Author(s): Takashi Asaeda; Md H. Rashid; Ma Dong; F. M. Jamil Uddin
Linked Author(s): Takashi Asaeda
Keywords: Bed load; Experiment; Step-pool system; Flow energy; Mountain stream
Abstract: River channel forestation is an important issue considering both environmental and flood protection managements. There are several factors that are thought to induce the forestation; however, the real cause has not been elucidated yet. This study was aimed to introduce the most effective factors for the recent increase in vegetation coverage of the river floodplain in Japan. In aerial photo images taken in 2010, 1978 and 1946, 1 - 5 unmodified reaches were selected in each of totally 109 rivers, close to hydrological and water quality monitoring stations (180 sites in total). The area between levees, watered area, and coverage area of total and woody vegetation were measured. Then the fraction of total and woody vegetation coverage in the floodplain area (FVC and FWC) were calculated and investigated as a function of possible factors of river channel. Since the growth of vegetation is highly affected by the climatic characteristics and local population in the catchment, all rivers were classified into 13 climatic regions and Tokyo and Kansai metropolitan regions. In all regions, FVC values were 3 - 5 times larger than those of FWC, indicating that most of vegetation in the floodplain was herbaceous plants rather than woody plants. Construction of dams stabilizes the channel, which may have increased the vegetation coverage. Low FVC was not observed in the downstream reaches close to the dams. However, such trend was not observed in areas 2-3 km further downstream, indicating that dams affect only 2 - 3 km in distance. River modification for the flood protection, such as widening of channel and reduction of flood peaks may cause vegetation coverage. In 2010, FWC had a significant negative correlation with the ratio between the annual maximum hourly flood peak and the distance between levees of both sides. No high coverage was observed at sites with higher ratio. This indicates the significance of flushing of woody vegetation during floods and the possibility of widening of river channel in increase of woody vegetation coverage as the flood volume did not change much. FWC was low in metropolitan regions due to the tree harvesting for the flood protection. FVC, on the other hand, had higher correlation with the ratio of distance of levees and water surface width. This is because wider floodplain area, which is relatively less frequently inundated, is easily covered with herbaceous vegetation. Nitrogen concentration of river water, which had high correlation with soil nitrogen concentration of the floodplain, had a significant correlation with FVC. Sediment production in the catchment and the amount of gravel mining did not have high correlations with FVC and FWC. Historically, FVC has increased nearly homogeneously in all climatic regions, which corresponds to the increasing trend of the FVC of the catchment area.
Year: 2013