<p> Spillway Modifications for Adapting with Revised FEMA FIRM at The Lake at Las Vegas</p><p>By: James A. Witnik, P.E., MWH Global, USA Angela I. MacKinnon, P.E., MWH Global, USA Edwin “Chip” Paulson, P.E., MWH Global, USA Steven P. Weber, Ph.D., MWH Global, USA Raul V. Valdez, P.E., MWH Global, USA</p><p>Abstract:</p><p>The Lake at Las Vegas (LLV) is a man-made lake located on the Las Vegas Wash within the limits of Henderson, Nevada. Approximately 10,392 acre-feet of water is impounded behind the dam at the normal operating stage. Approximately 3,600-acres of land are developed around the artificial lake which encompasses four major resort-casinos, three golf courses, residential communities, and marina facilities. </p><p>Hydrologic conditions within the watershed upstream of LLV have changed dramatically since the original design of the lake’s hydraulic structures. The official 100-year flood inflow hydrograph for the Las Vegas Wash upstream of LLV was estimated to have a peak flow of 11,500-cfs during the original 1988 dam design. In 2007, the Clark County Regional Flood Control District (CCRFCD) prepared a comprehensive re-study of the Las Vegas Wash and its tributaries and revised the 100-year flood inflow hydrograph upstream of LLV to a peak flow of 22,530-cfs. Revision to the inflow hydrology caused the base flood elevation (BFE) recognized by the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) to rise from 1406.80-feet to 1408.30-feet, lake datum. The change in BFE impacted existing residential communities, resorts, and habitable structures along the lake shoreline and also impeded further lakeside development. </p><p>In order to maintain the original BFE stage within the lake while accepting double the 100-year flood inflow rate, the modification included lowering the service spillway ogee crest by 6-feet and installation of two 65-foot long, 5-foot high pneumatically actuated hydraulic gates. </p><p>This paper discusses the design approach for lowering of the LLV service spillway ogee crest with precision structural demolition methods as well as and installation of pneumatically actuated spillway gates and the reservoir operating design methodology. </p>