Quantification of mean transport rates requires procedures that account for the erratic nature of bedload movement. Additionally, the total sampling period must be long enough to measure a suitable temporal average. There are, however, no hard and fast rules as to the procedures that will accomplish this, only some rough guidelines. Composite Versus Separated Samples Bedload samples may be either separated by vertical or composited into a single sample. Separated samples are emptied after measuring each vertical and are useful for assessing the contribution of bedload from various points within the cross-section.
Composite samples consist of sediment collected from all verticals combined into one sample.
Typically a bedload sample is a composite sample collected at a cross-section, unless indicated otherwise. As may be expected, separated samples are more expensive and time consuming to collect and analyze. Because bedload is transported unevenly within the channel, it is necessary to sample the entire cross-section over a variety of flows to identify the position and stability of transport zones. The more verticals used, the better the estimate of the mean rate and the more detailed cross-sectional patterns become. There is, however, a tradeoff between adding verticals and minimizing the time required to sample a cross-section.