The Westminster, East Garden Grove, CA Flood Risk Management Study is being conducted in accordance with the study resolution adopted by the Committee on Public Works, House of Representatives Committee on Public Works on May 8, 1964 (Flood Control Act of 1938). It includes analysis of multiple alternatives to reduce risks to life safety, damages to residential and commercial structures, damages to public infrastructure, and time lost due to traffic delays caused by flooded transportation routes.
Urbanization of the Westminster watershed since the 1950s has increased the potential for flood related damages, and life safety impacts associated with the overtopping of existing drainage channel systems during short duration, high intensity rainfall events. Urbanization has also increased the total amount of impermeable area, resulting in higher volumes of stormwater being directed to the drainage channels due to limited infiltration opportunities. The combination of increased runoff and underperforming conveyance channels results in increased flood risk for the residents of the Westminster watershed. Flooding also impacts traffic on area roadways. For example, portions of the Pacific Coast Highway, a major transportation route in the region, flood during storm events and/or during extreme tide conditions. Interstate-405, that connects Orange County to both Los Angeles County to the north and San Diego County to the south, is also affected by storm events. Interstate-405 within Orange County is one of the top three most heavily used freeways in California according to 2016 CalTrans Data. Flooding that inundates Interstate-405 during frequent storm events, causes major delays to commercial, commuter, and emergency traffic in the watershed.
An additional area of flood risk is related to the possible spread of contaminants in the Bolsa Chica Ecological Reserve resulting from large storm events that overtop drainage channels upstream of the reserve and inundate the active oil fields within it. If flood waters inundate these oil fields, widespread distribution of oil-laden runoff could potentially be transported to previously completed ecosystem restoration projects and eventually the Pacific Ocean as flood waters recede. Chemicals transported in flood waters from the oil fields could prove detrimental to sensitive natural areas.
The purpose of this study is to evaluate the flood risk within the Westminster watershed that is primarily attributable to underperforming drainage channels that collect surface runoff and convey it downstream towards eventual discharge into the Pacific Ocean. Subsequent to the completion of the Santa Ana River projects, the Westminster watershed is the largest remaining area of mapped 100-year floodplain in Orange County. Preliminary analysis shows that flood flows overtop the drainage channels in the study area between the 20% and 10% annual chance of exceedance (ACE) storm events (5 and 10 year recurrence intervals, respectively), putting approximately 400,000 area residents and 44,000 structures at risk. Overbank flooding also impacts traffic in the project area, causing closures on local roads as well as major routes, including Interstate 405 (I-405). In total, the study area experiences approximately $130,000,000 (FY2019 price levels, 2035 base year, 2.875% federal discount rate) in average annual equivalent (AAE) direct damages as a result of overbank flooding.
Features of the proposed project described in the draft report include modifying drainage channels to improve flow efficiency and/or capacity, widening an existing bottleneck at Warner Avenue at Huntington Harbour, replacing tide gates, and constructing a floodwall along a portion of the Pacific Coast Highway at Outer Bolsa Bay.