Tomohiro Naruoka

Comparative study of bedload sediment yield processes in small mountainous catchments covered by secondary and disturbed forests, western Japan

To clarify the sediment yield processes following a disturbance by a forest fire in a mountainous catchment, and considering the hydrological and geomorphological processes in the headwater, we measured bedload sediment yield at rainfall events in disturbed and secondary forest catchments in the western part of Japan. The three catchments were under different hydrogeological conditions. The IK, TB and TY catchments were disturbed by forest fires in 2000, 1994, and 1978, respectively. In the IK catchment, although runoff response to rainfall was fastest with high peak flows, the catchment also had the highest base flow. Moreover, the annual sediment yield there was about ten times as high as in the other two catchments, and it was found that there was a steep linear curve in the relationship between precipitation and bedload sediment yield. This is thought to be caused by overland flow generation following water repellency on the slopes, and by the accumulated sediment that forms the thick soil layer on the valley bottom. On the other hand, in the TB catchment runoff experienced high peak flows at rainfall events and low base flows, and there was a gradual linear curve in the precipitation–sediment yield relationship. This might be the result of there being a thin soil layer on the hillslope and on the valley bottom because of successive erosion after the fire. In the TY catchment, runoff had a low peak flow at rainfall events and a high base flow; and the bedload sediment yield increased exponentially with increasing precipitation. Therefore, sediment yield in the TB catchment was more than that in the TY during storm events with precipitation of less than 100 mm, whereas it was the opposite during heavier rainfalls. It indicates that there is a thick soil layer on the slope and a thin soil layer on the valley bottom in the TY catchment following the recovering of vegetation, and that the sediment yield process predominates only during big rainfall events, only then does subsurface flow generate.

Sediment Yield Variation in Small Catchments with Different Bedrock in a Humid Temperate Region

Sediment yield in mountainous catchments is closely associated with habitat stability of ecosystems in floodplain or valley bottom. The results of this study provide potential scenarios of future changes in sediment yield as a result of increasing rainfall due to climatic warming. We monitored stream discharge and sediment yield at four small catchments with different bedrock types and stream orders in a subalpine environment in central Japan. The results showed that bedload materials were yielded continuously in the granitic rock area, but only intermittently during the strong storm runoff in the sedimentary rock area. Our results indicate that future increase in the amount of bedload yield in sedimentary rock area will be proportional to rainfall in the granitic catchments, but may be variable in the sedimentary catchment depending on the sediment availability. The amount of bedload had no linear relation with catchment size. In the longterm, however, the impact on habitat stability can be substantial in both types of catchments, because annual amount of bedload will increase from year-to-year. Nevertheless, out results indicate that an improvement of the existing methods for predicting the trends of bedload yield is needed for the appropriate management and conservation of mountainous watersheds.