ABSTRACT :

In this paper, thirty years of negative storm surge data measured from Qingdao hydrologic station are selected to set up different statistical samples according to time division, such as year, season or month. Besides the Annual Maxima method and the Peak Over Threshold method, a combined distribution method is presented to estimate the return values of negative storm surge in shallow water area of Qingdao under the consideration of seasonal variation. The statistical results show that the negative storm surges occur in spring, autumn, and winter in Qingdao. This paper shows that, as the most frequently occurred negative storm surges are caused by cold waves, it is feasible to estimate the return values of the negative storm surge by sampling from dominant climate rather than from the climates occurred all the year around.

INTRODUCTION

Storm surge is the uncommon piling up of the seawater along the shoreline induced by intense atmospheric interference such as tropical or temperate cyclones. Coinciding with high astronomical tides, it generates serious coastal flooding and results in severe damages along the coastline. In the coastal area of medium and high latitude, cold waves often produce negative storm surges. For example, northerly winds, which may blow for several days and at considerable speed, produce negative storm surges in the coastal area of Qingdao (35°35" ~37°09" N, 119°30" ~121°00" E). When the negative surges occur, offshore winds blow the ocean water down from the coastline and a large area of shallow shoal expose to the air. The water level lowering is a potential threat to the navigation of super bulk carrier or giant oil tanker. Previous studies focus on the long-term prediction of extreme water level for structural design (i.e. Gumbel, 1954; Lennon, 1963; Yang et al., 1970; Ackers and Ruxton, 1975; Graff, 1981).

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