FLOOD11632 (2) - Laserfiche WebLink

overflow channel. Therefore, two models were run to represent various hydraulic conditions. The entire flow was routed in one model down the main channel and in a second model all of the flow was routed down the overflow channel. This would represent the worst case scenario that could result from either scenario for the 100-year floodplain. The Approximate 100-year Floodplain Map is located at ahe end of this report and represents both of these flow regimes. All hydraulic calculations are included in thf; appendix. As mentioned earlier, a berm exists on the eastern side of the Town of San Luis. This berm has been recently breached near Tenth Street by a private landowner and the Town is concerned how this will affect flooding from t:he Rito Seco. It has been brought to thE; attention of the project team that the County Commissioner, Ed Vigil, believes that the breach will allow flood flows to enter the town. The HEC-RAS output developed for this study demonstrates that the breach in the bern~ does not affect the town from the 100-year flood event under the current survey conditions. This does not imply that the breach in the berm could not have an effect on larger floods. It also does not imply that the breach will not negatively impact the Tow:n under certain changing conditions in the watershed. However, under current conditions,, the breach will begin to allow flow to enter the t~own at approximately twice the 500-year flow. It should also be emphasized that when FEMA and Costilla County undergo the proces:~ of updating the county Flood Insurance Rate Maps, the berm will not b~ considered a certified levee and therefore will not be allowed to be shown as protecting the town residents from flooding that could occur from the Rito Seco. The approximate floodplain map included in this report represents the physical conditions on the date of the survey and is not intended to represent the ultimate regulatory conditions. It should be noted that south of the breach (from 9th Street south to the County Road), the secondary channel shifts west so that the invert c>f this flowpath is essentially at the toe c-f the levee. For this reason, even for events below a 100-year event, the levee provides physical protection to the Town, although as noted earlier, this will not be reflected by FEMA when floodplain maps get formally updated. Conclusion The study of the floodplain associated with the Rito Seco demonstrates that the breach in the berm is not affected during the 100-year flood event. The hydraulic model demonstrated that it would take a flood at least twice the 500-year in magnitude to flow through the breach. However, this is an alluvial i'an floodplain and site conditions can change which could change these findings. The approximate floodplain map associated with this study is not intended to be a FEMA regulatory floodplain, but does show how the flood will behave with the site conditions that were surveyed as part of this project. The approximate floodplain map is included at the end of this repoR. San Luis, Colorado - Rito Seco Approximate Floodplain