Advisory Base Flood Elevations for St. Mary Parish, Louisiana
Hurricanes Katrina and Rita were both strong Category 5 hurricanes for several days in the Caribbean and Gulf of Mexico before pushing waters towards the Louisiana coast. Katrina made landfall on August 29, 2005 near the Mississippi-Louisiana border, and Rita made landfall on September 23, at the Texas-Louisiana border. These hurricanes caused extensive damage along the Gulf Coast Parishes of Louisiana.
To minimize the flood impacts of future events, the U.S. Department of Homeland Security’s Federal Emergency Management Agency (FEMA) is providing advisory information concerning coastal flood elevations that can be used to guide recovery efforts. This guidance is necessary because Hurricanes Katrina and Rita, along with other recent storms, have created concerns about the accuracy of the current flood risk information for St. Mary Parish (including incorporated areas) and whether or not it may be understated.
FEMA has completed an early assessment of the 1%-annual-chance (or 100-year) flood Stillwater Elevations (SWEL) which incorporate storm data from the past 35 years, including Hurricanes Katrina and Rita, new and existing long-term tidal gage records, and other existing engineering studies. The results of the analysis suggest that SWELs cited in the current Flood Insurance Study (FIS) for St. Mary Parish are adequate; however, due to land subsidence, loss of protective coastal barrier over the last 10-20 years and inclusion of wave setup, higher storm surges and larger waves can be expected to propagate further inland than previously estimated. For these reasons, FEMA expects that base flood elevation (BFE) will be increased at least 1 foot on future revised Flood Insurance Rate Maps (FIRMs) for St. Mary Parish.
FEMA is encouraging local government officials in St. Mary Parish to adopt a 1 foot freeboard and elevate structures to at least 1 foot above the current BFEs shown on the effective FIRMs for the reasons stated above. Additionally adding freeboard will provide extra flood protection to structures, reduce nuisance flooding, and may result in lower flood insurance premiums. Using freeboard is a prudent measure for ensuring structures are rebuilt using the best available information to protect lives and property, and is also a sound floodplain management practice which communities are encouraged to adopt and enforce.
"Freeboard" is defined as follows (from 44 CFR 59.1):
Freeboard means a factor of safety usually expressed in feet above a flood level for purposes of floodplain management. "Freeboard" tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed.
A FEMA coastal study of hurricane storm surge flooding is already underway, and FEMA intends to have an updated preliminary FIS and updated FIRMs for coastal areas of St. Mary Parish as early as 6 months from now. The maps will become effective following a formal appeals process and community adoption. The updated FIS and FIRM may show an increase of the SWELs, Special Flood Hazard Areas (SFHAs), and BFEs over existing flood data (including the storm data analysis and engineering studies used for this Flood Recovery Guidance), and may result in the coastal high hazard area (V Zone) moving further landward.
Until the study is completed, FEMA is encouraging communities within St. Mary Parish to use the Flood Recovery Guidance described herein. This Flood Recovery Guidance method can be used during recovery and reconstruction of the Louisiana coastal areas to determine site-specific Advisory BFEs.
Flood Recovery Guidance Method
This Flood Recovery Guidance method establishes the Advisory BFE necessary for recovery and rebuilding; for St. Mary Parish, the Advisory BFE is the effective BFE plus a recommended freeboard of 1 foot. The first step in applying the method is to use the effective FIRM to determine the SFHA and the highest BFE mapped for the footprint of the building site. FEMA requires, for National Flood Insurance Program rating purposes, the use of the most restrictive and highest BFE that encroaches on a building.
The next step is to have a surveyor determine the lowest adjacent ground elevation (also known as the lowest adjacent grade or LAG). The LAG is useful for ensuring the finished floor elevation is elevated above the Advisory BFE. For structures located in Zone VE on the effective FIRMs, the bottom of the lowest horizontal structural member must be at or above the Advisory BFE. For Zone AE slab-on-grade foundations that can’t be replaced with Zone VE pile elevated foundations, the finished first floor of the habitable area should be elevated at or above the Advisory BFE.
1. Approximate Method for Calculating Advisory BFE:
Advisory BFE = FIRM BFE + Freeboard
FIRM BFE = highest value from effective FIRM
Freeboard = 1 foot
Advisory BFE = 12 + 1 = 13 feet National Geodetic Vertical Datum 1929
St. Mary Parish FIRM BFE = Zone VE (EL 12 feet)
Freeboard = 1 foot
Compare Advisory BFE (egs., 13 feet) to building LAG. Assume building LAG (z) = 8 feet; therefore, building must be elevated 5 feet above ground surface.
Although the information provided here is advisory, communities should consider its use for rebuilding in a safer manner.
In addition to determining site-specific Advisory BFEs, community officials should consider additional protective measures to reduce future flood risks. These measures could include using additional freeboard and using FEMA's Coastal Construction Manual (CCM) (FEMA Publication 55). The CCM recommends the use of V Zone building standards in all areas subject to waves and velocity floodwaters caused by hurricane storm surges. For additional information on recommended practices, see the Coastal Construction Fact Sheet Series available at www.fema.gov/technology-transfer/home-builders-guide-coastal-construction-technical-fact-sheet-series-fema-p-499.
Ultimately it will be local officials, working with property owners, who will make final decisions regarding construction type and elevations that will apply during the recovery and rebuilding process. The Advisory BFEs will be a valuable tool until new detail studies can be developed and incorporated into the FIS and FIRMs. Within the next 2 to 3 months, FEMA will also publish a set of maps that will show detailed event information for Hurricanes Katrina and Rita, including flood inundation boundaries and high water elevations.