Author(s): Tetsuya Oishi; Shigeya Nagayama
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Abstract: Excavation of accumulated sediment in an inter-levee floodplain is undertaken to secure the capacity of flood control safety in the river. The purpose of our study is to investigate the dependence of vegetation shift on differences in initial conditions when excavation was conducted. In this paper, we analyzed in particular the relationship between changes in willows and the influence of excavation. Several methods were used for that purpose. More specifically, we analyzed vegetation changes for about 10 years after excavation in the Ibi River using vegetation maps and the height of the excavated face from river water surface level (hereinafter called"excavation height"). We used the GIS system for this analysis. As a result, we found that differences in excavation height have influenced changes in vegetation. In particular, the area of willows increased in many places, regardless of the excavation height. Even at higher ground level than the normal river surface level (mid water surface level), the area of willow plant communities increased by 50% in about 10 years. It is likely that the soil was wet because there was an underground water surface level near the excavation height. In conclusion, due to the relationship among the excavation height, the excavation timing, and the underground water surface level, the soil's water retention and the light conditions on the ground level were maintained, leading to the creation of a suitable environment for the germination and growth of willows. Based on these results, we propose that we should introduce high stem grass by burying their roots after excavation such that they grow faster than willows. Dominance of high stem grass will suppress willow growth by shading them.
Year: 2018