For those of us living in the Western U.S., the news is dominated by stories that describe the effects of water scarcity and how climate change is making the future of many water sources uncertain. Yet, drought is not the only potential hazard that climate change is contributing to. For instance, on October 24, 2021, Sacramento had 5.44 inches of rain in a 24-hour period, causing some local flooding and exceeding the previous record set in 1880. This massive rain event shows that even in a drought, flooding is still a real concern.
Across North America, rainstorms are growing more intense. More rain falls faster, flooding streets and gutters, and causing streams and rivers to overflow. Drinking water systems may be particularly vulnerable to flooding. Many are located in low lying areas that provide easy access to the lakes or rivers that serve as the drinking water source, but those locations are also vulnerable when the source water is impacted by flooding. During a flooding emergency, residents trapped in homes or shelters, possibly without power, may have no way to get bottled water and limited options to boil water if the drinking water supply becomes contaminated. How can a small water system assess its risk and start thinking about mitigation?
Inventory Assets in Flood Plains
Mitigating disasters starts with effective planning. For flood planning, the Federal Emergency Management Authority (FEMA) develops and publishes Flood Insurance Rate Maps (FIRM) for most of the country. The maps are used to designate flood insurance rates through federally-backed programs and are widely recognized as the standard official flood risk maps. These maps can be accessed via FEMA’s National Flood Hazard Layer Viewer and the area of interest exported either as a PDF of the map or as a Geographic Information Systems (GIS) layer.
While a PDF of the FIRM will show if a facility falls within a 100- or 500-year floodplain, the resolution is lacking (see image above). Using a GIS layer of the floodplain maps can make analysis much more powerful. As discussed in a previous blog entry, using GIS as part of asset management is particularly helpful when viewing assets spatially. Overlaying assets with a FIRM, as shown in the screenshot below, is a good example of this. Using tools built into GIS programs like ArcGIS, it is possible to update the attribute tables for all assets that fall within a floodplain layer with that information. This can save quite a bit of time when compared to manually updating hundreds or possibly thousands of asset attribute tables.
Determine an Acceptable Risk Using Asset Management Techniques
If assets are located in a flood plain, risk can be evaluated through a multi-step process to identify assets at risk, evaluate the level of risk, and identify ways to ensure essential drinking water systems can function during extreme events. An asset management approach provides a framework for this.
The EPA published Flood Resilience: A Basic Guide for Water and Wastewater Utilities, containing worksheets and advice that water and wastewater utilities can use to walk through the risk assessment process as well as mitigation strategies. Below is a sample table from the guide:
For each asset, risk is determined by its vulnerability to a flood event. As shown in the vulnerability section of the worksheet in the table above, risk is largely associated with whether or not flood water will reach the asset, making the elevation of each asset a critical attribute for risk determination. The consequence of failure should consider the impact to regular facility operations as well as emergency operations. This is an important distinction as regular drinking water plant operations may not be feasible during a flood event. It may be possible, however, to continue to deliver already treated water from a storage tank and maintain distribution system pressure with some flood mitigation actions.
A key difference between traditional asset management and flood risk evaluation are the mitigation actions that water system managers can take to ensure a minimum level of service. While traditional asset management may focus on preparing for the end of an asset’s predicted service life and then replacing the asset, flood mitigation actions can include waterproofing an enclosure or electrical connections or elevating the asset a few feet to place it above the predicted flood water elevation. In these cases, the flood mitigation actions can be inexpensive, performed by drinking water system maintenance staff, and focus more on protecting critical assets than replacing them.
In a time of increasingly extreme weather events, being aware of the risk that flooding poses to critical facilities is important for managers of small water systems. While the process may seem daunting, following the step-by-step guidance provided in the EPA flood resilience guide and leveraging resources like the FIRM GIS layers and an existing asset management database can greatly simplify the process.