An ephemeral drainage channel in La Jolla, California became severely incised due to stormwater and non-stormwater runoff from urban development that has increased over the last several decades. The progressive erosion of the channel threatened the stability of adjacent residential properties initiating a need for an emergency repair project.
This emergency action initiated Phase 1 of the Project which focused within the northern portion of the Project in an attempt to mitigate potential slope failure which was a direct threat to public health and safety. Phase 1 included grading activities to stabilize the approximately 85-foot high fill slope along the northwestern boundary of the Project by creating an earthen buttress along the toe of slope within the canyon bottom. Phase 2 of the Project includes finalizing the drainage repair and obtaining all of the required environmental and development permits for both phases of the Project.
Geosyntec was contracted by the City of San Diego (City), Engineering and Capital Projects Department to develop a Public Improvement Plan set of the channel design (Phase 2 Design), provide services to obtain CEQA approval, a Site Development Permit, and environmental permits (USACE-404, RWQCB-401, and CDFG-1602) for the project. The project also includes preparation of construction cost estimates, construction schedules and preparation of a Storm Water Pollution Prevention Plan in accordance with the new California State Water Resources Control Board Construction NPDES Permit.
Geosyntec coordinated with multiple City staff departments (maintenance, planning, and capital improvements), local agencies and planning groups (federal agencies, and the La Jolla Community Planning Association) to identify potential design alternatives to minimize short- and long-term maintenance while maintaining long-term function. Selected project alternative was evaluated for compliance related to Federal, State and local stormwater and environmental regulations and policies. Geosyntec managed a multi-disciplinary team to complete geotechnical, hydrologic, biologic, and archaeological technical studies to support the City Environmental Analysis Section staff for environmental determination.
Selected Phase 2 design is cost effective and provides long-term geotechnical stability, stream course protection, mitigation of hydromodification impacts, and improvement of watershed water quality. The design included innovative implementation of hydrologic flow management and stormwater BMPs while minimizing disturbance to the environment and local community. Geosyntec's stream (and hydraulic system) design provides a stable channel through diversion of storm flows equal to the pre-development runoff rates (within natural stream segments) while providing stormwater treatment through a storm drain diversion that routes excess flows caused due to development to a treatment detention basin. The basin is also designed to attenuate the peak flows during the high flow events to minimize the flooding impacts at the existing downstream storm drain system.