In-System Wet Weather Storage - An Innovative Solution to Unnecessary Expansion
- P.S. Arora PE, James Wilder PE, William Willard - City of Denton
- Brandt Miller PE, Chamindra Dassanayake PhD, PE, Curt Courter PE - Hazen and Sawyer
Like most municipal wastewater collection systems in the U.S., the City of Denton experiences large increases in flow during and immediately following large wet weather events due to infiltration and inflow. The peak flow rates for these large rain events can reach 4-6 times the average daily flow depending on the size of the rain event and the antecedent moisture conditions. This presentation will focus on how the first in-system wet weather storage facility in North Texas will manage peak flows while saving costs in collection system capacity enhancements and/or treatment plant capacity. In addition, some of the potential benefits to a collection system sized for normal flows, rather than extreme peaks, such as reduction is odor and solids deposition.
Denton’s primary trunk interceptors were built between the 1960’s and 1980’s and were not designed to handle these infrequent, large peak flows. As a result, Denton has been in the process of upgrading the size of its trunk interceptor system since the early 2000s.
The City utilizes a dynamic hydraulic computer model to simulate the impact of large rain events on the flow rates within the collection system for both present and future conditions. The primary design storm is a 5 year, 24 hour storm conforming to a SCS Type II storm. From the results of the design storm on the model, the City has developed a series of capacity enhancement projects which should serve the capacity requirements of the collection system out to 2033. Enhancement projects considered including increasing collection system capacity with larger pipelines, building peak flow storage at the Pecan Creek Water Reclamation Facility (PCWRP), expanding PCWRP and in-system peak flow storage.
In-system storage facilities were repeatedly explored during the development of the capacity enhancement program to detain or dampen the peak wet weather flows and reduce or eliminate the need for expensive downstream interceptor improvements that would otherwise be required to convey the peak flow to the treatment facility. This alternative also reduced the need for additional storage or diversion facilities at the treatment facility itself which was an important consideration for the City of Denton due to the limiting peak flow capacity of the PCWRP. However, the primary disadvantage was the potential for public opposition with siting the storage facilities close to existing residential and commercial properties.
One location emerged, however, that was sufficiently remote from existing residential and commercial neighbors and was also a poor location for future development. This site, hereafter referred to as the Hickory Creek Detention Facility (HCDF), is located in Denton’s southernmost collection system basin, the Hickory Creek Basin. The HCDF is screened by heavy woods from the nearest neighborhood and is located between a railroad and the Hickory Creek rendering it impractical to develop in the future.
The Decision for Denton:
After evaluating many alternatives throughout the collection system the HCDF became a critical piece of the collection system master plan and should save the City approximately 9 million dollars. Without it, the City would have to convey the peak flow 9 miles to the South Peak Flow Basin, which is an open air detention pond located on the site of the Pecan Creek WRP. Conveying the peak flow would require upsizing over 4 miles of interceptor and 2 miles of force main, upgrading the existing Hickory Creek Lift Station from an 11 million gallon per day (mgd) to a 36 mgd, and expanding the South Peak Flow Basin by 6 million gallons. Additionally, a lot of new easement located in developable land would need to be obtained which would escalate the cost. Finally, building the detention facility will not only save money, it will reduce the impact on public and private land by limiting construction activity to the facility site instead of being spread over 6 plus miles through residential and commercial neighborhoods.
National Lessons Learned from Implementing In-System Storage:
Hazen and Sawyer (Hazen) is currently designing the HCDF and has provided planning, design, construction and start-up services for more than 25 similar peak flow detention facilities around the country with up to 30 million gallons storage in one tank. In addition to discussing the benefits seen from in-system storage from the utility perspective, the lessons learned from siting, design, operation, monitoring, wash down, screening, odor control and more will be presented. These will be demonstrated through case studies from major utilities around the country that have implemented in-system storage to manage wet weather peak flows for example Renewable Water Resources (ReWa), Greenville, South Carolina; City of Raleigh, Raleigh, North Carolina; Metro Nashville, Nashville, Tennessee; Philadelphia Water Department, Philadelphia, Pennsylvania; Water and Sewerage Authority, Gwinnett County, Georgia and more.
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