For the past 30 years, freshwater flooding in the US resulted in an average of $8.2bn loss[1] per year, though this average accounts for an upward trend recently. According to a 2018 study titled “Estimates of present and future flood risk in the conterminous United States” published in Environmental Research Letters, nearly 41 million people live within the floodplain nationally, and damage estimates from a 100-yr flood is $1.2 trillion. With sea-level rise, the impacts of flooding will continue to grow with more people getting affected and the damage estimates increasing (Wing, 2018). The following graph shows future flooding trends and the expected damage.

The following graphs show the future flooding scenarios in various cities in the US.

Where to Start

You can make a difference in mitigating the flood impacts on buildings by adding a few simple steps to your design process.

1. Carry out high-level studies to determine if the project location is in a flood plain or can be affected by floods in the future. The following link provides flood maps for various locations for current and future climate scenarios. https://coastal.climatecentral.org/. In addition, FEMA flood maps indicate flood hazard areas for various locations.
2. It is recommended that projects in flood risk zones involve qualified consultants or engineers who can carry out accurate flood modeling and (a) identify Design Flood Elevation (DFE) and (b) identify all critical assets below the design flood elevation. (Tak, 2019). Design Flood Elevation (DFE) refers to the elevation that is used for determining the elevation of building elements in new construction[2]. Critical assets include components that support the function of the building during emergencies and otherwise.
3. The Resilience Design Toolkit that is compiled by AIA and HKS recommends performing a Benefit Cost Analysis for each of the identified strategies for the project. The analysis yields a Benefit Cost Ratio (BCR) which is the ratio of the initial costs to the savings during perilous events. If the BCR is one or more, the strategy can be considered beneficial.

What Designers Can Do Now

The figure below summarizes the various strategies that designers can use to “flood-proof” the building.

1. Building layout and access: As shown in the figure above consider elevating building access, emergency exits, and pedestrian infrastructure above the design flood elevation. Emergency exit stairways and corridors must be wet floodproofed and designed with flood vents to maintain the operation of the exit door.
2. Critical Services: For projects that are in flood risk zones, floodproof low-lying electrical and other critical components. Locate electrical components and other critical components such as emergency power backup, and chemical storage facilities at an elevation higher than the Design Flood Elevation. Alternatively, the project can use a “compartmentalizing” approach to protect spaces that hold critical components from flood water.
   
   1. Electrical: The project can enhance the redundant power supply which entails serving the project with multiple power sources. While each of the power supplies provides a fraction of the power depending on the number of power sources, one or more power supplies can provide the entire power when other power sources fail (Vechy, 2021).
   2. Potable water storage: Plan for potable water storage on-site if the municipal potable water supply is compromised during floods.
   3. Sewer Lines: Installing backflow prevention valves on sewer lines can minimize stormwater and sewage flowing into buildings (City of Hoboken, 2015).
   4. Stormwater systems: Design stormwater systems to manage excess stormwater runoff during flood events and infiltration, where feasible. The following resource provides an online stormwater assessment tool https://coast.noaa.gov/stormwater-floods/analyze/
3. Foundation:
   a. Construct floors above the design flood elevation with the grade sloping away from the building. Consider raised slab and elevated pier foundation, if applicable, as opposed to the slab foundations[4]. Placement of fill under floor slabs is not recommended in flood-prone areas, rather allowing for “wet levels” under DFE by accommodating flood openings are recommended to allow flood water to flow in and out of the space and prevent foundation collapse because of uneven hydrostatic pressure[5] as shown in the figure below.

Openings must be equally proportional to the floor area that it is enclosing as shown in the figures below. Areas exposed to flood water requires wet flood proofing.

4. Building Enclosure: The following table from Designing for Natural Hazards: A Resilient Guide to Builders and Developers Volume 2 summarizes some of the building enclosure strategies to mitigate flood impacts to the building.

5. Materials below DFE: The Resilient Building Design Guidelines published by the City of Hoboken recommends using reinforced concrete or steel below DFE and limiting the use of decay-resistant lumber, marine-grade wood, and carpeting. The guidelines recommend using only closed-cell insulation and avoiding the use of fiberglass insulation in areas that may encounter flood water. More detailed information is available in FEMA Technical Bulleting 2: Flood Damage Resistant Material Requirements.
6. Miscellaneous:
   a. Elevators: While elevators can access areas below the Design Flood Elevation (DFE), it is recommended to locate elevator control equipment above the DFE. Float switches can prevent a can from going below the DFE during floods. FEMA’s Technical Bulletin 4: Elevator Installation for Buildings Located in Special Flood Hazard Areas provides additional guidelines for elevator design and installation in flood-prone areas.

Tools and Resources

1. https://coastal.climatecentral.org/.
2. FEMA Flood maps
3. https://coast.noaa.gov/stormwater-floods/analyze/
4. https://buildingscience.com/documents/building-science-insights/bsi-128-designing-floods#:~:text=All%20flood%20risk%20is%20reduced,foundations%20and%20crawl%20space%20foundations.
5. https://longislandsoundstudy.net/our-vision-and-plan/thriving-habitats-and-abundant-wildlife/living-shoreline-projects/
6. https://coastalscience.noaa.gov/
7. https://opc.ca.gov/2024/01/draft-slr-guidance-2024/#:~:text=Key%20takeaways%20from%20the%20updated,in%20the%20next%2030%20years.
8. FEMA Technical Bulletin 2: Flood Damage Resistant Material Requirements
9. FEMA’s Technical Bulletin 4: Elevator Installation for Buildings Located in Special Flood Hazard Areas
10. Resilient Building Design Guidelines by the City of Hoboken
11. Resilience Design Toolkit by AIA and HKS
12. Flood Damage Resistant Materials Requirement- FEMA Technical Bulletin 2
13. Designing for Natural Hazards: A Technical Guide for Builders and Developers- Volume 2 Water

[1] (Wing, 2018)

[2] https://fontanarchitecture.com/base-flood-elevation-design-flood-elevation/

[3] Resilience Design Toolkit, HKS and AIA

[4] https://buildingscience.com/documents/building-science-insights/bsi-128-designing-floods#:~:text=All%20flood%20risk%20is%20reduced,foundations%20and%20crawl%20space%20foundations.

[5] Resilient Building Design Guidelines by City of Hoboken