The US EPA's Water Supply and Water Resources Division has partnered with the consulting firm of CDM to redevelop and modernize the Storm Water Management Model (SWMM) software. SWMM's computational engine has been completely rewritten using an object-based approach. Most existing modeling features have been kept and several new ones added. Special efforts were made to enhance the numerical stability of SWMM's dynamic wave flow routing routine. A rigorous Quality Assurance program was pursued to insure that results from the new SWMM matched those of the old SWMM or that reasons for any differences were justified. In addition, a graphical user interface was added to SWMM. It allows users to graphically edit system elements on a visual map of the drainage area and to view simulation results in a variety of formats such as time series plots and tables, profile plots, scatter plots, histograms, and statistical frequency tables. The revamped SWMM remains as public domain software and can be obtained from http://www.epa.gov/ednnrmrl/swmm.
Many municipalities throughout the US have sewer systems (separate and combined) that may experience exfiltration of untreated wastewater. A study was conducted by the US Environmental Protection Agency (USEPA) to focus on estimating the magnitude of leakage of sanitary and industrial wastewater from sewer pipes on a national basis. The method for estimating exfiltration amounts utilized groundwater table information to identify areas of the country where the hydraulic gradients of the wastewater are typically positive, i.e. the wastewater flow surface (within pipelines) is above the groundwater table. An examination of groundwater table elevations on a national basis reveals that the contiguous US comprises groundwater regions (established by the US Geological Survey) that are markedly different. Many parts of the northeastern, southeastern, and midwestern US have groundwater tables that are higher than the wastewater flow surface, resulting in inflow or infiltration. Conversely, the combination of a relatively low groundwater table and shallow sewer systems creates the potential for widespread exfiltration, a situation more commonly found in communities located in the western US.
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