Passive weir for emergency bypass

The weir system - which consists of a series of 19 passive weirs in a channel - will reduce reliance on the mechanically operated emergency bypass gate at the plant to relieve wastewater flowing into the treatment system during abnormal conditions. The series of passive weirs will divert wastewater without human intervention to protect the plant from potential flooding and provide an additional layer of life safety protection for plant staff. 

Project description

This project will modify existing treatment plant infrastructure to construct a passive weir system between the emergency bypass channel and the Influent Control Structure at West Point Treatment Plant.

This project was recommended as part of the independent review of the 2017 West Point flooding event, alongside other improvements at West Point Treatment Plant like the Power Quality Improvement Project.

What is a passive weir?

A passive weir is a structure designed to control and manage the flow of water without the need for operator intervention. It typically consists of a barrier or dam-like feature that allows water to flow over or through it at a controlled rate.

Think of a passive weir like an overflow drain in a sink or bathtub. Have you ever noticed the drain along the wall of the basin near the top? When water fills the sink or tub past its reasonable capacity, that drain will begin taking in excess water to prevent it from overfilling and flooding the room. The passive weir system at West Point will act in a similar manner, allowing excess wastewater to flow through when water levels are too high and the emergency bypass gate cannot be opened.

The passive weir system will allow wastewater to bypass the treatment plant if the emergency bypass gate is not able to be used. In emergency scenarios, operators may not be able to open the emergency bypass gate for a variety of reasons. This project will provide redundancy to protect the plant from possible flooding.

One of West Point’s goals is to reduce the number of untreated wastewater bypasses, but in extreme scenarios they are necessary to allow the plant to continue operations and avoid outages for an extended period of time from flooding damage. This is what occurred in 2017 at West Point. The emergency bypass gate was not able to be opened in time before the plant experienced dangerous flooding. This event shut down the plant and reduced its ability to treat wastewater for nearly three months. Constructing the passive weir system at West Point will help prevent scenarios like this from happening again.  

Project benefits

• Protect West Point from potential flooding in instances where the emergency bypass gate fails to operate.
• Protect West Point staff and equipment from dangerous flooding scenarios.
• Protect West Point from long-term flooding damage.
• Establish redundancy within the bypass system.
• Help reduce combined sewer overflow events at the Ballard Regulator station and 3rd Ave West Weir by lessening wastewater backups upstream. 

Project schedule

Cost breakdown

$13.8 million