Mill Creek Wet Weather Storage
JCW is constructing an underground wet weather storage facility that will be used to store wet weather flows during major storms. This project will provide capacity for continued growth in the Mill Creek Service Area and reduce the need for more costly downstream improvements.
Project Drivers and Benefits:
JCW is providing storage for wet weather flows during major storm events, to provide capacity for continued growth and reduce the need for more expensive and disruptive downstream improvements.
The project is currently in design, potential construction impacts will be updated at a later date. JCW is committed to mitigating impacts to the project area.
Winter 2023-Summer 2025
Wet Weather and Why it is a Concern
Wet weather—rain and snowmelt—can significantly increase flows at wastewater treatment facilities, creating operational challenges and potentially affecting treatment efficiency, reliability, and endangering the environment. As communities grow and current infrastructure ages, this can create additional stress on the system.
Sewage flows within a sanitary collection system are divided into two major categories: dry weather flows and wet weather flows.
Dry weather flows represent all flows within the collection system on a typical day without precipitation. This includes sewage collected from customers along with normal groundwater that enters through defective pipe joints, connections and/or manhole walls.
Wet weather flows include the dry weather flows AND additional rain and snowmelt runoff that enters the collection system during a storm event. Johnson County has separate collection systems for stormwater and wastewater, but during large rain events, stormwater and increased amounts of groundwater can find its way into the wastewater collection system through defects within the system and private sewer laterals, as well as illicit building connections from stormwater sources such as sump pits and driveway drains. This results in wet weather flow that can be several times greater than flow during dry weather. The additional flow dilutes the normal dry weather flow significantly.
While JCW continually works to fix defects within the system and eliminate these sources of flow into the system, there is no cost effective and practical way to remove all sources. Sometimes flows are still too high during peak wet weather, and collection system overflows can occur, overloading treatment plants and endangering the environment. One option to reduce peak weather flows (which is being planned for Mill Creek) are storage facilities (Wet Weather Storage) that temporarily store the excess flow and release it after the rainfall event, when downstream collection system capacity becomes available.
JCW evaluated this and other alternatives and determined the storage option is the most cost-effective option for handing these wet weather flows. Constructing the facility eliminates the need for a two-mile parallel construction system that would be required to run along Mill Creek from the site all the way to the Mill Creek Regional Wastewater Treatment Facility. It also reduces the magnitude and costs of future wet weather treatment capacity expansions at the treatment facility.
Dry weather flows will bypass the storage facility, it will only activate during wet weather events, and only when the event is large enough to exceed the capacity of the sewer pipe. As such, the storage facility will be empty the vast majority of the time, and when it does activate it will store the diluted flows associated with wet weather.
JCW, working with Johnson County Parks and Recreation District, has identified and procured property to place an underground Wet Weather Storage Facility for the Mill Creek Watershed and Mill Creek Wastewater Treatment Facility. The location will be just north of the Parks District’s Vista Drive maintenance facility. While a large part of the facility will be underground, portions of it will be visible above ground. Working with the Parks District, JCW will landscape the area to preserve the look and feel of the surrounding area. In addition, JCW and the Parks District will collaborate on development of native plantings and habitat enhancing features on the facility property. District access to the nearby trail system for maintenance activities will be maintained.
The potential for odorous air in the facility will be mitigated by a ventilation and scrubbing system, along with an automated cleaning system to move material out of the tank when not in use.
Schedule and Engagement
The project currently is early in the design phase. The construction timeline is anticipated to be roughly from the Winter of 2024 through the Summer of 2025, for an estimated 18-month construction period.
Keep checking back, as the schedule is developed and community outreach is planned; information will be posted on this website. You can also sign up for notifications.