A new geophone device for understanding environmental impacts caused by gravel bedload during artificial floods

To assess the contribution of gravel bedload on the removal of attached‐algae and aquatic plants from a cobble‐bed river during small floods, we propose a geophone type method for measuring the local bedload of non‐uniform sized gravel. Due to limited peak discharge for focused events during our study, a large fraction of bed material (here cobbles) was immobile and only a small fraction of bed material (sand and gravel) was expected to be transported during the flushing flows we analyzed. The device we developed has a size equivalent to immobile bed material and a shape similar to bed material (rounded cobbles) at the site. The instrument's design allows avoidance of disturbances in river bed micro‐topography during installation and local bedload transport during floods. A flume experiment was conducted in order to establish an empirical algorithm for estimating the diameter of impacted gravel, and uncertainty related to diameter estimation is discussed. The proposed method was utilized to quantify gravel bedload in a cobble‐bed river during flushing flows. We also discuss the contribution of measured gravel bedload during flushing flows on the removal of attached‐algae (up to a 37% reduction in chlorophyll‐a density) and aquatic plants (a reduction of 38% in dry mass per area). Based on time variation for the measured gravel bedload, we also suggest the propagation of a bed‐form composed of the fine sediment fraction migrating on immobile larger sediment and implications for the propagation of the fine sediment wave for attached‐algae removal. Key Points We developed a geophone device and established empirical models for estimating the size of grains impinged onto the device We installed a number of geophone devices in a cobble‐bed river and analyzed gravel transport during artificial floods In the text, we discuss our results and the consequences of gravel bedload on the removal of attached‐algae and aquatic plants