PARTIAL TRANSPORT OF A SAND GRAVEL SEDIMENT

Grains of a single size within a mixed-size bed are entrained over a r ange of flows. Within this range some grains exposed on the bed surfac e are active (entrained at least once over the duration of a transport event), while the remaining surface grains are immobile, a condition we define as partial transport. We demonstrate the existence and domai n of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted s and/gravel mixture. The active proportion of the bed surface increases with bed shear stress tau(0). At a given tau(0), 90% of the active gr ains are entrained when the cumulative mass transported exceeds approx imately 4 times the active mass on the bed. Mobilization of grains in a size fraction increases from 10% to 90% over a range of tau(0) of a factor of 2. The bounds of this range increase with grain size D-i so that at a given tau(0), sizes over a range of a factor of 4 are in a s tate of partial transport. Fractional transport rates are independent of D-i for fully mobilized fractions and decrease rapidly with D-i for partially mobile fractions. partial transport is associated with subs tantial transport rates of finer, fully mobile sizes, limits both the rate and size distribution of grain exchange with the bed subsurface, and may be the dominant transport condition in many gravel-bed rivers.