Studies1

Graduate

Studies1

Study of Diffusion of Seawater Spray Generated at Coastal Structures

photo

Field observation of wave run-ups
and splashing at a breakwater

This photo shows a large wave
run-up and a large number of
splashes generated at a vertical
breakwater. The device shown
in the photo is an anemometer.


photo

Visualization of model
experiments on wave run-up

The photo shows an example of the
visualized model experiments. The wave
run-up is reproduced by generating wind
and waves at the same time.
They are recorded on video so that
we can investigate the generation
and diffusion of the spray and other
factors in detail using image analysis.

ABSTRACT:
When wave overtopping becomes violent, a large amount of spray is generated by the collision of large waves against the front surfaces of breakwaters or seawalls, and the seawater spray is transported and diffused by strong winds. Consequently, in addition to the direct damage caused by wave overtopping, the salt damage (e.g. the corrosion of reinforced concrete structures, the withering of crops, etc.) is spread over a wide area. This violent wave activity is by no means an infrequent phenomenon. You can see a large amount of spray at coastal areas on any stormy day.

We are conducting field observations and model experiments to clarify the characteristics of spray diffusion caused by strong winds.

DETAILED:
Although breakwaters and seawalls are very effective at protecting coastal areas from severe waves, they often generate seawater spray when the waves collide with them. This aerosolized water spray diffuses and spreads to locations far from its point of creation due to onshore winds, and causes severe salt damage (e.g. the corrosion of reinforced concrete structures, the withering of crops, etc.) across wide areas. We do not yet have any effective fundamental measures to reduce this spray because many of the phenomena involved in spray from coastal structures, such as the mechanism of spray generation and transportation, are still unclear. In this study, field observations and hydraulic model experiments with a wind-wave flume are conducted in order to investigate the characteristics of generated seawater spray and, in the near future, to develop some techniques to reduce salt damage.

Field observations have been conducted at a small fishing port facing the Sea of Japan every winter since December 2005. In these observations, the waves, winds (velocity and direction), wave run-ups (height and frequency) and salinity concentrations of the spray have been measured. The fishing port has a vertical breakwater at the mouth of the port to keep the inside of the port calm. However, when large waves roll up against the breakwater violent wave run-ups can occur frequently, and a large quantity of the spray is transported landward by the strong wind. The residents in this area have a number of problems arising from salt damage. To cope with the salt damage, dissipation blocks were installed in front of the breakwater to reduce the generation of seawater spray in 2009. Thus, it was possible for us to conduct field observations of seawater spray with two different coastal structures at the same location, and the effects of the dissipation blocks were verified by the observed data.

At the same time, model experiments were conducted using a two-dimensional wind-wave flume (28m long, 0.5m deep, 0.3m wide) with the function of absorbing the reflected waves. The models of the structures and the bottom topography were based on the actual breakwater and bottom topography at the observation site, on a scale of 1/70. Generating waves and wind at the same time, the phenomena occurring in the actual field, including the wave run-ups and the splashes, were reproduced in the flume. With field observations alone, the mechanism of spray generation with large wave run-ups, the process of spray transportation, and the influences of winds and waves on the spray are not easily surmised. By using model experiments, all of these can be investigated in detail by using visualization techniques and image analysis.

photo

Run-up and spray transportation by wind
The photos show the process of spray transportation.
It is understood that a large wave run-up results in a large
amount of spray, and that the spray is transported
as a mist by a strong wind.


photo

Effect of dissipation blocks
Dissipation blocks prevent the large wave run-ups.
Consequently, the quantity of transported spray is
dramatically reduced.

photo

Two-dimensional wind-wave flume
Experiments are conducted using a 2-dimensional wind-wave flume (28m long, 0.5m deep, 0.3m wide) with the function of absorbing the reflected waves. The flume is partially composed of wide glass panes to assist observation.


Coastal and Ocean Engineering Laboratory, Disaster Risk Reduction Research Center
Engineering, Faculty of Engineering, Kyushu University
Professor Noriaki Hashimoto
Associate Professor Masaru Yamashiro
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