Raindrop-stimulated transportation in shallow water flows: An experimental study

Abstract

Subaqueous transportation of a poorly sorted sand by the combined action of shallow (<10 mm) water flows and falling water drops was investigated experimentally. Transportational effects of raindrops impacting shallow flows and of overland flows, acting alone, were compared over a large range of slopes (0.001–0.30). Raindrops, impacting flowing water shallow enough to allow bed particle disturbance, have transportational effects analogous to those of turbulence fluctuations in deeper water, causing suspension, saltation and contact load movement. We call this rain-stimulated, dominantly downslope, movement of solids ‘rain-flow transportation’. Bed-load sediments, texturally almost identical with many bed-load deposits of unimpacted water currents, are built from rain-flow transported particles. Rain-flow transportation, effective on slopes at least as low as 0.001, can operate in flows less than a millimetre deep, can proceed in both supercritical and subcritical flows and is able to transport quartz particles up to about 3 mm in diameter. On low slopes, raindrops impacting shallow water, tend to suppress channel formation and to promote sheet flow. In our experiments, rain-flow transportation moved particles downslope much more effectively than did rainsplash. For materials containing medium and coarse sand, the transporting capacity of unimpacted thin water flows is very low at slope angles of less than a degree but, as slope increases, transporting power rises very rapidly, reaching enormous values by slopes of 0.2–0.3 (11–16°C). In contrast, rain-flow transportation is relatively slope-insensitive and can operate at very low slopes. Consequently, it is relatively important at low slopes but becomes, in effect, over-shadowed by the independent transporting action of overland flows on medium and steep slopes. Some natural, sandy, sheet-like deposits, radiating from topographically high points but inclined at very low angles, may owe their origins, in part, to rain-flow transportation.