A Continuous Simulation Approach for Separate Sewered Areas

Abstract

The demand for large-scale watershed and sewershed planning studies in the United States has been increasing steadily over the past ten years. In large part, the demand is driven by major government programs regulating combined sewer overflows (CSO), sanitary sewer overflows (SSO), and storm water discharges. The implementation of these regulatory programs often results in local or regional public agencies embarking upon large multi-year studies requiring a comprehensive inventory of watershed and sewershed infrastructure, a characterization of the hydrologic and hydraulic function of that infrastructure, and analyses into the mechanisms by which pollutants are discharged into receiving waters. Significant monetary investments are made into comprehensive field investigations and surveys, hydrologic and hydraulic models, and regional facilities planning to develop and implement short-and long-term CSO and SSO control strategies.Accurately determining the quantity of extraneous flow that enters public sewers and private service laterals is a critical component of these comprehensive studies. The amount of rainfall dependent inflow and infiltration (RDII) entering the separate sewer systems varies from site to site and event to event as precipitation over a sewershed may produce different RDII responses within the sewers at different times of the year. The Lower Ohio hydrologic and hydraulic modeling project in Pittsburgh, Pennsylvania provided a unique opportunity to improve upon the accuracy and reliability of model simulations by incorporating monthly variations in sewer system responses to rainfall events. The completed analyses and model implementation were successful in quantifying site specific and seasonal variations observed in RDII responses.

Impact of Short Duration Intense Rainfall Events on Sanitary Sewer Network Performance

“…Therefore, we sought to establish a multi-variable linear regression equation to predict the remaining month's (January-October) RDII responses based on the limited data (measured during November-December, 2010). It is recommended in the literature to apply multi linear regression to predict better responses of RDII when long-term data is unavailable [26,41]. In the multi-variable linear regression analysis, the dependent variable is total R and the selected independent variables are total event rainfall, peak rainfall intensity and 7-day rainfall total before the event.…”

Rainfall Derived Inflow and Infiltration Modeling Approaches

“…· a minimum of 1 y, but a 4 month period including the spring season can provide a reasonable basis (Vallabhaneni et al, 2002); · 1 month or 2 month but with a high risk of incorrectly estimating RDII (Kurz et al, 2002); · a minimum of 42 d at 15 min recording intervals for a 400 parameter regression model (Zhang, 2005); and · at least 1 y (Loehlein et al, 2005).…”

“…Loehlein et al (2005) let RTK parameters vary from event to event with larger values for higher antecedent moisture conditions and lower values for lower AMCs. However, to use all of these calibrated RTK events in SWMM, it is necessary either to average individual storms into one composite set of RTK values or to average the storms for each individual month to develop a set of monthly-varying RTK parameters (Loehlein et al, 2005). Within SWMM it is possible to define initial abstraction values for each of the three RTK triangular hydrographs.…”

“…The second challenge, calibration, has been examined extensively (Bennett et al, 1999;Loehlein et al, 2005;Gheith, 2010), and the methods proposed include using brute force, regression models and genetic algorithms. In the BES case study, mentioned above, it took approximately 20 h per monitoring location to calibrate the nine RTK parameters to achieve the best fit (Bennett et al, 1999).…”

Impacts of Rainfall Data on Model Refinement in Greater Pittsburgh

“…To quantify RDII for each of the monitored sewersheds, a five-step process was used to perform dry-weather and wet-weather flow analyses for the monitored sewersheds in the study area. The analysis procedure and corresponding continuous simulation modeling approach, summarized briefly below, is described in greater detail in chapter 21 of this Monograph (Loehlein et. al., 2005).…”