Hill Canyon Treatment Plant Stormwater Diversion Project – Mitigated Negative Declaration
CI 5427
The City’s Hill Canyon Treatment Plant treats wastewater from the entire community of Thousand Oaks. The HCTP is located in a canyon environment in the northwest corner of the City, located between two creeks in a natural, unimproved environment. The drain inlets connect to an existing 30-inch diameter pipeline that captures storm flows and drains them to the sludge drying beds. This process is part of the zero-discharge storm flow capture program the City performs as part of its permit exemption.
Recent storms have caused significant runoff from the canyon wall and hillside, resulting in substantial flows that overwhelm the drain inlets. These flows deposit natural debris (rocks, vegetation, mud) onto the HCTP parking areas and access roads, creating unmanaged drainage issues that require substantial cleanup and debris removal. The debris flows pose a risk to HCTP operations because the location of the debris flow can prevent chemical deliveries to the plant. Additionally, the stormwater volume from the canyon wall reduces the detention basin’s available storage capacity, which is needed for managing system flows during heavy rain events.
This project will create a debris capture area on the eastern boundary of the plant by constructing elevated retaining walls and widening the drainage channel. Piping will be installed to redirect stormwater from the side canyon around the perimeter of the treatment plant. The diverted stormwater from the canyon will be diverted directly to the historical discharge point in the Arroyo Conejo Creek. The work to be performed under this project includes the installation of a 30-inch diameter reinforced concrete storm drainage pipe, construction of a retaining wall, open channel, and associated outlet structure, modification/abandonment of an existing storm drainage pump station, and modification to an existing 30-inch diameter storm drain pipeline.
The tentative schedule for completion of the project is as followed:
Winter 2025- Award of Construction Contract
Spring 2025- Start of Construction
Early 2026- Construction Completion