Continuous simulation of collection systems can quantify overflow frequency, pump runtime, storage utilization and dewatering characteristics, and other important indicators of wet-weather system performance. Compared to single event simulation using synthetic design storms, continuous simulation with long-term historical rainfall can better define storage facility performance and thereby avoid potential over-sizing or under-sizing. However, continuous simulation often requires long run times for complex computer models of large sewer system networks.
This paper presents an innovative approach to size a sanitary sewer storage facility as part of the Livingston/James Sewer System Infiltration and Inflow Remediation Project for Columbus, Ohio, using a 61-year continuous simulation period. The model includes 2,868 junctions and 2,915 conduits serving an area of 4,669 acres. The model was first developed with SWMM 4.4h, and then converted to SWMM 5. Using 2007-2008 flow monitoring data, seasonal (dormant and growth season) RTKs and seasonal initial abstraction parameters (IA) were calibrated to represent rainfall-derived infiltration and
inflow (RDII). Precipitation data for 1949 through 2009 (61 years) from the Port Columbus International Airport gauge were
applied to generate inflow hydrographs using the calibrated model. The model was used to size the storage facility to achieve a design objective of a 10-year level of service.
The 61-year inflow hydrographs were compressed using a threshold of full-capacity flow at the pipe downstream of the storage facility. Hydrographs for any period exceeding the threshold were extracted from the composite total inflow hydrograph, which is the sum of inflow hydrographs at loading nodes upstream of the storage facility. Each extracted hydrograph includes a complete RDII response starting and ending at dry weather flow condition with an extra dry day added before each storm hydrograph to allow sufficient time for emptying the storage facility. The same time periods of the extracted composite total hydrograph were used to compress the inflow hydrographs at each loading node. The compressed inflow hydrographs were then routed through the sewer system.
In this manner the hydrologic response of the study area to the full historical precipitation record is simulated to assure proper characterization of the inter-event hydrologic responses, and the full hydraulic routing of the storage events is also simulated to assure proper characterization of the storage responses. Full hydraulic routing is omitted during those periods for which there is no storage inflow or outflow, as this is unnecessary for the proper characterization of storage.
The results showed that using long-term continuous simulation to size storage facilities can be practical, even for large complex sewer models, but may require special techniques to reduce the size of the inflow hydrographs that are routed through the conduit network. An advantage of this continuous simulation approach using the 61-year inflow hydrographs is that it enables simulation of RDII without losing the influences from previous storm and moisture conditions, while maintaining reasonable runtimes to support generating and evaluating different design criteria for different levels of service.