A headland beach project was built on Mobile Bay, Alabama in September 1998 as a demonstration project of an alternative to bulkheading. This type of system, including sand fill and structures to extend the life of the fill, is recommended as a shoreline stabilization alternative that preserves the intertidal beach. The demonstration project survived the direct hit of Hurricane Georges one month after construction.
It has long been understood by the coastal engineering community that building a seawall along a receding shoreline will lead to the loss of the sandy beach in front of the wall (US Army, 1977). Because many people prefer a sandy beach to a seawall, the most popular solution to open coast beach erosion problems is beach nourishment.
On bays and estuaries, however, beach nourishment is rarely attempted. Property owners along eroding bay shorelines are usually faced with only two realistic choices. One, allow their property to erode. Or, two, build a bulkhead to protect the upland property. While a well-built bulkhead will protect the upland areas from erosion by waves, it doesn't address the sediment deficit along that stretch of shoreline that was causing the erosion problem. The sediment deficit that was resulting in landward recession of the shoreline as well as vertical erosion of sediment will be converted to vertical erosion in front of the wall. Areas in front of the wall that were intertidal or dry beach will eventually become underwater.
Mobile Bay, Alabama armoring characterization study
Figure 1 is a picture and surveyed profile on Mobile Bay, Alabama where this process has occurred. The bulkheads were initially built about 25 years ago when the shoreline was receding. At the time they were built, and for some time period after, there was an intertidal beach in front of the bulkheads. Now, there is no intertidal beach seaward of the bulkhead.
Douglass and Pickel (1999) used Mobile Bay, Alabama (see figure 2) as a case study of the rate and type of armoring of urban estuary shorelines. The armoring has increased to 30% (of the roughly 100-mile shoreline) in 1997 from 8% in 1955. The armoring corresponds with population growth in the area.
Vertical bulkheads are the most common type of armoring on Mobile Bay. "Trash revetments" and rubble-mound revetments are not as common. "Trash revetments" are defined as any loose debris; tires, broken concrete sidewalks, septic tanks, blocks, etc.; placed on an eroding bay bluff.
The loss of intertidal habitat due to bulkheading is roughly estimated at 10 to 20 acres in Mobile Bay. This includes a loss of about 4 to 6 miles of intertidal beach shoreline.
Will all urban estuaries become like bathtubs?
Is the fate of Mobile Bay and all urban estuaries to become like a "bathtub" (like Figure 1) with no intertidal beach areas?
The arguments and results presented above lead to a rather alarming concern for the long-term fate of urban estuaries. The only shorelines along Mobile Bay with no armoring and no intertidal loss were undeveloped shorelines or areas where the Bay was filling in due to sediment input. Where the bulkheads have been in place for the longest time, the shoreline is essentially a vertical wall, like a "bathtub."
Wetland protection regulations for the past several decades have dramatically slowed the loss of wetland intertidal areas due to direct filling and dredging. However, the process outlined in this paper is much less obvious. The loss of intertidal habitat of concern here is that due to the "passive" influence of bulkheads on local sediment budgets. Many other investigators (e.g. Kraus and McDougal, 1996; Fletcher, et al. 1997) have attributed the degradation and loss of open coast beaches to this process. It is argued here that the same process is damaging estuarine beaches and intertidal habitats.
This process has both ecological and societal implications. Stopping landward shoreline recession with a bulkhead eventually removes all of the intertidal habitat except for that up and down the vertical wall. Ecologists have long understood that the intertidal and adjacent sub-tidal areas can be some of the most productive habitat for organisms.
Loss of the intertidal zone also restricts public walking along the shoreline, particularly in those areas with a riparian right of access below mean high water. If there is no intertidal beach to walk on, walking requires trespassing at all times and is often difficult because of the water depths.
The societal implications of the loss of intertidal habitat may also include the loss of access to shellfish resources at low tide. Many parts of the country have different local customs and traditions regarding shellfish resources. For example, in Alabama, there are some areas that have exposed oyster beds at extremely low tides that are traditionally harvested for individual consumption. Vertical erosion as outlined above in front of a bulkhead can lead to the loss of this tradition. In the words of one Alabama resident, "the tide doesn't go out anymore" because there aren't any exposed oyster beds in front of their bulkhead anymore!
The implication is that the dual pressures of more people wanting to live on the water in the face of rising sea levels will continue to combine to reduce intertidal habitat. This will result in a loss to both the natural systems and society.
Alternatives to bulkheads
Beach nourishment with headland breakwaters to extend the life of the beach fill is one coastal engineering technique that has been used successfully to maintain beaches both on open coast locations (Bodge, 1992) and on bay shorelines (Hardaway, et al. 1995). The functional purpose of many of these constructed systems is a recreational beach that also protects upland property from further erosion. Several experimental systems have been developed to provide wave protection for fringe marsh construction on bay shorelines. A few offshore breakwater systems have been constructed along Gulf of Mexico estuaries (e.g. Appalachicola Bay National Estuarine Research Reserve). However, the technique is not commonly viewed as a realistic alternative to bulkheading by the general public, marine contractors, and government regulators.
This paper reports on a demonstration project of a headland beach system in Mobile Bay built as an alternative to bulkheading that preserves the intertidal beach habitat. The primary goal of this research was to demonstrate that the existing headland beach technology could be applied to the Gulf of Mexico estuaries in a cost efficient manner. A secondary goal of this research was to begin to develop design guidance for minimal levels of protection needed to maintain fringe beaches and wetlands.
A headland beach demonstration project was constructed in the northwest corner of Mobile Bay in Mobile, Alabama at the Gulf Pines Golf Course on the University of South Alabama's Brookley campus (Figure 3). The public golf course has an erosion problem threatening the 2nd green.
Historically, this shoreline was characterized by a small sandy beach adjacent to a vertical bluff. Bluff erosion has been estimated at about 5 feet/year during the past ten years.
The eroding bluff directly threatened the green and adjacent grassy areas along about 300 feet of shoreline. The adjacent shorelines are in front of a wetland area and a mature pine forest. These areas are "out of bounds" on the golf course. It was decided that some protection should be extended at least along 500 feet of shoreline to minimize future flanking of the green by erosion.
A primary constraint was the budget for the construction of the project. About $60,000 was available for the construction contract (not including engineering, permit application and monitoring costs).
The site, a public golf course, is not a recreational beach. However, fishing and wading in shallow water to throw cast nets along this beach are not uncommon. Sediment typical of the material along the shoreline was desired. There were some construction-related constraints placed on the contractor related to access roadways. They could not damage the golf course grasses or cart paths during construction. The golf course had to remain open, including the 2nd green, during construction (see Figure 4).
Breakwaters were included in the design to extend the life of the beach fill. The location of the structures was determined using the general principles for the stabilization of beach fills with artificial headland structures outlined in Silvester and Hsu (1993) and Bodge (1998). A number of alternative configurations of offshore breakwaters, t-head groins and groins were considered.
The final configuration (Figure 5); two offshore breakwaters, one of them at a higher elevation and connected to the shore with a "t" or stem, and one of them at a very low elevation not connected to the shore; was selected to meet two different design goals. The higher, shore-connected breakwater was designed as an artificial headland to stabilize the beach at the base of the bluff immediately in front of the 2nd green. The lower, offshore breakwater was designed to provide some limited, but less, protection for the sand at the southern, updrift, flank of the project. The sand in the lee of this breakwater may erode during high water back away from the structure to the form of a salient (bulge in shoreline) without compromising the 2nd green protection. Also, it is envisioned that this area is a good candidate for natural invasion of native wetland grasses.
The breakwaters were constructed between 50 to 70 feet seaward of the existing bluff along the mean low water location. This was chosen to protect submerged aquatic vegetation stands immediately offshore, because of geotechnical foundation concerns farther offshore, and because of the cost constraints on the whole construction project.
Both breakwaters are curved (in planview) for aesthetic reasons. The authors felt that the design of constructed shorelines is more pleasing to the eye when done with curved structures rather than straight structures along the waterfront. Perhaps the concave-seaward shape of the pocket beaches that form adjacent to the artificial headlands are complemented by the concave-seaward structures. In any case, the additional cost of a curved breakwater is small.
The curved breakwater idea came from Bodge (1992) and probably also has its roots in the pioneering Chicago waterfront designs of the last turn of the century.
Breakwater crest elevations
The larger, north breakwater is the shore-connected t-shaped breakwater. It is 100 feet long, about 20 feet wide, and has a crest elevation 2 feet above mean high water(+3.5 feet NGVD). The stem connection to land is approximately 60 feet long and 7 feet wide with a crest elevation of +2 feet NGVD. The southernmost breakwater is not shore connected with rocks. It is 80 feet long, about 15 feet wide, and has a crest elevation of only 1 foot above mean high water (+2.5 feet NGVD).
The crest elevations of the breakwaters were chosen to provide minimal protection in the water level environment of Mobile Bay. Since this project was built as a research demonstration project, some aspects were intentionally designed at minimal levels, i.e. with no factor of safety, to begin to develop some research understanding. One area where there is little available engineering guidance is the minimum elevation of offshore breakwaters for beach and wetland construction. The Gulf of Mexico has a microtidal range of about 2 feet between mean high tide and mean low tide. However, strong frontal wind events can set up the water level in the Gulf estuaries up to 2 feet more. Hurricane landfall can set the water level up to 10-15 feet more but only for a short duration. One question is whether or not a beach or constructed wetland can survive a few hours of much higher water levels during major storms. A related research issue is the development of an appropriate risk-based design methodology for these systems.
Frequency analysis of tide data shows that the annual high water level is about +3.5 feet NGVD. That is the elevation of the higher, northern breakwater. Thus, waves will completely overwash the lower breakwater several times annually.
Breakwater rock gradation
The rock size gradation initially specified was changed when the construction bids came in over budget. The contractors could not locally obtain the recommended gradation of stone size and included additional costs in their bids to hand-sort the rocks. The median weight of the rocks, based on Hudson's equation, coincided with median weight of Alabama Department of Transportation Class III rip-rap. However, the highway rip-rap is specified to include individual rock weights as low as 25 lbs. The use of such small rocks is questionable in the storm wave environment. However, since this is a research demonstration project and there are no structures landward of the breakwaters that could be damaged by rocks thrown by waves, it was decided to proceed with locally available rock gradations. Further efforts to educate the few rock producers about the marine environment market might alleviate this problem in the future. It should be possible to screen this material more appropriately at the quarry at little additional cost.
A composite geotextile-geogrid was placed under the rock breakwaters as a foundation. The composite came in rolls with the geogrid directly attached to the geotextile. The geogrid provides lateral support for bridging small soft pockets of underlying materials to reduce differential settlement. The geotextile prevents the sandy underlying material from being pulled directly out through the large rocks to reduce scour and settlement.
The beachfill was designed to "overfill" the structures initially beyond the predicted equilibrium shoreline position. The constructed beach planform was straight lines between the breakwater tips and from the breakwater tips to the adjacent shoreline along angles based on the predicted equilibrium shoreline position. Since the equilibrium shoreline configuration is curved at a location landward of this, this extra sand is the "overfill."
The beachfill sand source was stockpiled virgin dredged material from the construction of a nearby ship channel. The median grain size of the fill sand (d50=0.39 mm) was slightly larger than that on the site from the eroding bluff (d50=0.31 mm). The color was slightly darker but was the best commercially available sand in terms of color similarity with the site sand.
Approximately 1,400 yd3 of beach fill material was placed in the lee of the two offshore breakwaters. The fill was constructed with an elevation of approximately +2.5 feet NGVD at the bluff and had a slight slope down to the water. The seaward edge of the beachfill not behind the breakwaters was constructed at a 2:1 (horizontal to vertical) slope.
Biotechnical (grass) slope protection
The existing bluff varied in height from about 3 feet to 8 feet. The bluff was cut to a 4:1 (horizontal to vertical) slope and planted with sod. The sod extended from on top of the bluff down the slope to +3 feet NGVD. Since the project was built adjacent to the golf course, two types of sod were needed. On the slope, centipede, or St. Augustine grass was used; but on the top of the bluff, hybrid Bermuda grass was used. An irrigation system was installed for the slope. The intent of this sloped grass was to act as wave protection during high water events (above the typical annual high water level). Grass is a common biotechnical stabilization technique for stream side and construction site erosion. Observations indicate that mildly-sloped grassed lawns can be stable in Mobile Bay during the short durations typical of hurricane storm surges.
Predicted equilibrium shoreline position
A modified form of Silvester and Hsu's (1994) Static Equilibrium Bay (SEB) model was used to predict the equilibrium shoreline position of the final design configuration. A critical part of that model, a polynomial equation for the static equilibrium shoreline position, is the incident wave angle. To determine the incident wave approach angles, historic wind data for Mobile Airport from 1964-1991 were used to create a wind rose and wave rose. The two most commonly occurring onshore wind directions were the 40-degree azimuth (northeast) and the 140-degree azimuth (southeast). A wave rose was created for the project site by determining the significant wave height for each portion of the wind rose. Wave heights were estimated with the Shore Protection Manual's (US Army, 1984) shallow-water wave generation equations. The direction of wave approach was assumed as the wind direction. Wave refraction was ignored because of uncertainties in wave refraction modeling in shallow, limited fetch seas. The final estimate of equilibrium planform is a composite of the two shorelines from the two dominant incident angles.
The appropriateness of Silvester and Hsu's SEB model in fetch-limited multi-directional seas is questionable. The model equation is most appropriate when a single dominant swell direction is present. An alternative methodology for determining the principle incident wave angle was investigated. Potential longshore sand transport for the site was computed using the "CERC" equation. The "littoral drift" rose was then integrated to find the one incident wave angle (at the root-mean-square wave height) that would produce the same net littoral drift rate as the entire wave rose. The result was an azimuth of 135 degrees. Bodge (1998) has suggested a similar approach.
The project experienced design conditions one month after construction was completed. On September 28, 1998, Hurricane Georges made landfall in the Ocean Springs/Biloxi, Mississippi area approximately 50 miles west of the Brookley Headland Beach project. As Georges made landfall, it was a category 2 hurricane with maximum sustained winds of 105 mph just east of the eye. Measured maximum sustained wind speeds across Mobile Bay did not exceed 70 mph however. As Hurricane Georges progressed through the north-central Gulf of Mexico, the Brookley Headland Beach site faced increased onshore wind speeds, water levels, and wave heights. The wind and water level histories of the storm have been used to hindcast the wave conditions at the Brookley site. Water levels reached about +8 feet NGVD (based on tide gauge records and the debris line on the golf course) and peak significant wave heights probably exceeded 6 feet. The onshore winds on the right side of the storm set the Gulf of Mexico up several feet and these south to southeast winds caused more setup in the northern portion of the bay. The longest fetch length to the site is from the southeast also.
The rock breakwaters and the sand beachfill survived the storm well (Figure 6). Both breakwater cross-sections were the same after the storm as before. A few small rocks were found on the golf course behind the structures after the storm. These rocks were less than 100 lbs. This validates the initial design concerns about using such small rocks. It is believed that these few rocks came from the landward end of the "t" connection of the north breakwater. The rocks and sand were under 4 to 6 feet of water during the peak of the storm. Post storm profile surveys indicated that the sand elevations in the pocket beaches were the same after the storm as before.
The bluff slope that had been planted with sod was completely removed by the storm and the bluff eroded up to 35 feet landward. Given that the sod did not have time to establish any roots in one month, this is as expected. The adjacent unprotected bluffs eroded more than the bluff in the lee of the breakwaters. It appears that during the peak of the storm, the breakwaters functioned as submerged breakwaters. The breakwaters probably saved the green from being destroyed. The post-hurricane photograph shows much more sand around the area after the storm. This sand came from the bluff erosion. Grass has since been replanted on the existing post-storm slope (about 10:1).
A headland beach and breakwater system was built on Mobile Bay in September 1998 as a demonstration of an alternative to bulkheading. The project survived a direct hit of a hurricane one month after construction. The project showed that it is possible to engineer alternatives to bulkheads that preserve the intertidal habitat. The project will be monitored for several years. The cost per foot of shoreline protection is competitive with bulkheads built to survive similar storm levels. There are a number of engineering research issues related to the minimum level of design of these systems. One area for further research is the use of similar systems to stabilize fringe wetlands on bay shorelines.
The Gulf of Mexico Program funded this research demonstration project. Esfeller Construction Co. of Irvington, Alabama constructed the Brookley Headland Beach project. The assistance of the staff of Gulf Pines Golf Course, including Bruce Robinson and Frank Freel, is appreciated. Particularly appreciated was the unlimited use of golf carts to reach the project site. Brian Greathouse, Jennifer McClung, and Shawn Wozencraft helped collect the data.
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