Flow structure through groins in the Kiso River during a flood event

8^th International Conference of Hydro-Science and Engineering, Nagoya, Sept., 2008.

Ryota Tsubaki*1, Tetsuro Tsujimoto*2 and Hisashi Furuhata*3

*1 Researcher, Department of Civil Engineering, Nagoya University

Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan, e-mail: r.tsubaki@civil.nagoya-u.ac.jp

*2 Professor, Department of Civil Engineering, Nagoya University

Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan, e-mail: ttsujimoto@genv.nagoya-u.ac.jp

*3 PhD student, Department of Civil Engineering, Nagoya University

Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan, e-mail: furuhata.hisashi@h.mbox.nagoya-u.ac.jp

ABSTRACT

In the downstream segment of the Kiso River, the groins were constructed about one hundred

years ago. Since the installation of these groins, the back marsh, which was frequently inundated

and is now used for cultivation and human habitation, has been successfully protected

from the inundation. Over the last fifty years, the flow regime and the dynamics of the sediment

have been changed by the human activities, and altered the bed topography. Sediment has been

deposited around the groins, the accumulated sediment has changed the topography around the

groins, and an embayment has formed around the groins. In this study, a one-dimensional flow

model and a horizontal two-dimensional flow model are used to investigate the flow structure

around the groins, and the stability of the characteristic topography around the groins during

a flood is investigated. The results show that during the medium water stage of the flood, the

flow meanders with following the low-flow channel topography (bars), and the densely vegetated

regions in the groin section are associated with the flow structure during the medium water

stage periods. Then, in the peak discharge period, the flow shows a larger structure, following

the meandering of the river course, and the small channels in the groin section is parallel to the

flow pattern in the peak period. This means that the ribbed topography is formed during the

peak discharge period. Finally, from the result of the movable-bed calculation, it is found that

the topography in the groin section is stable because the intense flow resistance due to dense

vegetation reduces a velocity of the flow and the bed shear stress in the groin section , hence the

topography of the embayment formed in the groin section is stable due to the flow regulation

function, one of the ecological functions, provided by the riparian vegetation.

Keywords

groin, spur dike, embayment, flow structure, flood, river environment.