As part of the City’s adaptive management approach to assess and prioritize LTCP projects, Tetra Tech CSO was retained as the City’s modeling and hydraulic expert to provide services that include the development of an all-pipes detailed model of its entire combined sewer network in PCSWMM™ for the purposes of enhancing model prediction accuracy, facilitating future modeling developments, and accommodating future needs.
The City of Wilmington has been implementing a comprehensive combined sewer overflow (CSO) Long Term Control Plan (LTCP) since the late 1980’s. The City’s service area spreads over approximately 17 square miles and has a population of approximately 70,851 (source: US 2010 Census Data), mainly serviced by combined sewers with pockets of zones where partial separation has been undertaken, all tributary to the 11th Street Pumping Station and the Water Pollution Control Facility.
This all-pipes detailed model will be built sector by sector, in several phases and in the context of separate commissions that will include various tasks, such as detailed modeling, the planning of a measuring and monitoring campaign, and model calibration. Being an iterative process which depends on the accuracy of available information, the resulting detailed model will have to provide the most accurate evaluation of the local collection network, CSO conditions, inflows conveyed to the interception system, and hydraulic performance of the interceptors upstream of the 11th Street Pumping Station.
Model development included the loading of dry weather flows (sanitary and infiltration) and wet weather flows (fast and slow response components) in both the combined and separate sanitary sewer areas of the entire City. So far, Tetra Tech CSO has completed three of the commissions mentioned above, which have included the following tasks:
- Import of the Csoft® model into PCSWMM™;
- Development of the detailed model of the Interceptor B area;
- Development of the detailed model of the Interceptor A area;
- Planning and follow up of a temporary flow monitoring campaign of the Interceptor B area;
- Planning of a temporary flow monitoring campaign of the Interceptor A area;
- Calibration of the Interceptor B detailed model area and partial calibration of the Interceptor A detailed model area (west bank of the Brandywine River).
Tetra Tech CSO's activities within the framework of this project include:
- Model development and update from various data sources;
- Data conversion, transfer and engineering validation;
- Tracking of data sources providing detailed reference data to the modellers;
- Resolution of missing or suspect data within the source documents;
- As-built drawing research for most of the regulating structures;
- Detailed analysis of the model loading for both dry and wet weather flows;
- Model calibration for observed data from numerous flow meter sites for both dry and wet weather flows.
This commission included model data from multiple sources, the modeling of a high number of special structures, as well as the representation and calibration of wet weather responses using the PCSWMM™ Unit Hydrograph parameters (RTK) to generate the inflow and infiltration wet weather responses (I&I).