IV. Retreating Shorelines
297
As in any budget reserves may exist. Indeed sand can be withdrawn naturally
temporarily from the littoral zone transport system and stored on the backshore
and/or dunes. These "reserves" may remain untouched for months or years. They
should not be included in a budget calculated shortly after a storm. Eolean
transport being slower than water transport, the building up of dune reserves take
months or years, those of the backshore may accumulate in days or even hours
after a storm. Storms can reintegrate these reserves in the littoral sedimentary
transport system. The offshore zone seaward of the beach may be source or sink,
but little is known about it.
With worldwide erosion prevalent, it is quite obvious that many "accounts" are in
the red. In short, sediment budgets are sediment transport volume balances for a
specified coast segment, and many are "negative".
2
Budget Analysis
Analysis requires delimitation of boundaries and examination of the constituent
elements. From a sedimentologist's, and engineer's viewpoint, it is thus necessary
to identify processes involved, particularly those significant to the specific project
or the individual situation, estimate the sediment volume needed, and on the
hand of this information balancing the budget.
The area to be studied determines the boundaries, within time limits and
according to the aims pursued. It may be useful to gain some knowledge of
adjacent areas' sand budgets. The budget of a beach and its nearshore zone has
shore-parallel boundaries landward of the line of expected erosion and at, or
beyond, the seaward limit of significant transport. Barrier island sand dunes
might have a boundary at the island's bay side and the landward edge of the
backshore.
The credit lines of a budget are sources ; the debit lines are sinks. The total effect
of a line is obtained by multiplying the shown quantity by the length of shoreline
over which the "item" operates.
An assessment of the longshore transport rate requires that at least two of the
following variables be known" Qg, Qn, Qrt, Qlt, %
A balanced budget would be expressed by
tQi+ +tQ* = [~Qi- + !Q~ -] =0
i=I
i=i
I~ =I i
"=
(Iii. 1)
297
As in any budget reserves may exist. Indeed sand can be withdrawn naturally
temporarily from the littoral zone transport system and stored on the backshore
and/or dunes. These "reserves" may remain untouched for months or years. They
should not be included in a budget calculated shortly after a storm. Eolean
transport being slower than water transport, the building up of dune reserves take
months or years, those of the backshore may accumulate in days or even hours
after a storm. Storms can reintegrate these reserves in the littoral sedimentary
transport system. The offshore zone seaward of the beach may be source or sink,
but little is known about it.
With worldwide erosion prevalent, it is quite obvious that many "accounts" are in
the red. In short, sediment budgets are sediment transport volume balances for a
specified coast segment, and many are "negative".
2
Budget Analysis
Analysis requires delimitation of boundaries and examination of the constituent
elements. From a sedimentologist's, and engineer's viewpoint, it is thus necessary
to identify processes involved, particularly those significant to the specific project
or the individual situation, estimate the sediment volume needed, and on the
hand of this information balancing the budget.
The area to be studied determines the boundaries, within time limits and
according to the aims pursued. It may be useful to gain some knowledge of
adjacent areas' sand budgets. The budget of a beach and its nearshore zone has
shore-parallel boundaries landward of the line of expected erosion and at, or
beyond, the seaward limit of significant transport. Barrier island sand dunes
might have a boundary at the island's bay side and the landward edge of the
backshore.
The credit lines of a budget are sources ; the debit lines are sinks. The total effect
of a line is obtained by multiplying the shown quantity by the length of shoreline
over which the "item" operates.
An assessment of the longshore transport rate requires that at least two of the
following variables be known" Qg, Qn, Qrt, Qlt, %
A balanced budget would be expressed by
tQi+ +tQ* = [~Qi- + !Q~ -] =0
i=I
i=i
I~ =I i
"=
(Iii. 1)