw 4,2 Flood Hazards Based on the hydraulic analysis, field observations and input from the Project sponsors, the following flood hazards were identified: Reach 1 - Chatfield Reservoir to C-470: This reach is in Chatfield State Park on land owned by the U,S. Army Corps of Engineers. Downstream of the park access road, the channel slope is approximately 0.4 percent. There is extensive vegetation and the channel appears to be stable. Between the park access road and C-470, the channel slope is approximately 1.4 percent. The channel meanders immediately downstream of C-470, There are vertical banks along this reach up to 6 feet high. Massey Draw crosses under the park access road in three 7-feet diameter reinforced concrete pipe culverts, The road is overtopped by 1.9 feet during the 100-year event. It is estimated to overtop between the 25- and 50-year storm events. Reach 2 - C-470 to Wadsworth: This reach is in Chatfield State Park and is the site for the Massey Draw Watershed and Ecosystem Improvement Pilot project. Improvements were designed to stabilize the channel along this reach. Construction was completed in 2005. Reach 3 - Wadsworth to Carr: Massey Draw crosses private property between Wadsworth and Carr, Along the north bank, the houses were constructed close to the drainageway. On the south bank, at the east end, the houses are 100 to 200 feet from the center of the drainageway, At the west end, the houses are about 100 feet apart, with the drainageway passing between them. The average slope along this reach is 1.4 percent. There are vertical banks up to 6 feet high, There is an existing vertical rock drop structure, approximately 2.5 feet high, midway between Zephyr and Allison Streets. In 1999, 1. Channel slopes vary from 0.7 to 2.4 percent. These steep channels have resulted in high velocities, lateral and vertical instability. UDFCD recommends slopes of approximately 0.4 to 0.5 percent in the study area to achieve stable channels, 2. In many areas, the floodplain width has been reduced by encroaching development, resulting in higher flow depths. 3, Flood hazards in the study area include structure flooding and culvert overtopping, Jefferson County criteria do not allow culvert overtopping during the 5-year storm, Twelve inches of overtopping is allowed during the 1 OO-year event on local and collector streets. On arterial streets, a minimum clearance of 6 inches to 2 feet between the energy grade line and the culvert crown is required during the 1 OO-year event. 4, Although in many areas the drainageways flow through parks/tracts owned by the Foothills Park and Recreation District or homeowners associations, there are several reaches that flow through private property. Along these reaches there are no drainage easements or tracts dedicated for drainage maintenance, drainage improvements, or floodplain preservation. In some areas, development has encroached into the floodplain. 0\ 4-1