Linking criteria for incipient motion to field-based measures of bed load transport capacity

Abstract:

Early studies of sediment transport, such as those of Gilbert (1914) and Shields (1936), laid the groundwork for countless other studies of bed load entrainment and transport. Gilbert and Shields emphasized somewhat different aspects of sediment transport in their writing, but they had similar objectives in experimenting with conditions that affect both incipient motion and bed load transport capacity. The problem of predicting incipient motion is thus inextricably linked to the problem of predicting bed load transport capacity. In this talk I will discuss field-based approaches for evaluating incipient motion at channel-length scales ranging from a few km to many 10s of km. In the best-case scenario, where time and equipment are available to sample the bed load, the most robust approach for determining incipient motion is to extrapolate from measurements taken over a range of flows, and find the shear stress corresponding to a low but measureable transport rate. Alternatively, if channel properties (width, depth, slope and grain size) can be measured at a sufficient number of locations (say, more than 20), the reference shear stress can be estimated by assuming it scales with the channel-forming (bankfull) shear stress. Another new approach, which makes use of repeat aerial LiDAR and records of daily discharge, is to develop a two-parameter sediment rating curve that produces the same sediment flux for a time series of daily flows as the sediment flux estimated from topographic differencing. Last, in situations where a qualitative (yes/no) assessment of sediment motion is sufficient (e.g. during a reservoir release), longitudinal variations in bed load transport intensity can be detected with acoustical sensors- hydrophones- mounted on a kayak or boat equipped with a global positioning system (GPS). Pros and cons of these different approaches will also be discussed.