Groundwater-Sewer Interaction in Urban Coastal Areas

Abstract

In this paper, a study of the potential causes of the occurrence of high concentration of Enterococcus Faecalis in surface water within urban areas in dry-weather conditions (DWCs) is presented. Two hypotheses were formulated: (1) undersized sewer system; and (2) groundwater infiltration into damaged sewer pipes. In both cases, more frequent combined sewer overflows (CSOs) may occur discharging untreated sewage into surface water. To evaluate the first hypothesis, a hydraulic model of a sewer was developed assuming a water-tight system. The simulation results show that CSOs never occur in DWCs but a rain event of intensity equal to 1/3 of one-year return period may trigger them. To evaluate the second hypothesis, a model combining sewer failure with groundwater level was developed to identify the sections of damaged sewer below the water table and, therefore, potentially affected by infiltration. The risk of infiltration exceeds 50% in almost half of the entire network even at the lowest calculated water table. Considering 50% of infiltration distributed throughout that part of the network, CSOs can occur also in DWCs.

A Flow-Calibrated Method to Project Groundwater Infiltration into Coastal Sewers Affected by Sea Level Rise

“…De Benedittis and Bertrand-Krajewski [27] reported GWI of about 20% of ADWF in Lyon, France. Liu et al [17] summarized GWI ranges as: 73% in Nante, France, 108% in Rastatt, Germany, 39% in Zurich, Switzerland, 33% in Dresden, Germany, 14-50% in Rome, Italy, and 75% in Trondheim, Norway. The values reported in the present study are within the ranges reported in some of the prior studies.…”

“…The study area has both a wet and dry season, and although the study took place in the wet season, selection criteria were used to identify and isolate days of DWF. Other factors that can play a role in GWI include ocean tides, seasonal recharge rates, and fluctuations in nearby rivers or bodies of water [6,16,17]. Tidal effects on groundwater elevations were included in the present study.…”

“…Increases in GWI lead to higher maintenance and treatment costs, and potentially more incidents of sanitary sewer overflows (SSOs) [17]. Planning and management tools are needed to predict increases in GWI and prioritize sanitary sewer rehabilitation.…”

Groundwater Rise and Associated Flooding in Coastal Settlements Due To Sea‐Level Rise: A Review of Processes and Methods

“…In their particular case of a barrier island aquifer lens system common in the USA Atlantic coastline, the amount of groundwater discharge to streams increased rather than resulting in a water table rise, and the tidal influence propagated further inland decreasing the depth to the fresh-salt water interface. In addition to the typical groundwater flow numerical model development, the subsurface infrastructure interactions were added and simulated (Liu et al, 2018;Su et al, 2020). The groundwater infiltration into an aging sewage system was investigated and the impact of repairs on the damaged sewer pipes was modeled.…”

Sensitivity Analysis of a Groundwater Infiltration Model and Sea-Level Rise Applications for Coastal Sewers

“…Priority areas for managing I/I include low-lying areas, areas with unstable soils, areas with high groundwater, old pipe infrastructure, sewers located near storm drains, and sewers located near bodies of water. For collection systems with excessive I/I flows, potential impacts are sanitary sewer overflows (SSOs) from overloaded manholes and pump stations, and increased risk of bypasses at the wastewater treatment plant if flow capacity is exceeded [1][2][3].…”