Damage to ecosystems and threats to human health and the lack of effective mitigation require new approaches to mining regulation.

Twenty-eight coupled observations of flow, transport, and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted, bimodal grain size distribution. These observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface. The key to our observations is the use of a sediment in which each size fraction has been painted a different color, which permits reliable, repeatable, and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface. At a given flow, the fractional transport rates may be divided into two parts: a finer-grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate, and a coarser-grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run. We define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength. Complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction. Zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution, although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed. Critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude, a result strongly in contrast to many earlier observations, but consistent with our observations of incipient motion in sediments with bimodal grain size distributions. development of a general and consistent set of surface-based transport data. We present here many coupled observations of flow, transport, and bed surface size distribution from a series of flume runs at different flow strengths. The key to our observations is the use of an unusual sediment bed in which each size fraction has been painted a different color. This permits reliable, repeatable, and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface. Because our bed surface observations and direct scaling of transport with the bed surface texture are substantially different from previous work, we provide some emphasis in this paper on the methods used, our assessment of their accuracy and repeatability, and the degree to which our conclusions depend on the particular observations we make.Because the measurements we can make with the painted sediment are unlikely to be widely replicated, we attempt in this paper to organize our results in a fashion that may be readil...

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Abstract. We measured a wide range of transport rates for five different sand/gravel mixtures in a laboratory flume. Each mixture used the same gravel, and sand was added to produce mixtures containing 6, 15, 21, 27, and 34% sand. Control of other variables allows us to isolate the effect of bed sand content on transport. As sand content increases, gravel transport rates increase by orders of magnitude, even though the proportion of gravel in the bed decreases. The increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27%. The increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size, indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content. Bed surface grain size was measured at the end of each run. Surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate. This casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate. ] provides some indication that increased sand content can augment gravel transport rates beyond that which can be adequately accounted for in standard transport models. The purpose of the experiments described in this paper is to document the effect of river bed sand content on the transport rates of sand and gravel.We measured flow and transport in four series of flume runs using a gravel sediment to which sand was added to produce mixtures containing 6, 15, 21, and 27% sand.

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