CETN IV-15
Rev. September 1999
feature should be noted. The depth of closure is located where no significant depth changes
occur over times of engineering significance (typically, 10 to 50 years) (see Hallermeier
(1978); Birkemeier (1985); Kraus, Larson, and Wise (1998)). Kraus, Larson, and Wise
(1999) discuss the depth of closure in detail and extend its definition to cover varied
conditions as encountered in engineering practice. Profile surveys performed near structures
may indicate their condition. For example, a jetty that allows sediment transport over and
through it might be indicated by a berm-crest elevation adjacent to the structure that is
comparatively lower than the berm crest further away from the structure. Similarly, surveys
close to structures reveal whether the profile deviates from the average shape far from the
structures, improving estimates of sand volume. The investigator should be cautious in
interpreting beach-profile data near the inlet because of migrating shoal features that may
affect the profile shape.
Shoreline-Position Data. Shoreline-position data may be obtained from analysis of
topographic and HWL surveys, aerial photographs, beach-profile surveys, and bathymetric
data (Anders and Byrnes 1991; Byrnes and Hiland 1995). In a qualitative manner, beach
morphology indicated by shoreline position may imply sediment-transport pathways or
controls. As examples, a salient or bulge-type feature in the shoreline downdrift of an inlet
may represent the location for ebb-shoal bypassing to the adjacent beach. Rocky headlands
and outcroppings indicate geologic controls on sediment-transport pathways.
As quantified in a sediment budget, the change in shoreline position y av eraged over a
given longshore distance x c a n be converted to a volume change by assuming that the
shoreline translates parallel to itself over an active depth DA, given by
D A = B + Dc
(4)
in which B is the elevation of the seawardmost active berm relative to a datum, and DC is the
depth of closure measured from the same datum.1 The volume change2 V over a time
interval t is given by
∆y∆x D A
∆V =
(5)
∆t
If available, the impoundment rate at a shore-perpendicular structure such as a groin or jetty
that is sandtight gives an estimate of the longshore sediment-transport rate.
1
Estimates of uncertainties and their significance in coastal-sediment budgets are described in CETN IV-16 (Kraus
and Rosati 1998a).
2
Comparison of a shoreline position derived from aerial photography with a shoreline position derived from beach-
profile surveys should account for possible differences in the vertical datums to which each is referenced. For
example, it is likely that an aerially derived shoreline position represents a berm crest or HWL position, whereas a
beach-profile shoreline may represent a zero elevation relative to a standard datum (e.g., National Geodetic Vertical
Datum, or Mean Sea Level). See CETN II-39 (Kraus and Rosati 1998b).
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