WAVE OVERTOPPING 645
The correction factors rfor rough slopes have been updated but are only
slightly different from the values given in Table 15.1.
Waves breaking on a sloping structure causes run-down in addition
to run-up. Run-down is the vertical distance that the water level reaches
below the still water level. The average run-down Rd2for waves whose
height is exceeded by 2% of the incoming waves is estimated as (Coastal
Engineering Manual (US Army, 2002))
R
H
d
s2
0.33p for 0 p 41.5 for p 4 (15.29)Sea walls are often designed with composite slopes including a berm
which reduces the wave run-up. Hunt (1959) recommends a berm width of
at least one-fifth of the wavelength. For the effect of berm on run-up, refer
toCoastal Engineering Manual(US Army, 2002). Run-up on a composite
slope is estimated using Saville’s method (1957) where a single slope
extending from the point of wave breaking to the point of maximum run-
up is assumed and the wave run-up is estimated for this hypothetical slope
(Fig. 15.8(f )). Since the wave run-up is not known initially, it is determined
by trial and error. First, the maximum run-up on a slope is assumed. The
depthdbat which the wave breaks is 0.78Hb, where Hbis the height of the
wave at the instant of breaking. The run-up for the hypothetical slope is
found from equation (15.21) and compared with the assumed one (worked
example). As wall profiles have large effects on the reflection of the wave
and on the run-up on the wall, the design of large projects should involve
model studies (Novak and Cˇábelka, 1981). For fuller treatment of sea wall
design, the reader is referred to Thomas and Hall (1992).
15.5 Wave overtopping
Overtopping of the sea wall can take place as ‘solid water’ or as ‘sprays’.
Sea walls should be designed to satisfy their functional requirements
without causing undesirable impact on the surroundings. If the freeboard
is generous, the scheme can be too expensive. On the other hand, if the
freeboard is too low, the land that it is designed to protect may be sub-
jected to flooding and erosion. Under certain conditions occasional over-
topping is not unusual with irregular waves. It is important to estimate the
quantity of water overtopping a sea wall and to assess the consequences.
The mean overtopping discharge Qis usually expressed functionally as
Qfunction of (Hs,T,,R,d,g,d,s.. .) (15.30)