Japanese

Project name: Kurobe River
Place: Chiba pref. Japan
Company: ESPEC MIC Corporation

Background:
Kurobe River is a tributary of Tone River and the river is an artificial watercourse
The water is used as drinking and agriculture water. The artificial river is located at lowland then water velocity is very slow, therefore water pollution is a serious problem. Client is prefecture government and they wanted to purify the water by using wetland vegetation. Both sides of the river however were covered by sheet-pile revetment, thus there were no vegetation in river.

Solution:
Reed grass (Phragmites australis) was applied to their requirement by using pre-planted fiber rolls.

It was impossible to establish wetland vegetation along the river because there was no place to plant and water depth was about 2,5-3,0m. Then long piles (D180, L6500) were driven along river edges in order to create shallow wetland by retaining soil. Then pre-planted fiber rolls were installed. The length of the first installation was about 400m.

At first it looks easy to establish wetland vegetation however we can not find any vegetation in the site although well grown plants were installed. Therefore we started research so that we can find the reason why the plants were extinct. After the research we found two reasons. First is that the level of artificial wetland was settled down by compaction. Planed water depth of artificial wetland was about 0,5m but realized it was 1,0m. Second reason was grazing by Grass Carp (Ctenopharyngodon idellus).

We knew that there are so many Grass Carps in the river.

We had reported the results of our research to our clients and we made preventive measurement for the zone of 2000m.