This course provides an introduction to engineering design and economic guidance about what constitutes technically feasible and cost-effective retrofitting measures for flood-prone residential structures.
Objectives: At the end of this course, you will be able to:
Understand regulatory requirements that apply to retrofit projects
Describe the technical, regulatory and site-specific parameters to consider in flood protection design
Determine flood and other hazards present at the site that would impact design
Understand the basic parameters of each retrofitting measure and the associated design practices
This course is only intended to provide an introduction to retrofitting flood-prone residential structures. It is not intended to replace consultation with an engineer or architect.
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This lesson should take approximately 15-20 minutes to complete.
Unit Objectives
This unit includes a basic overview of the different flood retrofit options. Each option is defined and the pros and cons to each retrofit type are discussed. An overview of the general retrofitting process is also given.
List and define the five basic retrofitting measures for flood hazards
Explain some of the advantages and disadvantages of each retrofitting measure
Describe, in general, the considerations that should be taken when retrofitting
Identify the seven steps in the retrofitting process
Organization of the Design Manual (FEMA P-259)
Organization of the Design Manual (cont.) (FEMA P-259)
Icons Used in This Training
Retrofitting Methods
Retrofitting measures for flood hazards include:
Elevation (1 of 3)
Structure is raised so lowest floor is at or above the Design Flood Elevation (DFE)
Protects structure from damage in a Base Flood
Elevation techniques include:
Elevation on solid perimeter foundation walls
Elevation on open foundation systems
Must consider other hazards in design (i.e., seismic, wind)
Elevation (2 of 3)
Elevation on solid perimeter foundation walls
Used in areas of low to moderate water depth and velocity
Structure raised from current foundation, support walls extended
Elevation (3 of 3)
Elevation on open foundation systems
Vertical structural members (not a continuous foundation wall)
Includes piers, posts, columns, and piles
Figure 1-3. Elevation on piers
Figure 1-4. Elevation on posts or columns
Figure 1-5. Elevation on piles
Elevation Advantages and Disadvantages
Table 1-1
NFIP = National Flood Insurance Program; BFE = Base Flood Elevation
Relocation
Involves moving a structure to a location less prone to flooding and flood-related hazards such as erosion
Most structure types can be moved as a whole or in segments
Figure 1-8. Structure to be relocated
Relocation process fig 5R-2
Relocation Advantages and Disadvantages
Table 1-2
Dry Floodproofing
Is the strengthening of existing foundations, floors, and walls to withstand flood forces while making the structure watertight and substantially impermeable to floodwaters
Figure 1-9. Dry floodproofed structure
Dry Floodproofing Advantages and Disadvantages
Table 1-3
Wet Floodproofing
Modifying a structure to allow floodwaters to enter it in such a way that damage to the structure and its contents is minimized
Figure 1-10. Wet-floodproofed structure
Wet Floodproofing Advantages and Disadvantages
Table 1-4
Floodwalls and Levees
The placement of floodwalls or levees around the structure
Figure 1-11. Structure protected by floodwall and levee
Floodwalls and Levees Advantages and Disadvantages
Table 1-5
Considerations When Retrofitting
Some considerations when implementing a retrofitting strategy include:
Substantial damage/improvement requirements under the NFIP, local building codes, and floodplain management ordinances
Codes, ordinances, and regulations, for other restrictions
Retrofitted structures should not be occupied during conditions of flooding
Consult professionals for the design and construction of retrofitting measures
Maintenance of retrofitting measures should be included in plans
Purchase of or continuing flood insurance coverage
Plan of action for retrofit requiring human intervention
Retrofitting Process
The decision to retrofit is usually a result of:
Having witnessed or experienced a flooding event
Having sustained substantial damage from a flood or other event
Implementing a substantial improvement on the home that requires adherence to floodplain regulations
Information can be obtained through other homeowners, community officials, contractors, or design professionals
Figure 1-13. Primary steps in the retrofitting process.
This lesson should take approximately 15-20 minutes to complete.
This unit discusses the typical community floodplain management and building code environment. The role of local officials in a retrofitting project, the various tenets of the National Flood Insurance Program (NFIP), and the compatibility of items covered in the International Building Code (IBC) series are discussed.
Define the National Flood Insurance Program (NFIP)
Describe applicable flood hazard information in an NFIP participating community
Describe the community regulations and permitting process
Explain the influence of National Model Building Codes and consensus standards on flood-resistant design and construction
Regulatory Requirements
The components that make up a typical community floodplain management and building code environment include:
The NFIP
Local officials and community regulations
Compatibility of model building codes with the NFIP
Application of consensus standards
Flood Hazard Definitions
National Flood Insurance Program (NFIP): A voluntary program that operates through a partnership between the federal government and individual communities. The three components of the NFIP are flood insurance, floodplain mapping, and flood hazard mapping.
Base Flood Elevation (BFE): The water surface elevation resulting from the base, or 100-year flood, which is defined as the flood having a 1-percent chance of being equaled or exceeded in any given year.
Flood Insurance Study (FIS): Provides a narrative of the community's flood history and sources of flooding, as well as detailed information on the hydraulics and hydrology in the community.
Flood Hazard Definitions (cont.)
Flood Insurance Rate Map (FIRM): Official map of an NFIP community that delineates the SFHAs and the insurance risk premium zones applicable to the community.
Special Flood Hazard Area (SFHA): Area subject to inundation by the base flood.
Flood Insurance Rate Maps
Figure 2-1. Typical DFIRM for riverine flooding
Flood Insurance Rate Maps (Continued)
Figure 2-2. Typical DFIRM for coastal flooding showing the Limit of Moderate Wave Action
Even the latest FIRMs and FISs may be based on limited data, so the local floodplain manager should always be contacted for the latest information
Flood Insurance Studies (1 of 3)
FIRMs are based on the information provided in an FIS. An FIS is based on detailed engineering studies. The FIS contains information such as:
Detailed information on the hydrology and hydraulics of the community's floodplain
Narrative of the community's flood history and sources of flooding
Riverine floodplains: discharges and flood profiles
Coastal floodplains: stillwater elevations and wave height transects
Can be obtained through floodplain manager or online at FEMA Map Service Center http://www.msc.fema.gov
FISs describe how the flood hazard information was developed for a community (history and methodology)
FISs include:
Floodways
Discharges/ water surface elevations for 10-, 50-, 100-, and 500-year floods
Velocities
Flood profiles
Flood Insurance Studies (3 of 3)
Coastal Floodplains
FISs Include: storm surge stillwater elevations for 10-, 50-, 100-, and 500-year floods
FIS wave analyses include beach and dune erosion estimates
Figure 2-4. Wave height transect showing LiMWA, MoWA, and MiWA
Floodplain Management Regulations
To participate in the NFIP, communities must regulate development in floodplains in accordance with the NFIP
Must ensure two basic criteria are met before issuing a permit for development:
All new construction, substantial improvements, and repair of substantial damage will be protected from damage by the base flood
New floodplain development will not aggravate existing flood problems or increase damage to other properties
Floodplain Management Definitions
Basement: Any area of the building having its floor subgrade (below ground level) on all sides.
Enclosure: The portion of an elevated building below the lowest elevated floor that is either partially or fully closed in by walls.
Lowestfloor: Lowest floor of the lowest enclosed area. Should only be used for parking, access, or storage.
Post-FIRM/Pre-FIRM: Relates start of construction to the effective date of the community's floodplain management ordinance (i.e. if the start of construction was before the effective date, it is a Pre-FIRM structure).
Structure: A walled and roofed building or gas or liquid storage tank that is principally above ground. Also includes manufactured homes.
Substantial Damage/Substantial Improvement
Substantial Damage: Damage of any origin sustained by a structure whereby the cost of restoring the structure to its before-damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred.
SubstantialImprovement: Any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50 percent of the market value of the structure (or smaller percentage if established by the community) before the "start of construction" of the improvement.
Building Performance Requirements
NFIP has established minimum criteria and design performance requirements
Key NFIP Requirements for Zone A
Community Regulations and the Permitting Process
Regulation of the use of floodplain land is largely the responsibility of State and local governments
Communities may institute higher standards than those required by the NFIP, such as, but not limited to:
Adding freeboard requirements
Prohibiting building in certain areas
Requirements for building materials
Use and type of construction fill
Designer should always contact the local building official for building code and floodplain management requirements
National Model Building Codes
Include the I-Codes and the National Fire Protection Association (NFPA) Building Construction Safety Code (NFPA 5000, 2009)
The 2012, 2009, 2006, and 2003 I-Codes are consistent with the National Flood Insurance Program
As of January 2012, 47% of flood-prone communities had adopted a flood-resistant building code meeting or exceeding the NFIP
Consensus Standards
Relevant consensus standards for flood retrofit, developed by a committee of nationally recognized experts include:
ACI 530-08
ASCE 7-10
ASCE 24-05
Consensus Standards
Standards related to design and construction practices and construction materials are incorporated into a building code by reference rather than by inclusion of all of the text of the standard in the code
Relevant consensus standards include:
ACI 530-08, Building Code Requirements for Masonry Structures (referenced standard in IBC and IRC)
ASCE 7-10, Minimum Design Loads for Buildings and Other Structures (referenced in IBC, IRC, and NFPA 5000)
ASCE 24-05, Flood Resistant Design and Construction (referenced in IBC and NFPA; IRC allows, but does not require, provisions of ASCE 24)
This lesson should take approximately 15-20 minutes to complete.
This unit presents the factors that influence retrofitting decisions and the intimate role they play in choosing a retrofit method. The chapter provides two generic retrofitting matrices that were designed to help the designer narrow the range of floodproofing options.
Describe the importance of considering homeowner preferences in flood protection design
Identify the relevance of community regulations and permitting considerations in design
List and describe the technical parameters that should be considered in the initial evaluation
Identify the importance of taking historic preservation and other hazards (besides flood) into account in the design phase
Parameters of Retrofitting
This unit presents the factors that influence retrofitting decisions including:
Homeowner preferences
Community regulations and permitting requirements
Technical parameters
The Retrofitting Process
The process for retrofitting a residential structure requires the input of the homeowner, designer, and contractors
Technical parameters are key elements in identifying appropriate retrofitting measures
Understanding homeowner preferences helps the designer narrow the options once the technical parameters have been satisfied
Determining Homeowner Preferences
Initial homeowner meeting
Homeowner learns the reasoning and need for the proposed retrofit, the benefits, and what to expect of the retrofit
Designer learns about the building and site, as well as homeowner preferences, financial considerations, and any special needs
Aesthetic concerns
Designer should seek common ground with homeowner between aesthetics and function
Accessibility concerns
Dependent on the specific needs of the owner
Initial Site Visit
Perform low point of entry (LPE) determination
Identifies lowest floor and each of the structure’s openings
Identify:
Finished floor elevation (unless already identified as the lowest floor)
Other site provisions that may require flood protection
Elevation reference mark on or near the house
Homeowner Matrix
Helps the designer document the initial consultation with the homeowner
Helps identify appropriate retrofit measures for further consideration
Present the following information to the homeowner at this stage:
Cost estimates for each retrofit (shown in the table by $)
Residual risks after implementing the proposed retrofit (shown in the table by !)
Community Regulations and Permitting
Local codes: Designers should be aware of the local codes in place and any applicable amendments
Building systems/code upgrades: If the retrofit measure requires that the building be brought up to current code, new systems/utilities may be required
Off-site flooding impacts: If modification of site elements is required, designers should consider how adjacent properties will be affected
Retrofitting Screening Matrix
Completed once the designer has resolved preliminary retrofitting preference issues with the homeowner
Evaluates which measures are important for a structure
Depth and elevation: Critical during design considerations, as it is often the primary factor in evaluating the potential for flood damage.
Velocity: The speed at which the floodwaters are flowing. Flowing water often causes erosion and scour, as well as debris impacts and hydrodynamic forces.
Frequency: Probability that a flood of a specific size will be equaled or exceeded in any given year.
Rates of rise and fall: If an area is subject to flash flooding (high rate of rise), certain retrofitting methods may not be feasible, especially if they require human intervention.
Flooding Characteristics (Continued)
Duration: With long-duration flooding, certain measures such as dry floodproofing may be inappropriate due to increased chance of seepage.
Debris impact: A flooding characteristic directly related to depth, velocity, and rate of rise and fall. Impact may destroy retrofitting measures.
Site and Building Characteristics
Site characteristics
Site location in relation to flood hazard areas
Vulnerability to erosion and scour
Soil type and permeability
Building characteristics
Foundation type
Framing type
Utilities
Building condition
Figure 3-8. Large, fast-moving waves combined with erosion and scour to destroy this Gulf of Mexico home during Hurricane Opal
Building Condition Survey
Initial building condition survey is usually based on visual inspection, and later followed by a detailed analysis involving greater scrutiny
Designer will inspect walls, floors, roof, ceiling, doors, windows, and other superstructure and substructure components
Designer can use this worksheet to document findings during initial survey
Historic structures may be located in flood-prone areas and subject to significant damage if not retrofitted
Preventive measures can be carried out without harming historic character if supervised by a professional with historic preservation experience
At times, the best options may harm historical character. Designer should consider:
What is the risk if nothing is done?
Are there alternatives?
Is there a design treatment that could lessen the detraction of historical character?
Multi-Hazard Considerations
The chosen retrofit method could expose a structure to additional, non-flood-related hazards. For example, if a home is elevated, it may be subject to increased seismic loads.
Earthquake forces:
Earthquakes subject structures to lateral loads they may not have been designed to resist
FEMA 530, FEMA 232, and FEMA 454 provide additional guidance
Wind forces:
Damage potential increases when wind forces occur concurrently with flood forces
FEMA P-804 provides additional guidance
This lesson should take approximately 15-20 minutes to complete.
This unit presents guidance on how to focus on the specific retrofitting solution that is most applicable for the residential structure being evaluated.
Describe flood-related hazards that apply to all sites
Determine site-specific flood-related hazards that may be present
Identify non-flood-related environmental hazards, including earthquakes and high winds
Describe site-specific soil or geotechnical considerations
Determination of Hazards
Retrofitting measures should be designed to resist all loads associated with:
Flood-related hazards
Non-flood-related environmental hazards
Soil or geotechnical considerations
This unit presents techniques and guidance that designers can use to determine the hazards and site considerations listed above.
Analysis of Flood-Related Hazards
The following flood-related hazards will be covered in this unit:
Determining flood elevations
Flood forces and loads
Site-specific flood related hazards
Site drainage
Flood Elevation Terminology
RecurrenceInterval: An interval of time between flood events of a certain intensity or size (e.g., 5-,10-,100-, or 500-year flood).
1-Percent-Annual-Chance: A flood elevation that has a 1 percent chance of being equaled or exceeded in any given year; also referred to as the 100-year flood.
Determining Flood Elevations: Riverine (1 of 3)
To determine flood elevations in riverine areas:
FIRMs can be used, as they identify the specific flood zone(s) and BFEs of the project area
Flood elevations for the 100-year (1% annual chance) and 500-year (0.2% annual chance) floods are shown on the stream’s water-surface profile in the FIS
The cross sections can be used to locate a property on the water surface profile
Water surface profile shows the varying heights of the water surface along the stream
Figure 4-2. FIRM
Determining Flood Elevations: Riverine (2 of 3)
Determining Flood Elevations: Riverine (3 of 3)
To determine flood elevations for riverine areas using floodway data summary table:
Obtain table from the FIS report to determine the regulatory BFE at a given cross section, which can be interpolated for a given nearby project site.
A regulatory floodway is the channel of a river or watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated amount, shown in the table as the 'increase.'
Identify corresponding flooding source and location on the "Summary of Stillwater Elevation" table from FIS report (Table 4-2)
Select appropriate elevation for the recurrence interval (RI) in question
Flood Forces and Loads
Detailed explanations of the following flood forces and loads are included in FEMA P-259 (see referenced section numbers):
Flood Depth (4.1.2.1)
Floodproofing Design Depth (4.1.2.1)
Hydrostatic Forces (4.1.2.2)
Lateral Hydrostatic Forces (4.1.2.3)
Saturated Soil Forces (4.1.2.4)
Combined Saturated Soil and Water Forces (4.1.2.5)
Vertical Hydrostatic Forces (4.1.2.6)
Hydrodynamic Forces (4.1.2.7)
High Velocity Hydrodynamic Forces (4.1.2.8)
Impact Loads (4.1.2.9)
Riverine Erosion (4.1.2.10)
Flood Depth and Floodproofing Design Depth
Flood depth is calculated by subtracting the lowest ground surface elevation (grade) adjacent to the structure from the flood elevation for each flood frequency
Many communities have adopted more stringent regulations than the NFIP by:
Requiring freeboard (f) above the BFE
Regulating to a more severe flood than the base flood
These elevations above the BFE are called the design flood elevation, or DFE
Floodproofing design depth (H) = DFE – GS
DFE: design flood elevation
GS: lowest ground surface next to the structure
H = floodproofing design depth (ft)
d = depth of flooding (ft)
f = factor of safety (freeboard) (ft)
Figure 4-5. Flood depth and design depth
Hydrostatic Forces (1 of 3)
Hydrostatic forces can act:
Vertically downward on structural elements, such as flat roofs and similar overhead members having a depth of water above them
Vertically upward (uplift) from the underside of generally horizontal members, such as slabs, floor diaphragms, and footings (also known as buoyancy)
Laterally, in a horizontal direction on walls, piers, and similar vertical surfaces
Hydrostatic Forces (2 of 3)
Lateral water forces are applied to a structure by standing water above the ground surface.
Saturated soil forces must be included in design calculations if any portion of the structure is below grade.
Equivalent fluid pressures for various soil types are presented in Tables 4-3 and 4-4 of the manual
Combined saturated soil and water forces are the differential (f dif) between the water and soil pressures.
Hydrostatic Forces (3 of 3)
The computation of hydrostatic forces is vital to the successful design of floodwalls, sealants, closures, shields, foundation walls, and a variety of other retrofitting measures
The Hydrostatic Force Computation Worksheet can be used to perform hydrostatic calculations
Hydrodynamic Forces
Hydrodynamic loads are caused by moving floodwater around a building or structural element
Loads are a function of flow velocity and structure geometry, and include frontal impact, drag along the sides, and suction on the downstream side
Low velocity: floodwater velocities do not exceed 10 ft/sec
High velocity: floodwater velocities in excess of 10 ft/sec
Impact Loads
Impact loads are imposed on the structure by objects carried by moving water. The magnitude of these loads is very difficult to predict, but a reasonable allowance must be made for them in the design of retrofitting measures. Loads can be classified as:
No impact, which occurs in areas of little or no velocity or debris
Normal impact, which are isolated occurrences of typically sized debris or floating objects striking the structure
Special impact, which occurs when large objects or conglomerates of floating objects, such as ice floes or accumulations of floating debris, strike a structure
Extreme impact, which occurs when large, floating objects, such as runaway barges or collapsed buildings, strike a structure
Impact Loads: Calculations
Section 5.4.5 of ASCE 7-10 and the corresponding commentary contain an extensive discussion on computing impact loads for riverine and coastal flooding
FEMA 259, Section 4.1.2.9 includes equations and worksheets that can be used to calculate impact loads and forces
It is impractical to design residential buildings to resist extreme impact forces, so guidance on computing these loads is not discussed in this manual
Riverine Erosion
The variables that influence the stability (or erodibility) of stream banks in riverine erosion include:
Critical height of the slope
Inclination of the slope
Cohesive strength of the soil in the slope
Distance of the structure in question from the shoulder of the stream bank
Degree of stabilization of the surface of the slope
Level and variation of groundwater within the slope
Level and variation in level of water on the toe of the slope
Tractive shear stress of the soil
Frequency of rise and fall of the surface of the stream
Site Drainage
The drainage system for the area enclosed by a floodwall or levee must accommodate:
The precipitation runoff from the interior area (and any contributing areas such as roofs and higher ground parcels)
The anticipated seepage through or under the floodwall or levee during flooding conditions
Two general drainage methods are gravity flow systems and pump systems, and often both are used together
Equations for calculating runoff, seepage, and minimum discharge are shown in the manual (Section 4.1.3)
Site-Specific Flood-Related Hazards
Includes closed basin lakes, alluvial fan areas, and movable bed streams.
Closed basin lakes: Lakes with no outlets, such as the Great Salt Lake, and lakes with inadequate or elevated outlets, such as the Great Lakes. These lakes are subject to very large fluctuations in elevation and can retain persistent high water levels.
Alluvial fan areas: Flooding can occur at these locations, which are fan-shaped deposits of sediment eroded from steep slopes and watersheds and deposited on valley floors.
Movable bed streams: Streams where erosion (degradation of the stream bed), sedimentation (aggradation of the streambed), or channel migration cause a change in the topography of the stream sufficient to change the flood elevation or delineation of the floodplain or floodway.
Analysis of Non-Flood-Related Hazards
The following non-flood-related hazards will be covered in this unit:
Wind Forces
Damage potential increases when wind forces occur in combination with flood forces, often in coastal areas
When a structure is elevated to minimize the effects of flood forces, the wind loads on the structure may increase
Buildings must have sufficient strength to resist applied loads from positive and negative pressures
Wind Forces (Continued)
FEMA P-55, Coastal Construction Manual (FEMA, 2011), outlines parameters for determining wind loads as well as the main wind force resisting system (MWFRS) and the components and cladding (C&C) elements of structures.
MWFRS components:
Foundation
Floor supports
Columns
Roof rafters/trusses
Bracing
Walls
Diaphragms assisting in load transfer
C&C elements:
Roof sheathing
Roof coverings
Exterior siding
Windows
Doors
Soffits
Fascia
Chimneys
Seismic Forces
Seismic forces on structural elements can be significant
Earthquakes may trigger additional hazards such as landslides, soil liquefaction, and land subsidence
Should be considered in the design process
Structural and non-structural (building contents) retrofits should be used
Protection of the Structure and Contents
The most important step in protecting the structure from earthquakes is making sure the home is properly designed and constructed for seismic loads.
Proper design and anchoring of foundation
Consideration of retrofit concerns for wood and masonry structures
For non-structural elements, simpler methods include:
Anchoring and bracing fixtures, appliances, chimneys, tanks, cabinets, shelves, etc.
Land Subsidence
Land subsidence occurs in more than 17,000 square miles in 45 states each year—an area roughly the size of New Hampshire and Vermont combined
Causes of land subsidence vary, so the mitigation techniques vary as well
Typically due to withdrawal of fluids or gases, the existence of organic soils, or other geotechnical factors, land subsidence requires extensive engineering analysis to understand the risks
Subsidence may result in sudden catastrophic collapse of land surface or slow lowering of land surface
Geotechnical Considerations
During the design process, it is important to determine soil properties during flooding conditions for any surface intended to resist flood loads. These properties include:
Saturated soil forces
Allowable bearing capacity
Potential for scour
Frost zone locations
Permeability
Shrink/swell potential
Geotechnical Considerations (Continued)
Site investigations should be conducted and include surface and subsurface investigations
Relevant information can be obtained using USDA NRCS Soil Survey of the area, including:
Designers can use the decision matrix at right to compile information
Allowable Bearing Capacity
Weight of structure and backfilled soil (if present) creates a vertical pressure under the footing that must be resisted by the underlying soil
The allowable bearing capacity varies depending on the soil type
This directly affects the design of shallow foundations (larger area foundation required for lower bearing capacity soil)
Scour Potential
Scour is localized erosion caused by the entrainment of soil or sediment in the water as it flows around obstructions
Effects of flood loads on buildings can be increased by flood-induced erosion, localized scour, and long-term erosion because the water depth increases around the structure
Resistance to scour increases with clay content and/or the introduction of bonding agents, which help bond the internal particles of a soil together
Scour Potential (Continued)
Scour action is different depending on the type of building. These figures illustrate scour at an open foundation building and a ground level (continuous foundation) building.
Figure 4-23. Localized scour at piers, posts and piles as in an open foundation building graphic illustration.
Figure 4-24. Scour action on a ground level building graphic illustration.
Shrink/Swell Potential and Permeability
Certain soils under specific conditions expand upon freezing, so designers should consider the frost heave impact.
Normally, footing movement can be avoided by placing part of a foundation below the maximum frost penetration zone
In construction of retrofitting measures such as floodwalls and levees, the properties of the proposed fill material and/or underlying soil is important.
Ideally, homogeneous and impermeable materials are used
The advice of a professional engineer is vital.
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)
StructureSurvey: Vertical elevation assessment throughout the structure at openings where floodwater may enter.
TopographicSurvey: 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.
SiteUtilitiesSurvey: 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
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
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
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
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
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
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
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
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
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
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 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
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
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 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
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
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
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
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
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
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
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
A regulatory floodway is the channel of a river or watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated amount, shown in the table as the 'increase.' 
