Essential ancillary data requirements for the validation of surrogate measurements of bedload: non-invasive bed material grain size and definitive measurements of bedload flux

Achieving a significant advance in understanding the sedimentary dynamics of rivers, especially those with coarse-grained beds, depends upon the acquisition of data that adequately reflect sediment flux. In a similar vein, the successful development of a functional relation between bedload flux and contemporary hydraulics that is transferable from river to river requires an understanding of relations between the immediate source of bedload – the river bed – and the flow. This would benefit from deployment of a quick but efficient method of assessing the grain-size distribution of river-bed material, since this is one of several determinants of bedload flux and is a property that can be readily established in a previously ungauged channel. This paper reminds hydraulic engineers and geomorphologists of the need to deploy a method of data capture that allows the performance of surrogate measures of bedload to be assessed adequately. In particular, it highlights the performance and short-comings of the Birkbeck Sampler. This is an automatic slot sampler that provides a continuous, direct and sensitive measure of bedload flux that is as definitive as is feasible in the complex confusion of a river in spate. It has been deployed in a wide range of river environments and has proved to be both reliable and durable, having provided bedload records on ephemeral channels for more than a decade in one case. The paper also indicates a need for the use of a method that facilitates rapid and frequent surveys of river-bed materials in order to understand bedload dynamics that are measured by any means, not least those that detect bedload surrogates. The Digital Gravelometer is described, along with its advantages and limitations. Time-savings alone make this a valuable addition to the river scientist’s tool-kit.