Structural vs Non-Structural Mitigation Strategies (Elevation, Acquisition, Relocation, Dry/Wet Floodproofing)

Structural vs Non-Structural Mitigation Strategies (Elevation, Acquisition, Relocation, Dry/Wet Floodproofing)

Flood hazard mitigation is any sustained action taken to reduce or eliminate long-term risk to people and property from flooding. For Certified Floodplain Managers (CFMs), mitigation sits alongside regulation and insurance as a core pillar of modern floodplain management. When a community faces repeated inundation, rising residual risk behind levees, or a cluster of severe claims, the floodplain administrator must help decision-makers choose among measures that either change how water behaves or change how people occupy the floodplain. That distinction—structural versus non-structural—is foundational for the CFM exam and for real project pipelines.

Structural Mitigation: Modifying the Flood

Structural mitigation uses engineered works to contain, divert, store, or convey floodwater. Classic measures include:

• Levees and embankments that hold river or coastal surge water away from developed areas.
• Floodwalls (often concrete or masonry) used where space for earthen levees is limited.
• Dams and reservoirs that store peak runoff and release it more slowly.
• Channelization, diversions, and conveyance improvements that move water through or around a community more quickly.
• Stormwater detention/retention facilities that manage urban runoff peaks at a smaller scale.

Structural projects can provide substantial residual-risk reduction and protect large service areas. They also carry CFM-relevant caveats. Levees and floodwalls can create a false sense of security and induce development in the protected area; if the structure overtops or fails, damages can be catastrophic. Residual risk never goes to zero. Maintenance, freeboard design, interior drainage, and emergency action planning are ongoing obligations. From an NFIP perspective, areas behind levees may still be mapped and regulated depending on accreditation and mapping status—CFMs must not assume “behind the levee” equals “no floodplain management.” Channelization can increase velocities, transfer risk downstream, and degrade natural floodplain functions. Large structural works often involve USACE partnerships, multi-year planning, environmental review, and operations-and-maintenance agreements that outlive political cycles.

Non-Structural Mitigation: Reducing Vulnerability

Non-structural mitigation reduces damage without trying to fully control the water. It focuses on where and how buildings are sited and built, how land is used, and how residual risk is financed. Core non-structural strategies for flood include:

1. Elevation of structures above flood levels.
2. Acquisition (buyouts) of high-risk properties.
3. Relocation of buildings to safer ground.
4. Dry and wet floodproofing within NFIP limits.
5. Land-use controls, open-space preservation, and warning/evacuation systems that reduce exposure and life safety risk.

Non-structural measures often perform better in benefit-cost analyses for individual structures, preserve natural floodplain storage, and avoid transferring risk downstream. They are also the tools most closely tied to local ordinance administration, Increased Cost of Compliance (ICC), and FEMA Hazard Mitigation Assistance (HMA) project types.

Elevation

Elevation raises the lowest floor (or, in V zones, the lowest horizontal structural member) so floodwater passes beneath or around the building. Methods include piers, piles, columns, stem walls with fill, or continuous foundation walls with compliant openings in A zones. Elevation is the primary compliance path for residential structures in Special Flood Hazard Areas (SFHAs) and is the preferred long-term retrofit for many repetitive-loss homes. CFMs must ensure elevated buildings meet freeboard if required by ordinance, that enclosures below the elevated floor meet opening/vent and use limitations, and that utilities and mechanical equipment are also elevated or protected. Post-elevation Elevation Certificates document compliance for insurance and local files.

Acquisition (Buyouts)

Acquisition permanently purchases flood-prone properties (often after repetitive damage) and removes the structures. Deed-restricted open space typically remains in public ownership or under easement for parks, wetlands, or passive recreation. Buyouts eliminate future claims at that parcel, restore floodplain storage, and can create greenway corridors. Challenges include owner willingness, appraisal and relocation assistance under the Uniform Relocation Act when federal funds are used, demolition costs, and long-term stewardship of open space so it is not redeveloped. CFMs frequently identify candidate parcels from repetitive-loss lists, substantial-damage determinations, and mitigation plan priorities.

Relocation

Relocation moves an existing structure to a site outside the high-risk area (or to higher ground on a large parcel). It preserves building investment while eliminating flood exposure at the original site. Relocations require structural integrity assessments, temporary utility disconnects, foundation design at the new site, and careful handling of the vacated floodplain parcel—often with deed restrictions similar to buyouts if federal mitigation funds are involved.

Dry Floodproofing

Dry floodproofing makes a structure substantially impermeable to floodwater so that flood levels up to a design elevation do not enter. Techniques include sealants, waterproof membranes, closable shields or gates at openings, backflow valves, and sump systems. Under NFIP minimum standards, dry floodproofing is generally not an acceptable alternative to elevation for residential structures in the SFHA. It is commonly available as a compliance option for non-residential buildings in A zones when designed by a registered professional engineer or architect, with a design flood elevation that meets or exceeds the BFE (plus freeboard if required). Dry floodproofing has practical limits: hydrostatic and hydrodynamic loads increase with depth; FEMA guidance has long cautioned against relying on dry floodproofing for deep flooding (often cited around three feet or more depending on design), and continuous human intervention to install shields can fail during short-notice events. In V zones, dry floodproofing is not the standard path; coastal high-hazard construction relies on elevation on open foundations and breakaway construction concepts.

Wet Floodproofing

Wet floodproofing allows floodwater to enter and exit a structure to equalize hydrostatic pressure, using flood-damage-resistant materials, elevated utilities, and adequate flood openings. It is not a general solution for primary residential living space under NFIP rules. Wet floodproofing is typically limited to accessory structures, parking, building access, and limited storage below the elevated floor—uses already contemplated in many local ordinances for enclosures and accessory buildings. CFMs reviewing wet-floodproofed spaces must verify opening calculations, material specifications, and prohibited habitable uses.

Freeboard: Building in a Margin of Safety

Freeboard is additional height above the regulatory flood elevation (usually the BFE) required for the lowest floor or dry-floodproofed elevation. Communities adopt freeboard as a higher standard (for example, BFE + 1 foot or + 2 feet). Freeboard accounts for modeling uncertainty, debris and wave effects, future watershed development, and climate-driven rainfall trends. Freeboard also often improves flood insurance rating outcomes and is a common Community Rating System (CRS) credit activity. On the CFM exam and in practice, freeboard is a non-structural / building-standard tool that multiplies the effectiveness of elevation and dry floodproofing designs.

Choosing Among Strategies: A CFM Scenario

Consider a downtown corridor with mixed commercial buildings in Zone AE at 1–2 feet of estimated flood depth and a nearby residential neighborhood of slab-on-grade homes with three or more paid NFIP claims. For the commercial strip, engineered dry floodproofing or elevation of critical systems might be feasible if non-residential criteria are met. For the residential cluster, elevation or voluntary acquisition will usually dominate the long-term risk-reduction conversation—dry floodproofing of homes is not the NFIP compliance path. Structural channel improvements alone would not fix building vulnerability and could worsen downstream conditions without a watershed analysis. The CFM’s job is to match the measure to occupancy type, flood zone, depth/velocity, owner capacity, and available funding—not to treat “mitigation” as a single generic project type.

In short, structural measures alter flood behavior at a system scale; non-structural measures alter exposure and vulnerability at the parcel and community scale. Mastery of elevation, buyouts, relocation, dry/wet floodproofing limits, and freeboard is essential for both exam scenarios and local project counseling.