Los Peñasquitos Lagoon Restoration: Restoring Natural Processes to Address Multiple Objectives of Water Quality, Water Resource, Restoration and Flood Management
The Los Peñasquitos Lagoon Restoration project (Project) is located in the City of San Diego, San Diego County, California within a portion of Los Peñasquitos Lagoon (Lagoon) and the upstream riparian corridor that extends into Sorrento Valley. The lagoon receives drainage from a 59,212-acre watershed. Urbanization of the watershed since the 1970s has increased sedimentation from hydromodification of natural channels and perennial freshwater inputs to the lagoon, causing conversion of tidal and non-tidal salt marsh to brackish or freshwater marsh. Channelization of the canyon creeks that empty into the lagoon has impacted natural floodplain processes that provided sediment deposition to occur further upstream. Development adjacent to the channels experience periodic inundation during larger storm events due to accumulation of sediment and vegetation within these and downstream of these channels, and when flows exceed the design capacity of these channels.
The Project has multiple objectives that include addressing the targeted regulatory goals of the Los Peñasquitos Watershed Management Area Sediment Total Maximum Daily Load (Sediment TMDL) that is enforced through the National Pollutant Discharge Elimination System (NPDES) Permit and Waste Discharge Requirements for Discharges from the Municipal Separate Storm Sewer Systems (MS4s). These targeted goals include reductions in sediment loading to the lagoon, restoration of historical salt marsh and management of freshwater inputs to the lagoon. Freshwater management is an important element of the success of the project. This requires looking at these freshwater inputs as a resource that can be managed and part of a watershed water resource management approach. These goals also need to be integrated with flood management within Sorrento Valley. The Project aims to reduce fresh water ponding in the upper lagoon, provide for long-term sediment management, attenuate impoundment of floodwaters during storm events, and reduce breeding habitat for airborne vectors by improving hydrologic connectivity between the drainage basins of Los Peñasquitos Canyon and Carroll Canyon and Los Peñasquitos Lagoon. This technical presentation will provide lessons learned in developing a waterways infrastructure and habitat restoration project with multiple objectives that include Water Quality, Water Resource, Floodplain Expansion, Habitat Restoration and Flood Management.
The presentation also provides for how these waterway infrastructure projects need to integrate with the overall watershed management that in this case includes reducing sediment loading from hydromodification of natural channels from urbanization and gravel and sand mining. Downstream sediment management as part of the project includes restoring floodplain processes that allow for sediment to drop out before entering the restored salt marsh area. Expansion of floodplains and channels will also address flood management issues. Watershed management also includes reducing year-round dry weather flows freshwater flows to the Lagoon through reductions in irrigation use and possible diversions and beneficial use of these flows upstream of the Lagoon. Freshwater management in the upper lagoon will also be addressed through improved connectively between the upstream creek channels and tidal channels.
The approach and lessons learned through this design and environmental review process can be applied to other integrated multi-benefit waterway infrastructure and restoration projects that are becoming part of MS4 Permit compliance programs as watershed-based plans must meet multiple issues that include water resource management, water quality, natural resource protection and restoration, hydromodification and flood management.