2006-06-12_REVISION - M1982015 (23)

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CONDITIONAL LETTER OF MAP REVISION (CLOMR) FOR • ST. VRAIN LAKES Page 6 It was determined that the topography to the south of the study reach remained primarily unchanged since the ACOE study in 1977. Therefore the two-foot contours from the ACOE 1977 study work maps were digitized and translated to NAVD88 and used in the area south of the St. Vrain River. There were new lakes as a result of gravel mining south of the St. Vrain River, upstream of WCR13, that were not accounted for by the 1977 topography. Carroll & Lange, Inc. surveyed the high water surtace elevation of these lakes, assuming the lakes would fill up during the flood and they were included in the model as blocked obstructions. There is current gravel mining downstream of WCR13. The effects of this mining were approximately modeled with the use of blocked obstructions in the areas of the current lakes created by the mining. However, the high water elevation of these lakes downstream of WCR 13 was not surveyed because the mining is on-going and therefore continuing to change. It was also determined that the actual location of the river has moved in several locations since the 1977 ACOE study due to a combination of erosion and mining operations. The location of the existing Last Chance Ditch was used as a • reference basis when revising the river location as it was determined that the ditch had not moved. Cross-section 62715 and 59195 were added to model the oxbow configuration of the St. Vrain River. This was done by placing the main channel stations of the river at the upstream intersection of cross-section 62715 and the at the most downstream intersection of cross-section 59195. The channel downstream reach length was input at the length following the river, and the right and left overbank lengths were input as the straight length to the downstream cross- section. Since the area north of the St. Vrain River was mined for gravel and will be reclaimed as lakes, a berm .effect has resulted in the area between the excavated lakes and the St. Vrain River. This berm is not a certified flood levee and the 100-year and the 500-year flood overtops this berm at several low points along the top of the berm. Therefore, a lateral weir was modeled at four low- point locations along the berm to model this split flow scenario. The existing and proposed lateral weirs are modeled with a weir coefficient of 2.63. Documentation to support this determination is provided in Appendix A. The flows that overtopped into the lakes were then modeled as flowing into a separate reach named "Existing Lakes." There are existing berms that separate each of the three lakes up to an elevation of approximately 4,828 feet. Inline • weirs have been modeled at these ground-overtopping locations. The flow in this reach and the flow in the St. Vrain River join at a junction at cross-section 65735.