Preparing and protecting facilities from flood risks

by arslan_ahmed | February 9, 2024 3:11 pm

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By Kimberly Kilroy, PE, RRC, CDT and Peter Spanos, PE, CFM, LEED AP

As sea level continues to rise and weather patterns become more severe and less predictable, flood mitigation is becoming a critical part of new construction design and existing building renovations and maintenance. Whether the motivation is building code compliance, building protection and performance, or insurance premium reductions, it is necessary for facility owners and managers to address flood risk and determine how to protect their facilities.

 Sources of flooding

Typically, the first step in the flood mitigation process is assessing the flood risk for a given site. Sea level rise presents a potential source of flooding for coastal areas (i.e. coastal flood/storm surge). Flooding of streams and rivers presents a potential source of flooding for those in the vicinity of such waters 
(i.e. fluvial floods). However, even beyond these “obvious” areas of potential flooding, severe storm events catalyzed by climate change with increased rainfall rates and/or durations can increase the risk of flooding for many areas, even unexpected ones.

To assist in identifying areas susceptible to flooding, the Federal Emergency Management Agency (FEMA) developed flood maps in 1968 after Congress passed the National Flood Insurance Act. The maps delineate zones (listed below) to represent flood potential and identify areas susceptible to flooding in a 100-year storm (1 percent annual chance or higher) as “special hazard flood areas.”

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Zones B, C, and X are areas of low to moderate flood risk.
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Zone A has a high risk of flooding.
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Zone V has a high risk of flooding at a coastal area where storm waves present an additional hazard concern (i.e. velocity hazard/wave action).
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Zones A and V are supplemented with a secondary letter in some locations (where applicable) to provide more detailed information about the Zone.

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Zone AE indicates a Base Flood Elevation has been assigned to the area.

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Zone AH indicates flood depths of 0.3 to 0.9 m (1 to 3 ft) via sheet flow are anticipated.

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Zone AR indicates an area previously identified as high flood risk, which has since been decertified as maps were revised.

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Zone A99 indicates an area to be protected by a federal flood protection system that is in construction, such as a levee or a berm.

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Zone VE indicates a base flood elevation has been established for this Zone V area.

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Zone D identifies unstudied areas where flood hazards have not yet been identified.

These maps are publicly available and can be accessed virtually through an interactive tool on the FEMA website.¹

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While FEMA maps can identify areas of potential concern, classification as a low or moderate FEMA flood risk zone is not the end of the evaluation process. It is still important to consider potential flooding associated with thunderstorms, hurricanes in coastal areas, nor’easters in the northeast region, tropical storms, and frontal storm events. Precipitation from these storms may exceed the capacity of the existing stormwater drainage system and result in flooding.

Undersized or inadequately maintained stormwater drainage can result in standing water and sheet flow flooding at depths of 0.3 to 0.9 m (1 to 3 ft), as rainfall flows across sloped land (i.e. pluvial flood). As urban and suburban expansion persists, the growing network of roads and impermeable surfaces, replacing natural or landscaped spaces, places additional strain on stormwater drainage systems.

The National Oceanic and Atmospheric Administration (NOAA) provides National Hurricane Center Storm Surge Risk Maps to project the potential for storm-related flooding of less than 0.9 m (3 ft), greater than 0.9 m (3 ft), greater than 1.8 m (6 ft), or greater than 2.7 m (9 ft) for given coastal areas. In addition, various cities and towns provide interactive mapping through their websites to simulate flooding from storms and/or sea level rise to assist in planning for the current and potential future flood risk exposure. For example, Boston Water and Sewer Commission (BWSC) provides an interactive model where the user can choose various rainfall and sea level rise inputs.²

Although these maps offer a useful initial reference, designers also need to consider the level of detail and the data sources employed in their creation when using the results for a specific location. For example, if a map has the flooding potential broken out by town or county, it may provide a skewed perception of risk at or near town lines where adjacent properties with similar risk appear to have more varied flooding potential. Depending on whether topography (and its level of detail) is incorporated into the mapping, properties at the top and bottom of a hill may appear to have the same flood potential. However, water will flow downhill and infill low points until it reaches an equilibrium point, meaning the property at the bottom of the hill may be underwater before the property at a higher elevation has cause for concern.

Flood emergency response plan

If flooding is determined to be a concern, the project team should assist the owner in developing a flood emergency response plan (FERP). This plan should include:

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Steps that need to be taken to provide protection during a potential flood event.
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Implementing a plan for monitoring weather well in advance of potential flood event.
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Designation of who will have the authority to decide whether or not to mobilize the selected flood mitigation system for a given event.

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  Determination of when the decision needs to be made based on system mobilization time(s). For example, sandbag systems, water-filled barrier systems, and anchored panel systems will all take various times to deploy based on length required, availability of water/material, storage distance and/or availability of workers.

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How to maintain access to the building (if needed) while the flood mitigation system is in place. This is typically accomplished by incorporating an access door in the system or providing bridging of the system.
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Steps that should be taken post-event as part of the recovery process.
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Procedures for familiarizing personnel with the system installation methods and regularly scheduled system condition checks, maintenance, and installation dry-runs.

Distributing this plan to the appropriate team and designating a responsible individual for each task are crucial steps to prevent any oversight in maintenance.

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Flood mitigation design

Design parameters for flood mitigation measures will depend on whether the site is considered to be within a “flood hazard area” from a regulatory standpoint, insurance requirements, code requirements, local regulations, the risk category of the facility (I through IV), and the level of flood protection desired. ASCE 24 and FEMA technical bulletins provide flood design parameter guidance based on the Flood Design Class, elevations of the lowest floor and lower limits of horizontal structural members, and the FEMA Zone the facility falls within. This could result in designing protection to the design flood elevation (DFE), to 0.3 m (1 ft) above the base flood elevation (BFE), to 0.6 m (2 ft) above the BFE, or to the 500-year (0.2 percent annual chance) storm flood elevation.

Once the level of flood protection to be provided is established, vulnerable areas of the facility’s building envelope, such as windows, doors, louvers, wall penetrations, and sealant joints within this parameter, should be documented. In addition to flood depths, owners and designers must review the potential for stormwater and wind-driven rain that may impact the building envelope. For example, moisture could infiltrate the building walls through failed sealant joints. Equipment located on grade and/or in lower building spaces, and critical interior equipment/spaces should also be documented. This will allow for floodproofing to be designed to address the facility-specific vulnerabilities, both in the building facade and within the building interior. Part of the flood mitigation plan may include relocating critical mechanical and electrical equipment to upper floors of the building to reduce the potential operational impact or downtime during or after a storm event.

The type of system(s) selected for a site as part of this plan will depend on the frequency and extent of projected flooding, the facility’s vulnerabilities, the amount of notice anticipated prior to a flood event to allow mobilization of the system(s), available storage on site or near the facility for temporary system(s), and availability of personnel to mobilize the system(s) prior to an event. The two main protection options are wet floodproofing and dry floodproofing. As the names suggest, wet floodproofing involves building with flood-damage-resistant materials and construction methods able to accommodate flood waters and intentionally get wet, whereas dry floodproofing methods are intended to keep the site dry during flood events.

Wet floodproofing

Wet floodproofing is most suitable for new construction or major facility renovations where it is acceptable to allow area to flood up to the design flood elevation, given its use of flood-damage-resistant materials to accommodate flood waters that are not typical in building construction.

Flood-damage-resistant flooring may include concrete flooring with tile or vinyl-finished flooring glass block. Wood flooring or carpet would not be suitable given the potential for moisture damage during flooding. Flood-damage-resistant walls may include concrete, masonry, or appropriate lumber materials. Paper-faced gypsum board, fiberglass insulation, and other finishes susceptible to water damage should not be incorporated into the finished wall system. Rigid closed-cell insulation could be considered at/below the flood level. Metal doors and cabinets could be considered in lieu of wood doors and cabinets when selecting interior finishes.

Wet floodproofing also requires utilities, equipment, and other critical items within the building be raised above the selected elevation to be protected against flooding. To limit hydrostatic pressure on the building’s structure, wet floodproofing incorporates openings to allow floodwaters to enter/exit the wet floodproofed space. However, the anticipated hydrostatic load should be considered when designing structural elements or assessing the suitability of an existing building for wet floodproofing during renovations.

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Dry floodproofing

Dry floodproofing systems are intended to keep water from entering and can be temporarily deployed or permanent. They can be part of the original site design or incorporated into flood management plans for existing facilities. Sandbags have traditionally been used as a temporary barrier system in advance of storms to limit moisture infiltration. Temporary barrier systems can be flexible or rigid.

Flexible systems include water-filled flexible tubes in multiple configurations, self-inflating barriers, and barriers that rely on flood water pressure to open the barrier system completely and hold it in place. These products have multiple sizes and configurations based on the level of flood protection anticipated to be required. However, these systems can be susceptible to puncture and other damage if large debris is present in flood waters.

Rigid barrier systems include temporary modular walls, panels, or blocks joined together with gasketed connections to limit seepage. They can be installed in multiple configurations and come in multiple sizes to provide different levels of flood protection. Permanent flood walls with openings for access can also be considered. Access openings in a permanent wall would then be protected through installation of a temporary system. Flood glazings, which are reinforced glass elements, can be used in flood walls to maintain sightlines and/or provide an aesthetically pleasing permanent flood wall. However, these glazings are anticipated to require more regular cleaning and maintenance than other permanent wall materials, such as concrete. The advantages and disadvantages of each material should be considered during the design phase.

Many different products are available for the various system types discussed herein. To assist building owners with the selection of reputable flood protection products, Factory Mutual Global (FM Global) has developed a standard for testing flood protection systems (FM Approvals 2510, ANSI 2510) and allows products which have been tested and passed to indicate their FM approval on marketing materials. However, some products with this seal of approval may only be tested and approved in certain configurations.

In addition to dry floodproofing products, sump pumps and other accessory products may be incorporated into the flood plan to reduce stormwater within the protected areas and seepage beneath or through protection systems.

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Conclusion

It is critical to work with a project team focused on the development and implementation of a facility-specific FERP. This plan should include selection and maintenance of appropriate product(s) that work together as a flood mitigation system to protect against the agreed-upon flood protection design parameters. It is equally important to maintain this plan and periodically reassess the parameters and systems as the climate continues 
to change.

 Notes

¹ See the maps at msc.fema.gov/portal/home[8].

² Visit the Boston Water and Sewer Commission (BWSC) storm app at www.bwscstormviewer.com/stormapp[9].

Authors

Kimberly A. Kilroy, PE, RRC, CDT is a project manager at Gale Associates, Inc. She performs roof, wall, and window evaluations, analysis, and design, including the development of technical specifications, drawings, and details.

Peter Spanos PE, CFM, LEED AP is a senior project manager at Gale Associates, Inc. He is responsible for project engineering related to site design and permitting, with a focus on flood mitigation.

Source URL: https://www.constructionspecifier.com/preparing-and-protecting-facilities-from-flood-risks/

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