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Hurricane Opal : Results from overwash analysis Task E : Characterization of beach and dune response

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Title:
Hurricane Opal : Results from overwash analysis Task E : Characterization of beach and dune response
Series Title:
Hurricane Opal : Results from overwash analysis Task E : Characterization of beach and dune response
Creator:
Dean, Robert G.
Place of Publication:
Gainesville, Fla.
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Coastal & Oceanographic Engineering Dept. of Civil Engineering, University of Florida
Language:
English

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University of Florida
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University of Florida
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UFL/COEL-99/004

HURRICANE OPAL: RESULTS FROM OVERWASH ANALYSIS TASK E: CHARACTERIZATION OF BEACH AND
DUNE RESPONSE

by
Robert G. Dean and
Carrie L. Suter
March 8,1999

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

Coastal & Oceanographic Engineering Department
433 Weil Hall P.O. Box 116590. Gaineville, Florida 32611-6590

UNIVERSITY OF FLORIDA




UFL/COEL-99/004

HURRICANE OPAL: RESULTS FROM OVERWASH ANALYSIS

TASK E:

CHARACTERIZATION OF BEACH AND DUNE RESPONSE

Robert G. Dean and
Carrie L. Suter
March 8, 1999
Submitted to:
Bureau of Beaches and Coastal Systems Department of Environmental Protection Tallahassee, Florida 32399-3000




HURRICANE OPAL: RESULTS FROM WASHOVER ANALYSIS
TASK E: CHARACTERIZATION OF BEACH AND DUNE RESPONSE
March 8, 1999 by
Robert G. Dean and

Carrie L. Suter
Submitted to:
Bureau of Beaches and Coastal Systems Department of Environmental Protection Marjorie Stoneman Douglas Building 3900 Commonwealth Boulevard
Tallahassee, FL 32399-3000




TABLE OF CONTENTS
LIST OF FIGU RES ........................................................... iii
LIST OF TA BLES ............................................................ iii
1. Introduction ............................................................ 1
1.1 Purpose .......................................................... 1
1.2 M ethod .......................................................... 2
2. M ethods ............................................................... 2
2.1 Field M easurem ents ................................................ 2
2.2 A erial Photographs ................................................. 3
3. R eferences ............................................................. 4
APPENDIX
A TABLES DOCUMENTING WASHOVER THICKNESS DUE TO
HURRICANE OPAL .................................................. A-1




LIST OF FIGURES

FIGURE PAGE
1. Washover Due to Hurricane Belle (1976). The Lower Elevations After Belle are
Due to Wind Deflation (Dean and Perlin, 1977) ................................ 1
LIST OF TABLES
TABLE PAGE
1 Washover Volumes for Escambia and Santa Rosa Counties ....................... 4
A-1 Site 1. Santa Rosa Co. (near Monument R-210) ............................. A-1
A-2 Site 2. Santa Rosa Co. (near R-213, next to old Navarre Pass) ................. A-1
A-3 Site 3. Escambia Co. R-191 (about 1 foot of wind erosion estimated. Lag layer
present) ............................................................. A -2
A-4 Site 4. Escambia Co. R- 189 (about 1 foot of wind erosion estimated. Lag layer
present) ............................................................. A -2




HURRICANE OPAL: RESULTS FROM WASHOVER ANALYSIS
1. Introduction
1.1 Purpose
Hurricane Opal made landfall on Santa Rosa Island in Santa Rosa County on October 4, 1995. The maximum sustained surface winds at landfall were 110 mph, but more importantly, the storm surge reached between 8 and 20 feet along the Florida Panhandle. This large storm surge led to a loss of eight million cubic yards of sand from the beach due to breaking waves, extensive flooding and vast washover deposits in lower dune areas (Leadon, 1995).
Overwash is a process by which sand is transported from the nearshore and beach area landward and is deposited in a layer or "plaque" of sediment on the pre-storm surface. Overwash occurs during storms which include conditions of elevated water level and energetic wave conditions. The deposit of sand which occurs as a result of overwash is called washover (Schwartz, 1975). Overwash is the process (like overtopping) and washover is the associated deposit. Figure 1 presents a cross-section through a washover deposit.

L Vj U I I i a
0 50 00 1;0 200 250 300 350 400 450
Distance From Baseline (ft)
Figure 1. Washover Due to Hurricane Belle (1976). The Lower Elevations After Bell are Due
to Wind Deflation (Dean and Perlin, 1977).

Beach Profile: Sept. I, 1976 (post-Belle)Beach Profile: June I., 1977




Washover is important to sediment budgets since, similar to aeolian processes, in most cases the sand deposited by overwash is lost to the nearshore system. From considerations of long term coastal management, the most appropriate approach to dealing with washover deposits is not clear (Godfrey and Godfrey, 1973; Dolan, 1972; Leatherman, 1976). Combined with deposits from aeolian processes, washover deposits contribute to the upward growth of the barrier island. In this manner, the islands maintain their elevations relative to the rising sea level. If the washover deposits are removed and placed on the beach or removed from the near coastal area altogether, both common practices, the elevation of the barrier island will not keep pace with the relative rise of sea level and the frequency of overwash will increase with time due to the increased water levels. Washover deposits can occur as sheet deposits along significant segments of shorelines or can occur as fan shaped deposits through gaps in dunes.
For short term considerations, washover volumes represents a loss of sand from the nearshore system and this loss, unless compensated by beach nourishment, will result in shoreline recession. The purpose of this brief report is, through limited field measurements, to provide an estimate of the washover volumes on portions of Escambia and Santa Rosa Counties associated with Hurricane Opal.
1.2 Method
The volume of a washover fan is the product of its area and its depth. The area of the washover fan was estimated from aerial photographs obtained under contract by the Florida Department of Environmental Protection (FDEP) taken immediately after Hurricane Opal. The depths of the washover fans were established in the field by evaluating the representative depth of sand covering the pre-storm layer of vegetation or other debris/lag horizon. This representative depth was applied to the entire washover area to determine the volume of sand removed from the beach system. Additionally, informed simple sensitivity calculations were carried out to demonstrate the range of possible changes in washover depth.
2. Results
2.1 Field Measurements
Four sites were investigated in Escambia and Santa Rosa Counties in December 1997 to determine the representative depth of the washover deposit. The four sites were chosen because of their locations in unpopulated areas to avoid any interference due to washover removal or other activity. Any type of post-storm cleanup could result in an overestimate or underestimate of the deposited sand. The four sites were considered representative of most of the washover fans in location, depth, area and general appearance. The sites were also large enough to obtain a sufficient number of samples to accurately depict the representative depth of the washover fan. The washover fans were easily distinguishable through the absence of vegetation in the vicinity, due to burial by the washover deposits.




Once the appropriate site was chosen, holes were excavated in several locations on the washover fan to determine the depth to the old vegetation line and/or lag surface. The three definitive signs of an old vegetation level and/or lag surface are: (1) A clear black line, caused by the decomposition of vegetation buried and crushed flat by the heavy overlying sand, (2) Undecomposed vegetable matter, usually in the form of more fibrous tissue, such as the plant stalks, and/or (3) A lag layer of shells or other coarse materials. At some locations, a combination of indicators was found. The measured vertical distance between the existing sand surface and the indicator was accepted as the depth of the washover deposit. This is believed to represent a lower estimate due to winnowing of the fine sand and deflation by wind since the overwash event. Tables in Appendix A document the recorded depths and locations of the documented thicknesses for each of the four study sites.
2.2 Aerial Photographs
The areas of the washover fans were measured on the aerial photographs of Escambia and Santa Rosa Counties that were taken immediately post-Opal. The washover deposits are identified as characteristically bright white sandy areas. There is no vegetation, structures or roads present because of the overlaying layer of sand. The southern boundary of the washover (the side adjacent to the natural beach) was estimated by using old dune ridges, landscaping, houses or variations in the coloring of the sand that might indicate high water lines. Only the major fans were measured. There were some washover deposits located on the northern side of the island, but due to natural contours of the land, the depths of the sand deposits were considered to vary considerably. Therefore, the representative depth of washover could not be applied to these secondary deposits. The washover areas documented ranged from R-34 to R-214 in the Escambia and Santa Rosa Counties monument system.
In washover areas where houses or other structures were located, the technique for measuring the area of the deposit was modified slightly. The area of the houses was subtracted from the total area of the deposit. Any increase in the sand accumulation in front of the structures was considered to be balanced approximately by the reduced sand accumulation behind the structure.
Because the measurements to determine the representative depths of the washover deposits occurred 26 months after the storm event, a significant amount of sand is believed to have been removed from the washover fans by aeolian transport. This statement is supported by the frequent presence of a lag layer of shells on the washover deposits When aeolian processes have eroded the uppermost layer of a washover deposit, all measurements will underestimate the total amount of sand that was transported from the beach environment; this underestimation could be on the order of 25 to 50 percent. Therefore, it is highly desirable that future field evaluations of washover deposit thicknesses take place immediately following a storm event to reduce measurement uncertainties.
The average depth found from field measurements for the four sites is 16 inches. By multiplying the average depth and the areas estimated from the aerial photographs, the volume of




the washover fans was determined. The following table lists the results, in various units, allowing for varying amounts of erosion due to aeolian processes.
Table 1:
Washover Volumes For Escambia and Santa Rosa Counties

3. References
Dean, R. G. and M. Perlin (1977) "Coastal Engineering Study of Ocean City Inlet, Maryland", Proceedings, Coastal Sediments '77, American Society of Civil Engineers, Charleston, SC, pp. 520542.
Dolan, R. (1972) "The Barrier Dune System Along the Outer Banks of North Carolina, a Reappraisal", Science, pp. 1972-1974.
Godfrey, P. J. and M. M. Godfrey (1973) "Comparison of Ecological and Geomorphic Interactions Between Altered and Unaltered Barrier Systems in North Carolina", in Coastal Geomorphology, D. R. Coates, Editor, State University of New York, Binghampton, NY, pp. 329-358.
Leadon, Mark E., 1995. Hurricane Opal: Damage to Florida's Beaches, Dunes and Coastal Structures. Florida Bureau of Beaches and Coastal Systems.
Leatherman, S. P. (1976) "Quantification of Overwash Processes", Ph. D. Dissertation, Department of Environmental Sciences, University of Virginia.
Schwartz, R. K. (1975) "Nature and Genesis of Some Washover Deposits", U. S. Army, Corps of Engineers, Coastal Engineering Research Center, Technical Memorandum No. 61, Fort Belvoir, VA.

[ Units Measured Value 25% Increase 1 50% Increase]
cubic feet 76,695,115 95,868,893 115,042,673 cubic meters 2,171,764 2,714,705 3,257,646
cubic yards 2,840,560 3,550,700 4,260,840




APPENDIX A
TABLES DOCUMENTING WASHOVER THICKNESS
DUE TO HURRICANE OPAL




Introduction

In total, washover thicknesses were documented at twenty three different locations at four separate sites. The method of determining thickness was to excavate, using an ordinary shovel, down to an indicator which was either a vegetative or coarse sediment lag layer.
Results
The locations and washover deposit thicknesses at each of the twenty three locations are presented in the following four tables.

Table A-i: Site 1. Santa Rosa Co. (near Monument R-210)

Sample No. Thickness of Layer Is Defined By Location
Washover Deposit
1 20"
2 12" vegetable matter
3 12" black streak in sand
4 11.5" vegetable matter
5 11" vegetable matter and black streak
6 17.5" vegetable matter and black streak 100 ft S of road
7 17" 100 ft S of road
8 17" 200 ft S of road
Table A-2: Site 2. Santa Rosa Co. (near R-213, next to old Navarre Pass) Sample Thickness of Layer Is Defined By... Location
No. Washover Deposit
1 22" black streak dead end of road, 80 ft E
2 15" vegetable matter N edge of road
3 19" vegetable matter (entire plant) 75 ft N of edge of road
4 22" vegetable matter (entire plant) 110 ft S of road
5 22" vegetable matter 80 ft S, 15 ft W of road
6 15" large shell 140 ftN, 100 ftWof road




Table A-3: Site 3. Escambia Co. R-191 (about 1 foot of wind erosion estimated. Lag layer present) Sample Thickness of Layer Is Defined By... Location
No. Washover Deposit
1 13" gravel layer 130 ft N of road, 130 ft W ofmon.
2 6" shells 150 ft N, online
3 12" shells 150 ftN, 63 ft E ofmon
4 22" shells 300 ft N, online

Table A-4: Site 4. Escambia Co. R-189 (about 1 foot of wind erosion estimated. Lag layer present)
Sample Thickness of Layer Is Defined By... Location
No. Washover Deposit
1 15" shell 150 ft N, online
2 19" black streak 250 ft N, 50 ft E of mon
3 18" vegetable matter 350 ft N, 100 ft E of mon
4 14" vegetable matter 350 ft N, 250 ft E of mon
5 17" difference between sand color 350 ft N, 100 ft W of mon

A-2