Hill Canyon Treatment Plant Stormwater Diversion Project
Mitigated Negative Declarations
The City is proposing improvements to the existing Hill Canyon Treatment Plant (Treatment Plant), including storm drain improvements, the development of a new debris basin, and the development of a new stormwater channel to route the off-site stormwater flow through and around the Treatment Plant in a dedicated channel/pipe to an existing discharge location. All improvements will be contained within the existing site dedicated to the treatment plant.
As part of the self-contained storm drainage system, the Treatment Plant on-site stormwater system includes an existing 30-inch-diameter storm drain pipe that connects to a sump pump system and diverts flows to the adjacent sludge-drying beds. The project would demolish the existing sump pump area and structure and re-establish the connection to the existing 30-inch-diameter storm drain original outfall to transport off-site flows from the adjacent canyon in a dedicated pipeline. The project would include the addition of three storm drain pipes: Pipe A, Pipe B, and Pipe C. Pipe A, located upstream of the flow equalization basins, would divert on-site flows to the existing flow equalization basin’s overflow structure that is routed to the sludge-drying beds. Pipe A would be disconnected from the remaining downgradient 36-inch-diameter storm drain pipe. Pipe A would be 36 inches in diameter and have a full flow capacity of 55 cubic feet per second (cfs). Pipe B would receive off-site flows from the proposed channel at the upstream end and tie into the existing 30-inch-diameter storm drain at the downstream end just past the Pipe A disconnection point. Pipe B would be 24 inches in diameter and have a full flow capacity of 93 cfs. Pipe C would receive off-site flows from the proposed debris basin at the upstream end and outlet to the proposed channel at the downstream end. Pipe C would be 24 inches in diameter and would have a full flow capacity of 50 cfs.
The proposed debris basin would be incorporated at the upstream end of the proposed storm drain pipe on the eastern side of the treatment plant to intercept all debris and allow clear water flows from the off-site canyon to continue downstream into the storm drain system pipe, where it is routed to the outfall location. The proposed channel would connect to the proposed debris basin at the upstream end by Pipe C. The proposed channel would convey 50 cfs flows.
Pursuant to Government Code Section 65962.5, a search of the regulatory database listings of hazardous materials sites was conducted for the project site. The project site is not listed on the Hazardous Waste and Substances Sites List from the California Department of Toxic Substances Control. According to the List of Leaking Underground Storage Tank Sites from the State Water Resources Control Board, the site contained a leaking underground storage tank; however, the site has been remediated, and the case was marked closed as of 2004. Further, the project site is not identified on the list of solid waste disposal sites, nor is it on the list of active Cease and Desist Orders and Cleanup and Abatement Orders.