Analysis of Sea Level Rise in the Philippines using Coastal Tide-gauge Observations

Water level determines flooding and considerably affect wave climate. It is important to consider it in both disaster mitigation and coastal structure design. National Mapping and Resource Information Authority (NAMRIA) is the government agency that collects, process and utilize water level data along its 63 tide stations in the Philippines. The NAMRIA tide data are composed of tide time series and tide normals. The time series are in the interval of an hour and is also used to derive the tide normal. To assess the trends of the tide time series, the sea level rise in some tide stations are assessed. Monthly mean sea level from NAMRIA and Permanent Service on Mean Sea Level (PSMSL) are used in Least-square linear solution to obtain the Average Seasonal Cycle (ASC) and sea level trends in each stations. The ASC accounts for the steric effects on water level such as the temperature, salinity, and discharge of water from nearby river. Results showed that shorter period of observation (NAMRIA tide data) are lesser in magnitude (0 to 1 mm/year) and cannot capture the negative trends of sea level rates. Using longer period of observation (PSMSL data), negative sea level trends are obtained in 10 stations (Jolo, Legaspi, Pulupandan, San Fernando, San Jose, Subic, Surigao, Tacloban, Tandag, and Virac). Since generally, the sea level rates are increasing, falling sea level trends on these open coast areas can be accounted from the upward vertical land movement. It is recommended to use longer period of observations to deeply examine the long-term trends of sea level rise in the Philippines.