Monterey County presents Bolsa Knolls drainage study; board directs staff to refine options in 60 days

Monterey County presented a new drainage study for the Bolsa Knolls area on April 1, outlining modeled flood risks along Santa Rita Creek (also called Little Bear Creek) and several options to reduce recurring flood impacts.

Randy Ishi, director of Public Works, Facilities and Parks, said the study’s hydrologic and hydraulic modeling shows the creek’s carrying capacity upstream of Rogge Road is roughly 100 cubic feet per second. “If you imagine 100 basketballs coming down a second per second down the creek,” Ishi said, “that roughly represents the amount of flow the creek is able to carry before it starts to overtop.” The consultant used those models to evaluate alternatives ranging from routine sediment removal and targeted vegetation thinning to culvert upsizing and construction of upstream retention basins.

The draft report finds trade-offs for each approach. Staff described recurring maintenance such as culvert and sediment clearing (estimated at about $35,000–$40,000 per cleaning) and a selective vegetation‑management approach with planning-level costs of roughly $50,000–$150,000. Culvert upsizing would require state and federal permits (California Department of Fish and Wildlife, Regional Water Quality Control Board, U.S. Army Corps of Engineers) and was shown with a preliminary planning estimate of about $500,000–$1,000,000. A larger alternative—constructing upstream sediment‑ and stormwater‑retention basins—would require land acquisition, environmental review and permitting but, in modeling of the December 2022 event, could reduce structures in the study reach from 43 to 4; the report also shows that under a 100‑year flood scenario roughly 81 structures are currently affected and some alternatives leave dozens of structures still at risk.

Ishi warned extensive creek‑clearing has high permitting and mitigation costs: staff estimated roughly $2,000,000 just to secure environmental clearances and permits for large‑scale creek work and additional construction costs ranging in one scenario from about $1,000,000 to $4,000,000; combined estimates for that alternative could approach several million dollars and such work would likely have to be repeated because sediment input is ongoing.

The study also called out governance and jurisdictional issues: many parcels along the creek are privately owned, the Monterey County Water Resources Agency owns one parcel, and Monterey County owns only a few small parcels, complicating long‑term maintenance responsibilities. Staff outlined options including code enforcement, reclamation or levee districts, a flood‑control/protection district, a joint powers authority or special legislation, and said the Monterey County Local Agency Formation Commission (LAFCO) could be involved for some district formations.

At the March 24 Board meeting, supervisors received the presentation and “directed staff to come back in 60 days with a more detailed plan,” Ishi said, including clearer estimates for real‑estate acquisition, sediment‑removal scenarios, culvert‑upgrade costs, detention/retention basin acquisition and maintenance costs, and governance and funding structures; the board also asked staff to identify any immediate actions that could be taken—particularly selective vegetation removal where permits are not required.

The county cautioned that many of the larger options require CEQA and potentially NEPA review and endangered‑species mitigations (the report notes presence of the California tiger salamander and California red‑legged frog), which affect both permitting complexity and cost. Ishi said staff will use partner input from agencies such as the Salinas Valley Basin Flood Control Agency and the Resource Conservation District of Monterey County in preparing the refined package.

The board’s direction is procedural rather than a commitment to a single project; staff will return with the requested details and cost refinements 60 days after the March 24 meeting.