Adapting Collection System Infrastructure to Changing Flood Vulnerabilities - Southeastern Massachusetts Case Studies

Collection systems and treatment facilities serve as an important environmental line of defense against wastewater contamination of groundwater and coastal embayments. With more intense and frequent storm events and rising sea levels, many communities are facing an increased risk to critical wastewater infrastructure from flooding and storm damage affecting critical wastewater infrastructure (Figure 1). This presentation will use case studies for collection system infrastructure along multiple types of waterbodies to discuss flood resilience measures being evaluated and implemented in Southeastern Massachusetts. The case studies discuss collection system infrastructure along:
Complex coastal estuarine shorelines
Open ocean
Tidally influenced coastal ponds
Inland river flood plains
The presentation will also focus on how these communities are incorporating accommodations for projected changes in storm intensity and frequency into flood resilience evaluation, design and construction. A brief description of each case study is summarized below. Town of Wareham (complex coastal estuarine shorelines) - The Town of Wareham is a coastal community with over 54 miles of coastline. Hydraulic constrictions caused by the complex coastal estuarine system results in high anticipated flood levels, up to 21 feet above mean sea level. The Town recently completed a 'Risk and Vulnerability Assessment', which provided them with a valuable tool to quantify the anticipated costs borne to the Town and its citizens if any vulnerable pump station failed during a 100-year storm event. The assessment allowed the Town to prioritize which of its many competing coastal resilience wastewater infrastructure needs to address first. Building on the findings of the Assessment, the Town is proceeding with the design of coastal resilience mitigation measures for three pump stations which were among the highest priority stations because they serve critical infrastructure (a hospital, fire department and police department). Coastal resilience measures incorporated into the design include the use of flood planks, structural reinforcement of existing structures (Figure 2), sealing potential water entry points and installation of emergency bypass connections (Figure 3). Additionally, following multiple Nor'easter storms in 2018 that exceeded the regional 40-year flood stage record, the Town conducted an evaluation of the impact of anticipated increased rainfall from more frequent and intense storms on the infiltration/inflow entering its collection system. The repetitive, consecutive nature of the 2018 storms elevated the groundwater table, producing more head pressure in the gravity collection system and dramatically increasing infiltration rates entering the collection system and being conveyed to the Wareham Water Pollution Control Facility. The Town is currently completing final design of retrofit measures recommended by the I/I projections evaluation (Figure 4). Town of Oak Bluffs (infrastructure exposed to the open ocean) -The Town of Oak Bluffs is a coastal community on the island of Martha's Vineyard. Its three vulnerable stations serve over 90% of the Town's sewered population, including many residences and the Town's primary commercial district (Figure 5). The Town recently completed the construction of mitigation measures to increase the coastal resilience of the stations through installation of a new emergency generator on a raised platform (Figure 6) and relocation of critical pump station equipment outside of the flood zone. Town of Chatham (tidally influenced coastal pond) - The Town of Chatham is a coastal community on Cape Cod. One of the Town's wet pit/dry pit pumping stations (Mill Pond) is located at the base of a tidally influenced coastal pond; updated FEMA flood maps show that the location is vulnerable to storm surges and flood events. During the Mill Pond Pumping Station Upgrade design, multiple flood protection measures were incorporated to increase the coastal resilience of the station. These measures include the establishment of a design flood elevation for the project which incorporates anticipated effects of sea level rise, installation of hydrophilic water stops and a flood proof entry door with stop blocks to provide flood mitigation measures (Figure 7). Town of Uxbridge (inland river flood plain). The Town of Uxbridge is an inland community in Central Massachusetts. Since construction of its centralized collection system the Blackstone River flood plain maps have changed and recent storm events have indicated that one of the Town's major pump stations is now vulnerable to the 100 year flood storm. During one storm event water was observed entering the station's wet well through conduits and rising within a couple inches of the top of the entrance tube (Figure 8). Additionally, the existing generator room was flooded, rendering equipment in the room inoperable during the storm event. The Town is currently constructing mitigation measures to increase the flood resilience of this equipment, including raising vulnerable infrastructure above flood levels.