##### Case Study

##### SLOSH Model 

#### **SLOSH**

#### 

#### (The following description is provided by the Department of the Army, New Orleans District, Corps of Engineers, "Technical Data Report: Southeast Louisiana Hurricane Preparedness Study," August, 1994.) A copy of this case study is available in Readings: 9-SLOSH Case Study---a Word document)

**Activity for the Class:** Class members may be asked to read "Hazards
Analysis -- Modeling" for discussion in class. The following questions
may be discussed with the class concerning modeling natural disaster
events.

-   How could the SLOSH model be used by emergency managers?

```{=html}
<!-- -->
```
-   Do you see the information drawn from the model as a planning,
    response, recovery, or mitigation tool?

```{=html}
<!-- -->
```
-   As an emergency manager, what do you consider to be the strengths
    and limitations of this model for understanding hurricanes?

The purpose of hazards analysis and modeling is to quantify the adverse
impact of an event. The primary object of a hazards analysis is to
determine the probable worst-case effects from various events including
chemical releases, hurricanes, floods, earthquakes, and other unplanned
events. An excellent example of modeling as applied to disaster is the
Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model developed
by the National Weather Service. SLOSH calculates the potential surge
heights from hurricanes. It is a two-dimensional model that was designed
for real-time forecasting of surges from actual hurricanes within
selected Gulf and Atlantic coastal basins. In addition to furnishing
surge heights for the open coast, the SLOSH model has the added
capability of computing the routing of storm surge into bays, estuaries,
or coastal river basins as well as calculating surge heights for
overland locations. Significant natural and man-made barriers are
represented in the model and their effects simulated in the calculations
of surge heights within a basin.

### Forecasting Inaccuracies

An analysis of hurricane forecasts made by the National Hurricane Center
indicates the magnitude of error that can be expected in forecasting the
track of approaching hurricanes. The average error in the official
hurricane track forecast between 1970 and 1979 was 51 miles for the
12-hour forecast, 109 miles for the 24-hour forecast, 247 miles for the
48-hour forecast, and 377 miles for the 72-hour forecast. Thus if a
storm were forecast to make landfall due south of New Orleans in 24
hours, and if, in fact, it made landfall anywhere between Dauphin
Island, Alabama and Marsh Island, Louisiana, the error in the forecast
landfall position would be no worse than average. There has been a small
but significant downward trend in the forecast errors in the Atlantic
basin over the period 1970-1991. The downward trend is found not only in
the 24-hour forecast, but also in the 48- and 72- hour forecasts.

Errors also occur in forecasting the maximum sustained wind speed of an
approaching hurricane. During the period 1970-1979, the average error in
the official 24-hour wind speed forecast was 15 miles per hour (mph),
and the average error in the official 24-hour wind speed forecast was 15
mph, and the average in the 12 hour forecast was 10 mph. Hurricane
evacuation decision makers should note that an increase in wind speed of
10 to 15 mph could easily raise the intensity category of the
approaching hurricane on category on the Saffir/Simpson Hurricane Scale.
To account for inaccuracies in forecasting the behavior of approaching
hurricanes, the National Hurricane Center recommended that public
officials faced with an imminent evacuation prepare for the evacuation
as if the approaching hurricane will intensify one category above the
strength forecast for landfall.

### Storm Surge

A hurricane moving over the continental shelf produces a buildup of
water at the coastline which is commonly referred to as storm surge.
Storm surge is the increase in height of the surface of the sea due to
the forces of an approaching hurricane. Storm surge normally occurs over
a coastline for distances of 100 miles or more. The winds associated
with a hurricane are the largest single component responsible for the
buildup of storm surge within a basin. The wind blowing over the surface
of the water exerts a horizontal force, which induces a surface current
in the general direction of the wind. The surface current, in turn,
induces currents in subsurface water. This process of current creation
continues to a depth which is determined by the depth of the water and
by the intensity and forward motion of the hurricane. For example, a
fast moving hurricane of moderate intensity may only induce currents to
a depth of a hundred feet, whereas a slow moving hurricane of moderate
intensity might induce currents to several hundred feet. These
horizontal currents are impeded by a sloping continental shelf, as the
sloping continental shelf is particularly conducive to the formation of
large storm surges. The amount of rise increases shoreward to a maximum
level at, or some distance inland from, the shoreline.

Waves and swells breaking at or near the coast also transport water
shoreward. During storms when there is an increase in wave height, water
cannot flow back to the sea as rapidly as it is brought shoreward. This
results in a phenomenon known as "wave setup" and causes a further
increase of water level along the coastline. Waves are directly affected
by water depth and will break and dissipate their energy in shallow
water. A steep continental shelf will allow large ocean waves to
approach the coastline before breaking, thus increasing wave setup. This
phenomenon is primarily a concern near the coastline because large waves
are generally not transmitted inland.

The elevation of the storm surge within a coastal basin depends upon the
meteorological parameters of the hurricane as well as the physical
characteristics existing within the basin. The meteorological parameters
affecting the amount of storm surge generated include the intensity of
the hurricane measured by the central barometric pressure and maximum
surface winds at the center of the storm, path or forward track of the
storm, forward speed, and radius of maximum winds (storm size). The
radius of maximum winds is measured from the center of the hurricane to
the location of the highest wind speeds within the storm. This distance
can vary from as little as 4 miles to as much a 50 miles. Due to the
counter-clockwise rotation of the wind field, the highest recorded surge
levels are generally located to the right of the forward track of the
hurricane.

The physical characteristics of a basin also influence potential surge
heights. These factors include the roughness of the continental shelf,
configuration of the coastline, and the existence of significant natural
or man-made barriers. Another factor which affects the storm surge
heights in the initial water level exists within the basin at the time
of arrival of a hurricane includes the tide plus any anomalous.

Numerous methods and models have been utilized to quantify the potential
storm surge generated by hurricanes. One of the earlier guides developed
for that purpose is the Saffir/Simpson Hurricane Scale. The
Saffir/Simpson Hurricane Scale is a descriptive scale which categories
hurricanes based upon the hurricane intensity and its damage potential.
The Saffir/Simpson Hurricane Scale also provides a range of wind speeds
and normal surge heights associated with each of the five categories of
hurricanes.

**Table 1**

**Saffir/Simpson Hurricane Scale**

**Category 1:** Winds of 74 to 95 miles per hour. Damage primarily to
shrubbery, trees, foliage, and unanchored mobile homes. No real damage
to other structures. Some damage to poorly constructed signs. Storm
surge 4 to 5 feet above normal. Low lying coastal roads inundated, minor
pier damage, some small craft in exposed anchorage torn from moorings.

**Category 2:** Winds of 96 to 110 miles per hour. Considerable damage
to shrubbery and tree foliage; some trees blow down. Major damage to
exposed mobile homes. Extensive damage to poorly constructed signs. Some
damage to roofing materials of building; some window and door damage. No
major damage to buildings. Storm surge 6 to 8 feet above normal. Coastal
roads and low lying escape routes inland cut by rising water 2 to 4
hours before arrival of hurricane center. Considerable damage to piers.
Marinas flood. Small craft in unprotected anchorages torn from moorings.
Evacuation of some shoreline residences and low-lying inland areas
required.

**Category 3:** Winds of 111 to 130 miles per hour. Foliage torn from
trees; large trees blown down. Practically all poorly constructed signs
blown down. Some damage to roofing materials of buildings; some window
and door damage. Some structural damage to small buildings. Mobile homes
destroyed. Storm surge 9 to 12 feed above normal. Serious flooding at
coast and many smaller structures near coast destroyed; larger
structures near coast damaged by battering waves and floating debris.
Low lying escape routes inland cut by rising water 3 to 5 hours before
hurricane center arrives.

**Category 4:** Winds of 131 to 155 miles per hour. Shrubs and trees
blown down; all signs down. Extensive damage to roofing materials,
windows, and doors. Compete failure of roofs on many small residences.
Complete destruction of mobile homes. Storm surge 13 to 18 feet above
normal. Major damage to lower floors of structures near shore due to
flooding and battering waves and floating debris. Low lying escape
routes inland are cut by rising water three hours before hurricane
center arrives. Major erosion of beaches.

**Category 5:** Winds greater than 155 miles per hour. Shrubs and trees
blown down; considerable damage to roofs of buildings; all signs are
blown down. Very severe and extensive damage to windows and doors.
Compete failure of roofs on many residences and industrial buildings.
Extensive shattering of glass in windows and doors. Some complete
buildings failures. Small buildings overturned or blown away. Complete
destruction of mobile homes. Storm surge possibly greater than 18 feet
above normal. Major damage to lower floors of all structures less than
15 feet above sea level. Low lying escape routes inland cut by rising
water 3 to 5 hours before hurricane center arrives.

The Saffir/Simpson Hurricane Scale was intended as a general guide for
use by public safety officials during hurricane emergencies. It does not
reflect the effects of varying localized coastline configuration,
barriers, or other factors which can greatly influence the surge heights
that occur at differing locations during a single hurricane event.

### The SLOSH Model

The SLOSH Model (Sea, Lake, and Overland Surges from Hurricanes) is the
latest and most sophisticated mathematical model yet developed by the
National Weather Service to calculate potential surge heights from
hurricanes. SLOSH is a two dimensional model that was developed for real
time forecasting of surges from actual hurricanes within selected Gulf
and Atlantic coastal basins. It furnishes surge heights for open coast,
and has the added capability to compute the routing of storm surge into
bays, estuaries, or coastal river basins as well as calculating surge
heights for over land locations. Significant natural and man-made
barriers are represented in the model and their effects simulated in the
calculations of surge heights within a basin.

The SLOSH model is designed for use in an operational mode; that is, for
forecast runs without controlled, local calibration, or observed winds.
The rationale for this design is to avoid having the forecaster predict
unavailable input data. The SLOSH model contains a storm model into
which simple, time-dependent meteorological data are input and from
which the driving forces of a simulated storm are calculated. These data
are as follows:

-   Latitude and longitude of storm positions at six-hour intervals for
    a 72-hour tract.

```{=html}
<!-- -->
```
-   The lowest atmospheric sea level pressure in the eye of the
    hurricane at six-hour intervals.

```{=html}
<!-- -->
```
-   The storm size measured from the center to the region of maximum
    winds; commonly referred to as the "radius of maximum winds". Wind
    speed is not an input parameter since the model calculates a wind
    field for the modeled storm by balancing forces according to weather
    input parameters.

Input data to the SLOSH model also includes the initial height of the
water surface well before the storm directly affects the area of
interest. This initial height is the observed still water level
occurring about two days before storm arrival and includes an existing
anomalous rise in the water surface. Water surface elevations correspond
to the vertical datum used to specify land elevations and water depths
within the model. All water surface elevations, land elevations, and
water depths were referenced to NGVD. Tidal fluctuations immediately
prior to landfall have not been accounted for because a small error in
predicting the phasing of storm track and astronomical tide would likely
invalidate model results. The possible effects of landfall occurring at
a particular phase of the tide, such as at the time of high or low tide,
are evaluated as an increment to the surge values predicted by the SLOSH
model.

The values or functions for the coefficients within the SLOSH model are
generalized to serve for modeling all storms within all basins and are
set empirically through comparisons of computed and observed
meteorological and surge height data for numerous historical hurricanes.
It is probable that the coefficients are a function of differing storm
parameters and basin characteristics; therefore, calibration of the
model based on a single storm event within a basin is avoided since
there is no guarantee that the same coefficient values will serve as
well for alternate storms.

**SLOSH Grid Configuration.** The SLOSH model utilizes a telescoping
polar coordinate (fan-shaped) grid system within which a particular
coastal basin is represented. The grid configuration of the "Lake
Pontchartrain Basin" SLOSH model is illustrated in Figure 1. The
resolution of the model for inland locations near the focus is
approximately ½ square miles per grid square and increases to
approximately 50 square miles at the outer fringe of the grid. The grid
squares in Figure 1, constantly expand in size and become progressively
larger out from the coastline. The larger grid cells in the off shore
region permits the inclusion of a large geographic area in the model so
that the model boundary effects on the dynamics of the storm are
diminished. The advantage of this grid system is that it offers good
resolution in areas of primary interest while conserving computer
resources by minimizing the number of calculations required to model a
storm.

![](media/image1.png){width="5.99375in"
height="3.6645833333333333in"}**Figure 1**

**Local Streets, Levees, and Parish Boundaries**

**Basis for Analysis of the SLOSH Model**

The characteristics of a particular basin are constructed as input data
within the model. These characteristic include the topography of inland
areas; river basins and waterways; bays and large inland water bodies;
significant natural and man-made barriers such as barrier islands,
dunes, roadbeds, floodwalls, levees, etc.; and a segment of the
continental shelf. The SLOSH model simulates inland flooding from storm
surge and permits the overtopping of barriers and flow through barrier
gaps.

**Verification of the Model.** After a SLOSH model has been constructed
for a coastal basin, verification experiments are conducted. The
verification experiments are performed in a "hindcast" mode, using the
real-time operational model code and storm parameters and an initial
observed sea surface height occurring approximately 48 hours before the
storm landfalls or affects the basin. Ideally there would be a large
number of actual storm events with well-documented meteorology and storm
surge histories which could be compared to the storm surge histories
hindcast by the SLOSH model for the same storms. In reality, hurricanes
are a rare meteorological event for any given region, and it is even
rarer to find adequate, reliable measurement of storm surge elevations
over a representative number of sites within a region due to the
difficulty in making such measurement during hurricane conditions.

The computed surge heights are compared with those measured from
historic storms and, if necessary, adjustments are made to the input or
basin data. These adjustments are not made to force agreement between
computed and measured surge heights from historical storms but to more
accurately represent the basin characteristics or historic storm
parameters. In those instances where the model gave realistic results in
one area of a basin but not in another, closer examination of the basin
often revealed inaccuracies in the representation of barrier heights or
missing values in bathymetric or topographic charts. In the case of
historic storms, most of the data was coarse, with parameters prescribed
invariant with time and with an unrealistically smooth storm track. When
necessary, further analysis and subjective decisions are employed to
amend the track or other parameters of the historic storms used in the
verification process.

**Model Output.** The SLOSH model output for a modeled storm consists of
a tabulated storm history containing hourly values of storm position,
speed, direction of motion, pressure and radius of maximum winds; a
surface envelope of highest surges; and, for pre-selected grid points,
time-history tabulations of values for surge heights, wind speeds, and
wind directions. If desired, the model can also furnish two-dimensional
snapshot displays of surges at specified times during a simulation.

![](media/image2.png){width="5.99375in" height="3.6645833333333333in"}

**Figure 2**

**Vulnerable Sites: Hospitals.**

**LaFouch Parish**

The highest water level reached at each location along the coastline
during the passage of a hurricane is called the maximum surge. Maximum
surges along the coastline do not necessarily occur at the same time.
The time of the maximum surge for one location may differ by several
hours from the maximum surge that occurs at another location. A plot of
the maximum water surface elevation attained at each grid cell over the
duration of the simulated storm does not represent a "snapshot" of the
storm surge at a given instant of time. Instead, it represents the
highest water level at each grid cell during a hurricane irrespective of
the actual time of occurrence.

The printed envelope of highest surges from the SLOSH model shows the
computed surge heights in feet NGVD in the center of each grid square.
Other information depicted includes symbols for natural and man-made
geographic features, latitude and longitude lines, and the storm path
through the basin. In order to output surges on a line printer, the
polar grid for a basin is transformed onto an image plane of equal
spacing. Cells near the origin of the polar grid are thus expanded
relative to their original size and cells near the outer portion of the
polar grid are contracted relative to their original size. The result is
that the model grid is represented by equally spaced parallel lines
while latitude and longitude lines and all other geographic features
within the basin are distorted.

The time-history data of surge heights, wind speeds, and wind directions
are tabulated for each pre-selected grid point in the model. These data
are listed for each grid point at ten-minute intervals for a 72-hour
segment of a simulated storm track, starting 48 hours prior to landfall
and continuing for 24 hours after landfall or closest approach. The
surge heights are in feet NGVD; the wind speeds in miles per hour; and
the wind directions in degrees azimuth from which the wind is blowing.
Water depths are not computed because terrain heights varies within a
grid square. The depth of flooding is deduced by subtracting the actual
terrain height from the model-generated surge height.

The Southeast Louisiana SLOSH Modeling Process

The Lake Pontchartrain basin SLOSH model was the primary model used for
the Southeast Louisiana Hurricane Preparedness Study. The Lake
Pontchartrain SLOSH basin covers the Louisiana and Mississippi coastline
from Vermilion Bay to the Mississippi Sound, and extends inland to cover
Lake Pontchartrain.

The accuracy of the SLOSH model depends heavily on the ability to
accurately model the topographic and bathymetric features of the basin.
Inaccuracies in modeling these features will contribute directly to
errors in the modeling of storm surge. This is of particular importance
for the Lake Pontchartrain basin, which is composed primarily of
sea-level marsh, swamp and open water. The major barriers to storm surge
in southeast Louisiana are generally man-made features such as levees,
floodwalls, highways, and railroad embankments.

As part of the Southeast Louisiana Hurricane Preparedness Study, a total
of 1,640 simulated hurricanes were modeled using the Lake Pontchartrain
basin SLOSH model. The characteristics of the simulated hurricanes were
determined from an analysis of historical hurricanes which have occurred
within the study area. The parameters selected for the modeled storms
were the intensities, forward speeds, directions of motion, and radius
of maximum winds. These parameters were defined based on the probability
of occurrence of weather conditions within the Lake Pontchartrain basin.

A total of 164 storm tracks were modeled for the study. These tracks had
combinations of parameters representing five categories of hurricane
intensity, as described by the Saffir/Simpson Hurricane Scale; nine
approaches directions for landfall (west, west-northwest, northwest,
north northwest, north, north northeast, northeast, east northeast, and
east); two forward speeds of 5 and 15 miles per hour; and numerous
landfall or closest approach locations separated by 20 miles or less
along the coastline.

**Model Output**

One of the outputs from the SLOSH model is a plot of maximum water
surface elevation at each grid cell with the basin affected by the
storm. The imaginary surface defined by the maximum water level in each
grid cell is termed the "envelope" of maximum water surface elevations.
The largest individual water surface elevation within the entire basin
for a particular storm is termed the "peak" surge. The location of the
peak surge depends on where the eye of a hurricane crosses the
coastline, its intensity, the topography of the basin, configuration of
the coastline, the approach direction, and the size or radius of maximum
winds of the hurricane. In most instances, the peak surge from a
hurricane occurs to the right of the storm path near the radius of the
maximum winds.

The objective of the analysis of the model is to determine the potential
peak surges. For this purpose, a maximum envelope of water (MEOW) is
utilized. A total of 90 MEOW's were developed for the study. Each MEOW
represented a different combination of hurricane intensity, direction,
and forward speed. The MEOW shows the peak surge heights for each grid
cell within the basin, independent of where the hurricane actually
crosses the coastline.

![](media/image3.png){width="5.397916666666666in" height="3.3in"}

**Figure 3**

**Shelter Locations, LaFouch Parish**

In most instances, a change in storm category accounted for the greatest
change in the peak surge heights calculated for grid cells. Changes in
the forward speed of the simulated hurricanes resulted in the greatest
peak surge height differences. Faster moving (15 miles per hour) storms
produced higher surge levels at the open coast while the slower (5 miles
per hour) storms resulted in higher surge levels within most lakes and
over land locations.

The surge maps depict the limits of inundation from peak storm surge
heights potentially generated by the five categories of storm intensity.
The displays from the study do not predict the limits of inundation from
a single storm, but rather delineate the areas that are threatened by
storm surge.

A major output of the model is a vulnerability analysis to identify the
areas, populations, and facilities which are potentially vulnerable to
flooding associated with hurricanes. The storm surge data from the
hazards analysis was used to develop inundation maps; to determine
evacuation zones and evacuation scenarios for local areas; to quantify
the population at risk under a range of hurricane intensities; and to
identify major medical institutional and other facilities that are
potentially vulnerable to storm surge.

# ![](media/image4.png){width="5.99375in" height="3.6645833333333333in"}

# Vulnerability Zones by Type of Storm