HomeMy WebLinkAbout01072016 BSC Agenda Item 4 STAFF REPORT
Business of the Building and Standards Commission
City of West University Place, Texas
AGENDA OF: January 7, 2016 DEPARTMENT OF ORIGIN: Development
Services
DATE SUBMITTED: November 30, 2015
PRESENTER: Staff
PREPARED BY: Clay Chew, CBO, CFM
City Building Official
STAFF PRESENTER:
SUBJECT: Discuss finished floor height Clay Chew, CBO, CFM
above Base Flood Elevation City Building Official
ATTACHMENTS: Ordinance Article IX Section 18-271 through 18-275. Recent rainfall
damage report, Documents from previous meetings.
STAFF SUMMARY
Background:
City Council has requested the Building and Standards Commission (BSC) to
review the finished floor requirements in relation to Base Flood Elevation (BFE).
Presently the City's ordinance requires that finished floor elevation to be at or above
BFE. Some cities require an additional 1 to 2 feet above BFE which is known as
"freeboard".
The advantages and disadvantages of requiring freeboard were discussed jointly
between the ZPC and BSC in 2006-2007. Documentation from those meetings has been
included for reference. Some of the key concerns were:
• The need for fill to elevate structures.
• The impact on drainage of surrounding properties.
• The possible impact to maximum height requirements for structures. Presently it
is 35 feet above Standard Base Level (natural grade).
• The hardships that may be created for existing structures that are substantially
improved and would be required to meet the higher finished floor elevation.
From 2012 — present, 212 new home permits have been issued. 26 of these were
built in the floodplain or 12%.
1
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D West University Place Southside Place
2013 - 2015 Building Height of Completed Homes in the Floodplain with BFE and
Finished Floor Elevations
Address Height above BFE Finished Difference
Base Level Floor
6619 Belmont 34.6 48.50 49.66 1.16
6424 Rutgers 34.6 48.40 49.80 1.40
3926 Swarthmore 31.03 49.80 51.69 1.89
6330 Brompton 34.59 48.00 49.40 1.40
6340 Mercer 34.77 49.00 50.21 1.21
3020 Cason 34.13 48.50 49.80 1.30
3829 University 32.69 49.00 51.51 2.51
3118 Pittsburg 34.93 48.40 49.20 0.80
6520 Wakeforest 33.21 48.10 49.35 1.25
6616 Belmont 31.11 48.50 49.85 1.35
6401 Brompton 34.68 48.50 49.96 1.46
3502 Corondo Ct. 34.72 49.00 49.30 0.30
6527 Westchester 34.77 48.50 48.59 0.09
3826 Villanova 34.98 49.50 50.24 0.74
3323 Cason 34.77 49.0 50.26 1.26
6624 Rutgers 34.69 48.50 50.02 1.52
6530 Wakeforest 34.78 48.50 49.74 1.24
6344 Mercer 34.68 49.00 50.62 1.62
6426 Sewanee 34.93 48.50 49.63 1.13
6415 Westchester 34.82 48.60 49.95 1.35
Address Height above BFE Finished Difference
Base Level Floor
6502 Westchester 34.27 48.00 49.47 1.14
6544 Sewanee 34.6 49.00 50.38 1.38
3117 Carnegie 32.31 48.50 50.70 2.20
6424 Westchester 35.00 48.80 48.90 0.10
6627 Rutgers 34.47 48.60 49.73 1.13
CITY OF WEST UNIVERSITY PLACE [WU]—BUILDING AND STANDARDS COMMISION
BASE FLOOD ELEVATION (BFE)ANALYSIS—C. R. MARSHALL 17 DECEMBER 2015
Mapped Floodplains of WU [quoted material taken from FEMA,ASCE, NFIP, & Harris Co.websites]
^'1/4 of WU area (some northernmost parts) is outside of the 0.2%(500 year floodplain)
"'1/2 of WU area is inside of the 0.2%(500 year floodplain) FEMA Zone "X"
^'1/4 of WU area is inside of the 1%(100 year floodplain) FEMA Zone "AE"
See Exhibit A(Harris County Flood Education Tool showing floodplain boundaries in WU)
Flood Insurance Rate Maps Showing WU Mapped Floodplains:
Panel 48201C0855L(WU West of Auden Street)
Panel 48201C0860L(WU East of Auden Street)
See Exhibits B and C(Screenshots from maplmsc.FEMA.gov/....)
Zone "X" is defined as the flood insurance rate zones outside the 100 year floodplain (i.e., in the 500
year floodplain). Flood insurance is available, but optional.
Zone "AE" is defined as the flood insurance rate zone that corresponds to the 100 year floodplain.
Mandatory flood insurance purchase is required.
See Exhibit D(FEMA Floodplain Zone definitions)
Highlights (and Excerpts) from ASCE 24-14 Applicable to This Analysis
"Elevation and Freeboard (additional height above National Flood Insurance Plan's Base Flood Elevation
(BFE) are specified as a function of the Flood Design Class* and the nature of the flood hazard areas
(Table 4 of the Highlights)."
*Flood Design Class for residence property in WU is"Class 2."
Flood Design Classification is based on degree of risk to the public:
Class 1: Minimal risk or unoccupied buildings and structures
Class 2: Moderate risk(most residential, commercial and industrial buildings and structures
Class 3: High risk, disruptive to the community
Class 4: Essential facilities such as for emergency response,fire departments, etc.
Exhibit E (ASCE publication"Highlights of ASCE 24-14 Flood Resistance Design and Construction")
Designing for Flood Levels Above the BFE's [FEMA Homebuilder's Guide,Ex. F]
Required Base Flood Elevation (BFE)for most buildings in WU is zero inches. That means damage can
occur if the flood is a fraction of an inch above the BFE which is not a hard number. However, if the
minimum BFE is set to zero as it is in WU (i.e,to the top of the concrete slab),then wood or non-flood-
resistant material, i.e., subflooring, hardwood, and carpeting is highly likely to get wet and be damaged.
Conclusion: It would seem to be prudent for WU to increase the Minimum BFE to more than zero inches
above BFE to provide some freeboard to protect structures and avoid unnecessary damages. Damages
would not only include water damage, but also to mold and mildew throughout the structure that water
has entered, which is it likely to do in a 100 year storm,which can happen at any time.
NOTE: "Floods more severe and less frequent than the 1%flood can occur in
any year."
"There are design and construction practices that can eliminate or minimize damage to buildings when
flood levels exceed the BFE. The most common approach is to add freeboard to the design(i.e.,to
elevate the building higher than required by the FIRM [Flood Insurance Rate Maps]. [See Methods
outlined in ASCE 24-05]
Other benefits of designing for flood levels above the BFE [from FEMA Technical Fact Sheet 1.6, Ex. F]:
1. Reduce building damage and maintenance
2. Longer building life
3. Reduced flood insurance Premium
4. Reduced time occupants are displace, need for temporary shelter, assistance
5. Reduced job loss
6. Increased retention of tax base
"Some coastal storms will result in flood levels that exceed the BFE, and buildings constructed to the
minimum elevation [such as zero above BFE in WU] could sustain flood damage....There is an 18%
chance that the 100 year flood level will be exceeded in 20 years, 39%in 50 years, and 51%in 70 years."
"Unlike wind—where exposure to a wind speed slightly above the design speed does not generally lead
to severe wind damage—occurrence of a flood level even a few inches above the lowest floor elevation
generally leads to significant flood damage."
"Where additional freeboard is prohibited or not provided use flood damage-resistant building materials
and methods above the lowest floor."
"A comprehensive study of freeboard (American Institutes for Research, 2006)demonstrated that
adding freeboard at the time of house construction is cost-effective. Reduced flood damage yields a
benefit-cost ratio greater than one over a wide range of scenarios."
Exhibit F(FEMA Technical Fact Sheet 1.6)
RECOMMENDATION
Based on the above information it is recommended that WU adopt and require BFE plus one foot
freeboard on all new construction in WU.
The objection that such an increase intrudes upon the 35 foot maximum building height in WU is
outweighed by the logic of having all damageable parts of a structure above, not at the level of expected
floods. The belief that"100 year"floods only occur once every one hundred years is unfounded. WU
has had three or four"100 year" storms in the last ten years. Houses in the"100 Year"floodplain(Zone
AE) have been flooded several times in the last ten years.
Further,the lower floor of a building can be constructed below the BFE through appropriate
construction and waterproofing(See Exhibit F). It has also been pointed out that by slightly altering roof
slope,the same internal ceiling heights can be achieved.
Finally it should be observed that both Southside Place and Bellaire Ordinances require BFE plus one
foot. Most of those two cities are Zone EA, 100 year floodplain, as is about half of WU. [The area
around Brays Bayou is Zone AE, 100 year floodplain (same as half of WU) and flooded extensively in
2105.]
Author's Note: In order to compile this analysis, it was necessary to go to many confusing government
and private websites. Even with Google, it is not easy to locate the information, let alone in one or two
locations(websites). As far as I could tell,there is not a single site that consolidates all maps,term
definitions, zone information and technical papers,which this analyses attempts to do. In order to
access some sites it is necessary to register, and one site [Harris County Flood Control District's RBD
(Regional, Best Management Practices, Database)]requires further authorization,why you need access,
and County approval (I registered as an WU BSC member, and was approved). Many of the map sites
are not copyable or printable, so screenshots are needed which are difficult to enlarge.
FEMA: Federal Emergency Management Agency
NFIP: National Flood Insurance Program
ASCE:American Society of Civil Engineers
12/17/15
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ZONE A
The flood insurance rate zone that corresponds to the 100-year
floodplains that is determined in the Flood Insurance Study by
approximate methods. Because detailed hydraulic analyses are not
performed for such areas, no Base Flood Elevations or depths are shown
within this zone. Mandatory flood insurance purchase requirements
apply.
Zone A99
The flood insurance rate zone that corresponds to areas of the 100-year
floodplains that will be protected by a Federal flood protection system
where construction has reached specified statutory milestones. No Base
Flood Elevations or depths are shown within this zone. Mandatory flood
\\- insurance purchase requirements apply. n�
Zone rAEEand Al-A30 �ST U�‘U EP-St\y Ir t��.XX
The flood insurance rate zone that corresponds to the 100-year
floodplains that is determined in the Flood Insurance Study by detailed
methods. In most instances, Base Flood Elevations derived from the
detailed hydraulic analyses are shown at selected intervals within this
zone. Mandatory flood insurance purchase requirements apply.
Zone AH
The flood insurance rate zone that corresponds to the areas of the 100-
year shallow flooding with a constant water-surface elevation (usually
areas of ponding) where average depths are between 1 and 3 feet. The
Base Flood Elevations derived from the detailed hydraulic analyses are
shown at selected intervals within this zone. Mandatory flood insurance
purchase requirements apply.
Zone AO
The flood insurance rate zone that corresponds to the areas of 100-year
shallow flooding (usually sheet flow on sloping terrain) where average
depths are between 1 and 3 feet. The depth should be averaged along the
cross section and then along the direction of flow to determine the extent
of the zone. Average flood depths derived from the detailed hydraulic
analyses are shown within this zone. In addition, alluvial fan flood
hazards are shown as Zone AO on the Flood Insurance Rate Map.
Mandatory flood insurance purchase requirements apply.
Zone AR
The flood insurance rate zone that results from the decertification of a
previously accepted flood protection system that is being restored to
provide protection from the 100-year or greater flood event.
Zone D
Designation on National Flood Insurance Program maps used for areas
where there are possible, but undetermined, flood hazards. In areas
designated as Zone D, no analysis of flood hazards has been conducted.
Mandatory flood insurance purchase requirements do not apply, but
coverage is available. The flood insurance rates for properties in Zone D
are commensurate with the uncertainty of the flood risk.
Zone V
The flood insurance rate zone that corresponds to the 100-year coastal
floodplains that have additional hazards associated with storm waves.
Because approximate hydraulic analyses are performed for such areas,
no Base Flood Elevations are shown within this zone. Mandatory flood
insurance purchase requirements apply.
Zone VE
The flood insurance rate zone that corresponds to the 100-year coastal
floodplains that have additional hazards associated with storm waves.
Base Flood Elevations derived from the detailed hydraulic analyses are
shown at selected intervals within this zone. Mandatory flood insurance
purchase requirements apply.
Zone B, C, and \Y), S1 VO.M CK `Y ()� LED
Zones B, C, andLLX are the flood insurance rate zones that correspond to
areas outside the 100-year floodplains, areas of 100-year sheet flow
flooding where average depths are less than 1 foot, areas of 100-year
stream flooding where the contributing drainage area is less than 1
square mile, or areas protected from the 100-year flood by levees. No
Base Flood Elevations or depths are shown within this zone.
-S441t1
HIGHLIGHTS OF ASCE 24-14 Flood Resistant Design and Construction
Published by the American Society of Civil Engineers (ASCE),Flood
Resistant Design and Construction, ASCE 24, is a referenced standard in the
International Codes®(I-Codes). ASCE 24 states the minimum requirements pi.diteithaw
and expected performance for the siting and design and construction of
Construction
buildings and structures in flood hazard areas that are subject to building code row +l
requirements. Types of buildings and structures are described in ASCE 24-14,
Table 1-1 (see page 5 of these Highlights), and include commercial,
residential, industrial, educational, healthcare, critical facilities, and other
occupancy types. Buildings and structures designed according to ASCE 24 are
better able to resist flood loads and flood damage.
FEMA deems ASCE 24 to meet or exceed the minimum National Flood
Insurance Program (NFIP) requirements for buildings and structures. ASCE 24 includes additional
specificity, some additional requirements, and some limitations that are not in NFIP regulations.
Buildings and structures within the scope of the IBC and proposed to be located in any flood hazard area
must be designed in accordance with ASCE 24. The 2015 I-Codes reference ASCE 24-14, while the
2006 through 2012 I-Codes reference ASCE 24-05. The International Residential Code® requires
dwellings in floodways to be designed in accordance with ASCE 24, and the 2015 edition of the IRC
allows use of ASCE 24 for dwellings in any flood hazard area(the 2012 and 2009 editions allow use of
ASCE 24 in Coastal High Hazard Areas).
A summary of significant technical
Highlights of ASCE 24-14 that complement the NFIP revisions from ASCE 24-05 to ASCE
minimum requirements are described below. 24-14 is reproduced on page 6 of these
Highlights.
Building Performance
• Flood loads and other loads and load combinations are specified in ASCE 7-10, Minimum Design
Loads for Buildings and Other Structures. Performance of foundations exposed to flooding is
specified in ASCE 24. Soil characteristics and underlying strata, including soil consolidation,
expansion or movement, erosion and scour, liquefaction and subsidence must be considered, as
applicable.
• Flood Design Classes replace Occupancy/Risk Categories for the purpose of establishing elevations of
lowest floors, flood-resistant materials, equipment and floodproofing. The 2015 International Building
Code requires designers to identify the Flood Design Class assigned in accordance with ASCE 24-14.
• Elevation and Freeboard (additional height above the NFIP's base flood elevation) are specified as a
function of the Flood Design Class and the nature of the
flood hazard areas (see table on page 4 of these Highlights). ASCE 24 uses 'design flood' and
Essential facilities (Flood Design Class 4)must be elevated `design flood elevation'to acknowledge
that some communities adopt flood
or protected to the BFE + 2 ft or 500-year flood elevation, hazard maps that depict flood hazard
whichever is higher. areas in addition to Special Flood
• Elevation requirements in Zone V and Coastal A Zones are Hazard Areas shown on FEMA's Flood
Insurance Rate Maps (FIRM).
independent of orientation of the lowest horizontal structural
member(relative to direction of wave approach)as a factor The design flood elevation (DFE)
in determining the required eboard (ASCE 24-05 made equals the base flood elevation (BFE)
elevation a function of orientate of the lowest horizontal in communities that regulate based on
structural member relative to the ection of wave FIRMs. The DFE is always equal to or
higher than the BFE.
approach).
Highlights of the ASCE 24-14(rev. July 2015) 1
• Fill is required to be stable under conditions of flooding, including rapid rise and rapid drawdown,
prolonged inundation, and erosion and scour. Compaction of structural fill is specified unless
otherwise required by the building code or in a geotechnical or engineering report. Fill side slopes
must be no steeper than 1:1.5.
• Two methods are specified to meet the requirements for flood openings in walls of enclosures below
elevated buildings,to allow for the automatic entry and exit of floodwater: nonengineered openings
that do not require certification(1 sq in per sq ft of enclosed area) and engineered openings that must
be certified by a registered design professional. The performance of engineered openings must account
for the presence of louvers, blades, screens, grilles, faceplates, or other covers and devices and must
ensure that the difference between the exterior and interior floodwater levels does not exceed 1 foot.
Installation of all flood openings must be in at least two walls and must be no more than 1 foot above
the higher of the interior grade or floor and the finished exterior grade immediately under each
opening.
• All breakaway walls in all flood hazard areas must have flood openings(ASCE 24-05 did not require
flood openings in Zone V breakaway walls).
• Provisions are included for attached and detached decks and porches, and for garages, carports, and
accessory storage structures.
• Provisions are listed for concrete slabs-on-grade, depending on the purpose and location of the slabs.
• Stairways and ramps must be designed and constructed to resist flood loads and to minimize transfer
of flood loads to foundations, or to break away without causing damage to the main structure, or to be
retractable/able to be raised.
• Where stairways are located inside areas enclosed with breakaway walls, exterior doors are required at
the main building entry at the top of the stairs, to minimize entry of wind-driven rain and wave splash
after breakaway walls have failed.
• In Coastal High Hazard Areas (Zone V) and Coastal A Zones: N R ' To U L(,
— Coastal A Zones are treated like Coastal High Hazard Areas if FEMA has delineated a Limit of
Moderate Wave Action, or if the community has designated a Coastal A Zone.
— Buildings must be supported-bp piles, drilled shafts, caissons, or other deep foundations(including
columns, and shear walls)and fbtindation depth must take into account erosion and local scour.
— Stem walls supporting floors and backfilled with soil or gravel are allowed in Coastal A Zones if
designs provide for the effects of local scour and erosion.
— Requirements are included for shallow fqundations in circumstances where soil conditions prevent
deep foundations. N.
— Provisions are provided for pile foundations,attachment to piles, and different types of piles(wood,
steel H, concrete-filled steel pipe, prestressed cncrete, precast concrete, cast-in-place concrete).
— Provisions are provided for pile design (capacity, apacity of supporting soils, minimum
penetration, spacing, connections, splicing, and mi ,d and multiple types of piles).
— Provisions are provided for footings, mats, rafts, slab`s\on-grade, pile caps, grade beams, bracing,
and shear walls.
— Walls designed to break away must not produce debris that is capable of damaging structures.
— Erosion control structures(bulkheads, seawalls, revetments)lnust not be attached to buildings or
direct floodwater into or increase flood forces or erosion imp cts on structures.
— Pools must be elevated, or designed to breakaway without prod\ cing damaging debris, or designed
to remain in the ground without obstructing flow that causes dam e. Pools must be structurally
independent of buildings and structures unless pools are located in r on elevated floors or roofs
that are above the design flood elevation.
Highlights of the ASCE 24-14 (rev.July 2015) 2
• Dry floodproofed nonresidential buildings and non-residential portions of mixed-use buildings:
— The terms"mixed use"and "residential portions of mixed use"now are defined in Commentary.
— Dry floodproofing measures are not permitted in Coastal High Hazard Areas, Coastal A Zones,
High Risk Flood Hazard Areas,where flood velocities exceed 5 ft/sec, and where conformance with
certain human intervention limits cannot be achieved.
— At least one exit door or emergency escape/rescue opening must be provided above the elevation
specified for dry floodproofing.
— If dry floodproofing measures specified require human intervention to activate or implement, there
must be a minimum warning time of 12 hours unless a community warning system provides a
warning time sufficient to accomplish certain activities. If removable shields are approved as part of
design, flood emergency plans must address specified elements and actions and must be posted in at
least two conspicuous locations.
Flood Damage-Resistant Materials
• Flood damage-resistant materials must be used below specified elevations (see table on page 4).
• Metal connectors and fasteners exposed to salt water, salt spray or other corrosive agents must be
stainless steel or equivalent corrosion resistant material, or hot-dipped galvanized after fabrication.
.
• Where preservative treated wood is required,treatment must be in accordance with AWPA
p q
requirements.
Attendant Utilities and Equipment
• Attendant utilities and equipment must be at or above specified elevations (see table on page 4), or
must be specifically designed, constructed, and installed to prevent floodwaters from entering or
accumulating within components.
• Fuel supply lines must be equipped with float operated automatic shut-off valves.
• Where required to meet life safety provisions of the code, certain exterior electrical components may
be installed below the design flood elevation, provided they are installed on a non-breakaway
structural element on the landward or downstream side of structures.
• Tanks that are below the design flood elevation and that are attached to or beneath buildings must be
installed and anchored to resist at least 1.5 times the potential buoyant and other flood forces assumed
to act on empty tanks.
• Elevator cabs that descend below the design flood elevation must be equipped with controls that
prevent the cab from descending into floodwaters. Elevator shafts must be designed to resist flood
loads, but are not required to have flood openings or breakaway walls.
Siting Considerations
• New buildings must not be built(1) seaward of the reach of mean high tide, or(2) in areas subject to
flash flooding(floodwaters rise to 3 feet or more above banks in less than 2 hours). Unless protected,
new buildings must not be built(1) in erosion-prone areas (determined by analysis); or(2) in mudslide
areas (determined by analysis); or(3) in certain portions of alluvial fan areas; or(4) in high velocity
flow areas(faster than 10 ft/sec); or(5) in ice jam and debris areas.
• Buildings in proximity to flood protective works (dams, levees, floodwalls, diversions, channels,flood
control structures)must not have adverse effects on, or conflict with, maintenance and repairs of those
protective works.
Prepared by FEMA Building Science Branch. Content from ASCE 24-14 used with permission from
ASCE. Purchase ASCE 24 at www.asce.org.
Highlights of the ASCE 24-14 (rev. July 2015) 3
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Flood Flood Flood Design ''
See next page for description of Flood Design Classes 4 a ,- a:.?�S4+11 Class 4
Minimum Elevation*of Zone A not identified DFE BFE+1 ft or BFE+1 ft or BFE+2 ft or DFE, or
Lowest Floor as Coastal A Zone DFE, DFE, 500-year flood elevation,
(Zone A: ASCE 24-14 whichever is whichever is whichever is higher
Table 2-1) higher higher
Minimum Elevation of Coastal High Hazard DFE BFE+1 ft or BFE+2 ft or BFE+2 ft or DFE, or
Bottom of Lowest Areas (Zone V)and DFE, DFE, 500-year flood elevation,
Horizontal Structural Coastal A Zone whichever is whichever is whichever is higher
Member higher higher
(Zone V: ASCE 24-14
Table 4-1)
Minimum Elevation Zone A not identified DFE BFE+1 ft or BFE+1 ft or BFE+2 ft or DFE, or
Below Which Flood- as Coastal A Zone DFE, DFE, 500-year flood elevation,
Damage-Resistant whichever is whichever is whichever is higher
Materials Shall be Used higher higher
(Table ASCE 24-14 5-1) Coastal High Hazard DFE BFE+1 ft or BFE+2 ft or BFE+2 ft or DFE, or
Areas(Zone V)and DFE, DFE, 500-year flood elevation,
Coastal A Zone whichever is whichever is whichever is higher
higher higher
Minimum Elevation** of Zone A not identified DFE BFE+1 ft or BFE+1 ft or BFE+2 ft or DFE, or
Utilities and Equipment as Coastal A Zone DFE, DFE, 500-year flood elevation,
(ASCE 24-14 Table 7-1) whichever is whichever is whichever is higher
higher higher
Coastal High Hazard DFE BFE+1 ft or BFE +2 ft or BFE+2 ft or DFE, or
Areas (Zone V)and DFE, DFE, 500-year flood elevation,
Coastal A Zone whichever is whichever is whichever is higher
higher higher
Minimum Elevation of Zone A not identified BFE+1 ft or BFE+1 ft or BFE+1 ft or BFE+2 ft or DFE, or
Dry Floodproofing of as Coastal A Zone DFE, DFE, DFE, 500-year flood elevation,
non-residential whichever is whichever is whichever is whichever is higher
structures and non- higher higher higher
residential portions of Coastal High Hazard Not permitted Not permitted Not permitted Not permitted
mixed-use buildings Areas(Zone V)and
(ASCE 24-14 Table 6-1) Coastal A Zone
Minimum Elevation of Zone A not identified BFE+1 ft or BFE+1 ft or BFE +1 ft or BFE+2 ft or DFE, or
Wet Floodproofing*** as Coastal A Zone; DFE, DFE, DFE, 500-year flood elevation,
(ASCE 24-14 Table 6-1) Coastal A Zone; whichever is whichever is whichever is whichever is higher
Coastal High Hazard higher higher higher
Areas(Zone V)
* Flood Design Class 1 structures shall be al l owed below the mi ru man elevation if the structure meets the wet f l oodproof i ng
requirements of ASCE 24-14 Section 6.3.
** Unless otherwise permitted by ASCE 24-14 Chapter 7
*** Only if permitted by ASCE 2414 Section 6.3.1
Highlights of the ASCE 24-14 (rev. July 2015) 4
ASCE 24-14 Table 1-1 Flood Design Class of Buildings and Structures
Flood
Use or Occupancy of Buildings and Structures Design
Class
Buildings and structures that normally are unoccupied and pose minimal risk to the public or mi ru mal
di srupti on to the community should they be damaged or fail due to flooding. Rood Design Class 1 includes(1)
temporary structures that ae in place for less than 180 days, (2) accessory storage buildings aid minor storage
facilities(does not include commercial storage faci l iti es), (3)small structures used for pa-king of vehicles,and
(4)certain agricultural structures.[Note(a)]
Buildings aid structures that pose a moderate risk to the public or moderate disruption to the community should 2
they be damaged or fail due to flooding,except those l i sted as Flood Design Classes 1, 3,and 4.Flood Design
-d Class 2 i nd udes the vast majority of buildings and structures that are not specifically assigned another Flood
Design Class,induding most residential,commerd al,and industrial bti ldi ngsa
Buildings aid structures that pose a high risk to the public or significant di srupti on to the community should 3
they be damaged,be unable to perform their intended functions after flooding,or fa I due to flooding.Flood
Design Class 3 indudes(1) buildings aid structures in which a large number of persons may assemble in one
place,such as theaters, lecture hats,concert halls,aid religious institutions with large seas used for worship;
(2)museums; (3)community centers ald other recreational facilities; (4)athletic fadlitieswith seating for
spectators;(5)elementary schools,secondary schools,aid buildings with college or adult education
dassrooms; (6)jails,correctional facilities, and detention facilities; (7)healthcare faci lities not having surgery
or emergency treatment capabilities; (8)care facilities where residents have limited motility or ability,
induding nursing homes but not induding care facilities for five or fewer persons; (9)preschool aid child care
facilities not located in one-aid two-fanily dwellings; (10) buildings and structures associated with power
generating stations,water aid sewage treatment plaits,telecommunication facilities,aid other uti Iities which,
if their operations were interrupted by a flood,would cause signifi cart disruption in day-to-day life or
significant economic losses in a community; aid(11) buildings and other structures not induded in Flood
Design Class 4(induding but not limited to facilities that manufacture,process, handle,store,use,or dispose of
such substances as hazardous fuels,hazardous cherni cal s,hazardous waste,or explosives) c ontani rig toxic or
explosive substances where the quality of the material exceeds a threshold quality establ i shed by the
authority having jurisdiction and is suffi d ent to pose a threat to the public if released.[Note(b)]
Buildings aid structures that conta n essential facilities and services necessay for emergency response and 4
recovery,or that pose a substanti al risk to the community at l age in the event of failure,disruption of function,
or damage by flooding.Flood Design Class 4 i nd udes(1) hospitals and health care faci lit es having surgery or
emergency treatment facilities; (2)fire,rescue,a-nbuiance, aid police stations aid emergency vehide garages;
(3)designated emergency shelters; (4)designated emergency preparedness;communication, and operation
centers and other facilities requi red for emergency response; (5) power generating stations and other publ i c
utility fadlitiesrequired in emergencies; (6)critical aviation facilities such as control towers,sir traffic control
centers,and hangars for arcraft used in emergency response; (7)ancillary structures such as communication
towers,electrical substations,fuel or water storage tanks, or other structures necessary to allow continued
f uncti our ng of a Rood Design CI ass 4 facility during and after an emergency; and(8) bti l di ngs and other
structures(induding, but not limited to,fad lities that manufactue,process,handle,store,use,or dispose of
such substances as hazardous fuels,hazardous chemicals,or hazardous waste) c onto ri ng sufficient qualities
of highly toxic substances where the quantity of the material exceeds a threshold quantity established by the
authority having j uri sdi cti on aid is sufficient to pose a threat to the public c if released.[Note(b)]
[Note(a)] Carta n agricultural ltuaa structures may be exempt from some of the provi si ons of this standard;see ASCE 24-14 Section C1.4.3.
[Note(b)] Buildings and other structures contai rr ng toxic,highly toxic,or explosive substaices shall be eligible for assignment to a
lower Flood Design Cl ass if it can be deinonstrsed to the sati sfacti on of the authority having jurisdiction by a hazard assessment as
described in ASCE 7-10 Section 1.5.3 of Minimum Design Loads for Buildings and Other Structures that a release of the substances is
commensurate with the risk associated with that Flood Design Class.
Highlights of the ASCE 24-14 (rev. July 2015) 5
Significant Technical Revisions
ASCE 24-14 lists a number of significant technical revisions from the 2005 edition:
1. Defines Flood Design Class rather than use Risk/Occupancy Classification assigned under
ASCE 7 and requires each building or structure governed by the standard to be assigned to Flood
Design Class 1, 2, 3, or 4. Uses the assigned Flood Design Class to apply elevation requirements
specified in Chapters 2,4, 5, 6 and 7. Flood Design Class 4 buildings and facilities are
equivalent to Occupancy Category/Risk Category IV buildings, which ASCE 7-10 identifies as
essential facilities.
2. Adds definitions for Mixed Use and Residential Portions of Mixed Use in commentary to clarify
limitations on use of dry floodproofing measures.
3. Changes the Coastal A Zone determination requirement from the designer's responsibility to one
depending on either: 1)delineation of a Limit of Moderate Wave Action(LiMWA) on a Flood
Insurance Rate Map, or 2)designation by the Authority Having Jurisdiction.
4. Separates specifications for flood openings from the installation requirements. Requires the
presence of louvers, blades, screens, faceplates, or other covers and devices to be accounted for
in determining net open area for non-engineered openings and in determining the performance of
engineered openings. Revises coefficient of discharge table for engineered flood openings. Adds
commentary regarding selection of coefficient of discharge and for grouping or stacking of flood
openings.
5. For Flood Design Class 4 buildings, requires the minimum lowest floor elevation(or
floodproofing level of protection)to be the higher of: the Base Flood Elevation plus freeboard
specified in Chapters 2, 4 and 6,the Design Flood Elevation, or the 500-year flood elevation.
The 500-year flood elevation requirement is new.
6. Clarifies text pertaining to alluvial fan high risk flood hazard areas.
7. In coastal high hazard areas(V Zone)and Coastal A Zones (if delineated):
a. Makes explicit that designs must account for local scour and erosion
b. Provides for shallow foundations in Coastal A Zones under certain circumstances
c. Requires flood openings in breakaway walls
d. Eliminates orientation of the lowest horizontal structural member as a factor to determine
elevation for lowest floors, equipment, and flood damage-resistant materials
e. Requires exterior doors at the top of stairways that are located inside enclosed areas with
breakaway walls
f Consolidates requirements for all nonstructural concrete slabs
g. Allows substantial improvement of existing buildings seaward of the reach of mean high
tide in V zones(makes ASCE 24 consistent with NFIP) and Coastal A Zones.
8. Updates flood damage-resistant material requirements.
9. Clarifies emergency escape and rescue opening requirements for dry floodproofed buildings.
10. Clarifies requirements for garages, carports, and accessory storage structures. Adds new section
for multistory parking structures.
11. Consolidates requirements for tanks and more clearly distinguishes between requirements based
on flood hazard area.
Highlights of the ASCE 24-14 (rev. July 2015) 6
Designing Flood Levels
Above the BFE
HOME BUILDER'S GUIDE TO COASTAL CONSTRUCTION Technical Fact Sheet No. 1.6
Purpose: To recommend design and construction practices that reduce the likelihood of flood damage
in the event that flood levels exceed the Base Flood Elevation(BFE).
C)
m
Key Issues z
r
BFEs are established at a flood level, includ- The cost of adding freeboard at the time of home M
ing wave effects, that has a 1-percent chance construction is modest, and reduced flood insur- r-'
of being equaled or exceeded in any given year, ance premiums will usually recover the freeboard
also known as the 100-year flood or base flood. cost in a few years' time.
Floods more severe and less frequent than the
1-percent flood can occur in any year.
Flood levels during some recent storms have ex-
ceeded BFEs depicted on the Flood Insurance - 4
Rate Maps (FIRMs), sometimes by several feet. •'--- _:-� ,x
In many communities, flooding extended inland, -
well beyond the 100-year floodplain (Special
Flood Hazard Area [SFHA]) shown on the FIRM 'c : ••_
(see Figure 1). }
Flood damage increases rapidly once the el-
evation of the flood extends above the lowest r -
floor of a building, especially in areas subject to �, °�`
o� <
coastal waves. In V Zones, a coastal flood with rte , '_ ' -y' """
a wave crest 3 to 4 feet above the bottom of
the floor beam (approximately 1 to 2 feet above Figure 1. Bridge City,Texas,homes were flooded dur-
the walking surface of the floor) will be sufficient ing Hurricane lke,even though they were constructed
to substantially damage or destroy most light outside the SFHA and in Zone B.The fibod level was
frame residential and commercial construction approximately 4'above the closest BFE.
(see Figure 2).
There are design and construction practices that ;' "` ;,4, ` = =# w
can eliminate or minimize damage to buildings . ,....„.,;, ,,.. "u T--. -
when flood levels exceed the BFE. The most ,.v., '`` : -
common approach is to add freeboard to the
6 h x
design (i.e.,to elevate the building higher than
required by the FIRM). This practice is outlined lli
in American Society of Civil Engineers(ASCE) 24- I1, -
05, Flood Resistant Design and Construction. "' - f
There are other benefits of designing for flood ' s .. S -. ': '�
levels above the BFE: reduced building damage r° .,, 5 "' ii , °` 3
and maintenance, longer building life, reduced s.� " ' � : 1 i � '� ' L t
flood insurance premiums, reduced period of z
time in which the building occupants may need A'; ifs :lis, 40 .
to be displaced in the event of a flood discs Figure 2.Bolivar Peninsula,Texas,V Zone house
ter (and need for temporary shelter and assis constructed with the lowest floor(bottom of floor
tance), reduced job loss, and increased reten beam)at the BFE(dashed line).The estimated wave
tion of tax base. crest level during Hurricane Ike(solid line)was 3'to 4'
above the BFE at this location.
FEMAHOME BUILDER'S GUIDE OTO COASTAL CONSTRUCTION of 8
How High Above the BFE Should a Building
be Elevated? If the 500-year stillwater elevation (feet North
American Vertical Datum of 1988 [NAVD) or
Ultimately, the building elevation will depend on sev- feet National Geodetic Vertical Datum of 1929
eras factors, all of which must be considered before [NGVDj) is not available, a rule of thumb can be
a final determination is made: used to approximate it as 1.25 times the 100-
The accuracy of the BFE shown on the FIRM: year stillwater elevation(feet NAVD or feet NGVD).
If the BFE is suspect, it is probably best to ele- The 500-year wave crest elevation can be ap-
vate 3 or more feet above the BFE; if the BFE is proximated as 1.25 times the BFE.
deemed accurate, it may only be necessary to el-
evate a couple of feet above the BFE.
If historical high water levels are above the Flood Insurance Rate Maps and Flood Risk
BFE, the historical high water levels should be Hurricanes Ivan (2004), Katrina (2005), Rita (2005),
considered in building elevation decisions. and Ike (2008) have demonstrated that constructing
Availability of preliminary Digital Flood Insur- a building to the minimum National Flood Insurance
ance Rate Maps (DFIRMs): As new Flood Insur- Program (NFIP) requirements—or constructing a
ance Studies (FISs) are completed, preliminary building outside the SFHA shown on the FIRMs—is
DFIRMs will be produced and available for use, no guarantee that the building will not be damaged
even before they are officially adopted by those by flooding.
communities. This is due to two factors: 1) flooding more severe
Future conditions: Since the FIRM reflects con- than the base flood occurs, and 2) some FIRMs, par-
' ditions at the time of the FIS, some owners or ticularly older FIRMs, may no longer depict the true
jurisdictions may wish to consider future condi- base flood level and SFHA boundary.
tions (such as sea level rise, subsidence, wet- Even if the FIRM predicted flood levels perfect-
land loss, shoreline erosion, increased storm
frequency/intensity, and levee settlement/fail- ly, buildings constructed to the elevations shown
ure) when they decide how high to elevate. on the FIRM will offer protection only against the
1-percent-annual-chance flood level (BFE). Some
State or local requirements: The state or local coastal storms will result in flood levels that exceed
jurisdiction may require a minimum freeboard the BFE, and buildings constructed to the minimum c4i4ce
through its floodplain management requirements elevation could sustain flood damage. The black line cx(,eedeM
or building code. in Figure 3 shows the probability that the level of the %r cbt�►i
Building code requirements: The International flood will exceed the 100-year flood level during time P
periods between 1 year and 100 years; there is an
Building Code (IBC) requires buildings be de- 18 percent chance that the 100-year flood level will Y �$
signed and constructed in accordance with ASCE be exceeded in 20 years, a 39 percent chance it will 60 31/
UJ U i5 24. ASCE 24 requires between 0 and 2 feet of be exceeded in 50 years, and a 51 percent chance
freeboard, depending on the building impor- it will be exceeded in 70 years. As the time period 701 Stq,
x°we tance and the edition of ASCE 24 referenced.1 increases,the likelihood that the 100-year flood will
X $ A c The 2009 International Residential Code (IRC) be exceeded also increases.
requires 1 foot of freeboard in V Zones and in
Coastal A Zones. Figure 3 also shows the probabilities that floods of
other seventies will be exceeded. For example, tak-
Building owner tolerance for damage, displace- ing a 30-year time period where there is a 26 percent
ment,and downtime: Some building owners may chance that the 100-year flood level will be exceeded
wish to avoid building damage and disruption, and a 6 percent chance that a flood more severe
and may choose to elevate far above the BFE. than the 500-year flood will occur.
In V Zones and A Zones, FEMA 499 recommends
considering elevation of residential structures to the Elevation Recommendation
500-year flood elevation, or to the requirements of
ASCE 24-05, whichever is higher. FEMA 499 recommends new and reconstructed
residential buildings be elevated above the effec-
tive BFEs with freeboard equal to that specified
in ASCE 24-05, plus 3 feet. When new DFIRMs
are available and adopted, 499 additionally rec-
ommends new and reconstructed residential
1 The 1998 edition of ASCE 24 is referenced by the 2003 edition of the buildings be elevated to or above the freeboard
IBC,and requires between 0 and 1 foot of freeboard.The 2005 edition
of ASCE 24 is referenced by the 2006 and 2009 editions of the IBC, elevation specified by ASCE 24-05.
and require between 0 and 2 feet of freeboard.
1.6: DESIGNING FOR FLOOD LEVELS ABOVE THE BEE
=1 HOME BUILDER'S GUIDE TO COASTAL CONSTRUCTION
FIRMs depict the limits of flooding,flood Probability of a Flood Exceeding the n-Year Flood Level
elevations, and flood zones during the
base flood.As seen in Figure 3,buildings During a Given Period of Time
elevated only to the BFEs shown on the 100% - '
FIRMs have a significant chance of being w 90% _�r.
flooded over a period of decades. Users C 80k
should also be aware that the flood lim- 11 70%
its, flood elevations, and flood zones 60% �._ =
shown on the FIRM reflect ground eleva- m''' 50%
tions, development, and flood conditions ° aoi° r�� ���
at the time of the FIS.2 �!„ � �af�_,..
0 20% ril tom— —
FIRMs do not account for the n. 10% .. G)
m
following: 0% Z
0 10 20 30 40 50 60 70 80 90 100 ,
Shoreline erosion,wetland loss, Period of Time(years) 73
subsidence, and relative sea r-
level rise ---10-yr flood -8--100-yr flood -+-200-yr Hood
Upland development or -m-`50-yrflood ---1,--150-yrflood -x-500-yrflood
topographic changes
Degradation or settlement of Figure 3. Probability that a flood will exceed the n-year food level
levees and floodwalls over a given period of time.
Changes in storm climatology (Note:this analysis assumes no shoreline erosion,and no increase in sea level
(frequency and severity) or storm frequency/severity over time.)
The effects of multiple storm
events
Thus,what was once an accurate de- 90 -
piction of the 100-year floodplain and 80
flood elevations may no longer be so. 70 V Zone
(No Obstruction)
as 60
Consequences of Flood Levels o 50 ,
Exceeding the BFE = 40
Buildings are designed to resist most 2 30 A Zone —
environmental hazards (e.g., wind, a (Two-Story Without
—
seismic, snow, etc.), but are generally 20 Basement)
designed to avoid flooding by elevat- 10 1 11H
ing the building above the anticipated 0
flood elevation. The difference in design -2 -1 0 1 2 3 4 5 6 7 8 9 1011 12 13 14 15 16 17 18 19 20 21 22
approach is a result of the sudden on- Depth in Feet Relative to
set of damage when a flood exceeds Bottom of Lowest Horizontal Structural Member*
the lowest floor elevation of a build-
ing. Unlike wind—where exposure to a .Assumes distance between top of floor and bottom of lowest
wind speed slightly above the design horizontal structural member=2'for A Zone buildings.
speed does not generally lead to se-
vere building damage—occurrence of a Figure 4. Flood depth versus building damage curves for V Zones and
flood level even a few inches above the riverine A Zones.
lowest floor elevation generally leads to (SOURCE:FEMA 55,COASTAL CONSTRUCTION MANUAL).
significant flood damage. Therefore,the
recommendation is to add freeboard.
This is especially true in cases where waves accompany coastal flooding. Figure 4 illustrates the expected
flood damage(expressed as a percent of a building's pre-damage market value)versus flood depth above the
bottom of the lowest horizontal structural member supporting the lowest floor(e.g.,bottom of the floor beam),
for a building in a V Zone and for a building in a riverine A Zone.3
2 Sections 78.1.3 and 7.9 of FEMAs Coastal Construction Manual(FEMA 55, 2000 edition)provide guidance on evaluating a FIRM to determine
whether it still provides an accurate depiction of base flood conditions,or whether it is obsolete.
3 Since the normal floor reference for A Zone buildings is the top of the lowest floor,the A Zone curve was shifted for comparison with the V Zone curve.
1.6: DESIGNING FOR FLOOD LEVELS ABOVE THE BFE
HOME BUILDER'S GUIDE TO COASTAL CONSTRUCTION 3 of 8