front major foredune Systems in different geographical locations. For
example, fine sand beaches front a massive foredune System on Mustang
Island, Texas, and coarse sand beaches front dunes on the Cape Cod spits.
Godfrey (1972) discusses the effect of a foredune System on the
long term stability of the barrier islands of the Cape Hatteras and Cape
Lookout National Seashores, North Carolina. Important implicit assumptions of the discussion are that no new supply or inadéquate new supplies
of sand are available to the barrier island System, and that rising sea
level is, in effect, creating a sand déficit by drowning some of the
available island volume. The point of the geomorphic discussion is that
under such conditions the islands must migrate landward to survive. A
process called "oceanic overwash” (the washing of sand from low foredunes
or from the beach over the island crest onto the déflation plain by overtopping waves) is described as an important process in the landward migra
tion of the islands. Since a foredune System blocks overtopping and prevents oceanic overwash, foredunes are viewed as a threat to barrier island
stability.
Granted the implicit assumptions and a géologie time frame, the
geomorphic concept presented has convincing logic and probably has merit.
However, the assumptions are not valid on ail barrier islands or at ail
locations in most barrier islands or at ail locations in most barrier
island Systems. Too, most Coastal engineering projects are based on
a useful life of 100 years or less. In such a short period, géologie
processes, such as sea-level rise, hâve a minor effect in comparison with
the rapid changes caused by wind and waves. Therefore, the island crest
élévation and foredune System will maintain their élévation relative to
the mean water level on stable or accreting shores over the life of most
projects. On eroding shores, the foredunes will eventually be eroded
and overwash will resuit in shoreward migration of the island profile;
sand burial and wave and water damage will occur behind the original
duneline. Therefore, planning for and évaluation of the probable success of a foredune System must consider the general level of the area of
the déflation plain to be protected, the rate of sea level rise, and the
rate of beach recession.
4.7 SEDIMENT BUDGET
4.71 INTRODUCTION
4.711 Sédiment Budget. A sédiment budget is based on sédiment removal,
transportation and déposition, and the resulting excesses or deficiencies
of material quantities. Usually, the sédiment quantities are listed
according to the sources, sinks, and processes causing the additions and
subtractions. In this chapter, the sédiment is usually sand, and the
processes are either littoral processes or the changes made by man.
The purpose of a sédiment budget is to assist the Coastal engineer
y* 1 enti )ing relevant processes; estimating volume rates required for
design purposes; smgling out significant processes for spécial attention;
and on occasion, through balancing sand gains against losses, checking
the accuracy and completeness of the design budget.
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example, fine sand beaches front a massive foredune System on Mustang
Island, Texas, and coarse sand beaches front dunes on the Cape Cod spits.
Godfrey (1972) discusses the effect of a foredune System on the
long term stability of the barrier islands of the Cape Hatteras and Cape
Lookout National Seashores, North Carolina. Important implicit assumptions of the discussion are that no new supply or inadéquate new supplies
of sand are available to the barrier island System, and that rising sea
level is, in effect, creating a sand déficit by drowning some of the
available island volume. The point of the geomorphic discussion is that
under such conditions the islands must migrate landward to survive. A
process called "oceanic overwash” (the washing of sand from low foredunes
or from the beach over the island crest onto the déflation plain by overtopping waves) is described as an important process in the landward migra
tion of the islands. Since a foredune System blocks overtopping and prevents oceanic overwash, foredunes are viewed as a threat to barrier island
stability.
Granted the implicit assumptions and a géologie time frame, the
geomorphic concept presented has convincing logic and probably has merit.
However, the assumptions are not valid on ail barrier islands or at ail
locations in most barrier islands or at ail locations in most barrier
island Systems. Too, most Coastal engineering projects are based on
a useful life of 100 years or less. In such a short period, géologie
processes, such as sea-level rise, hâve a minor effect in comparison with
the rapid changes caused by wind and waves. Therefore, the island crest
élévation and foredune System will maintain their élévation relative to
the mean water level on stable or accreting shores over the life of most
projects. On eroding shores, the foredunes will eventually be eroded
and overwash will resuit in shoreward migration of the island profile;
sand burial and wave and water damage will occur behind the original
duneline. Therefore, planning for and évaluation of the probable success of a foredune System must consider the general level of the area of
the déflation plain to be protected, the rate of sea level rise, and the
rate of beach recession.
4.7 SEDIMENT BUDGET
4.71 INTRODUCTION
4.711 Sédiment Budget. A sédiment budget is based on sédiment removal,
transportation and déposition, and the resulting excesses or deficiencies
of material quantities. Usually, the sédiment quantities are listed
according to the sources, sinks, and processes causing the additions and
subtractions. In this chapter, the sédiment is usually sand, and the
processes are either littoral processes or the changes made by man.
The purpose of a sédiment budget is to assist the Coastal engineer
y* 1 enti )ing relevant processes; estimating volume rates required for
design purposes; smgling out significant processes for spécial attention;
and on occasion, through balancing sand gains against losses, checking
the accuracy and completeness of the design budget.
4-116