identifying and addressing flood vulnerabilities in ...€¦ · clogged gutters and downspouts...

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

3

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

4

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)

5

Key Flood Characteristics

Hazard probabilities over a

30-year mortgage:

▪ 100-year Flood = 26%

▪ House Fire = 10%

Riverine vs. Localized Flooding

6

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 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


measures can help

▪ Consider wet or dry

floodproofing (if outside

floodplain)

Flood Characteristics

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Riverine Flooding

▪ The most effective retrofit

options tend to be very

expensive


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


measures can help

▪ Consider wet or dry

floodproofing (if outside

floodplain)

Flood Retrofit Options

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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

13

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

14

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


▪ Eliminate subgrade driveway

and garage space ($$$)

▪ Wet floodproof garage space

▪ Add floodwall to driveway

entrance with retaining walls


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


▪ Eliminate subgrade space

and window openings ($$$)

▪ Floodproof framing around

subgrade window opening

and add drainage tied to

sump pump


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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▪ Relocate basement utilities to

a higher floor ($$$)

▪ Elevate exterior equipment


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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