Abstract:
Sediment transport measurements on Flynn Creek, a headwater stream in the Oregon Coast Range, have illustrated the magnitude of
fluvial transfer processes, primarily of the bedload component, during a moderate storm runoff season (1979 water year). The total dissolved solids concentration of storm runoff averaged 40 mg/L, and was independent
of water discharge. Most particulate export from the 202 ha forested watershed occurred during the annual peak flow of 0.75 m³/sec-km² (2-year return interval). Suspended load was the most important transport mode, with a total yield of 5.7 x l0⁴ kg during the 24 h peak flow period. The export of coarse particulate organic matter
during the same period was 1.5 x 10³ kg.
Bedload discharge, as measured with vortex tube and Helley-Smith samplers, occurred in pulses of short duration that did not necessarily coincide with peak streamflow. For the same 24 h storm runoff period, the total bedload yield at the mouth of the watershed was only 2.6 x 10³ kg (particles > 0.25 mm), and consisted primarily of
sand-size material. Bedload discharge at an upstream site (1.3 x 10⁴ kg) was dominated by gravel-size particles. Channel morphology
and in-stream obstructions (organic debris, fish trap) appear to cause significant spatial and temporal variations in sediment transport and streambed composition. Although streamflow was the principal variable controlling sediment transport, results suggest that supply limitations exist. The
supply of transportable materials is dependent on the retention characteristics of the stream channel and on local hydraulic conditions. The discharge of both suspended solids and coarse particulate organic matter peaked early in the storm; however, pulses of the latter component on the recession limb appeared to be related to streambed
disturbances. Bedload movement is an important process regulating both bed composition and particulate yields in this Coast Range stream. The Helley-smith sampler was the preferable method for use in streams having a large fraction of bedload in the sand-size range. The vortex tube trapped from 60-70% of the bedload sediment measured by an upstream Helley-smith sampler, with trapping efficiency of the former increasing as transport increased. The variability in bedload movement suggests that monitoring designs must address: (1) rapid temporal fluctuations in bedload discharge, (2) variations in bedload transport along a stream, (3) lateral variations in bedload discharge at a cross section, and (4) the type of sampler used.
