Flood Mitigation forStructures in the Gulf Coast

Region

Norma Jean Mattei, Ph. D., P.E.University Of New Orleans

Special thanks to: Chris Jones, PE and Gordon Boutwell, PhD, PE

A Quiz -- Which Hurricane?

Construction: “…the fault lies in the actual construction in the field, such as lack of attention to small detail, anchors, ties, bracing, reinforcing, carpentry, and masonry work.”

Which Hurricane?

Designers: “…Engineers and architects are too prone to write specifications in which everything is covered to the minutest detail, and to draw plans on which requirements are shown with hair-splitting accuracy, and then allow the contractor to build the building, sewer, pavement, or structure in general with little or no supervision.”

Which Hurricane?

Codes: “…we should not lose sight of the value of good building codes…. Every city in the State, whether damaged by the storm or not, would do well to carefully analyze the existing codes and strengthen them where weak.”

Answer:1926 Hurricane, Miami FL

Article by T. Eefting, Article by T. Eefting, Florida Engineer Florida Engineer and Contractor (1927)and Contractor (1927)

“Hope for the best, prepare for the worst”

Designing for Flood:All you really need to know. . .

Flood Characteristics(Flood source, BFE & DFE, velocity, duration, waves, erosion, & debris)

Flood Loads and ConditionsLowest Floor Elevation

FoundationFlood-Resistant Materials

Flood Characteristics

Riverine flooding:

inundation, velocity, debris, duration (A zones)

Coastal flooding: waves, velocity, erosion, debris, inundation, wind (V zones)

and Flooding Behind Levees. . .

(Zones A & B)

Let’s Define Terms. . . Base Flood: Flood having a 1% chance of being equaled or exceeded in any year (100-yr flood)

Base Flood Elevation (BFE): Flood elevation having a 1% chance of being equaled or exceeded in any year

Advisory Base Flood Elevation (ABFE): Reassessment of BFE after a catastrophic event

Codes use Design Flood, DFE

NFIP Flood Hazard Zones

VE (V, V1 through V30)

AE (A, A1 through A30)

AO

AH, AR

shaded X (B)

unshaded X (C)

Special Flood Hazard Area, SFHA (code uses FHA)

Outside 1% Flood Area}

Flood Hazard Zones

• A Zones

• V Zones

• Coastal A Zones

Shown on the FIRM}

Used in ASCE 7, 24, not shown on FIRM

3-ft breaking wave3-ft runup depthWave overtopping

1.5-ft breaking wave

Minimum Standards: FIRM, BFE & Advisory Flood Elevations

FIRM zones and BFE’s reflect “existing conditions” at the time of the study, not today’s conditions many maps are old

They do not consider FUTURE CONDITIONS long-term erosion, subsidence, sea level rise, subsequent construction

Flood Requirements for Buildings

Also: State of LA Building Code

Opal (1995): New Building in Area Mapped on FIRM as A Zone

Orange Beach, AL (before Ivan)

Orange Beach, AL (Ivan)

Buoyancy Failure

Chalmette, LA

Pointe-a-La-Hache, LA

Hurricane Opal:Effects of High-Velocity Flow

Hurricane Opal:Effects of High-Velocity Flow

Flood Depth and Velocity Effects

Flood Depth and Velocity Associated with Building Failure/Collapse

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0.0 5.0 10.0 15.0 20.0 25.0

Velocity (ft/sec)

De

pth

(ft

)

McBean (1988) TypicalResidential Wall Failure

USACE NFPC (1993)Foundation Wall Limit

Portland Dist. (1985), Class DBldg. (Wood) Collapse

Kelman (2002) 1-StoryResidence Failure (DS4)

Kelman (2002) 2-StoryResidence Failure (DS4)

Black (1975) 1 storyframe/drywall

Black (1975) 1.5 storyframe/drywall

Black (1975) 2 storyframe/drywall

Black (1975) brick veneer

proposed du2=200 ft3/sec2

dv2 = 200 ft3/sec2

High Velocity Flow:At Levee Breach in Orleans Parish

Observations and Thoughts

Wave effects

Only slabs remaining

Deb

ris L

ine

1,100 ft

SF Home Success

Wave Effects

Debris (Katrina)

Wave and Erosion Damage: Don’t Rely on Erosion-Control Structures

Hugo (1989)

Opal (1995)

Ivan(2004)

But what about New Orleans?. . .

New Orleans’ dilemma

-100

-80

-60

-40

-20

0

+20

EL

EV

AT

ION

(fe

et,

NG

VD

)

MISS.RIVER

LAKEPONTCHARTRAIN

CITY OF NEW ORLEANSGENERALIZED TOPOGRAPHY AND GEOLOGY

STIFF CLAY(PLEISTOCENE)

SEA LEVEL

WHEREFRENCH

BUILT

SOFT CLAY

NATURALLEVEE

SAND

PEAT

EXPANSION 1850 - 1950

New Orleans’ levee system

FEMA Flood Zones (part of New Orleans)

Flood Mitigation:New Orleans

50% Damage Rule: Must elevate lowest floor above

highest of Advisory Flood Elevation (~ old

BFE) 3 foot above adjacent grade

Even if structure has never flooded before in the past 100 years

Levees must be certified

Flood Mitigation: FoundationsV Zone (add Coastal A Zone)

Must:

• resist erosion and scour

• resist lateral forces

• support the building

NFIP Wet FloodproofingBreakaway walls should (cleanly)

NFIP Dry Floodproofing

Accumulation of no more than 4 inches of water depth during a 24-hour period if there were no devices provided for its removal

** dry floodproofing option not permitted in V zone

Conclusions

Coastal mitigation Foundation design and tie downs important Advisory Flood Elevations are way up Add Coastal A Zone designation to FEMA

FIRM What is successful mitigation (if a

structure still stands, is that a good thing?)

Mitigation within a hurricane protection system Levee certification 3’ above adjacent grade area dependent