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Group Title: Hurricanes Erin and Opal Hydrodynamics and erosion potential
Title: Hurricanes Erin and Opal Hydrodynamics and erosion potential
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Title: Hurricanes Erin and Opal Hydrodynamics and erosion potential
Series Title: Hurricanes Erin and Opal Hydrodynamics and erosion potential
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Table of Contents
    Front Cover
        Front Cover
    Title Page
        Page i
    Table of Contents
        Page ii
        Page iii
    Main
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Appendix
        Page A-0
        Page A-1
        Page A-2
        Page A-3
Full Text



UFL/COEL-99/OO1


HURRICANES ERIN AND OPAL
HYDRODYNAMICS AND EROSION POTENTIAL




by




Robert G. Dean


January 8, 1999



Project Sponsor:

Bureau of Beaches and Coastal Systems
Department of Environmental Protection
Tallahassee, Florida 32399-3000












HURRICANES ERIN AND OPAL

HYDRODYNAMICS AND EROSION POTENTIAL




January 8, 1999



Prepared by:

Robert G. Dean







Project Sponsor:

Bureau of Beaches and Coastal Systems
Department of Environmental Protection
Tallahassee, Florida 32399-3000









Submitted by:

Department of Coastal and Oceanographic Engineering
University of Florida
Gainesville, Florida 32611








TABLE OF CONTENTS


LIST OF FIGURES .......................................................... iii

LIST OF TABLES ............................................................ iii

1. INTRODUCTION ..................................................... 1
1.1 Purpose ..................... ................................ 1
1.2 Background ................ ................................... 1

2. CHARACTERIZATION OF MODEL HURRICANE .......................... 2
2.1 Pressure Field ................................................... 3
2.2 Wind Field ................................................... 3

3. CHARACTERISTICS OF HURRICANES ERIN AND OPAL COMPARED TO
PREVIOUS HURRICANES ............................................... 6
3.1 G general .............. .................... .............. .... 6
3.2 Characteristics of Hurricanes Erin and Opal and Previous Hurricanes
Affecting the Panhandle ....................... ..................... 10
3.3 Parameters Relating to Hurricane Damage and Erosion Potential ........... 10
3.3.1 W ave Height .......... ........... ......................... 10
3.3.2 Storm Surge ............................................. 10
3.4 Structural Damage Index ......................................... 12
3.4.1 Local Structural Damage Index ............. .................. 12
3.4.2 Global Structural Damage Index ................................... 13
3.5 Hurricane Erosion Index .... ... ..................................... 13
3.5.1 Local Hurricane Erosion Index ................................ 14
3.5.2 Global Hurricane Erosion Index ..... ........... ............ 14

4. RESULTS .............................. .............................. 14
4.1 Structural Damage Indices ...................... ...................14
4.2 Hurricane Erosion Indices ............................... ......... 18

5. SUMMARY ................. ...................... .................... 18

6. REFERENCES .............................. ............. ............ 21

APPENDIX A MODEL HURRICANE CHARACTERISTICS ................. A-1








LIST OF FIGURES


FIGURE PAGE

1. Tracks of Hurricanes Erin and Opal ...................................... 1

2. Definition Sketch of Model Hurricane Coordinate System ....................... 2

3. Characteristics of Model Pressure Field ...................................... 4

4. Characteristics of Model Wind Field. Without Translation or Coriolis Force ......... 5

5. Characteristics of Model Wind Field. With Translation But Without Coriolis Force .... 7

6. Characteristics of Model Wind Field. Without Translation But With Coriolis Force .... 8

7. Characteristics of Model Wind Field. With Translation and Coriolis Force ........... 9

8. Variation of Local Hurricane Erosion Index With Non-Dimensional Longshore
Distance, x/R .......................................... ......... ...... 15

9. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Global
Structural Damage Index, SDI ............................................ 16

10. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Local
Structural Damage Index, sdi ............................................. 17

11. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Global
Hurricane Erosion Index, HEI ............................................ 19

12. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Local
Hurricane Erosion Index, hei ............................................. 20

A-1 Definition Sketch of Model Hurricane Coordinate System ..................... A-1


LIST OF TABLES

TABLE PAGE

1. Characteristics of Historical Hurricanes Affecting the Panhandle Area ............. 11








HURRICANES ERIN AND OPAL
HYDRODYNAMICS AND EROSION POTENTIAL


1. INTRODUCTION

1.1 Purpose

The purpose of this report is to characterize Hurricanes Erin and Opal in terms of their hydrodynamic
characteristics and structural damage and erosion potentials. The intent of this effort is to both
develop a basis for assessing the structural damage and erosional potentials of these two storms
relative to past storm events for which data are available and to provide a basis for rapidly evaluating
the potential of future hurricanes to cause widespread damage to the State's upland structures and
beach and dune system.

1.2 Background

Hurricanes Erin and Opal made landfall in the Florida Panhandle area on August 3, 1995 and
October 4, 1995, respectively. A depiction of their tracks is presented in Figure 1. The Central
Pressure Deficit at landfall of 2.16 inches of mercury (in. Hg.) ranks Opal as the most severe to
impact the Florida Panhandle over the 84 period of record. The hurricane decreased in strength from
a Category 4 hurricane offshore such that at landfall it was a marginal Category 3 hurricane
(Lawrence, et al 1998). Opal was one of the most destructive hurricanes to impact the Panhandle
area. The maximum winds on the coast due to Opal were in excess of 100 miles per hour (Powell
and Houston, 1998). The radius to maximum winds of 46 n. mi associated with Opal also contributed
to the wide spread damage.


350



25 0 058045
300
8/03




25* V,


Figure 1. Tracks of Hurricanes Erin and Opal







Storm surge data obtained from a National Oceanic and Atmospheric Administration (NOAA) tide
gauge located on the Panama City Beach pier showed a peak water level of 8.3 feet above National
Geodetic Vertical Datum (NGVD), which was nearly 8 feet above the normal predicted astronomical
tide. High water mark surveys conducted by Department of Environmental Protection (DEP) staff
documented a storm surge ranging from 8-11 feet above NGVD between Pensacola Beach and Fort
Walton Beach and approximately 12-20 feet above NGVD between Destin and Seagrove Beach
(FEMA, 1996). In Panama City Beach, evidence of wave impacts and sand deposition were found
in first-floors of structures up to 17-18 feet above NGVD during post-storm inspections conducted
by the DEP (Leadon, et. al., 1997).

Hurricane Opal caused extensive damage to the beach and dune systems. Eight million cubic yards
of sand were lost from above sea level due to breaking waves, extensive flooding, a substantial storm
surge and extensive overwash in lower dune areas. East of Fort Walton Beach, portions of Highway
98 were washed away, and most of the survey control monuments maintained by the BBCS were
destroyed (Leadon, et. al. 1997). The approximately $2 billion damage to structures during
Hurricane Opal rank it as one of the most costly natural disasters to affect the United States (FEMA,
1996). It caused more structural damage along the Florida coast than all of the hurricanes and
tropical storms combined in the last 20 years (Leadon, et. al., 1997).

2. CHARACTERIZATION OF MODEL HURRICANE

This section illustrates the model hurricane that will be used in the development of structural damage
indices and hurricane erosion indices. The model hurricane is one that was first proposed by Wilson
(1957) and that is currently used by the Florida Beaches and Shores Resource Center in calculation
of storm surges for development of the Coastal Construction Control Line recommendations. The
model hurricane is described in Appendix A. A definition sketch of the model hurricane is presented
in Figure 2. Several results follow which illustrate the characteristics of the model hurricane.

y V,














Figure 2. Definition Sketch of Model
Hurricane Coordinate System.








2.1 Pressure Field


The pressure field is characterized by circular isobars and defined by the Central Pressure Deficit,
(CPI), Ap and the radius to maximum winds, R. The pressure field is defined in terms of the CPI and
the radius to maximum winds as
p(r) =p + Ape -Rr (1)

where

Ap =P_ -Po (2)


and r is the radial distance from the hurricane center to any location of interest, po is the pressure at
the eye of the hurricane and p. is the ambient pressure. Substituting r = 0 and r =0 will demonstrate
that Eqs. (1) and (2) provide the correct limits, p, and p., respectively.

Figure 3 presents a section through the pressure field and a plan view of the pressure field. It is seen
that at a radius, r = 3R, the pressure deficit has been reduced to 28% of its maximum. The pressure
field in Figure 3 is plotted in the following non-dimensional form (where primes denote non-
dimensional quantities)

p(r) -Po -R(r
p'(r) =e (3)
Ap


2.2 Wind Field

The wind field is more complicated than the pressure field due to the effect of the forward translation
speed of the hurricane and the Coriolis effect, which represents the rotational effect of the earth. If
the hurricane were stationary and the earth were not rotating, the isobars of wind speed would be
concentric circles as are the isobars of pressure. If there were no friction, the wind vectors would be
tangential to circular isobars of wind speed; however, the effects of friction of the water surface
cause the wind vectors to be rotated in toward the center of the hurricane system.

Four graphs are presented to illustrate the characteristics of the hurricane wind field and the effects
of translation speed and Coriolis force. Figure 4 presents a section and planview of the non-
dimensional wind field for a stationary hurricane system on a non-rotating earth over a frictionless
water surface. It is seen that since the system is not moving and the effect of Coriolis force is not
included, the isolines of wind speed are concentric circles. In these graphs, the wind speed has been
normalized by the maximum velocity. This same normalizing term will be used in all subsequent
plots of wind fields.






10
8 o
6 0
o 4
0
W 2 4 4-
o

O -2 C-
C 0


E c
c -6
C
0 3
Z -8 I

-10 I I I I
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Pressure Field


S1.0
) 0.9
5 0.8
c 0.7
0- 0.6
0.5
0.4
D 0.3
E
5I 0.2
c 0.1
0
z 0.0
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-Section of Presure Field at y = 0.

Figure 3. Characteristics of Model Pressure Field.




















-6

-8
A t


I ,III I I I. I I II I


- u
-10 -8 -6 -4 -2 0 2 4 6
Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Wind Field


1.0
S0.9
a)
U) 0.8
0.7
3 0.6
S0.5
0.4
a 0.3
.E 0.2
S0.1
z 0.0 I I I I I I I
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-Section of Wind Field at y = 0.

Figure 4. Characteristics of Model Wind Field.Without
Translation or Coriolis Force.
5








Figure 5 presents the wind field for a hurricane system translating with a speed equal to 20% of the
normalizing velocity but with no Coriolis force effect. It is seen that the maximum wind speed to
the right of the hurricane center is increased and the wind speed to the left of the hurricane center
is decreased. The amount of increase and decrease are approximately one-half of the forward speed
of hurricane translation.

Figure 6 presents results in a similar form as for Figure 5 except the Coriolis effect has been included
and the hurricane system is stationary, Vf =0. For this case, the wind speed field is composed of
concentric circles

Finally, Figure 7 presents results for a hurricane translating with a speed equal to 20% of the
reference wind speed and the Coriolis force effect is included.

The effect of friction is to reduce the velocity by approximately 17% and to cause the velocity vector
to be rotated toward the center of the hurricane by approximately 18 degrees.

3. CHARACTERISTICS OF HURRICANES ERIN AND OPAL COMPARED TO
PREVIOUS HURRICANES

3.1 General

The hydrodynamic and erosion potentials associated with a particular hurricane depend on the
detailed pressure and wind fields, speed and direction of movement of the hurricane, the state of the
tide and the bathymetric characteristics. However, hurricanes can be idealized by a set of five
parameters which capture their principal characteristics. These parameters are the central pressure
deficit, Ap, the radius from the center of the hurricane to maximum winds, R, the forward speed, Vf,
and two parameters which fix the track and direction of the hurricane. The CPI is a measure of the
intensity of the hurricane and it can be shown that the wind speeds are proportional to the square root
of the CPI. The radius to maximum winds is a measure of the size of the hurricane. There is a weak
inverse correlation between the size and intensity of hurricanes. Both of these parameters are
relevant to potential damage to structures and to beach erosion potential. Waves generated by the
hurricane depend on both the CPI and the size as represented by R. The duration of the forces
associated with a hurricane is also relevant to the erosion potential of a hurricane, since the offshore
sediment transport which results in erosion requires time to occur. The duration is related to the ratio
of the radius to maximum winds, R, to the forward translation speed, Vf, since the slower the
translation speed of the hurricane system, the longer the erosion potential will remain in a particular
area. In the following sections, parameters will be developed to characterize the erosion and
structural damage potentials and the characteristics of Hurricanes Erin and Opal will be examined
relative to the historical hurricanes which have affected the Florida Panhandle area and to
characterize the return periods of these hurricanes based on parameters relevant to damage potential
and erosion potential.






10
8 .0

-i 6e a) 1
'4 4
*n 2



a) -4 ,
E
I 0.6 i
-6


-10
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Wind Field

a) 1.2
C)
1. 1.0 -

0.8




I 0.4

I- 0.2
Z
0.0 '
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-Section of Wind Field at y = 0.
Figure 5. Characteristics of Model Wind Field.With
Translation But Without Coriolis Force.
7






10
8
-" 6 \
ci
4 o
Ni 4

@2
CD
0
0 1.0
O -2 \
F 0.6 o.
a) -4
E -4 0.5
-6 0.4

z -8-

-10
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Wind Field

- 1.2
a)
c) 1.0

S0.8

S0.6

u 0.4
E
S 0.2
0
Z 0.0
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-Section of Wind Field at y = 0.

Figure 6. Characteristics of Model Wind Field.Without
Translation But With Coriolis Force.
8








8 o


Q 4
U

0 A
0
F -2
c 0.6 a





-10 -8 -6 -4 -2 0 2 4 6 8 10

Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Wind Field

02 1.2
'()
S41.0






E
S0.8


o 0.6








-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-Section of Wind Field at y 0.






Figure 7. Characteristics of Model Wind Field.With
Translation and Coriolis Force.
19
' 0.2








3.2 Characteristics of Hurricanes Erin and Opal and Previous Hurricanes Affecting the
Panhandle

Table 1 presents the parameters discussed previously associated with hurricanes affecting the
Panhandle area including Hurricanes Erin and Opal. The length of the record, from 1911 to 1995
encompasses a period of 84 years. It is seen that based on the CPI, Ap, Hurricane Opal is the most
intense hurricane on record.

3.3 Parameters Relating to Hurricane Damage and Erosion Potential

In developing indices that are appropriate for the characterization of the damage potential of a
hurricane, two competing objectives emerge. The first is that the index be as representative of the
potential damage as possible. The second is to keep the indices relatively simple and straightforward.
To fulfill the first objective completely, it would be necessary to account for the individual beach
and offshore profiles and the locations of the structures, etc. However, realizing that we are
comparing potential damage in a particular area, all of the factors would be the same and thus basing
the damage potential on the hurricane conditions alone is the optimum compromise to achieve the
goals in the development of the indices.

In evaluating the rarity of Hurricanes Erin and Opal, it is necessary to develop parameters that
provide measures of the potential damage and erosion by a hurricane. Two parameters will be
considered. The first relates to structural damage and the second to beach and dune erosion.
However, first it is useful to examine the interrelationships between meteorological variables and
hydrodynamic response. The two hydrodynamic response variables of interest are wave height and
storm surge.

3.3.1 Wave Height

For a uniform wind field, the wave height at the downwind end of the fetch is related to the wind
speed, W, and the fetch, F, by (Shore Protection Manual, 1984)

H= 1.6x10-3 1 (4)



in which Ho is the deep water significant wave height and g is the gravitational constant.

3.3.2 Storm Surge

The storm surge is a result of several effects, including: (1) Onshore wind stress, (2) Pressure
reduction, and (3) Wave set-up. The onshore wind stress is proportional to the integral, over the







Table 1


Characteristics of Historical Hurricanes Affecting the Panhandle area


Date Name Ap (in. Hg.) R (n. Mi.) VF (knots)
August 9, 1911 0.74 N/A 7.0

Sept. 11, 1912 0.74 N/A 12.0

August 31, 1915 0.85 N/A 16.0

June 29, 1916 1.86 26.0 25.0

October 12, 1916 1.16 19.0 21.0

September 21, 1917 1.44 33.0 13.0

September 13, 1924 0.64 N/A 5.0

September 11, 1926 1.72 17.0 7.0

September 22, 1929 1.12 N/A 6.0

August 26, 1932 0.74 N/A 10.0

August 29, 1935 1.57 21.0 10.0

July 27, 1936 1.46 19.0 9.0

August 7, 1939 0.64 N/A 7.0

October 3, 1941 0.94 18.0 11.0

August 20, 1950 Baker 1.00 21.0 23.0

September 23, 1953 Florence 0.97 N/A 9.0

September 21, 1956 Flossy 1.16 18.0 10.0

June 4, 1966 Alma 1.07 20.0 13.0

June 14, 1972 Agnes 1.04 20.0 11.0

September 1, 1975 Eloise 1.72 14.0 22.0

August 29, 1979 Frederic 1.99 33.0 11.0

August 28, 1985 Elena 1.77 N/A 12.0

November 15, 1985 Kate 1.36 30.0 13.0

June 3, 1995 Allison 0.68 N/A 14.0

July 31, 1995 Erin 1.07 21.0 11.0

September 27, 1995 Opal 2.16 46.0 23.0








continental shelf, of the square of the wind speed multiplied by the cosine of the angle which the
wind vector makes with a normal to the coastline. The pressure reduction is proportional to the
square of the local wind speed. Finally, the wave set-up is proportional to the wave height which
includes the product of the wind speed and the square root of the fetch.

In summary, parameters governing the storm surge at the shoreline include wind speed with
exponents ranging from the first to the second power. Since the purpose here is to develop an index
for comparison purposes and since two of the three components of the storm surge depend on the
square of the wind speed, the square of the wind speed integrated over the dominant wind field will
be selected as the appropriate parameter for storm surge, i.e.


Y2
S(x)af W2cosedy (5)



Where the cosine term accounts for the angle that the wind vector makes with a normal to the
shoreline. The above representations for the wave height and storm surge will be used below in
developing the "Structural Damage Index" and the "Hurricane Erosion Index".

3.4 Structural Damage Index

Structural damage is due to a combination of the storm surge, the longshore distance over which it
occurs, the waves generated by the hurricane and the duration of the storm. As noted earlier,
structural damage occurs as a result of waves reaching up to structural members. The formulation
of a relevant parameter will be assisted by understanding the relationships between hurricane and
hydrodynamic parameters.

Both local, sdi, and global, SDI, structural damage indices will be proposed. The basis for both of
these indices is the wave height cubed which is related to the total wave force which occurs on a
structure which extends from the bottom through the water column. The only difference between the
two indices is that the global index is a measure of the overall hurricane index. That is, it is the
integrated effect over the entire right hand side of the hurricane.

3.4.1 Local Structural Damage Index

The local structural damage index is defined as

sdia H3 (6)

which can also be expressed, from Eq.(4), as









sdi(x)= K2 W3cosOR 3/2 (7)




3.4.2 Global Structural Damage Index

The global index, SDI representing the effect of the entire hurricane, is simply the integral of Eq.
(7) over the dominant shore parallel dimension



10R
SDI= sdi(x)dx (8)
0


3.5 Hurricane Erosion Index

As for the structural damage indices, a local and a global index will be presented for the Hurricane
Erosion Indices and they are denoted hei and HEI, respectively. The basis for the erosion indices is
the so-called Bruun Rule (1962)


W
AY= -S (9)
h +B


where S is the storm surge, Ay is the shoreline change, W. is the shore normal distance over which
the sediment motion is active and here will be taken as the width out to the breaking zone, h. is the
depth associated with W. and B is the berm height. The distance W. can be related to the depth, h.,
through the equation for the equilibrium beach profile


h =AW2/3 (10)


which, when inserted in Eq. (9), yields


h 1/2
Ay = -s (11)
A 3/(1 +B/h )








3.5.1 Local Hurricane Erosion Index


Since h. is proportional to the breaking wave height, the storm surge, S, at the shoreline is
approximately proportional to the integral of the square of the wind speed and the water depth, h.,
is proportional to the breaking wave height, the local hurricane erosion index, hei, is defined as
10R
hei(x) = f W52(xy)cos dy/ V3 (12)
0


where the cosine term accounts for the angle of the wind relative to the shoreline and the Vf in the
denominator results in the slower moving hurricanes acting on the shoreline for a longer period of
time. Figure 8 is an example of a local hurricane erosion index.

3.5.2 Global Hurricane Erosion Index

With the local hurricane erosion index established, the global hurricane erosion index, HEI, is simply
the integral of the local index over the shoreline on the right hand side of the hurricane,
1OR
HEI= fhei(x)dx (13)
0


4. RESULTS

The structural and erosional indices will be presented as extreme value plots (Gumbel, 1958) which
should assist in assessing the return periods and associated index values of the various historical
storms and Hurricanes Opal and Erin in particular. The main value of extreme value plots such as
are to be presented is that for many natural phenomena, the extreme values will plot as a straight line.
For those hurricanes in Table 1 for which no value for the radius to maximum winds was available,
the average radius determined from the other hurricanes (23.5 n. mi.) was used. The scale of the
indices on each of the four plots to be presented is arbitrary, that is, only the relative magnitude of
the index for each of the hurricanes on a particular index is relevant.

4.1 Structural Damage Indices

The results from the Global Structural Damage Index, SDI and the Local Structural Damage Index,
sdi are presented in Figures 9 and 10, respectively.

Referring to Figure 9 for the Global Structural Damage Index, SDI, it is seen that Hurricane Opal
had by far the most damaging potential. Hurricane Erin ranked 14th with a return period of
approximately 6 years. The hurricane with the next second greatest damage potential was Hurricane
Frederic, although this storm made landfall at Dauphin Island, AL considerably to the west of Florida









1.6 I I I


2" 1.4

.c 1.2


| 1.0
0
CD
0 0.8
w

0.6
,--
I 0.4

0
-0.2


0.0
0 1 2 3 4 5 6 7 8 9 10

Non-Dimensional Distance, x/R


Figure 8. Variation of Local Hurricane Erosion Index

With Non-Dimensional Longshore Distance, x/R








2.0

1.8 -- ------.---.-- ------------------- -------- -Opal-

S1 .6 .- ............................................... ................................................. ... ......-

1 .6 .4 - -- --- ---- --- -- --. : ... .. .. ... ...... ... ....... .... .. .. .. ....... : ......... ... .... : ... .. .... .. .. ..-
0 .4

1.2

1, 0 .................. ... ....... .. ...........................
( 1.0

Frederic
1917
0 .6 .. ........... ...- .... ........ .... .... .... ... .. .. .... .................... .... ... ... .... ......... .. ..
0 Erin 1916
Kate
0.2 -------------------------

0.0
2 5 10 20 50 100
Return Period (Years)

Figure 9. Return Periods of Historic Storms in Florida's Panhandle Area.
Based on Global Structural Damage Index, SDI









10

9








5 8 ...... ....................... ......... ........ ..... ..... ....................Op a
CD





I-


o4

7.............................------- ------------------------------------







2 5 10 20 50 100
o0







Return Period (Years)

Figure 10. Return Periods of Historic Storms in Florida's Panhandle Area.
Based on Local Structural Damage Index, sdi.








and thus did not have such a great effect on the Florida coastline. It is clear that a single straight line
would not provide a reasonable fit to the results in Figure 9. Possibly two or three straight line
segments would provide reasonable fits to the results; however, there are no evident advantages to
carrying out such fits. It is possible that Hurricanes Opal and Frederic represent members from a
different population of storms. At the present stage of knowledge, much more information would
be required to interpret Figure 9 further.

Referring to Figure 10 for the local Structural Damage Index, sdi, it is seen again that Hurricanes
Opal and Frederic ranked first and second in terms of their damage potentials. The results in Figure
10 would be better represented by one or two straight line segments. However, at this stage, there
appears to be no merit in carrying out such fitting without further investigation of the cause of the
nonlinear relationship on this plot.

4.2 Hurricane Erosion Indices

The results from the Global Hurricane Erosion Index, HEI and the Local Hurricane Erosion Index,
hei are presented in Figures 11 and 12, respectively.

Referring to Figure 11 for the Global Hurricane Erosion Index, HEI, it is seen that Hurricanes Opal
and Frederic ranked first and second in terms of their erosive potentials. The third ranked storm is
an unnamed hurricane which occurred in 1917. The general nature of this plot is similar to Figure
9 and comments made of that plot apply here also.

Referring to Figure 12 for the Local Hurricane Erosion Index, hei, it is seen that Hurricanes Opal and
Frederic ranked first and second in terms of their local erosive potentials and Hurricane Eloise
(1975) ranked third. The general form of these results is similar to that in Figure 10 and similar
comments apply.

5. SUMMARY

This report has examined four indices which relate to the magnitude of potential damage that
historical hurricanes can impart on the Florida Panhandle shoreline and attendant structures. Two
of these indices relate to the potential for structural damage and two relate to the potential for
erosion. Both global and local indices are presented with the global index representing the potential
impact in the entire area affected by the storm and the local index representing the potential damage
at that location along the shoreline where the impact is the greatest. The hurricane characteristics are
represented by a model storm defined by three parameters: Central pressure deficit which represents
the intensity of the storm, readius to maximum winds, representing the storm size, and forward speed
of translation of the storm which along with the radius to maximum winds, represents the duration
of the storm. The forward speed also contributes to the wind velocity. The damage potentials are
presented in extreme value plots for all historical hurricanes to impact the Florida Panhandle area
through the 1995 hurricane season. Hurricanes Opal and Frederic were found to have the first and
second highest potentials for shoreline and upland structure damage on the basis of all four indices.
Valuable extensions of this work would include: (1) An effort to verify and/or calibrate the indices
with actual storm damages and erosion quantities associated with particular storms for which such









10

9.
91-- ---- --- ------- ---- --- ---- ---- --- ---- ------- ----*-- ---- --- ------- ---- ---




7 . . . . . . . . * . .. .. . . . . . . . . . . . . . .. . ... .. .. . . . . . . .. . .. . . .. . . ..




. 5 . . . . . . . . . .. . . . . . .. . . . . . . . .. . . . . . . . .. . . .... . .... . . . .. . . .. .
1 7
Opal
"16












1 ------- ------------ --------------'------'*--------'-------------------*-------I
0











Based on Global Hurricane Erosion Index, HEI.
I
3-
.0 Erin Eloise

0





2 5 10 20 50 100
Return Period (Years)

Figure 11. Return Periods of Historic Storms in Florida's Panhandle Area.
Based on Global Hurricane Erosion Index, HEI.









10

9
10 -------------------------------------------------------------------- --------


... ........ ................ .. ..... ..... .... .. ... .... .... .... ... .... ... ... ... .... .. ...
7

-6


O i"Opal
7 5 .-.- .- .-.-.-. .-.-.-.-.-.-.-. . . . . . ... : . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


Sa
0)
0 Opal

F Frederic ..
B o 1917
3 ..... ..... ....... ............................... s e ..................... -
SErin \. .lena
o 2 ............... .1.3 5 ........ ... 9 3 5 ....... .................... -....

..-------.-- .-..------ --... --------------------

0
2 5 10 20 50 100
Return Period (Years)

Figure 12. Return Periods of Historic Storms in Florida's Panhandle Area.
Based on Local Hurricane Erosion Index, hei.








data are available, and (2) Further development of the four indices with a goal of making them more
representative and with a capability to tailor them to a particular area such that damage and erosional
potentials could be compared for hurricanes impacting different areas.

6. REFERENCES

Bruun, P. (1962) "Sea Level Rise as a Cause of Shore Erosion", Journal of Waterway, Port, Coastal
and Ocean Engineering, American Society of Civil Engineers, Vol. 88, pp. 117-130.

FEMA (1996) "Hurricane Opal in Florida: A Building Performance Assessment", Federal
Emergency Management Agency, Mitigation Directorate, Washington, D. C.

Gumble, E. J. (1958) "Statistics of Extremes", Columbia University Press, New York, NY.

Lawrence, M., M. Mayfield, R. Avila, R. Pasch and E. Rappaport (1998) "Atlantic Hurricane Season
of 1995", Monthly Weather Review, vol. 126, pp. 1124-1151.

Leadon, M.E., N. T. Nguyen and R. R. Clark (1997) "Hurricane Opal Beach and Dune Erosion and
Structural Damage Along the Panhandle Coast of Florida", Bureau of Beaches and Coastal Systems,
Department of Environmental Protection, State of Florida, Tallahassee, FL.

Powell, M. D. and S. H. Houston (1998) "Surface Wind Fields of 1995 Hurricanes Erin, Opal, Luis,
Marilyn, and Roxanne at Landfall", Monthly Weather Review, pp.1259- 1273.

U. S. Army Corps of Engineers (1984) "Shore Protection Manual", Volumes I and II, Coastal
Engineering Research Center, Superintendent of Documents, U. S. Government Printing Office,
Washington, D. C. 20402
































APPENDIX A

MODEL HURRICANE CHARACTERISTICS








APPENDIX A


MODEL HURRICANE CHARACTERISTICS

A-1. General

The model hurricane is defined by five parameters: central pressure deficit, radius to maximum
winds, hurricane forward translation speed and two additional parameters which fix the hurricane
track and direction. Figure A-1 presents a definition sketch. The model hurricane consists of a
pressure field and a wind field. Each of these is described below.

y v,
















Figure A-1. Definition Sketch of Model Hurricane Coordinate System.


A-2. Pressure Field

The pressure field is composed of concentric circles defined by


p(r) =o + Ape -R',


(A-l)


in which po is the pressure at the center of the hurricane and R is the radius to maximum winds. The
Central Pressure Deficit, Ap is defined as








Ap =p. -Po (A-2)

in which p. is the ambient pressure. Figure 2 in the main body of this report provides a plan view
and section through a sample pressure field.


A-3. Wind Field

It is useful to define several wind velocities.

A-3.1 Gradient Wind Speed

The wind speed of interest is the gradient wind speed, UG, and is the speed at approximately 30 feet
above the water surface. The gradient wind speed is the speed that is used in computations of wave
height and storm surge. Prior to presenting an equation for the gradient wind speed, it is necessary
to define several other wind speeds.


A-3.2 Cyclostrophic Wind Speed

The cyclostrophic wind speed, U, is the wind speed that is associated with the pressure field without
any effect of friction or the Coriolis force. Based on the circular isobaric pattern, it can be shown that
the cyclostrophic wind field is also composed of concentric circular isovels and is given by


Uc= Ap Re -Rr (A-3)
Pa r

where Pa is the mass density of air (=2.4x10" slugs/ft3).

A-3.3 Geostrophic Wind Speed

The geostrophic wind speed, Ug, is the speed that would occur if the pressure field defined earlier
were in balance with the Coriolis force which is due to the rotational effects of the earth. The
geostrophic wind speed is defined as

Ap (R)2 -R/r

U =Pa r (A-4)
g 2Rsin(b


A-2








where o and 4 are the rotational speed of the earth in radians/second (=27n/(24x3600)) and the
latitude of the location of interest, respectively.

A-3.4 Gradient Wind Speed

The gradient wind speed, UG is determined in terms of the other speeds through a parameter, y,
defined as
1 Vsinp U
Y=-- + (A-5)
2 U U
c g

and p is the angle defined in Figure A-1.

Finally, the geostrophic wind speed is

UG =0.83Uc(yi +1 -y) (A-6)


and the wind vectors are rotated inward toward the center of the hurricane by approximately 180.




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