Board of Adjustment Agenda and Packet 2003 04 16

Home Browse Search

SH 42/96t Street Connection Drainage Report for Coal Creek Bridge FLOODPLAIN REPORT FOR 96`h STREET EXTENSION / CONNECTION COAL CREEK BRIDGE CITY OF LOUISVILLE, COLORADO Introduction U i3 - MAr: 2 8 200 _i The City of Louisville will be constructing a roadway improvement project for 96th Street from south of Pine Street to a connection with the Northwest Parkway south of Dillon Road. The alignment of 96th Street is generally north -south. The project will include a new crossing of Coal Creek about 800 feet south of Colorado State Highway 42 and 750 feet east of County Road. At the Coal Creek crossing, the new 96th Street roadway will ultimately include two lanes in each direction, a raised median, with outside shoulders and separated pedestrian / bicycle trails on each side. Construction of the project is scheduled to begin in the summer of 2003. This report summarizes the floodplain related issues of the bridge to be constructed at this new roadway crossing of Coal Creek. Coal Creek Drainase Basin Description Coal Creek begins as a foothills stream many miles west of the project site. The drainageway flows northeasterly to join Boulder Creek in east -central Boulder County. Snowpack, springs, and intense thunderstorms contribute to this perennial stream. The upper portion of the basin is covered with mixed mountain forest, and foothills shrubs and native grasses. The lower basin is agricultural and open space land with some areas being rapidly urbanized. The drainage basin of Coal Creek at the project site is about 30 square miles. Soils in the watershed are shallow, and range from very gravelly and stoney in the foothills areas to gravelly, sandy, and clayey in the lower areas. These soils have been designated by the Soil Conservation Service to be primarily in Hydrologic Soil Groups C and D with moderate to high runoff potential. Coal Creek Channel Description Coal Creek has a well defined meandering low flow channel with wide gentle overbanks. The overall slope of the drainageway is about 1.4 percent. At the location of the proposed 96th Street bridge, Coal Creek is a riparian corridor with dense trees and shrubs. The low flow channel and inner overbanks are lined with woody and herbaceous wetland vegetation, and the outer overbanks have native prairie grasses. The corridor is relatively undisturbed and the vegetation is in good condition. There is an existing gravel surface trail following Coal Creek on the north edge of the riparian corridor. The floodplain area at the proposed bridge site consists of silty sands, gravelly sand, and gravel overburden and underlying sandstone and claystone bedrock. At the low flow channel the bedrock is about 13 feet deep. Much of the low flow channel bottom is armored with 4 to 6 inch cobble. There is no evidence of significant erosion or sedimentation along the drainageway within the project reach. There is an existing concrete irrigation diversion structure about 3 feet high, 125 feet upstream of the proposed bridge. There is also an existing soil and rubble irrigation diversion structure about 3 feet high, 200 feet downstream of the proposed bridge. When low flows are high and the diversion gate is closed, the elevation of the top of this downstream diversion structure can back water up co the bottom of the diversion structure upstream of the new bridge. W1LSOPJ &COMPANY EllensgN[ennecllon