The four reduction factors involved in calculating overtopping rate are
discussed further in the following subsections.
Reduction factor for influence of a berm
The presence of a berm in the nearshore profile can significantly reduce
runup and overtopping relative to a continuously sloped profile. The reduction
factor which accounts for the influence of a berm in the nearshore profile is
given by VJ as
γb = 1 - rB (1 - rdh )
(13)
where
rB = factor based on berm width
rdh = factor based on berm location relative to the still-water level (swl)
The factor γb is confined to the range 1.0 ≥ γb ≥ 0.6.
The factor based on berm width is given by
tan αeq
rB = 1 -
(14)
tan α
where
tan αeq = equivalent slope gradient to represent berm width effects
tan α = representative slope gradient without consideration of berm
Equivalent slope gradient is determined as the slope of a straight line between
two points on the nearshore profile: one point is at elevation Hs above the most
seaward point of the berm; the other point is at elevation Hs below the most
seaward point of the berm (Figure 8). Equivalent slope gradient is independent
of water level.
Representative slope gradient is determined by extending an imaginary line
up from the most seaward point of the berm (Figure 9). Slope of the imaginary
line is equal to the slope of the nearshore profile shoreward of the berm. Effec-
tively, the berm is removed from the nearshore profile. Representative slope
gradient is defined as the slope of a straight line between two points: one point
is at elevation Hs below the swl; the other point is at elevation Hs above the swl.
If the upper elevation is higher than that of the most seaward point on the berm,
then the upper point is taken on the imaginary line. This approach is based on the
consideration that the slope most influencing wave runup is that within one
significant wave height above and below the swl.
22
Chapter 3
Modeling Approach
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