City of Helena Tenmile Diversion Project Decision Notice

The purpose of the project is to remove and replace six (6) existing concrete diversion structures within the Ten Mile Creek drainage. The structures were constructed in 1921 have reached their serviceable lives and need to be replaced. The structures are identified as follows: Ten Mile Creek Diversion; Walker Creek Diversion; Moose Creek Diversion; Minnehaha Creek Diversion; Beaver Creek Diversion; and Banner Creek Diversion. The project is proposed to be constructed starting in early fall 2024.

The Ten Mile Water Treatment Plant (WTP) and the associated raw water collection structures are located 6.5 miles west to 15 miles southwest the City of Helena in Lewis and Clark County Montana. The raw water collection system is located within the Tenmile Creek drainage. The Ten Mile Raw Water Collection System components consists of two surface water reservoirs (Scott & Chessman), a five-mile long diversion flume/canal (Red Mountain Canal), and a 7.5-mile-long 18-inch diameter concrete supply pipeline to the Ten Mile Water treatment facility which captures water at six intakes along the creeks intercepted by the pipe alignment.

The existing water intake structures are designed and operated to intercept the flow of water in their respective drainage channels until the water depth rises at each location to a level where it can be diverted into an intake box and enter the raw water concrete pipeline that transports water to the City’s Tenmile water treatment plant (WTP). Each intake box is screened to limit the amount and size of debris that is flushed through the pipeline and into the WTP. During high flows at each diversion structure, water flow rates exceeding the intake capacity of the pipeline can be released through gates and/or overflow spillways at the intake structures into the downstream natural drainage channels. When the intake structures are not in use and/or during flood events, the diversion gates are raised out of the way to allow the natural drainage flow or high flow to continue without interruption. The intakes are operated, cleaned, and maintained by the City of Helena WTP operations personnel. The City uses the intakes in various combinations as necessary to obtain the desired plant intake flow up to the City’s water right allowance and to intercept the highest quality water available at a given time.

The existing concrete headwalls and structures were constructed in 1921 with the slide gates replaced and/or updated at later dates. The intake structures are over 100 years old and in poor condition with areas of concrete delamination, spalling, deterioration, and exposed rebar. The slide gates and winch assemblies are functional in some instances but require frequent maintenance and repair. In addition, bank erosion which has occurred at several of the intake structures is threatening the stability of the structure(s).

The diversion structure replacements will consist of similar cast-in-place concrete diversion structures, vertical slide/sluice gates designed to back water up behind each diversion structure and redirect the flows to the new concrete intake vaults, and subsequently to connect to existing downstream pipelines. The replacement structures will generally be in-kind replacement structures within the same footprint of the existing structures and set to match the existing structure invert elevations and heights.

Based on the environmental review provided in the Draft EA, and, to the greatest extent possible, in accordance with all applicable laws, rules, regulations, and policies, FWP determined the proposed action (Alternative 2), will not have significant adverse impacts on the human environment associated with the proposed action and constitutes a reasonable and appropriate strategy to achieve identified objectives. Therefore, preparation of an environmental impact statement or EIS is unnecessary.

FWP hereby adopts the Draft EA as final and approves Alternative 2, the Proposed Action.

Related Documents: City of Helena Tenmile Diversion Project