Breaking waves at the shore intermittently push water up the beach, creating
wave runup. For both existing and plan nearshore profiles in the project area,
runup during an intense typhoon can reach the seawall crest and continue over
the top of the seawall. This wave overtopping can create problems along the
commercial port road due to flooding, debris, and damage to the road surface. It
can also cause flooding in the Apra Harbor container yard. Wave overtopping
rate was calculated with the methodology described in Chapter 3, developed by
van der Meer and Janssen (1994). Reduction factors used in the calculation were
determined to fit this application and produce overtopping rates consistent with
qualitative observations of storm damage in the project area, as discussed in the
following section.
Wave setup and overtopping rates were computed along 15 transects within
the study area. Transects were specified by elevation profiles surveyed and pro-
vided by the Honolulu District. Plan profiles were also provided. Elevations
were specified relative to mllw. The project stations modeled are at 61-m
(200-ft) intervals, beginning with Sta 00+00 and ending with Sta 28+00.
Implementation of Overtopping Method
The methodology used to estimate overtopping rates along the north shore of
Cabras Island includes four reduction factors to represent a variety of physical
factors which can reduce overtopping. Implementation of these reduction factors
requires a calibration/validation process to ensure that the methodology is giving
a reasonable representation of the project area. This section describes the
implementation for Cabras Island, Guam.
Three historical typhoons which caused damage to the commercial port road
in the study area are considered in the feasibility report (USAED, Pacific Ocean,
1995). Typhoon Roy (0188) and Typhoon Koryn (0190) were reported to cause
significant overtopping of the seawall and washing of rubble and debris onto the
road. Typhoon Omar (1592) caused similar damage but to a lesser extent due to
rapidly changing conditions as the eye passed almost directly over the study area.
The same three storms were used in this study to help calibrate overtopping
rate calculations to be consistent with documented experience. Since no
quantitative overtopping information is available, published information about
dangerous overtopping rates on roadways was used to assess calculated over-
topping rates. Ward and Ahrens (1992) report on a study by Fukuda, Uno, and
Irie (1974) which determined that an overtopping rate greater than about
0.00002 m3/sec/m (0.0002 cfs/ft) would prohibit a vehicle from driving past at
high speed a distance of 3 m (10 ft) behind the coastal structure. At a distance of
9 m (30 ft) behind the structure, the threshold overtopping rate is
0.0002 m3/sec/m (0.002 cfs/ft). CIRIA/CUR (1991) give threshold overtopping
rates of about 0.000001 m3sec/m (0.00001 cfs/ft) for the onset of unsafe high
speed traffic and minor structural damage to buildings and around
0.00003 m3/sec/m (0.0003 cfs/ft) for the onset of unsafe driving at any speed and
significant structural damage.
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Chapter 5
Development of Overtopping and Stage-Frequency Relationships
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