Flood Hazard Analysis and Protection Plan for a Residential Development

An investigation was carried out to develop a flood hazard analysis and flood protection plan for a proposed residential development located in Indio, California. The project area is part of an existing floodplain that straddles portions of the alluvial fans derived from Thousand Palms Canyon and Pushawalla Canyon. Floodwaters emanating from a channel located west of the project area impact the project site. A two-dimensional flood routing model was applied to study the progression of a flood flow in the existing floodplain. Processes simulated included overland flow, infiltration, and bottom boundary roughness. Elevations were based on USGS DEM, rough grading plans, high-resolution LiDAR survey data, and data from a field reconnaissance survey. Model results were analyzed to determine the peak discharges into the proposed development. A channel was proposed to intercept the peak flows crossing the western boundary of the proposed development and convey these floodwaters southward along the western boundary and then eastward along the southern boundary of the project. The U.S. Army Corps of Engineers' River Analysis System (HEC-RAS) was utilized to predict water surface elevations in the proposed channel, and to establish freeboard conditions with respect to pad elevations. To account for the interception of flows along the western boundary, the flows in the proposed channel were progressively increased to the peak flows. To simulate the outflows across the southern boundary, lateral weir flow was assumed to occur. The results of the analysis show that the proposed flood control channel accept and discharge the floodwaters at the historical locations. The U.S. Army Corps of Engineers' HEC-6 model was used to conduct sediment transport simulations in the floodplain and onsite channels to determine the potential for sediment deposition. Model results were analyzed to estimate the sediment deposition for each cross section at the peak flow of the event, and at the end of the event. The results of our model studies indicate that sediment is transported through the system well and does not interfere with the function of the proposed stormwater channel.