Building on a Floodplain: Steps to Mitigation 

Floodplains, Floodways, and Other Natural Resources

Floodplains and floodways are just some of the waters that may be present on a project site. These areas have fertile soil, and their waters have unique characteristics, purposes, and potential risks. Each water feature may be subject to different regulations and must be carefully identified. Should the feature be protected, proper mitigation and development steps must be taken to ensure a responsible and safe outcome.

Floodplains

Floodplains are regions of land adjacent to a stream or river. These land areas are usually flat and can become covered with water. Floodplains can be home to marshes, vegetated lowlands, and various species. These waters help control the downstream flow following a flooding event, ensuring the central water systems are not overwhelmed by the abrupt surplus of water.

These areas change over time due to erosions, but their most significant threat is development. Floodplains are altered intentionally to control a channel's flow or unintentionally via adjacent development without proper mitigation.

Floodways

The floodway is a floodplain's main channel. These channels can be ephemeral or seasonal, meaning they appear dry or not at all during portions of the year. A seemingly flat, open space may experience rapid inundation of water during a flash flood or because of melting snowfall. Due to the nature of these ephemeral streams and waterways, these channels can be challenging to identify on a project site. Misidentification of these features during development can lead to drastic effects on the floodplain.

Flood Fringe

The flood fringe is the region from the outer bank to a river valley's bluff lines that can vary in width. Some floodplains have a steep gradient from the floodway to the bluffs, while others yield a narrower width. The flood fringe's size correlates to the amount of additional stormwater the floodplain can hold. Development alterations to the gradient or width of the fringe can prove dangerous to those who rely on the floodplain.

Adverse Impacts of Floodplain Development

Altering a floodplain can significantly negatively impact the surrounding ecosystems, watershed quality, and community safety. Many species of plants and animals rely on floodplains for shelter, food, and water. Additionally, many communities rely on their surrounding floodplain for flood protection, drinking water, and activities such as fishing. Intentionally changing the hydrologic nature of the floodplain can cut water systems off from their main supply, starve irrigation sources, and drain areas of the water and nutrients needed to sustain life.

These features are natural hazards due to their flood risk. Developing a floodplain that alters the structure of the banks and flow of the stream can result in severe flooding events that cause property damage and potential loss of life. These flooding events can wreak havoc on cities, agriculture, and other surrounding life.

Additionally, sediment, debris, and other pollutants from development can endanger the surrounding life forms and impact the water quality of the floodplain. During a flooding event, these additional pollutants can end up in drinking water, agricultural regions, and public waterways.

Who Regulates Floodplain Mitigation Measures

U.S. Army Corps of Engineers

The United States Army Corps of Engineers (USACE) plays a significant role in protecting our nation's waters. In addition to providing Section 401 Clean Water Act Permits, the USACE oversees the floodplain management program. This power is provided via the Flood Control Act of 1960. This program aims to minimize flood damage while maintaining the integrity of the floodplain. The program includes flood warnings, floodproofing (mitigation efforts), and floodplain delineation.

FEMA

The Federal Emergency Management Agency (FEMA) oversees community-wide floodplain management efforts. The National Flood Insurance Program (NFIP) is one of the FEMA-based efforts that involves government and community outreach efforts in mitigation projects for a flood zone. These zones exist within the floodplain and can change over time. FEMA outlines standards for minimizing flood risk and changes to the floodplain and can provide financial support for these efforts via programs such as the Hazard Mitigation Assistance Grant Program (HMGP).

Additionally, the agency provides extensive data for state and local governments regarding present floodplains and their potential impacts. FEMA provides Flood Insurance Rate Maps that outline the Special Flood Hazard Areas (SFHAs), or areas prone to mudslides or flooding events and the Base Flood Elevations for a site. FEMA defines Base Flood Elevations as "The elevation of surface water resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year." This information can help environmental experts advise land developers on the best steps for a project being developed near these areas and help local governments create ordinances that support the management of these regions.

Types of Floodplain Management

Mitigation actions to ensure flood control and a safe environment do not depend solely on the steps of homeowners. Developers must take flood hazard mitigation planning seriously when developing in a way that can impact floodplains.

After identifying a waterway, the next steps must be drafted to follow the required actions for the specific land use. These steps can be required at the state, local, and federal levels. Once a mitigation plan has been recommended or outlined, the land developer must begin breaking ground on these requirements.

When developing in a way that may impact a floodplain, land developers may have to build structures such as levees or floodwalls to ensure their actions do not put a community or environment at risk. Levees and floodwalls act as structural mitigation efforts that hold water flood waters. These structures may naturally be required over time due to erosion or as a result of human activity.

These requirements come from national standards as interpreted and implemented by local officials. Many states offer further regulations for mitigation efforts when developing a floodplain. Some states offer a mitigation grant program to support FEMA's funding via HMA grants to aid in flood preparedness.

State or local governments complete many mitigation efforts. These efforts involve non-structural actions such as implementing relocations, buyouts of real estate, and changing building codes. As the risk of flooding in some areas is increasing with climate change, having a sound management plan is key to limiting flood losses.

Additional mitigation efforts may be required if development impacts a protected species or other natural resources. The respective organizations oversee those mitigation efforts. A comprehensive assessment of all the environmental risks requiring permitting or mitigation efforts is completed during the land development's environmental due diligence process.

How to Detect Protected Water Resources

Environmental Consultants

Environmental consultants are professionals with extensive training and expertise in natural resources and the regulations that protect them. These professionals survey a project site to look for protected environmental features, such as a floodplain or floodway. Consultants write comprehensive reports on their findings of the site, the regulations that apply to that site, and the next steps for mitigation. These consultants work in-house or, more commonly, through a consulting firm where they charge an hourly rate. These reports, called a Desktop Report, Feasibility Study, or Critical Issues Analysis (CIA), can take weeks to months to complete and cost upwards of $5,000.

Transect 

Transect is a software platform that automates the permitting process, providing due diligence in minutes- not weeks or months. The platform contains all federal, state, and local regulations regarding the nation's protected natural resources. After outlining their potential sites, Transect provides users with site-specific data regarding the environmental red flags, regulatory requirements, and next steps for a project. This report will include all the data found in a traditional CIA.

The software has an extensive waters section, mapping all known and suspected floodplains, wetlands, and other water features on a project site. The software's Ghost Waters tool allows seasonal or ephemeral streams and waters, such as some floodways, to be seen on a site. This floodplain mapping feature denotes the risks shown within the reports section. Seeing flood-prone areas on a project site and how impacting floodplain functions can derail a project helps simplify decision-making. Though property owners can access other floodplain mapping tools, they will need more environmental context and next steps for the specific project land use. Whether developing where there are existing buildings or starting new construction, developers should always be aware of their project risks and requirements.