By Henry Bokuniewicz
School of Marine and Atmospheric Sciences
Stony Brook University
Stony Brook, New York 11794-5000
CHOOSING A RESPONSE to shore erosion due to a rise in relative sea level is a dilemma compounded by at least four levels of accumulated uncertainty. The recent increase in the atmospheric “greenhouse” gases is well documented, but the evidence for a concomitant global warming may be less compelling. Anticipated climatic changes are reasonable but uncertain in detail and reliant on the confidence placed in complicated climatic models. Convincing evidence is not yet available for a global acceleration in the rate of sea level rise, but the risk is widely recognized.2 Forecasts of a rise in sea level require a second level of model predictions to be built on the climatic models. If the rate of coastal submergence should dramatically increase in the near future, the response of the shoreline is the subject of yet a third layer of models that are largely empirical and sitespecific. Finally, choosing a viable response relies on a fourth level of prediction, a forecast of how the shoreline will respond to a wide range of possible human actions.
The uncertainty can be debilitating, yet the potential risks are so high, the costs of an effective response so great and the time required to implement a coordinated effort is so long that the problem deserves attention even while work continues to establish confidence in long-range predictions. The broad range of strategies for coping with this threat has been reviewed, 2 but appropriate, regional tactics need to be adopted. Ideally, the regional tactics should not require any great, immediate departure from any well-founded, existing policies and, given the level of uncertainty, they should not demand an irreversible commitment at this time. Action at the regional level should be begun now but must be proportionate to the uncertainty in the prediction of future sea level rise and the shoreline response. The commitment should be a gradual one and fairly predictable for very long periods.
Retreat from the present shoreline is one extreme response to rising sea level. Deserting the shoreline incurs little or no expenses, although the costs in abandoned property and facilities could be large; relocating at a more landward position may require installing or expanding the infrastructure to accommodate the new population. Holding the shoreline by structures or beach nourishment lies at the other extreme. The technology is available, as demonstrated by the successes of the Netherlands and elsewhere, but the initial expense and continuing commitment to maintenance would be immense. In addition, the cost of holding the fastland could include the loss of wetlands and beaches unless these too were artificially maintained.
Each strategy has an appropriate application. Tenaciously holding the shoreline has been the economically defensible choice along urban, heavily developed coasts and ports. Unabashed retreat has been a responsible tactic along some undeveloped coasts especially parkland. Most of the coastline, however, falls somewhere in between these two categories and, as a result, a compromise strategy of orderly withdrawal behind defensible positions may be required. The coastal lagoons along the south shore of Long Island might be considered to illustrate the point.
Great South Bay is the largest of a series of coastal lagoons along Long Island’s south shore. It’s about 63 km long and 10 km across at its widest point, with a median depth of about 1 meter. The north shore of the bay is fairly well developed. The slope of the bay’s north shore is about 0.002, so the present rate of relative sea level rise of about 3 mm/ yr would push the shoreline landward at a rate of 1.5 m/yr by passive submergence.3
The barrier island, Fire Island, is relatively lightly developed. There are no paved roads on most of the island and access to Fire Island’s twenty communities is almost entirely by ferries. These communities are clusters of primarily seasonal, one or two story, wooden houses separated by stretches of undeveloped coast, part of which is National Seashore.
Over the last few thousand years, the barrier island has been migrating landward across the shelf. Its progress, however, has not necessarily been one of gradual and continuous rollover. There is some evidence that the island has been drowned in place at some time in the past and reformed, or “jumped” at a more landward position.6 Comparisons of maps and aerial photographs over a century suggests recession rates of several feet per year near the inlets at either end of the island but a stable, or slightly accreting, beach along the central stretch of the island.4 Severe, unpredictably localized storm erosion poses a constant threat, however. During a hurricane in 1938, 22% of the houses on the island were reported lost. Individual or community efforts to protect shoreline property are common. Such measures include bulkheads, small groins, sandbags, dune fencing and beach scraping. There are no large rock groins or revetments in part because of the expense of supplying such materials to the site. In the face of rising sea level, the frequency of storm-induced erosion damage should be expected to increase and any chronic recession will be more rapid but the future severity of the problem is unknown except in these general, qualitative terms.
The local management approach for this situation could have two primary goals: (a) hold the position of the north shore of the bay and (b) maintain the integrity of the barrier island.
About 33% of the north shore of the bay is already bulkheaded. In the face of rising sea level, it is conceivable that the north shore will eventually be completely bulkheaded, diked, or otherwise protected given the degree of existing development in the area. These measures are effective because of the regional protection provided by the bay and barrier island. Furthermore, they are not prohibitively expensive to either local governments or individual property owners. As long as this option is available, it seems reasonable to assume that it will continue to be employed effectively. Over many decades, beaches and marshland along the north bay shore would be squeezed out as the shoreline everywhere reached the artificial barriers. The region’s principal recreational beaches, however, are already on the barrier island and new marshland in the bay could be created.
Large scale public works projects would not need to be implemented on the north shore until the bay had risen to a level on the barriers so that freshwater drainage and seawater seepage landward of the barriers became a problem. As sea level rises above the land level behind the north bay-shore barriers, about 350 million cubic meters of fresh water will have to be discharged to the sea per year. The windmills of the Netherlands began doing this type of job in the fifteenth century and, of course, pumping systems for coastal drainage continue to be an integral part of the Netherlands defenses. Relatively inexpensive but effective protection is possible on the bay’s north shore only while the bay and barrier island continue to exist. As a result, maintaining the integrity of the barrier is the second objective. This objective does not preclude individual or community efforts specifically to protect structures but neither does it require such protection be done as public works. On the other hand, letting nature take its course would not be sufficient. The integrity of the barrier could be lost for long intervals by prevalent breaching or drowning of the barrier. Historically, new inlets have apparently survived for decades before closing naturally.4 Initial tactics would at least require that breaches be closed and beach nourishment could also be done to forestall the formation of new inlets. Such operations are routinely done on a case-by-case basis today but contingency plans could be developed now to make future work more economical. This approach would be designed to preserve the barrier for decades but would not necessarily preserve existing structures or even necessarily keep the barrier in its same position.
If sea level continues to rise, the offshore gradient would steepen and artificially maintaining the barrier in its present position would become more and more costly. For this reason, landward migration by controlled rollover or managed jumps would be preferred. This might be done at first by pre-emptive beach nourishment on the bay shore of the barrier island. It seems that such man-made rollover has already happened on Jones Island, immediately to the west of Fire Island, where the bay shoreline of the barrier island was filled in order to construct a roadway in the 1930’s’. 8 The process of landward migration might eventually be better controlled by the construction of artificial reefs along the bay shore of the barrier. Their function would not be to dissipate ocean wave energy, the barrier island would still be relied upon for that, but rather to serve as traps for sand to build up the bay shore and armor the barrier against breaching outward from the bay to the ocean. A recent breach at nearby Moriches Inlet was cut initially by bay water rushing seaward over the barrier and the measures used to close the breach included armoring the bay shore. Bayside structures could also be designed to promote the growth of wetlands as replacements for marshland lost at the north shore of the bay.
Since the interior reefs would not be intended to stand up to the brunt of ocean waves, they could be much less substantial than ocean breakwaters and might even be constructed with stabilized fly ash from incinerators.1 Demonstration projects have been done using stabilized fly ash to construct offshore fishing reefs in the ocean. The material is stable in the marine environment and any contaminants bound in the ash do not leach into the seawater or otherwise pose an environmental threat. Garbage incinerators on Long Island produce about 900,000 tons of ash per year as a waste produce; after preprocessing to remove ferrous metal about 630,000 tons could be used as an aggregate substitute to construct a section of reef 4 meters high, 10 meters wide and 5 kilometers long every year.1 The entire interior shore of the barrier could be reinforced in 12 years at this rate.
If the future rate of sea level rise becomes about 1cm/yr, the barrier island might be managed to migrate at rates of 100 to 200 meters per century. A gradual decrease of bay area would also be unavoidable with this strategy but the rate at which it shrinks would be extremely slow. Some bay environment could be preserved for a millennium. The bay’s clamming industry would begin to diminish proportionately and it might be appropriate to include in the management strategy a shift to mariculture using either the endemic species or perhaps other, more suitable species. Mariculture may not be economical at present5 but could become cost effective as the habitat changes.
The loss of private property on the barrier is also inevitable if the rate of sea level rise accelerates. A policy of beach nourishment to preserve the integrity of the barrier would help to preserve this property for many decades. In the long term, however, the barrier would move out from under existing property lines. Although there will always be a special risk to living on the barrier island, there would be no inherent reason why properly constructed seasonal houses could not continue to be allowed. Some development increases the value of the barrier as a recreational resource. The landward migration might be controlled in a series of jumps, say every 50 years. Land on the island could be leased for private use with the terms of the lease corresponding to the period of stability between the controlled changes in the island’s position. Land-lease arrangements already exist in four communities on the barrier to the west of Fire Island although, admittedly, the situation is controversial.
The suggested long-term regional tactics are summarized in Figure 1. Such first steps would be to allow, or even encourage, well designed shore protection on the bay’s north shore, and to develop an institutional commitment to maintaining the barrier island including contingency plans for beach nourishment during crises. Sources of sand must be identified, equipment be made readily available, funding mechanisms must be developed, and institutional arrangements must be made ahead of time to allow emergency work to proceed in a timely, and therefore cost effective, manner following crises. As this is done the design of future tactics outlined here could be pursued. Whatever approaches are adopted, however, more attention needs to be given now to tailoring the array of possible responses to regional conditions.
- Chesner Engineering, Long Island Regional Planning Board and the New York State Energy Research and Development Authority. Long Island Ash Management Study: Long Island Materials Market Survey, Long Island Regional Planning Board Technical Memorandum Task 1.3. Hauppauge, New York: 26 p. plus appendices. 1987.
- Dean, R.G., R.A. Dalrymple, R.W. Fairbridge, S.P. Leatherman, D. Nummedal, M.P. O’Brien, O.H. Pilkey, W. Sturges, R. L. Wiegel. Responding to Changes in Sea Level, National Academy Press, Washington, DC. 148 p., 1987.
- Giese, G., Bokuniewicz, H., Hennessy, J., Smith, G., Tangren,S., Zarillo, G. and Zimmerman, M. ” Hypsometry as a Tool for Calculating Coastal Submergence Rates.” Coastal Zone ’85. Proceedings of the 4th Symposium on Coastal and Ocean Management., American Society of Civil Engineers, pp. 1971-1978. 1985.
- Leatherman, S.P. and Allen, J.R. Fire IslandInlet toMontauk Point, Long Island, New York. Reformulation study, Final Report. National Parks Service, Boston, MA, 530 p. plus appendices. 1985.
- McHugh, J.L. “The Hard Clam Fishery Past and Present,” Chapter 7 in Great South Bay. Schubel J.R.andCarter, H.H., eds. In press.
- Sanders, J. and Kumar, N. ” Evidence of Shoreface Retreat and In-place “Drowning” During Holocene Submergence of Barriers, Shelf off Fire Island, New York.” Geological Society ofAmerican Bulletin , Vol.86, pp. 65-76, 1975.
- Schneider, S.H. “Me Greenhouse Effect: Science and Policy,” Science, Vol. 243, pp. 771-781, 1989.
- Wolff, M. “An Environmental Assessment of Human Interference on the Natural Processes Affecting the Barrier Beaches of Long Island, New York,” Northeastern Environmental Science Vol. 8, pp. 119-134, 1989.