GIS as a Tool for Modeling Sanitary Sewer Expansions

Proper sizing and location of sanitary sewer interceptors is of great value to a sanitary sewer district, both in terms of monetary value and ecological costs. This paper discusses the methods and advantages of using a Geographic Information System (GIS) in the planning and design of a future sanitary sewer expansion for the Louisville and Jefferson County Metropolitan Sewer District (MSD). For this particular project the Louisville and Jefferson County Information Consortium (LOJIC) GIS database was used.The detail of the information available through GIS can vary greatly. A small community may only have information that is available on the internet, while a major metropolitan area may have many well developed layers of information including digital elevation models (DEM), hydrography data sets, zoning maps, vectorized street segments, floodplain/floodway boundaries and land use maps, existing drainage facilities etc. Regardless of the level of complexity, GIS can help the planner to create a better model of future needs.Sanitary sewer interceptors should be of adequate capacity so as to minimize the likelihood of sanitary sewer overflows (SSO's). Additionally, the pipe must be small enough to be economically feasible as well as to maintain adequate flow characteristics early in the life cycle. Geographic Information Systems give the engineer/planner the data to assess current and future sanitary sewer needs and assist in preliminary design.Modeling to determine MSD's future needs began with a digital elevation model and streams GIS. These layers were used to create sub-basin boundaries. A zoning layer was then used to determine residential, commercial, and industrial areas. Areas that were considered non-developable were subtracted from the residential acreage in each sub basin. These non-developable areas include parks, cemeteries, floodplains, right-of-way, wetlands, steep sloped areas, and buffer zones.A parcel analysis of was performed to identify un-served, small parcels (less than one acre) with houses. These parcels are unlikely to experiences further development and the expected flows from these parcels can be determined from water use records. The remaining larger tracts of land were then projected to be fully developed based on current zoning. The average parcel size for existing residential lots was determined for each subbasin at this time to give the designer some idea of how many parcels will ultimately be created in a particular sub basin. Future flows were projected based on the number of proposed new homes with existing flows being added as a known quantity. With this projected flow data, and slope estimates from the digital elevation model, interceptors, pump stations and force mains can be located and preliminarily sized.Growth prediction and sanitary sewer modeling with GIS is still in its infancy. And these early projects should be monitored closely to determine their accuracy and to gain better tools for future modeling.