Sediment concentration was sampled in streams using standard methods described in U.S. Geological Survey publications (Guy and Norman, 1970) and in engineering literature (Vanoni, 1975). Depth-integration techniques were used to define the sediment concentration at vertical transit points in the cross section.
To obtain a representative sediment concentration for the entire cross section, samples are collected at a number of points across the stream by one of two weighting methods. The cross-section sediment samples discussed in this report were collected most often by the equal-discharge increment (EDI) method. The equal-width increment (EWI) method was used less frequently.
The EDI method was standardized for streams near Mount St. Helens to define sediment concentration at five individual increments of equal discharge, each representing 20 percent of the stream discharge. Sediment concentration for the stream was computed from an average of concentrations sampled at five increments.
The EWI method usually was applied at lower flows where a single bottle might be filled by sampling at several equally spaced points. This process continues until the entire cross section has been sampled. Sediment concentration is computed from the total sediment weight of the samples divided into the total volume of water collected in the samples and provides no lateral definition of concentration.
Cross-section samples of sediment concentration were collected during regular visits to gaging stations. Samples also were collected at a single fixed point in the cross section. At several streams, a U.S. D-49 or D-74 sampler was installed in a protective box at a fixed point on a bridge (fig. 9). Samples at the fixed point (referred to informally as "box samples") were collected repetitively during storm flows by field personnel. Local citizens also were employed by the U.S. Geological Survey to collect daily or twice daily suspended-sediment samples at gaging stations near their homes. Samples collected by local observers (referred to informally as "observer samples") are a common method for obtaining daily sediment concentrations. Concentration at the fixed point was compared with the cross-section concentration to derive coefficients for adjustment of the samples.
Particle-size distributions of suspended sediment were obtained by pipet analysis, visual-accumulation tubes, and wet sieving (Guy, 1969). The percentage of suspended sediment finer than 0.062 mm ("sand division") was routinely determined for most suspended-sediment samples. This percentage can be used to compute the concentration of sediment both finer and coarser than 0.062 mm. In this report, the two fractions are called "fine concentration," which includes silt and clay sizes, and "sand concentration," which includes all sand sizes up to 2 mm. Fine sediment is more likely to be transported at or near stream velocity than sand sediment (Allen, 1985, p. 129), which makes the division useful for sediment-transport studies.
Sediment concentrations and size analyses were entered into WATSTORE (National Water Data Storage and Retrieval System), which is the digital data repository for the U.S. Geological Survey. Size analyses were stored with averaged sediment concentrations and associated stream discharges.
Continuous operation of automatic pumping samplers eventually eliminated the need for observer samples and reduced the need for repetitive box samples during storm flows. As automatic sampling reduced the need for repetitive manual sampling, more measurements of stream discharge could be made during storm flow. Also, more cross-section samples of suspended sediment, bedload, and bed material were collected.
Fixed-point pumped samples, however, are not depth-integrated samples. The proportion of fine and sand concentration in a pumped sample does not represent the flow, and the sand concentration may be under-represented. Although the temporal pattern of concentration curves was well defined, the sand concentration in the stream could not be reliably estimated from the sand concentration of the automatic sample. Automatic samples were only analyzed for concentration, and sand and fine concentrations were defined by cross-section and box samples.