DRAFT


Advisory 1% Annual Chance Flood Elevations for Hancock County, Mississippi

Hurricane Katrina made its first landfall on 
August 25, 2005 between Hallandale Beach 
and North Miami Beach, Florida. From 
there, the storm continued into the Gulf of 
Mexico, increasing in strength to a Category 
5 hurricane. Katrina made a second landfall 
as a Category 4 hurricane on August 29 at 
Grand Isle, Louisiana. The hurricane caused 
extensive damage along the Gulf Coast of 
Mississippi.

In order to minimize the impacts of future 
catastrophic events, the Federal Emergency 
Management Agency (FEMA) is providing 
advisory information that can be used by 
local officials to help guide recovery efforts 
(including building elevations). 

The information provided is for advisory 
purposes only. In addition to determining 
site specific flood elevations, community 
officials should consider additional protective 
measures to reduce future flood risks such 
as coastal setbacks, freeboard and use of 
the Coastal Construction Manual (FEMA 
Publication 55). This publication contains the 
recommended use of coastal high hazard (VE 
Zone) building standards in all areas subject 
to 1% annual chance flood elevations caused 
by hurricane storm surges. 

The currently effective coastal flood 
elevations for Hancock County are based on 
a study performed more than 20 years ago. 
Significant hurricane activity has occurred 
throughout the Gulf Region since that study. 
Additionally, observed flood levels from 
Hurricane Katrina suggest that the currently 
effective flood elevations may not accurately 
reflect the actual flood risks. 

Consequently, FEMA has completed a 
preliminary assessment of the 1% annual 
chance flood elevations incorporating the 
observations from Hurricane Katrina as well 
as additional tide and storm data from the 
past 20 years. This preliminary analysis 
concludes that the coastal flood data depicted 
on the Flood Insurance Rate Map (FIRMs) 
for Hancock County need to be updated and 
that flood elevations are understated. The 
preliminary flood data analysis confirms that 
storm surge stillwater elevations (SWELs) 
in the current Flood Insurance Study should 
increase by 6 to 8 feet. It is also expected that 
the coastal high hazard areas (VE Zone) will 
move inland significantly and that the general 
inland extent (boundary) of the Special Flood 
Hazard Area will increase substantially when 
the FIRMs are updated. 

The preliminary analysis of historic data 
indicates that the 1% annual chance SWELs 
for Hancock County should be increased 
to 20 feet for the Gulf Coast and 18 feet for 
back bay areas. The difference between 
these elevations is wave setup that increases 
SWELs in the open coast area.

To help guide the immediate needs in the 
recovery and rebuilding process, a simplified 
method has been developed to calculate a 
site specific advisory building elevation using 
the above SWELs and the ground elevation 
at the building site. The first step in applying 
the method is to determine if the building site 
is in the open coast or a back bay area and 
selecting the appropriate advisory SWEL. 
The next step is to determine the ground 
elevation at the site so that the wave height 
for the area can be estimated. The wave 
height is estimated by dividing the flood 
depth resulting from the advisory SWEL by 
two. The Advisory Building Elevation is then 
calculated by adding the estimated wave 
height to the advisory SWEL.

This method is illustrated in the 
example below and in Figure 1.

If use of the above guidance results 
in elevation or foundation type 
requirements that appear severe, it 
is suggested that local community 
officials consider coastal setbacks 
and relocating homes and businesses 
from that area. Other factors to 
consider in situations like this include 
the frequency of flooding at higher 
frequencies (i.e. 20%, 10%, and 5% 
annual chance floods), anticipated 
future damages from other hazards 
(such as wind), and the frequency 
of historical flood losses in the area. 
High risk areas should be avoided 
whenever possible for all type of 
natural disasters.

Ultimately it will be home and business 
owners and state/local officials who make final 
decisions regarding construction type and 
elevation requirements during the recovery 
process. The guidance in this publication will 
assist them in making these decisions during 
the period in which the approximate 1% 
annual chance flood elevation estimates are 
refined and improved upon as more data is 
collected and analyzed.

Approximate Method for Calculating 
Advisory Building Elevation:

Advisory Building Elevation = SWEL + Wave

Wave = ½ depth = d/2

Example: 

Back Bay SWEL = 18 ft

Ground Elevation (z) = 10 ft

Depth = SWEL – z = 18 ft – 10 ft = 8 ft

Wave = ½ (8) = 4 ft

Advisory Building Elevation = 18 + 4 = 22 feet 
NGVD29

Figure 1 shows an example of how to apply the estimate 1% annual chance flood elevation equation.