Flood Risks

Communities can reduce flood risks (i.e., the risk of having too much water) through three general approaches: Protection, Accommodation, and Avoidance.^{See footnote 1} 

Protection

Living shoreline pilot project along the Oyster River at Wagon Hill Farm in Durham, New Hampshire. (Credit: Kirsten Howard. Source: New Hampshire Department of Environmental Services)

Protection against flooding involves steps to repel or absorb flooding in a manner that does require that communities must rebuild or relocate. The most common type of protection measure is a floodwall or levee. These typically keep floodwaters from inundating areas inland of the defenses and send the floodwaters downstream. An alternative is use of natural systems such as wetlands or other vegetation to absorb floodwaters. This is known as “green infrastructure” and is widely discussed in the toolkit, including in this chapter.

A key drawback to protection measures is that they have limits — as demonstrated in New Orleans during Katrina.^{See footnote 2} A very large flood can breach or overtop a floodwall or exceed the amount of water that green infrastructure can absorb. Thus, a strategy relying solely on defenses against flooding can leave communities with some residual risk of flooding (although a lower risk than if there were no protection or other measures taken).

Policymakers can address equity considerations by investing in defensive measures that provide equal protection to frontline communities, ensuring that flood risks do not shift from privileged areas to frontline areas. Programs can include providing financial support for the implementation of defensive measures, such as green infrastructure, for frontline communities. The support should include maintenance to ensure systems are not degraded over time.

Green infrastructure (GI) uses vegetation and soils, permeable surfaces, and landscaping to allow runoff from storms to be stored, infiltrate into soils and aquifers, or evapotranspirate (i.e., evaporation from surface water and soils into the atmosphere and transpiration through vegetation).^{See footnote 3} GI can rely on “grey infrastructure” of drainage systems and pipes to convey water to treatment plants. Installing green infrastructure can cost less than installing grey infrastructure, but can require more maintenance.^{See footnote 4} A distinct advantage of GI is the multiple benefits it provides -- benefits that grey infrastructure does not provide. The vegetation in GI can lower temperatures, which can be critical and life-saving in urban areas in the summertime. It also absorbs carbon, reduces air pollution, and provides aesthetic benefits.^{See footnote 5} This chapter presents case studies using green infrastructure in frontline communities to address flood risks; see the Natural Resilience Chapter for additional case studies and further discussion of the benefits of green infrastructure. 

Considerations of Green Infrastructure as a Tool for Flood Protection

Accommodation

Accommodation involves developing strategies to live with (i.e., accommodate) flooding. A common approach is elevating structures such as houses and buildings, thereby allowing floodwaters to flow under them.^{See footnote 6} Other structural changes might include placing key equipment such as heating and cooling systems on upper floors of multi-story buildings. Comprehensive accommodation policies can also include emergency management systems, (e.g., warning systems and evacuation procedures) to enable people to evacuate an area that is forecast to be flooded.

Accommodation approaches such as elevating structures to reduce their exposure to flooding, avoids dislocation of communities and has fewer adverse environmental impacts than protection, but is an expensive resilience option. As a result these options are often adopted by wealthier communities that can better afford to elevate structures.^{See footnote 7} For frontline communities, expensive options can be particularly challenging to implement, so a key factor in elevating communities is to obtain financial support from other sources. An equitable approach to adopting accommodation as a policy strategy means that frontline communities are given opportunities to finance construction or elevation of homes and buildings, including through access to federal and state funding sources, so they can withstand future floods.

This chapter presents a case study on elevating homes in a highly flood-prone area.

Considerations of Elevating Structures as a Tool for Flood Accommodation

Avoidance

An avoidance strategy reduces exposure of people and structures in areas at risk of flooding. One approach is to limit or prohibit development in floodplains.^{See footnote 10}

Another approach is to “retreat” or relocate structures and neighborhoods to areas with a lower risk of flooding. Avoidance reduces vulnerability but can come at an economic cost. Foregoing use of a land area near a water body such as a riverfront for commercial or residential purposes can have economic and lifestyle costs.^{See footnote 11} In addition, compensation may be needed for property owners who are prohibited from developing certain areas. Relocation of communities to avoid the risk of flooding can be expensive and disruptive of neighborhoods, community connections, and commerce.

One strategy for moving structures out of harm’s way is through government buyouts, whereby the structures in flood areas are purchased from homeowners by the government.

Frontline communities in flood-prone areas can be disproportionately identified for relocation because the relative costs of buyouts are lower than buyout costs in wealthier communities. As a result, it is important to ensure that buyouts do not happen only in frontline communities and that frontline community residents are treated fairly and not made worse off by buyouts. Perhaps most critical is that frontline communities are relocated to areas that do not diminish access to livelihoods and public transportation and do not otherwise diminish quality of life. Some key equity considerations for buyouts include: ensuring that relocation is applied equitably, not disproportionately to frontline communities; ensuring that property owners are given fair compensation for the value of their properties; and ensuring there is adequate financial and administrative support for the purchase of new properties, relocation costs, and addressing issues such as mental health.

This chapter presents a case study on pursuing buyouts as a potential avoidance strategy.

Considerations of Buyouts as a Tool for Flood Avoidance

It is important to note that all three of these approaches (Protection, Accommodation, and Avoidance) involve determining what level of flooding to protect against. Flood protection and elevation are done to a particular height. Should future floods exceed these levels, the protections may prove to be inadequate. Raising structures to historic flood levels such as the 1:100 flood (sometimes referred to as the 1-in-100-year flood) risks inadequately protecting structures in the future because climate change is likely to increase flood levels. So, what is currently a 1:100 flood event might in future decades become a 1:50 year event (i.e., happen twice as frequently). This is challenging when attempting to anticipate climate change because while it is virtually certain that future floods will be larger, exactly how much larger is not known.^{See footnote 13} A presidential executive order in 2015 required that flood protection for federal projects add two feet of elevation above the current 1:100 flood level for non-critical infrastructure, which includes housing.^{See footnote 14} The executive order was revoked in 2017.^{See footnote 15} 

Lessons Learned

Protection: Green Infrastructure

• Green infrastructure is an attractive approach for addressing risks from flooding such as combined sewer overflow risks because GI has multiple benefits such as reducing heat stress and air pollution and providing aesthetic benefits.
• Targeted finance programs are often needed to ensure that frontline communities can implement GI.
• GI requires maintenance but this provides an opportunity to employ contractors and labor from frontline communities.

Accommodation: Elevating Structures

• Elevation of structures can keep a community in place and avoid environmental harm from flood protection measures.
• Financial support from the federal government (and if available state governments) for the elevation of homes is needed to make the option affordable for frontline communities.
• As is seen in the Tehama case study, not all homeowners may elevate their homes, leaving some structures still vulnerable to flooding.

Avoidance: Buyouts

• For areas at risk of repeated flooding, buyouts to move structures and people out of harm’s way may be better for well being, safety, and public finances than taking other measures to protect the structures.
• Buyouts give municipalities the option to use areas for other purposes such as parks.
• It is critical that frontline communities are not made worse off through buyouts and if anything, gain an equal or improved quality of life and access to livelihoods and city services such as public transportation.