The effect of stress history on fluctuation of bedload transport rate and bed topography in gravel-bed streams

Mountainous gravel-bed stream characterized by large-scale roughness generally goes through the seasonal flow, including flood and interevent periods. Recent studies suggest the effect of the interevent period (termed "stress history") is significant for the bedload incipient motion and bed topography evolution. However, the research on bedload transport fluctuation and protruding microtopography (a fundamental element in the riverbed) under the effect of stress history is limited, especially for quantitative description. As such, this study evaluated experimentally the effect of stress history on bedload transport and bed topography in gravel-bed streams. The long-duration interevent period restrains the variation of bedload transport fluctuation by decreasing the microtopography spatial density, while the large-magnitude interevent period enhances the variation of bedload transport fluctuation through the increasing spatial density. Meanwhile, the grain distribution of transported bedload is significantly affected by the magnitude of the interevent period, rather than the duration. Bed topography becomes smoother through an intermittent adjustment of structure-scale bed topography in the interevent period. Moreover, the decreasing bedload cumulative transport rate and lower variation coefficient of bedload transport fluctuation after short-duration and small-magnitude interevent period are attributed to the stable riverbed with the decreasing microtopography spatial density of less flow drag. An assumption of the larger rate and higher fluctuation after the long-duration and middle/large-magnitude interevent period is proved that the magnitude of the interevent period can promote the microtopography spatial density to generate large flow drag and carry coarse particles of microtopography to randomly hit the bed to generate abundant sediment ejection and transport. Additionally, the relations among microtopography spatial density, charac-teristic grain sizes, and statistical parameters of bed topography at the different scales are further explored, indicating bed topography at the grain-scale has more relation with bed topography.