in different periods, including seabed mapping and
profile measurements; (2) aerial photographs of the
research area in different periods, which are especially
helpful for indicating morphological changes and
major transport pathways of the area; (3) records and
data of anthropogenic activities in the area and related
change of sediment properties and volumes; (4) records
and data of natural hazards (e.g., storms, floods) occurring in the area within the time span of interest;
(5) meteorological conditions of the area (e.g., wind
and wave monitoring data) and possible climate
change impacts (e.g., sea level change data);
(6) tectonic movement map of the area; (7) map of sediment grain-size distribution on the seabed; (8) measurements of suspended sediment concentration and
discharge in representative periods (e.g., flood and
ebb tides, dry and wet seasons); and (9) estimates of
waves, currents, and sediment transport aided by
numerical models.
3. Dividing the research area into a series of cells. Each
cell acts as a control unit with clearly defined boundaries. Sediment budget is cell-dependent, and calculation of sediment volume change is performed only at
the boundaries of each cell. Cells can be defined by
geological or morphological controls, available data
resolution, coastal structures, and knowledge of the site
(Rosati, 2005). Depending on the specific sources,
sinks, and transport processes in the research area, cells
with different sizes can be assigned. Sub-cells can also
be defined within a cell to better measure the sediment
budget of a region with varying rates of accretion and
erosion. A well-known example is given by Bowen
and Inman (1966) who introduced the concept of littoral cells. A littoral cell is usually a zone parallel to the
shoreline, bounded by the foot of the foredune or cliff
in the landward direction and depth-closure point in
the shoreward direction. Changes of the sediment volume in a littoral cell directly induce changes in the
coastline, and ideally they are defined to minimize
longshore sediment exchange with adjacent cells, for
example, setting the lateral boundary of a littoral cell
at the nodal points where the net longshore transport
rate is zero or defining a pocket beach bounded by
rocky headlands, which is able to conserve its majority
of sediment, as a littoral cell. An example of cell division of an estuary and its adjacent coast is shown in
Figure 1a, b.
4. Defining sediment transport pathways at the cell
boundaries. Sediment transport pathways specify the
sediment transfer direction and corresponding processes between adjacent cells. The transport pathways
can be estimated through knowledge of the processes
(natural and anthropogenic) occurring at the site.
Interpretation of aerial photographs, trend analysis of
sediment grain-size distribution, and particle tracking
also aid to define sediment transport pathways.
Figure 1b shows possible sediment transport pathways
for an estuary and its adjacent coast.
5. Calculating the volume of sediment transfer in each
transport pathway. A general equation for the sediment
volume change (DV) of a cell is expressed by:
DV ¼
X
Q source À
X
Q sink þ I À O
ð1Þ
where Q source and Q sink represent the amounts of natural
sources (i.e., input) and sinks (i.e., output) transferred at
the cell boundary, respectively. I and O are the amounts
of artificial sediment input (e.g., beach nourishment) and
output (e.g., dredging) transferred at the cell boundary,
respectively. The quantity of sediment volume transfer
on each transport pathway should be evaluated based
on the acquired data. For example, comparison among
aerial photographs in different periods gives the detailed
information on the rates of coastline change. Field measurements on currents, waves, and sediment concentrations help to quantify the typical rates of sediment
transport caused by natural processes. Analysis of
extreme events provides information on the frequency
and magnitude of consequent sediment transport. Comparison among DEMs, coastal profiles, or aerial photographs provides information on calibration and
validation of the estimated quantities in (1).
6. Estimation of uncertainty in the calculated sediment
budget. One should consider that uncertainty always
exists in the calculated sediment budget. Uncertainty
of a sediment budget mainly comes from two sources:
(1) measurement error and (2) true uncertainty due to
temporal and spatial variability of the transport processes (Rosati, 2005). Measurement error contains the
error made during measurement processes (e.g.,
improper positioning of the instruments) and the error
in the measured signals (e.g., noises). Net sediment
flux across an estuary mouth or an inlet is difficult to
measure, particularly when the sediment flux on the
ebb and flood tides are high. In estuaries, sources and
sinks can be expressed as rates of exchange in a tidal
cycle. However, many of the exchanges are not at
steady rates, for example, sediment exchange in stochastic extreme events (e.g., storms, floods) can be
several orders of magnitude higher than in normal conditions. Another source of uncertainty comes from
numerical models which are applied. For example, littoral drift is normally estimated on the basis of standard
equations (e.g., the Coastal Engineering Research Center (CERC) (USACE, 1984) and Kamphuis
(Kamphuis, 1991) methods) or more complicated
models (e.g., Zhang et al., 2013). However, different
models may yield quite different results on the rate of
longshore sediment transport even though they
are based on the same boundary inputs. Thus, the
estimated littoral drift is often biased and should be calibrated by field measurements. Impacts of unknowns
552
SEDIMENT BUDGETS
profile measurements; (2) aerial photographs of the
research area in different periods, which are especially
helpful for indicating morphological changes and
major transport pathways of the area; (3) records and
data of anthropogenic activities in the area and related
change of sediment properties and volumes; (4) records
and data of natural hazards (e.g., storms, floods) occurring in the area within the time span of interest;
(5) meteorological conditions of the area (e.g., wind
and wave monitoring data) and possible climate
change impacts (e.g., sea level change data);
(6) tectonic movement map of the area; (7) map of sediment grain-size distribution on the seabed; (8) measurements of suspended sediment concentration and
discharge in representative periods (e.g., flood and
ebb tides, dry and wet seasons); and (9) estimates of
waves, currents, and sediment transport aided by
numerical models.
3. Dividing the research area into a series of cells. Each
cell acts as a control unit with clearly defined boundaries. Sediment budget is cell-dependent, and calculation of sediment volume change is performed only at
the boundaries of each cell. Cells can be defined by
geological or morphological controls, available data
resolution, coastal structures, and knowledge of the site
(Rosati, 2005). Depending on the specific sources,
sinks, and transport processes in the research area, cells
with different sizes can be assigned. Sub-cells can also
be defined within a cell to better measure the sediment
budget of a region with varying rates of accretion and
erosion. A well-known example is given by Bowen
and Inman (1966) who introduced the concept of littoral cells. A littoral cell is usually a zone parallel to the
shoreline, bounded by the foot of the foredune or cliff
in the landward direction and depth-closure point in
the shoreward direction. Changes of the sediment volume in a littoral cell directly induce changes in the
coastline, and ideally they are defined to minimize
longshore sediment exchange with adjacent cells, for
example, setting the lateral boundary of a littoral cell
at the nodal points where the net longshore transport
rate is zero or defining a pocket beach bounded by
rocky headlands, which is able to conserve its majority
of sediment, as a littoral cell. An example of cell division of an estuary and its adjacent coast is shown in
Figure 1a, b.
4. Defining sediment transport pathways at the cell
boundaries. Sediment transport pathways specify the
sediment transfer direction and corresponding processes between adjacent cells. The transport pathways
can be estimated through knowledge of the processes
(natural and anthropogenic) occurring at the site.
Interpretation of aerial photographs, trend analysis of
sediment grain-size distribution, and particle tracking
also aid to define sediment transport pathways.
Figure 1b shows possible sediment transport pathways
for an estuary and its adjacent coast.
5. Calculating the volume of sediment transfer in each
transport pathway. A general equation for the sediment
volume change (DV) of a cell is expressed by:
DV ¼
X
Q source À
X
Q sink þ I À O
ð1Þ
where Q source and Q sink represent the amounts of natural
sources (i.e., input) and sinks (i.e., output) transferred at
the cell boundary, respectively. I and O are the amounts
of artificial sediment input (e.g., beach nourishment) and
output (e.g., dredging) transferred at the cell boundary,
respectively. The quantity of sediment volume transfer
on each transport pathway should be evaluated based
on the acquired data. For example, comparison among
aerial photographs in different periods gives the detailed
information on the rates of coastline change. Field measurements on currents, waves, and sediment concentrations help to quantify the typical rates of sediment
transport caused by natural processes. Analysis of
extreme events provides information on the frequency
and magnitude of consequent sediment transport. Comparison among DEMs, coastal profiles, or aerial photographs provides information on calibration and
validation of the estimated quantities in (1).
6. Estimation of uncertainty in the calculated sediment
budget. One should consider that uncertainty always
exists in the calculated sediment budget. Uncertainty
of a sediment budget mainly comes from two sources:
(1) measurement error and (2) true uncertainty due to
temporal and spatial variability of the transport processes (Rosati, 2005). Measurement error contains the
error made during measurement processes (e.g.,
improper positioning of the instruments) and the error
in the measured signals (e.g., noises). Net sediment
flux across an estuary mouth or an inlet is difficult to
measure, particularly when the sediment flux on the
ebb and flood tides are high. In estuaries, sources and
sinks can be expressed as rates of exchange in a tidal
cycle. However, many of the exchanges are not at
steady rates, for example, sediment exchange in stochastic extreme events (e.g., storms, floods) can be
several orders of magnitude higher than in normal conditions. Another source of uncertainty comes from
numerical models which are applied. For example, littoral drift is normally estimated on the basis of standard
equations (e.g., the Coastal Engineering Research Center (CERC) (USACE, 1984) and Kamphuis
(Kamphuis, 1991) methods) or more complicated
models (e.g., Zhang et al., 2013). However, different
models may yield quite different results on the rate of
longshore sediment transport even though they
are based on the same boundary inputs. Thus, the
estimated littoral drift is often biased and should be calibrated by field measurements. Impacts of unknowns
552
SEDIMENT BUDGETS