elevation of the most seaward berm point. As the swl deviates from this eleva-
tion, rdh soon reaches a value of one and the berm has no further impact on runup
and overtopping. When swl is near the elevation of the seaward berm point, then
berm width has an additional effect in reducing runup and overtopping. As berm
width increases, the value of rB approaches one, and the overall reduction factor
γb becomes smaller.
Reduction factor for influence of shallow foreshore
When water depth immediately seaward of the nearshore profile for runup
and overtopping calculations is relatively shallow, high waves in the train of
irregular incident waves will break. Thus, waves attacking the nearshore profile
will be somewhat diminished due to the presence of a shallow foreshore. VJ
suggest the following approximation for the reduction factor:
2
d
γh = 1 - 0.03 4 - s
(16)
H ss
where
ds = water depth at toe of nearshore profile
Hss = significant wave height at toe of nearshore profile
If ds/Hss ≥ 4, then γh = 1.0.
Reduction factor for influence of roughness
Runup and overtopping on a rough slope is reduced relative to a comparable
smooth slope. This effect has traditionally been represented by a factor repre-
senting the ratio of rough slope runup to smooth slope runup. A table of
reduction factors, γf , is given by VJ for various types of slope. This factor is
comparable to reduction factors given in the Shore Protection Manual (1984) in
Table 7-2, Chapter 7, Volume II. VJ recommend that this factor be set to one for
cases in which the breaker parameter is relatively large, that is, γf = 1 when
ξop ≥ 4.
Reduction factor for influence of angle of wave attack
Waves approaching perpendicular to a nearshore profile can be expected to
cause higher runup and overtopping than waves approaching from an oblique
angle. For the case of long-crested waves approaching within 30 deg of normal
to shore, VJ suggest that obliquity has no impact and these cases can be treated
as directly approaching. For short-crested waves, even small oblique angles of
24
Chapter 3
Modeling Approach
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