AKM prepared the City’s first Master Plan of Drainage in 1999 and the Master Plan Update in 2008. The study included developing and updating of the existing system facility information, creation of flood protection criteria, hydrologic and hydraulic analyses, formulation and evaluation of alternative mitigation measures, prioritization, cost estimates and report preparation.
The City area includes 11.2 square miles, and drains to San Gabriel River, Los Alamitos Channel, Kempton Storm Channel, Montecito Storm Channel, Bixby Storm Channel, Federal Storm Channel, and the Pacific Ocean. Portions of the City of Los Alamitos and the unincorporated Rossmoor area drain through the City facilities. AKM collected and reviewed all existing Project Reports from Orange County Public Works and the US Army Corps of Engineers to establish downstream water surface control elevations in the regional facilities.
AKM developed drainage criteria in cooperation with the City. The 1986 Orange County Hydrology Manual and subsequent addenda, and the Local Drainage Manual formed the basis of the criteria, including modifications for the existing facility constraints. Hydrologic studies were conducted for the high confidence 10 and 25-year (Expected Value 100-year) frequency design storms.
One of the key tasks in the project was the detailed hydraulic analyses that enabled AKM to accurately define the deficiencies in the storm drain system. The development of accurate computer models of the drainage system allowed AKM to maximize the capacity of the existing storm drains, thereby minimizing the amount of additional facilities required. AKM modeled the entire drainage system utilizing the Water Surface Pressure Gradient (WSPG) program developed by the Los Angeles County Department of Public Works.
Recommended relief and replacement facilities were identified and prioritized, including improvements to the West End Storm Water Pump Station. System extension recommendations were also made to meet the flood protection criteria. Cost estimates were developed based upon recent construction costs, and a final report was prepared summarizing the analyses and recommendations.
AKM also formulated projects to comply with NPDES requirements, including urban runoff diversion to the sanitary sewer system. AKM participated in study sessions and public presentations.
The City of Seal Beach retained AKM to conduct focused studies of the Marina Drive and College Park East drainage areas to formulate detailed improvement recommendations addressing the frequent flooding issues in these areas that had not been addressed due to lack of funding. AKM conducted detailed hydrologic studies, formulated alternative mitigation projects, evaluated them through detailed hydraulic analyses, and recommended improvement projects, along with their implementation costs and phasing. AKM prepared power point presentations and participated in Council study sessions and public presentations. AKM documented the studies in individual Preliminary Design Reports (August 2019) including plans and profiles of the recommended improvements.