Changes of the material transport depending on the flood scale

摘要

Nutrients transported in rivers are important in terms of river ecosystems. Understanding of Nutrients behavior in the river environment is important in the maintenance and management of the rivers. Transport of Nutrients and other materials increases during floods. To better understand the influence of flooding on the river environment, this study aims to clarify the relationships between suspended solids (SS) and Nutrients, both of whose behaviors in the river water are not clear, and to estimate the amount of Nutrients deposited in the high water channel as a result of flood. The study area was the Saru River basin, and the study was conducted in three steps. First, toward understanding the differences in material transport in four floods (two rainfall floods and two snowmelt floods) observation data were examined. Second, equations were derived from the observation data in order to estimate the amounts of Nutrients deposited on the flood plain as a result of flooding events. Third, calculation for unsteady flow in a compound cross-section was performed to clarify how the amounts of Nutrients deposited on the flood plain differ by the flood scale. The following were found: (1) The volume of material supplied from the channel relates to the discharge, and we could hypothesize that when the discharge reaches a certain threshold, the material being transported reaches a saturation concentration. (2) From the estimation equation that we derived for Nutrients, we found a close correlation between the amount of silt deposition and the amount of water quality components in particulate form. (3) The calculations revealed that the influence of floods of different scale on the river environment varies by location.