(13) Experiments of the formation of tidal channels

From Stratodynamics

Authors

Haruka KUSE, Norihiro IZUMI, and Adriano Coutinho de LIMA

River & Watershed Engineering Laboratory Field Engineering for the Environment, Graduate School of Engineering Hokkaido University

E-mail : ajyasanjp0603@yahoo.co.jp

Abstract

Various characteristic landscapes are observed especially at river mouths by combined effects of tide, wave and river flow. Among a variety of landscapes, tidal flats and marshes formed by tidal flow have been known to have an important role from an environmental point of view. Because of landfill and reclamation of coasts, area of tidal flats and marshes is decreasing everywhere in the world. In order to ensure the preservation of tidal flats and marshes, it is important to understand the formation process of tidal flats and marshes. The formation of tidal flats often accompanies the formation of complex channel networks (tidal creeks), and therefore it is considered that tidal creeks has an important role for the formation and development of tidal flats. Though there have been some studies on the formation of tidal creeks, there are still a large number of things left unclarified. In this study, we investigated the formation process of tidal channels and the effect of unidirectional river flow on the formation of tidal channels from an experimental point of view. The experimental apparatus consists of a part of a tidal flat (slope covered with vinyl chloride powder) and a part of deep ocean (basin). The part of the tidal flat is 4 m long and 5 m wide. We used vinyl chloride powder instead of natural sand in order to facilitate the movement of sediment. Tidal flow is generated by a tide generator. The tide generator and a depth probe are installed in the basin. A variety of tidal periods and amplitudes are obtained in this apparatus. Unidirectional river flow can also be generated by pump installed in the basin and the upstream end of the tidal flat. Vinyl chloride powder as sediment can be transported by tidal flow and river flow. We performed three different cases of experiments. The first series of experiments corresponds to the formation of tidal channels only by oscillatory tidal flow, the second series to that only by unidirectional river flow, and the third series to that by both tidal and river flows. The landscapes observed on the surface composed of vinyl chloride powder are photographed every few minutes. In all the three experimental cases, we observed tidal channel networks to be formed on the tidal flat. In the first series of experiments with only oscillatory tidal flow, we found spacing between channels to be approximately 30 cm, and found channels to meander. In the second series with only unidirectional river flow, we found spacing between channels to be approximately 160 cm, and channels to be linear. In the third series with both tidal and river flows, we found that spacing between channels were approximately 70 cm, and that channels were found to meander. These results suggest that unidirectional river flow tends to make channels more straight, and tends to increase spacing between channels.