UNIT III
Information and Data Provided through the National Flood Insurance Program The delineation of floodplains on maps is a basic necessity for floodplain management. Federal Interagency Floodplain Management Task Force, 1992 Floodplain Management in the United States: An Assessment Report
Overview This unit describes the Flood Insurance Study reports and floodplain maps that the NFIP provides to communities to assist them in carrying out a local floodplain management program.
Contents A. Introduction.............................................................................................................................. 3 B. Flood Insurance Study Reports and Maps............................................................................. 4 Flood Hazard Boundary Map............................................................................................................4 Flood Insurance Study Report ..........................................................................................................6 Flood Insurance Rate Map ................................................................................................................7 Flood Boundary and Floodway Map ................................................................................................8 FIRM New Format............................................................................................................................9 Countywide FIRM ..........................................................................................................................11 Digital FIRM...................................................................................................................................11 Digital Flood Insurance Rate Map....................................................................................11 Digital Flood Insurance Rate Map—Digital Line Graph .................................................12 Q3 Flood Data ..................................................................................................................14
C. Detailed Studies ...................................................................................................................... 16 Riverine Studies ..............................................................................................................................16 Cross Sections...................................................................................................................16 Elevation Reference Marks...............................................................................................18 Hydrology.........................................................................................................................19 Hydraulics.........................................................................................................................20 Profile ...............................................................................................................................20 Floodplain Map.................................................................................................................22 Floodway Analysis ...........................................................................................................23 Coastal Flood Studies .....................................................................................................................26 Hydrology.........................................................................................................................26 Waves ...............................................................................................................................26 Hydraulic Analysis ...........................................................................................................27
Information and Data
November 1997
Coastal High Hazard Area ................................................................................................28 Coastal Floodplain Map....................................................................................................29 Shallow Flooding Studies ...............................................................................................................29
D. Approximate Studies.............................................................................................................. 30 Requirements for Developing BFE Data ........................................................................................32
Unit Learning Exercise................................................................................................................ 36 Answers to Unit Learning Exercise............................................................................................ 38
References Federal Emergency Management Agency 1995 Managing Floodplain Development in Approximate Zone A Areas: A Guide for Obtaining and Developing Base (100-Year) Flood Elevations. Includes the computer program QUICK-2: Computation of Water Surface Elevations in Open Channels, Version 1.0. FEMA 265.
4-2
1995
How to Use a Floodmap to Determine Flood Risk for a Property: A Guide for Interested Private Citizens, Property Owners, Community Officials, Lending Institutions, and Insurance Agents. FEMA 258.
1995
Flood Insurance Study: Guidelines and Specifications for Study Contractors. FEMA 37.
November 1997
Information and Data
A. Introduction The planning and implementation of floodplain management measures requires a solid technical basis in order for there to be effective flood damage reduction and to ensure equitable treatment of all concerned citizens. Information is needed on the magnitude and frequency of flooding for a specific site or area, i.e., which areas are subject to flooding, to what depth, and how often do floods occur. The Flood Insurance Study (report and maps) provides the technical basis for implementation of floodplain management programs. The National Flood Insurance Act of 1968 directed the Federal Insurance Administration (FIA) to: •
Identify all floodprone areas within the United States
•
Establish flood-risk zones within floodprone areas
These two goals were established in an effort to reduce loss of life and property, as well as disaster relief costs, by: •
Guiding future development away from these flood hazard areas
•
Requiring that new and substantially improved building in these areas be constructed to resist flood damages
An important step toward meeting these goals is the preparation of Flood Insurance Study reports (FIS) and floodplain maps for floodprone communities. These studies provide sufficient technical information to •
Enable communities to adopt floodplain management measures required to participate in the NFIP
•
Designate floodprone areas and determine flood elevations and flood risk zones needed to establish actuarial flood insurance premiums
4-3
Information and Data
November 1997
B. Flood Insurance Study Reports and Maps Flood Hazard Boundary Map
New Term FHBM
In keeping with the 1968 Congressional directive, initial study efforts in the Emergency Phase of the National Flood Insurance Program were focused on identifying all floodprone areas within the United States. Utilizing flood data and floodplain information from a variety of sources (e.g., soils mapping, actual high water profiles, aerial photographs of previous floods, topographic maps, etc.), the approximate outline of the extent of the one-percent-chance (base or 100-year) floodplain for specific stream reaches was overlaid on available community maps, usually United States Geological Survey topographic quadrangle maps. In this manner, Flood Hazard Boundary Maps (FHBM) were prepared, without benefit of detailed studies or hydraulic analyses, for nearly all floodprone communities in the nation (over 18,000). However, some communities were not mapped initially, about 214 in North Carolina alone. On the FHBM, the Special Flood Hazard Area (SFHA) is designated as a shaded area labeled “Zone A,” and no flood elevations are given (see Figure 4-1). The FHBMs are often referred to as “flat maps” because they were published in a rectangular 11” x 17” format that was folded in the middle. More detailed mapping is published in a Z-fold format. These FHBMs were intended for interim use in most communities until more detailed studies could be carried out.
New Terms Detailed studies Approximate studies
As money was appropriated by Congress, FEMA performed more detailed studies for many communities, resulting in the publication of Flood Insurance Study reports and Flood Insurance Rate Maps (FIRMs). These subsequent studies were carried out to provide communities with the information and data needed to adopt and implement local floodplain management measures and to enter the Regular Phase of National Flood Insurance Program. These studies can be grouped into two broad categories: detailed studies and approximate studies. Detailed studies were carried out for developed communities and for those areas experiencing rapid growth. They are used to guide future development within flood hazard areas and to provide the information needed for new construction allowed in already developed areas. While Congress appropriated money for the original flood studies in the 1970s and 1980s, all floodplain mapping is now funded out of NFIP insurance policy service charges. Funds allocated annually for mapping
4-4
November 1997
Information and Data
are very limited. In North Carolina, typically only one to two studies are funded per year.
New Terms Zone A Unnumbered A Zone
Approximate studies are utilized where there is presently little or no development, or expectation of development, in identified floodprone areas. In some cases FEMA simply converted the FHBM to a FIRM by issuing a letter to the community stating that the FHBM shall be considered a FIRM. In those cases the community was instructed to line out FHBM on the map’s title block and write in FIRM; the SFHAs were also formally converted to Zone A in communities with extremely low risk of flooding, little or no existing development in the floodplain, and little potential for future development. Zone A without a flood elevation is called an “unnumbered A Zone.”
Figure 4-1. Flood Hazard Boundary Map
4-5
Information and Data
November 1997
Flood Insurance Study Report
FLOOD INSURANCE STUDY A Flood Insurance Study (FIS) is a compilation and presentation of flood risk data for specific water courses within a municipality. The FIS report and associated maps delineate flood hazard areas and establish flood elevations, thereby serving as the basis for regulating floodplain development and providing flood insurance.
A detailed Flood Insurance Study provides the data and maps needed for both the flood insurance and floodplain management aspects of the NFIP. The study consists of three components: •
The Flood Insurance Study report, (the “FIS”)
•
The Flood Insurance Rate Map (the “FIRM”)
•
The Flood Boundary and Floodway Map (the “FBFM” or “Floodway Map,” included in studies prepared before 1985)
A flood insurance study (FIS) is a compilation and presentation of flood risk data for specific watercourses, lakes, and coastal flood hazard areas within a municipality or other local unit of government. It delineates flood hazard areas, designates flood risk zones, and establishes base flood elevations, thereby serving as the basis for providing flood insurance and for regulating floodplain development and carrying out other floodplain management measures. The Flood Insurance Study report includes: •
An appraisal of the community’s flood problems in a narrative that describes the purpose of the study, historic floods, the area and streams studied, the engineering methods employed to determine flood-hazard areas, and use of the study for both floodplain management and flood insurance purposes
•
Maps of the study area and, often, photographs of historic floods
•
Tables summarizing various study data
•
Computed flood profiles of the 10-, 50-, 100-, and 500-year floods for the stream reaches studied
The FIRMs and FBFMs, which are an essential part of the study, portray:
4-6
•
Floodplain boundaries
•
Delineation of floodways, where applicable
•
Designation of flood insurance risk zones
November 1997
Information and Data
Flood Insurance Rate Map New Term FIRM
The Flood Insurance Rate Map (FIRM) is an essential product of the FIS which shows floodplain boundaries and flood data for insurance purposes (see example in Figure 4-2). The FIRM: •
Identifies the Special Flood Hazard Area (SFHA), and often the 500year flood zone
•
Shows flood risk zones
•
Shows base flood elevations
•
Identifies the location of elevation reference marks
•
Shows certain landmark features in the community
•
Shows floodway boundaries and study cross sections on most maps prepared since 1985
Figure 4-2. Flood Insurance Rate Map
4-7
Information and Data
November 1997
There are five types of FIRMS: •
A flat FIRM, which is in black and white on 11” x 17” panels, was the format used in older studies. Often this may simply be a “converted FBFM.
•
Z-fold FIRMs replaced flat FIRMs in the late 1970’s. They come in large panels and are folded up like a road map; most FIRMs are in this format. The floodplain is shown as a blue or gray shaded area. The SFHA is a darker shade, while the 500-year floodplain is lighter. Older FIRMS refer to the SFHA as Zones A1-A30, and refer to the 500-year floodplain as a Zone B. Minimally floodprone areas are called Zone C.
•
Newer FIRMs have AE and X Zones and floodways.
•
Countywide FIRMs are Z-fold FIRMs that include all the cities, villages, and towns in a county, as well as the unincorporated areas.
•
Digital or DFIRMs are computer generated from GIS databases.
Flood Boundary and Floodway Map New Term FBFM
The Flood Boundary and Floodway Map (FBFM) contains the data needed by the local permit official to carry out regulation of the floodplain areas and for other community floodplain management measures. These maps were included in flood insurance studies started before 1985, and, in some instances, in studies after that date. The FBFM looks like the FIRM because it shows some of the same information. The Flood Boundary and Floodway Map (Figure 4-3) shows: •
Floodplain boundaries
•
The floodway (described later) for rivers and streams, shown in white down the middle of the floodplain
•
Study cross sections
The FBFM does not show:
4-8
•
Base flood elevations (BFEs, described in Part C of this unit)
•
Flood risk zones
November 1997
Information and Data
It is essential that communities use both the FBFM and the FIRM when administering the NFIP regulations.
Figure 4-3. Flood Boundary and Floodway Map
FIRM New Format Since Flood Insurance Studies were first prepared in the late 1960s, many improvements have been made to the design of flood maps so that they are easier to use and meet user’s needs. Starting in 1985, the following changes have been made to the FIRM: •
Inclusion of floodway and other floodplain management information that was previously provided on separate Flood Boundary and Floodway Maps (FBFMs). (Except in a few instances, new FBFM maps are no longer being prepared.) All floodplain management and flood insurance information are now included on one map, the FIRM.
4-9
Information and Data
November 1997
•
Simplified flood insurance zone designations (e.g., the previous Zones A1-A30 and V1-V30) have been replaced by the designations AE and VE; Zones B and C are replaced by Zone X. The 500-year floodplain is still shown as “shaded” portions of Zone X.
•
Identification of undeveloped portions of barrier islands and adjoining areas designated as units in the Coastal Barrier Resources System, established by Congressional Acts in 1982 and 1990, and subsequent revisions, called “COBRA zones.” These areas are identified because no new flood insurance coverage may be provided after specified dates for new or substantially improved structures on any coastal barrier in the system.
New Terms Zone X COBRA zones
The above features appear on newer FIRMs. Figure 4-4 shows an example of a FIRM illustrating some of the above changes. With these changes, the FIRMs can more easily be used by community officials for floodplain management, by lenders to determine the need for flood insurance, by insurance agents to rate policy applications, and by land surveyors, engineers, property owners, etc. In other words, all the players are using the same map and flood information.
Figure 4-4. FIRM (New Format)
4-10
November 1997
Information and Data
Countywide FIRM
New Term Countywide FIRM
Since the late 1980s, many flood insurance restudies have been produced in a countywide FIRM format. This makes it easier for administration because the SFHAs outside a municipal boundary or extraterritorial jurisdiction (ETJ) are also shown. Thus, the municipality knows what SFHAs are being included in any annexation or extension of the ETJ. Figure 4-5 shows a portion of the countywide FIRM for Wake County, with the corporate limits and ETJ of the Town of Cary, as well as unincorporated county areas. Countywide FIRMs have their own 6-digit map ID number ending in “C,” e.g., 37153C for Richmond County. Figure 4-5. Countywide FIRM
Digital FIRM Digital Flood Insurance Rate Map The Digital Flood Insurance Rate Map (DFIRM) is intended to be the primary means of transferring flood-risk data depicted by FIRMs to Geographic Information Systems (GIS) through a public domain data 4-11
Information and Data
November 1997
exchange format. The DFIRM is generally produced in a countywide format, where all flood hazards for the county and incorporated communities are shown on one set of maps. It can be used for floodplain management purposes in a manner similar to other flood maps. The DFIRM is comprised of all digital data required to create the hardcopy FIRM. This includes base map information, graphics, text, shading, and other geographic and graphic data. An example of a hardcopy paper DFIRM is shown in Figure 4-6.
Figure 4-6. Hard Copy DFIRM
Digital Flood Insurance Rate Map—Digital Line Graph The Digital Flood Insurance Rate Map—Digital Line Graph (DFIRMDLG) is a database created by extracting certain flood risk data from the DFIRM. The DFIRM-DLG does not include base map information, nor does it include graphic data required to create a hardcopy FIRM. Communities whose digital base mapping files were utilized as the base map for the DFIRM will find that they may easily use the DFIRM-DLG files for determination of flood zones and for enforcement of regulations. A graphic image of a DFIRM-DLG is shown in Figure 4-7.
4-12
November 1997
Information and Data
Figure 4-7. Graphic Image of a DFIRM-DLG
The digital data captured from the hardcopy paper DFIRM consists of FEMA hydrography, flood hazard zones, base flood elevations, cross section locations, and elevation reference marks. All lines and area features in DLG files are encoded with one or more seven-digit attribute codes that provide the user with detailed information about the features. FEMA intends to make the DFIRM-DLG available on CD-ROM compatible with Insurance Services Office (ISO) 9660 standards and over the Internet. Many commercially available GIS software packages allow the direct conversion of DLG data into vector data usable within the GIS environment. Third party conversion software is also available that will convert DLG data to other proprietary GIS formats not supported with their own conversion software. 4-13
Information and Data
November 1997
The conversion of FIRMs to a digital format is expected to have many benefits. However, users must bear in mind that the simple conversion of FIRMs to a digital format does not inherently improve the engineering quality of the product. Many of the same difficulties with interpretation of flood risk data and the requirement that users apply sound judgment in methods selected for decision making and map interpretation remain unchanged. The DFIRM-DLG, when coupled with digital base map files or the local community digital base, can be used in a GIS to determine whether a structure is located within a Special Flood Hazard Area. It should be noted that if a structure is determined to be within or near the area by using a GIS, and a different base map source was used to generate the hardcopy DFIRM, this determination should be confirmed by referencing the printed hardcopy DFIRM. The DFIRM-DLG may also be used for engineering and planning studies.
Q3 Flood Data New Term
Q3
In the Q3 Flood Data product, FEMA has developed a graphical representation of certain features of the FIRM. The Q3 flood data are in three formats that are usable with desk-top mapping and GIS software packages. These formats are: •
Digital Line Graph
•
ARC/INFO
•
MapInfo
The Q3 flood data are created by scanning (raster or grid data files) the FIRM paper maps and vectorizing (converting to lines and areas) select data features onto one countywide data layer. The following vectorized (lines and areas) data features are included:
4-14
•
1% and 0.2% annual chance flood boundaries (SFHA and 500-year floodplain)
•
Flood insurance zone designations
•
Floodway boundaries (if available)
•
Coastal Barrier Resources System boundaries
•
Political boundaries
November 1997
Information and Data
•
Community/map panel identification numbers
•
Boundaries between FIRM panels (called neatlines)
•
U.S. Geological Survey 7.5 minute (1:24,000 scale) quadrangle neatlines
The following features are not included: •
Hydrographic features
•
Base flood elevations
•
Cross section lines
•
Roads, road names, or address ranges
•
Elevation reference mark locations and elevations.
FEMA has produced Q3 Flood Data for approximately 900 counties nationwide, 53 in North Carolina. The data are distributed on CD-ROM and over the Internet from FEMA’s Map Service Center. The Q3 flood data are designed to be used for a variety of types of planning applications including: broad-based review for floodplain management, land-use planning, commercial site analysis, insurance target marketing, natural resource and environmental analyses, and real estate development targeting and planning. The data are designed to provide guidance and a general proximity of the location of Special Flood Hazard Areas. The data are not suitable for applications such as detailed site design and development plans or flood risk determinations. They cannot be used to determine absolute delineations of flood boundaries, but instead should be seen as portraying zones of uncertainty and possible risks associated with flood inundation. Any questions regarding digital FIRM data products may be directed to: Federal Emergency Management Agency Mitigation Directorate Q3 Flood Data Program 500 C Street, S.W. Washington, DC 20472
4-15
Information and Data
November 1997
C. Detailed Studies Floods come in different sizes. The NFIP regulations are written and applied to the base flood. The “base flood,” or “100-year flood,” is defined as a flood that has a one-percent chance of being equaled or exceeded in any given year, and is more properly termed the “one-percent annual chance flood.”
BASE FLOOD A flood that has a onepercent chance of being equaled or exceeded in any given year. It often is referred to as the 100year flood
The base, or 100-year, flood is a statistical concept denoting the average length of time between floods of this magnitude. The term does not mean that this flood will occur only once every 100 years. It is a flood which, over a very long period of time, can be expected to occur, on an average, once in each 100 years, although the flood may occur in any given year. The 100-year flood could conceivably occur more than once in a year, two or three years in a row, or it may not occur for several hundred years. Because this can be confusing, the NFIP program uses the term base flood. The elevation to which floodwater is anticipated to rise during the base flood is the base flood elevation (BFE). The 100-year flood was chosen for use as a compromise between excessive exposure to flood risk from using a lower standard (e.g., 25-year flood) and applying such a high standard (e.g., 1000-year flood) that it would be considered excessive and unreasonable for the intended purposes of requiring the purchase of flood insurance and regulating new development. An engineering study must be conducted to calculate and map the base flood. Flood studies are conducted differently for different types of flooding. This unit discusses how three types of flood hazards are studied and mapped: •
Riverine flooding of rivers, streams, ditches or other waterways
•
Coastal flooding caused by hurricanes or severe storms
•
Shallow flooding, ponding, and sheet flow
Riverine Studies Flood studies involving rivers, streams, or other areas that have defined channels follow a number of basic steps.
Cross Sections 4-16
November 1997
Information and Data
The first step in a riverine flood study is to examine the areas through which floodwater will flow. This requires a determination of ground elevations and obstructions (such as buildings, bridges, and other developments) for these areas. Accurate data on the shape of the floodplain is obtained from ground surveys, aerial photomaps, or topographic maps. New Term Cross section
Surveys are taken of cross sections of the floodplain at locations that are representative of local conditions. Cross section locations are shown on the Flood Boundary and Floodway Map and on the new format FIRMs with lines that have letters at each end (see Figure 4-8). The cross section lettering system starts downstream and goes upstream. Cross sections are taken at each bridge or other major obstruction and at other locations, depending on how much the channel or adjacent floodplain conditions change. At each cross section, the engineer has accurate information on the floodplain shape and the elevation of the ground, e.g., see the surveyed cross section in Figure 4-9.
New Term Roughness factor
The engineer also estimates the roughness factor along the floodplain to determine how fast floodwater will flow through the area. Roughness factors are related to ground surface conditions. They account for the fact that water will flow faster over mowed grass and pavement than it will over an area covered in bushes and trees, or planted in tall crops.
Figure 4-8. Cross Section Locations
4-17
Information and Data
November 1997
Figure 4-9. Surveyed Cross Section
Elevation Reference Marks New Terms Elevation reference mark Datum
A common reference (or elevation reference mark) is needed when referring to flood elevations to ensure uniformity of use and avoid any confusion. A datum is a reference surface from which elevations are measured. For the NFIP, datums are used to define the elevation of the base flood and the elevation of the lowest floor of a structure. Originally, mean sea level (MSL) was the accepted elevation reference datum. Because this datum is local in nature, it was not ideal for a nationwide program for flood insurance. During the 1920s, the U.S. government created a network of 21 tidal gauges in the U.S. and five in Canada to provide a fixed continental datum that would bring a consistent relationship to all vertical determinations in the U.S. This new datum, known as the National Geodetic Vertical Datum (NGVD) of 1929, is the base elevation to which all relief features and elevation data are referenced in the contiguous United States. NGVD is also the datum of reference for the vast majority of Flood Insurance Studies. An ultimate goal of the NFIP is to convert all FISs to a newer standard called the North American Vertical Datum (NAVD) of 1988. This standard eliminates inconsistencies caused by assuming that 0 NVGD is the same as mean sea level at all 26 tidal stations. There are now 600,000 permanent bench marks associated with the NAVD. See Flood Insurance Study: Guidelines and Specifications for Study Contractors, FEMA-37 (1995), for further information.
4-18
November 1997
Information and Data
Figure 4-10. Typical Bench Mark
Most permanent elevation reference marks (bench marks) are referenced to the NGVD (see Figure 4-10 for an example). Their locations are identified on flood maps with a small “x” and the designation “ERM” or “RM” followed by a number (see Figure 4-11 for an example). Descriptions of the marks, including their elevations, appear either on the FIRM panel, on the FIRM index, or in the Flood Insurance Study report. Reference marks are not always brass caps; they can be chiseled squares or other designated markers. The North Carolina Geodetic Survey has a comprehensive inventory of all bench marks in the state. Reference marks of known, recorded elevation are used by surveyors to determine elevations at nearby locations. For example, they can be used to establish first floor elevations at a development site and to determine whether a proposed site is subject to flood risk.
Figure 4-11. Elevation Reference Marks
Hydrology New Terms Discharge CFS Hydrology Hydraulics HEC-2
The amount of water that will flow downstream during a flood is termed the flood discharge. It is measured in cubic feet per second or cfs. A cubic foot of water is approximately 7.5 gallons. The science of determining water discharge is called hydrology. Discharges are calculated by utilizing hydrologic computer programs. Hydrological analysis utilizes information from historical stream records, rainfall and snowmelt data, ground cover, and slope. Runoff will vary depending on the ground characteristics, e.g., generally more runoff 4-19
Information and Data
November 1997
occurs on steeper slopes, on unforested land, and on paved and built-on urban lands.
Hydraulics HYDROLOGY A science that deals with the distribution and circulation of water in the atmosphere, on land surfaces, and underground. It is used to determine flood flow frequencies.
HYDRAULICS A science that deals with fluids in motion.
• It is used to determine how a quantity of water will flow through a channel or floodplain.
Hydraulics is the study of moving water. Hydraulic analysis combines (1) the flood hydrology, or discharge; (2) the cross section data on how much area there is to carry, or convey, the flood; and (3) then estimates how fast the flood will move through the floodplain. The data are usually processed using a computer model, HEC-2, developed by the U.S. Army Corps of Engineers. The result of the hydraulic study is the determination of the elevation of one or more flood levels (see Figure 4-9). A Flood Insurance Study produces elevations for the 10-, 50-, 100-, and 500-year floods. Elevations for the 10-year and 50-year floods are typically used for other floodplain management purposes, e.g., the 10-year flood for septic systems and the 50-year flood for bridges and culverts.
Profile New Term Flood profile
There is obviously a need for flood elevations at locations between the cross sections. This is done by plotting the elevations at the cross sections on a graph and connecting the plotted points. Such a graph is called a flood profile. The more changes there are in topography, the more cross sections are needed to reflect accurate base flood elevations. A flood profile is shown in Figure 4-12. The bottom of the graph shows the distance along the stream. The distance is measured above the mouth of the stream or above its confluence with another stream. The left side of the graph shows elevation. Bridges are indicated with an “I” shaped symbol. The bottom line of the “I” represents the elevation of the top of the bridge opening over the stream, and the top line represents the elevation of the bridge floor or top of rail. A flood elevation at any point along the stream can be determined by reading the profile. Flood profiles are located at the end of the Flood Insurance Study report (usually as foldout pages). FIS profiles show the streambed and four flood levels: the 10-, 50-, 100-, and 500-year floods. The bottom of the grid shows the distance upstream from a starting point. Elevations above NGVD are shown at the left edge.
4-20
Figure 4-12. Typical Flood Profile
Information and Data
November 1997
Floodplain Map New Terms Topographic map Contour map
The next step in the mapping process is to transfer the flood elevation data onto a map that shows ground elevation data. This is called a topographic map or contour map because points with the same elevation are connected by a contour line. The most common topographic maps used are those produced by the United States Geological Survey (USGS) at a scale of one inch equals two thousand feet with ten-foot contour intervals. Only within the last decade has FEMA started having detailed four-foot contour interval topographic mapping prepared along the floodplains as part of the Flood Insurance Study, e.g., Watauga and Transylvania County studies. Also, some communities have prepared their own detailed topographic mapping and provided it to FEMA to improve the accuracy of their new flood maps, e.g., Durham and Charlotte/Mecklenburg County. The BFEs from the cross sections and profile are plotted on the topographic map. Floodplain boundary lines are drawn connecting these plotted points using the contour lines as a guide. The result is a flood map (Figure 4-13). It is important to remember that floodplain map boundaries are only as accurate as the topographic map on which they are drawn. Since the USGS topographic quadrangle maps have so large a scale, the SFHA boundaries cannot be precisely mapped. Some judgment must be used in correlating map features with ground features because maps do not always represent exact conditions on the ground.
Figure 4-13. Topographic Floodplain Map
4-22
Information and Data
November 1997
FLOODWAY The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to allow the discharge of the base flood without increasing the water surface elevation more than a designated height.
Where there is an apparent discrepancy between flood boundaries shown on a map and actual ground conditions, rely on the elevation data to resolve the matter by locating the flood elevation on the ground via an elevation survey. This represents the actual extent of flooding for that particular flood. Elevation data are found in the Floodway Data Table, the flood profile, and on the Flood Insurance Rate Map (FIRM). To correct discrepancies, see Letters of Map Amendment in Unit 9. Only FEMA, not a surveyor or local administrator, can correct or revise an NFIP map. The NFIP’s floodplain map is called the Flood Insurance Rate Map or FIRM. The map key is located in the upper right corner of the FIRM. The plotted base or 100-year floodplain is shown as dark gray shading. Many FIRMs show the area between the 100-year and 500-year floodplain boundary as a light gray shaded area, sometimes as a Zone B or on newer FIRMs as a shaded portion of Zone X. The zone terminology and how to read a FIRM and other maps are covered later in Unit 5. The BFEs along rivers and streams are usually denoted on the FIRM using a wavy line that extends across the width of the floodplain. The number shown on the wavy line is the BFE, rounded to the nearest whole number, at that location.
Floodway Analysis The final step in preparing most riverine flood studies is the floodway analysis. New Terms Floodway Flood fringe
Division of the floodplain into two separate parts, the floodway and the flood fringe, recognizes their natural functions. The floodway is the stream channel and that portion of the adjacent floodplain that must remain open and unobstructed to permit passage of the base flood. The floodwater is deepest and swiftest in the floodway, and structures and other uses located in this area are subject to the greatest danger during times of flooding. The remainder of the floodplain is called the flood fringe. Here the water is shallower and may have little or no movement. Therefore, most communities permit development in the flood fringe portion of the floodplain if the development is elevated or otherwise protected to the base flood level (or any higher state or local standards). A floodway analysis is based on the following floodplain management concepts:
4-23
Information and Data
November 1997
a. Continued development in the floodplain will likely further obstruct flood flows which will back water up or divert it to other properties. b. Properties on both sides of a river or stream should be treated equitably. The degree of obstruction permitted now for one should be permitted in the future for the other. c. Property owners should be allowed to develop their land provided they do not obstruct flood flows and cause damage, or create a nuisance, to others. A community may allow development in the flood fringe that cumulatively increases the base flood elevation, but NFIP regulations specify that such total increases cannot exceed one foot at any point along the stream. Some states or communities have more restrictive standards that must be met. The floodway is that area that must be kept open to allow the passage of the base flood. NFIP minimum standards provide that other areas outside the boundaries of the floodway can be developed, as long as the cumulative impact of the developments do not raise the base flood by more than one foot (or to a more restrictive state or local standard). A floodway analysis identifies where encroachment by development can and cannot be allowed. The analysis is usually done utilizing a computer program that can make the necessary calculations of the effects of further development. The model assumes that ultimately total development of the flood fringe will occur, restricting water from flowing through the developed area, and requiring all floodwater to flow through the floodway (see Figure 4-14). Beginning at both edges of the floodplain, the computer model starts “filling” the floodplain. This critically “squeezes” the floodwater toward the channel and causes the flood level to rise. At the point where this process reaches a one-foot rise, the floodway boundaries are drawn. The floodway boundaries at each cross section are transferred to a map. The plotted points are connected to delineate the floodway and flood fringe on the floodplain map. States and communities have the option of using a more restrictive standard for delineating a floodway. Some may allow only a 0.5-foot or 0.1-foot rise in the 100-year flood elevation in the floodway analysis. This results in wider floodways and less “developable” area in the flood fringe.
4-24
November 1997
Information and Data
Figure 4-14. Computer Floodway Analysis
In the older Flood Insurance Studies, mapped floodways are shown on the Flood Boundary and Floodway Map (FBFM), also called a Floodway Map. Check the map key in the upper right corner on the map. The floodway is shown as a white area on each side of the channel. The flood fringe is shown as a dark gray, the same color as the floodplain shown on the FIRM. The floodway is usually wider in flatter floodplain areas and narrower in steeper areas. One problem with this method of delineating floodways is that sometimes people confuse the white floodway with the white area that represents land free from flooding. Also, because the floodway was mapped separately, often property owners, lenders, real estate agents, and others did not have easy access to the floodway maps and did not know of the severe flood hazard associated with the floodway. Newer Flood Insurance Studies have corrected this problem—they do not have separate FIRM and Floodway Maps. Floodways are delineated on the newer FIRMs as a diagonally hatched area. A floodway is a floodplain management tool prepared through close coordination between the modeling engineer and those responsible for community planning and floodplain management. The number of possible floodway configurations is almost limitless. Therefore, in choosing the regulatory configuration, the interest of both individual property owners and the community as a whole must be weighed. 4-25
Information and Data
November 1997
Coastal Flood Studies FLOOD INSURANCE STUDY
TOWN OF MOREHEAD CITY, NORTH CAROLINA CARTERET COUNTY
APRIL 18, 1983
The basic steps to a coastal study are described on the following pages. Because coastal studies do not have floodways, all of the data needed are shown in the Flood Insurance Study report and on the Flood Insurance Rate Map. Note that coastal communities, particularly counties, may also have riverine floodplains with designated floodways.
Hydrology
COMMUNITY NUMBER - 370048 B
Most coastal floods result from storm surge and wave action caused by coastal storms, usually hurricanes and northeasters. The storms bring very strong winds that “pile” water up against the shore. Air pressure changes add to this change in water level that is called a New Terms storm surge. This results in the ocean being higher in some areas than in other areas. Storm surge AStillwater computer simulation of a coastal storm is developed based on data from elevation past storms and past flood heights. For coastal North Carolina, the computer model uses data on wind speeds, wind direction, and air pressure from historical hurricanes. The results are correlated with the odds or chances of whether the ocean will be at high or low tide during the storm. The coastal hydrology computer program produces stillwater elevation, (i.e., the elevation of various coastal floods, not counting waves). The computer model is checked or “calibrated” by being able to reproduce the historical stillwater elevations. The program extrapolates the base flood stillwater elevation from the historical data.
Waves New Terms Wave crest elevation Wave envelope New Terms Wave runup Transect 4-26
In addition to storm surge, wave action is an important aspect of coastal storms. Wind causes waves which push water higher than the stillwater elevation. This is called wave setup. The coastal flood study analyzes how much higher the wave crest elevation will be above the stillwater elevation as water is driven onshore. The term wave envelope is often used in place of wave crest elevation. When the waves hit the shoreline, the water is moving with such force that it keeps moving inland. This is called wave runup, i.e., when areas on land above the stillwater elevation are flooded. Wave runup elevations are typically one to two times the height of the wave.
Information and Data
November 1997
Hydraulic Analysis As with riverine studies, a coastal hydraulic analysis also determines where moving water goes. Instead of surveying a cross section, the coastal flood engineer’s survey data is called a transect. Figure 4-15 shows surveyed transects in a typical flood insurance study.
Figure 4-15. Transect Locations
A transect shows the elevation of the ground, both onshore and offshore. The ground elevation data are used by the computer program to determine what happens to the stillwater and wave crest (wave runup) elevations. A typical transect schematic is shown in Figure 4-16. Underwater
4-27
Information and Data
November 1997
topography, called bathymetry, and the shape and location of coastal islands, headlands, estuaries, harbors, etc., are also entered into the modeling. Note that the boundary between the V Zone and the A Zone is where the wave height is greater than 3 feet. Also note that the base flood elevation is comprised of the 100-year stillwater elevation and the wave crest elevation.
Figure 4-16. Typical Transect Schematic
Coastal High Hazard Area
New Term V Zone
Waves pack a lot of power. They are much more destructive than standing or slow-moving water and that power increases exponentially with their height. For the purposes of the NFIP, the flood study identifies the “coastal high hazard area” as that part of the coastal floodplain where the wave heights during the base flood will be three feet or more. These areas are designated as “V Zones” where the “V” stands for “velocity wave action.” V Zones are subject to more stringent regulatory requirements and a different flood insurance rate structure due to increased degree of risk. The three-foot wave height threshold was selected because it was found to be the smallest wave with enough power to break a wall panel away from a floor to which it has been nailed. Coastal flood areas with wave heights of less than three feet are mapped as AE or A# Zones with the BFE indicated in parentheses.
4-28
Information and Data
November 1997
Figure 4-16 shows how the BFE with wave effects and the 100-year stillwater elevation get closer to each other as the water goes inland (toward the right side of the illustration). Typically, obstructions such as dunes or buildings break the waves, dissipating wave energy so that wave height and BFEs are reduced as one goes inland. Coastal high hazard areas are delineated on FIRMs based on transect data and topographic mapping.
Coastal Floodplain Map The coastal flood engineer now has accurate stillwater elevation and wave height data at the transects. As with a riverine flood study, the next job is to transfer the elevation data to the best available topographic map. Flood elevations between transects are interpolated, taking into consideration local topography. An example of a coastal floodplain map is the Town of Wrightsville Beach Flood Insurance Rate Map, a portion of which is shown in Figure 4-17. The applicable BFE is shown on the map in parentheses below the zone designation.
Figure 4-17. Coastal Floodplain Map
Shallow Flooding Studies For the NFIP, shallow flooding is defined as flooding with an average depth of one to three feet in areas where a clearly defined channel does not exist. Shallow flooding can exist in any of the following situations:
4-29
Information and Data
New Terms Base flood depth Zones AO or AH
November 1997
•
In flat areas, water collects or “ponds” in depressions. This is known as “ponding.”
•
In steeper areas where there are no defined channels or on flat plains, water will spread out over the land surface. This is known as “sheet flow.”
•
In coastal areas, wave runup will send water inland over flat areas or over dunes. Often it may collect or pond behind an obstruction which keeps it from draining back into the ocean.
For the purposes of the NFIP, shallow flooding is a type of flooding that is distinguishable from riverine or coastal flooding. Therefore, areas such as the edges of riverine or coastal floodplains where the base flood is less than three feet deep are not designated as shallow flooding. Shallow flooding is mapped based on historic flood experiences and a study of the topography. In some areas, the techniques used for riverine studies are used. The result will either be a BFE (i.e., in NGVD) or a base flood depth, i.e., in feet above the ground (2 feet in Figure 4-18). A shallow flooding study will usually only produce data for the base flood and not for the 10-year or other floods. Shallow flooding areas are denoted as Zones AO or AH.
D. Approximate Studies Part C of this unit explained what is called a detailed riverine, coastal, or shallow flood study (all three types may be included in a community’s flood insurance study). Detailed studies produce BFEs. In riverine areas, they usually produce a floodway map. In coastal areas, they identify wave heights and coastal high hazard areas (V Zones). Detailed studies are Figure 4-18. FIRM with Shallow Flooding Area 4-30
November 1997
Information and Data
expensive; for example, a riverine study typically costs $5,000 to $10,000 per mile. It is not cost effective to spend that kind of money to produce BFEs where there is presently little or no development and none is anticipated. Therefore, many FIRMs include floodplains mapped using approximate study methods. Also, many Flood Hazard Boundary Maps have been converted to FIRMs without a detailed Flood Insurance Study. These are usually rural areas or areas where little development is occurring. In addition, many flooding sources have been studied by other federal, state, or local agencies. Such studies that do not meet the NFIP standards for a Flood Insurance Study often contain valuable flood hazard information, which may be incorporated into NFIP maps as approximate studies. Those types of studies, referred to as “existing data studies,” typically cover developed or developing areas. They often contain flood elevation profiles that can be used as “best available data.” Use of these data will be discussed in later units. APPROXIMATE FLOODPLAIN • No floodways defined • No BFEs • Need other data source
An approximate study shows the estimated boundary of the Special Flood Hazard Area. It does not show a BFE and there are no elevation data in the FIS report. The boundary was drawn on the best available topographic map based on knowledge of past floods and the judgement of experienced engineers. In Figure 4-19, Zone A corresponds to the 100-year or base floodplain determined by approximate methods. This is often called an unnumbered A Zone or an approximate A Zone. According to NFIP requirements, federal, state, and other sources of flood data are to be obtained, when possible, in determining floodways and BFEs in approximate floodplain areas. See Managing Floodplain Development in Approximate A Areas: A Guide for Obtaining and Developing Base (100-Year) Flood Elevations (FEMA-265, 1995), available by contacting the DEM, the FEMA regional office, or the Internet at http://www.fema.gov/home/library/index.htm. Portions are contained in Appendix F.
4-31
Information and Data
November 1997
Figure 4-19. Unnumbered A Zone
Requirements for Developing BFE Data Under 44 CFR §60.3(b)(3) of the NFIP regulations, communities must: Require that all new subdivision proposals and other proposed development (including proposals for manufactured home parks and subdivisions) greater than 50 lots or 5 acres, whichever is the lesser, include within such proposals BFE data. This means that any subdivision which meets this threshold must be evaluated to determine if the subdivision proposal is affected by an approximate Zone A area and whether BFE data is required. BFE data is required for the affected lots in the subdivisions shown in Figure 4-20 and Figure 4-21. Figure 4-20 shows a 76-lot subdivision with several lots clearly affected by an approximate Zone A area. The subdivision depicted in Figure 4-21 is only 12 lots, but because the subdivision is greater than 5 acres and clearly shows buildable sites affected by an approximate Zone A area, BFE data is required.
4-32
Information and Data
November 1997
Figure 4-20. Proposed 76-Lot Subdivision
Figure 4-21. Proposed 6.7-Acre Subdivision
Communities are encouraged to address the flood hazards at the earliest stages of subdivision planning rather than at the actual placement of individual structures. If a community can work with the developer and others when land is being subdivided, many long-term floodplain management benefits can be achieved, particularly if the floodplain is avoided altogether. In Figure 4-22, “Proposed 76-Lot Subdivision,” the entire approximate Zone A area is to be dedicated as open space. If the planned subdivision shows the floodplain is contained entirely within an open space lot, it may not be necessary to conduct a detailed engineering analysis to develop BFE data.
4-33
Information and Data
November 1997
Figure 4-22. Proposed 76-Lot Subdivision
Also, it may not be necessary to develop detailed BFE data in large-lot subdivisions or single-lot subdivisions that are within the thresholds in 44 CFR §60.3(b)(3) of the NFIP regulations when the actual building sites are clearly outside of the Zone A area. In Figure 4-23, it is evident from the topographic features of this 5.6-acre subdivision that the building sites would be clearly out of the floodplain since the proposal indicates a steep grade between the approximate Zone A area and the building sites which are located on natural high ground. Figure 4-23. Proposed 5.6-Acre Subdivision
4-34
November 1997
Information and Data
If the grade between the actual building sites and the approximate Zone A area of the proposed subdivision is relatively gradual, as shown in Figure 4-24, the floodplain could extend beyond what is shown on the Flood Insurance Rate Map. It is very likely that flooding could affect the building sites. In this case, an analysis should be conducted to determine the location of the 100-year floodplain and the BFE.
Figure 4-24. Proposed 6.7-Acre Subdivision
4-35
Information and Data
November 1997
Unit Learning Exercise Purpose:
To review what you have learned in this unit.
Directions: Answer the following questions. 1.
The three components of a Flood Insurance Study are a a , and a
.
2.
The Flood Insurance Rate Map is used for
purposes.
3.
The Flood Boundary and Floodway Map is used for
purposes.
4.
The new format FIRMs can now be used by
,
. 5.
Cross sections are . They are shown on
maps
and on 6.
A Flood Insurance Study produces elevation for the and -year floods.
7.
Elevation reference marks are
maps. -,
-,
,
. They are used by to determine elevations at nearby locations, such as first floor elevations at development sites. 8.
A flood profile is used by the local administrator and others to .
9.
In riverine situations the floodplain is divided into two separate parts for floodplain management purposes. These parts are the and the .
. It is
10. Strict controls over further development within the floodway exist because here floodwaters are and further development will . 11. NFIP regulations specify that floodway boundaries be established so that the BFE is not increased more than at any point along the stream. 4-36
Information and Data
November 1997
12. Most coastal flooding result from caused by coastal storms, such as hurricanes. 13. In coastal areas, the BFE is comprised of the elevations.
and
,
and
14. A transect shows
.
15. For the purpose of the NFIP, the coastal high hazard area (V Zone) is that part of the coastal floodplain where wave heights are . 16. Instead of using a BFE, some shallow flooding areas are defined using a base flood . 17. An approximate study just shows the base
.
18. The map used to show coastal high hazard areas (V Zones) is the .
4-37
Information and Data
November 1997
Answers to Unit Learning Exercise
1.
The three components of Flood Insurance Study are a study report, a Flood Insurance Rate Map, and a Flood Boundary and Floodway Map.
2.
The Flood Insurance Rate Map is used for flood insurance purposes.
3.
The Flood Boundary and Floodway Map is used for floodplain management purposes.
4.
The new format FIRMs can now be used by community officials for floodplain management, lenders and insurance agents to determine the need for flood insurance and to rate policy applications, and by land surveyors, engineers, property owners, etc.
5.
Cross sections are surveys to obtain the shape of the floodplain, i.e., its ground elevations and obstructions. They are shown on Flood Boundary and Floodway maps and on new format Flood Insurance Rate maps.
6.
A Flood Insurance Study produces elevation for the 10-, 50-, 100-, and 500-year floods.
7.
Elevation reference marks are permanent references to which elevation data are referred, usually in terms of NGVD. They are used by surveyors to determine elevations at nearby locations, such as first floor elevations at development sites.
8.
A flood profile is a plotting on graph of computed flood elevations at cross sections and connecting the plotted points with straight lines. It is used by the local administrator and others to determine flood elevations at any point along the river or stream.
9.
In riverine situations the floodplain is divided into two separate parts for floodplain management purposes. These parts are the floodway and the flood fringe.
10. Strict controls over further development within the floodway exist because here floodwaters are deep, swift, and the most dangerous and further development will increase flood levels on other properties. 11. NFIP regulations specify that floodway boundaries be established so that the BFE is not increased more than 1 foot at any point along the stream. 12. Most coastal flooding result from storm surge and wave action, caused by coastal storms, such as hurricane. 13. In coastal areas, the BFE is comprised of the 100-year stillwater and wave crest elevations. 4-38
November 1997
Information and Data
14. A transect shows the elevation of the ground, both onshore and offshore. 15. For the purpose of the NFIP, the coastal high hazard area (V Zone) is that part of the coastal floodplain where wave heights are three feet or more. 16. Instead of using a BFE, some shallow flooding areas are defined using a base flood depth. 17. An approximate study just shows the base floodplain. 18. The map used to show coastal high hazard areas (V Zones) is the Flood Insurance Rate Map.
4-39
Overview This unit describes the Flood Insurance Study reports and floodplain maps that the NFIP provides to communities to assist them in carrying out a local floodplain management program.
Contents A. Introduction.............................................................................................................................. 3 B. Flood Insurance Study Reports and Maps............................................................................. 4 Flood Hazard Boundary Map............................................................................................................4 Flood Insurance Study Report ..........................................................................................................6 Flood Insurance Rate Map ................................................................................................................7 Flood Boundary and Floodway Map ................................................................................................8 FIRM New Format............................................................................................................................9 Countywide FIRM ..........................................................................................................................11 Digital FIRM...................................................................................................................................11 Digital Flood Insurance Rate Map....................................................................................11 Digital Flood Insurance Rate Map—Digital Line Graph .................................................12 Q3 Flood Data ..................................................................................................................14
C. Detailed Studies ...................................................................................................................... 16 Riverine Studies ..............................................................................................................................16 Cross Sections...................................................................................................................16 Elevation Reference Marks...............................................................................................18 Hydrology.........................................................................................................................19 Hydraulics.........................................................................................................................20 Profile ...............................................................................................................................20 Floodplain Map.................................................................................................................22 Floodway Analysis ...........................................................................................................23 Coastal Flood Studies .....................................................................................................................26 Hydrology.........................................................................................................................26 Waves ...............................................................................................................................26 Hydraulic Analysis ...........................................................................................................27
Information and Data
November 1997
Coastal High Hazard Area ................................................................................................28 Coastal Floodplain Map....................................................................................................29 Shallow Flooding Studies ...............................................................................................................29
D. Approximate Studies.............................................................................................................. 30 Requirements for Developing BFE Data ........................................................................................32
Unit Learning Exercise................................................................................................................ 36 Answers to Unit Learning Exercise............................................................................................ 38
References Federal Emergency Management Agency 1995 Managing Floodplain Development in Approximate Zone A Areas: A Guide for Obtaining and Developing Base (100-Year) Flood Elevations. Includes the computer program QUICK-2: Computation of Water Surface Elevations in Open Channels, Version 1.0. FEMA 265.
4-2
1995
How to Use a Floodmap to Determine Flood Risk for a Property: A Guide for Interested Private Citizens, Property Owners, Community Officials, Lending Institutions, and Insurance Agents. FEMA 258.
1995
Flood Insurance Study: Guidelines and Specifications for Study Contractors. FEMA 37.
November 1997
Information and Data
A. Introduction The planning and implementation of floodplain management measures requires a solid technical basis in order for there to be effective flood damage reduction and to ensure equitable treatment of all concerned citizens. Information is needed on the magnitude and frequency of flooding for a specific site or area, i.e., which areas are subject to flooding, to what depth, and how often do floods occur. The Flood Insurance Study (report and maps) provides the technical basis for implementation of floodplain management programs. The National Flood Insurance Act of 1968 directed the Federal Insurance Administration (FIA) to: •
Identify all floodprone areas within the United States
•
Establish flood-risk zones within floodprone areas
These two goals were established in an effort to reduce loss of life and property, as well as disaster relief costs, by: •
Guiding future development away from these flood hazard areas
•
Requiring that new and substantially improved building in these areas be constructed to resist flood damages
An important step toward meeting these goals is the preparation of Flood Insurance Study reports (FIS) and floodplain maps for floodprone communities. These studies provide sufficient technical information to •
Enable communities to adopt floodplain management measures required to participate in the NFIP
•
Designate floodprone areas and determine flood elevations and flood risk zones needed to establish actuarial flood insurance premiums
4-3
Information and Data
November 1997
B. Flood Insurance Study Reports and Maps Flood Hazard Boundary Map
New Term FHBM
In keeping with the 1968 Congressional directive, initial study efforts in the Emergency Phase of the National Flood Insurance Program were focused on identifying all floodprone areas within the United States. Utilizing flood data and floodplain information from a variety of sources (e.g., soils mapping, actual high water profiles, aerial photographs of previous floods, topographic maps, etc.), the approximate outline of the extent of the one-percent-chance (base or 100-year) floodplain for specific stream reaches was overlaid on available community maps, usually United States Geological Survey topographic quadrangle maps. In this manner, Flood Hazard Boundary Maps (FHBM) were prepared, without benefit of detailed studies or hydraulic analyses, for nearly all floodprone communities in the nation (over 18,000). However, some communities were not mapped initially, about 214 in North Carolina alone. On the FHBM, the Special Flood Hazard Area (SFHA) is designated as a shaded area labeled “Zone A,” and no flood elevations are given (see Figure 4-1). The FHBMs are often referred to as “flat maps” because they were published in a rectangular 11” x 17” format that was folded in the middle. More detailed mapping is published in a Z-fold format. These FHBMs were intended for interim use in most communities until more detailed studies could be carried out.
New Terms Detailed studies Approximate studies
As money was appropriated by Congress, FEMA performed more detailed studies for many communities, resulting in the publication of Flood Insurance Study reports and Flood Insurance Rate Maps (FIRMs). These subsequent studies were carried out to provide communities with the information and data needed to adopt and implement local floodplain management measures and to enter the Regular Phase of National Flood Insurance Program. These studies can be grouped into two broad categories: detailed studies and approximate studies. Detailed studies were carried out for developed communities and for those areas experiencing rapid growth. They are used to guide future development within flood hazard areas and to provide the information needed for new construction allowed in already developed areas. While Congress appropriated money for the original flood studies in the 1970s and 1980s, all floodplain mapping is now funded out of NFIP insurance policy service charges. Funds allocated annually for mapping
4-4
November 1997
Information and Data
are very limited. In North Carolina, typically only one to two studies are funded per year.
New Terms Zone A Unnumbered A Zone
Approximate studies are utilized where there is presently little or no development, or expectation of development, in identified floodprone areas. In some cases FEMA simply converted the FHBM to a FIRM by issuing a letter to the community stating that the FHBM shall be considered a FIRM. In those cases the community was instructed to line out FHBM on the map’s title block and write in FIRM; the SFHAs were also formally converted to Zone A in communities with extremely low risk of flooding, little or no existing development in the floodplain, and little potential for future development. Zone A without a flood elevation is called an “unnumbered A Zone.”
Figure 4-1. Flood Hazard Boundary Map
4-5
Information and Data
November 1997
Flood Insurance Study Report
FLOOD INSURANCE STUDY A Flood Insurance Study (FIS) is a compilation and presentation of flood risk data for specific water courses within a municipality. The FIS report and associated maps delineate flood hazard areas and establish flood elevations, thereby serving as the basis for regulating floodplain development and providing flood insurance.
A detailed Flood Insurance Study provides the data and maps needed for both the flood insurance and floodplain management aspects of the NFIP. The study consists of three components: •
The Flood Insurance Study report, (the “FIS”)
•
The Flood Insurance Rate Map (the “FIRM”)
•
The Flood Boundary and Floodway Map (the “FBFM” or “Floodway Map,” included in studies prepared before 1985)
A flood insurance study (FIS) is a compilation and presentation of flood risk data for specific watercourses, lakes, and coastal flood hazard areas within a municipality or other local unit of government. It delineates flood hazard areas, designates flood risk zones, and establishes base flood elevations, thereby serving as the basis for providing flood insurance and for regulating floodplain development and carrying out other floodplain management measures. The Flood Insurance Study report includes: •
An appraisal of the community’s flood problems in a narrative that describes the purpose of the study, historic floods, the area and streams studied, the engineering methods employed to determine flood-hazard areas, and use of the study for both floodplain management and flood insurance purposes
•
Maps of the study area and, often, photographs of historic floods
•
Tables summarizing various study data
•
Computed flood profiles of the 10-, 50-, 100-, and 500-year floods for the stream reaches studied
The FIRMs and FBFMs, which are an essential part of the study, portray:
4-6
•
Floodplain boundaries
•
Delineation of floodways, where applicable
•
Designation of flood insurance risk zones
November 1997
Information and Data
Flood Insurance Rate Map New Term FIRM
The Flood Insurance Rate Map (FIRM) is an essential product of the FIS which shows floodplain boundaries and flood data for insurance purposes (see example in Figure 4-2). The FIRM: •
Identifies the Special Flood Hazard Area (SFHA), and often the 500year flood zone
•
Shows flood risk zones
•
Shows base flood elevations
•
Identifies the location of elevation reference marks
•
Shows certain landmark features in the community
•
Shows floodway boundaries and study cross sections on most maps prepared since 1985
Figure 4-2. Flood Insurance Rate Map
4-7
Information and Data
November 1997
There are five types of FIRMS: •
A flat FIRM, which is in black and white on 11” x 17” panels, was the format used in older studies. Often this may simply be a “converted FBFM.
•
Z-fold FIRMs replaced flat FIRMs in the late 1970’s. They come in large panels and are folded up like a road map; most FIRMs are in this format. The floodplain is shown as a blue or gray shaded area. The SFHA is a darker shade, while the 500-year floodplain is lighter. Older FIRMS refer to the SFHA as Zones A1-A30, and refer to the 500-year floodplain as a Zone B. Minimally floodprone areas are called Zone C.
•
Newer FIRMs have AE and X Zones and floodways.
•
Countywide FIRMs are Z-fold FIRMs that include all the cities, villages, and towns in a county, as well as the unincorporated areas.
•
Digital or DFIRMs are computer generated from GIS databases.
Flood Boundary and Floodway Map New Term FBFM
The Flood Boundary and Floodway Map (FBFM) contains the data needed by the local permit official to carry out regulation of the floodplain areas and for other community floodplain management measures. These maps were included in flood insurance studies started before 1985, and, in some instances, in studies after that date. The FBFM looks like the FIRM because it shows some of the same information. The Flood Boundary and Floodway Map (Figure 4-3) shows: •
Floodplain boundaries
•
The floodway (described later) for rivers and streams, shown in white down the middle of the floodplain
•
Study cross sections
The FBFM does not show:
4-8
•
Base flood elevations (BFEs, described in Part C of this unit)
•
Flood risk zones
November 1997
Information and Data
It is essential that communities use both the FBFM and the FIRM when administering the NFIP regulations.
Figure 4-3. Flood Boundary and Floodway Map
FIRM New Format Since Flood Insurance Studies were first prepared in the late 1960s, many improvements have been made to the design of flood maps so that they are easier to use and meet user’s needs. Starting in 1985, the following changes have been made to the FIRM: •
Inclusion of floodway and other floodplain management information that was previously provided on separate Flood Boundary and Floodway Maps (FBFMs). (Except in a few instances, new FBFM maps are no longer being prepared.) All floodplain management and flood insurance information are now included on one map, the FIRM.
4-9
Information and Data
November 1997
•
Simplified flood insurance zone designations (e.g., the previous Zones A1-A30 and V1-V30) have been replaced by the designations AE and VE; Zones B and C are replaced by Zone X. The 500-year floodplain is still shown as “shaded” portions of Zone X.
•
Identification of undeveloped portions of barrier islands and adjoining areas designated as units in the Coastal Barrier Resources System, established by Congressional Acts in 1982 and 1990, and subsequent revisions, called “COBRA zones.” These areas are identified because no new flood insurance coverage may be provided after specified dates for new or substantially improved structures on any coastal barrier in the system.
New Terms Zone X COBRA zones
The above features appear on newer FIRMs. Figure 4-4 shows an example of a FIRM illustrating some of the above changes. With these changes, the FIRMs can more easily be used by community officials for floodplain management, by lenders to determine the need for flood insurance, by insurance agents to rate policy applications, and by land surveyors, engineers, property owners, etc. In other words, all the players are using the same map and flood information.
Figure 4-4. FIRM (New Format)
4-10
November 1997
Information and Data
Countywide FIRM
New Term Countywide FIRM
Since the late 1980s, many flood insurance restudies have been produced in a countywide FIRM format. This makes it easier for administration because the SFHAs outside a municipal boundary or extraterritorial jurisdiction (ETJ) are also shown. Thus, the municipality knows what SFHAs are being included in any annexation or extension of the ETJ. Figure 4-5 shows a portion of the countywide FIRM for Wake County, with the corporate limits and ETJ of the Town of Cary, as well as unincorporated county areas. Countywide FIRMs have their own 6-digit map ID number ending in “C,” e.g., 37153C for Richmond County. Figure 4-5. Countywide FIRM
Digital FIRM Digital Flood Insurance Rate Map The Digital Flood Insurance Rate Map (DFIRM) is intended to be the primary means of transferring flood-risk data depicted by FIRMs to Geographic Information Systems (GIS) through a public domain data 4-11
Information and Data
November 1997
exchange format. The DFIRM is generally produced in a countywide format, where all flood hazards for the county and incorporated communities are shown on one set of maps. It can be used for floodplain management purposes in a manner similar to other flood maps. The DFIRM is comprised of all digital data required to create the hardcopy FIRM. This includes base map information, graphics, text, shading, and other geographic and graphic data. An example of a hardcopy paper DFIRM is shown in Figure 4-6.
Figure 4-6. Hard Copy DFIRM
Digital Flood Insurance Rate Map—Digital Line Graph The Digital Flood Insurance Rate Map—Digital Line Graph (DFIRMDLG) is a database created by extracting certain flood risk data from the DFIRM. The DFIRM-DLG does not include base map information, nor does it include graphic data required to create a hardcopy FIRM. Communities whose digital base mapping files were utilized as the base map for the DFIRM will find that they may easily use the DFIRM-DLG files for determination of flood zones and for enforcement of regulations. A graphic image of a DFIRM-DLG is shown in Figure 4-7.
4-12
November 1997
Information and Data
Figure 4-7. Graphic Image of a DFIRM-DLG
The digital data captured from the hardcopy paper DFIRM consists of FEMA hydrography, flood hazard zones, base flood elevations, cross section locations, and elevation reference marks. All lines and area features in DLG files are encoded with one or more seven-digit attribute codes that provide the user with detailed information about the features. FEMA intends to make the DFIRM-DLG available on CD-ROM compatible with Insurance Services Office (ISO) 9660 standards and over the Internet. Many commercially available GIS software packages allow the direct conversion of DLG data into vector data usable within the GIS environment. Third party conversion software is also available that will convert DLG data to other proprietary GIS formats not supported with their own conversion software. 4-13
Information and Data
November 1997
The conversion of FIRMs to a digital format is expected to have many benefits. However, users must bear in mind that the simple conversion of FIRMs to a digital format does not inherently improve the engineering quality of the product. Many of the same difficulties with interpretation of flood risk data and the requirement that users apply sound judgment in methods selected for decision making and map interpretation remain unchanged. The DFIRM-DLG, when coupled with digital base map files or the local community digital base, can be used in a GIS to determine whether a structure is located within a Special Flood Hazard Area. It should be noted that if a structure is determined to be within or near the area by using a GIS, and a different base map source was used to generate the hardcopy DFIRM, this determination should be confirmed by referencing the printed hardcopy DFIRM. The DFIRM-DLG may also be used for engineering and planning studies.
Q3 Flood Data New Term
Q3
In the Q3 Flood Data product, FEMA has developed a graphical representation of certain features of the FIRM. The Q3 flood data are in three formats that are usable with desk-top mapping and GIS software packages. These formats are: •
Digital Line Graph
•
ARC/INFO
•
MapInfo
The Q3 flood data are created by scanning (raster or grid data files) the FIRM paper maps and vectorizing (converting to lines and areas) select data features onto one countywide data layer. The following vectorized (lines and areas) data features are included:
4-14
•
1% and 0.2% annual chance flood boundaries (SFHA and 500-year floodplain)
•
Flood insurance zone designations
•
Floodway boundaries (if available)
•
Coastal Barrier Resources System boundaries
•
Political boundaries
November 1997
Information and Data
•
Community/map panel identification numbers
•
Boundaries between FIRM panels (called neatlines)
•
U.S. Geological Survey 7.5 minute (1:24,000 scale) quadrangle neatlines
The following features are not included: •
Hydrographic features
•
Base flood elevations
•
Cross section lines
•
Roads, road names, or address ranges
•
Elevation reference mark locations and elevations.
FEMA has produced Q3 Flood Data for approximately 900 counties nationwide, 53 in North Carolina. The data are distributed on CD-ROM and over the Internet from FEMA’s Map Service Center. The Q3 flood data are designed to be used for a variety of types of planning applications including: broad-based review for floodplain management, land-use planning, commercial site analysis, insurance target marketing, natural resource and environmental analyses, and real estate development targeting and planning. The data are designed to provide guidance and a general proximity of the location of Special Flood Hazard Areas. The data are not suitable for applications such as detailed site design and development plans or flood risk determinations. They cannot be used to determine absolute delineations of flood boundaries, but instead should be seen as portraying zones of uncertainty and possible risks associated with flood inundation. Any questions regarding digital FIRM data products may be directed to: Federal Emergency Management Agency Mitigation Directorate Q3 Flood Data Program 500 C Street, S.W. Washington, DC 20472
4-15
Information and Data
November 1997
C. Detailed Studies Floods come in different sizes. The NFIP regulations are written and applied to the base flood. The “base flood,” or “100-year flood,” is defined as a flood that has a one-percent chance of being equaled or exceeded in any given year, and is more properly termed the “one-percent annual chance flood.”
BASE FLOOD A flood that has a onepercent chance of being equaled or exceeded in any given year. It often is referred to as the 100year flood
The base, or 100-year, flood is a statistical concept denoting the average length of time between floods of this magnitude. The term does not mean that this flood will occur only once every 100 years. It is a flood which, over a very long period of time, can be expected to occur, on an average, once in each 100 years, although the flood may occur in any given year. The 100-year flood could conceivably occur more than once in a year, two or three years in a row, or it may not occur for several hundred years. Because this can be confusing, the NFIP program uses the term base flood. The elevation to which floodwater is anticipated to rise during the base flood is the base flood elevation (BFE). The 100-year flood was chosen for use as a compromise between excessive exposure to flood risk from using a lower standard (e.g., 25-year flood) and applying such a high standard (e.g., 1000-year flood) that it would be considered excessive and unreasonable for the intended purposes of requiring the purchase of flood insurance and regulating new development. An engineering study must be conducted to calculate and map the base flood. Flood studies are conducted differently for different types of flooding. This unit discusses how three types of flood hazards are studied and mapped: •
Riverine flooding of rivers, streams, ditches or other waterways
•
Coastal flooding caused by hurricanes or severe storms
•
Shallow flooding, ponding, and sheet flow
Riverine Studies Flood studies involving rivers, streams, or other areas that have defined channels follow a number of basic steps.
Cross Sections 4-16
November 1997
Information and Data
The first step in a riverine flood study is to examine the areas through which floodwater will flow. This requires a determination of ground elevations and obstructions (such as buildings, bridges, and other developments) for these areas. Accurate data on the shape of the floodplain is obtained from ground surveys, aerial photomaps, or topographic maps. New Term Cross section
Surveys are taken of cross sections of the floodplain at locations that are representative of local conditions. Cross section locations are shown on the Flood Boundary and Floodway Map and on the new format FIRMs with lines that have letters at each end (see Figure 4-8). The cross section lettering system starts downstream and goes upstream. Cross sections are taken at each bridge or other major obstruction and at other locations, depending on how much the channel or adjacent floodplain conditions change. At each cross section, the engineer has accurate information on the floodplain shape and the elevation of the ground, e.g., see the surveyed cross section in Figure 4-9.
New Term Roughness factor
The engineer also estimates the roughness factor along the floodplain to determine how fast floodwater will flow through the area. Roughness factors are related to ground surface conditions. They account for the fact that water will flow faster over mowed grass and pavement than it will over an area covered in bushes and trees, or planted in tall crops.
Figure 4-8. Cross Section Locations
4-17
Information and Data
November 1997
Figure 4-9. Surveyed Cross Section
Elevation Reference Marks New Terms Elevation reference mark Datum
A common reference (or elevation reference mark) is needed when referring to flood elevations to ensure uniformity of use and avoid any confusion. A datum is a reference surface from which elevations are measured. For the NFIP, datums are used to define the elevation of the base flood and the elevation of the lowest floor of a structure. Originally, mean sea level (MSL) was the accepted elevation reference datum. Because this datum is local in nature, it was not ideal for a nationwide program for flood insurance. During the 1920s, the U.S. government created a network of 21 tidal gauges in the U.S. and five in Canada to provide a fixed continental datum that would bring a consistent relationship to all vertical determinations in the U.S. This new datum, known as the National Geodetic Vertical Datum (NGVD) of 1929, is the base elevation to which all relief features and elevation data are referenced in the contiguous United States. NGVD is also the datum of reference for the vast majority of Flood Insurance Studies. An ultimate goal of the NFIP is to convert all FISs to a newer standard called the North American Vertical Datum (NAVD) of 1988. This standard eliminates inconsistencies caused by assuming that 0 NVGD is the same as mean sea level at all 26 tidal stations. There are now 600,000 permanent bench marks associated with the NAVD. See Flood Insurance Study: Guidelines and Specifications for Study Contractors, FEMA-37 (1995), for further information.
4-18
November 1997
Information and Data
Figure 4-10. Typical Bench Mark
Most permanent elevation reference marks (bench marks) are referenced to the NGVD (see Figure 4-10 for an example). Their locations are identified on flood maps with a small “x” and the designation “ERM” or “RM” followed by a number (see Figure 4-11 for an example). Descriptions of the marks, including their elevations, appear either on the FIRM panel, on the FIRM index, or in the Flood Insurance Study report. Reference marks are not always brass caps; they can be chiseled squares or other designated markers. The North Carolina Geodetic Survey has a comprehensive inventory of all bench marks in the state. Reference marks of known, recorded elevation are used by surveyors to determine elevations at nearby locations. For example, they can be used to establish first floor elevations at a development site and to determine whether a proposed site is subject to flood risk.
Figure 4-11. Elevation Reference Marks
Hydrology New Terms Discharge CFS Hydrology Hydraulics HEC-2
The amount of water that will flow downstream during a flood is termed the flood discharge. It is measured in cubic feet per second or cfs. A cubic foot of water is approximately 7.5 gallons. The science of determining water discharge is called hydrology. Discharges are calculated by utilizing hydrologic computer programs. Hydrological analysis utilizes information from historical stream records, rainfall and snowmelt data, ground cover, and slope. Runoff will vary depending on the ground characteristics, e.g., generally more runoff 4-19
Information and Data
November 1997
occurs on steeper slopes, on unforested land, and on paved and built-on urban lands.
Hydraulics HYDROLOGY A science that deals with the distribution and circulation of water in the atmosphere, on land surfaces, and underground. It is used to determine flood flow frequencies.
HYDRAULICS A science that deals with fluids in motion.
• It is used to determine how a quantity of water will flow through a channel or floodplain.
Hydraulics is the study of moving water. Hydraulic analysis combines (1) the flood hydrology, or discharge; (2) the cross section data on how much area there is to carry, or convey, the flood; and (3) then estimates how fast the flood will move through the floodplain. The data are usually processed using a computer model, HEC-2, developed by the U.S. Army Corps of Engineers. The result of the hydraulic study is the determination of the elevation of one or more flood levels (see Figure 4-9). A Flood Insurance Study produces elevations for the 10-, 50-, 100-, and 500-year floods. Elevations for the 10-year and 50-year floods are typically used for other floodplain management purposes, e.g., the 10-year flood for septic systems and the 50-year flood for bridges and culverts.
Profile New Term Flood profile
There is obviously a need for flood elevations at locations between the cross sections. This is done by plotting the elevations at the cross sections on a graph and connecting the plotted points. Such a graph is called a flood profile. The more changes there are in topography, the more cross sections are needed to reflect accurate base flood elevations. A flood profile is shown in Figure 4-12. The bottom of the graph shows the distance along the stream. The distance is measured above the mouth of the stream or above its confluence with another stream. The left side of the graph shows elevation. Bridges are indicated with an “I” shaped symbol. The bottom line of the “I” represents the elevation of the top of the bridge opening over the stream, and the top line represents the elevation of the bridge floor or top of rail. A flood elevation at any point along the stream can be determined by reading the profile. Flood profiles are located at the end of the Flood Insurance Study report (usually as foldout pages). FIS profiles show the streambed and four flood levels: the 10-, 50-, 100-, and 500-year floods. The bottom of the grid shows the distance upstream from a starting point. Elevations above NGVD are shown at the left edge.
4-20
Figure 4-12. Typical Flood Profile
Information and Data
November 1997
Floodplain Map New Terms Topographic map Contour map
The next step in the mapping process is to transfer the flood elevation data onto a map that shows ground elevation data. This is called a topographic map or contour map because points with the same elevation are connected by a contour line. The most common topographic maps used are those produced by the United States Geological Survey (USGS) at a scale of one inch equals two thousand feet with ten-foot contour intervals. Only within the last decade has FEMA started having detailed four-foot contour interval topographic mapping prepared along the floodplains as part of the Flood Insurance Study, e.g., Watauga and Transylvania County studies. Also, some communities have prepared their own detailed topographic mapping and provided it to FEMA to improve the accuracy of their new flood maps, e.g., Durham and Charlotte/Mecklenburg County. The BFEs from the cross sections and profile are plotted on the topographic map. Floodplain boundary lines are drawn connecting these plotted points using the contour lines as a guide. The result is a flood map (Figure 4-13). It is important to remember that floodplain map boundaries are only as accurate as the topographic map on which they are drawn. Since the USGS topographic quadrangle maps have so large a scale, the SFHA boundaries cannot be precisely mapped. Some judgment must be used in correlating map features with ground features because maps do not always represent exact conditions on the ground.
Figure 4-13. Topographic Floodplain Map
4-22
Information and Data
November 1997
FLOODWAY The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to allow the discharge of the base flood without increasing the water surface elevation more than a designated height.
Where there is an apparent discrepancy between flood boundaries shown on a map and actual ground conditions, rely on the elevation data to resolve the matter by locating the flood elevation on the ground via an elevation survey. This represents the actual extent of flooding for that particular flood. Elevation data are found in the Floodway Data Table, the flood profile, and on the Flood Insurance Rate Map (FIRM). To correct discrepancies, see Letters of Map Amendment in Unit 9. Only FEMA, not a surveyor or local administrator, can correct or revise an NFIP map. The NFIP’s floodplain map is called the Flood Insurance Rate Map or FIRM. The map key is located in the upper right corner of the FIRM. The plotted base or 100-year floodplain is shown as dark gray shading. Many FIRMs show the area between the 100-year and 500-year floodplain boundary as a light gray shaded area, sometimes as a Zone B or on newer FIRMs as a shaded portion of Zone X. The zone terminology and how to read a FIRM and other maps are covered later in Unit 5. The BFEs along rivers and streams are usually denoted on the FIRM using a wavy line that extends across the width of the floodplain. The number shown on the wavy line is the BFE, rounded to the nearest whole number, at that location.
Floodway Analysis The final step in preparing most riverine flood studies is the floodway analysis. New Terms Floodway Flood fringe
Division of the floodplain into two separate parts, the floodway and the flood fringe, recognizes their natural functions. The floodway is the stream channel and that portion of the adjacent floodplain that must remain open and unobstructed to permit passage of the base flood. The floodwater is deepest and swiftest in the floodway, and structures and other uses located in this area are subject to the greatest danger during times of flooding. The remainder of the floodplain is called the flood fringe. Here the water is shallower and may have little or no movement. Therefore, most communities permit development in the flood fringe portion of the floodplain if the development is elevated or otherwise protected to the base flood level (or any higher state or local standards). A floodway analysis is based on the following floodplain management concepts:
4-23
Information and Data
November 1997
a. Continued development in the floodplain will likely further obstruct flood flows which will back water up or divert it to other properties. b. Properties on both sides of a river or stream should be treated equitably. The degree of obstruction permitted now for one should be permitted in the future for the other. c. Property owners should be allowed to develop their land provided they do not obstruct flood flows and cause damage, or create a nuisance, to others. A community may allow development in the flood fringe that cumulatively increases the base flood elevation, but NFIP regulations specify that such total increases cannot exceed one foot at any point along the stream. Some states or communities have more restrictive standards that must be met. The floodway is that area that must be kept open to allow the passage of the base flood. NFIP minimum standards provide that other areas outside the boundaries of the floodway can be developed, as long as the cumulative impact of the developments do not raise the base flood by more than one foot (or to a more restrictive state or local standard). A floodway analysis identifies where encroachment by development can and cannot be allowed. The analysis is usually done utilizing a computer program that can make the necessary calculations of the effects of further development. The model assumes that ultimately total development of the flood fringe will occur, restricting water from flowing through the developed area, and requiring all floodwater to flow through the floodway (see Figure 4-14). Beginning at both edges of the floodplain, the computer model starts “filling” the floodplain. This critically “squeezes” the floodwater toward the channel and causes the flood level to rise. At the point where this process reaches a one-foot rise, the floodway boundaries are drawn. The floodway boundaries at each cross section are transferred to a map. The plotted points are connected to delineate the floodway and flood fringe on the floodplain map. States and communities have the option of using a more restrictive standard for delineating a floodway. Some may allow only a 0.5-foot or 0.1-foot rise in the 100-year flood elevation in the floodway analysis. This results in wider floodways and less “developable” area in the flood fringe.
4-24
November 1997
Information and Data
Figure 4-14. Computer Floodway Analysis
In the older Flood Insurance Studies, mapped floodways are shown on the Flood Boundary and Floodway Map (FBFM), also called a Floodway Map. Check the map key in the upper right corner on the map. The floodway is shown as a white area on each side of the channel. The flood fringe is shown as a dark gray, the same color as the floodplain shown on the FIRM. The floodway is usually wider in flatter floodplain areas and narrower in steeper areas. One problem with this method of delineating floodways is that sometimes people confuse the white floodway with the white area that represents land free from flooding. Also, because the floodway was mapped separately, often property owners, lenders, real estate agents, and others did not have easy access to the floodway maps and did not know of the severe flood hazard associated with the floodway. Newer Flood Insurance Studies have corrected this problem—they do not have separate FIRM and Floodway Maps. Floodways are delineated on the newer FIRMs as a diagonally hatched area. A floodway is a floodplain management tool prepared through close coordination between the modeling engineer and those responsible for community planning and floodplain management. The number of possible floodway configurations is almost limitless. Therefore, in choosing the regulatory configuration, the interest of both individual property owners and the community as a whole must be weighed. 4-25
Information and Data
November 1997
Coastal Flood Studies FLOOD INSURANCE STUDY
TOWN OF MOREHEAD CITY, NORTH CAROLINA CARTERET COUNTY
APRIL 18, 1983
The basic steps to a coastal study are described on the following pages. Because coastal studies do not have floodways, all of the data needed are shown in the Flood Insurance Study report and on the Flood Insurance Rate Map. Note that coastal communities, particularly counties, may also have riverine floodplains with designated floodways.
Hydrology
COMMUNITY NUMBER - 370048 B
Most coastal floods result from storm surge and wave action caused by coastal storms, usually hurricanes and northeasters. The storms bring very strong winds that “pile” water up against the shore. Air pressure changes add to this change in water level that is called a New Terms storm surge. This results in the ocean being higher in some areas than in other areas. Storm surge AStillwater computer simulation of a coastal storm is developed based on data from elevation past storms and past flood heights. For coastal North Carolina, the computer model uses data on wind speeds, wind direction, and air pressure from historical hurricanes. The results are correlated with the odds or chances of whether the ocean will be at high or low tide during the storm. The coastal hydrology computer program produces stillwater elevation, (i.e., the elevation of various coastal floods, not counting waves). The computer model is checked or “calibrated” by being able to reproduce the historical stillwater elevations. The program extrapolates the base flood stillwater elevation from the historical data.
Waves New Terms Wave crest elevation Wave envelope New Terms Wave runup Transect 4-26
In addition to storm surge, wave action is an important aspect of coastal storms. Wind causes waves which push water higher than the stillwater elevation. This is called wave setup. The coastal flood study analyzes how much higher the wave crest elevation will be above the stillwater elevation as water is driven onshore. The term wave envelope is often used in place of wave crest elevation. When the waves hit the shoreline, the water is moving with such force that it keeps moving inland. This is called wave runup, i.e., when areas on land above the stillwater elevation are flooded. Wave runup elevations are typically one to two times the height of the wave.
Information and Data
November 1997
Hydraulic Analysis As with riverine studies, a coastal hydraulic analysis also determines where moving water goes. Instead of surveying a cross section, the coastal flood engineer’s survey data is called a transect. Figure 4-15 shows surveyed transects in a typical flood insurance study.
Figure 4-15. Transect Locations
A transect shows the elevation of the ground, both onshore and offshore. The ground elevation data are used by the computer program to determine what happens to the stillwater and wave crest (wave runup) elevations. A typical transect schematic is shown in Figure 4-16. Underwater
4-27
Information and Data
November 1997
topography, called bathymetry, and the shape and location of coastal islands, headlands, estuaries, harbors, etc., are also entered into the modeling. Note that the boundary between the V Zone and the A Zone is where the wave height is greater than 3 feet. Also note that the base flood elevation is comprised of the 100-year stillwater elevation and the wave crest elevation.
Figure 4-16. Typical Transect Schematic
Coastal High Hazard Area
New Term V Zone
Waves pack a lot of power. They are much more destructive than standing or slow-moving water and that power increases exponentially with their height. For the purposes of the NFIP, the flood study identifies the “coastal high hazard area” as that part of the coastal floodplain where the wave heights during the base flood will be three feet or more. These areas are designated as “V Zones” where the “V” stands for “velocity wave action.” V Zones are subject to more stringent regulatory requirements and a different flood insurance rate structure due to increased degree of risk. The three-foot wave height threshold was selected because it was found to be the smallest wave with enough power to break a wall panel away from a floor to which it has been nailed. Coastal flood areas with wave heights of less than three feet are mapped as AE or A# Zones with the BFE indicated in parentheses.
4-28
Information and Data
November 1997
Figure 4-16 shows how the BFE with wave effects and the 100-year stillwater elevation get closer to each other as the water goes inland (toward the right side of the illustration). Typically, obstructions such as dunes or buildings break the waves, dissipating wave energy so that wave height and BFEs are reduced as one goes inland. Coastal high hazard areas are delineated on FIRMs based on transect data and topographic mapping.
Coastal Floodplain Map The coastal flood engineer now has accurate stillwater elevation and wave height data at the transects. As with a riverine flood study, the next job is to transfer the elevation data to the best available topographic map. Flood elevations between transects are interpolated, taking into consideration local topography. An example of a coastal floodplain map is the Town of Wrightsville Beach Flood Insurance Rate Map, a portion of which is shown in Figure 4-17. The applicable BFE is shown on the map in parentheses below the zone designation.
Figure 4-17. Coastal Floodplain Map
Shallow Flooding Studies For the NFIP, shallow flooding is defined as flooding with an average depth of one to three feet in areas where a clearly defined channel does not exist. Shallow flooding can exist in any of the following situations:
4-29
Information and Data
New Terms Base flood depth Zones AO or AH
November 1997
•
In flat areas, water collects or “ponds” in depressions. This is known as “ponding.”
•
In steeper areas where there are no defined channels or on flat plains, water will spread out over the land surface. This is known as “sheet flow.”
•
In coastal areas, wave runup will send water inland over flat areas or over dunes. Often it may collect or pond behind an obstruction which keeps it from draining back into the ocean.
For the purposes of the NFIP, shallow flooding is a type of flooding that is distinguishable from riverine or coastal flooding. Therefore, areas such as the edges of riverine or coastal floodplains where the base flood is less than three feet deep are not designated as shallow flooding. Shallow flooding is mapped based on historic flood experiences and a study of the topography. In some areas, the techniques used for riverine studies are used. The result will either be a BFE (i.e., in NGVD) or a base flood depth, i.e., in feet above the ground (2 feet in Figure 4-18). A shallow flooding study will usually only produce data for the base flood and not for the 10-year or other floods. Shallow flooding areas are denoted as Zones AO or AH.
D. Approximate Studies Part C of this unit explained what is called a detailed riverine, coastal, or shallow flood study (all three types may be included in a community’s flood insurance study). Detailed studies produce BFEs. In riverine areas, they usually produce a floodway map. In coastal areas, they identify wave heights and coastal high hazard areas (V Zones). Detailed studies are Figure 4-18. FIRM with Shallow Flooding Area 4-30
November 1997
Information and Data
expensive; for example, a riverine study typically costs $5,000 to $10,000 per mile. It is not cost effective to spend that kind of money to produce BFEs where there is presently little or no development and none is anticipated. Therefore, many FIRMs include floodplains mapped using approximate study methods. Also, many Flood Hazard Boundary Maps have been converted to FIRMs without a detailed Flood Insurance Study. These are usually rural areas or areas where little development is occurring. In addition, many flooding sources have been studied by other federal, state, or local agencies. Such studies that do not meet the NFIP standards for a Flood Insurance Study often contain valuable flood hazard information, which may be incorporated into NFIP maps as approximate studies. Those types of studies, referred to as “existing data studies,” typically cover developed or developing areas. They often contain flood elevation profiles that can be used as “best available data.” Use of these data will be discussed in later units. APPROXIMATE FLOODPLAIN • No floodways defined • No BFEs • Need other data source
An approximate study shows the estimated boundary of the Special Flood Hazard Area. It does not show a BFE and there are no elevation data in the FIS report. The boundary was drawn on the best available topographic map based on knowledge of past floods and the judgement of experienced engineers. In Figure 4-19, Zone A corresponds to the 100-year or base floodplain determined by approximate methods. This is often called an unnumbered A Zone or an approximate A Zone. According to NFIP requirements, federal, state, and other sources of flood data are to be obtained, when possible, in determining floodways and BFEs in approximate floodplain areas. See Managing Floodplain Development in Approximate A Areas: A Guide for Obtaining and Developing Base (100-Year) Flood Elevations (FEMA-265, 1995), available by contacting the DEM, the FEMA regional office, or the Internet at http://www.fema.gov/home/library/index.htm. Portions are contained in Appendix F.
4-31
Information and Data
November 1997
Figure 4-19. Unnumbered A Zone
Requirements for Developing BFE Data Under 44 CFR §60.3(b)(3) of the NFIP regulations, communities must: Require that all new subdivision proposals and other proposed development (including proposals for manufactured home parks and subdivisions) greater than 50 lots or 5 acres, whichever is the lesser, include within such proposals BFE data. This means that any subdivision which meets this threshold must be evaluated to determine if the subdivision proposal is affected by an approximate Zone A area and whether BFE data is required. BFE data is required for the affected lots in the subdivisions shown in Figure 4-20 and Figure 4-21. Figure 4-20 shows a 76-lot subdivision with several lots clearly affected by an approximate Zone A area. The subdivision depicted in Figure 4-21 is only 12 lots, but because the subdivision is greater than 5 acres and clearly shows buildable sites affected by an approximate Zone A area, BFE data is required.
4-32
Information and Data
November 1997
Figure 4-20. Proposed 76-Lot Subdivision
Figure 4-21. Proposed 6.7-Acre Subdivision
Communities are encouraged to address the flood hazards at the earliest stages of subdivision planning rather than at the actual placement of individual structures. If a community can work with the developer and others when land is being subdivided, many long-term floodplain management benefits can be achieved, particularly if the floodplain is avoided altogether. In Figure 4-22, “Proposed 76-Lot Subdivision,” the entire approximate Zone A area is to be dedicated as open space. If the planned subdivision shows the floodplain is contained entirely within an open space lot, it may not be necessary to conduct a detailed engineering analysis to develop BFE data.
4-33
Information and Data
November 1997
Figure 4-22. Proposed 76-Lot Subdivision
Also, it may not be necessary to develop detailed BFE data in large-lot subdivisions or single-lot subdivisions that are within the thresholds in 44 CFR §60.3(b)(3) of the NFIP regulations when the actual building sites are clearly outside of the Zone A area. In Figure 4-23, it is evident from the topographic features of this 5.6-acre subdivision that the building sites would be clearly out of the floodplain since the proposal indicates a steep grade between the approximate Zone A area and the building sites which are located on natural high ground. Figure 4-23. Proposed 5.6-Acre Subdivision
4-34
November 1997
Information and Data
If the grade between the actual building sites and the approximate Zone A area of the proposed subdivision is relatively gradual, as shown in Figure 4-24, the floodplain could extend beyond what is shown on the Flood Insurance Rate Map. It is very likely that flooding could affect the building sites. In this case, an analysis should be conducted to determine the location of the 100-year floodplain and the BFE.
Figure 4-24. Proposed 6.7-Acre Subdivision
4-35
Information and Data
November 1997
Unit Learning Exercise Purpose:
To review what you have learned in this unit.
Directions: Answer the following questions. 1.
The three components of a Flood Insurance Study are a a , and a
.
2.
The Flood Insurance Rate Map is used for
purposes.
3.
The Flood Boundary and Floodway Map is used for
purposes.
4.
The new format FIRMs can now be used by
,
. 5.
Cross sections are . They are shown on
maps
and on 6.
A Flood Insurance Study produces elevation for the and -year floods.
7.
Elevation reference marks are
maps. -,
-,
,
. They are used by to determine elevations at nearby locations, such as first floor elevations at development sites. 8.
A flood profile is used by the local administrator and others to .
9.
In riverine situations the floodplain is divided into two separate parts for floodplain management purposes. These parts are the and the .
. It is
10. Strict controls over further development within the floodway exist because here floodwaters are and further development will . 11. NFIP regulations specify that floodway boundaries be established so that the BFE is not increased more than at any point along the stream. 4-36
Information and Data
November 1997
12. Most coastal flooding result from caused by coastal storms, such as hurricanes. 13. In coastal areas, the BFE is comprised of the elevations.
and
,
and
14. A transect shows
.
15. For the purpose of the NFIP, the coastal high hazard area (V Zone) is that part of the coastal floodplain where wave heights are . 16. Instead of using a BFE, some shallow flooding areas are defined using a base flood . 17. An approximate study just shows the base
.
18. The map used to show coastal high hazard areas (V Zones) is the .
4-37
Information and Data
November 1997
Answers to Unit Learning Exercise
1.
The three components of Flood Insurance Study are a study report, a Flood Insurance Rate Map, and a Flood Boundary and Floodway Map.
2.
The Flood Insurance Rate Map is used for flood insurance purposes.
3.
The Flood Boundary and Floodway Map is used for floodplain management purposes.
4.
The new format FIRMs can now be used by community officials for floodplain management, lenders and insurance agents to determine the need for flood insurance and to rate policy applications, and by land surveyors, engineers, property owners, etc.
5.
Cross sections are surveys to obtain the shape of the floodplain, i.e., its ground elevations and obstructions. They are shown on Flood Boundary and Floodway maps and on new format Flood Insurance Rate maps.
6.
A Flood Insurance Study produces elevation for the 10-, 50-, 100-, and 500-year floods.
7.
Elevation reference marks are permanent references to which elevation data are referred, usually in terms of NGVD. They are used by surveyors to determine elevations at nearby locations, such as first floor elevations at development sites.
8.
A flood profile is a plotting on graph of computed flood elevations at cross sections and connecting the plotted points with straight lines. It is used by the local administrator and others to determine flood elevations at any point along the river or stream.
9.
In riverine situations the floodplain is divided into two separate parts for floodplain management purposes. These parts are the floodway and the flood fringe.
10. Strict controls over further development within the floodway exist because here floodwaters are deep, swift, and the most dangerous and further development will increase flood levels on other properties. 11. NFIP regulations specify that floodway boundaries be established so that the BFE is not increased more than 1 foot at any point along the stream. 12. Most coastal flooding result from storm surge and wave action, caused by coastal storms, such as hurricane. 13. In coastal areas, the BFE is comprised of the 100-year stillwater and wave crest elevations. 4-38
November 1997
Information and Data
14. A transect shows the elevation of the ground, both onshore and offshore. 15. For the purpose of the NFIP, the coastal high hazard area (V Zone) is that part of the coastal floodplain where wave heights are three feet or more. 16. Instead of using a BFE, some shallow flooding areas are defined using a base flood depth. 17. An approximate study just shows the base floodplain. 18. The map used to show coastal high hazard areas (V Zones) is the Flood Insurance Rate Map.
4-39
