Field Measurements of Bed-Load Transport Distances Using Painted Sediment Tracers in an Urban Stream in the Missouri Ozarks

Predictions of bed-load mobility and transport in stream channels are useful for restoration and management purposes. This study uses native gravel tracers to determine transport distances for bed-load in an urban stream in the Ozark Highlands. The objectives of this project are to: (i) determine downstream transport distances of painted tracers of different sizes over a range of flow conditions; (ii) evaluate the influence of channel morphology and thalweg location on transport; and (iii) compare field results to those predicted by mobility equations. The study site is located on South Creek, which drains Springfield, Missouri. The study reach is 132 m long and averages 5.8 m wide with a confining bank height of 1.5 m. A USGS discharge gage (#07052120, drainage area = 27.2 km2) is located 80 m above the study reach. Painted tracers of four sizes were released at pool, glide, and riffle locations along the channel bed with intermediate diameters based on pebble count survey results as follows: D50, 16-22.6 mm; D75, 22.6-32 mm; D84, 32-45 mm; and D90, 45-64 mm. As expected, higher flows resulted in a higher percent and larger distance of tracer movement. After a 7-year recurrence interval flood, 71% of tracers were either carried out of the study reach or buried in bar deposits. However, no tracers were found downstream with some later found after other flow events, suggesting that the tracers were most likely buried in bar deposits either within the study reach or beyond it. Overall, the riffle location and mid-channel deployment points produced the highest percentage of sediment mobility. Annual transport distances were estimated as follows: D50, 306 m; D75, 170 m; D84, 87 m; and D90, 39 m. Observed mobility trends validated the use of Shield’s and Wolman’s equations for the estimation of bed- load mobility and transport