FEMA 259 Update
Engineering Principles and Practices for
Retrofitting Flood-Prone Residential Structures
Gregory Wilson, CFM
Adrienne Sheldon, PE, CFM
Amit Mahadevia
Engineering Principles and Practices for Retrofitting
Flood-Prone Residential Structures
FEMA P-259
• Provides guidance for selecting and
implementing flood retrofitting
measures that are feasible and cost
effective.
• Describes:
• Elevation
• Relocation
• Barriers (floodwalls and levees)
• Dry floodproofing
• Wet floodproofing
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Organization of the Design Manual
Chapter
Title
I
Introduction to Retrofitting
II
Regulatory Requirements
III
Parameters of Retrofitting
IV
Determination of Hazards
V
General Design Practices
VD, VE, VF, VR, VW
VI
Types of Flood Retrofits
Case Studies
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Elevation
• Structure is raised so lowest floor is at or
above the Design Flood Elevation (DFE)
• Protects structure from damage in a
base flood
Base flood elevation
(BFE): Flood having a
1-percent chance of
being exceeded in any
given year.
Design flood elevation
(DFE): Regulatory flood
elevation adopted by a
local community.
Typically, the DFE is the
BFE plus any freeboard
adopted by the
community.
FEMA P-55, Figure 2-14
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Residential structures that can be elevated
Houses over a crawlspace
• Elevated on either solid or open foundation walls
Easier
Houses over basements
• Elevated on either solid or open foundation walls
Houses on piers, post, piles, or columns
• Temporary relocation of home may be necessary
Slab-on-grade houses
• Wood frame vs. masonry; different elevation
methods
Harder
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Elevation on Solid Perimeter
Foundation Walls
Before
After
6
Elevation on Piers
Cross-section of elevated
wood-frame house on new
or extended pier foundation
7
Elevation on Posts and Piles
Piles
Posts
8
Relocation
Structure is moved to a
location less prone to
flooding and floodrelated hazards
9
Relocation Process
Most structure types can be moved
as a whole or in segments
10
Select New Site (Step 3)
Examine potential sites for:
• Location outside of floodplain
• Utility extension feasibility
• Accessibility for both the house movers and the new site
construction crews
• Permitting feasibility for the existing house on the new lot
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Prepare the Structure (Step 6)
• Disconnect utilities
• Cut holes in
foundation wall
for beams
• Install beams
• Install jacks
• Install bracing
• Separate structure
from foundation
Structure is separated from its foundation
(photograph courtesy of Wolfe House Movers)
12
Preparing the New Site (Step 7)
• Design foundation
• Design utilities
• Excavate and prepare
new foundation
• Construct support
cribbing
• Construct foundation
walls
New foundation wall construction begins
(photograph courtesy of Wolfe House Movers)
13
Move the Structure (Step 8)
Trailer wheel sets are
placed beneath the
lifting beams
(photograph courtesy of
Wolfe House Movers)
14
Dry Floodproofing
The portion of a structure below the flood protection level (walls and other
exterior components) is sealed to make it impermeable to floodwaters
15
Dry Floodproofing Examples
• Watertight shields for doors and windows
• Reinforced walls
• Membranes and sealants
• Drainage collection systems and sump pumps
• Check valves
• Anchoring
16
Select and Design Sealant System
• Selection centers on the ability
of the manufacturer’s product to
be compatible with the length
and depth of flooding expected
as well as the type of
construction materials used
• Processes for selection of the
following systems are presented
in the manual
• Coatings
• Wrapped Systems
• Brick Veneer Systems
17
Select and Design Shield System
Selected material must
be able to:
• Structurally secure the
opening
• Be compatible with
existing construction
materials
• Be responsive to the
duration and depth of
flooding expected
18
Select Drainage Collection Systems
Typical sump detail
Underdrain systems:
reduce flood loads for
short duration flooding
by moving floodwater
away from building
foundation
Sump Pumps: used to
prevent accumulations
of water within
residence, typically
around important utilities
19
Select Backflow Valves & Emergency Power
Backflow valves: helps
prevent backflow
through sanitary sewer
and/or drainage systems
Emergency power: if
used, observe proper
guidelines
20
Dry Floodproofing Pitfalls
• Active mitigation
• Increases buoyancy
• Increases lateral
forces potential
• Potential for total
house failure
• Power dependency
(sump pump)
• Maintenance
requirements
21
Wet Floodproofing
Involves modifying
structure to allow
floodwaters to enter
in such a way that
damage is minimized
22
Wet Floodproofing Concepts
FEMA 259 discusses the following concepts related to wet
floodproofing:
• Protecting the structure
• Designing openings for intentional flooding of enclosed areas
below the DFE
• Use of flood-resistant materials below the DFE
• Adjusting building operations and maintenance procedures
• Emergency preparedness for actions that require human
intervention
• Designing protection for the structure and its contents,
including utility systems and appliances
23
Use of Flood Resistant Materials
All materials exposed to floodwater must be:
• Durable
• Resistant to flood forces
• Retardant to deterioration
caused by repeated
exposure to floodwater
24
Protecting Utility Systems
• Electrical systems
• Heating, ventilation,
and air conditioning
(HVAC) systems
• Fuel supply/storage
systems
• Sewer systems
• Water systems
25
Floodwalls and Levees
26
Floodwall Design
Types of floodwalls include:
• Gravity wall
• Cantilever wall
• Buttress
• Counterfort
27
Levee
Levee: A manmade
compacted earthen
structure built
parallel to a
waterway to contain,
control, or divert the
flow of water.
System may include
non-earthen
components, such as
floodwalls,
floodgates, and
pump stations.
Home protected by a levee
28
Levee Design
Standard levee design criteria was established to provide a
conservative design while eliminating several steps in the
USACE design process, thereby minimizing design cost
29
Relative Costs and Risks of Floodproofing
Methods
FEMA 259, Table 3-5
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Engineering Principles and Practices for Retrofitting
Flood-Prone Residential Structures
FEMA P-259
• An important resource for
engineers, architects, and local
code officials in flood-prone areas
since the first edition was published
in 1995.
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FEMA Building Science
Flood/Wind Building Science Helpline:
FEMA-BuildingScienceHelp@fema.dhs.gov
866-927-2104
http://www.fema.gov/rebuild/buildingscience
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Questions?
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