CETN IV-20
September 1999
A sediment budget with an effectively limitless number of cells, sources, sinks, and engineering
activities can be copied to a new alternative. The new alternative may be modified to reflect
different assumptions about sediment-transport magnitudes and pathways, and engineering
activities within the budget, or a different time period as compared with the original formulation.
SBAS has an option to append and pre-pend alternatives from another project file to the
alternative within the topology window.
Uncertainty provides a powerful means of comparing cells within the budget and quantifying the
reliability of the budget as a whole (see CETN-IV-16 for discussion of uncertainty). The
uncertainty of sediment budgets for different reaches can also be compared. Within SBAS, a
total cell uncertainty is calculated by adding all uncertainty values, u, within that cell. This value
is normalized by the absolute-value magnitude of sources, sinks, and engineering activities
within that cell and expressed as a percentage U (percent) as follows:
∑u
U (%) =
(100),
∑ Qsource + ∑ Qsink + ∆V + P + R
∑ Qsource + ∑ Qsink + ∆V + P + R > 0 ;
for
(2)
∑ Qsource + ∑ Qsink + ∆V + P + R
U (%) = 0
=0
for
Similarly, a percent uncertainty is calculated for the entire alternative using Equation 2. The
percent uncertainty for various alternatives can then be compared, revealing the degree to which
various assumptions are known and thereby giving a means to quantify the reliability of the
budget as a whole.
SBAS has several features designed to streamline the process of formulating a sediment budget.
Transport rate quantities may be calculated as associated with relative sea-level change and
onshore or offshore transport or based on values of other transport rate values. Scanned
photographs or bathymetric maps may be included as wallpaper within the topology window,
allowing calculation cells to be drawn on top of the regions of interest in the proper location.
Sediment budget cells may be rotated to more closely approximate the shoreline and bathymetric
orientation. An extensive help file provides on-line guidance for formulating a sediment budget
and estimating various quantities.
With the SBAS installation package, Tutorial and Example files are provided. The Tutorial
simply provides an on-line presentation of the basic methodology within SBAS and is self-
explanatory. The Example file pertains to the following discussion below, and the user is
encouraged to read through the example while performing calculations in the on-line Example.
EXAMPLE: Although SBAS is a convenient tool for formulating a sediment budget, any
systematic solution approach, such as a spreadsheet, could be taken. Within this example,
uncertainty in length and time values are assumed to be negligible, although this is not necessary.
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