Site Management Feature: Floodplain Models And Facilities

The nation’s floodplains are expected to grow by 45% by the end of this century: Work to reduce risk from floods by reassessing the conditions around building sites.


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The nation’s floodplains are expected to grow by 45% by the end of this century: Work to reduce risk from floods by reassessing the conditions around building sites.
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Site Management Feature: Floodplain Models And Facilities

Work to reduce risk from floods by reassessing the conditions around building sites.

Site Management Feature: Floodplain Models And Facilities

By Carolyn Howard, P.E.
From the December 2020 Issue

It’s not just 2020. Extreme weather events have become more common in recent years. A 2018 report from Science Daily found that the number of floods and other hydrological events across the globe have doubled since 2004 and quadrupled since 1980. In the United States, a 2013 FEMA report on climate change and the National Flood Insurance Program concluded that the nation’s floodplains are expected to grow by 45% by the end of this century. And it’s not just coastal communities that are affected. Inland flooding events are on the rise as well.

floodplain
Smith Creek runs along the east wall of the Masonic Theater in Clifton Forge, VA. The creek’s west bank is the theater’s east wall. Flooding concerns required a new floodplain model to determine the proper elevation needed before renovations could begin. (Photo: Draper Aden Associates)

News reports from flooded communities often show harrowing images of the destruction. What is often overlooked in those stories, although not overlooked by facility management and building owners, is that facilities are on the front line of flooding. Regardless of the type of structure, facilities in many parts of the United States in close proximity of a water body face a heightened danger of flooding. Owners and managers, though, may not be fully aware of the threat or have the most accurate information to assess their risk.

It’s incumbent upon facility management and owners across the country to reassess floodplain models for their buildings. This action will help them better assess their risk and have a more accurate picture of that risk. Furthermore, for those who plan to renovate an existing facility or construct a new building in or near a mapped floodplain, taking a second look at approved flood models could save a lot of headaches and extra costs, and help to protect your investment in the future.

Why Floodplain Delineations Matter

Many facility manager and owners are familiar with the concept of the 100-year storm. Insurers often incorporate calculations related to the likelihood of a 100-year storm when underwriting a facility. However, what most people don’t understand that floodplain elevations are really based on the percent chance that a particular rainfall depth over a specific duration might happen. The Federal Emergency Management Agency (FEMA) regulatory floodplains are based on the rainfall intensity that has a one percent chance of occurring in a year, which is what many refer to as the 100-year storm event.

Understanding where your facility stands relative to the elevation at which a riverine floods during the one-percent-chance storm event and what precautions need to be put in place are crucial. This knowledge is necessary to protect building and infrastructure investments, secure insurance coverage, and meet various regulatory standards (depending on the type of facility).

As demonstrated by recent reports, and visible to most, a one-percent-chance flood event is increasing by the year. As the likelihood of extreme weather events grow more frequent, existing floodplain models and regulatory base flood elevations should be confirmed and run with updated rainfall intensities to ensure proper safeguards are in place to protect facilities now and into the future.

Floodplain Management 101

Floodplains and floodways are regulated by FEMA; states and local governments may impose stricter standards than FEMA. When considering improvements to existing facilities within floodplains, consult your locality’s floodplain manager for guidance on specific local provisions.

Floodplains are generally defined by FEMA as areas with a one-percent annual chance of flooding. Floodways are defined by FEMA as “the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.”

Development and redevelopment within a floodway are prohibited, except for certain situations and uses. Floodplain models are tools used to calculate the water surface elevations of a lake, creek, or river.

Federal Rate Insurance Maps (FIRM) define the areas and base flood elevation of the regulatory floodplains and floodways for each community and along each significant waterbody.

FEMA defines the base flood elevation 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.” A FIRM is supplemented by a Flood Insurance Study, which provides more detail on the basis and history of the hydrologic and hydraulic studies, and profiles of the studied riverines.

It is important to note that not all floodplain areas or areas prone to flooding may be designated as a floodplain and/or have an assigned base flood elevation. FIRMs often include areas designated as a Zone A, which are regulatory yet undefined floodplains. If your project lies within a Zone A floodplain or near a waterbody without a designation, you might want to consider, or may be required to for a construction project, completing a floodplain study to determine the base flood elevation, and accurately define the flood inundation area. A study typically includes the development of models to calculate the amount of upstream stormwater runoff (hydrologic model using XP-SWMM, HEC-HMS, or other computer models), and the velocity of and elevation and area needed to carry that amount of stormwater through a riverine section (hydraulic model using XP-SWMM, HEC-RAS and other computer models). The data from these models are used to develop a map on a topographic survey that depicts areas prone to flooding and the corresponding base flood elevations.

It’s easy to find whether your facility or property is within a floodplain or floodway. Visit FEMA’s Flood Map Service Center and enter the address or longitude/latitude coordinates of the property, select “Search”, and a map will appear. Typically, urban areas have updated regulatory maps based on the latest topographic and hydrologic information; however, in the developing suburban and rural areas, models have not been updated, and likely need revision based on historical knowledge and better data.

If you think the FIRM in your area is incorrect, the process to update your model involves contacting an engineer experienced in floodplain modeling. Your engineer will identify sources of historical rainfall and flood elevation data for model calibration, obtain the FEMA models, update the models with current topographic and land cover information, and, if a change to the FIRM is warranted, submitting to FEMA for a Letter of Map Change (LOMC).

Risk Management Implications

Creating accurate flood models or maps for facilities is about proper risk management and cost management. A key factor in the success of any business venture is the ability to prepare for the future and predict challenges so you can anticipate and address them. For facility management and owners throughout the country, climate change and severe rain events are challenges that need to be prioritized.

floodplain
The front view of the renovated Masonic Theater in Clifton Forge, VA. Located in the downtown area, the theater is adjacent to Smith Creek (seen on opposite page). (Photo: Jay Sink)

There are a few components to the risk management implications of flooding. For existing structures, it’s important for owners to protect their investment. In most of the U.S., flood elevations aren’t going to decrease over time. They’re going to continue rising as storm intensity increases.

For facilities that are under renovation or new buildings (either in the planning or construction stages), decisions need to be made about where to site a facility on a specific property. Decision-makers also must consider and balance the finished floor elevation and its relation to the regulatory floodplain elevation with accessibility requirements. Many states and local governments require the finished floor elevations of new buildings to be one to three feet above the regulatory floodplain elevation. If the project is only required to be at or one foot above the base flood elevation, you might want to consider raising the finished floor elevation even more.

The decision comes down to your risk tolerance today and into the future. Can expensive equipment or offices be located at the base elevation, or should they be raised a few feet higher? Making an informed decision is much easier with an updated and current regulatory floodplain elevation. Developing an accurate flood model early in the process is vital to proper planning. Don’t just rely on the approved regulatory flood model. It’s possible that could be out of date, and then you will be making assumptions based on old or inaccurate data.

For all types of facilities—existing, under renovation, and under construction, insurance plays a major role in flood plain models. As an owner or manager, you’ll want to ensure your facility has the proper level of insurance. An accurate flood elevation is crucial in making that determination.

Ultimately, facility managers and owners would be wise to review existing FEMA floodplain maps and elevations that impact their buildings. If based on historical knowledge and data the maps appear incorrect, explore the potential to update the models and maps to more accurately depict potential flooding of the adjacent waterway.

FEMA Compliance Considerations

In addition to helping manage risk and preparing for proper flood proofing measures, updated flood models can yield cost savings for new facilities or those planning renovations. As described above, approved regulatory flood models are not always accurate. In many cases, they may underestimate the potential flood elevation, but in other instances older models have been shown to overestimate the threat of flooding, which could result in significant costs placed on facility owners.

floodplain
Seen here is a sample floodplain map from FEMA indicating floodways, floodplains, and base flood elevations in different areas. The map is of downtown Clifton Forge, following the Letter of Map Revision completed by Draper Aden Associates. The red pin is the Masonic Theater. Not shown here is the map legend that accompanies each floodplain map from FEMA. (Photo: provided by Draper Aden Associates)

Again, the best advice is to always check and verify the existing flood model for an area where a facility sits. An example from a Virginia town is illustrative. A private foundation wanted to renovate and transform the historic Masonic Theater in Clifton Forge as part of a broader revitalization effort in the downtown area. In many communities, especially historic areas, a creek or river runs along the town, and sometimes the creek runs underneath part of the downtown area, as in the case of Clifton Forge. The west bank of Smith Creek is the east wall of the Masonic Theater.

The regulatory Federal Rate Insurance Map (FIRM) showed the creek overtopping Main Street in the downtown area during flooding events and was based on a 1978 model that had not been updated. Anecdotally, no one could ever recall the creek overtopping the road, even during significant rain events. This led to further investigations into the accuracy of the existing model. A new model that incorporated updated topographic data for the area demonstrated that the actual flood elevation should be up to 5.5 feet lower upstream of Main Street than what was in the FIRM.

The foundation, which owns and manages the theater, presented this new information and model to FEMA. After a thorough review, FEMA accepted this updated model and adopted it as the approved regulatory flood model for the area. With the newly lowered flood elevation, the foundation saved significant costs on flood prevention and proofing of five feet along the building face. These cost savings were crucial to financing the project and bringing it to fruition.

Whether revised floodplain models lower or raise the flood elevation in an area, it’s prudent to check your local map and update existing floodplain maps, as needed, to better inform your decisions. Facilities of any size and type could benefit from cost savings associated with lower flood elevations. Likewise, facility owners and managers that update flood models for their buildings will have the most accurate data to use when assessing threats to their investment and planning for proper risk management needs.

floodplainsHoward, P.E. is a regional manager for site development and infrastructure with Draper Aden Associates, an engineering and environmental services firm based in the Mid-Atlantic. She has over 28 years of experience working on floodplain modeling, stormwater management and compliance, and site development. She oversees the firm’s floodplain modeling projects and has been active across the country in her career, including in Virginia, Illinois, and Michigan. 

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