Gregory Wilson, CFM

Adrienne Sheldon, PE, CFM

Amit Mahadevia

FEMA 259 UpdateEngineering Principles and Practices for

Retrofitting Flood-Prone Residential Structures

2

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

3

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 Types of Flood Retrofits

VI Case Studies

4

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

5

Residential structures that can be elevated

Houses over a crawlspace• Elevated on either solid or open foundation walls

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

Easier

6

Elevation on Solid Perimeter Foundation Walls

Before

After

7

Elevation on Piers

Cross-section of elevated wood-frame house on new or extended pier foundation

8

Elevation on Posts and Piles

Posts

Piles

9

Relocation

Structure is moved to a

location less prone to

flooding and flood-

related hazards

10

Relocation Process

Most structure types can be moved

as a whole or in segments

11

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

12

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)

13

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)

14

Move the Structure (Step 8)

Trailer wheel sets are placed beneath the lifting beams(photograph courtesy of Wolfe House Movers)

15

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

16

Dry Floodproofing Examples

• Watertight shields for doors and windows

• Reinforced walls

• Membranes and sealants

• Drainage collection systems and sump pumps

• Check valves

• Anchoring

17

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

18

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

19

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

20

Select Backflow Valves & Emergency Power

Backflow valves: helps prevent backflow through sanitary sewer and/or drainage systems

Emergency power: if used, observe proper guidelines

21

Dry Floodproofing Pitfalls

• Active mitigation

• Increases buoyancy

• Increases lateral forces potential

• Potential for total house failure

• Power dependency (sump pump)

• Maintenance requirements

22

Wet Floodproofing

Involves modifying

structure to allow

floodwaters to enter

in such a way that

damage is minimized

23

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

24

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

25

Protecting Utility Systems

• Electrical systems

• Heating, ventilation, and air conditioning (HVAC) systems

• Fuel supply/storage systems

• Sewer systems

• Water systems

26

Floodwalls and Levees

27

Floodwall Design

Types of floodwalls include:

• Gravity wall

• Cantilever wall

• Buttress

• Counterfort

28

Levee

Home protected by a 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.

29

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

30

Relative Costs and Risks of Floodproofing Methods

FEMA 259, Table 3-5

31

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.

32

FEMA Building Science

Flood/Wind Building Science Helpline:

FEMA-BuildingScienceHelp@fema.dhs.gov

866-927-2104

http://www.fema.gov/rebuild/buildingscience

33

Questions?