Optimizing netting properties for unflared bedload samplers

Mountain streams transport a wide range of bedload particle sizes (sand to cobbles), and most studies like to collect as wide a range as possible. Large sampler opening sizes and coarse-mesh nets facilitate trapping coarse particles, but selecting a net to capture sandy and cobble bedload is problematic. This study experimented with various 0.5 - 4 mm nets, comparing depth and velocity of flow at the sampler entrance and analyzing the relationships between mesh opening size, thread width, and through-flow rates. Coarse nets with 3.6 mm mesh openings do not significantly hamper through- flow. But fine netting required to capture sand tends to decrease the through-flow rate, to retard the entrance velocity below ambient flow, and to compromise sampling efficiency. Mesh clogging by fine particulates further jeopardizes the through-flow rate. Flared sampler openings and short sampling times have been offered as mitigation (e.g. Helley-Smith); however, both invite other problems. For sampling a wide range of particle sizes, an optimum combination of oppos- ing netting attributes is required: fine mesh size, sturdy thread, and high through- flow rate. The ratio of opening size to thread width needs to be large for fine nets to accommodate water viscosity, requiring the thinnest possible thread. This be- comes less critical as mesh openings approach several mm. Other important net- ting attributes to evaluate are mesh shape, stiffness which makes for poor handling, abrasion resistance for longevity, and particle adhesion ("clinginess").