Development and Application of Method to Project Groundwater Infiltration in Sanitary Sewer Systems Affected by Sea Level Rise

Abstract

Coastal sanitary sewer networks are increasingly at risk of groundwater infiltration (GWI) into pipes due to sea level rise (SLR). As sea level increases, so does the groundwater table, causing sanitary sewer pipes to become submerged. If structural defects or deterioration, such as cracks, are present, submerged pipes will be subject to GWI. Therefore, there is a strong need for GWI projections as SLR persists. While previous methods relied on specialized modeling expertise or general infiltration parameters, this thesis describes a user-friendly procedure based on a two-dimensional (2D) GWI model with explicit parameters that better visualize circumstances of the network. These parameters include hydraulic head, which is proportional to SLR, and pipe defect size, which is used to prioritize pipes for improvement. Using flow monitoring data, the 2D GWI model was calibrated for the network in three-meter-pipe intervals. Data analysis and calibration were performed using Microsoft Excel and U.S. Environmental Protection Agency’s Sanitary Sewer Overflow Analysis and Planning Toolbox. Pipes that are currently submerged and pipes that will be eventually submerged were investigated individually. Firstly, for pipes that are currently under the groundwater table and subject to GWI, the flow meter measurement provides a sum of GWI in the upstream pipes. To determine the flow in each of the three-meter pipe segments, two methods of calibration were explored. Following calibration, future GWI was projected by increasing hydraulic head depending on the level of SLR. Secondly, different methods of GWI projections were tested for pipes that will be eventually submerged due to future SLR. These methods included a severity matrix of varying defect sizes and lengths of pipes with defects, a mathematical relationship based on previous calibration results, and an effective defect size that is representative of the porosity of a given sewer area. In conclusion, a flow chart is presented with GWI projection methods that are recommended based on availability of flow monitoring data or system knowledge.