Keeping our heads (and infrastructure) above water

Recently, I had the opportunity to collaborate with a coastal community in the US, with over 85 kilometres of coastline. Due to its unique coastal geography, this area faces a strong likelihood of severe flooding, potentially reaching up to 6.5 metres above sea level. As sea levels continue to rise due to climate change, many residents find themselves in a vulnerable position, especially considering the 1-in-100-year flood event (a 1% annual chance), referred to as the base flood in this article.

While this scenario could describe numerous coastal areas around Aotearoa New Zealand, this example is from my experience working on a project in the Town of Wareham, a coastal community in southeastern Massachusetts, with a population of approximately 20,000. Although in a different country, the approach to evaluate and prioritise both the risks and mitigation measures can be universally applied.

The US Federal Emergency Management Agency (FEMA) produces flood maps that identify zones with a 1% base flood and a 0.02% annual chance of flooding. These maps are periodically updated based on the latest flood projection models. 

One particularly vulnerable area is the low-lying coastal town of Wareham, Massachusetts. The town is nestled around the coastal estuaries of Buzzards Bay, which creates several throttling points where strong waves are anticipated to develop from Atlantic Ocean storms. Wareham’s wastewater infrastructure, typically located at the lowest elevations, is especially at risk under the most recent base flood projections.

To assess the status of vulnerable coastal wastewater infrastructure, it’s important to first determine the safe elevation for these facilities, known as the Design Flood Elevation (DFE). The DFE is based on a number of factors, including the base flood elevation, a required minimum freeboard and other factors such as projected sea level rise.

Most of Wareham’s pump stations are situated in areas vulnerable to base floods, with risks heightened by inundation and exacerbated by anticipated sea level rise due to climate change. Out of the town’s 43 pump stations, 29 are located within the Special Flood Hazard Area. Additionally, the infrastructure and multiple pump stations outside of these flood zones rely on vulnerable stations to transport wastewater to the Wastewater Treatment Facility. Many of these outlying pump stations route wastewater through up to four other pump stations, many of which are in the vulnerable zones.

The anticipated impacts from the base flood ranged from minimal damage to inundation, which could render a pump station inoperable. With so many pump stations in vulnerable positions, a pragmatic prioritisation methodology needed to be developed.

A Risk and Vulnerability Assessment was developed using the risk and consequence approach outlined in “Risk Analysis and Management for Critical Asset Protection (RAMCAP®), a document commonly referred to as “J100”.

Flood risk is calculated for each pump station based on the expected monetary total loss for vulnerable infrastructure for both Wareham and its residents. J-100 defines flood risk as a combination of two factors: The likelihood of a specific natural disaster and the total estimated loss that can be anticipated from the disaster. While determining the likelihood is reasonably simple, being equivalent to the base flood, the calculation of loss involves a more complex process.

J-100 defines total loss as the “sum of repair and replacement costs and losses due to lost production capability and other first effects”.

Due to the lack of readily available data, we couldn’t proceed with the complex and costly modelling required by the Hazards United States (HAZUS) process. Instead, we opted for a Benefit Cost Analysis (BCA) using the BCA 5.1 software, produced by the Federal Emergency Management Agency (FEMA).

1. Once flood water has entered the base floor of the structure, all electrical components are at risk. Specifically, for pump stations with wet/dry well configurations, the pumps are vulnerable to damage.  
2. The project evaluated whether existing structures were designed to withstand the hydrostatic pressure from flooding. It was found that if the pressure on an unreinforced masonry wall is greater than 75 cm, the integrity of the structure is compromised.  
3. For stations positioned downstream from important facilities like fire departments, police stations, or hospitals, it was assumed that these essential services would face operational challenges if the wastewater services were disrupted.  
4. The monthly displacement costs were estimated for each pump station. This takes into account the number of people dependent on that pump station and all sewer sheds upstream that would be affected if the pump station was not operational.