4.713 Sédiment Budget Boundaries. A sédiment budget is used to identify
and quantify the sources and sinks that are active in a specified area.
By so doing, érosion or déposition rates are determined as the balance
of known sinks and sources. Boundaries for the sédiment budget are determined by the area under study, the time scale of interest, and study purposes. In a given study area, adjacent sand budget compartments (control
volumes) may be needed with shore-perpendicular boundaries at significant
changes in the littoral System. For example, compartment boundaries may
be needed at inlets, between eroding and stable beach segments, and between
stable and accreting beach segments. Shore-parallel boundaries are needed
on both the seaward and landward sides of the control volumes. They may be
established wherever needed, but the seaward boundary is usually establishec
at or beyond the limit of active sédiment movement, and the landward boundary beyond the érosion limit anticipated for the life of the study. The
bottom surface of a control volume should pass below the sédiment layer
that is actively moving, and the top boundary should include the highest
surface élévation in the control volume. Thus, the budget of a particular
beach and nearshore zone would hâve shore parallel boundaries landward of
the line of expected érosion and at or beyond the seaward limit of significant transport. A budget for barrier island sand dunes might hâve a boundary at the bay side of the island and the landward edge of the backshore.
A sédiment budget example and analysis are shown in Figure 4-43.
This example considers a shoreline segment along which the incident wave
climate can transport more material entering from updrift. Therefore
the longshore transport in the segment is being fed by a continuons ly
eroding sea cliff. The cliff is composed of 50 percent sand and 50 percent clay. The clay fraction is assumed to be lost offshore while the
sand fraction feeds into the longshore transport. The budget balances
continuity équation:
the sources and sinks using the following
Sun of Known Sources - Sum of
An example calculation is shown in Figure
4.72 SOURCES OF LITTORAL MATERIAL
Known Sinks = Différence
4-43.
4-119
•
ri vers of the world bring about 3.4
4.721 Rivers. It is estimated that river
t0 the coast each year
cubic miles or 18.5 billion cubic yar
. frOm strakhov, 1967.)
(volume of solids without voids). ( 0
> the’sand size range that is
Only a small percentage of this.sedimen
flCC0Unt for most of the volume
common on beaches. The large rivers w
example, it is estimated
of sédiment carry relatively little san .
the Gujf of Mexico each
(Scruton, 1960) that the sédiment load
g
cent clay> 48 percent silt,
year by the Mississippi River consists 0
of sanj seem probable for
and only 2 percent sand. Even lower P®
67
1218, for information
other large river discharges.
(See 1
» owing through sandy drainage
on the Amazon River.) But smaller ri
fChow, 1964, p. 17-20.) In
areas may carry 50 percent or more or s
•
*he fiOods of small
Southern California, sand brought î?4oraî°material. (Handin, 1951; and
rivers is a significant source or
Norris, 1964.)
and quantify the sources and sinks that are active in a specified area.
By so doing, érosion or déposition rates are determined as the balance
of known sinks and sources. Boundaries for the sédiment budget are determined by the area under study, the time scale of interest, and study purposes. In a given study area, adjacent sand budget compartments (control
volumes) may be needed with shore-perpendicular boundaries at significant
changes in the littoral System. For example, compartment boundaries may
be needed at inlets, between eroding and stable beach segments, and between
stable and accreting beach segments. Shore-parallel boundaries are needed
on both the seaward and landward sides of the control volumes. They may be
established wherever needed, but the seaward boundary is usually establishec
at or beyond the limit of active sédiment movement, and the landward boundary beyond the érosion limit anticipated for the life of the study. The
bottom surface of a control volume should pass below the sédiment layer
that is actively moving, and the top boundary should include the highest
surface élévation in the control volume. Thus, the budget of a particular
beach and nearshore zone would hâve shore parallel boundaries landward of
the line of expected érosion and at or beyond the seaward limit of significant transport. A budget for barrier island sand dunes might hâve a boundary at the bay side of the island and the landward edge of the backshore.
A sédiment budget example and analysis are shown in Figure 4-43.
This example considers a shoreline segment along which the incident wave
climate can transport more material entering from updrift. Therefore
the longshore transport in the segment is being fed by a continuons ly
eroding sea cliff. The cliff is composed of 50 percent sand and 50 percent clay. The clay fraction is assumed to be lost offshore while the
sand fraction feeds into the longshore transport. The budget balances
continuity équation:
the sources and sinks using the following
Sun of Known Sources - Sum of
An example calculation is shown in Figure
4.72 SOURCES OF LITTORAL MATERIAL
Known Sinks = Différence
4-43.
4-119
•
ri vers of the world bring about 3.4
4.721 Rivers. It is estimated that river
t0 the coast each year
cubic miles or 18.5 billion cubic yar
. frOm strakhov, 1967.)
(volume of solids without voids). ( 0
> the’sand size range that is
Only a small percentage of this.sedimen
flCC0Unt for most of the volume
common on beaches. The large rivers w
example, it is estimated
of sédiment carry relatively little san .
the Gujf of Mexico each
(Scruton, 1960) that the sédiment load
g
cent clay> 48 percent silt,
year by the Mississippi River consists 0
of sanj seem probable for
and only 2 percent sand. Even lower P®
67
1218, for information
other large river discharges.
(See 1
» owing through sandy drainage
on the Amazon River.) But smaller ri
fChow, 1964, p. 17-20.) In
areas may carry 50 percent or more or s
•
*he fiOods of small
Southern California, sand brought î?4oraî°material. (Handin, 1951; and
rivers is a significant source or
Norris, 1964.)