The Arroyo Grande Creek Flood Control Project was designed and constructed in 1961 by the former U.S. Soil Conservation Service. The main feature of the project was a 3.5 mile levee system and trapezoidal channel that confined Arroyo Grande Creek on both sides from its confluence with Los Berros Creek downstream to the Pacific Ocean. In addition, the lower portion of Los Berros Creek from the Valley View Bridge to the confluence with Arroyo Grande Creek was diverted from its pre-1960 channel which ran along the southern edge of the Cienega Valley to its current confluence upstream of the Highway 1 Bridge. Runoff from the Meadow Creek Watershed, which runs through Pismo Lake, was designed to enter Arroyo Grande Creek through a pair of flap gates near the Pismo Dunes State Vehicular Recreation Area. After the project was constructed, maintenance of the flood control channel became the responsibility of the San Luis Obispo County Flood Control and Water Conservation District, under the purview of the County Public Works Department.
Maintenance of the flood control channel consisted of vegetation and sediment removal and routine maintenance of the levee system and infrastructure. Since the 1990s maintenance efforts have been restricted due to lack of funding and environmental concerns about the impacts of vegetation removal on aquatic and riparian habitat. To increase funding for channel maintenance, special tax assessments paid by Zone property owners were increased in July 2006. However, habitat protection for the Federally listed California red-legged frog and steelhead trout remained the limiting factor to vegetation and sediment removal efforts. As a result of not being able to remove dense vegetation and accumulated sediment, the channel capacity has been severely reduced.
In addition to decreased channel capacity there has been an increase in stormwater runoff volume due to significant development within the watershed since 1961. In turn, this has
increased the probability for when the channel capacity may be exceeded. It is estimated that under existing conditions, the channels can provide flood protection from a storm with less than a 5-year recurrence interval. This means that the channel has the probability to overtop once every 5 years.
In order to alleviate the existing flood risk, Public Works developed the Arroyo Grande Creek Levee Failure Emergency Response Plan. This plan is activated when there is a forecast for significant rainfall or when the water surface in the impacted water bodies (Arroyo Grande Creek or Oceano Lagoon) reaches flood levels.
Additionally, the District is focused on the goal to improve maintenance of the Zone 1/1A flood control facilities through coordination and input from the Zone 1/1A Advisory Committee. As a result, the District has developed a long-term maintenance plan for the channel. This plan is called the Arroyo Grande Creek Channel Waterway Management Program (AGWMP). The AGWMP was adopted and the associated Environmental Impact Report was certified by the Board of Supervisors on November 2, 2010. The AGWMP includes the following components to increase channel capacity to provide 10-year flood protection and improve creek habitat:
- Vegetation management will be performed throughout the project area to increase the channel capacity and improve the health and diversity of the riparian corridor.
- Sediment management will be done to remove accumulated sediment and create secondary channels or overflow paths during high flow events for
- Habitat log structures will be constructed where the secondary channels join the low-flow primary channel to set the location of the primary channel, promote pool scour, encourage sediment sorting, provide deep pools and cover habitat for species.
- Flood control walls will be constructed where needed to protect the residential areas north of the channel.
- Turf reinforcement mat along the outside slope of the south levee provides erosion control and slope stabilization.
- Levee raise and in place stabilization along the top of levees to provide erosion control and overtopping protection to the levee system.
The AGWMP project is being partially funded with State and Federal grants awarded to the project:
$2.8M Proposition 1E Stormwater Flood Management Grant
$2.2M Proposition 84 Integrated Regional Water Management Implementation Grant
$3.0M FEMA Hazard Mitigation Grant
Construction of the AGWMP has commenced and is expected to be completed in 2021.