Switching Gears with the Tools You Have: Sizing SSO Storage Tanks vs. Relief Sewers

Traditionally, sanitary sewer relief projects have been designed on a capacity basis. Given this, most design tools that the hydraulic engineer uses are capacity driven. One of the main tools in the toolbox is the hydraulic model. These models vary in complexity from steady state that use peak flows, to fully dynamic that solve the Saint-Venant equations. Typically, sewer design does not require a fully-dynamic model, but a simple steady-state model or one that uses a hydro logic method of flood routing. The traditional solution to providing sanitary relief for collection systems with significant inflow and infiltration (I/I) has been transport and treat. However, the cost of constructing new sewers and expanding treatment capacity is increasing significantly. A cost-effective solution currently gaining more acceptance is providing temporary storage within the collection system, at treatment plants or pump stations. With this change, the design basis has shifted from mainly peak flow, to volume or flow over a period of time. This change in design basis presents some questions. In general, a design storm is used to determine the peak flow when considering I/I in a sanitary sewer. Is this same method of using a design storm still appropriate for volume-based designs? If not, then a continuous simulation is needed. The next question to be asked is how rigorous of a model is necessary based on the nature of I/I modeling, data available, and existing models? Since the traditional solution has been to transport and treat, most existing hydraulic models have been built around the design storm. These models either are not capable of performing continuous simulations, or require additional data that may not be readily available and can be computer intensive. Is there a way to utilize these existing models and do a continuous simulation that doesn't require large computer resources? A procedure is outlined that uses existing single-event Hydra models and historical rainfall data to develop continuous flow records for sizing storage facilities. To summarize the procedure, HEC-HMS is used to develop continuous wet weather flow at the storage facilities by using sub-basins with user-defined unit hydrographs derived from the Hydra models. The groundwater infiltration is modeled with the recession baseflow method. With a continuous flow record, a frequency analysis of the required storage volume for each activation is then made.