transport of sediment along the shore. Knowledge of the sediment trans-
port capacity of the longshore currents is important in preventing erosion
of the beach or in siting a harbour entrance.
The sediment transport capacity is related to the wave power at the
instant of wave breaking:
Rb H^2 sbCgbsinbcosb (15.1)where the subscript ‘b’ refers to the quantities at the instant of wave
breaking, and is the angle of the crests with the bed contour. The sub-
merged weight of the sediments transported per unit time is
Gs( (^) s )gaQs (15.2)
where (^) sis the density of the sediments, Qsis the volume rate of sediment
plus water transported and ais the ratio of the volume of sediments to the
total volume (typically 0.6).
An empirical relationship between GsandRbis
GskRb. (15.3)
Shore Protection Manual(1984) recommends a value of 0.39 for the coeffi-
cientk. Some authors use r.m.s value of wave height Hrmsbat breaking
point instead of Hsbfor wave power Rbin equation (15.1). By virtue of
equation (14.54), k0.78 in equation (15.3) when Hrmsbis used.
The coefficient kis not a constant. Its dependence on the sediment
diameter is discussed in detail in Coastal Engineering Manual(US Army,
2002) to suggest Bailard’s formula
k0.052.6 sin^2 (b)0.007ub/ws (15.4)
in which ubis the maximum velocity of particle at breaking point and wsis
the fall velocity of sand. Using shallow water approximation
ub0.5(Hb/db)(gdb)0.5 (15.5)
The value of kobtained from equation (15.4) is applicable when Equation
(15.3) is expressed in Hrmsbinstead of Hsb.
Most of the drift occurs in the breaker zone. The direction of littoral
drift or transport is determined by the direction of the longshore current.
A temporary reversal of its direction may take place as a result of the vari-
ation of the approach angle of the waves. As the wave attack usually pre-
dominates in one direction over a period of time, there is a net transport
of sediment in that direction. Wave climate at a particular site on the
g
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