Many rivers are competent to erode both bed and banks during moderate flows. The extreme velocities associated with infrequent events are compensated for by their rarity, and it is found that the greatest bulk of sediment is transported by more moderate events. Transport of sand and dust by wind in general follows the same laws. In many basins 90 per cent of the sediment is removed by storm discharges which recur at least once every five years. As the variability of the flow increases and hence as the size of the drainage basin decreases, a larger percentage of the total load is carried by less frequent flows. Analysis of records of sediment transported by rivers indicates that the largest portion of the total load is carried by flows which occur on the average once or twice each year. The frequency at which this maximum occurs provides a measure of the level at which the largest portion of the total work is accomplished. Because the frequency distributions of the magnitudes of many natural events, such as floods, rainfall, and wind speeds, approximate log-normal distributions, the product of frequency and rate, a measure of the work performed by events having different frequencies and magnitudes will attain a maximum. For many processes, above the level of competence, the rate of movement of material can be expressed as a power function of some stress, as for example, shear stress. We also have a sponsorship program for business and corporate philanthropy.Ĭlick here to get more information about your business or organization becoming a sponsor.The relative importance in geomorphic processes of extreme or catastrophic events and more frequent events of smaller magnitude can be measured in terms of (1) the relative amounts of 'work' done on the landscape and (2) in terms of the formation of specific features of the landscape.

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