Measurement and Theory of Soil Creep

Although soil creep is widely thought to be an important agent in hillside development, its rate has rarely been measured. The role of creep therefore has been a subject for speculation, unsupported by evidence. In this paper, a theoretical model of soil creep is proposed, using soil mechanics principles. This model was tested in a laboratory study, which provided moderate support for the theory. Creep was also measured in two ways at each of fifty sites in Kirkcudbrightshire, southwest Scotland, over a period of 2 years. Soil moisture was shown to be significantly related to changes in soil moisture, and was significantly downhill, averaging 2. 1 cm^3/cm. year for slopes of which the median angle was 13°. The high variability between individual measurements, at least part of which is inherent in the process of soil creep, disguised the existence of possible correlations with slope, aspect, or soil type. Estimates of the relative magnitudes of soil creep due to various agents indicated that, for southwest Scotland, soil moisture, freeze-thaw, burrowing animals, and temperature changes were the most important, in that order. Measurements of soil wash showed that its average rate was not more than 0. 09 cm^3/cm. year, which is significantly less than the rate of soil creep. Thus the sum of creep and wash together, if acting at their present rates, would only have lowered the drainage basin by an average of one-third of an inch in the last 10, 000 years.