A Stepped Approach to Generating a Single Set of RTK Parameters for Continuous Calibration
This chapter presents a straightforward approach to generating a constant, single set of RTK parameters per monitored sewershed which provides good wet weather flow (WWF) continuous simulation for all monitored storm events. This single set is function of collection system deterioration conditions and sewershed physical dimensions, which are constant in each monitored basin. A constant single set of RTK parameters which successfully regenerates observed WWF responses from all monitored storm events will ensure accurate flow prediction when applied to non-monitored periods. The difficulty in calculating a single set of RTK parameters comes from the lack of knowledge of how much initial abstraction is available at the start of each storm event. Initial abstraction refers to the amount of rainfall that will be lost to wet the surface and subsurface before the excess rain (runoff) arrives to the receiving collection system. The stepped approach presented in this chapter resolves this difficulty since it relies on selecting a monitored storm event where the initial abstraction is almost negligible. This can be achieved by selecting a monitored storm event which was shortly preceded by another large storm. The first storm will bring the sewershed into a saturated condition. The period between the two storms should be short to avoid large recovery of the system storage. This saturated condition will ensure that most of the second storm’s rain is runoff since the initial abstraction term in the R-equation is negligible. Generating R based on this second storm event will allow R to be almost exclusively a function of the collection system’s size and asset defects and not affected by initial abstractions. Similarly, in a saturated condition, the T and K parameters will not be affected by delay time to wet the surface and subsurface storages. The constant, single set of RTK parameters from that storm represents the collection system response to runoff. To accurately introduce the effect of initial abstraction between the storm events, values of the storage recovery rate parameter should then be adjusted to account for climatic change conditions.
The proposed approach was successfully used to characterize WWF conditions in the West Fifth Avenue Sanitary Sewer System Inflow and Infiltration Remediation Project in the city of Columbus, Ohio. Sixty flow meters were used to monitor the flow response from the basin’s sewer-sheds. Monitoring activities were conducted between August 2007 and June 2008. Monitored tributary sewershed areas varied in size from less than ten acres to more than 300 acres. A constant, single set of RTK pa-rameters was generated for each monitored sewershed following the proposed stepped approach. Recovery rates and maximum storage were adjusted monthly to allow for climatic and groundwater changes. The generated RTK sets accurately simulated WWF responses in each monitored sewershed for all year-long monitored storm events. The generated single set of RTK parameters closely described the collection system’s deterioration condition. It can be applied to non-monitored and synthetic design storms with confidence and will ultimately lead to more accurate recommendations and sizing for capital improvement projects.
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