CETN IV-15
Rev. September 1999
Bathymetry. Historical and recent bathymetric data sets are a valuable resource for
determining the rate of volume change in the inlet channel and on the ebb- and flood-tidal
shoals. If coverage is sufficient, differences in bathymetric surfaces give the subaqueous
volume change on the adjacent beaches and channel and ebb- and flood-tidal shoals. It is
noted that, in the past, typical bathymetric coverage has been limited to the inlet channel.
However, the benefits of increasing the survey area to include the ebb- and flood-tidal shoals
far outweigh the additional costs, particularly in view of reductions in the cost of bathymetric
surveys (e.g., SHOALS bathymetric survey system, Parson and Lillycrop 1998).
Bathymetric data can also indicate sediment-transport pathways. As examples, a finger shoal
extending from the tip of a jetty likely indicates a dominant sediment-transport pathway, and
the morphologic form of an ebb-tidal shoal that connects to the adjacent beaches may
indicate inlet bypassing. Aerial photography of flood-tidal shoals at different but known tidal
elevations can be referenced to create a contour map of the shoals, and thereby to estimate a
shoal volume.
Engineering History. Engineering activities of significance to a sediment budget fall into
two categories: (a) those that are of a descriptive nature and must be quantified within the
sediment budget and (b) those that are a priori quantified. Rehabilitation of a jetty is an
example of a descriptive activity that requires quantification. The morphology of the inlet
and adjacent beach before and after structure rehabilitation, as well as the type of
rehabilitation (e.g., raising the jetty crest elevation, inserting a sandtight core, adding armor
stone), and other pertinent data sets indicate the effectiveness of the structure. Consideration
should be given as to the degree of sediment transport through, over, and around the structure
before and after rehabilitation. Another example of descriptive data is the grain size of
dredged material placed on the adjacent beaches. From this information, the engineer must
estimate percentage of material that would remain in the active littoral zone.
Engineering activities that are a priori quantified (although sometimes only partially) include
the following: volumes, locations, and times of dredged and placed material; volume of
material mined from ebb- and flood-tidal shoals, the locations, and times of mining;
configuration of the placement; volume of fill on adjacent beaches and its placement location
and time period of placement; and records of mechanical bypassing (volume, placement
location, and time periods). These quantities will enter the sediment budget calculations by
adjusting measured volume changes to account for either the removal or placement of
material through engineering activities. The adjustment of an initial beach fill can be used to
infer rates of longshore and cross-shore sediment transport.
Coastal Processes. Data on the acting coastal processes are a resource for understanding
and quantifying inlet- and sediment-transport pathways and quantitites. Examples are
discussed here.
Net, left-, and right-directed potential longshore sand-transport rates can be calculated from
wave gauge, Wave Information Study (WIS), and Littoral Environment Observation (LEO,
see Schneider (1980)) wave height, period, and direction data. CETN-II-19 (Gravens 1989)
discusses the methodology for calculating net potential longshore sand-transport rates from
WIS data. The components of the net transport, directed to the left or right as noted by a
8
Previous Page
