FLOOD11632 (2) - Laserfiche WebLink

Executive Summary This report summarizes the findings of the flooding extents from the Rito Seco located in San Luis, Colorado. The town of San Luis is located in Costilla County. 'The study was undertaken to see how a breach in a berm located on the west overbank of the Rito Seco affects the floodplain. The berm is not a FEMA certified levee and according to FEMA policy, will not be considered when a more formal map adoption takes place. At the request of the local jurisdictions, this report demonstrates how the 100-year floodplain will appear if the berm is in place using a survey from January 5, 2007, but does not necessarily reflect future FEMA regulatory conditions. The findings of this report demonstrate that it will take a flood of approximately twice the 500-year event to flow through the breach. However, the Rito Seco floodplain is an alluvial fan and the flow paths can change altering these results and should be considerE:d in making a decision about the breach in the levee. The approximate floodplain map associated with this study is included at the end of this repoR. Introduction HDR Engineering Inc. was contracted by Costilla County and the Colorado Water Conservation Board (CWCB) to survey and provide an approximate floodplain map for the Rito Seco located in the vicinity of San Luis, Colorado. On January 5, 2007, HDR personnel surveyed the Rito Seco. This report is a summary of the work done and information gathered for the Rito Seco and provides an approximate floodplain map. Ttiis approximate floodplain map is not intended to be a FEMA regulatory f~oodplain map nor to be used in future regulatory updates due to the condition of the adjacent berm. The project had originated from a land owner breaching a berm that is located on the east side of San Luis that protects the town from flood flows. Site The stream is located on the east side of San Luis, Colorado. The floo~ing that can occur on the Rito Seco originates in the Sangre de Cristo Mountains and flows out onto an alluvial fan. The alluvial fan consists of highly erodable soils, and aerial photos show th.at the channel has moved to different locations in the pas~ than the current configuration. The main channel of the Rito Seco flows directly through the town, anc~ a flood of any significant magnitude would severely impact its citizens and businesses. In response to an unusually large snowpack during the winter of 1978/79, the U.S. Army Corps of Engineers (COE) built a berm under emergency measures to protect the Town of San Luis from flood-producing runoff. As part of this project, a diversion s7ructure was constructed northeast of town that allows small flows to remain in the main channel while redirecting large flows east of town in an overflow channel south to Culebra Creek. Th~. berm was built between the overtlow channel and the town limits to prevent the bypasse;s floodwaters from reentering the town prior to the channel's confluence with Culebra San Luis, Colorado - Rito Seco Approximate Floodplain