protection configurations. Some configurations included low-crested berms. No
configurations ideally matched the plan configuration for Cabras Island, but
USAED, Pacific Ocean (1995) provided a reasonable initial assessment of the
plan's impact on overtopping rates by considering several test configurations
with berm features.
A method for estimating storm wave runup and overtopping on berm profiles
has been developed for application to dikes in The Netherlands (van der Meer
and Janssen 1994; De Waal and van der Meer 1992). The method has general
applicability and it was chosen for application in the present study. Based on
extensive laboratory testing, it inherently includes important irregular wave
effects on overtopping rates. It provides a well-tested, unified methodology for
evaluating both existing and plan beach profiles along Cabras Island. A few of
the existing profiles include a natural berm which can be expected to function
similarly to the plan berm. The van der Meer and Janssen methodology, hence-
forth abbreviated as VJ, can realistically accommodate those profiles. The VJ
methodology is described in the remainder of this section.
A distinction is made between breaking and nonbreaking wave conditions, as
identified by the surf similarity parameter or breaker parameter
tan α
ξop =
(6)
sop
where
>op = breaker parameter
α = nearshore slope angle
sop = wave steepness; = 2B Hs/gTp2
Hs = significant wave height near toe of the slope
Tp = peak period of the wave spectrum
When >op < 2, waves are considered to be breaking on the slope. Otherwise, they
are considered to be nonbreaking.
The overtopping rate for breaking waves is calculated from an empirical
relationship between dimensionless overtopping rate
sop
q
Qb =
(7)
tan α
3
gH
s
and dimensionless crest height of the beach profile
20
Chapter 3
Modeling Approach
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