Effects of Landward Slope Protection and Toe Protection Works Shape at Coastal Dikes on Landward Bed Scouring caused by Tsunami Overflow

Tomihiro Iiboshi, Shiro Maeno, Keisuke Yoshida, Daisuke Takata, Akira Yamamura

Wednesday 1 july 2015

11:15 - 11:30h at Africa (level 0)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 9F. Engineering - River

Coastal dikes in Iwate, Miyagi, Fukushima prefectures in Japan were severely damaged by tsunami consequent to the 2011 off-the-Pacific-coast of Tohoku Earthquake. One of the main factors of the failure was landward toe bed scouring caused by tsunami overflow. Coastal dikes were destroyed completely or partially because of embankment material outflow caused by scouring of landward toe bed. Based on this severe damage, the tsunami committee in Japan recommended several methods to produce resilient structures for coastal dikes. One of these methods is landward toe protection works which can reduce the scouring of landward bed, and can improve the stability of coastal dikes. In recent years, the relations between the scouring characteristics and flow field near landward toe bed were clarified by several investigations. In these studies, blocks with flat surface were used for the protection works. However, in designing the protection works, a variety of the shapes of protection works were required considering the landscape and accessibility. Therefore, the relation between a shape of protection works and scouring characteristics is extremely important knowledge. So, we carried out small scale tests to clarify the effect of a shape of landward slope and toe protection works on the landward bed scouring. With respect to the slope protection works, large flat blocks and stair blocks were used. As for the landward toe protection works, three types of blocks were used. Two of them are large and small flat blocks and the other is blocks with protuberance. For the experiment, a combination the landward slope protection and toe protection works were investigated. The obtained results show that a large scouring hole was formed around the landward toe area in any shape of slope protection works without bed protection works. Landward bed protection works have an effect to reduce the size of scouring hole, and surface protuberance of landward toe protection works is more effective to reduce the scouring depth and area.