Identifying and Addressing Flood
Vulnerabilities in Residential
Structures
Flood Resilient Arlington Workshop
October 2019
John Squerciati, PE, CFM (Dewberry Engineers Inc.)
Agenda
▪ List of Acronyms
▪ Key Flood Characteristics
▪ Riverine vs. Localized Flooding
▪ Overview of Flood Design Principles
▪ Identifying and Addressing Key Flood Vulnerabilities
▪ Additional Flood Retrofit Tips
▪ Post-Flood Cleanup Reminders
▪ Additional Resources
▪ Questions
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List of Acronyms
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BFE – Base Flood Elevation
FEMA – Federal Emergency Management Agency
FIRM – Flood Insurance Rate Map
FIS – Flood Insurance Study
HVAC – Heating, ventilation and air conditioning
MSC – FEMA Map Service Center
NFIP – National Flood Insurance Program
SD – Substantial Damage
SI – Substantial Improvement
Flood Source
▪ Riverine:
▪ Occurs along rivers or
streams
▪ Caused by rainfall runoff
or ice melt that
accumulates
and exceeds channel
capacity (includes flash
flooding)
▪ Other Flood Sources
include Coastal, Coastal/
Riverine, Dam Break
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Key Flood Characteristics
Flood Depth
▪ Riverine Flood Depth
influences (community
factors):
▪ Watershed characteristics (size,
shape of valley, topography)
▪ Development density
▪ Stormwater management
Frequency
▪ Annual probability of flooding
▪ “100-year flood” terminology
can be misleading (BFE)
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Key Flood Characteristics
Hazard probabilities over a
30-year mortgage:
▪ 100-year Flood = 26%
▪ House Fire = 10%
Riverine vs. Localized Flooding
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Arlington Central Library
(1015 N Quncy Street)
GMU Van Metre Hall
(3351 Fairfax Dr)
Virginia Flood Risk
Information System (VFRIS)
Flood Hazard Zones from
FIRMs
Riverine Flooding areas shown along
waterways in purple, aqua and orange
Localized Flooding areas include
pretty much everywhere else
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Riverine vs. Localized Flooding
Riverine Flooding
▪ Occurs in/around mapped
floodplains - check FIRM!
▪ Flooding characterized by
inundation throughout the
floodplain
▪ Drainage and landscaping
measures can’t help
▪ Avoid dry floodproofing
measures (use wet
floodproofing instead)
Localized Flooding
▪ Can occur anywhere
▪ Flooding characterized by
sheet (overland) flow
flooding of select structures
in low-lying, poor drainage
areas
▪ Drainage and landscaping
measures can help
▪ Consider wet or dry
floodproofing (if outside
floodplain)
Flood Characteristics
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Riverine vs. Localized Flooding
Riverine Flooding
▪ The most effective retrofit
options tend to be very
expensive
▪ Drainage and landscaping
measures can’t help
▪ Avoid dry floodproofing
measures (use wet
floodproofing instead)
Localized Flooding
▪ All Riverine Flooding retrofit
options typically effective
▪ Some effective retrofit
options can be less
expensive
▪ Drainage and landscaping
measures can help
▪ Consider wet or dry
floodproofing (if outside
floodplain)
Flood Retrofit Options
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Riverine vs. Localized Flooding
Riverine Flooding and Localized Flooding
▪ SI/SD restrictions apply to pre-FIRM buildings (i.e.,
buildings built before May 3, 1982) in the floodplain.
▪ If your residence is determined to be SI/SD, you must
either relocate out of the floodplain or elevate the lowest
floor to BFE + 1 foot to ensure NFIP compliance
Substantial Improvement/Substantial Damage (SI/SD)
Substantial Improvement (SI): An improvement of a building (such as
reconstruction, rehabilitation, or an addition) where the cost equals or
exceeds 50% of the pre-improvement market value of the building.
Substantial Damage (SD): Damage of any origin, where the cost of
restoring the building to its before damage condition would equal or
exceed 50% of the pre-event market value of the building.
Riverine Flood Protection for Individual Structures:
1) Leave (Acquisition/Demolition, Relocation)
2) Elevate (Elevation, Mitigation Reconstruction)
3) Shield (Floodwall, Dry floodproofing restricted)
4) Partial Protection (Wet floodproofing)
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Overview of Flood Design Principles
DRY FLOODED
Localized Flood Protection for Individual Structures:
1) Leave (Acquisition/Demolition, Relocation)
2) Elevate (Elevation, Mitigation Reconstruction)
3) Shield (Floodwall, Dry floodproofing)
4) Partial Protection (Wet floodproofing)
5) Site Landscaping and Drainage Improvements
(Vegetation management, Positive drainage,
Increased runoff storage, Reduce impervious surfaces)
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Overview of Flood Design Principles
Identifying and Addressing Key Flood
Vulnerabilities
▪ Basements
▪ Subgrade driveways and garages
▪ Subgrade window openings
▪ Subgrade or at grade utilities
▪ Landscaping and drainage issues
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Basements
Identifiers:
▪ Pretty much any basement can flood
▪ Subgrade vs. walkout basements
Cautions:
▪ Do not dry floodproof basements in
floodplains; Pump out with caution
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Basements
Riverine Flood Solutions:
▪ Infill basement ($$$)
▪ Wet floodproofing – flood-
damage-resistant materials,
elevate basement utilities
▪ Move critical contents upstairs
Additional Localized Flood
Solutions:
▪ Clear walkout basement
drains; Cover walkout areas
▪ Install sump pumps and
perimeter drains
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Subgrade Driveways and Garages
Identifiers:
▪ Subgrade driveways often
connected to garages
▪ Subgrade driveways with
and without retaining walls
Cautions:
▪ Subgrade driveways can
direct stormwater into
garages and towards homes
▪ Residents with vehicles in
subgrade driveways may be
unable to evacuate
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Subgrade Driveways and Garages
Riverine Flood Solutions:
▪ Eliminate subgrade driveway
and garage space ($$$)
▪ Wet floodproof garage space
▪ Add floodwall to driveway
entrance with retaining walls
Additional Localized Flood
Solutions:
▪ Raise curb at street entrance
to subgrade driveway
▪ Add drains at top and bottom
of subgrade driveway
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Raised curb
Subgrade Window OpeningsIdentifiers:
▪ Typical windows for
basements
▪ Window openings framed
by metal arches or timber
boxes
▪ Drain(s) in window well
Cautions:
▪ Common point of floodwater
or rainwater entry
▪ Potential fire evacuation
point from basement space
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Subgrade Window Openings
Riverine Flood Solutions:
▪ Eliminate subgrade space
and window openings ($$$)
▪ Floodproof framing around
subgrade window opening
and add drainage tied to
sump pump
Additional Localized Flood
Solutions:
▪ Install covers over subgrade
window wells
▪ Clear drains in window wells
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Subgrade or At Grade Utilities
Identifiers:
▪ Exterior HVAC equipment at
grade
▪ Interior utility equipment
located in basements
Cautions:
▪ Loss of utilities at an
otherwise undamaged home
can lead to displacement
▪ Check with utility company
before moving/installing
equipment
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Subgrade or At Grade Utilities
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Riverine Flood Solutions:
▪ Relocate basement utilities to
a higher floor ($$$)
▪ Elevate exterior equipment
Additional Localized Flood
Solutions:
▪ Add a flood-resistant
enclosure
▪ Install sanitary sewer
backflow prevention valve
Landscaping and Drainage Issues
Identifiers:
▪ Negative drainage (towards
house instead)
▪ Obstructions to storm flow
(fences, blocked storm drains,
drainage easements)
▪ Excess impervious surfaces
▪ Dense vegetation too close to
the house
▪ Clogged gutters and downspouts
Cautions: Riverine Flood
Solutions Not Applicable
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Landscaping and Drainage Issues
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Localized Flood Solutions:
▪ Slope soil away from foundation;
add downspout extensions
▪ Remove obstructions to storm
flow (open fences, unblock storm
drains, access easements)
▪ Trim vegetation away from home;
more absorptive plants, trees
▪ Use permeable pavements; add
perimeter drains
▪ Clean and maintain gutters;
consider removing gutter guards,
upsize gutters
Additional Flood Retrofit Tips
Riverine Flooding (i.e., you’re in a mapped floodplain)
1) Focus on passive measures not active measures
2) Purchase flood insurance and add sewer backup
insurance to homeowners policy
3) Relocate valuable contents above design flood (BFE)
4) Elevate utilities above BFE whenever possible
5) Avoid dry floodproofing - never apply it to basements
6) Flood insurance coverage for basements is minimal; so
avoid expensive wall and floor finishes, furnishings
7) If house is declared SI/SD, then elevation and relocation
are your only options for NFIP compliance.
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Additional Flood Retrofit Tips
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Localized Flooding (i.e., you’re outside the floodplain)
1) Focus on passive measures not active measures
2) Purchase flood insurance and add sewer backup
insurance to homeowners policy
3) Relocate valuable contents above anticipated flood level
4) Elevate utilities above anticipated flood levels whenever
possible
5) Consider dry floodproofing with caution
6) Flood insurance coverage for basements is minimal; so
avoid expensive wall and floor finishes, furnishings
7) Landscaping and drainage improvements can help
Post-Flood Cleanup Reminders
Remember the five “outs”:
1) Air out – Open all doors and
windows; use fans (if available)
2) Move out – Remove salvageable
contents, saturated porous items
3) Tear out – Remove wet carpets,
saturated drywall, fibrous insulation
4) Clean out – Clean up debris; clean
and sanitize materials/surfaces
5) Dry out – Allow time for drying;
check moisture content of wood
framing, other materials
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Additional Resources - FEMA
▪ FEMA Flood Map Service
Center for FIRMs, FIS
▪ FEMA P-312,
Homeowner’s Guide to
Retrofitting
▪ FEMA P-348, Protecting
Building Utility Systems
From Flood Damage
▪ FEMA-ICC, Reducing
Flood Losses Through the
International Codes®
▪ NFIP Technical Bulletins
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Additional Resources – Arlington
County Website▪ Links to latest FIRMs and FIS
for Arlington County
▪ County Watershed Map
▪ Home Floodproofing Checklist
▪ Pre-Storm Preparation and
Post-Storm Recovery Tips
▪ Flood Resilient Arlington
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Questions?
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