| Profile Data | Regression Data |
by
Scott L. Douglass
Civil Engineering Department
University of South Alabama
Mobile, AL 36688
To
Coastal Programs Office
Alabama Department of Economic and Community Affairs
Riley Boykin Smith, Commissioner
January 31, 2001
This report is written for citizens interested in the understanding of Alabama’s Gulf beach erosion that has been developed through research by the Coastal Engineering and Science Research Program at the University of South Alabama. The report attempts to explain the analysis tools and results in a format for laymen. Some suggestions for future management decisions are included in the report.
The author of the report is Dr. Scott Douglass, Professor of Civil Engineering. He can be reached through the Department of Civil Engineering by telephone at 334-460-6174 and by email at sdouglas@jaguar1.usouthal.edu.
This report was prepared under contract with the Coastal Programs Office of the Alabama Department of Conservation and Natural Resources (ADCNR). Funding for this research was provided by a grant (NA970Z0152) from the National Oceanic and Atmospheric Administration, Office of Ocean and Coastal Resource Management. Mr. Phillip E. Hinesley, ADCNR, was the technical monitor for the contract.
The City of Gulf Shores and Olsen Associates, Inc. provided the figure on the cover of the report that shows a typical beach fill design section for the 2001 Gulf Shores beach nourishment project.
This report can be found on the internet by going through the University of South Alabama homepage and finding the “research” program entitled “Coastal Engineering and Science Research.”
The Gulf of Mexico beaches are one of the top economic and environmental assets in coastal Alabama. The coastal/beach tourism “industry” is one of the fastest growing parts of the Alabama economy. Beautiful, white sandy beaches are the linchpin of this “industry” as well as a part of the quality of life for many Alabama citizens.
The coming year, 2001, will be an interesting time for the beaches of the state. The first major, engineered beach nourishment project in the state’s history will be built in Gulf Shores. Beach nourishment, the direct addition of sand to the beaches, has been both praised and criticized elsewhere as a way to maintain wide, sandy beaches along eroding coasts. The National Research Council concluded that beach nourishment can successfully widen beaches in some locations if done correctly.
“What you see (initially) is not what you get” with beach nourishment! The beach in Gulf Shores will look much different one or two years from now than it will during and right after construction. The beach initially will be much wider than its true design width (see a typical section on the cover of this report). The first few big storms after construction will permanently pull fill sand from the dry beach berm out to the underwater, nearshore sand bar system. This is part of the design and it will be important to realize that the project is not “washing away.” Also, the sand initially will not look as white as it will after several months in the sun and surf. There will be more broken seashells in the filled beach but these will probably be washed away in the surf through time. Gradually, over the next 5 to 10 years, the new sands will move down the coast and the beaches within the project limits will narrow back toward the pre-nourishment widths.
About half of Alabama’s Gulf beaches have become narrower in the past three decades. All of the Gulf of Mexico beaches of Alabama naturally fluctuate in width in response to the variable wave climate. The dry portion of the beach often builds wider when the waves are milder and gets narrower when the seas are stormier. The long-term trends underlying these fluctuations are important. Half of the Alabama beaches, while they fluctuated in width, did not have a trend in either direction. Only a few miles of beaches have widened.
We are responsible for most of the Gulf beach erosion in Alabama in the past three decades. We have removed millions of cubic yards of beach quality sand from the littoral system at Perdido Pass and Mobile Pass to maintain navigation channels. The amount of sand removed at the passes just about equals the total amount of beach erosion losses during this time!
We can have great beaches and safe navigation channels: indeed, for the long-term economic health of southern Alabama, we must. If we better manage our beach sands at the passes, we can preserve our beautiful beaches for our own enjoyment and for the enjoyment of future generations. The engineering solutions to the beach erosion problems in Alabama are better sand bypassing at the inlets and beach nourishment where needed to restore the already narrowed beaches. Recommendations concerning new legislation to accomplish this and a new university research center to support it are included in this report.
This annual report presents data, analysis and recommendations for managing the health of the narrow strip of beach sand between the buildings and the water. Emphasis is placed on outlining the trends in beach width and, where possible, explaining those trends in terms of the physical, coastal processes that are causing the changes. Notes are also made as to how well the trends and their causes are understood and where further research is warranted to improve our understanding of the “state of the beaches.”
Individual reaches, or stretches, of beach are discussed in the “Beach by Beach” section of this report. These stretches of beaches, from 1 to 13 miles long, generally behave similarly.
The extensive beach systems of the bays, sounds, and bayous of Alabama are not discussed in this report. Only the Gulf of Mexico beaches are discussed.
"Golden is the sand"
The Robert Louis Stevenson poem with the line “golden is the sand” was not written with the Alabama beaches in mind - but it could have been! Alabama’s Gulf of Mexico beaches are one of the greatest economic and environmental assets of the state. The tourist economy in south Baldwin County provides over 40,000 jobs and over $1 billion in revenue annually. Billions of dollars in private wealth have been created along the coast. All of this business ultimately rests on the thin strip of beach sand, the strand, along the Gulf of Mexico.
Alabama citizens throughout the state consider the Gulf beaches to be one of the state’s prettiest areas. Indeed, people from throughout the nation and the world acknowledge that the beauty of the white sand and gentle surf creates some of the prettiest beaches in the world.
Like sugar attracts ants, the sugar-white sands of Alabama attract people. The Alabama Gulf front properties have experienced tremendous development in the past few decades. The percentage of Gulf front lots with condominiums or hotels rose from 3% to 22% between 1970 and 1996. The percentage of condominiums will continue to increase as more single family houses are replaced with condominiums. The density of the housing of the beachfront properties is extremely high. Considered along the beach, these densities approach only several inches of coastline per person! Several very large condominium projects have recently been built and are more are planned in west Baldwin County along the Fort Morgan Peninsula.
Terminology
The terms, “recession” and “accretion” are used in this report to describe the direction of shoreline movements. A “receding” or “recessional” shoreline is where the beach is eroding and getting narrower. In other words, since this report only considers the Alabama Gulf of Mexico, a receding shoreline is moving north. An “accreting” or “accretional” shoreline is where the beach is gaining sand and getting wider. In other words, an accreting shoreline is moving south.
The relationship between inlets and beaches
Most of the dramatic shoreline change in Alabama is near the tidal passes or inlets. Some of this shoreline change is probably in response to natural inlet fluctuations. And, man’s engineering has influenced much of it. Alabama has 3½ tidal inlets: Perdido Pass, Little Lagoon Pass and Mobile Pass break through the barrier island system of the state. Petit Boit Pass is at the western end of Dauphin Island and straddles the Mississippi-Alabama border. Each of the three passes in the state has a significant impact on the beaches in the vicinity of the pass and a jetty or seawall structure with regular dredging to maintain adequate water depths for safe navigation.
The sand that is dredged from these passes is sand that came off the adjacent beaches. It is also sand that was on its way back to the beaches before it was removed by dredging operations. The removed sand is part of the same littoral system as the sand on the beaches. The details of our understanding of the relationship between the beaches and passes are discussed throughout this report.
Littoral drift
Along the Alabama Gulf beaches, the dominant direction of longshore sand transport, or littoral drift, is from east to west. However, often and for sustained periods of time, significant amounts of sand move toward the east. The dominant process, or mechanism, for moving sand in the littoral system is longshore sand transport. Longshore sand transport is the wave-driven movement of sand along the coast. As waves approach a beach at an angle, they break and move sand in that direction. Thus, when waves approach the beach from the other direction, the longshore sand transport direction reverses. Winds and tidal currents, while important in some situations, are clearly secondary processes in terms of sand movement along the coast. Many of the long-term shoreline change trends in Alabama, even those very near the inlets, can be explained in terms of changes in longshore sand transport.
Two types of data are regularly used to measure the beaches in Alabama: aerial photography and beach profile surveying. Aerial photography allows for an estimate of the dry beach width at a specific moment in time. However, because of cross-shore sand transport mechanisms such as sand bar migration, the dry beach is only a small portion of the complete littoral system. The sand underwater immediately offshore of any sandy beach is also part of the beach system since it breaks wave energy and feeds sand to and from the dry beach. Surveyed beach profiles measure the underwater portion of the littoral system as well as the elevations of the dry beach and dune system.
The University of South Alabama’s air photo shoreline change database was used to calculate average annual erosion rate. The data are shoreline positions based on 16 sets of air photos from 1970 to 2000. Five of the 16 sets of air photos were taken during the 1970’s and 80’s. Eleven sets of air photos were taken annually, in late September or early October, since 1990 by the Alabama Coastal Programs office (now ADCNR). Post-storm photos, such as the October 1998 photos taken immediately following Hurricane Georges, are not used in this analysis. The air photos are part of the Alabama Coastal Shoreline Archives that is housed in the College of Engineering at the University of South Alabama. The archives contain these and other air photos as well as most of the historical coastal charts and surveys of the Alabama coast.
Beach width was measured at 91 fixed locations from arbitrary but consistent points (e.g. the corner of a house, the centerline of a road) to the visible wetline. The measurements were adjusted for the actual scale on a photo-by-photo basis using Global Positioning System (GPS) technology. Sources of error include those due to interpretation of the wetline; water level fluctuations due to tides, winds, barometric pressure, and waves; and photogrammetric errors of tilt and lens distortion.
It is obvious from Figure 2 that the answer to the question “is the beach eroding” depends greatly on which Alabama beach is being considered. Many are eroding and many are not. Only a few are gaining sand.
This year, at many beaches in the state there were significant changes in the underwater portion of the beach profiles. In general, it appears that the offshore sand bar has moved shoreward back to its location prior to Hurricane Georges. Most of the biggest changes in dry beach width seen this year were positive changes on the developed portion of west Dauphin Island. Roughly 300,000 cubic yards of sand were placed along over two miles of shoreline in the surf and as a dune to prevent the overwashing of the island that continued after Hurricane Georges in 1998.
The following discussion summarizes the results of the monitoring data described above on a beach-by-beach basis. The individual beach reaches discussed are shown on Figure 4. The discussion also attempts to explain the results in terms of the author’s understanding of the coastal processes of that area based on a decade of research on the Alabama coast and general principles of coastal science and engineering. There is an incomplete understanding of the Alabama coastal processes because of the short duration and limits of our research. We have a better understanding at some places than others. The data for each location are available on the internet.
The western two miles of Perdido Key are in Alabama. The analysis of the air photos shows no significant trend in shoreline position. There are significant fluctuations, over 150 feet, in shoreline position. The beach profile data here also show no large shoreline change trends.
There are two major engineering projects that are important to the long-term health of the Perdido Key beaches. The jetties at Perdido Pass were built in 1968-69 immediately before the beginning of the air photo database used in this report. The jetties successfully “stabilized” the inlet. Prior to construction, the inlet had migrated along a stretch of coastline extending about 4 miles to the east of its present location in the past several hundred years. The present-day locations of all of the beaches of Alabama on Perdido Key were in the water of Perdido Pass at some point during the last two hundred years. The shoreline position just east of Perdido Pass is essentially fixed in place by the eastern Perdido Pass jetty. The eastern jetty has a low weir section that allows sand to pass over the rocks into a deposition basin in the pass when sand is moving westward in the wave-driven littoral current of Perdido Key.
A large beach nourishment project was placed in Florida at the eastern end of Perdido Key in the early 1990’s. The project was part of a sand bypassing effort with sand dredged from the deepening of the entrance to Pensacola Bay. This is the sort of sand bypassing that is needed in Alabama. A 400-foot wide beach was built along several miles of the Johnson Beach unit of the Gulf Islands National Seashore and even more sand was placed offshore in a constructed sand bar. It is now likely that some of that sand has moved out of that area to the western portion of Perdido Key.
Alabama Point
Immediately west of Perdido Pass (for about 1 to 1½ miles) is one of the few beaches in the state that has gotten significantly wider since 1970. It has gotten wider because of the engineering at the pass. The western Perdido Pass jetty, built in 1968-69 just before the beginning of the air photo database used in this report, is a high, rock jetty designed to keep sand from moving back east into the pass. Figures 1 and 2 show the beaches for about a mile west of the pass have accreted since 1970. Figure 2 shows annual trends of up to 15 feet per year of accretion near the western jetty. Roughly three million cubic yards of sand have accreted here since 1970.
This deposition of sand adjacent to the western jetty is called a “fillet” by analogy with a fillet weld in the corner of two pieces of metal. The sand fillet formed as the beach planform (shape of the beach as viewed from above) moved toward a new equilibrium position adjacent to the jetty. When waves are approaching the Alabama coast from the southwest and the longshore sand transport is to the east, sand is free to move off the beaches of Orange Beach into this area. However, when waves are approaching the Alabama coast from the southeast and the longshore sand transport is to the west, the sand in this area is partially sheltered by both the rock jetty itself and the shoals around the mouth of Perdido Pass. Essentially, this sand is partially, permanently trapped in this fillet.
The sand fillet is also the location of the disposal area for the sand-bypassing operation at Perdido Pass. Much of the sand dredged from the deposition basin and main channel during the past thirty years has been placed on the beaches within several hundred yards of the jetty or immediately offshore of these beaches. The beaches have fluctuated dramatically in response to sand bypassing episodes.
It is recommended that the approved disposal area for the dredged sands be moved from this beach to the beach about two miles west of the Pass. This beach near the pass does not need the sand. There is an over-abundance of sand here that has led to several recurring problems including standing water on an unnaturally wide beach, unnaturally high scarps (vertical cliffs) on the beach face that are unsafe for humans and prevent successful sea turtle nesting, and complaints from tourists about the very long walk to the beach. The beaches about 1½-2 miles west of the pass need the sand. Also, moving the disposal area west will reduce the likelihood of this sand ending up back in Perdido Pass to be dredged again.
West Orange Beach
The beaches of the western portion of Orange Beach appear to be receding from 1970 to 1999 (see Figures 1 and 2). This recessional reach extends roughly from the west end of Cotton Bayou to the western city border at the main unit of the Gulf State Park. This recession is probably due to trapping of sand by the engineering at Perdido Pass. The trapping of sand by inlet jetties is expected and found everywhere in the world. Some trapping is unavoidable. At Perdido Pass, the trapping includes sand dredged and placed on dry land in the vicinity of the pass, sand stockpiled and removed by trucks, sand trapped in the western fillet as described above, and probably sand stored in the ebb-tidal shoal that formed farther south after construction of the jetties. The total trapping is between 4 and 8 million cubic yards of sand since 1968. The uncertainty is due to a lack of sand disposal records and a lack of survey and monitoring data.
Long-term erosion due to jetties typically is displaced some distance downdrift of the jetties due to the fillet formation as described above. At other jettied inlets of comparable size, the downdrift erosion begins to show up 1 to 4 miles from the jetty. Essentially, the formation of an equilibrium shoreline and new ebb-tidal shoal after jetty construction traps large amounts of sand in the inlet vicinity and starves the downdrift beaches. The pattern of accretion near the jetty and recession farther downdrift (Figures 1 and 2) is consistent with the signature patterns of erosion caused by jetties.
As mentioned in the previous section, it is recommended that sands dredged from Perdido Pass be placed on these beaches (over 1½ -2 miles from the pass) in the future. This is essentially just a modification of the sand bypassing operations. This would nourish these beaches while re-establishing the littoral drift along the beaches. It is also recommended that formal, independent monitoring of the bypassed sand be implemented to learn how this works for future management decisions.
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East Gulf Shores
The beaches of the eastern, developed portion of Gulf Shores, from the western border of Gulf State Park to just west of the main beach area, show varying levels of recession mixed with areas that show no trend (see Figure 2).
During the past decade, this portion of the coast had the narrowest beaches in the state. For example, in the September 1995 photos used in this study, the beach widths, defined as the distance between the buildings or bulkheads and the high water line, averaged about 70 feet and varied from 0 to 130 feet along the central 2.5 miles of Gulf Shores beaches. They averaged over 200 feet and varied from 120 to 700 feet in the rest of Baldwin County. The narrow beaches in Gulf Shores may be due to encroachment of the buildings more than erosion. To the citizen or the tourist, the result is the same… the beaches are narrower.
The narrowness of the beach in this area may also be partially due to the trapping and incomplete sand bypassing at Perdido Pass. Jetty trapping and littoral system blockage has been found to cause beach erosion up to 20 miles away at other locations. One way to estimate the extent of the impact of inlet sand trapping on the beaches is to look downdrift for a quantity of beach erosion that matches the quantity trapped. Such an approach implies that beach erosion in Gulf Shores could be due to sand trapping at Perdido Pass.
The engineering of Little Lagoon Pass has impacted the beaches around the pass for at least a mile on either side. When the pass was stabilized by jetty construction in 1981, no sand bypassing system was established. In a classic response to jetty construction, the beaches accreted on the eastern side and receded on the western side for about a decade. In the early 1990’s a lawsuit settlement included pumping sand onto the western beaches and shortening of jetties. Dredging is now needed on a fairly regular basis (about 6 times per year) to maintain depths in the pass. The dredged sand is placed on the beaches to the immediate west. Essentially, the current operations are a form of sand bypassing.
The court-ordered coastal engineering has worked to a major extent. The beaches have been widened on the western side of the pass and the pass has remained open. However, because of the constant dredging and shoaling process, there is some desire to further modify the jetty system. The Alabama Department of Transportation, the agency with responsibility for the jetties, has proposed a modification to widen the jetties. Also, the 2001 beach nourishment project being built to the immediate east of the pass may lead to a higher rate of shoaling and thus increased dredging costs in Lagoon Pass.
A comprehensive coastal engineering and planning analysis should be done for Little Lagoon Pass. The pass is an outstanding resource for the people in many ways. Its flushing has a major impact on the water quality in Little Lagoon, it has significant impacts on the beaches to the immediate east and west, and it is a highly used municipal park. The pass also provides boat access from the Lagoon to the Gulf. However, the access is often severely limited by the shoaling and dredging operations. Navigation is often treacherous. The longshore sand transport along the beach, the tidal flushing of the pass, and the interactions between the two make for a complex situation that is traditionally addressed by coastal engineers.
The history of Little Lagoon pass is a good example of how decisions that are not based on sound coastal engineering and comprehensive planning can cause problems. Both the downdrift erosion problem before the court-ordered jetty shortening and the present-day shoaling problems in the pass were predicted by coastal engineers. An open, sound, comprehensive coastal engineering and planning analysis for the pass could address the concerns of all the multiple users of the pass and evaluate options to provide better information for management decisions while avoiding the negative impacts experienced in the past.
West Gulf Shores
The western beaches of Gulf Shores, from Lagoon Pass to the end of West Beach Boulevard, have been recessional since 1970. The extent of the recession, shown in Figures 1 and 2, includes most of the beaches to the west of Little Lagoon Pass.
The causes of this recession are unclear and require further research. Part of the recession may be due to the engineering at Little Lagoon Pass. Another possible cause of the erosion along these beaches is the removal of sand from the beach system that occurs when a large storm hits the area. For example, when Hurricane Opal brushed this coast in 1995 on its way to Pensacola, the storm surge crossed over the barrier island allowing waves to move sand from the beaches and dunes across the road and into Little Lagoon. A layer of sand up to several feet deep was deposited on the lots and West Beach Boulevard at the end of the storm. Little of this overwashed sand was returned to the beaches from which it came. Hurricane Georges resulted in similar overwash. Some portion of the overwashed sand was returned to the beaches but the portion that was on private property, or in Little Lagoon after the storm, was not returned to the beach. Thus, there was a significant removal of sand from the beach and dune system. The volume of sand removed from these beaches via this overwash process during Opal and Georges was very roughly equivalent to 20 to 30 feet of permanent beach width.
Most of beaches of western Baldwin County, from the west end of Little Lagoon to the end of Fort Morgan Peninsula, have no shoreline change trends for the past thirty years. Figures 1 and 2 show that most of these beaches have no significant trend. There has been some speculation that these beaches may be growing over the past century. These data, however, do not show any significant accretion in the past three decades. The beach widths along some of these beaches are the widest in the state because the construction was set back so far. The perception of healthy beaches may be partially because they are so wide as measured from the construction line. When the waterline is 300 feet from the buildings, shoreline fluctuations of plus or minus 100 feet are hardly noticeable.
Some very wide construction setbacks along these beaches have been established recently in front of large condominiums because of endangered species habitat (beach mouse). A secondary benefit of these large setbacks (up to 700 feet) is that these locations will probably never experience beach erosion problems like Gulf Shores. Setbacks are an outstanding beach management practice for developing coastlines. Setbacks of this width (200-500 feet) should be maintained here and elsewhere new construction is planned.
Figure 2 shows that a few locations have recessional trends. The western tip of Fort Morgan Peninsula, in the state park, in particular has extremely large recession rates. The recession is threatening to uncover some historically important artifacts. These recession rates are probably related to the dynamics of Mobile Pass. The elevations of the shoals offshore of this area have decreased. Part of this decrease may be natural fluctuations but part of it is probably also due to the removal of sand from the outer bar of Mobile Pass. The same problem is influencing the beaches of Dauphin Island.
East Dauphin Island
The beaches of the east end of Dauphin Island have experienced some of the most dramatic shoreline recession on any inhabited barrier island in the United States in the past 30 years. The shoreline recession is over 500 feet in the vicinity of the Coast Guard R&R facility. Several studies have shown that the location and elevation of the ebb tidal delta of Mobile Pass influence these beaches. Sand or Pelican Island as well as Dixie Bar are part of the ebb-tidal delta.
Shoreline change analysis shows that roughly the easternmost mile of the island is receding while the next mile of beaches to the west is accreting! This pattern is due to a shift of sand from the easternmost mile to the next mile to the west. This pattern of sand shift is consistent with one due to wave driven longshore sand transport along the beaches. Essentially, these beaches are re-aligning themselves to be more perpendicular to the incident wave climate. They are rotating to face more south-southeast instead of south.
About 400,000 cubic yards of sand shifted from the easternmost mile of Gulf beaches to the next mile of beaches to the west between 1984 and 1996. The probable cause of this shift is a change in longshore sand transport rates due to changes in the wave climate caused by the northwestward migration of Sand/Pelican Island and the loss of elevation of the shoals around the outer portion of the ebb-tidal delta (near the lighthouse on both Dixie Bar and Sand Island Bar).
A wave-driven longshore sand transport model has been used to evaluate the sensitivity of the beach shift to the shoal elevations. The beaches of Dauphin Island were found to be so sensitive to the sheltering afforded by the shoals that a two-foot increase in the elevation of the shoals near the lighthouse would have reduced the longshore sand transport rate, and thus the erosional shift of sand along the beach, to roughly 50% of what was experienced. The implication is that the most landward recession on these beaches is attributable to the removal of sand from those shoals for maintaining the ship channel to Mobile. Over 16 million cubic yards of sand have been removed from the outer bar of the ebb-tidal delta by dredges and dumped offshore in the past 30 years.
Sand was dredged from the Fort Gaines channel during the summer of 2000. This sand (14,000 cubic yards) was moved to the north towards the Pass Drury area and the artificial sand bar that protects Government Cut. That disposal location was used instead of one to the south on the Gulf beaches in the lee of the flanked groin field at the east end of Dauphin Island. This practice of moving the sand dredged from Fort Gaines channel to the north was common in the 1960-80’s. However, in the 1990’s that sand was used for several small beachfills on the east end Gulf beaches that then fed the beaches to the west. The Gulf site, in the lee of the flanked groins, is an approved disposal site that was created just for this purpose. This much sand would have made a significant difference in the beach widths along the Gulf. This is an example of competition for the same sand resource that will probably become more common in Alabama’s future.
About two years ago, at the request of the Town of Dauphin Island, the U.S. Congress began funding a study by the U.S. Army Engineer District, Mobile of the entire north-central Gulf of Mexico coast from Dauphin Island, Alabama through the Florida panhandle. This study, the Regional Sediment Management Initiative, was funded (over $2 million to date) as a pilot project to investigate putting dredged sands (such as those from Ft. Gaines channel) back in the littoral system instead of just disposing of them in the cheapest way to maintain the navigation channels. The study will be a success only if it results in more sand on the Gulf beaches in the future.
West Dauphin Island
The west end of Dauphin Island, from the little red schoolhouse to the end of the road, has been receding since 1970. The data analysis for this study stopped at the west end of Bienville Road and did not include the undeveloped portion of the island. The recession rate has averaged 2 to 5 feet per year. The shoreline change was so severe that the pilings supporting several houses were in the surf consistently throughout 1999. These houses were very close to the surf occasionally prior to Hurricane Georges. After the hurricane, they were in the surf much more regularly until the construction of the storm “berm” project in 2000 discussed below.
The primary cause of erosion on Dauphin Island in the last several years appears to be the navigation project at the Mobile Ship Channel. Specifically, the disposal of sand offshore, out of the littoral system, has caused both the west and east end beaches of the island to erode. Millions of cubic yards of sand have been removed from the littoral system near the Sand Island Lighthouse in the past several decades. The lighthouse is several miles “upstream” in the littoral system from the west end beaches.
Sand naturally moves via wave driven processes along the outer edge of the ebb-tidal delta (Sand/Pelican Island) from the area near the lighthouse towards the fishing pier. It then naturally moves from Sand Island to the beaches of Dauphin Island in the form of migrating sandbars between the pier and the general vicinity of Ponchatrain Street. From there, some of the sand is moved west via wave driven longshore sand transport toward the west end of Dauphin Island and some of it moves back toward the fishing pier. In essence, the “river of sand” that feeds the beaches of the west end of Dauphin Island is being interrupted by the dredging removal near the lighthouse. It appears that the west end beaches are beginning to suffer severely from the decades of complete littoral blockage out by the lighthouse. There is also some natural component of this fluctuation. Shorelines near inlets often naturally fluctuate more than other shorelines because of inlet dynamics. Separating out the man-induced portions of erosion can be difficult.
The patterns of the west end erosion are generally consistent with those found at other places where the littoral system is interrupted. And it is consistent with what should be expected based on sound coastal engineering principles. One expected aspect is that there is some lag time between the beginning of the interruption and the downdrift erosion problem. This is due to the time it takes for sand to move down the littoral system. For example, at Little Lagoon Pass in Gulf Shores, the downdrift recession did not get critical until about a decade after construction of a jetty that trapped sand. The Dauphin Island/Mobile Pass littoral system is much larger than that one.
A more appropriate comparison for Dauphin Island may be Ocean City, Maryland where a new inlet for navigation created in 1933 trapped littoral sand in a new ebb-tidal delta and starved the downdrift barrier island. The starvation became critical within several decades and is still continuing to be felt over sixty years later up to 20 miles away. Initially at Ocean City Inlet, the downdrift shoreline began to recede. Then some large hurricanes caused significant island overwash. Eventually, numerous small winter storms caused frequent overwash as the dune field was not able to re-establish itself on the downdrift island (the north end of Assategue Island). Essentially, the downdrift barrier island was destroyed to the extent that it has migrated an entire island’s width landward and is no longer inhabited. The scenario played out at Ocean City Inlet fifty years ago is beginning to be played out again at Dauphin Island, Alabama. A total blockage of the littoral system gradually destroys the downdrift island.
Another aspect of the Dauphin Island erosion pattern that is consistent with what is to be expected downdrift of a littoral interruption is the distance. Dauphin Island’s west end erosion due to this starvation mechanism begins several miles away from the ship channel and extends at least to the end of the road and probably much farther west (we have no data out on the undeveloped portion of the island).
Another aspect of Dauphin Island’s erosion problem that is consistent with what is to be expected downdrift of a littoral interruption is the fact that most of the obvious erosion and recession occurs during storms. Most littoral drift occurs during storm events and all island overwash occurs during major storms. It must be emphasized that the storm was not the underlying cause of the erosion but just the agent that hurried it along. The beach is responding to the long-term starvation and most of the responding occurs during storms. Hurricane Georges was a big storm but Dauphin Island has experienced much bigger storms for centuries without the level of recession seen this time. Also, the shoreline position recovery of the rest of the state’s beaches after Hurricane Georges indicates that the west end of Dauphin Island is being stressed more in other ways. Beaches that are starved prior to a storm will erode more due to the storm.
In March 2000, the Dauphin Island Property Owners Association filed a lawsuit against the federal government concerning this beach erosion. From a technical perspective, a solution is clear: future sand bypassing of the dredged sands with a limited beachfill on the beaches. We can have great beaches and safe navigation channels in Alabama: indeed we must.
During the past year, a construction project added sand to the west end of Dauphin Island. The project is locally referred to as “the berm” project because the funding authority was a post-disaster emergency response to stop the regular overwashing of the island that occurred during 1998 and 1999 because Hurricane Georges removed the sand dunes. Sand was placed on the upper portion of the beach profile to widen the beach and create some dune elevation. The behavior of the project since construction is consistent with accepted scientific principles and engineering models. Sand has eroded from the seaward side of the “dune” and moved both offshore and alongshore.
In spite of the fact that the project apparently performed as expected, and further overwashing has been prevented to date, there is still much criticism of the project. The project was not designed using available state-of-the-art analysis tools for predicting its fate. One of the biggest problems is that the fate of the sand placed here has not been adequately monitored. Adequate monitoring of coastal engineering projects should be a required part of any future beach project in the state. We need to learn as much as we can about these beaches for future management decisions.
The fate of the sand from the sand bypassing attempt at Mobile Pass in 1999 is not presently known and may never be adequately known. After Hurricane Georges, three million cubic yards of sand dredged from near the lighthouse was not placed in the usual offshore disposal site (that is out of the littoral system). Instead it was placed immediately west of the Mobile Pass Ship channel south of the lighthouse in an attempt to place it in the littoral system. However, the selected disposal site was not the best location for returning the sand to the state’s littoral system based on state-of-the-art coastal engineering analysis. Better locations would have been in the shallower shoals to the northwest of the site selected or directly on the beaches of Dauphin Island where the sand is needed.
Some of the fondest memories of many Alabamians are of trips to the beach. Alabama has some of the prettiest beaches in the world today but many citizens would argue that they were prettier twenty years ago. The real issue is… how pretty will they be twenty years or fifty years from now?
This section of the State-of-the-beaches report presents suggestions for the management of Alabama’s beaches based on the technical findings presented above. The present day state-of-the-beaches has been influenced by management decisions made along the coast during the past several decades. Likewise, the future state-of-the-beach will be influenced by management decisions made in the next few years. The philosophy underlying these suggestions is that beach management and development decisions should be made either:
The management issues are: