This lesson should take approximately 30-40 minutes to complete.

This unit provides an overview of the general design practices and briefly discusses specific design practices that are unique to each retrofitting measure.

Describe the components of a field investigation
Explain the process for analyzing an existing structure for retrofitting
Identify the five retrofitting measures for flood-prone residential structures

Design Practices

This unit focuses on applying the anticipated loads discussed in Unit 4 to the existing site/structure and designing an appropriate retrofit measure

The unit includes:

General design practices common to all projects
Elevation
Relocation
Dry floodproofing
Wet floodproofing
Floodwalls and levees

The Design Process

The design process is straightforward, but technically intensive. It will result in:

Generation of construction plans to submit for building permitting
Mitigation of potential damages from flood and other natural hazards

Field Investigation (1 of 3)

Detailed information is required to make decisions about, and calculations for, retrofit measures, including:

Local building requirements
Surveys (structure, topographic, site utilities)
Hazard determinations
Documentation of existing mechanical, electrical, and plumbing systems
Homeowner preferences
Homeowner coordination
Maintenance programs and emergency action plans

Field Investigation (2 of 3)

Designers should review the selected retrofitting measure concept with the local building official to identify local design standards or practices that should (or must) be integrated.
A detailed survey of the site will supplement the information gathered during the low point of entry determination.* This includes structure survey, topographic survey, and site utilities survey.
The risk determinations (Unit 3) and hazard determinations (Unit 4) should be reviewed to confirm the flood protection design level and required height of retrofitting method.
For elevation, relocation, and dry/wet floodproofing measures, documentation of the condition of the existing structure and building systems is important.

Mechanical, Electrical, Plumbing, and Related Building Systems Data Sheet can be used to document the condition of the existing systems

Designers should confirm the homeowner’s preferences as outlined in Unit 3.
The process of homeowner coordination involves reviewing design options, costs, specific local requirements, and other applicable design components with the homeowner.
Retrofitting measures will remain useful throughout the life of the measure with the implementation of appropriate maintenance programs.

The next screen explains the three types of surveys

* Low point of entry determination identifies the lowest floor and each of the structure’s openings

Field Investigation (3 of 3)

Structure Survey: Vertical elevation assessment throughout the structure at openings where floodwater may enter.

Topographic Survey: A site plan or map of the area developed by a State-registered Professional Land Surveyor. The plan should include the low point of entry determination information, as well as general topographic and physical features.

Site Utilities Survey: Identification of above- and below-ground site utilities.

Analysis of Existing Structure

The structure’s ability to withstand the additional loads created as a result of retrofitting is an important design consideration.

The steps involved in the analysis include:

Structural reconnaissance
Determine the capacity of the existing footing and foundation system; analyze the loads that would be imposed by the retrofitting measure
Calculate the capacity of the existing structure to resist the additional loads imposed by the retrofitting measure

Structural Reconnaissance

Gather information from the following sources to complete structural reconnaissance:

Construction drawings
Building permits office
Renovation records
Contractors who have recently performed work
Home inspection report, if home is newly purchased

Sample of a Structural Reconnaissance Worksheet

Figure 5-3 structural reconnaissance worksheet

Footings and Foundation Systems

The foundation system of a house:

Supports the house by transmitting loads to the ground
Serves as an anchor against uplift and other loads

Retrofitting measures change the dynamics of the forces acting on a house

graphic showing Foundation Systems loading and footing reactions

Loads Imposed by Retrofitting Measure

If the stress caused by the expected loads is greater than the code-allowable stresses for the expected failure mode, reinforcing is required
Loads to consider include but are not limited to building dead, live, snow, flood, wind, and seismic loads (if applicable)

For dead loads, the worksheet in Figure 5-5 can be used to estimate the weight of a structure
Live loads are produced by the occupancy of the building, not including environmental loads
Roof snow loads vary according to the geography, roof slope, thermal exposure, and importance factors

Calculations for dead, live and snow loads are explained in Sections 5.2.10 – 5.2.14 of FEMA P-259. Flood, wind and seismic loads are explained in Chapters 3 and 4 of FEMA P-259.

Retrofitting Methods

Elevation

Raising a structure to place the lowest floor at or above the designated design flood elevation (DFE) on an extended support structure or fill

Elevation

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 piles, piers, or columns

Temporary relocation of the home may be necessary

Slab-on-grade houses

Wood frame vs. masonry—different elevation methods

Houses Over a Crawlspace

Generally the easiest and least expensive houses to elevate

Usually one- or two-story houses built on a masonry crawlspace wall, allowing for access in placing steel beams underneath the house for lifting

In most cases, the low clearance in crawl spaces prevents utilities from being placed under the home, limiting the need to relocate utilities during elevation

Can be elevated on:

Extended solid foundation walls
Open foundation such as masonry piers

Houses Over Basements

More difficult to elevate than houses over crawlspaces, as mechanical and heating, ventilation, and air conditioning (HVAC) equipment are often located in the basement

Basement walls may have already been extended to the point where they cannot structurally withstand flood forces

Can be elevated on:

Solid foundation walls by creating a new masonry-enclosed area on top of an abandoned and filled-in basement
Open foundation, such as masonry piers, by filling in the old basement

Elevation on Solid Perimeter Foundation Walls

A cross section of an elevated wood-frame house with extended masonry-enclosed area on top of an abandoned and filled-in basement
Similar to the cross section of house elevated on extended solid foundation walls over a crawlspace

Elevation on Piers

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

Similar to the cross section of a house on new reinforced piers on top of existing filled-in basement

Cross section of Elevation house on piers-Note: Flood-resistant materials and methods required below DFE

Elevation of Slab-on-Grade Houses

Most difficult type of house to elevate

Can raise the structure with or without the slab and use a first floor that is typically composed of wood or masonry

There are different elevation methods for wood-frame and masonry slab-on-grade houses

Slab-on-Grade Houses: Wood Frame

The following alternatives apply for this elevation method:

Elevating without the slab and using a new first floor constructed of wood trusses
Elevating with the slab intact

sample cross section of a slab on grade house-wood frame-Note: Flood resistant materials and methods required below DFE

Slab-on-Grade Houses: Masonry

The following alternatives apply to this elevation method:

Elevate with slab intact
Elevate without the slab, using first floor constructed of wood framing
Install elevated concrete slab within structure
Install elevated wood-frame floor system within structure
Create new masonry livable area on top of existing home
Create new wood-frame livable area on top of existing home

Field Investigation Concerns

Field investigations for elevating a building should include:

Property inspection and existing data review
Code search

Designers can use the Elevation Field Investigation Worksheet to record information

Sample Elevation Field Investigation Worksheet

Design

The design process for an elevated structure is presented here

Detailed descriptions of each step can be found in Section 5E.3 of FEMA 259

Design Process- Step 1-Calculate the vertical loads, Step 2-Calculate the lateral loads, Step 3-check ability of existing structure to withstand additional loading, Step 4-Analyze the existing foundation , Step 5-design the new foundation walls, Step 6-design top of foundation wall connections, step 7-design sill plate connections, step 8-design new access, step-9-design utilities extensions, step 10-specify the increased insulation requirements

Construction Considerations

Prior to elevating any house:

Obtain all required permits and approvals
Ensure all utility hook-ups are disconnected (plumbing, phone, electrical, cable, mechanical)
Estimate the lifting load of the house
Identify the best location for the principal lift beams, lateral support beams, and framing lumber, and evaluate their adequacy

Additional construction considerations, organized by structure and elevation type, are located in Section 5E.4

Relative Costs for Elevation

Link to a larger version of Table 3-1: Relative Costs for Elevation Pdf

Table 3-1 Relative costs of elevating a home with headings stating construction type, existing foundation, retrofit, relative cost

Relocation

Moving a structure to a location that is less prone to flooding and flood-related hazards such as erosion

Relocation Process

Relocation is the retrofitting measure that can offer the most protection from future flooding
The new site is often selected by the homeowner in consultation with the designer or community officials

Select the House Moving Contractor

Critical step in relocation

The designer can help the homeowner select a home moving contractor
The Relocation Contractor Selection Checklist can be used to record the key elements of selection

Sample of a Relocation Contractor Selection Checklist

Analyze the Existing Site and Structure

When analyzing the existing site and structure, consider:

Is there enough space around the structure to accommodate lifting beams and truck wheels?
Can the structure be lifted as one piece?
How much bracing will be required to successfully move the structure?
Will the structure survive the lift and move proposed?
Which utilities must be disconnected and where?
What local regulations govern demolition of the remaining portions of the structure (foundation and paved areas)?
To what standard must the site be restored?

Select New Site and Prepare Existing Site

Examine potential sites for:

Floodplain location
Utility extension feasibility
Accessibility for both the house movers and the new site construction crews
Permitting feasibility of the existing house on the new lot

Preparation of the existing site includes clearing all vegetation from the area in and around the footprint of the house to clear a path to allow the insertion of beams for lifting supports.

Analyze and Prepare the Moving Route

Identify route hazards, including narrow passages, bridge height and weight limits, utility conflicts, fire hydrants, road signs, steep grades, traffic signals, and tight turns around buildings, bridges, and overpasses

Obtain approvals for the area from which the structure is being moved and also from jurisdictions through which the structure will pass and its ultimate destination

The moving contractor should be responsible for route preparation, including the raising or relocation of utilities by utility companies, road/highway modifications, traffic lights, signage, etc.

Clear/grub overland areas, where necessary

Preparing the Structure and New Site

The steps for preparing the structure include:

Disconnect utilities
Cut holes in foundation wall for beams
Install beams
Install jacks
Install bracing
Separate structure from foundation

house on pallets and frame ready to be moved

Preparing the New Site

The steps for preparing the new site include:

Design foundation
Design utilities
Excavate and prepare new foundation
Construct support cribbing
Construct foundation walls

picture of a cement block foundation with a house ready to relocated.

Move Structure and Restore Old Site

Movement of the existing structure includes the excavation / grading of a temporary roadway, attachment of the structure to a trailer, transport, attachment of the structure to the new foundation, and landscaping.

Restoration of the old site should be conducted in accordance with local regulations. More details on restoring the old site can be found in Section 5R.9 of the manual.

Relative Costs for Relocation

Dry Floodproofing

A flood retrofitting technique in which the portion of a structure below the flood protection level (walls and other exterior components) is sealed to be watertight and substantially impermeable* to floodwaters

Dry Floodproofing Selection

Dry floodproofing mitigation measures:

Watertight shields for doors and windows
Reinforced walls
Membranes and sealants
Drainage collection systems and sump pumps
Check valves
Anchoring

Dry flooding Selection-Field investigation, selection and design, confirm ability of structure to accommodate dry floodproofing measure(s), select and design sealants and shields, select and design drainage collection systems, select and design sump pumps, select and design backflow valves, provide for emergency power for drainage system operation, prepare emergency operations plan, prepare operations and maintenance plan, construction of dry floodproofing measures.

Dry Floodproofing Applications

Buildings that are dry floodproofed may be subject to enormous hydrostatic pressure and imbalanced forces against the foundation and exterior walls and floor surfaces. It should only be used under the following conditions:

Short duration flooding
Low velocity flooding
Depth less than 3 feet

Dry floodproofing is NFIP- compliant for non-residential structures only

sample of Dry Floodproofing Applications

Evaluation of Existing Structure

Important step in the development of type, size, and location of the sealant and shield systems
Design process used to determine the ability of the existing structure and foundation to resist the expected flood- and non-flood-related forces

Figure 5D-10 presents a flow chart representation of the design process for a sealant or shield system

Selection and Design of Sealant Systems

Selection depends on the ability of the manufacturer’s product to withstand the depth and duration of flooding expected and the type of construction materials required

Processes for selecting the following systems are presented in the manual:

Coatings
Wrapped Systems
Brick Veneer Systems

Cross section of Design of Sealant Systems

Selection and Design of Shield System

Selection is based on the ability of the selected material to:

Structurally secure the opening
Be compatible with existing construction materials
Be effective for the duration and depth of flooding expected

The processes for selecting plate shields is presented in Section 5D.7.1 of the manual

Drainage Collection Systems

Underdrain systems may reduce flood loads for short duration flooding by moving floodwater away from the building’s foundation. Varieties include:

French drains, exterior underdrain systems, or interior drain systems

Sump Pumps prevent accumulations of water within the residence, often around important utilities.

They can be submersible or pedestal
If relying on for dry floodproofing, verify the pumps are functioning
Detailed field investigation is needed to determine if a sump pump is feasible
Sump pumps should be coordinated with other floodproofing methods

Backflow Valves and Emergency Power

Backflow valves can help prevent backflow through the sanitary sewer and/or drainage systems into the house.

Detailed information must be obtained about the existing structure to determine if backflow valves are feasible
Design process is outlined in Section 5D.10.2

Emergency power (generators) can be installed in homes if the proper guidelines are observed.

Small, portable residential generators can be used
Unit capacity should match the anticipated maximum load
Essential equipment/appliances are outlined in Table 5D-1

Non-Residential Construction

Dry floodproofing options for non-residential construction include:

Permanent closure of non-essential vulnerable openings
Watertight core areas
Enhanced flood shields
Pressure relief systems to protect against structural failure

Wet Floodproofing

Modifying a structure to allow floodwaters to enter in a way that damage to the structure and its contents is minimized

Figure 1-10. Wet floodproofed structure

Wet Floodproofing (Continued)

This section introduces the following concepts related to wet floodproofing:

Protection of the structure
Design of openings is for intentional flooding of enclosed areas below the DFE
Use of flood-resistant materials below the DFE
Adjustment of building operations and maintenance procedures
Emergency preparedness for actions that require human intervention
Design of protection for the structure and its contents, including utility systems and appliances

collage of different types of wet floodproofing-fence around ac units, vents in basement to let water pass though.

Protection of the Structure

Failure of structural components when subjected to inundation is a major cause of structural damage. Those components, typical failure modes, and related design considerations are briefly explained below.

Floodwater can affect a structure’s foundation by eroding supporting soil, scouring foundation material, and undermining footings. Footing depth, anchoring of the structure to the foundation, and lateral support in foundation walls should be considered in design.
Wet floodproofing will not be successful if the cavity space does not fill with water and drain at a rate equal to the floodwater rate of rise and fall. Insulation within cavity walls subject to inundation should be designed of flood-damage-resistant material.
Solid walls may absorb moisture and associated contaminants and should have both exterior and interior protective cladding to guard against absorption.

Use of Flood-Resistant Materials

All materials exposed to floodwater must be durable, resistant to flood forces, and retardant to deterioration caused by repeated exposure to floodwater

Interior materials such as wall finishes, floors, ceilings, roofs, and building envelope openings can suffer damage from inundation, which can lead to failure or an contamination and mold issues

List of appropriate materials can be found in NFIP TB 2-08, Flood Damage-Resistant Materials Requirements

Cover of Technical Bulletin 2-Flood Damage-Resistant Materials Requirements

Operations, Procedures, Preparedness

Consider the following when wet floodproofing:

Flood Warning System. Wet floodproofing, in most cases, requires some human intervention. It is extremely important to have adequate time to execute actions. May be accomplished by monitoring local weather reports, the NWS alert system, or a local warning system.
Inspection and Maintenance Plan. Wet floodproofing design requires periodic inspection and maintenance to ensure components can operate under flood conditions.
Emergency Operations Plan. Includes adjustments to or relocation of structure contents and utilities. A list of specific actions and the location of materials needed to perform these actions should be developed and provided to those responsible for executing the plan.

Protection of Utility Systems-Electrical

Electrical systems:

Raise/relocate equipment and devices above the DFE
Seal outside wall penetrations; mechanically protect wiring system in flood-prone locations
Seal out moisture
Add Ground Fault Circuit Interrupting (GFCI) breakers

Figure 5W-1. Elevated air conditioning system

Protection of Utility Systems-HVAC Systems

HVAC systems:

Maintain equipment clearances and access by code and/or manufacturer
Provide adequate combustion air for fuel-burning equipment
Modify and/or maintain proper venting for fuel-burning equipment
Eliminate ductwork below the DFE

Figure 5W-1. Elevated air conditioning system

Protection of Utility Systems-Fuel Supply/Water System

Fuel Supply/Storage Systems:

Use flexible connections
Support and anchor tanks to resist flood forces (assume tank is empty in design)
Move fuel tank with relocated equipment
Use automatic cut-off valves

Water System:

Minimize plumbing fixtures below the DFE
Modify fixtures to prevent backflow
Protect system components from high-velocity flow
Modify the well top using watertight casing

graphic of a propane tank with galvanize 48-inch long, 3/4 inch diameter, double-headed ground anchor with 6-inch single helix auger.

Figure 5W-6. Fuel tank anchored from two sides

Protection of Utility Systems-Sewer System

Sewer Systems:

Install and/or maintain a check valve or sewer backflow prevention valve
Install an effluent ejector pump
Provide a backup electrical source
Seal septic tanks to prevent contamination
Adequately anchor septic tank to withstand buoyancy forces

graphic of Sewer System with a typical installation of an exterior backflow valve.

Figure 5W-7. Backflow valve – a check valve and gate valve with an effluent pump bypass

Floodwalls and Levees

Floodwall: A flood retrofitting technique consisting of barriers designed to keep floodwaters from coming into contact with the structure.

Levee: A manmade structure built parallel to a waterway to contain, control, or divert the flow of water. A levee system may include concrete or steel floodwalls, fixed or operable floodgates and other closure structures, pump stations for rainwater drainage, and other elements, all of which must perform as designed to prevent failure.

Floodwalls and Levees (Continued)

The design and construction process for both floodwalls and levees includes:

Field investigation
Design
Seepage concerns
Leakage concerns (floodwalls only)
Construction
Drainage

Residential floodwall

Residential Levee

Field Investigation Considerations

Floodwalls:

Using previous floods to define affected areas
Evidence of seepage in foundation walls
Plan of action for relief of hydrostatic pressure on foundation/exterior walls
Floodwall options
Adjustment of utilities
Construction activities and level of disruption

Floodwall Design

Types of floodwalls:

Gravity
Cantilever
Buttressed
Counterfort

graphic of different Floodwall Design-gravity wall, cantilever wall, buttress, counterfort

Types of Floodwalls

Gravity: Uses its weight for stability. Structural stability is attained by effective positioning of the mass of the wall.

Cantilever: A reinforced-concrete wall (cast-in-place or built with concrete block) that relies on a vertical support and horizontal footing to retain the mass behind the wall.

Counterfort: Similar to a cantilever retaining wall except that it can be used where the cantilever is long or when very high pressures are exerted behind the wall.

Buttressed: Similar to a counterfort wall except that the transverse support walls are located on the side of the stem, opposite the retained materials.

Floodwall Design

Design of floodwalls can be summarized in an 8-step process:

Determine wall height and footing depth
Assume dimensions
Calculate forces
Calculate factor of safety against sliding
Calculate factor of safety against overturning
Calculate eccentricity
Calculate soil pressures
Select reinforcing steel

The process can be simplified by assuming certain design parameters.

Floodwall Seepage and Leakage Considerations

Seepage through the floodwall

Expansion and construction joints must be constructed with appropriate waterstops/sealants

Seepage under the floodwall

Structure design may include impervious barriers or cutoffs under floodwalls

Leakage between the floodwall and residence

The gap between floodwall and residence should be filled with a waterproof material

Construction Considerations

Inspect/observe the following:

Adequate slope drainage
Proper floodwall foundation construction
Sealants applied per manufacturer’s requirements
Sump pump
Sample brick/decorative block
Maintenance requirement checklist

Field Investigation Considerations

Levees:

Whether natural topography lends itself to levee construction
Availability of fill material
Federal, State, and local regulations/ordinances
Coordination with Federal, State, and local officials
Effect of levee on natural flow of floodwaters
Flood velocities along the water side of the levee embankment

Levee Design

Standard levee design criteria were established in FEMA 259 to provide a conservative design while eliminating several steps in the USACE design process, thereby minimizing design cost:

Minimum settled levee height of 6 feet
Minimum levee crest width of 5 feet
Levee floodwater side slope of 1: 2.5
Levee land side slope (varies)
One foot of levee freeboard

cross section of a typical residential Levee Design

Relative Costs for Wet Floodproofing

Levee Seepage Considerations

Levee foundation seepage

Install and backfill the inspection trench with impervious material

Levee embankment seepage

Mandatory inclusion of a drainage toe

Scouring and levee slope protection

Stabilize embankments with vegetation or sod

Interior levee drainage

Install drain pipes through levee having backflow prevention (flap gate) as well as sump pump

Construction Considerations

Levees:

Remove all ground vegetation and topsoil over full levee footprint
Ensure suitability of levee soil
Levee should be constructed in layers with proper compaction
Construct levee at least 5% higher than the height desired to allow for soil settlement
Avoid using a borrow area within 40 feet of landward toe
Ensure access across levee