Channel bed adjustments following major aggradation in a steep headwater setting: findings from Oyabu Creek, Kyushu, Japan

摘要

A typhoon in 1993 induced major aggradation along Oyabu Creek, a steep, gravel bed mountain stream in Kyushu, Japan. Processes of sediment reworking are inferred from a 7-year monitoring program that measured adjustments to channel cross-sections, the longitudinal profile, and the extent/distribution of bedrock outcrops along a 3-km study reach. Over time, the reach adopted a riffle and pool structure, with notable increase in the area of exposed bedrock on the bed. This adjustment process was characterised by progressive reduction in sediment storage change per unit flow. The relaxation pathway following disturbance induced by the typhoon was shaped by the magnitude and frequency of subsequent rainfall events, the capacity of these events to transport available sediments, and physical linkages between reaches. Adjacent subreaches demonstrated differing relaxation pathways in response to these influences, induced by spatial and temporal variability in threshold conditions along the channel. Longer-term evidence indicates that responses to major disturbance, such as the 1993 typhoon, occur as 'cycles' of around 20-year duration. A relaxation period of 7 years is required to attain a quasi-equilibrium bed configuration and rate of sediment flux. The timeframe of cycles is considered to reflect changes to hillslope-channel bed coupling, marking the period required to generate sufficient sediment stores to reactivate phases of aggradation and subsequent degradation.