Page 89 - Soil Degradation, Conservation and Remediation

76 3 Soil Erosion by Water

Fig. 3.5 Gully erosion
(Photo Courtesy of
USDA-NRCS)

typical wash prevails with a marked backward or retrograde erosion and vertical or
depth erosion. Gullies have larger dimensions and their development is more com-
plicated. Besides retrograde and vertical erosion, lateral erosion also appears here,

together with accessory landslide, soil flow, and other phenomena. Gullies may
grow into gorges and canyons in high altitudes and very steep slopes. Gullies may
be flat, narrow, broad, and round. Flat forms occur mostly on shallow soil or in con-

nection with a specific lithic structure of the slope. In this form, characterized by a
broad V-section, lateral erosion prevails over vertical erosion. Narrow, acute forms
are created with a narrow V-section, the breadth of the gully usually being equal to
its depth or smaller. Broad gullies have a wide bottom and are U shaped. Here, lat-
eral erosion prevails over depth erosion. Active gullies maintain steep or even per-
pendicular sides. It happens frequently that recent forms replace older forms so that
their origin and age cannot be assessed from superficial observation. The main fea-

ture of gully erosion is the volume and velocity of water at the lowest level. The
energy of flowing water increases its cutting and smashing power and often results

in bank erosion.