Modeling Approach

2_Guam
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Cabras Island, Apra Harbor, U.S. Territory of Guam, requires application of

several standard CHL numerical models and many additional processing steps.

The objective of this chapter is to explain the modeling approach and document

models and procedures used in the study. An overview of the modeling approach

is given in the following paragraphs. More detailed descriptions of key modeling

steps are given in following sections of the chapter.

The main modeling steps are as follows. First, a Planetary Boundary Layer

(PBL) wind model simulates the time-history of typhoon-induced wind and

atmospheric pressure fields for each selected storm during its general proximity

to the study area. The time-history of wind information serves as input to both a

long-wave hydrodynamic model ADCIRC and a wind-wave model WISWAVE.

The ADCIRC model provides a refined time-history of typhoon-induced water

levels at the study location for each storm. The WISWAVE model provides a

time-history of deepwater wave parameters in the general vicinity of Apra

Harbor.

For the exposed coastal study area along Cabras Island, offshore WISWAVE

information is adjusted to provide a time-history of waves incident to the near-

shore coral reef. The adjustment is done with the wave-transformation model

WAVTRAN. These wave parameters are subsequently matched in time with

nearshore water level information from ADCIRC and used to calculate a time-

history of wave ponding over the reef and nearshore setup, runup, and over-

topping. Both existing and proposed project configurations are used for the

nearshore profiles. Maximum overtopping rates are extracted for each nearshore

profile in each storm. The EST analysis is applied and overtopping rates are

calculated for various return periods.

For the harbor side along the commercial docks, offshore WISWAVE infor-

mation incident to Apra Harbor entrance is adjusted to account for the break-

water gap and propagation to the dock area. Because of the possibility of very

strong typhoon winds, the dock can be exposed to a second wave component,

locally-generated waves inside the harbor. The time-history of local wind from

WISWAVE serves as input for calculating local wave growth inside the harbor.

Both wave components, the effect of wave reflection from the dock face, and

water-level information from ADCIRC are combined at the dock to give a time-

history of water level along the dock for each storm. Maximum water levels,

Chapter 3

Modeling Approach