Background
The Arroyo Grande Creek Flood Control Project (also referred to as Flood Control Zone 1/1A – Arroyo Grande Creek Channel) 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. The design capacity in 1961 carried 50-year flood protection. Additional capacity due to freeboard could carry a 100-year storm or 10,000 cfs of flood protection before overtopping.
In the 1990s, maintenance efforts were 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.
Additionally, with increased development, the original Project cannot provide the same 50-year flood protection as original intended. A flood estimated to occur once every 50 years in 1955 is now estimated to have a recurrence interval of 15-20 years due to changes in the hydrology of the lower watershed. Development affects a watershed’s hydrology by increasing the amount and timing of runoff through an increase in impervious surfaces.
Waterway Management Program
After an extensive process to evaluate alternatives and obtain regulatory permits, the District developed an Arroyo Grande Creek Channel Waterway Management Program (WMP) and implemented a project alternative that was completed in 2020 and designed to carry a discharge of 5,400cfs, which is equivalent to a 10-year event today. A minimum freeboard is provided above the design maximum water surface, which could contain up to 8,000cfs. The ongoing WMP program includes annual vegetation maintenance, improvements to habitat, and sediment removal in designated areas. See the video below for more information on the 2020 project.
Annual vegetation management maintains the flood capacity and reduces the potential of debris jams and blockages in the channel during high flow scenarios. The District uses several methods to reduce the amount of vegetation that could inhibit flows and diminish capacity while also adhering to environmental regulations. Vegetation is removed from the low flow channel and the secondary channels to maintain the targeted flood capacity and freeboard. Weed abatement by grazing is conducted in the Los Berros Diversion Channel, and in sections of the Arroyo Grande Creek Channel between the Union Pacific Railroad and Highway 1 bridge crossings. Goats are used to remove dense weedy vegetation on the tops and slopes of the channels. Willow trimming is performed by hand crews who cut and trim woody vegetation from within the channel flow areas. The hand crews also remove all non- native invasive species and clean up trash throughout the Arroyo Grande and Los Berros Channels.
Sediment accumulation monitoring is performed annually. The results are used to guide sediment removal per the WMP. Prior to 2023 rain events no significant sediment accumulation was indicated.
Culvert and Flap Gate Maintenance is completed every year in September prior to the storm season. Culverts and flap gates are inspected and then cleaned and maintained for proper operation. Accumulated sediment and debris are removed from inside of the culverts and from in front of the flap gate at the culvert outlet. Culverts and flap gates are routinely checked throughout the winter and before every storm.
Levee Repair and Maintenance is completed as necessary and prior to the on-set of winter storms. The levee system is routinely inspected and needed repairs such as fill of low areas and or slope repair are performed using County crews.
The County Public Works Department coordinates the maintenance of the channels. If you need to report a maintenance issue or problem in the channels, please call (805) 781-5252 or submit a request by clicking here.
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