A Multi-Tiered Modeling Approach for Planning, Design, and Regulatory Compliance in Mixed Combined and Separate Sewersheds

A hydrologic and hydraulic modeling system was applied to a mixed combined and separate sewershed to support system expansion planning and design, and to support Combined Sewer Overflow (CSO) regulatory compliance. The model was developed in two stages, or tiers. Tier 1 results, based on a detailed representation of CSO regulator and interceptor hydraulics and a simplified representation of system hydrology, aided in the planning phase for facilities needed to meet combined sewage capture requirements. The refined Tier 2 results, based on more detailed representations of both hydrologic and hydraulic elements, supported the final design stage of a new relief sewer. In addition, initially Tier 1, and now Tier 2 models are used for quarterly discharge monitoring reporting required by a National Pollutant Discharge Elimination System permit for CSO discharges.