Studies of grain fragmentation in natural streams have the limitation that the full size range of the debris produced is virtually unobtainable. Experiments described here for grain fragmentation in a rotating drum permitted the study of all of the debris, and a fragmentation load technique was used to relate experimentally and naturally fragmented material. The present investigation has been focused on granitic quartz.Relatively gentle collective movement in water can cause significant fragmentation of coarse, nascent, granitic quartz grains. The debris produced by rotating in a drum a range of single sieve fractions, taken from gravel in the headwaters of a stream draining granite, had continuous size distributions down to (and probably beyond) 0.06 wm. Quartz was the dominant fragmentation product in all fractions down to 2 pm and present in finer fractions. When pebbles moved with sand in these experiments, breakage of the latter was greatly increased. In comparison with that of breakage, the effect of attrition on granitic quartz was negligible. At least a proportion of granitic quartz grains are subject to a fatigue effect as a result of impacts in water. Evidently they are thus progressively weakened prior to being broken. Size analysis of debris showed a significant break at 20 pm, suggesting some special production of quartz particles just below this size.Granitic quartz is criss-crossed with partially healed cracks acquired before the zone of weathering is reached. The wholesale breakage that affects it, particularly in pebbly streams, is largely due to the reopening of these cracks. Progressive fragmentation of this material must eventually reach a stage wherein grains comprising single original crack-bounded volume elements are produced. Such grains, lacking significant internal weaknesses, must strongly resist further breakage. Possibly the preferential production of quartz grains just below 20 pm in size may represent an accumulation of these single, crack-bounded volume elements.
Rainfall intensities of 45, 100, and 150 mm/hr with systematically varied kinetic energies were applied to a saturated noncohesive, sandy bed 3 m long and set at slopes of 0.5 and 5%. Detailed size analyses of solids discharged showed that the <31‐µm fraction was most readily mobilized and behaved as a suspended load; the 31‐ to 250‐µm fraction was transported slowly, much apparently as saltating bed load; the 0.25‐ to 4‐mm fraction was transported rapidly, grains tending to move as rolling bed load; the >4‐mm fraction behaved as a lag gravel. The sedimentary properties of bed deposits also reflected the differentiation of various size fractions and minerals in the original mixture. The effects of raindrop impacts within the flow were more important in promoting transport of solids than the aerial component of splash. Under conditions where overland flow had developed, transport of solids was related directly to rainfall intensity and variations in rainfall energy that were associated with variations in raindrop impact frequency. Increases in rainfall energy due to increasing raindrop sizes did not result in increases in solids discharged.
The 20-100 mm portion of a yellow podzolic soil (Albaqualf) from the Ginninderra Experiment Station (A.C.T.) was used in a rainfall simulator and flume facility to elucidate the interactions between raindrop impact, overland water flow and straw cover as they affect soil erosion. A replicated factorial design compared soil loss in splash and runoff from 50 and 100 mm h-1 rainfall, the equivalent of 100 mm h-1 overland flow, and 50 and 100 mm h-1 rainfall plus the equivalent of 100 mm h-' overland flow, all at 0, 40 and 80% straw cover on a 9% slope. As rainfall intensity increased, soil loss in splash and runoff increased. Within cover levels, the effect of added overland flow was to decrease splash but to increase total soil loss. This is due to an interaction between raindrops and runoff which produces a powerful detaching and transporting mechanism within the flow known as rain-flow transportation. Airsplash is reduced, in part, because of the changes in splash characteristics which accompany changes in depths of runoff water. Rain-flow transportation accounted for at least 64% of soil transport in the experiment and airsplash accounted for no more than 25% of soil transport The effects of rainfall, overland flow and cover treatments, rather than being additive, were found to correlate with a natural log transform of the soil loss data.
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