Sitka Critical Secondary Water Supply

Sitka Critical Secondary Water Supply

Background:

The City and Borough of Sitka, Alaska constructed a new fish-safe intake and raw water pump station on Sawmill Creek to serve as a secondary water supply to an existing unfiltered source. To protect anadromous salmon, trout, and other species in Sawmill Creek, the facility installed a mechanical brush-cleaned wedge wire drum screen at the secondary intake.

ISI designed and fabricated a D36-36TA-R retrievable brushed-cleaned cylindrical wedge wire drum screen for the project to provide a maximum flow rate of 14.2 CFS (9.2 MGD; 1,448 m³/h) at a 0.5 ft/s approach velocity. The 36-inch diameter, 36 inch-long wedge wire cylinder provides 28.3 ft² of screen surface area and was fabricated from type 304 stainless steel continuous slot wedge wire with a 2.5-mm slot opening and 59% open area.

This screen system is located in a remote location and therefore was designed to operate without a power supply. The screen surface is cleaned by rotating the screen cylinder between the external and internal brushes powered by a turbine drive assembly mounted to the cylinder screen. As water flows through the screen unit, it turns a turbine blade attached to a shaft and high ratio gear box which slowly rotates the wedge wire cylinder. This screen unit is mounted on a type 304 stainless steel retrieval rail system with a hand winch which allows the screen to be raised and lowered between the submerged operational position and the inspection position at the top of the intake structure. The track also includes an integrated slide gate with hand winch for raising and lowering the slide gate.

Benefit of ISI System

ISI's brush-cleaned screen system provides the following key advantages at this site:

1. The ISI turbine drive system provides mechanical cleaning of the wedge wire screen surface without a power supply. This ensures reliable water withdrawal at the facility and maintenance of a clean screen to retain compliance with fish protection requirements.
2. The cylindrical screen shape provides a large screen surface area in a small project footprint.
3. The retrieval track with manual hoist provides a simple and cost-effective system that allows for easy inspection and servicing of the screen system.
4. The manually actuated slide gate integrated into the retrieval track provides fish and debris protection at the intake when the screen is in the raised position.

Owner:

City and Borough of Sitka

Engineer(s):

CRW Engineering Group

General Contractor:

Dawson Construction

Location:

Sawmill Creek, Sitka, AK