Selecting Appropriate Mitigation Measures for Floodprone Structures FEMA 551 February 2007 CHAPTER 7 – DRY FLOODPROOFING 7.1 Introduction A dry floodproofed structure is made watertight below the level that needs flood protection to prevent floodwaters from entering. Making the structure watertight requires sealing the walls with waterproof coatings, impermeable membranes, or a supplemental layer of masonry or concrete (Figure 7-1). [text box] CAUTION: Dry floodproofing may not be used to bring a substantially damaged or substantially improved residential structure into compliance with the community’s floodplain management ordinance or law. [End of text box] Figure 7-1. A typical dry floodproofed structure (Source: FEMA 312) Table 7-1 includes a summary of advantages and disadvantages for using dry floodproofing as a mitigation measure. Table 7-1. Considerations for Using Dry Floodproofing Advantages --Dry floodproofing is less costly than other retrofitting methods. --Does not require the additional land that may be needed for levees and floodwalls. --May be fundable under FEMA mitigation grant programs. Disadvantages --May not be used to bring a substantially damaged or substantially improved residential structure into compliance with the community’s floodplain management ordinance or law. --Dry floodproofing requires human intervention and adequate warning to install protective measures. --Does not minimize the potential damage from high-velocity flood flow and wave action. --Ongoing maintenance is required. --Flood shields may not be aesthetically pleasing. Dry floodproofing a structure includes the following: --Using waterproof membranes or other sealants to prevent water from entering the structure through the walls --Installing watertight shields over windows and doors --Installing measures to prevent sewer backup 7.1.1 Waterproof Membranes Installing heavy plastic sheeting or waterproof membrane along a wall’s exterior surface is an effective means of waterproofing (Figure 7-2). The waterproof membrane can be installed relatively quickly; however, it does require human intervention. The membrane is unsightly and cannot remain in place indefinitely. Furthermore, the plastic will deteriorate with continued exposure to solar radiation. Figure 7-2. Photograph of membrane providing flood protection (Source: U.S. Army Corps of Engineers) 7.1.2 Closures In conjunction with a waterproof membrane, openings in the walls need to be closed, either with temporary closures or permanently sealed shut (Figure 7-3). Figure 7-3. Permanently sealed opening (Source: FEMA 259) Low window openings at ground level can either have a pre-sized closure fitted over their surface or have a low wall constructed around the opening to a height above the flood protection elevation (Figure 7-4). Figure 7-4. Low wall construction Various closure systems can be manufactured to fit the individual openings, providing a way for it to be quickly closed and have a watertight seal. These types of closures can either be stored in a readily accessible location or permanently remain in place. Many of these closures have rubberized seals and other components that will require periodic care and maintenance (Figure 7-5). Figure 7-5. Small patio gate (Source: W.A. Wilson Consulting Services) 7.1.3 Sewer Backup Protection Backup of sanitary sewers into a structure is a major concern due to the health hazards. Even after floodwaters have receded, contents and belongings that have been exposed to sewage are severely contaminated and can be nearly impossible to clean. The five main approaches to protect a structure against sewer backup are floor drain plugs, floor drain standpipes, overhead sewers, backup valves, and grinder pumps. For a detailed discussion of sewer backup protection, see FEMA 511, Reducing Damage from Localized Flooding, Chapter 10 (pages 10-9 to 10-11) and FEMA 259, Engineering Principles and Practices of Retrofitting Floodprone Residential Structures, Section VI-D. 7.2 Technical Considerations The flood characteristics that affect the success of dry floodproofing are flood depth, flood duration, flow velocity, amount of warning time, and floodborne debris. 7.2.1 Flood Depth The depth of the floodwaters affects the hydrostatic pressure that is exerted on walls and floors. Because water is prevented from entering a dry floodproofed structure, the exterior pressure on walls and floors is not counteracted from the opposite side as it is in a wet floodproofed structure. The ability of walls to withstand the hydrostatic pressures depends partly on how the walls are constructed: --Typical masonry and masonry veneer walls, without reinforcement, can usually withstand the pressure exerted by water up to about 3 feet deep. --In flood depths exceeding 3 feet, unreinforced masonry and masonry veneer walls are much more likely to crack or collapse. An advantage of masonry and masonry veneer walls is that their exterior surfaces are resistant to damage by moisture and can be made watertight relatively easily with sealants. --Typical frame walls are likely to fail at lower flood depths, are more difficult to make watertight, and are more vulnerable to damage from moisture. 7.2.2 Structures with Basements If a structure has a basement, the walls and floors must be specifically designed to resist hydrostatic pressure. Otherwise the risks associated with dry floodproofing a basement are high. Figure 7-6 illustrates how hydrostatic pressure operates on a structure during a flood. Structure “a” has no basement and the forces acting upon the structure are relatively small. However, for structure “b,” the forces are significantly greater due to the presence of a basement. Figure 7-6. Hydrostatic pressures on a structure (Source: FEMA 312) 7.2.3 Flood Duration The longer a structure is exposed to floodwaters, the more likely it is that structural systems and floodproofing measures will begin to leak or fail. Most sealing systems will begin to allow some amount of seepage after prolonged periods of exposure to water. If the structure is in an area where high floodwaters can remain for days, a different retrofitting method should be used. 7.2.4 Flow Velocity A seemingly “low” flow velocity of 1 or 2 feet per second can exert tremendous forces on a structure with only a few feet of flooding. Structures can be easily moved off of their foundations, which results in the total loss of the structure. Walls are easily damaged and subject to collapse by moving water. 7.2.5 Warning Time Since dry floodproofing is often best suited for flooding conditions that last for a relatively short period of time, warning time and the time it takes to deploy the various components of a dry floodproofing system must be considered. Streams that are at flood levels for a short period of time generally reach flood levels quickly, thus the warning and preparation time is limited. Incorporating any type of flood warning system into the project design will enhance its ability to perform properly. 7.2.6 Floodborne Debris The impact forces from debris in the moving water, such as trees, can compound the hydrodynamic forces of the moving floodwaters. 7.3 Relative Costs The relative cost ranking is based on the combination of the estimated costs for the dry floodproofing project and a determination of cost-effectiveness. 7.3.1 Estimated Cost Dry floodproofing a structure is generally an inexpensive mitigation measure. The costs for dry floodproofing a structure will depend on the following factors: the size of the structure, the height of the Flood Protection Elevation (FPE), types of sealant and shield materials used, number of plumbing lines that have to be protected by check valves, and number of openings that have to be covered by shields. Examples of cost estimating items that may need to be considered include the following: --Analysis to determine the effective dry floodproofing method and design of the dry floodproofing method --Dry floodproofing method selected, including: ----Watertight shields for doors and windows ----Reinforcing walls to withstand floodwater pressures and impact forces generated by floating debris ----Drainage collection systems and sump pumps to control the interior water level, collect seepage, and reduce hydrostatic pressure on slab and walls ----Membranes and/or other sealants to reduce seepage of floodwater through walls and wall penetrations ----Anchoring the structure to resist flotation, collapse, and lateral movement --Construction To estimate the relative cost of a dry floodproofing project, examples of general cost estimates have been provided below and are included in FEMA 312, Homeowner’s Guide to Retrofitting: Six Ways To Protect Your House From Flooding and FEMA 259, Engineering Principles and Practices of Retrofitting Floodprone Residential Structures. The figures in Table 7-2 are example cost estimate numbers used in a study for the St. Louis Metropolitan Sewer District. These numbers were generated using the U.S. Army Corps of Engineers’ publication, Flood Proofing - How to Evaluate Your Options, and updated to 2002 and adjusted for the St. Louis area. It is important to note that the cost estimate numbers are location and time dependent. Table 7-2. General Estimates of the Unit Costs for Typical Dry Floodproofing Projects Dry Floodproofing Waterproofing a concrete block or brick-faced wall by applying a polyethylene sheet or other impervious material and covering with a facing material such as brick. $3.50/square foot Acrylic latex wall coating. $3.00/ square foot Caulking/sealant - a high performance electrometric “urethane” sealant is recommended. $2.50/linear foot Bentonite grout (below grade waterproofing, 6 feet deep) $20/linear foot Appendix C, Cost Estimating, provides guidance and references for conducting a more detailed cost estimate. Additional cost estimates can be obtained from R.S. Means’ Contractor’s Pricing Guide. A blank preliminary cost estimating worksheet (Worksheet D) is provided in Appendix B. 7.3.2 Determination of Cost-Effectiveness A component of the relative cost scoring is to include a determination of cost- effectiveness. Table D-1 in Appendix D, Determining Cost-Effectiveness, provides a quick screening for the cost-effectiveness of a project. The attributes included in the table are frequency of flood, level of damage, project cost, project benefits, and criticality (impact or loss of function). For example, if the frequency is the 10-year flood, the project will have a very high likelihood of cost-effectiveness. Based on the combination of the estimated cost of the project and the likelihood of cost-effectiveness, a relative cost ranking will be assigned on Worksheet B, Appropriate Mitigation Measures. If the likelihood of cost-effectiveness is low, the ranking of relative cost will be either moderate or high, based on the estimated cost of the project. However, if the estimated cost is low and the likelihood of cost-effectiveness is very high or high, the relative cost ranking will be low. 7.4 Additional Considerations Most floodproofing projects use more than one technique; this is especially true in dry floodproofing. A good dry floodproofing project can be enhanced through the use of small flood barriers or modifications to the structure’s foundation drainage system. The small flood barriers will move the floodwaters away from the structure, thereby reducing the forces exerted on the subsurface portions of the structure. Additional considerations for using dry floodproofing are human intervention, annual maintenance, non-residential buildings, and structures without basements. 7.4.1 Human Intervention Property owners must be able to install all flood shields and physically perform the activities required for the successful operation of the dry floodproofing system before floodwaters arrive. 7.4.2 Annual Maintenance The components must be inspected and maintained on a regular basis. Because dry floodproofing has window and door closures as part of the system, closures must be available and in good condition. Some considerations to facilitate a successful maintenance schedule are as follows: --Develop an inventory and location list of all flood shields and closures. --Develop an inspection plan to ensure flood shields and closures fit properly. --Check walls, floors, and floodproof coatings for cracks and potential leaks. 7.5 Available Resources FEMA 259. Engineering Principles and Practices of Retrofitting Floodprone Residential Structures. See Chapters VI-D, Dry Floodproofing. FEMA 312. Homeowner’s Guide to Retrofitting: Six Ways to Protect Your House from Flooding. See Chapter 3, An Overview of the Retrofitting Methods and Chapter 7, Other Methods – Dry Floodproofing. FEMA 511. Reducing Damage from Localized Flooding. See Chapter 10, Retrofitting. The Louisiana State University (LSU) Extension Center website (http://www.louisianafloods.org) lists many retrofitting publications, provides advice on floodproofing methods, and flood insurance and links to online shopping for retrofitting products and contractors. R.S. Means Contractor’s Pricing Guide. USACE. Flood Proofing - How to Evaluate Your Options. [End of Chapter 7]