10:45 AM - 11:00 AM
[ACG45-07] Unit peak discharge increase with increasing catchment area in a high-relief catchment composed of accretionary sedimentary rocks
Keywords:High-relief mountain, peak unit discharge, permeable bedrock, radar-rain gauge analyzed precipitation, storm runoff
To improve the accuracy of flood-flow prediction, understanding the storm-flow generation at various scales in a catchment is essential. Flood-flow prediction is generally conducted assuming that peak unit discharge decreases with catchment area, based on previous observations. However, this assumption may not be applicable in some catchment. For instance, in catchment with permeable bedrock, flow path in bedrock can contribute largely to flood flow. In such a catchment, peak unit flow can increase with catchment area. However, this has not been confirmed due to the difficulty of flood-flow measurement in nested catchment, especially in high-relief mountains.
We measured discharge at three nested catchments CS (0.58 km2), CM (2.2 km2) and CL (94 km2) in the high-relief Chichibu Mountains, which were underlined by permeable sedimentary bedrock, and we tested the hypothesis that peak unit discharge and quick flow (or direct runoff) per unit area (hereafter referred to as unit quick flow) were larger in larger catchments. The influence of the spatially distributed rainfall on the differences in flood flow among catchments was evaluated based on radar-rain gauge analyzed precipitation data. We collected data for 69 storms, with total storm rainfall depth ranging from 26.5 to 231.5 mm and a return period ranging from 10-3 to 3.2 y. Flood flow varies depending on the storm magnitude and for each catchment. During significant floods with certain level of discharge increase, peak unit discharge and unit quick flow were always larger at larger catchment. Differences in peak unit discharge and unit quick flow among measured catchments were several times and sometimes more than an order of magnitude. While the “catchment mean storm total rainfall depth” for each catchment was greatest at the largest CL catchment but it was at most 1.26 times larger than that of the two smaller catchments, indicating that spatial variations in storm rainfall have little influence on the differences in flood flow among catchments. In our studied catchments, some rainfall that infiltrates bedrock on hillslopes move to downstream within the bedrock to generate storm flow in larger catchments. The results of our study and previous research indicate that this increasing unit flood flow with catchment area should be characteristic of catchments with permeable bedrock.
We measured discharge at three nested catchments CS (0.58 km2), CM (2.2 km2) and CL (94 km2) in the high-relief Chichibu Mountains, which were underlined by permeable sedimentary bedrock, and we tested the hypothesis that peak unit discharge and quick flow (or direct runoff) per unit area (hereafter referred to as unit quick flow) were larger in larger catchments. The influence of the spatially distributed rainfall on the differences in flood flow among catchments was evaluated based on radar-rain gauge analyzed precipitation data. We collected data for 69 storms, with total storm rainfall depth ranging from 26.5 to 231.5 mm and a return period ranging from 10-3 to 3.2 y. Flood flow varies depending on the storm magnitude and for each catchment. During significant floods with certain level of discharge increase, peak unit discharge and unit quick flow were always larger at larger catchment. Differences in peak unit discharge and unit quick flow among measured catchments were several times and sometimes more than an order of magnitude. While the “catchment mean storm total rainfall depth” for each catchment was greatest at the largest CL catchment but it was at most 1.26 times larger than that of the two smaller catchments, indicating that spatial variations in storm rainfall have little influence on the differences in flood flow among catchments. In our studied catchments, some rainfall that infiltrates bedrock on hillslopes move to downstream within the bedrock to generate storm flow in larger catchments. The results of our study and previous research indicate that this increasing unit flood flow with catchment area should be characteristic of catchments with permeable bedrock.