Case 5 - North-Northwest Typhooon Waves

The Bureau of Environmental and Coastal Quality, Division of Coastal Resource Management (BECQ-DCRM) has contracted Sea Engineering, Inc. (SEI) to conduct a hydrodynamic study of Saipan's western lagoon. The study approach included measuring waves and currents in the lagoon, and then using that data to develop and calibrate combined wave and hydrodynamic models of current and circulation in the lagoon. The field measurements included deployment of 9 wave and current meters throughout the lagoon for a three-month period during summer of 2017. This report present results of the wave and hydrodynamic numerical modeling. The models provides a basic understanding of the primary currents and circulation patterns within the lagoon and will assist BECQ-DCRM with the development of sampling and protocols for marine and water quality monitoring in the lagoon. Five cases were therefore selected to encompass the range of wave conditions most typically impacting the lagoon: 1) typical winter tradewind wave conditions, 2) typical summer tradewind wave conditions, 3) typical summer westerly wave conditions, 4) a 1-year return period typhoon wave from the southwest, and 5) a 1-year return period typhoon wave from the north-northwest. Download/View the Full Report or Data Report for more detailed information. A typical typhoon passing annually to the north of Saipan could generate waves approaching the island from 345 degrees with deepwater wave heights of 4.3 meters and periods of 12 seconds. The typhoon waves directly impact the reefs along Tanapag Lagoon and refract with substantial energy into Garapan Lagoon. Waves drive strong flow over the reef, into the Tanapag Lagoon, and out the main channel. Velocities over the reef are greatest at sections of reef with direct exposure to the north and have speeds of approximately 0.75-1.50 m/s. The currents flow straight across the reef to the north, and along the reef to the southwest from Managaha Island to the main channel. Flow out of the main channel has a maximum velocity of approximately 1.05 m/s. Flows around the Port of Saipan remain small with magnitudes less than 0.02 m/s. The patch reef area to the west of Garapan experiences wave driven currents with speeds up to 0.9 m/s.