Hydraulic Performance and Vulnerability on Sanitary Sewer Overflow in Southern Pinellas County, FL

Rain-induced sanitary sewer overflow due to high infiltration is a significant challenge for many utilities, including Pinellas County utilities. The main aim of this study is to develop a hydraulic model to analyze the performance of the existing sanitary sewer system, especially during intense rainfall events. To calculate the flow inputs for the model, a times series analysis was performed to separate the inflow and infiltration from the actual sewer flow. Using the Stevens-Schutzbach method, daily Base Infiltration (BI) was calculated and was subtracted from the total observed flow to give the Dry Weather Flow (DWF). Adjusting the DWF by the diurnal pattern, residual flows were calculated to test the flow variability in the system and compare to rain events (\u3e 0.5 inches); the residual flow help deduced if there is a significant surface inflow into the system. Using PC SWMM as the hydraulic model, the average DWF was simulated using the average value and the diurnal daily and weekend pattern during the dry weather periods. The calculated BI was added to the model as a direct contribution from the statistical model. Both the average value of DWF and BI were distributed throughout the system for simulation. The simulated flow shows that few downstream manholes surcharges during extreme rainfall events and remained surcharge for over 48 hours. Cross-correlation analysis suggests the rainfall of the past seven days still impacts the BI, with the highest impact on days 1, 4 and 5. The correlogram results were used to develop a regression model, to predict the BI for different rainfall depths, which in turn was used for hydraulic performance analysis. Increasing the rainfall depths and routing the flow using PC SWMM, showed that the hydraulic grade line, number and hours of the surcharged manholes increases as total rainfall depths increases, but no sanitary sewer overflow. Sanitary sewer overflow occurred at the lift station with a design capacity of 200 GPM for all increased rainfall depths. Furthermore, the analysis results can help locate areas where overflow is more likely to occur, and can also help plan and implement a cost-effective rehabilitation program for the existing sewer network