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Flood Mitigation for Structures 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?.
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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, Florida Engineer and Contractor (1927)
Designing for Flood:All you really need to know. . . Flood Characteristics (Flood source, BFE & DFE, velocity, duration, waves, erosion, & debris) Flood Loads and Conditions Lowest Floor Elevation Foundation Flood-Resistant Materials
Coastal flooding: waves, velocity, erosion, debris, inundation, wind (V zones) Riverine flooding:inundation, velocity, debris, duration (A zones) Flood Characteristics
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) } Outside1% Flood Area
A Zones • V Zones • Coastal A Zones } Shown on the FIRM Flood Hazard Zones 3-ft breaking wave 3-ft runup depth Wave overtopping Used in ASCE 7, 24, not shown on FIRM 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
Buoyancy Failure Pointe-a-La-Hache, LA Chalmette, LA
dv2 = 200 ft3/sec2 Flood Depth and Velocity Effects
1,100 ft Only slabs remaining Debris Line SF Home Success Wave effects
Opal (1995) Ivan(2004) Hugo (1989) Wave and Erosion Damage: Don’t Rely on Erosion-Control Structures
WHERE FRENCH BUILT EXPANSION 1850 - 1950 MISS. RIVER LAKE PONTCHARTRAIN +20 SEA LEVEL 0 NATURAL LEVEE -20 ELEVATION (feet, NGVD) -40 SOFT CLAY PEAT -60 SAND -80 STIFF CLAY (PLEISTOCENE) -100 CITY OF NEW ORLEANS GENERALIZED TOPOGRAPHY AND GEOLOGY
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 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