Implementation of Storm Surge Model

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validation, and production runs. Accuracy of model results is greatly influenced

by the accuracy of boundary and forcing conditions, representation of the

geometry of the study area (i.e., bottom topography and land/water interface),

and to a lesser degree, the values of certain calibration parameters. Model

calibration involves adjustment of the calibration parameters to maximize

agreement between model results and measurements.

Upon completion of calibration, the model is subject to a validation that

consists of applying the model over a different segment of time from that of the

calibration and where no changes have been made to the calibration parameters.

The model is considered validated if results agree with measurements within an

acceptable degree of error. The validation procedure provides confidence that

the model can accurately simulate hydrodynamic processes in the study area.

The strategy for calibrating and validating the storm surge model requires

that the model accurately simulate both tidal propagation and storm surge in the

study area. The model was first tested for simulation of tidal motion, then tested

with the typhoon wind and pressure fields. Procedures applied in conducting

model testing and the results of these tests are presented in the following

sections.

Calibration of Storm Surge Model

The model was calibrated for tidal propagation and subsequently verified for

storms. Water-level measurements were available from one gauge within the

study area located in Apra Harbor at the entrance to Sumay Cove. The gauge is

operated by the U.S. National Ocean Service (NOS). The gauge location is

13.4435 N, 144.6566 E. Tide gauge data for the month of January 1997 illustrate

tidal behavior at Apra Harbor (Figure 10). Mean tide range is only 0.5 m (1.7 ft).

The tide is characterized by a large diurnal inequality. High tides are generally

relatively close to msl and lower low tides typically drop well below msl. The

probability distribution of hourly tide levels during the year 1991, a relatively

Chapter 4

Implementation of Storm Surge Model