North Fayetteville Sanitary Sewer Study

The Public Works Commission of the City of Fayetteville, NC (PWC) identified a need to expand upon their existing 1996 Sanitary Sewer Master Plan in the North Fayetteville area to better prepare for future growth and potential land use changes. The following tasks were undertaken:

Study Model Assumptions

The existing Sanitary Sewer Master Plan is a tool from which PWC can manage the long-range needs of its wastewater collection system for the next 20 years. The North Fayetteville Study would use 2010 Land Use Zoning to calculate sewer flows based upon the ultimate build-out conditions. The 2010 Land Use Plan is a guide, and may not represent actual zoning changes that may occur. Therefore, the model was constructed to allow the user to define land use changes within a specific sewershed and re-calculate the associated sanitary sewer flows.

The North Fayetteville model is a tool from which PWC can project flows within the sewersheds of the study area and determine what the most financially feasible methods of expansion. The model would permit the user to make land use changes to a sewershed area and determine what impact those changes would have on the recommended collection system. Soil types are also integrated into the model, allowing the user to eliminate areas within a sewershed that could not support growth due to their hydric nature.

Creation of Land Classification Data

The land classification data layer was created using the Land Use 2010 Plan, Cumberland County Soils, Watershed Protection Area, Flood Zones, Sewershed. The common unit in the study was acres and when the total amount of acres of each land type were determined, the total daily flow could be generated quickly. This process was automated through ArcView’s Avenue programming language.

Example Sewershed

Sewershed #11 has an area of 742 acres. Unlike many of the sewersheds encountered in this study, this sewershed contains several different land uses, including Heavy Commercial WPA (38 acres), Heavy Industrial WPA (116 acres), Office/Institutional (10 acres), Low Density Residential WPA (15 acres), and Rural Residential WPA (302 acres).

Varied topography makes providing sewer to this area difficult because the areas with the most existing development are associated with the steeper contoured areas.

The soils in this area consist mostly of Dunbar Loams (117 acres), Blaney Loamy Sands (92 acres), Gilead Loamy Sands 103 acres), Duplin Sandy Loam (76 acres) and Vaucluse Loamy Sands (68 acres). There are approximately 113 acres of associated hydric soil areas that are deemed to be unusable. These varied soil classifications when combined with the differences in topography across the site makes it difficult to analyze for potential sewer improvements, since the hydric soils areas are scattered throughout the area.

Example Sewer Improvement

Slocomb Outfall, Sewer Improvement #8

This outfall is designed to serve the areas within sewershed 11, 13 and E. Collectively, these sewersheds could be expected to generate a peak flow of 741,654 gpd using the currently proposed land uses. Factoring in a potential increase in the expected flows from the sewersheds due to changes in land uses of around 25% would increase the total expected peak flow to 927,968 gpd.

The costs associated with this improvement are as follows:

1. 6500' 12" Gravity Sewer @ $104.00 $676,000

2. 3500' 8" Gravity Sewer @ $64.00 $224,000

Estimated Construction Cost $900,000

Estimated Project Cost $1,143,000

Conclusion

The use of GIS technology greatly enhances the ability of PWC to analyze and manage the wastewater infrastructure needs of a given area and to adjust land uses to predict potential flow quantities generated by different types of development activities. The model used for this study is applicable to any sewershed identified within the service areas currently served and proposed to be served by PWC, and can be manipulated to consider various land use conditions in order to determine the best alternative available to provide sewer service to any given area. The ability exist to potentially integrate this model into other hydraulic models to provide a real time simulation of conditions and needs for the sewer system. With a comprehensive database in place, PWC will be able to consider existing collection systems and treatment plant capacities and implement a cost effective expansion policy of leading development into an area rather than reacting to development.