Question Of The Week: Have You Addressed Flood Mitigation Strategies?

flood mitigation

As stated by the U.S. Geological Survey this week: “Persistent precipitation accompanied by strong winds and tornadoes in some areas blanketed two large bands of area from Texas to Ohio and Mississippi to North Carolina. As the resulting large-scale flash flooding begins to recede, the nation heads into 2016 with major floods predicted on the Mississippi River that are approaching or even exceeding 2011 floods in some reaches.” Have you been impacted, and have flood mitigation measures put in place helped to protect your facilities?

The article below from experts at engineering firm Simpson Gumpertz & Heger Inc., delves into the issue of flood mitigation, options for facilities, and financing tools that may be available.

By Filippo Masetti, P.E. and Milan Vatovec, P.E., Ph.D., LEED AP

Natural disasters can cause significant damage to the built environment and infrastructure, and could result in prolonged business and “life” interruptions. “Superstorm” Sandy in 2012 showed that the northeast, and in particular the New York City metropolitan area, is highly vulnerable to flood- and wind-related damage and its consequences. To successfully tackle the inevitable future events, building owners and managers need to be prepared (and some have already started) by introducing resiliency and redundancy to their structures, systems, and components. To stay ahead of the game, federal and private agencies (e.g., FEMA, NFIP, and others) are also revising their assessment of the flood risk for certain communities most likely to be affected in the future.

This article aims to assist building owners and managers in their preparation for future extreme events. Several publications on flood mitigation techniques are currently available at both national and local levels. However, these publications address a wide-range audience and the outlined techniques typically need to be adapted to building-specific needs. This article provides a review of the state-of-the-art methodology related to flood-risk assessment and the associated mitigation techniques (and products), and is particularly aimed at essential facilities, mid-rise, and high-rise buildings.

A step-by-step approach will outline all that decision makers should consider in evaluating their flood mitigation options. In particular, this article will discuss the most recent information on the design flood-elevation maps and FEMA technical articles, the pros and cons of main flood mitigation techniques, and will summarize the authors’ research on specific flood mitigation products that are now populating the market.

Finally, by including a review of current tax relief legislations aimed at properties laying in flood-prone areas, this article will provide guidance to building owners and managers for selecting holistic flood mitigation options that best suit the site-specific building needs.

Step 1: Assessing Flood Risk

When evaluating flood risk of a structure, one must first understand the inherent vulnerabilities of the site upon which the structure is built. For example, coastal communities in proximity of the coastline may be subjected to both tidal surge and battering wave-action, while inland areas may be subjected to groundwater surges. After Hurricane Sandy occurred in 2012, FEMA posted the Advisory Base Flood Elevations (ABFEs) to determine the expected elevation of water surge for certain flood events (which typically include the 100-year and the 500-year return events). FEMA also evaluated (and provides information for) the extent of the areas affected by the battering-wave action, and has posted maps showing the extent of the Limit of Moderate Wave Action (LiMWA), defined as the inland limit of the area affected by waves greater than 1.5 feet. As a result, if the exact location and elevation of a particular building and its components is established (e.g., through surveying or review of existing records), these maps can be very helpful to owners interested in evaluating risks and mitigation approaches.

It is important to note, however, that the ABFEs are not intended to support regulatory flood-hazard zone designations or insurance ratings. Over the next one to two years, FEMA will complete updated Flood Insurance Rate Maps (FIRMs) for the communities for which the ABFEs have been produced. To date, preliminary FIRM maps are already available for some areas (e.g., the majority of the coastal areas around Manhattan), but the review process is not yet complete. Until detailed analyses are completed, the ABFEs and the preliminary FIRM maps are considered the best available data and the best available tool to assess the flood risk in a determined area.

Unfortunately, flood hazard prediction is an evolving science, as our understanding of issues like storm prediction and climate change continues to evolve. In addition, site-specific phenomena need to be included in the risk analysis and considerations. For example, the effects of above-ground water surges on the elevation of groundwater (below-grade water), which can affect sewer and other below-grade systems, depend on the site-specific soil characteristics and cannot be evaluated solely through usage of the FIRM maps (new or old). As a result of all this, owners should understand that there are inherent uncertainties in current maps and science. In some cases, it may be desirable or necessary to engage consultants to prepare a site-specific flood hazard assessment.

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Figure 1: FEMA Advisory Base Elevation Map of downtown Manhattan
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Figure 2: Pre-Flood Mitigation

Step 2: Reviewing Flood-Mitigation Techniques

Once the risk of water surge and wave-battery action is evaluated, building owners and managers should understand the available flood mitigation options that have been developed and successfully implemented in projects of similar magnitude and complexity.

To date, FEMA has released several publications that describe methods for floodproofing structures. The most common approaches include:

  • building relocation
  • building elevation
  • wet floodproofing
  • dry floodproofing
  • subslab drainage system construction
  • levees
  • flood walls

Building relocation and building elevation approaches are typically cost-prohibitive for the types of buildings this article focuses on, and are not discussed in this work. The main characteristics of the remaining flood-mitigation methods are briefly summarized below.

Wet Floodproofing – General Approach

Wet floodproofing is a method that allows floodwaters to enter the building. The building interior, however, is partially floodproofed around critical areas or around equipment, which minimizes damage to specific elements.

A variant of the wet floodproofing method includes allowing water to enter the basement or sub-basement at large, while keeping it out of specific, “dry” areas (or rooms) within the basement. This approach typically requires both installation of floodproof doors at rooms containing equipment that needs to be protected, as well as, in some cases, structural strengthening of the walls separating the “dry” and “wet” areas (to allow them to withstand the hydrostatic pressure). Wet floodproofing can also be enhanced by providing redundancy to critical pieces of equipment (e.g., by storing spare parts on site), in case some specific elements cease to operate due to sustained exposure.

The main advantage of the wet floodproofing method is that it typically costs less than other floodproofing methods.

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Figure 3: Wet Floodproofing

Wet Floodproofing – Elevating Critical Equipment

Essential equipment includes electrical, mechanical, fuel, and life safety systems. To arrive at an effective equipment relocation methodology, a holistic review of critical equipment, critical systems, and critical functions is required to determine which pieces of equipment need to be relocated above the design flood elevation (DFE). Typical approaches for elevating utilities, controls, and equipment in existing buildings include:

  • Installing platforms on the floor to elevate the equipment in place.
  • Relocating systems from below grade or the first floor to a higher floor (or even the rooftop).
  • Relocating systems to a higher elevation in a different building.

In addition, consideration should be given to emergency power availability. When planning the needed emergency supply for a building, it is important to consider the following:

  • How long may the building be without utility power.
  • Which equipment is critical to functionality, and needs to be connected to the emergency power supply.
  • Whether equipment will be damaged or lost during rapid, unexpected power losses, or during power restoration (surges, spikes).
  • How quickly will the backup power be needed (seconds, minutes, hours, etc.).

Dry Floodproofing

In the dry-floodproofing method, floodwaters are not allowed to enter the building. The exterior building walls are floodproofed, above and below the grade level. Some examples of dry floodproofing include: sealing all openings in the building’s exterior walls (such as windows, doors, utility penetrations, and vents); applying negative-side waterproofing to the foundation walls; and/or installing interior drainage collection systems to collect leakage that passes the primary barrier (the exterior walls). The main advantage of this approach is that it generally retains the building in its present state, without requiring significant changes in its appearance or functionality.

Figure 4: Dry Floodproofing

On the other side, some of the disadvantages of this approach include the following:

  • This approach can be very challenging and expensive.
  • The exterior wall of the building acts as a structural flood wall. Typically, exterior cladding systems are not designed to withstand loads induced by the hydrostatic pressure, and a certain level of structural strengthening would likely be required.
  • In buildings with a basement, cracks and openings in the foundation walls and slabs below the design flood elevation (DFE), typically constructed of stone, masonry, and/or concrete, would require floodproofing (positive- or negative-side waterproofing). This approach requires inspection and/or testing of the perimeter exterior wall and below-grade concrete surfaces to identify and seal all potential points of water entry. To this end, several approaches, including grout or chemical injections, are commercially available today, but none are likely to fully and permanently seal-off the building perimeter without multiple application iterations and troubleshooting.
  • During flood events, the community storm and sewer drainage system will likely be surcharged and check-valve devices will be required to prevent the community drainage system from discharging into the building’s drainage system.

Subslab Drainage System

A subslab drainage system is typically used to dewater below the ground or basement slab level, and to discharge seepage and collected water away from a building’s foundation system. It may not eliminate lateral water pressures on the building walls, but it will significantly reduce the uplift water pressures below a basement slab. The installation of this system would require extensive excavation around the building, which is likely impractical in highly developed urban areas (such as Manhattan).

Levees and Embankments

Levees are embankments built of compacted soil or other materials that keep floodwater away from a building. The levees would be built around the building and/or on the sidewalks to prevent above-ground water from accessing the building. For this reason, the installation of this system would require loss of use of valuable real estate space, which is often impractical (or the space does not exist) in highly developed urban areas.

Flood Walls

Flood walls are walls constructed to keep floodwater away from the buildings. Some advantages of using flood walls include the following:

  • They keep floodwater away from isolated openings in the building’s exterior walls, without requiring waterproofing of the openings.
  • No significant changes to the building structure are required.
  • They are designed to be strong enough to resist the lateral hydrostatic forces caused by the floodwater and wave action.
  • They can be temporary (they are installed only before flooding occurs), and generally retain the building in its present state without requiring significant permanent changes in its appearance.

Some disadvantages of using flood walls include the following:

  • They may restrict access to and egress from the building (requiring construction of additional means of access and egress to meet local building-code requirements).
  • Drainage for the space between the flood wall and the building must be provided to collect and remove rainfall and runoff in that area.
  • Without appropriate waterproofing detailing below the flood wall, seepage of floodwater will occur below the flood wall, and water will find an alternate path into the building.
  • Certain types of flood walls require excavation to be constructed, and are expensive.
  • Check-valve devices will still be required to prevent the community drainage system from discharging into the building’s drainage system.

After Hurricane Sandy, several off-the-shelf, flood mitigation barrier products came into high demand. These can be categorized as “permanent” and “temporary” flood barriers. Permanent products typically require installation of a permanent supporting infrastructure, but the barrier itself requires installation only prior to an anticipated flood event. The supporting infrastructure typically consists of a continuous concrete footing installed such that the top of the footing is at the same elevation as the top of the sidewalk (or grade). Metal embed plates are permanently installed at the top of the footing. Vertical supports are then manually connected to the embedded plates, and aluminum planks are installed between the vertical supports to create the flood barrier system.

On the other side, temporary flood barriers are portable fence systems that require manual setup around the perimeter of the building prior to an anticipated flood event. The fence can be set up with no structural changes to the building or sidewalk, and the water pressure of the flood provides the panel stability against overturning. Therefore, no footing is required for this system. Based on the author’s experience, temporary systems are typically preferred by building owners as they are less costly.

Step 3: Understanding Financial Implications Associated with Flood-Mitigation — Tax-Relief Programs

Subsequent to evaluation of benefits (and costs) of the available flood mitigation techniques, building owners and managers should consider other financial implications associated with the implementation of chosen techniques. In particular, building owners and managers are interested in public funding (direct or in form of tax-relieves) options.

Several bills have been proposed in New York City, New York State, and New Jersey, and at the federal level (FEMA). These bills have not been passed yet, but their review is a useful glimpse into the upcoming state and federal legislations. It appears that these bills mainly focus on homeowners, small dwellings, and small building; the authors are currently not aware of any similar effort being under way also for mid- and high-rise buildings. Nevertheless, below we summarize the results of our research to date, as applicable to New York:

New York State Bill S1787-2013. The bill creates a tax credit for homeowners and business owners who construct a permeable surface (such as paved driveways and parking lots, which allow for the movement of water and air around the materials), during the taxable year, as part of or near their real property.

New York State Bill S1787-2013. This bill enacts the “sewage flooding prevention act”; it authorizes certain homeowners and not-for-profit agencies in cities having a population of one million or more to receive a credit against real property taxes up to two thousand dollars for the cost of installing sewer improvement check-valve devices in certain dwellings and buildings; and directs the department of environmental protection of the city of New York to promulgate any necessary rules and regulations.

House of Representatives Bill 1268: Flood Mitigation Expense Relief Act of 2013. This bill provides relief to homeowners and small businesses from scheduled flood insurance rate increases by providing tax credits and grant funding for flood mitigation expenses. Tax credits require incurring in qualified flood mitigation expenses and being policyholder of a qualified flood insurance program. Grant funding (for a total of $200 million nationally) would cover flood mitigation activities and voluntary property buy-outs, and it would assign additional funding to recipients who undergo revised flood maps and either fall below the base flood elevation or are mapped into a higher risk flood zone.

FEMA’s Hazard Mitigation Assistance (HMA) grant programs provide funding for eligible mitigation activities that reduce disaster losses and protect life and property from future disaster damages. Currently, FEMA administers the following HMA grant programs:

  • Hazard Mitigation Grant Program (HMGP). HMGP assists in implementing 
long-term hazard mitigation measures following Presidential disaster declarations.
  • Pre-Disaster Mitigation (PDM). PDM provides funds on an annual basis for hazard mitigation planning and the implementation of mitigation projects prior to a disaster. The goal of the PDM program is to reduce overall risk to the population and structures, while at the same time, also reducing reliance on federal funding from actual disaster declarations.
  • Flood Mitigation Assistance (FMA). FMA provides funds on an annual basis so that measures can be taken to reduce or eliminate risk of flood damage to buildings insured under the National Flood Insurance Program (NFIP).
Step 4: Selecting the Optimal (Holistic) Solution

Very often, financially or operationally driven considerations may ultimately be the deciding factor on what risks get to be addressed, and when. In addition, when building owners own and manage more than one building (even a campus), they need to look at their risk on a “portfolio” basis; vulnerability of one building could be made up in the redundancy or reserve of another building. Consideration of the “big picture” is paramount, and once all cards are on the table, building owners and managers need to decide which flood mitigation option, if any, meets their needs.

This is not a trivial task, as each flood mitigation technique can be riddled with potential drawbacks. Wet floodproofing, if chosen as a standalone option, would keep water out of specific “dry” areas (or rooms), but significant amounts of water would be allowed to enter the building at large, and could still cause extensive damage to the areas left unprotected. Dry-floodproofing can be cost prohibitive due to required excavation and excessive investigation and mitigation of potential leakage. Flood walls alone do not stop below-ground water seeping through the foundation system walls and slabs.

Based on the authors’ experience, almost every building requires a holistic solution, an approach that embraces components of different techniques: for example, wet floodproofing associated with elevation and/or relocation of selected equipment is combined with partial 
dry-floodproofing of the most deficient areas of the foundation system and installation of flood walls.

A combined approach is typically the most effective for several reasons. First, the combination of different techniques allows focusing on the most critical areas, so that the implementation of resources can be prioritized. This allows more flexibility, less disruption of business operations and everyday activities of the occupants, and more cost-savings when compared to the implementation of one, standalone flood-mitigation option. Secondly, this strategy allows for redundancy, in case one system performs poorly due to installation or other unforeseen issues. Lastly, this approach allows meeting multiple requirements for future government or local funding. Finding the optimal balance between wet-floodproofing, 
dry-floodproofing, and flood walls is typically the result of a collaborative effort between owners, managers, and consultants.

1) FEMA P-55. 2011. Coastal Construction Manual: Principles and Practices of Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in Coastal Areas (4th ed.). Federal Emergency Management Agency. Washington, DC, Aug. 2011.
2) FEMA P-85. 2009. Protecting Manufactured Homes from Floods and Other Hazards: A Multi-Hazard Foundation and Installation Guide (2nd ed.). Federal Emergency Management Agency. Washington, DC, Nov. 2009.
3) FEMA 102. 1986. Floodproofing Non-Residential Structures. Federal Emergency Management Agency. Washington, DC, May 1986.
4) FEMA. 2013. Hurricane Sandy Recovery Advisory RA4. Reducing Interruptions to Mid- and High-Rise Buildings During Floods. Federal Emergency Management Agency. Washington, DC, March 2013.
5) FEMA. 2013. Hurricane Sandy Recovery Advisory RA2. Reducing Flood Effects in Critical Facilities. Federal Emergency Management Agency. Washington, DC, April 2013.
6) ASCE/SEI 24. 2005. Flood Resistant Design and Construction. American Society of Civil Engineers, Reston, VA, 2006.

Filippo_MasettiMasetti, PE joined Simpson Gumpertz & Heger Inc. (SGH) in 2005. He has been involved in design, investigation, strengthening, and rehabilitation projects involving concrete, steel, masonry, fiber-reinforced-polymer, and wood structures. He has worked on load-testing of existing structures since he completed his master-thesis “Structural Implications of Field Load Testing Using Patch-Loads.” He has also conducted several flood-mitigation studies of buildings in the New York City metropolitan area.Milan_Vatovec

Vatovec, PE, PhD, LEED AP, is a senior principal at Simpson Gumpertz & Heger Inc. (SGH) and the head of structural engineering at the firm’s office in New York. In his more than 18 years with SGH, 
he has been involved with numerous design, investigation, forensic analysis, repair and rehabilitation, and research projects. He has worked on more than 400 different projects involving evaluation and structural design for repair or modification of various existing wood, concrete, masonry, and steel structures. Vatovec is a contributing member of the Wood Committee at American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) and of ACI Committee 440 on Fiber-reinforced Polymer Reinforcement.

Have you addressed flood mitigation strategies for your facilities? What experiences do you have with this issue? Please share in the Comments section below.