Reconciling Time with Sediments in the River

Abstract:

Fluvial sediment transport is constructed in different ways for differing time scales. For event time, we seek to understand the physics of sediment transport. Most effort has been expended on this problem, which directly leads to understanding of local changes in river channels and the creation of certain river habitat features. For regime time, it is more pertinent to ask for the sediment budget at reach scale, whence we may come to understand the morphodynamics of the reach. This is often the principal river management problem, though it has not often been recognized as such. On time scales of river and landscape evolution, the topographic profile of the river assumes primary importance and sediment supply from the land surface is the primary (and ultimate) driver of change. The relevant time scales, for river systems of intermediate size (10⁴km²), are of order 1a, 10a and 10⁴a. In view of these times, it is unlikely that any major river is in secular equilibrium along its entire course. Events across time scales are, of course, linked. River morphodynamical change is driven by major evolutionary effects on the river system, such as climate change, seismic events, or dam construction, that drive secular changes in sediment supply. On the other hand, computing the sediment budget requires event-scale input of information – derived from observations or physical calculations – at some points in the system. Accordingly, we attempt to make the links across scales by embedding our understanding of transport physics in numerical models in which events at one scale predict outcomes over longer time scales.