Floods are one of many common hazards in the United States. It can occur anytime and anywhere, especially in the low-lying area, near water or downstream from a dam, coastal area and 100-year flood plain.
There are different types of floods which are generally grouped into riverine flooding, urban drainage, ground failures, fluctuating lake levels, and coastal flooding and erosion . The nature of their occurrence is different each time. Some floods may develop slowly, sometimes up to several days, and other can develop in just a few minutes (known as flash flood) and sometimes with no visible sign of rain. Flash flood is often accompanied by high velocity roaring water that carries rocks, mud and other debris and it can destroy almost any things in its path . The damages it caused are more severe than ordinary riverine floods due to its speed and the debris load. The pressure exerted by water moving at 10 miles per hour is equal to the pressure exerted by wind gusts at 270 miles per hour .
The extent of damage to structures that come in contact with floodwaters depends on the depth of the water in and around the structure, the length of time of inundation, the toxic extent of contaminants in floodwaters, and how rapidly the water is moving. Floodwater attacks and compromises the integrity of every part of a structure, particularly the wood foundation pilings, structural beams, mechanical systems and walls. The dynamic forces of rapidly moving floodwater, of course, will worsen the damage of the above structural components.
Damages continue to develop even during post-flood period due to retained floodwater in the structure and high humidity. Power outage and blocked access, which commonly occurs during and after flooding, have made the flooded structures to be inaccessible; thereby no immediate recovery/restoration steps can be taken.
Thus, every structure in flood prone areas must be designed in such a way that it does not only sustain from damages but must also able to reduce the possible threat of damages.
Identifying Flood Zones and Flood Hazards
Flood Zones
The first step should be the determination whether the building is located in Special Flood Hazard Area by checking maps. If the building is subject to the hazard, the Flood Zone (expressed by a letter) and a Base Flood Elevations (expressed in feet) should be obtained before proceeding to the next step.
Federal Emergency Management Agency (FEMA) is one of many federal agencies that has been highly involved in identifying flood hazards, creating development regulations for flood hazard areas as well as responding to flood disasters. FEMA Mitigation Division manages the National Flood Insurance Program (NFIP).
Flood hazards in more than 20,000 communities within the United States have been recorded by the NFIP in Flood Insurance Rate Maps (FIRM), that highlight Special Flood Hazard Areas (SFHA), their boundaries, and Base Flood Elevations (BFEs), and accompanied by Flood Insurance Study (FIS) reports to reflect data that is not found on the map.
Flood Insurance Rate Maps (FIRM are available online by typing the address of the property at the FEMA webpage. As there is often no address available for the new development, a zip code would suffice.
The Special Flood Hazard Area is the area known as the 100-year floodplain, or the area subject to flooding during the Base Flood. The Base Flood Elevation (BFE) is the water surface elevation associated with the 100-yr flood, and has a one-percent chance of being equaled or exceeded in any year.
The SFHA is the floodplain that has a one-percent annual chance of being equaled or exceeded in any given year.
This area is divided into the “A” zone and the “V” zone, indicated on the maps. “V” zones are found along coastal floodplain where high velocity wave action can occur during the Base Flood. The “A” zones covered the inland and coastal 100-yr flood plain where high velocity wave action is not expected during the Base Flood.
The most hazardous floodplain areas are mostly found close to coastal shorelines (V zone, also called the coastal high hazard area – CHHA), floodways (close to major streams and river), and alluvial fans. Different design restrictions and requirements are given for each of these areas.
A Few Cautions about Flood Hazard Maps
Many of the existing flood hazard maps are old, and conditions might have changed since the maps were produced (e.g., increased development and runoff in riverine areas, shoreline erosion in coastal areas). Nowadays, topographic data and more years of climate and flood data are available compared to the time when the maps are produced. Hydrologic and hydraulic models have evolved and produced different results than previous models.
Data Acquisition
The latest adopted FIRM can be obtained via the NFIP’s Community Status Book. Copies of the FIRMs and FIS reports can be purchased at FEMA’s Map Service Center or viewed on-line. The Digital FIRMs (DFIRMS) are available for some communities and contain more data compared to the traditional copy. Flood risks can be found by street address at FEMA’s Map Service Center and at FloodSmart.gov.
Community officials should have office copies of FIRMS and FIS report, and should serve as the best source of any local data and the governing floodplain management ordinance.
Elevation
The second step is to verify the applicable requirements based on the Flood Zone in which the project is located. See the section titled Codes and Standards at the end of this text. In most cases the elevation of the building structure should be determined and compared with the Base Flood Elevation (BFE), on which the specific code requirements would be based on.
The Design Target: A “Successful” Building
A “successful” building (Jones, 2008) is able to withstand flooding effects for long period of time and has the following characteristics after a flood event:
Repairable flood induced damages
Undamaged foundation
No structural damages inflicted from the breakaway enclosures constructed below BFE
Intact building envelope
Minor or no flood induced damages to utility connections
Accessible and functional
The fundamental goal of flood-resistant construction is for buildings to resist flotation, collapse and permanent lateral movement during flooding.
The basic requirement is to have the lowest floor elevated to or above the design flood elevation or, in cases of non-residential buildings, be designed so that the building is watertight below the design flood elevation (dry-floodproofed) with impermeable walls and able to resist hydrostatic and hydrodynamic effects.
However, dry-floodproofing is not permitted in the V zones. For areas outside the V zone and for buildings not dry-floodproofed, incorporate flood openings in walls forming an enclosure below the DFE, to equalize the flood levels.
For areas that are subject to extreme flood loads, elevate the building on piles or columns; keep the area below the DFE free of obstructions that could transfer flood loads to the elevated building, and use breakaway construction for any non-structural building elements.
Floodproofing
Floodproofing is the process of making a building resistant to flood damage, either by taking the building out of contact with floodwaters or by making the building resistant to any potential damage resulting from contact with floodwaters. Designers should be able to determine specific floodproofing elevations for their projects, based on applicable regulations and code requirements.
There are 2 types of floodproofing, dry-floodproofing and wet-floodproofing. Dry-floodproofing involves the use of sealants, coatings, and making the building watertight. Wet-floodproofing allows certain part of the building to be flooded while keeping the damages to its minimum.
Dry Flood-Proofing
Dry flood-proofing a structure involves getting a solution to prevent any water entry. In this case, by elevating the lowest floor to or above the base flood level or by designing the structure below the base flood level watertight such that it's able to resist water entry as well as hydrostatic and hydrodynamic effects.
Residential structure in the A zones must be dry flood-proofed at least 1 foot above the BFE. The lowest flood within the AO zones must be elevated above or as high as the depth specified in the community’s FIRM.
Flood-resistant buildings should also have adequate connections between structural elements to resist flotation, collapse and permanent lateral movement. Uplift forces during a flood event are often great enough to separate an insufficiently anchored structure from its foundation. Floating debris, such as stairs, porches and other poorly constructed elements can cause structural damages upon impact. The level of damages depends on the flood depth and its flow velocity. Based on guidance from FEMA 55 and FEMA 259, debris impact load is estimated by assuming a 450kg mass traveling at the floodwater velocity, striking a 0.09m2 area, with impact times range from 0.2 to 1.1 seconds.
In flood-prone areas, a building should be designed or constructed based on the type of flood that occurs in the building’s location. If it is located in coastal areas, flood is primarily caused by storm surges and waves. While in the inland areas, flood occurs due to over-bank flooding along rivers/lakes, poor drainage (particularly in low elevated areas), and flash flooding in steeply sloped areas. Understanding the above characteristic of flood helps to determine the right protection method for a structure to withstand the maximum flood level, floodwater velocity, duration, and even movement of ice during winter floods.
Figure 1. Example of a dry-floodproof office building. The ground floor serves as a parking garage.
Wet Flood-Proofing
Wet floodproofing is designing a building to allow floodwaters to enter in order to equalize hydrostatic forces. The NFIP does not allow wet floodproofing in lieu of meeting the lowest floor elevation requirements. The acceptably flooded interiors include enclosed areas below the BFE that are used solely for parking, building access, or limited storage, attached garages, or accessory structures. The contents of these areas is generally not eligible for insurance. However, compared to dry flood-proofing, the wet flood-proofing presents a cheaper solution.
Flood Openings
Enclosed areas below the BFE must be equipped with flood openings or flood vents to equalize water levels and hydrostatic loads. Covers, such as grates, louvers and grills, are used for these openings for pest control.
The location, number and size of the openings are specified in section 2.6 of ASCE/SEI-24, which, in turn, is referenced by both the IBC and NFPA 5000.
The prescriptive (non-engineered) method and engineered method are used for sizing the openings in multiple walls. These openings are to be located no more than one foot above the adjacent ground level with a minimum opening of 3 inches in plane of the wall.
However the two above methods employ different procedure in calculating the total net area of required openings. The prescriptive method requires 1 square inch of opening per square foot of enclosed area below the BFE. The engineered method allows designer to determine the size of openings to suit the type of flood in the specified area and usually results in a smaller total net opening compared to the prescriptive method.
Equalizing hydrostatic pressure
Wall openings must be provided to allow water to fill in all spaces in the structure, equalizing internal and external hydrostatic forces that may cause wall collapse or flood-induced uplift. Provisions must be made to prevent air trapped within the structure during periods of inundation from becoming pressurized and damaging on the exterior walls and roof. If the cavity wall air space is filled with insulation, it should be a type that is not subject to damage from inundation. The air space within the cavity wall will also be inundated by floodwater and the contaminants it carries which may remain in the cavity space after the floodwater has receded.
Choosing the Right Materials
The choosing of flood-resistant material, or as defined by NFIP, the building product that is capable of withstanding direct and prolonged contact with floodwaters without sustaining significant damage is the basic requirement of The International Building Code (IBC) and the International Residential Code (IRC). The term “prolonged contact” means at least 72 hours and the term “significant damage” means any damage requiring more than cleaning, sanitizing and resurfacing, or simply known as cosmetic repair. The cost of repair should be less than the cost of replacement of affected materials and systems. Flood damage-resistant materials must also resist degradation.
Figure 2. Examples of materials not resistant to water damage: plywood panelling and laminated wood flooring.
Figure 3. Examples of materials not resistant to water damage: wooden siding and doors.
Flood-Resistant Classification of Materials
The following table specifies building materials based on the ability to resist flood damage.
Specific lists of materials and their classifications are contained in FEMA Technical Bulletin 2.
Material Drying
Good movement of air aids in drying process after a flood event when there is power outage. Cavities should be well ventilated which allow vent openings at the top and bottom to improve gravity airflow. The materials used must be vapor open and non-water sensitive to facilitate drying by diffusion.
Figure 4. Drying of a wall by the gravity airflow after the wet base of gypsum wall was knocked off.
Post-flood Considerations
It is a good practice to clarify the post-flood considerations with the owner, by reminding about those aspects that may not be immediately obvious in peaceful times.
Flood is considered to be the most devastating hazard. Private insurance companies reached this conclusion on basis ot their actuarian data and sophisticated risk analyses; therefore, they no longer underwrite this risk. Ironically, as a side effect, the flood is underrated by the general public because it is absent in the advertising media constituting a large percentage of the information intake.
Certain coastal areas are subject to mandatory evacuation; therefore no staff would be available to attend or mend the structures. Also, community services, such as police and firemen would be stretched thin, leaving the structure vulnerable in case of emergency.
The infrastructure may be destroyed or obstructed, including power, water, sewage, telephone, and transportation hubs and conduits for a prolonged period, which may vary from weeks to months. This in turn may cause a shortage of supplies and services, and a restricted access to the site. To address power outages during and post flood event, pumps and generators should be installed. A separate storage space should be provided for supplies such as fuel and water tanks for potable water. Critical items, such as electrical and communication equipments rooms, must be located on the higher level of the building to keep it safe from flooding.
Most insurance policies require the owner to undertake prompt steps to avoid further damage. This typically requires erection of a temporary protection as well as removal of water and moisture from the interiors. Contaminants such as silt, chemicals, and organic materials can be hazardous to the structure and the occupants; caustic chemical can deteriorate building materials and debris that harbors organic growth may bring associated bacterial problems and odors.
Floods, unlike tornados, affect large areas; therefore, everybody else in the area would shop for construction supplies, restoration and construction contractors, causing their limited availability in the months following the flood event.
The damage often causes shortage of finances, as the business was interrupted, the repair of damage and the insurance adjustment process are under way. The insurance deductible should be considered as a minimum cash reserve. Insurance policies contain numerous restrictions that would further affect the damages*. As the result of frequent disputes, the insurance claims are sometimes seen taking up to 5 years to fully resolve.
* In the insurance vocabulary the term "damages” as opposed to “damage” means an amount of money.(The author is also a licensed insurance adjuster.)
Code Requirements
Compliance
The designer should always verify with the pertaining requirements with the Authority Having Jurisdiction (AHJ) and the owner. (Some owners, particularly federal institutions may require compliance with in-house standards and may be immune from local codes.) Residential and nonresidential buildings are subject to different requirements; we limited our discussion below to the nonresidential construction. The local and project-specific requirements may vary; however, the general requirements are typicaly found in the two volumes: The International Building Code (IBC) and NFPA 5000 Building Construction and Safety Code, which typically govern the design and construction. Both reference two standards important to flood-resistant design: ASCE/SEI 7 (Minimum Design Loads for Buildings and Other Structures), and ASCE/SEI24 (Flood Resistant Design and Construction). Both codes refer to the flood loads and load combinations contained in ASCE/SEI-7 and both codes require design and construction within flood hazard areas to comply with ASCE/SEI-24.
The vast majority of communities participate in the NFIP, and the NFIP floodplain management requirements (44 CFR 60) are administered at the community level (usually through the adoption of a floodplain management ordinance, zoning or subdivision regulations, and a recent model building code), with some states and regional governments mandating additional requirements.
Floodplain management requirements, when reduced to their simplest form, govern the lowest floor (including basement) elevation and foundation of a building, the materials and methods used to construct lower portions of the building, and (sometimes) the location of the building. Most of the requirements are performance requirements rather than prescriptive requirements (exceptions include the lowest floor elevation in all flood hazard zones, and the foundation type in a V zone).
Codes and Standards
IBC
Most of the mandatory provisions are contained in Section 1612 (Flood Loads), but others pertaining to Exterior Walls Performance and Flood Resistance also occur in Sections 1403.6 and 1403.7 (Exterior Walls, Performance, Flood Resistance). Optional Provisions are incorporated into Appendix G (Flood Resistant Construction) and Section J101.2 (Grading, Flood Hazard Areas).
Codes issued by individual states may vary, and so the Section 1612 in State of Florida is dedicated generally to High-Velocity Hurricane Zones. State of Florida has added Section 3110 (Flood-Resistant Construction) and Section 3109 (dedicated to Structures Seaward Of A Coastal Construction Control Line) instead.
NFPA 5000
Most of the mandatory provisions are found in Chapter 39 (Flood-Resistant Design and Construction). Requirements pertaining to Exterior Walls Flood Resistance are also recorded in Section 37.1.1.2 . Optional Provisions, that jurisdictions may or may not adopt, are included in: Annex C (Flood-Resistant Construction).
ASCE/SEI-7
ASCE/SEI-7 (Minimum Design Loads for Buildings and Other Structures) represents the national standards for loads and load combinations which has been used in drawing up the design and construction requirements by ASCE/SEI-24. It consists of flood load calculation methods which are referenced by the IBC, NFPA 5000 and ASCE/SEI-24.
ASCE/SEI-24
ASCE/SEI-24 (Flood Resistant Design and Construction) created basic requirements for design and construction in the A and V zones, which mostly correlate to NFIP minimum requirements. It exceeds the minimum requirement of those stated in NFIP by requiring freeboard for lowest floor and equipment, for the use of flood-resistant materials and for floodproofing.
Federal Buildings: Executive Order 11988
Federal structures and facilities are also governed by Executive Order 11988, whose pertinent sections are summarized below:
Before taking an action, each Federal agency shall determine whether the proposed action will occur in a floodplain.
Requires Federal agencies to avoid, to the extent possible, the long- and short-term adverse impacts associated with the occupancy and modification of floodplains and to avoid the direct and indirect support of floodplain development whenever there is a practicable alternative.
The construction of Federal structures and facilities is to be in accordance with the standards and criteria of, and to be consistent with the intent of those promulgated under, the National Flood Insurance Program.
Federal structures and facilities shall deviate only to the extent that the standards of the Flood Insurance Program are demonstrably inappropriate for a given type of structure or facility.
National Flood Insurance Program (NFIP)
The flood resistance requirements, like many modern roofing requirements, are tied to the insurance underwriting requirements. In order to be insurable, a building must be eligible and located in the eligible community. While the requirements are codified for the new construction, bringing the existing structure in compliance may either make the building eligible for the coverage, or reduce the insurance premiums.
All flood insurance coverage in the U.S. is either directly or indirectly underwritten by the NFIP, created by Congress in 1968 to provide federally backed flood insurance coverage.
Specific Requirements
Lowest Floor Elevation
Generally, buildings within the Special Flood Hazard Area must have their lowest floor (including basement) elevated above the BFE (except for those non-residential buildings which are dry-floodproofed). Specific lowest floor elevation requirements—tied to the flood hazard zone and the building classification in Section 1 of ASCE/SEI-7 building classification—are contained in ASCE/SEI-24, which is referenced by the IBC and NFPA 5000.
In A zones, the reference level for the lowest floor (including basement) is at the top of the floor; in V zones, the reference level for the lowest floor is set at the bottom of the lowest horizontal structural member supporting the lowest floor.
Dry and Wet Flood-Proofing
The floodplain management regulations and building codes do not allow dry flood-proofing for certain types of structures and specific flood hazard areas. Dry flood-proofing for areas below the BFE is not allowed for buildings in V zones due to potential structural failures and for residential buildings in A zones.
The dry flood-proofed parts of the building envelope should not allow more than 4 inches of water depth accumulated within 24-hour if no mechanism for water removal is installed. The usage of sump pumps may be required. This requirement applies to any areas of the building envelop below the BFE.
Below is a table classifying flood-proofed space:
NFIP Classification
USACE Floodproofing Class
USACE Class Description
Dry Flood-Proofed
W1
completely dry: envelope impermeable to water and water vapor; designed for hydrostatic and hydrodynamic loads
W2
essentially dry: envelope substantially impermeable to water, but may pass some water vapor or seep slightly; designed for hydrostatic and hydrodynamic loads
flooded by flood water: wet flood proofing; hydrostatic pressure equalized
Not Flood-Proofed
W5
non-flood proofed: (fails to meet any of above)
Lateral hydrostatic and vertical buoyant forces must be calculated for a structure that has dry flood-proofed area to ensure adequate anchorage.
Certifications
Different certifications by a licensed construction professional may be required for constructing in different flood zones. Designers can consult the AHJ to determine the right type of certification for their projects.
Lowest Flood Elevation Certification.
Measurement of the lowest floor elevation and certification form completion is needed.
Flood Openings Certification.
This certification is required for openings constructed by engineer method. It will state that the openings allow for automatic entry and exit of floodwaters and meet the requirements of ASCE/SEI-24.
Dry Flood-Proofing Certification.
It is required for non-residential areas below BFE. The certification must state: 1) the elevation to which the building has been dry flood-proofed, 2) that the building, together with utilities and sanitary facilities, is watertight to the flood-proofed elevation, with walls that are substantially impermeable to the passage of water, and 3) that the structure is capable of resisting hydrostatic and hydrodynamic and debris impact forces, including the effects of buoyancy.
V Zone Design and Construction Certification.
It is required for buildings constructed in the V zone. It must state 1) the design and methods of construction complies to the accepted standards of practice, 2) the bottom of the lowest horizontal structural member supporting the lowest floor (including basement) is at or above the DFE, and 3) the foundation and the structure attached thereto are anchored to resist flotation, collapse and permanent lateral movement due to the effects of the design flood in combination with other loads, as specified by the building code.
Breakaway Wall Certification.
It is required for breakaway walls below the DFE that are designed to break away at lateral loads greater than 20psf. It must state that these walls are designed to collapse from a flood load less than that during the base flood.
International Building Code (IBC), International Code Council (ICC), 2009.
Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05), ASCE, 2005
NFPA 5000: Building Construction and Safety Code, NFPA, 2009
Flood Resistant Design and Construction (SEI / ASCE 24-05), ASCE 2005
Literature List
ABCDos and Don’ts of Flood-Proofing. Construction Specifier, December 2001.
Non-Residential Floodproofing – Requirements and Certification for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program. FEMA Technical Bulletin 3-93, April 1993. http://www.fema.gov/pdf/fima/job6.pdf
Flood Damage – Resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program. FEMA Technical Bulletin 2, August 2008. http://www.fema.gov/pdf/fima/job4.pdf
Wet Floodproofing Requirements for Structures Located in Special Flood Hazard Areas in accordance with the National Flood Insurance. FEMA Technical Bulletin 7, December 1993. http://www.fema.gov/pdf/fima/job14.pdf
Non-Residential Floodproofing — Requirements and Certification for Buildings Located in Special Flood Hazard Areas. FEMA Technical Bulletin 3 - 1993. http://www.fema.gov/pdf/fima/job6.pdf
Karol Kazmierczak (Kaz) CDT, ASHRAE, NCARB, LEED AP, is the senior building science architect and the leader of the BEC Miami. He has 17 years of experience in building enclosure technical design, engineering, consulting, and inspection. He can be contacted via e-mail at info@b-e-c.info.
updated on 05-19-2010
DISCLAIMER
The author is not an attorney and any information derived or found in this material is NOT to be construed as legal advice and is for educational purposes only. Contact an attorney if you have any questions about statements in this material before you take any action on your own. The author’s comments about architects, engineers, attorneys, contractors, manufacturers, managers, administrators, and any other profession mentioned in this material are not meant to be any indictment of character or generalization about any group or any individuals. The author believes any and all statements in this material are true to the best of his knowledge. For no specific reason, other than to save space, we have used the male gender (his, he) in all places where the female gender (her, she) would fit equally as well.
Any similarities to the actual situations, persons, jobs, constructions, and discussions are strictly coincidental.
Limits of Liability and Disclaimer of Warranty: The author and publisher of this material have used their best efforts in preparing this information. They make no representation or warranties with respect to the accuracy, applicability, or completeness of this material and its contents. They disclaim any warranties either expressed or implied, merchantability, or fitness for any reason or particular purpose. The author, publisher, and the above mentioned companies shall in no event be held liable for any loss or other damages, including but not limited to special, incidental, consequential, or other damages. The advice of competent legal and construction professionals should be sought if the reader has any questions what-so-ever.
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Special Flood Hazard Areas (SFHA)
There are 7 types of A zones:
A
The base floodplain mapped by approximate methods; BFEs are not determined. It is often referred to as unnumbered A Zone or an approximate A Zone.
A1-30
Or also known as numbered A Zones (e.g., A7 or A14). This is the base floodplain where the FIRM show a BFE (old format).
AE
Base Flood Elevations are provided. This is now more commonly used on the new FIRMs format instead of the numbered A Zones.
AO
Consists of sheet flow, ponding or shallow flooding. Base flood depths (feet above ground) are provided.
AH
Shallow flooding base floodplain. BFEs are provided.
A99
Areas that need levees or Federal Flood Protection Systems to be constructed. BFEs are not determined.
AR
The base floodplain that results from the decertification of a previously accredited flood protection system that is in the process of being restored to provide a 100-year or greater level of flood protection.
The V zones are divided into:
V and VEV
The coastal area subject to a velocity hazard (wave action) where BFEs are not determined on the FIRM.
VE
The coastal area subject to a velocity hazard (wave action) where BFEs are provided on the FIRM.
Outside SFHA or Undetermined Flood Hazard, Zone B and Zone X, are areas of moderate flood hazard. They are found between the limits of the 100-year and 500-year floods. The B Zones are used to designate base floodplains of lesser hazards, such as areas protected by levees from the 100-year flood, or shallow flooding areas with average depths of less than one food or drainage areas less than 1 square mile.
Zone C and Zone X are areas of minimal flood hazard and are usually plotted as above the 500-year flood level on the FIRMs. Zone C may have ponding and local drainage problems that don’t warrant a detailed study of designation as base floodplain. Zone X is the area determined to be outside the 500-year flood and protected by levee from 100-year flood.
Zone D are area of undetermined but possible flood hazards.
The NFIP sometimes maps flood hazard zones outside the SFHA (B, C, X zones) or maps areas of possible but unknown flood hazards the D zones. On newer maps, the B zones are replaced with shaded X zones and depicts areas expected to flood at flood recurrence intervals between 100 years and 500 years (0.2 percent to 1.0 percent annual chance of being equaled or exceeded). The C zones are replaces with unshaded X zones, which highlights areas with less than a 0.2 percent annual chance of flooding.
