Stephen P. Leatherman, A. Todd Davison, Robert J. Nicholls
University of Maryland
College Park, Maryland
Coastal geomorphology, by definition, is the study of the morphological development and evolution of the coast as it acts under the influence of winds, waves, currents, and sea-level changes. This study of physical processes and responses in the coastal zone is often applied in nature, but it also involves basic research to provide the fundamental understanding necessary to address the pertinent questions.
A principal coastal concern today and in the foreseeable future is beach erosion. It is estimated that 70 percent of the world's sandy shorelines are eroding (Bird, 1985). In the United States the percentage may approach 90 percent (Leatherman, 1988). This worldwide extent of erosion suggests that eustatic sea-level rise is an important underlying factor, although many other processes contribute to the problem. In many low-lying coastal areas, human impacts, such as the maintenance of tidal inlets and subsidence induced by groundwater and hydrocarbon withdrawals, have also made a substantial contribution to the erosion problem (National Research Council, 1990). At the same time, coastal populations are burgeoning, and this trend seems set to continue (Culliton et al., 1990). This raises the fundamental question — what is the best response to the problem of shoreline recession?
Faced with progressive shoreline retreat and the inevitable loss of protective and recreational beaches, coastal communities have three basic alternatives: (1) retreat (relocate buildings and other infrastructure in a landward direction), (2) accommodate (e.g., raise buildings to the projected higher flood levels), or (3) protect (build hard structures or use beach nourishment methods). In areas of dense population and highly developed infrastructure, protection is the preferred alternative. Hard structures are costly and inflexible and often have environmentally and aesthetically undesirable effects such as loss of the recreational beach. Thus, beach nourishment has become the coastal management tool of choice over the last several decades (Leatherman, 1991).
To date, it is estimated that over 640 km of U.S. coastline have been nourished, largely through public funding, at a total cost of about $8 billion (Dixon and Pilkey, 1989). The use of beach nourishment as a coastal management tool will probably continue its significant growth over the next few decades. The contemplated economic commitments to this management alternative by federal, state, and local governments is unprecedented. For instance, in northern New Jersey a Congressionally authorized nourishment project proposes to reinstate 19 km of beach at a cost of
approximately $200 million with projected maintenance costs over 50 years of about $300 million (Bocamazo, 1991). Similarly, the total cost of the recently (1991) completed Ocean City, Maryland, nourishment project, including renourishment every four years for 50 years, is estimated at $342 million (Kelly, 1991).
Predictability of the performance of beach nourishment is still poor in spite of its increasing use. This lack of understanding exists because: (1) predictive models of beach behavior in response to varying hydrodynamic forces are still relatively crude tools for engineering purposes and (2) most completed projects did not include adequate post-emplacement monitoring to allow for objective project assessment and necessary adjustment of designs (Davison et al., 1992). Therefore, each beach fill remains, in part, an educated experiment. Although many believe that there is sufficient understanding and inherent flexibility within the procedure to produce practical and successful designs (Delft Hydraulics, 1987), this confidence is not universally accepted.
During the 1980s, because of the actual or perceived failure of numerous projects, beach nourishment began receiving heavy criticism as an ill-advised use of taxpayers' money (e.g., Gilbert, 1986). During this time, several researchers (e.g., Leonard et al., 1989) began to contradict the traditional coastal engineering methods used to design and evaluate such projects. Such criticisms are not isolated, and many coastal environmental groups advocate planned retreat as the only true solution to coastal erosion.
The conclusions of Leonard et al. (1989) have been challenged by many in the scientific and engineering communities (e.g., Strine and Dalrymple, 1989; Houston, 1991a). Nonetheless, contentions from the Pilkey camp have focused attention on the lack of high-quality monitoring of U.S. beach nourishment projects and acted as a catalyst for renewed research efforts. This controversy places beach nourishment in the forefront of public policy decisions in the coastal zone. The basic aim of beach nourishment is to advance the shoreline a given distance and hence realize all the consequent benefits such as increased storm protection. Accurate designs are essential for predicting beach fill longevity and maintenance requirements, which necessitates quantitative understanding of the transport processes. Other pertinent questions involve the volume and grain size of sand required to attain a specific increase in subaerial beach width. Also, what is the lifetime and thus the renourishment frequency of a particular beach?
A major problem is that many of the present design concepts remain relatively untested against actual field performance. A Delft Hydraulics (1987) report summarizes our present understanding: ''an exact forecast of the behavior of the beach fill is not possible, not even in the case where a large number of data of the relevant areas is available''. At the present stage of technology, beach nourishment is more art than science (Egense and Sonu, 1987). The behavior of nourished and natural beaches is subject to the same uncertainties, and Wiegel (1987) argues that our present inadequate quantitative knowledge of natural beach processes handicaps dependable estimates on how well nourished beaches will perform. From a fundamental perspective, future shoreline evolution will always be stochastic, even with complete understanding of all the processes, because the underlying driving forces (e.g., waves, storms) are themselves stochastic (National Research Council, 1990). Thus, probabilistic predictions of nourishment performance must be the goal.
Evaluation of beach nourishment projects requires knowledge of cross-shore sand transport limits as well as delineation of the profile of equilibrium; these are fundamental concepts in coastal geomorphology (Leatherman, 1991). It is not often appreciated that most of the active beach profile is submerged. The entire profile must be moved seaward for nourishment to be successful. Thus, the seaward limit of the active beach profile for the purposes of beach nourishment is a problematic but very important determination (Bruun, 1986). Early nourishment projects did not consider the offshore profile (Jarrett, 1987), or if they did, utilized unrealistic slopes, which caused excessive losses of the subaerial beach (Hansen and Lillycrop, 1988). Hailermeier (1981) developed a wave-based profile zonation, including the depth definition (d1), which he proposed as the seaward limit for beach fill design. Limited field observations support this recommendation (Houston 1991b). The equilibrium profile concept can also be applied to beach nourishment design (Dean, 1983; 1991). But clearly, more field data are required, and routine post-project monitoring should include measuring the entire active profile to the depth of closure, which is generally less than 10 meters of water depth on U.S. coasts. Such basic data as time-series surveys of beach profiles are difficult or impossible to obtain for most of the 155 nourished beaches considered by Pilkey (1988). Therefore, the effectiveness of beach nourishment projects, particularly actual versus predicted performance, is debatable, and substandard sources, such as the local media, have been used to declare project success or failure.
Another problem involved in assessing the performance of beach nourishment is the widespread lack of post-project monitoring by independent, objective parties. Conflicting statements concerning the success or otherwise of beach nourishment are common in the literature (Davison et al., 1992). It is clear that project performance can only be objectively assessed if high-quality monitoring data are available and considered using commonly agreed upon criteria of success and failure (Stauble and Hoel, 1986).
There is also frequently a lack of commitment or inability of project sponsors to properly maintain nourished beaches. This raises important questions about the accreditation of beach nourishment projects, particularly when such projects are being used as a means to potentially lower 100-year flood levels and hence to reduce the cost of federal flood insurance. Also, many states now petition the Federal Emergency Management Agency for funds to restore their eroded beaches after Presidential disaster declarations. Clearly there need to be established criteria for design, maintenance, and financial commitment for the accreditation of beach nourishment projects (Davison, 1992; Davison et al., 1992).
The increasingly developed character of the nation's coastline will undoubtedly lead to increasing demand for beach nourishment. It is hoped that this will be undertaken within the context of sensible management plans. In addition to population and development pressure, accelerated sea-level rise will also increase the demand for beach nourishment (Weggel, 1986; Leatherman and Gaunt, 1989; Stive et al., 1991). This raises a number of new questions, particularly regarding the seaward limit of the beach profile over long time scales and the long-term availability of sufficient sand. These fundamental concepts in coastal geomorphology will undoubtedly receive considerable attention in the coming decades.
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