Benefits of design modifications to the Birkbeck bedload sampler illustrated by flash-floods in an ephemeral gravel-bed channel

The benefits of three simple modifications to the design of a Birkbeck bedload slot-sampling system that has been continuously operating in Nahal Eshtemoa, Israel, since the early 1990s are demonstrated. The modifications include the deployment of a removable slot cover which delays the accumulation of sediment, so allowing sampling at late stages of a flood and, in conjunction with other samplers, extending the period of sampling during a flood wave; inclusion of a slot the size of which is adjustable so that that the probability of sampling the largest clast sizes in transit as bedload can be increased post-installation, once knowledge is gained about the bedload grain-size distribution; and a sampler side-wall door that allows stratification and textural changes within the accumulated bedload to be identified, so promoting intelligent sampling of the deposit for grain-size determination. Results from seven flash-floods are presented and discussed, with recommendations for bedload monitoring, particularly in rivers where sediment flux is high and dynamic sediment records are inevitably short because of instrumental limitations.