Managing Coastal Erosion (1990) / Chapter Skim
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6. Predicting Future Shoreline Changes
6. Predicting Future Shoreline Changes
Pages 120-142
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From page 120... ...
coastlines are required. There are essentially two approaches for the acquisition of such data: analysis of historical shoreline changes to forecast future evolution and a statistical method (Monte CarIo simulations)
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From page 121... ...
This task of establishing a comrnonaTity of risk is not easy, considering the following points: 1. A single common method of predicting shoreline changes, valid for all situations, does not appear to be possible, owing to
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From page 122... ...
This chapter reviews the validity and Irritations of the two major approaches for predicting future shoreline changes. The recommended approach for FEMA in determining shoreline change is the Emote Cario" method, but its utility presently is limited by the lack of sufficient correlated oceanographic and shoreline change data.
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From page 123... ...
When historical data are compared to modern high-quality maps, long-term rates of coastal erosion and accretion can be computed. These maps can be augmented and updated with historical aerial photographs (late 1930s to present)
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From page 124... ...
It is preferred to any other tidal boundary for the shoreline indicator because this wetted bound can be recognized in the field and approximated from air photos. This allows for the direct comparison of data obtained from the NOS T sheets and vertical aerial photographs.
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From page 125... ...
Projection of Shoreline Positions Extrapolation of trends bayed on historical shoreline change analysis takes into consideration the inherent variability in shoreline response based on differing coastal processes, sedimentary environments, and coastal exposures. The following discussion concerns the validity of determining long-term shoreline position changes from limited observations (e.g., snapshot views of the beach through time via time-series air photos)
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From page 126... ...
Where beach erosion rates are calculated to be in the low range (1 foot or less per year) , it must be realizecl that the reliability of this measurement is probably low owing to natural fluctuations in beach width.
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E~tmg Nationwide Data on Beach Erosion Maps depicting shoreline changes along portions of the U.S. coasts have been made for a host of reasons and by literally hundreds of researchers using a range of methodologies.
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From page 128... ...
An example of the unplementation of the historic shoreline change method is illustrated by the New Jersey coastal erosion project. The raw data used included all available NOS T sheets, New Jersey Department of Environmental Protection orthophotos, and a specially acquired set of large-scale vertical aerial photographs for the Atlantic coast beaches.
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From page 129... ...
for quantitative prediction of future shoreline changes as a result of natural or human-induced effects. These models, which include both analytical and numerical types, are well beyond the infancy stage and provide a sound foundation for the recommended longer-term methodology, yet they are not presently at a level where they can be applied and interpreted without substantial effort and skill.
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From page 130... ...
(1987) have summarized a number of such solutions, including the effect of constructing a groin along the shoreline, evolution of a beach nourishment project, and shoreline changes from delivery of sedunent to the coast by a river.
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From page 131... ...
Finally, littoral transport is a nonlinear, sensitive function of wave approach direction, and wave rises are reported too coarsely for an accurate determination of littoral drift. Given reliable functional relationships between wave-surge-current and sediment transport, only short-term episodic events can in theory be analyzed deterministically based on the short time history of the incident waves and surge during these individual storms.
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From page 132... ...
Then, formulation of the functional relationships relating sediment transport to these forcing events can allow for determination of shoreline changes. These functional relationships can be calibrated from the site-specific shoreline data under extreme conditions and the poststorm recovery time.
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From page 133... ...
This method translates the random nature of the sea state into deterministic events, the sum of which gives the same wave energy rose as provided by summary atlases. Shoreline evolution then is deterrn~ned statistically by a succession of Monte CarIo simulations of wave climatology, including both prevailing wave conditions and storms (Le Mehaute et al., 1983~.
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In principle, the Monte CarIo simulations provide as many shoreline positions as needed] for determining confidence bands and standard deviations.
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From page 135... ...
The distance from that line defining the zone of imminent collapse is obtained by the sum of five times the annual rate of erosion and an additional distance defined by a 50 percent probability that the distance will be exceeded within the next 3 years. For lack of accurate determination of the confidence bands allowing an accurate definition of the 50 percent probability deviation from the mean, a [(}foot distance will be added to the five times annual rate of erosion.
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From page 136... ...
The earliest information dates from the m~-1800s and extends to the 1970s; for many coastal areas, approximately four sets of historical maps are available at 30~year intervals. All rectifiable NOS T sheets should be utilized in a long-term analysis of historical shoreline changes.
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From page 137... ...
The compiled data on historical shoreline changes should in any case meet or exceed National Map Accuracy Standards. Shoreline change maps and the derived rates of beach erosion must be interpreted by professionals; otherwise, misleading or even wrong conclusions can be drawn from a causal inspection of the data.
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From page 138... ...
Beach erosion trends can be determined by the historical shoreline mapping methodology until this preferred long-term approach of using oceanographic data and statistical treatments can be undertaken. In actuality, the preferred methodology involves utilizing available records of shoreline recession for analysis of the time history of oceanographic forces (e.g., wind waves, storm surges, etc.~.
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From page 139... ...
ESTABLISHMENT OF A COMPUTERIZED NATIONAL DATA BASE A tremendous amount of shoreline change data must be assembled, analyzed, and interpreted properly for implementation of erosion-based setbacks by FEMA. High-quality data obtained from historical shoreline change analysm should be acquired at close intervals along the coast.
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From page 140... ...
monitoring of beach response to wave climate variations and episodic events; and 3. more research on predictive mathematical and probabilistic models of probability distribution of shoreline locations by a Monte CarIo simulation of the wave climate, taking into account both the longshore and cros~shore transport.
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1978. Analysis of coastal erosion and storm surge hazards.
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From page 142... ...
Proceedings, Sixteenth Conference on Coastal Engineering, ASCE, New York, pp.
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