2006-06-12_REVISION - M1982015 (23)

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CONDITIONAL LETTER OF MAP REVISION (CLOMR) FOR • ST. VRAIN LAKES Page 7 This junction is also modeled as an inline weir. This flow is therefore effectively conveyed through the lakes and back into the river, not retained. This floodplain has been plotted on the pre-project conditions CLOMR work map provided within the map section of this report. The 10-, 50-, 100-, and the 500- year events have been modeled and the results of these models have been presented within Appendix B. Post-Proiect Conditions Model - Apost-project conditions HEC-RAS computer model has been created to model the proposed grading within the floodplain. The proposed grading lies exclusively within the reach "Existing Lakes" and at northern edge of cross-sections 57401 and 56496. The grading at cross- sections 57401 and 56496 is to facilitate a future detention pond for the future phases of St. Vrain Lakes Filing No. 1. All other cross-sections are identical between the pre-project and post-project models. • The lateral weirs modeled in the post project condition model have a weir coefficient of 2.63. The Manning's roughness coefficient for the along the shoreline of the "Existing Lakes" reach is 0.035 because the proposed grading will be gently rolling and native grass and manicured turf. The Manning's coefficient will remain 0.04 in the middle of the "Existing Lakes" reach as there is no proposed change to this area. The Manning's coefficient was increased on the cross-section for the inline weirs to account for the proposed riprap spillways at cross-sections 69479 and 65125. Documentation to support this determination of Manning's roughness coefficient is provided in Appendix A. The berm effect for the post-project condition was modeled in the same manner as in the Pre-Project condition model. The 500-year model will be assumed to over-top the berm in the same manner as the 100-year model. The proposed grading for the lateral weirs will regulate the quantity of flow that overtops the berm and will keep this quantity of flow above the flow rate that overtopped the berm during the pre-project condition. The results of the post-project conditions model have been presented in Appendix A. The 10-, 50-, 100-, and the 500-year events have been modeled. The resulting floodplains have plotted and shown on the post-project conditions CLOMR work map provided within the "Appendix D - CLOMR Work Map" section of this report. •