Page 91 - Soil Degradation, Conservation and Remediation

78 3 Soil Erosion by Water

Fig. 3.6 Landslide on a hillslope (Photo courtesy of Dr. Animesh Biswas)

It is often dominant in smaller streams and the upper reaches of larger streams and
rivers. Mass failure occurs when large chunks of bank material become unstable and
topple into the stream or river (Fig. 3.7 ). Riverbank erosion can be accelerated by
lowering streambed, inundation of bank soils followed by rapid drops in water fl ow,
saturation of banks from off-stream sources, removal of protective vegetation from
stream banks, poor drainage, readily erodible material within the bank profi le, wave
action generated by boats, excessive sand and gravel extraction, and intense rainfall.

3.3.8 Effects of Water Erosion

Soil erosion may be either harmless (benignant) or harmful (malignant). In well-
vegetated forestlands, in pasturelands, and in level and mulched croplands, natural
erosion is low, gradual, and harmless. Here, the rate of soil loss is less than the rate
of soil formation. The estimated average soil erosion tolerance level ( T ) used in soil
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and water conservation planning in the USA is 11 Mg ha year . The T value is the
amount of soil erosion that does not significantly decrease soil productivity. The

specific rates of maximum tolerable limits of erosion vary with soil type (Blanco
and Lal 2008 ). A survey of soil loss at 70 sites throughout Western Australia com-
pleted as a part of a National Reconnaissance Survey has shown that erosion is
occurring at an unsustainable rate. Results indicated that about 10 % of the sites had
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soil losses in excess of 20 t ha year , about 30 % of the sites had soil losses of