Simple Method for Calculating Hydraulic Behavior of Combined Sewer Overflow from Rainfall Event Data

Abstract

This paper proposes a simple method for estimating the hydraulic behavior of combined sewer overflow (CSO) using only rainfall data. The method analyzes rainfall event time-series data to extract the critical rainfall intensity (the threshold for the occurrence of CSO) and uses the results to calculate quantities such as response time, duration, and runoff depth. Thresholds at 67 urban outfalls in Tokyo were identified from combined sewer simulation modeling using real-world rainfall data. CSO hydraulic behavior at these outfalls during 117 rainfall events, calculated using the simplified method, was strongly correlated with simulated results (R2 value >.8 in >8% of outfalls). The aggregated CSO depths from the whole 3,481 ha drainage area, determined for seven types of rainfall events using simulations and the simplified calculation method, differed by no more than 18%. These results indicate that event-based time-series rainfall data can yield accurate predictions of CSO behavior, and the simplicity and speed of calculation of the proposed method may make it an effective alternative to complex simulation models for stormwater management. Furthermore, the process of calculating CSO parameters reveals the underlying relationship between rainfall and CSO hydraulic behavior.