Determining a consistent peak flow level of service for a wet weather management plan

The City of Columbus, Ohio, completed a comprehensive Wet Weather Management Plan (WWMP) to mitigate hydraulic deficiencies in the City’s main trunk sewers and to address sanitary sewer overflow (SSO) in waterways and water in basements (WIB). One of the main problems we overcame was to determine the desired level of service (LOS), or what is more commonly called the most reasonable level of control. At the outset of Columbus’ WWMP production, the design team was well aware of the high model sensitivity to hydraulic and hydrologic parameters such as storm recurrence intervals and durations, antecedent moisture conditions, and rainfall distributions. Federal guidelines do not mandate specifics on these. And the impact these would have had on program costs made determining this reasonable level of control the most important question we faced. To address this problem, and to ensure a consistent LOS, the design team used “peak-flow events” or the maximum peak-flow rate level of service. The focus of this paper is to describe this procedure, and the many innovative steps we used to generate this consistent peak-flow LOS in the main interceptors of Columbus’ large wastewater collection system (approximately 3000 miles). The proposed procedure specifically provides an alternative to the traditional application of one system-wide design storm where the storm recurrence interval is assumed to be the LOS. That is, this is an alternative to devising and sizing remediation actions using a 10-year design storm which implies that it would provide a 10-year level of service.

The proposed procedure first evaluated flow meter records to determine the peak flow rates at desired recurrence intervals (e.g., two-, five-, and 10-year peak flow levels – to provide a “knee-of-the-curve” analysis) in each trunk sewer using traditional statistical analysis techniques; making use of the City of Columbus’ long record of flow metering. The identified flows at each LOS were then used them iteratively in SWMM modeling to determine the appropriate synthetic (design) storm generating the desired flow rate. Innovative procedures to augment flow anomalies in collected data such as flow backups, overflows, and capacity limitations were also devised. After defining the correct design storms required to generate associated peak flow LOS for each segment, the collection system model was updated to reflect the desired end-of-program year tributary conditions. The collection system model was then in a position to be evaluated as one system-wide model to ensure accuracy of the complex system-wide operation strategy and the nature of the system-wide alternative solution.

Technologies included in system-wide alternatives were express sewers, relief pipes, local storage facilities, and large storage and conveyance tunnels. The recommended system-wide solution for the City’s collection system was a combination of parallel relief pipes and two long deep tunnels (in-line storage) which cross and relieve several main trunk sewers. The developed procedure is a straightforward approach that can easily be adopted into other large and complex collection systems similar to the one found in Columbus, Ohio.