Initially, waves were transformed to a water depth of 10 m (33 ft). If maxi-

mum significant wave height during the typhoon exceeded 4 m (13 ft) (0.4 times

the water depth), WAVTRAN was rerun to transform to a deeper nearshore

depth. Nearshore depth was increased in 5-m (16-ft) increments until maximum

significant wave height during the storm was less than 0.4 times the depth or

until the nearshore depth reached 30 m (98 ft). This transformation approach is

expected to produce realistic incident significant wave heights for calculation of

nearshore processes.

Wave setup on reefs occurs due to overtopping of waves onto the reef

platform. As waves break on the reef, water is deposited causing an elevated

water level, which is typically referred to as ponding over the reef. Seelig (1983)

conducted a set of laboratory experiments for fringing reefs typical of Guam to

investigate hydraulics of reef-lagoon systems. Seelig's formulations were applied

in this study. Parameter ranges in Seelig's study were: still-water depth at the

reef crest was 0 m (0 ft) and 2 m (6.6 ft); wave periods ranged from 8 to 16 sec;

and irregular deepwater significant wave height ranged from 2.5 to 10.7 m (8.2

to 35.1 ft). These conditions are representative of the typhoon events impacting

Cabras Island.

Seelig (1983) found that the ponding water level is a function of the swl

(astronomical tide and storm surge), incident deepwater significant wave height,

and wave period. Gourlay (1996) confirmed these findings. Contributions to the

ponding level include waves breaking on the reef, but do not include setup from

reformed waves at the shoreward end of the reef. Ponding level can be estimated

by (Seelig 1983)

(

)

η = *a*1 + *a*2 log *H * 0 T

2

(27)

where η is the ponding level in m, *H*0 is the deepwater significant wave height in

m, *T *is the wave period in sec, and *a*1 and *a*2 are empirical coefficients dependent

on the swl and wave spectrum (monochromatic or irregular). Table 7 gives

values of the empirical coefficients for irregular waves.

0 (0)

-0.92

0.77

2 (6.6)

-1.25

0.73

Note: Depth measured relative to reef crest; coefficients are based on SI units.

38

Chapter 5

Development of Overtopping and Stage-Frequency Relationships

Integrated Publishing, Inc. |