Citation
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
Creator:
Dean, Robert G.
Place of Publication:
Gainesville, Fla.
Publisher:
Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
Language:
English

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University of Florida
Holding Location:
University of Florida
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All applicable rights reserved by the source institution and holding location.

Full Text
UFL/COEL-99/001

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 FIGU RES ........................................................... iii
LIST OF TAB LES ............................................................ iii
1. INTRODU CTION ....................................................... I
1.1 Purpose .......................................................... 1
1.2 B ackground ...................................................... 1
2. CHARACTERIZATION OF MODEL HURRICANE ........................... 2
2.1 Pressure Field ..................................................... 3
2.2 W ind Field ....................................................... 3
3. CHARACTERISTICS OF HURRICANES ERIN AND OPAL COMPARED TO
PREVIOUS HURRICANES ............................................... 6
3.1 G eneral ........................................... 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 H eight .............................................. 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. R ESU LTS ............................................................ 14
4.1 Structural Damage Indices .......................................... 14
4.2 Hurricane Erosion Indices .......................................... 18
5. SU M M AR Y ........................................................... 18
6. REFEREN CES ........................................................ 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 M odel 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
D istance, x/R .......................................................... 15
9. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Global
Structural Dam age Index, SDI ............................................. 16
10. Return Periods of Historic Storms in Florida's Panhandle Area. Based on Local
Structural Dam age 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
I 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
VA 10/05
300 A 10/05
&03
aO2
2501

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.
D-X
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 -Ilr(1
where
Ap =p_~ -p0 (2)
and r is the radial distance from the hurricane center to any location of interest, p0 is the pressure at the eye of the hurricane and p-. is the ambient pressure. Substituting r = 0 and r willl demonstrate that Eqs. (1) and (2) provide the correct limits, p0, and p.0, 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 nondimensional quantities)
p'(r)- p =e p (3)
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 fiction, 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 nondimensional 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
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0 4
W 2 44-a
o
5 "
- 0 C
CC 02
CO -2 C3
E -6
C
0
Z -8
-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
21.0 ) 0.9
L..
5 0.8 cn 0.7
0- 0.6
" 0.5
~O*
0
0.4 D 0.3 .
E
5- 0.2 O.
- 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

I I I I I I I I I I III I I I I I I I

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

1.0 0.9
aI)
U) 0.8 S0.7 3:0.6 S0.5
S0.4
a 0.3 .E 0.2 S0.1
0
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
8'4
-8 0
Cn 2 CU
E -4)
0
0
S-62
-10
a)
) -10 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(a) Plan View, Isolines of Wind Field
E (U
a) 1.2
0. 1.0 /
_ 0.6. S0.4
Z .2
0.0 "
-10 ,I
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-SPlan View, Isolines of Wind Field at y = 0.
Figure 5. Characteristics of Model Wind Field.With
Translation But Without Coriolis Force.
V7
a) 1.2
CL
0.8
.0.
C
E 0.4
E
sL 0.2
0
z
0.0 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 5. Characteristics of Model Wind Field.With
Translation But Without Coriolis Force.
7




10
8
-- 6 \
ai
C., 4
@2
Cl .D 2 0
0
O0 -2 F0 0.6
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)
CL
0) 1.0
0.8
S0.6 o0.4
E
0
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




10
8 o
d 45
- 0 U 0
E
6 .Q
-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
co 1.0 -k
o 0.6
E
0.
oW
Z
0.0
z -80 .
-60
-10 -8 -6 -4 -2 0 2 4 6 8 10
Non-Dimensional Distance, x/R
(b) Cross-SPlan View, Isolines of Wind Field at y = 0.
Figure 7. Characteristics of Model Wind Field.With
Translation and Coriolis Force.
a9 1.2
C)
C) 1.0
~0.8
o 0.6
0.4
E
E
c' 0.2
0
z
0.0 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 7. Characteristics of Model Wind Field.With
Translation and Coriolis Force.
9




3.2 Characteristics of Hurricanes Erin and Opal and Previous Hurricanes Affecting the Panhandle
Table I 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)
1.6XIO-3 F 1/2 w (4)
9
in which H. 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




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 3 1 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 1 2.16 46.0 23.0

Table I

Characteristics of Historical Hurricanes Affecting the Panhandle area




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.
Y,
S(x)Cc f W2 cosedy (5)
Y,
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
sdi a H' (6)
which can also be expressed, from Eq.(4), as




sdi(x) = K2 W3cosOR 32 (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
1OR
SDI= f 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)
AY=-S (9)
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 3/2(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 lOR
hei(x) =f W512(Xy) cosO dy/ V (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, [TEI, is simply the integral of the local index over the shoreline on the right hand side of the hurricane, 1OR
HEI= f hei(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 Eri 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
"1.4
x: 1.2
x
S1.0
0
CD
0 0.8 L_.
w
a)
0.6
.
,
3:: 0.4
0
0
" 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 ----------------------- ................. ------------ ------------------------ ---- O p a l ------C 1 .6 ................................................ ........................................... .......... ......
U)
1 .4 ----------------------------------- ...................................................... ........... .........
a)
R 1 2 ................................................ .................. --------------------- --- ------------E
CU
1 .0 ---------------------------- ................... .................. ..........................................
0 .8 ............... ----------- ..................... .................. -------------------- --- -----------------
Frederic
1:917
U)
0 .6 .. .. .... ..... ... --- -- -- ---- ... ..... .... .. ... ---- -- --- -- --- --- --- ----- -- -- -- --- --- ---- --- --- -- --- -0 Erin 1916
0 0 .4 .. .... .. ....... ..... ... .. ...... ...... ..... .. ... ... ... ..... .. ... ....... .. ...... .. .. .... ....... .. .. ... ... .
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
CD
Cts
=3
3O .. . . . . .- - - - - . . . . ..-- - - - F re d e ric -
C.) rn ln
091
2I - - - - - - . . . . . . . . . . . . . ... ..
0
2 510 20 50 100
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, radius 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 --- ---------- -- ---- --- -- -- ---- ---- --- ---- --- -- -- ------ --- --- -- -- -- --- ---- -- -- ---
w 8 ................................................ .............................................................
- - - - - -- -- - . .. . . . . . . .. : . . . . . . . . . . . .. .. . . . . . . .. . .. . . . . . .
7 ------------Opal
0 6 ---------------------------- ..... ------- ------------------------- --0
w 5 ............................ ..................... ..... ......... ....... .. ..............
a)
CU Frederic
.. . . . . . . . . . . . . . . . . . . . .
4 - --- ----- ---- ------ -- .... .... ... .. .. ... ... ........ ....
3 - - -- - - - - - -- -- -- - --- - -- - - - -- - . . . . . . .. . . . . . . . . . . . .. .. . . .
Erin Elo[se
1917
2 --- ----- --- --- -- ---- ------------ -- -- --- ----- ....... ... .. .. ........ ..... .. .. ....... ........... ... .
Elena
1 - ----- --- --- ---------- --------- -- ---- --- --- -- --- -- -- -------- ---- -- -- -- ---- --- ----- -- -- ---
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, HEL




10
a)
0
0 Oa
2 5 10---- 20 50-------- ........00......
Basedon Lcal urriane rosin Fneeic




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-i. 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-i presents a definition sketch. The model hurricane consists of a pressure field and a wind field. Each of these is described below.
Vfx
Figure A-i. Definition Sketch of Model Hurricane Coordinate System. A-2. Pressure Field
The pressure field is composed of concentric circles defined by

p(r) =p 0+ Ape -Rir

(A-1)

in which p0 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- -P0 (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
U= Ap Re -Rrr (A-3)
Pa r
where Pa is the mass density of air (z2.4x 103 slugs/ft). 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)2e -I/r
U Pa r (A-4)
U 2wRsin4

A-2




where w and 4i are the rotational speed of the earth in radians/second (=27c/(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 Vpsinp U
(A-5)
2 U U
c g
and 3 is the angle defined in Figure A-1. Finally, the geostrophic wind speed is UG =0.83 UC(Py +1 -Y) (A-6)
and the wind vectors are rotated inward toward the center of the hurricane by approximately 180.