In this study, elevation of the reef crest was taken as 0.3 m (1 ft) below
mllw. Depth over the reef crest is calculated based on a water level due to the
sum of astronomical tide and storm surge. When depth over the reef is less than
2 m (6.6 ft), which is typically the case, ponding is calculated for both cases in
the table and an interpolated ponding value is used.
Astronomical tide was included as a single level in this study for three
reasons. First, the tide range is relatively small. Second, tide levels at this loca-
tion are characteristically in a very narrow range between msl and mean high
water (mhw) most of the time, as discussed in Chapter 4. Finally, extreme
overtopping rates are not directly related to tide level, as with stage-frequency
analysis, so the customary introduction of varying tide levels in EST analysis is
no longer possible. The tide level used in this study was mhw, a representative,
but not extreme, high tide level.
Wave Setup, Runup, and Overtopping
Although high waves break on the reef, a significant amount of wave energy
can remain and continue to propagate across the reef to shore. The height of
these waves is limited by shallow water depths over the reef. Previous investiga-
tions have indicated that significant wave height across a relatively flat reef is
limited to approximately 0.4 times the local water depth (e.g., Smith 1993). That
limit was applied in this study, with water depth over the reef including tide,
storm surge, and ponding. During an intense nearby typhoon, depth over the reef
can exceed 3.04 m (10 ft), giving nearshore significant wave heights of over
1.21 m (4 ft).
Waves which have propagated across the reef lagoon encounter the
nearshore slope approaching the seawall. Again, they break and cause a local
increase in water level. This contribution to water level, referred to as wave
setup, is not included in the ponding calculation. It is calculated as (Shore
Protection Manual 1984)
′
g (H o )2 T
Sw = 0.15db -
(28)
64πdb.5
1
where
Sw = nearshore wave setup
db = water depth at breaking
′
H o = equivalent normally incident deepwater significant wave height
Water depth over the reef and significant wave height over the reef were used for
′
parameters db and H o , respectively.
39
Chapter 5
Development of Overtopping and Stage-Frequency Relationships
Previous Page
