Listen to the sound of moving sediment in a small gravel-bed river

摘要

In order to assess the dynamics of rivers,a reliable characterization of bedload transport particularly during unsteady flow regimes is required.In contrast to highly energetic cases in hillslope areas,we aim to answer the question whether the usage of acoustic measurements can improve the characterization of bedioad in small rivers draining low land mountains with comparatively low water discharge and bedioad.In addition to the investigation of natural flood events,controlled floods were generated by releasing water from a reservoir into a small gravel-bed stream.The controlled releases allow for an evaluation of bedioad solely from channel storage or bank erosion.For acoustical in-situ characterization of bedioad transport,hydrophones were mounted onto the bottom side of steel plates,thus recording the impacts of sediments via the acoustic vibrations on the surface of the plates while at the same time minimizing the disturbing noise resulting from water turbulence.Corresponding bedioad traps are removable boxes with open lids fixed in the riverbed so that bedioad material registered by the hydrophone is trapped.The acoustic signals correlate well with the quantity of the transported material.During summer flood events the highest transport rates occur at the beginning of the rising limb featuring clockwise hysteresis.This is due to the rising transport energy of the flow and the presence of loose,unconsolidated material.During typical winter flood events bedioad shows anticlockwise loops.The intensification of bedioad conveyance after the runoff peak can be explained by a decreasing stability of the bed material from the beginning to the end of a transport event.Anticlockwise behavior also results from a combination of bedioad exhaustion in the vicinity of the monitoring station with a delayed arrival of new material from distal sources later in the hydrograph.