2006-06-12_REVISION - M1982015 (23)

Home Browse Search

CONDITIONAL LETTER OF MAP REVISION (CLOMR) FOR • ST. VRAIN LAKES Page 4 VII. DRAINAGE IMPROVEMENTS Drainage improvements to the existing gravel mined lakes will serve as the detention requirements of the St. Vrain Lakes development. The lakes will regulate the release rate from the St. Vrain Lakes development for the 10-year and 100-year event by the use of an outlet structure. This outlet structure will release detained flows at the 5-year onsite historic rate. The outlet structure elevation will be set at 4,819.0 feet and is sized so that 0.7 feet of head will convey the 5-yr historic release rate. The emergency spillway will be set at a crest elevation of 4,819.7 feet. The 100-year offsite flows will be conveyed over the spillway to the St. Vrain River. The portions of the flood from the St. Vrain River, which overtop the berm, will be conveyed through the lakes through proposed inline spillways and back to the St. Vrain River. • VIII. HYDRAULIC ANALYSIS The hydraulic analysis along the St. Vrain River was performed using the U.S. Army Corps of Engineers' Water Surtace Profile computer program, HEC-RAS. Manning's n-values -The typical Manning's n-values used for the hydraulic models was 0.04 for the main river channel and 0.07 for the river overbanks. These n-values are consistent with the ACOE model. In the "Existing Lakes" reach, the n-values were adjusted according to the proposed condition. Documentation to support this determination of the Manning's n-value is provided in Appendix A. More about this is discussed in the Post-Project Conditions Model. Pre-Proiect Conditions Model -The original floodplain model data of the lower St. Vrain River has been obtained from the ACOE. This model was titled Vrain1.dat. This model was in the HEC-2 format prior to the bridge routines changing from the Omaha District Bridge Routine to the Special Bridge or Normal Bridge routine. The model was therefore updated and converted into HEC-RAS format. This model was run with a downstream boundary condition of a normal depth with a friction slope of 0.0025 ft/ft. This is the approximate slope of the channel • in this location as determined from the USGS Quad map. The model stretches from the downstream edge of the I-25 Bridge to the fully effective portion of the river downstream of the effects from the WCR13 bridge.