11:55 AM - 12:10 PM
[ACG48-10] Observation of longshore migration of mega-cusps and longshore current
Keywords:mega-cusp, longshore current, observation
Observations using X-band radar and radio velocimeters are being made at the observation pier HORS of the Port and Airport Research Institute (PARI) at Suda Beach in the southern part of the Kashima Coast.
The southern part of the Kashima Sea is a 17 km long sandy beach with Kashima Port at the northern and Hasaki Fishing Port at the south end. Five headlands are installed along the coast, but no strong erosion areas exist. The observation pier is located about 4 km from the northern end.
The radar captures the shoreline profile and topography of the swash zone for an area of about 2,000 m in the longshore direction. A radio velocimeter installed at the pier measures the hourly coastal current velocity within the wave-breaking zone.
During periods when the incident waves are relatively small, the shoreline becomes wavy and mega-cusps in the swash zone are captured. Their wavelengths are several hundred meters and constantly move in the coastal direction. When the incident waves become large, the coastal reset occurs, the shoreline becomes straight, and the topography of the swash zone becomes uniform in the coastal direction. After a while, the wavy shoreline and mega-cusp reappear.
The longshore current U is directed to the down-wave, and its magnitude can be well regressed by the wave power P (~ H^2-T-sin θ) and the longshore wind speed V. The magnitude of the longshore current U has an upper bound on the magnitude of the longshore current. There is an upper limit to the magnitude of the longshore current velocity. This is because even if the offshore incident waves become large, wave breaking occurs, and the water depth limits the wave height in the wave breaking zone.
The annual wave shoreline movement and mega-cusps showed a high correlation between the time integration of U. This suggests that longshore currents may be responsible for the longshore movement of the wavy shoreline shape. Currently, no model can describe the movement of wavy landform patterns in shallow water while maintaining their shape. Our next research goal is to propose a model that describes sediment transport and landform change based on the observations presented here.
The southern part of the Kashima Sea is a 17 km long sandy beach with Kashima Port at the northern and Hasaki Fishing Port at the south end. Five headlands are installed along the coast, but no strong erosion areas exist. The observation pier is located about 4 km from the northern end.
The radar captures the shoreline profile and topography of the swash zone for an area of about 2,000 m in the longshore direction. A radio velocimeter installed at the pier measures the hourly coastal current velocity within the wave-breaking zone.
During periods when the incident waves are relatively small, the shoreline becomes wavy and mega-cusps in the swash zone are captured. Their wavelengths are several hundred meters and constantly move in the coastal direction. When the incident waves become large, the coastal reset occurs, the shoreline becomes straight, and the topography of the swash zone becomes uniform in the coastal direction. After a while, the wavy shoreline and mega-cusp reappear.
The longshore current U is directed to the down-wave, and its magnitude can be well regressed by the wave power P (~ H^2-T-sin θ) and the longshore wind speed V. The magnitude of the longshore current U has an upper bound on the magnitude of the longshore current. There is an upper limit to the magnitude of the longshore current velocity. This is because even if the offshore incident waves become large, wave breaking occurs, and the water depth limits the wave height in the wave breaking zone.
The annual wave shoreline movement and mega-cusps showed a high correlation between the time integration of U. This suggests that longshore currents may be responsible for the longshore movement of the wavy shoreline shape. Currently, no model can describe the movement of wavy landform patterns in shallow water while maintaining their shape. Our next research goal is to propose a model that describes sediment transport and landform change based on the observations presented here.