Environmental Engineering Reference
In-Depth Information
Figure 1
Evolution of the British coastline since the last glacial
maximum. (a) Approximate sea level below the present and rate of
rise at key intervals as global ice sheets melted. (b) Extended
continental shelf and British ice sheet at 18,000 BP. The modern
coast took shape from the start of the Holocene (c) and through
separation from Europe (d). Exposed land surface shown by mid-
tone, recently abandoned shoreline by broken line. Source: After
Smith (1992).
geomorphic consequence, or
dissipated
in the intertidal zone through turbulence, bed
friction and the movement of rock and sediment. Wave energy, normally measured in
joules, varies with the square of wave height (see Chapter 11) and can reach levels up to
20 kJ m
−2
s
−1
. It is delivered either through hydraulic pressure, capable of compressive
stresses reaching 10-100 MN m
−2
, or water jets and sheets applying bed shear stress. The
dynamics of breaking waves determine how energy is likely to be transformed and
dissipated.
Prior to breaking, the increase in
H
/
L
converts potential to kinetic energy, with some
lost as friction against the bed, at a rate determined by velocity and bed roughness.
Dissipation on and after breaking varies according to breaking style. Turbulence
commences at the crest of spilling waves and, over several wavelengths, 'spills' down the
wave front, leaving little energy for geomorphic work. Collapsing and surging waves
may be 'failed' plunging waves but their turbulence pushes a water sheet onshore.
Plunging waves are the most dramatic and set up a vortex led by a water jet below its
breaking tip. If this is powerful enough, it penetrates the trough ahead of the wave and
scours the bed, throwing up a cloud of sediment and trapped air bubbles clearly visible
behind the crest (Figure 17.5). Water continues to move onshore as small bores in the
surf zone and as sheet flow at 1-10 m s
−1
in swash run-up. The energy available to move
sediment depends on the velocity, depth, turbulence (through swash/backwash
impedance) and extent of percolation in this zone. Run-up endows backwash and
percolated water with potential energy capable of further sediment movement as they
