Particle transport by continental water flows in relation to erosion, deposition, soils, and human activities

Abstract

Field and laboratory studies, and the results of an experiment, combine to show that erosional, transportational and depositional processes in shallow, overland flows are basically the same as those of streams. Thus essentially fluviatile processes can operate, ephemerally, over entire landscapes. Separation into behaviourally distinct suspended and bed loads occurs even in flows a millimetre deep. Normally, suspended-load transport rate is limited by detachability or availability, whereas bed-load transport rate is governed by highly slope-sensitive capacity limits. Because most slopes must ultimately decline in gradient downhill and because bedload capacity falls rapidly with decreasing slope, 'hydraulic mantles', deposits built of excess bed load, are almost universal features around the bases of hills. Most natural slopes are consequently divisible into an upper zone of net erosion and a lower zone of net deposition. In these depositional zones, solid-fluid interactions exert major control over their own physical environments by adjusting such parameters as slope, depth, and velocity. Consequently, given a supply of heterogeneous detritus, an extremely consistent succession of sediment types differentiates downhill, with distally decreasing surface slopes, as transporting power and turbulence decline. This succession is represented in the well-known catenary sequence of Milne (1936a, b) and also occurs, with little modification, in rivers. Overland flow transportation is strongly inhibited by dense plant cover. Thus, whereas landscapes with sparse vegetative cover can maintain hydraulic adjustment to overland flows, heavily vegetated landscapes are less able to make such adjustments and tend to remain protected from erosion, often for long periods. During such periods tectonic movements may modify slopes, pedogenesis may alter textures, or deposition of fine material may result from the dissipation of fluid energy by plant cover. A backlog of needed hydraulic adjustments is thus built up and sudden baring of such long-protected surfaces can lead to drastic readjustment by surface flows. Much of the more severe human-activated water erosion evidently results from the baring of surfaces in such naturally disequilibrated states. Many hillslope soil materials must have developed on bed-load sediments. Some examples have been shown to have this origin. Fine organic and inorganic materials, with which soil fertility is closely associated, are, if entrained by flowing water, maintained largely in suspension in steeply sloping hillside environments. However, where turbulence has waned sufficiently, probably from a stage in which the bed surface grains are first immersed in the viscous sublayer, redeposition of these particles takes place, often in high concentration. This evidently happens on footslopes and many floodplains and is part of a natural system of fertility transference and renewal. Diversion by man of natural flows and the building of dams interfere with this natural process and can therefore have a long-term detrimental effect on natural soil fertility.