coastline can be expressed in terms of bands of wave heights in sectors of
wave direction. It is possible to use this statistical information along with
equation (15.1) to determine the net sediment discharge and its direction
and then evaluate whether erosion or deposition takes place at the site
taking account of the tidal currents which will have an influence on the
sediment transported.
Structures such as detached breakwaters and groynes interfere with
the littoral drift. A detached breakwater is normally built parallel to the
shore to provide an area sheltered from the wave action (Fig. 15.3(a)). The
refraction and diffraction effects produce a decrease in energy behind
the breakwater; hence deposition takes place in the protected area. In the
presence of a structure, the wave height varies in the direction alongshore.
Such variation influences the longshore sediment transport in addition to
the contribution from wave breaking.
Shore-connected breakwaters (Fig. 15.3(b)), which extend across the
littoral zone, act as a barrier. If the littoral drift was originally in equilib-
rium, the breakwater will initially cause deposition on the updrift side, and
because of the reduced sediment concentration in the longshore current,
erosion on the downdrift side of the breakwater. As time proceeds, the
632 COASTAL ENGINEERING
Fig. 15.3 Effects of breakwaters on the littoral drift
