Chapter 2. The Flood Hazard

Chapter 2. The Flood Hazard
This chapter reviews the flood hazard that faces Gretna. By definition, flooding is caused
by more water than the drainage system can convey. Flooding is dependent on three
factors: precipitation, conditions in the watershed (where the rain comes from), and
conditions in the drainage system (where the water goes).
The first four sections of this chapter discuss the physical nature of flooding: precipitation, the watershed, the drainage system, and where floods occur. Section 2.5 covers a
special source of floodwaters that is not always dependent on local precipitation: levee
failure. The rest of the chapter reviews the impact of flooding on people and property.
2.1. Precipitation
7
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Jefferson Parish receives an
average of 60 inches of rain
each year. The rain comes from
tropical storms, thunderstorms,
and storms caused by the interaction of warm moist air with
colder air from the north. As
seen in Table 2-1, precipitation
occurs throughout the year, but
is heavier in the spring and
summer.
Table 2-1 Monthly Distribution of Rainfall
8
National Weather Service .
Tropical storms and hurricanes: The greatest amount of precipitation is caused by
tropical storms and hurricanes. These are large-scale systems of severe thunderstorms
that develop over tropical or subtropical waters and have a defined, organized circulation.
The larger storms generally form over the eastern Atlantic Ocean and move westward.
The hurricane season runs from May through November, with the peak activity in
September.
In June and October, storms are more likely to come from the Gulf, while in July −
September, they generally form in the South Atlantic. The peak recorded wind speed in
the city was 125 miles per hour during
Hurricane Camille in 1969.
Table 2-2 Saffir-Simpson Scale
Tropical storms and hurricanes are
categorized by their wind speed, as
shown in Table 2-2. While best known
for their winds, these storms can also
bring flooding of coastal regions, heavy
rains that cause inland flooding, thunderstorms, lightning, and tornadoes.
The Flood Hazard
Category
Winds
(mph)
Depression
TD
< 39
Tropical Storm
TS
39-73
Type
Surge (ft)
Hurricane
1
74-95
4-5
Hurricane
2
96-110
6-8
Hurricane
3
111-130
9-12
Hurricane
4
131-155
13-18
Hurricane
5
>155
>18
2–1
April 2007
Tropical storms and hurricanes get their energy from warm waters and lose strength as
the system crosses land. However, because Gretna is so close to the Gulf, there is not
enough land for the winds to dissipate. The City receives the full strength of a storm or
hurricane when it makes landfall.
History: The first recorded hurricane struck in 1711.
The state has had an average of 3 or 4 each decade
since detailed records have been kept (Table 2-3).
Only four hurricanes have made landfall in Louisiana
as major hurricanes of category 4 or 5 intensity:
unnamed hurricanes in 1909 and 1915, Hurricane
Audrey in 1957, and Hurricane Camille in 1969.
Camille is the only category 5 hurricane to hit Louisiana since the 1850’s (Katrina was a Category 3
when it made landfall with winds of 110 miles per
hour in the area).
In the last six years, southeast Louisiana has received
six storms and hurricanes. Here is a short history of
these recent storms.
Tropical Storm Allison: In June 2001, Tropical
Storm Allison hit Texas. After it vented, it moved
across Louisiana, causing more flooding than wind
damage. Up to 30 inches of rain fell in some areas.
Table 2-3 Louisiana Storm History
Decade Hurricanes T.S.s Total
1850's
3
1
4
1860's
7
2
9
1870's
6
3
9
1880's
7
3
10
1890's
3
6
9
1900's
2
7
9
1910's
3
2
5
1920's
3
2
5
1930's
2
8
10
1940's
3
9
12
1950's
2
7
9
1960's
4
1
5
1970's
4
3
7
1980's
4
5
9
1990's
3
2
5
Totals
57
61
118
National Weather Service
Tropical Storm Isidore: Isidore had been a
hurricane over Mexico. It was a tropical storm by
September 26, 2002. In a few hours it went from
landfall at Grand Isle to central Mississippi and
was downgraded to a tropical depression. While
Isidore had more wind than rain, it caused a significant storm surge over a large area. At Lake
Pontchartrain, storm surges of 4 to 5 feet above
normal were measured.
Tropical Storm Isidore
Hurricane Lili: On the heels of Isidore, Lili hit
St. Tammany Parish
the State on October 2, 2002. It had been a
category 4 storm, but dropped to a category 1 hurricane just before landfall. Lili caused 3
to 5 feet of storm surge tides across most of coastal southeast Louisiana. Up to 10 inches
of rain fell. Heavy rainfall was not widespread, in part due to the rapid movement of the
hurricane away from the area. Flash flooding occurred in only a couple of areas.
Tropical Storm Bill: This storm moved into southeast Louisiana on June 30, 2003. It
caused a storm surge of 3 to 5 feet above normal along Lake Pontchartrain. In 48 hours, it
rained 6 to 10 inches.
The Flood Hazard
2–2
April 2007
Hurricane Katrina: Although earlier estimated to be a Category 5 storm, Hurricane
Katrina made landfall at Buras, Louisiana, on August 29, 2005, as a Category 3.
Maximum winds were 110 miles per hour. Jefferson Parish’s levees held, but there was
extensive flooding because of the rainfall (estimated at 7 – 14 inches in the area) and lack
of pumping (the pump stations had been shut down when the operators were told to
evacuate).
Hurricane Rita: At one time a category 5 storm, Hurricane Rita weakened to a category
3 hurricane by the time she made landfall on September 23, 2005, just east of the Texas/
Louisiana border. In southeast Louisiana, the main affect from Hurricane Rita was storm
surge flooding in coastal areas. Storm surges of 5 to 7 feet above normal overtopped or
breached local drainage levees, including the Industrial Canal in New Orleans. There was
concern about high water in the Harvey Canal and the potential for a levee failure there,
which would have impacted Gretna. Rainfall ranged from 6 to 12 inches.
Unnamed Storms: While named tropical storms and
hurricanes are the best known causes of significant
flooding, sometimes heavy rains from “regular”
storms can cause a lot of damage. Table 2-4 shows
that heavy rains of greater than 9 inches over 1 – 4
days occurred 8 times over a 20 year period.
In fact, the most damaging storm before Katrina was
an unnamed storm in May 1995. Over 9 inches of rain
fell in 3 hours, an intensity that exceeded a 500-year
storm for that time period, flooding some homes up to
two feet deep. Before Katrina, it was the single most
expensive flood event for the National Flood Insurance Program in Louisiana.
Table 2-4. Historical
“Unnamed” Rainstorms
Date
May 3,1978
April 12 – 13, 1980
April 24, 1982
April 6, 1983
April 1 – 2, 1988
Nov 7 – 8, 1989
May 8 – 9, 1995
Sept 10 – 13, 1998
Rainfall
10.50”
11.86”
12.92”
10.20”
9.74”
16.01”
12.70”
13.18”
Jefferson Parish Mitigation Plan
2.2. The Watershed
A “watershed” is an area of land that drains into a lake, stream or other body of water.
The runoff from rain is collected by swales, storm drains, and ditches which send the
water to the stream or other body of water. In Gretna’s case, the water is sent to canals
which drain to pumps near the levees. The water is pumped over the levees where it
eventually drains to the Gulf.
Because it is nearly surrounded by drainage canals, the City of Gretna can be considered
its own watershed − almost all the water that falls on Gretna either stays in the City or
drains to the Heebe or Whitney Canals on the City’s borders.
There are three main watershed conditions that affect local flooding:
1. The size of the watershed. Smaller watersheds will flood more quickly, Gretna is a
small watershed.
The Flood Hazard
2–3
April 2007
2. The ground surface. More water runs off
pavements than off grass or natural
ground cover (see graphic). Even in
natural conditions, the soil in Gretna is
not very permeable.
3. The slope of the land. More rain will run
off the land and into the streams if the
terrain is steep. Because much of Gretna
is flat, water tends to pond where it falls
and run off slowly.
Therefore, given the soil conditions and the
great extent of pavement and other impervious surfaces, much of the rain that falls on
Stormwater runoff increases
Gretna is not absorbed into the ground and
as areas are urbanized
must be handled by the drainage system.
Association of State Floodplain Managers
Because of the small watershed and flat
slope, the water will collect quickly in low spots, but will not drain away quickly. This
results in very localized flooding conditions, soon after and even during rainstorms.
2.3. The Drainage System
Water flows downhill. The highest ground in Gretna is to the north, where the Mississippi
River formed a “natural levee” over the centuries as it deposited sediment along the riverbank during floods. Under natural conditions, most of the rainfall that falls on Gretna
would flow south. However, human development has disrupted the terrain with levees,
streets, and other construction. For example, Map 2-2 (page 2-7) shows how the Westbank Expressway forms a barrier to surface flows going from north to south.
Being so flat and surrounded by a levee, Gretna cannot rely on a natural drainage system
to carry water away. Over the years, a man-made system has been constructed to collect
stormwater, convey it to the canals, and pump it over the levee. This system was designed
to carry the 10-year storm under good conditions.
Floods will occur when Nature drops more than a 10-year storm. The 10-year “design
storm” is nine inches of rain in 24 hours. As noted in Table 2-4, the City has received
more than 9” of rain many times in recent years, although usually over longer periods
than 24 hours.
The drainage system will not be able to carry even the 10-year storm when:
─ The storm drains are obstructed or broken, especially when the ground under the
pipes settles from subsidence.
─ The ditches are obstructed by trash jams or small bridge or culvert openings. If
there is no room in the channels, even smaller storms will cause floods.
The Flood Hazard
2–4
April 2007
─ The pumps do not operate. This happened during Hurricane Katrina when the
Parish ordered pump operators to evacuate for their own safety.
In sum, the drainage system in Gretna is a man-made system that depends on constant
maintenance to ensure that it can carry water away and pump it outside the levee. Even if
it works to full capacity, it can only handle a 10-year storm. The City’s 1998 Floodplain
Management Plan commented on this situation:
The City of Gretna has experienced substantial flooding in recent history, with as many as
eight (8) rain events producing flooding of buildings in a 17 -year period. During high intensity
rainfall events, the subsurface drainage system becomes surcharged and runoff collects or
ponds within the low-lying ground areas of the City before it can reach the pumps and be
discharged into one of the outfall canals.
Since the City must be pumped for drainage, conditions dictate that the 10-percent-probability
or 10-year design storm event (defined for the area as 9 inches of rainfall in 24 hours) serve
as the basis of storm water design. Any greater level of protection would be difficult to obtain.
From a logistics standpoint, the flat ground and slight gradients makes it difficult to adequately bury large subsurface pipes or lay pipes on steep slopes. From an economic standpoint,
costs of increased pumping capacity and deeper excavations, which must necessarily be
braced due to poor soil conditions, are restrictive. (pages 8 – 9).
Gretna’s drainage system consists of storm drains, roadside ditches (left) and larger channels that
convey stormwater to the main collector canals, such as the Heebe Canal (right). The system will not be
able to convey the 10-year storm where there are obstructions or blockages (left).
2.4. Flood Risk and Floodprone Areas
Mitigation plans are based on the risk of future flooding, even to areas that have not
flooded in recent memory. Studies extrapolate from historical records to determine the
statistical potential that storms and floods of certain magnitude will recur. Such events
are measured by their “recurrence interval,” i.e., a 10-year storm or a 50-year flood.
These terms are often misconstrued. Commonly, people interpret the 50-year flood
definition to mean “once every 50 years.” This is incorrect. Statistically speaking, a 50year flood has a 1/50 (2% or .02) chance of occurring in any given year. A 50-year flood
could occur two times in the same year, two years in a row, or four times over the course
of 50 years. It is possible to not have a 50-year flood over 100 years (see box, next page).
The Flood Hazard
2–5
April 2007
Base flood: FEMA uses the “base”
flood as the basis for its regulatory
requirements and flood insurance rate
setting. The base flood is the one
percent chance flood, i.e., the flood
that has a one percent (one out of 100
or .01) chance of occurring in any
given year.
Another term used is the “500-year
flood.” This has a 0.2% or .002
chance of occurring in any given
year. While the odds are more
remote, it is the national standard
used for protecting critical facilities,
such as hospitals and fire stations.
What are the Odds of a Flood?
Chance of Flooding over a Period of Years
Time
Flood Size
Period 10-year 25-year 50-year 100-year
1 year
10%
4%
2%
1%
10 years
65%
34%
18%
10%
20 years
88%
56%
33%
18%
30 years
96%
71%
45%
26%
50 years
99%
87%
64%
39%
These numbers do not convey the true flood risk
because they focus on the larger, less frequent, floods.
If a house is low enough, it may be subject to the 10or 25-year flood. During the proverbial 30-year
mortgage, it may have a 26% chance of being hit by
the 100-year flood, but the odds are 96% (nearly
guaranteed) that a 10-year flood will occur during the
30 year period. Compare those odds to the only 5%
chance that the house will catch fire during the same
30-year mortgage.
FIRM: The area affected by the base
flood in Gretna has been mapped. The official floodplain study for insurance and
regulatory purposes is FEMA’s Flood Insurance Study for Jefferson Parish and the
official map is the Parish’s Flood Insurance Rate Map (FIRM), dated March 23, 1995.
Gretna is on panels 135, 145, and 155. A portion of the FIRM is shown in Map 2-1.
The base or 100-year floodplain shown on the FIRM panels is the AE Zone shown in
blue on Map 2-2. This is called the Special Flood Hazard Area on the FIRM and is
designated as an “AE Zone.” Areas outside the AE Zones are called X Zones, i.e., the
white areas on Map 2-2.
The FIRM also shows the base flood elevations (BFE). In the AE Zones north of the
Westbank Expressway, the BFE is 1.5 feet above sea level. South of the Expressway, the
BFE in the AE Zones is -1.5 feet, or 1.5 feet below sea level.
Map 2-1. Excerpt from the Gretna portion of the Jefferson Parish FIRM
The Flood Hazard
2–6
April 2007
Map 2-2. City of Gretna AE Zones
The Flood Hazard
2–7
April 2007
It should be noted that the FIRM is based on the conclusion that the Mississippi River
levee will hold up to at least the river’s 500-year flood level (Flood Insurance Study,
page 10). The Study also states
The hydraulic analyses for this study were based on unobstructed flow. The flood elevations
show on the maps are this considered valid only if hydraulic structures [i.e., ditches, canals, and
pump stations] remain unobstructed, operate properly, and do not fail. (page 17)
Other floods: It should be noted that the FIRM shows the relatively rare, but very
possible, base flood. Based on the historical record, a tropical storm or hurricane should
be expected somewhere within the state every 1.2 years (0.83 chance). A hurricane
should make landfall every 2.8 years.
Storms hit Gretna even more often and smaller floods that affect smaller areas are more
likely to occur. An intense localized storm can cause flooding in one part of the City and
an obstruction in a ditch can result in a flood from any rainfall. An even bigger potential
hazard is levee failure, which would cover the whole city. Accordingly, the mapped
floodplain or AE Zone should be viewed only for its regulatory and insurance
implications, not as an exact designation of what areas will be flooded.
The future: Another reason to not rely on the FIRM as a definitive statement of the
flood hazard is that things are predicted to get worse. Whether it’s attributed to global
warming or other factors, signs point to increased precipitation. The State Mitigation Plan
states “Over the past century, there has been an apparent increase in large rainstorms and
resultant flooding, particularly in the late winter and spring.” (p. I-33)
Subsidence compounds the problem of increased rain. The land is sinking. Again, from
the State Mitigation Plan, “Subsidence is already occurring throughout much of coastal
Louisiana. An acre of land along the coast disappears every 24 minutes.” (p. I-48)
2.5. Levee Failure
The previous sections discussed how and where “normal” or “historical” flooding occurs.
Flooding can also be caused by levee failure. For the purposes of this Plan, levee failure
includes overtopping, breach, or collapse of the levee that protects the City. Some of the
causes of levee failure are:
–
–
–
–
–
Overtopping due to flood heights exceeding
a levee’s design protection elevation
Flooding from sources internal to the levee
Erosion caused by embankment leaking or
piping
Erosion of the levee base caused by moving
waters
Improper operation and maintenance,
including failure to inspect and repair
seepage problems
Failure of the Industrial Canal levee in
New Orleans following Hurricane Katrina
FEMA
The Flood Hazard
2–8
April 2007
Technically, overtopping is not a “failure,”
it is simply a case of water going higher
than the design protection level. However,
the results are the same to the people and
properties affected. Levees have been
overtopped or breached during flood
events and non-flood events in Louisiana.
While the failure of the levees protecting
New Orleans during Hurricane Katrina is
the nation’s greatest example of the impact
of a broken levee, the City of Gretna has
had experiences with such a flood in the
past (see photos).
Lafayette Street, 1891 levee failure flood
635 Lafayette Street during the 1891 levee failure flood and today
Gretna is protected by two levee systems.
Along the Mississippi River, the Corps of
Engineers has constructed and maintains
levees that are rated to protect the area
from the “standard project flood.” This is
usually equated to a 250- to a 500-year
flood.
Map 2-3. Hurricane Protection Levees
The rest of the area is protected by a
hurricane protection system that runs along
the Harvey and Algiers Canals. A recent
survey of levee heights showed them to be
lower than desired. Map 2-3 shows how
high they are. Based on these elevations,
the hurricane protection system would not
be rated as having 100-year levees.
West Jefferson Levee District website
The Flood Hazard
2–9
April 2007
Even the higher and stronger Mississippi River levees are not safe from damage. In July
1985, a section of levee along the Mississippi River near Marrero failed in a non-floodrelated event. The “Celotex Failure” was named for the factory at the site. The failure
was caused by scouring and erosion of sand along the river bank and created a hole 600
feet long.
It took three months before repairs were completed. There is no guarantee that other
sections of the levee system are free of similar trouble spots. Map 2-4 shows the location
of the 1985 “Celotex Failure” and its proximity to Gretna.
Map 2-4. Location of the Celotex Levee Failure
A Case History of Embankment Failure, page 3
Levee failure can be a great hazard where the levee is large. People assume they will
never be flooded, and no flood protection measures are taken for new construction. This
hazardous situation is often the case where FEMA has mapped leveed areas as being
outside the 100-year floodplain. Being outside the AE Zone, there are no Federal or State
flood protection requirements for new construction.
An interesting case involves Jackson, Mississippi, on the Pearl River. The US Army
Corps of Engineers had constructed levees in the 1960s to protect the town from
flooding. The levees were overtopped in the 1979 flood, with some 40% of the damage
being inflicted on buildings constructed after the levee was built.
Levee failure is a greater threat to Gretna than the flooding described in the previous
sections. The AE Zone in Map 2-2 is based on rainfall flooding, so flood heights are
nowhere near as high as a levee failure flood. Over half the City is outside this mapped
Special Flood Hazard Area, where there is no flood insurance purchase requirement and
no flood protection requirements for new construction.
The Flood Hazard
2–10
April 2007
2.6. Threat to People
Life safety: The hazard presented by
floodwaters is dependent on how deep it
is and how fast it moves. The speed of
moving water, or velocity, is measured in
feet per second. In Gretna, velocities
outside of the ditches and canals are
generally less than one foot per second.
The relationship between depth and
velocity is shown in the graph to the right.
While high velocities don’t often occur in
Gretna, it doesn’t take much depth to be
Depth – Velocity danger levels for people
dangerous. A car will float in less than 2
feet of moving water and can be swept downstream into deeper waters (see graphic). This
is one reason floods kill more people trapped in vehicles than anywhere else. Victims of
floods have often put themselves in perilous situations by ignoring warnings about travel
or mistakenly thinking that a washed-out culvert is still there.
Effects of shallow water on cars
Flash Floods and Floods. The Awesome Power
The Flood Hazard
2–11
April 2007
A levee failure flood should not come as a surprise. It will happen during high water
when levee conditions would be monitored by the levee district. Therefore, the City
should not have a high level of exposure to life threatening problems. The most important
factor for protecting people is a timely warning. However, if people consider themselves
safe from flooding and do not evacuate, then the results could be deadly.
Electrocution is the number two cause of flood deaths, claiming lives in a flooded area
that is carrying a live current created when electrical components short. People die of
heart attacks, especially from exertion during a flood fight.
Floods also can damage gas lines, floors
and stairs, creating secondary hazards
such as gas leaks and unsafe structures.
Propane tanks can float, causing gas
leaks and sending explosive “torpedoes” downstream (see photo).
Fire can be a result of too much water:
floods can break gas lines, extinguish
pilot lights, and short circuit electrical
wiring—causing conditions ripe for a
fire. Fire equipment may not be able to
reach a burning building during high water.
During a flood, a propane tank floated into
this building, exploded, and destroyed it
Floodplain Management Desk Reference, p. 2-3
Health: Two general types of health hazards accompany floods. The first comes from
the water itself. Floodwaters carry whatever was on the ground that the runoff picked up,
including dirt, oil, animal waste, and lawn and industrial chemicals.
Rain and floodwaters saturate the ground which leads to infiltration into sanitary sewer
lines. When wastewater treatment plants are flooded, there is nowhere for the sewage to
flow. Infiltration and lack of treatment lead to overloaded sewer lines which back up into
low lying areas and some homes. Even though diluted by flood waters, raw sewage can
be a breeding ground for bacteria, such as E. coli, and other disease causing agents.
The second type of health problem comes after the water is gone. Stagnant pools become
breeding grounds for mosquitoes, and wet areas of a building that have not been cleaned
breed mold and mildew. A building that is not
thoroughly and properly cleaned becomes a health
hazard, especially for small children and the elderly.
These health problems can be aggravated when
heating ducts in a forced-air system are not properly
cleaned after inundation. When the furnace or air
conditioner is turned on, the sediments left in the
ducts are circulated throughout the building and
breathed in by the occupants. If the water system
loses pressure, a boil order may be issued to protect
people and animals from contaminated water.
The Flood Hazard
2–12
Post-flood silt, mold and mildew
April 2007
Mental health: There is a long-term psychological impact of having been through a
flood and seeing one’s home damaged and irreplaceable keepsakes destroyed. The cost
and labor needed to repair a flood-damaged home puts a severe strain on people,
especially the unprepared and uninsured. There is also a long-term problem for those who
know that their homes can be flooded again. The resulting stress on floodplain residents
takes its toll in the form of aggravated physical and mental health problems.
2.7. Property Damage
The City of Gretna has 6,689 primary
structures (buildings other than garages,
sheds, etc.). Of these, 855 are in the FEMA
mapped floodplain (AE Zones). Table 2-5
shows these numbers in more detail.
Table 2-5. Primary Buildings
Type
Single Family Home
Commercial
Other
Total
In City
5,376
554
759
6,689
AE Zones
721
98
36
855
As with the threat to people, depth and velocity of flooding determine property damage.
Flood velocity is important because the faster water moves, the more pressure it puts on a
structure and the more it will erode channel banks and scour the earth around a building’s
foundation. In a few situations, deep or fast moving waters will push a building off its
foundation, but this is rare and Gretna has no areas where the depths and velocities are
that high.
The most common type of damage inflicted by a flood in Gretna is caused by soaking.
When soaked, many materials change their composition or shape. Wet wood will swell
and, if dried too quickly, will crack, split or warp. Plywood can come apart. Gypsum
wallboard will fall apart if it is bumped before it dries out. The longer these materials are
wet, the more moisture, sediment and pollutants they will absorb.
Soaking can cause extensive damage to household goods. Wooden furniture may become
so badly warped that it cannot be used. Other furnishings such as upholstery, carpeting,
mattresses, and books usually are not worth drying out and restoring. Electrical
appliances and gasoline engines will not work safely until they are professionally dried
and cleaned.
In short, while a building may look sound and unharmed after a flood, the waters can
cause a lot of damage. As shown in the photo below, to properly clean a flooded building,
the walls and floors should be stripped, cleaned, and allowed to dry before being
recovered (see photo, next page). This can take weeks and is expensive.
2.8. Flood Insurance Data
As of July 2006, there are 3,084 National Flood Insurance Program (NFIP) flood
insurance policies on properties in Gretna. This number overstates the amount of
coverage because the records are kept based on the NFIP community number. Many
policies in unincorporated Jefferson Parish have been incorrectly listed under the Gretna
number because their mailing addresses have “Gretna” in them.
The Flood Hazard
2–13
April 2007
Gretna resident throwing out soaked carpet
after Katrina
Since 1978, there have
been over 4,200 flood
insurance claims
submitted for policies
under the Gretna community number. Just over
1,000 were not paid,
leaving 3,197 paid
claims. Even though
some number of these
claims are for properties
not in the City, the data
can provide useful
information for this Plan.
The total and average
claim payments for major
flood events are shown in
Table 2-6.
Table 2-6 shows that the
Hurricanes Katrina and
Rita were the worst in
terms of number of claim
payments and average
dollar damage. They also
show a pattern of claims
from Spring and Fall
storms. The maps on the
next two pages show
where the claims were
located.
Proper cleaning after a flood requires stripping
Walls and floors and letting them dry thoroughly
Table 2-6 Flood Insurance Claims Payment Data
Date
Number
May 1978
1979
April 1980
1981
April 1982
July/August 1982
December 1982
April 1983
1983 – 1984
October 1985
April 1988
November 1989
May 1990
1991 – 1993
May 1994
May 1995
1996
May 1997
1997 – 1998
September 1998
2000
June 2001 (Allison)
Sept 2002 (Isidore)
October 2002 (Lili)
2003 – 2004
June 2005
July 2005
August 2005 (Katrina)
September 2005 (Rita)
All Claims
353
9
683
8
113
5
30
358
2
17
124
264
94
20
22
213
2
10
5
116
2
38
21
6
8
11
3
658
2
3,197
Total Paid
$1,928,679
$19,712
$5,962,176
$45,929
$431,581
$3,218
$56,661
$2,149,511
$2,511
$269,231
$538,737
$2,538,281
$488,776
$70,996
$29,063
$2,551,078
$9,911
$30,162
$13,839
$989,429
$9,910
$144,101
$100,861
$20,609
$32,413
$34,148
$18,426
$19,360,257
$70,306
$37,920,510
Average Claim
$5,464
$2,190
$8,729
$5,741
$3,819
$644
$1,889
$6,004
$1,256
$15,837
$4,345
$9,615
$5,200
$3,550
$1,321
$11,977
$4,955
$3,016
$2,768
$8,530
$4,955
$3,792
$4,803
$3,435
$4,052
$3,104
$6,142
$29,423
$35,153
$11,861
FEMA-NFIP .
The Flood Hazard
2–14
April 2007
Map 2-5. Historical Flood Insurance Claims
This series of maps shows the locations of
the properties for which flood insurance
claims were paid for that event. While there
are some concentrations of flood losses in
the shaded AE Zones, all parts of the City
have been subject to flooding, especially
during the more severe storms, such as
1980, 1995, and 2005 (Katrina).
FEMA-NFIP data
May 3, 1978
April 6, 1983
April 13, 1980
The Flood Hazard
2–15
April 2007
Map 2-5. Historical Flood Insurance Claims
November 7, 1989
May 8, 1995
September 11, 1998
August 29, 2005 (Katrina)
The Flood Hazard
2–16
April 2007
The numbers in Table 2-6 show that between the May 1995 unnamed storm and Katrina,
there was only one significant event, September 1998. This was Tropical Storm Frances
which dropped up to 12 inches of rain in the New Orleans area. The lack of major
damage since 1995 can be attributed to drainage improvements that have been
constructed throughout Jefferson Parish.
Another interesting fact from Table 2-6 is the amount of the claim payments. Over the
years, claims have averaged $11,861. Only 168 claims (5%) were paid for $50,000 or
more. When Katrina and Rita are not counted, the average claim has been for $7,288.
There were only 17 claims paid for $50,000 or more − less than 1% of the 2,537 claims
paid before Katrina.
It must be kept in mind that claim payments do
not include the standard $500 or $1,000 deductible. They are only for what is covered by a
flood insurance policy and do not include
vehicles, landscaping, items kept outdoors, and
certain valuables or jewelry. Further, many
people do not purchase the optional contents
coverage, so many claim payments do not
include the cost of damage to furniture, throw
rugs, clothes, etc.
Privacy of Flood Insurance Data
The Privacy Act of 1974 (5 U.S.C. 522a)
restricts the release of certain types of
data to the public. Flood insurance policy
and claims data are included in the list of
restricted information. FEMA can only
release such data to state and local
governments, and only if the data are
used for floodplain management, mitigation, or research purposes. Therefore, this
report does not identify the repetitive loss
properties or include claims data for any
individual property.
After Katrina, FEMA reviewed flood insurance claim payments and identified 65
properties in Gretna that appeared to be damaged more than 50% of the building’s value
based on the amount of claim payment and an estimate of the building’s value. FEMA
and the City reviewed these sites more closely using FEMA’s Residential Substantial
Damage Estimator software.
This work concluded that 27 buildings were damaged more than 50% of their market
value. Of the 27, 22 (81%) were damaged between 50% and 60%. It is not clear how
much of the damage is also attributed to wind from Katrina.
2.9. Repetitive Losses
A “repetitive loss” property is an NFIP-insured property where two or more claim
payments of more than $1,000 have been paid within a 10-year period since 1978. To
focus resources on those properties that represent the best opportunities for mitigation,
two subcategories have been defined: the Target Group and Severe repetitive losses.
Target Group properties are repetitive loss properties that have had four or more claims
of more than $1,000 since 1978, or two or three claims that equal or exceed the building’s
value. While the Target Group is no longer used in FEMA’s programs, FEMA had field
checked these properties and collected a lot of data on them for analysis over the last few
years. Unlike the data on other repetitive loss properties, the Target Group information
has been confirmed on the ground.
The Flood Hazard
2–17
April 2007
The Target Group term has been replaced by “Severe” repetitive loss properties. These
are 1-4 family residences that have had four or more claims of more than $5,000 or at
least two claims that cumulatively exceed the reported building’s value. This definition
comes from the Flood Insurance Reform Act of 2004, which established new funding
mechanisms to help mitigate flood damage for the worst case repetitive loss properties.
There are 426 repetitive loss properties coded to the Gretna NFIP community number. As
explained earlier, it is likely that some number of these are not in the City limits. Deleting
zip codes other than 70053 leaves 404 repetitive loss properties in the City. These were
plotted by the University of New Orleans repetitive loss project. The results are shown on
Map 2-6.
Table 2-7 has some statistics about
these 404 properties, based on April
30, 2006, data from FEMA. Over
half of the 404 properties have only
submitted two or three claims. The
average payment was $11,255.
FEMA’s repetitive loss records
show that of the 404 properties, two
have been mitigated by being elevated. An additional 156 are reported
to now be protected from repetitive
flooding by one or more drainage
improvement or stormwater
management projects.
Table 2-7. Repetitive Loss Data
Number of
Claims
2
3
4
5
6
7
8
9
10
11
15
Total
Number of
Properties
Total Claim
Payments
Average Claim
Payment
128
108
58
32
23
22
12
11
5
4*
1
404
$2,614,589
$3,246,179
$2,476,995
$1,915,833
$1,424,502
$1,819,641
$1,289,115
$1,389,066
$506,792
$872,552
$92,070
$17,647,334
$10,213
$10,019
$10,677
$11,974
$10,322
$11,816
$13,428
$14,031
$10,136
$19,831 *
$6,138
$11,255
* One property with 11 claims skews these figures. It is not a
single family home and it had very high contents claims. If it is
not counted, the average claim for this group would be $15,600
Map 2-6 does not reflect these
designations because a check of the
NFIP Data
addresses doesn’t reveal any differentiation by street address. For example, FEMA data show 25 sites on Hero between 21st
and 33rd as “mitigated due to drainage improvements” and 46 on the same blocks as not
mitigated.
In a 2004 study by FEMA, Gretna was ranked in the top 20 communities of the country
with the most Target Group properties. Seventy-eight (78) of the 404 properties are
Target Group properties that are still insured. The insurance policies were transferred to
the “Special Direct Facility” and managed directly by FEMA. This group does not
include some of the worst cases, including the property with 15 claims (which is no
longer insured, at least not under the same address).
The 78 properties have submitted a total of 488 claims, or an average of 6.25 each. The
total payments have been $6.2 million. Nineteen percent (19%) of the repetitive loss
properties account for 35% of the total claim payments. The average claim for the Target
Group was $12,688.
The Flood Hazard
2–18
April 2007
Map 2-6. Repetitive Loss Properties
The Flood Hazard
2–19
April 2007
Repetitive loss areas: FEMA
planning guidance and credit
under the Community Rating
System calls for designating
repetitive loss areas. “The
repetitive loss areas must
include the properties on the
repetitive loss list obtained
from FEMA and all adjacent
properties with the same or
similar flooding conditions.”
(CRS Coordinator's Manual,
page 500-7).A review of the
addresses and Map 2-6 shows
seven concentrations of
problem sites. These are shown
on Map 2-7.
Map 2-7. Repetitive Loss Concentrations
While there are some
concentrations, there are still
both regular and Target Group
repetitive loss properties
scattered throughout the City.
Therefore, for the purposes of
flood hazard mitigation and the
CRS requirements, the entire
City should be considered a
repetitive loss area. This means
that all properties in Gretna
will get equal attention from
the City and the CRS.
2.10. Critical Facilities
When dealing with flooding, some properties are more important than others. “Critical
facilities” are buildings and infrastructure whose exposure or damage can affect the well
being of the community. For example, the impact of a flood on a hospital is greater than
on a home or most businesses. Critical facilities are not strictly defined by any agency.
Generally, they fall into two categories:
–
Buildings or locations vital to public safety and the disaster response and recovery
effort, such as police and fire stations and telephone exchanges, and
– Buildings or locations that, if damaged, would create secondary disasters.
Examples of such buildings or locations are hazardous materials facilities and
schools.
This planning effort identified 41 critical facilities in six general categories. The distribution of these facilities is shown in Table 2-8. Their locations are plotted on Map 2-8.
The Flood Hazard
2–20
April 2007
Map 2-8. Critical Facilities
The Flood Hazard
2–21
April 2007
The six categories of critical facilities are:
─ Government buildings, i.e., City Hall and
the Parish office building
─ EPA facilities (hazardous materials)
─ Schools (because they house an
especially vulnerable segment of society
and they served as shelters following a
disaster)
─ Public safety (fire stations, police and
Parish correctional facility)
─ Utilities, water works, sanitary sewer lift
stations, etc.
─ Elderly housing
Table 2-8. Critical Facilities
Category
Government
EPA Facilities
School
Public Safety
Utilities
Elderly Housing
Total
Number
in City
2
2
12
6
18
1
41
Number in
AE Zone
7
3
14
24
Table 2-8 shows that most critical facilities are schools and utility facilities, like sanitary
sewer lift stations. Table 2-8 and Map 2-8 show that most are not in the deeper areas of
the mapped AE Zones. While all could be damaged by a levee break flood, the shallower
stormwater and drainage problems will not do much damage to most sites.
The exceptions are the seven schools, three fire stations, the wastewater treatment plant,
and 13 sanitary sewer lift stations that are in the mapped AE Zones. They would
especially benefit from special flood protection measures and/or flood emergency plans.
Examples of Critical Facilities in Gretna
Gretna Police Station
Gretna Water Works
Hazardous materials storage
Gould Fire Station
The Flood Hazard
2–22
April 2007
2.11. Summary
The City of Gretna is subject to two flood hazards:
─ Flooding from rain and stormwater that overloads the drainage system, and
─ Flooding from the failure of one of the levees that surround the area.
Much of the rain that falls on Gretna is not absorbed into the ground and must be handled
by the drainage system. Because of the small watershed and flat slope, the water will
collect quickly in low spots, but will not drain away quickly.
The drainage system is a man-made system that depends on constant maintenance to
ensure that it can carry water away and pump it outside the levee. Even if it works to full
capacity, it can only handle a 10-year storm. If there are breaks or obstructions in the
system or if the pumps don’t run, even smaller storms will create flooding conditions.
The City’s situation means it faces localized flooding conditions, soon after and even
during rainstorms. The AE Zone mapped on the Flood Insurance Rate Map is not an
exact designation of what areas will be flooded. The entire City should be considered
subject to the hazards of both levee break flooding and flooding from rain and stormwater. The latter has been the cause of the City’s designation by FEMA as one of the top
repetitive flooding problems in the country.
While damaging and causing safety and health problems, most floods in Gretna do not
threaten lives, destroy buildings or cause substantial structural damage. Flooding from a
levee failure is a different story − such a hazard can kill people and wipe out large areas
of the City. All of the flood hazards cause health and mental health problems.
Because the City has not experienced a levee failure in recent history, flooding to date
has been primarily a problem of property damage. The City is one of the most repeatedly
floodprone communities in the country with over 3,000 flood insurance claim payments
and over 400 repetitive loss properties. Flooding also affects critical facilities and key
City services and utilities.
While most of the repetitive loss properties are concentrated in seven areas, no part of
the City is free from repetitive shallow flooding caused by an overloaded drainage system
and no part of the City is free from damage by a levee failure.
2.12. References
─ A Case History of Embankment Failure: Geological and Geotechnical Aspects of the
Celotex Levee Failure, New Orleans, Louisiana, US Army Engineer Research and
Development Center, Technical Report GL-99-11, 1999
─ City of Gretna Floodplain Management Plan, Burk-Kleinpeter, Inc, 1998
─ City of Gretna web site, www.gretnala.com/
─ CRS Coordinator's Manual
─ Flood insurance claims records, Federal Emergency Management Agency, 2006
The Flood Hazard
2–23
April 2007
─ Flood Insurance Rate Map, Jefferson Parish, Louisiana, Federal Emergency
Management Agency, 1995.
─ Floodplain Management Desk Reference, Louisiana Department of Transportation
and Development, 2004.
─ Interview with Jack Griffin, Director of Public Works
─ Interview with Jerry Spohrer, Executive Director, West Jefferson Levee District
─ Interview with Mike Chopin, City Engineer
─ Jefferson Parish Hazard Mitigation Plan, and Appendix C, City of Gretna, July 2005
─ National Weather Service websites
─ NCDC: website of the National Climatic Data Center,
www.ncdc.noaa.gov/oa/ncdc.html
─ RSDE Community Report, FEMA, November 8, 2005
─ State of Louisiana Hazard Mitigation Plan, Louisiana Office of Homeland Security
and Emergency Preparedness, April 15, 2005
─ The Public Health Consequences of Disasters, U.S. Department of Health and Human
Services, Public Health Service, 1989.
─ Websites of the US Army Corps of Engineers, Jefferson Parish, City of Gretna, and
West Jefferson Levee District
The Flood Hazard
2–24
April 2007