Sediment trapping characteristics of a pit trap and the Helley-Smith sampler in a cobble gravel bed river

[1] Although designed for granule gravels, the Helley-Smith (HS) bed load sampler is often used in streams with much coarser beds. To compare magnitudes and grain size distributions of sediment samples collected by a pit trap and a standard HS sampler, we conducted 22 "sampling events'' over a wide range of flows during a snowmelt freshet in a stream with a coarse gravel bed. Each sampling event consisted of three simultaneous measurements: one made with a pit trap installed in the bed, one with a HS sampler placed on the bed directly beside the pit trap (HS-I), and one with a HS sampler placed on the downstream rim of the pit trap (HS-P). We summed the catches of the pit trap and HS-P to estimate "actual'' bed load transport. The pit trap and HS-I catches were then compared to the summed sample. The pit trap yielded a remarkably consistent, positively skewed, sigmoidal distribution of catch efficiency for all 22 measurements, with near 100% efficiency for material larger than 2.8 mm. The HS-I sampler was more variable in its catch and trapping efficiency, exhibiting low trapping efficiency for midrange material (0.71 to 16 mm) but high efficiency for finer material. The results cast doubt on the accuracy of bed load data sets collected by Helley-Smith samplers in coarse gravel channels.