WAVES AND CURRENTS IN THE VICINITY OF HARBORS – AN EXAMPLE FROM SENDAI PORT, TOHOKU, JAPAN

Port and harbor facilities located along shelf coasts are often causing modifications to nearshore current and sediment transport patterns. As a consequence, local erosion in the vicinity of such structures is a common problem that often requires additional counter-measures leading to long-term maintenance and design costs.

Sendai Port in Miyagi prefecture, Japan, is exposed to energetic waves from different sources such as daily swells from the open Pacific Ocean as well as waves from typhoons and tsunamis. Although the predominant swell direction induces northward longshore sediment transport in Sendai Bay, the shoreline south of Sendai Port has been facing significant erosion problems over the past years that compromise the completion of a new set of tsunami seawalls. In the wake of the 2011 Great East Japan Earthquake and Tsunami, a massive reconstruction effort is underway but shoreline changes as a result of daily wave conditions were not considered in the planning stage.

This study uses phase-resolving wave modeling in combination with high-resolution topography data derived from UAV surveys to analyze the fundamental mechanisms, which lead to intensified erosion rates in the vicinity of Sendai Port. The technique can be applied to similar coastal structures elsewhere. The case of Sendai Port highlights how harbor geometry and wave direction affect the local surf zone waves and, subsequently, alter the current patterns.

Multiple scenarios of wave-by-wave computations show that reflection and superposition are the driving forces for a local change in the nearshore currents. Counter-intuitively, swell from southern directions, as in the case of bypassing typhoons, causes intense southward-directed currents leading to a local deficit in sediment budget. The findings were confirmed by data from a field survey of nearshore waves during typhoon swell in August 2016.