Slope Indicator Measurements of Subsurface Movement in Gully Walls

Abstract

ABSTRACT -.SLOPE indicator access tubes were installed in gully banks along an actively eroding channel and upstream of an actively advancing gully headcut in western Iowa. Deflection of these aluminum access tubes due to soil movement was measured with a slope indicator probe capable of sensing small horizontal displacement. Surface soil movement adjacent to the channel was toward the channel; deflections of the tubes at greater depths were generally toward the channel and indicated slab or toppling failure. Bank failure was related to high peak flows and passing channel scarps that decreased bank stability, especially during spring runoff. All recorded bank failures showed greatest displacement at the surface with most of the movement occurring above a depth of 3 to 4 m (10 to 13 ft). At distances of 6 and 10 m (19 and 33 ft) from the channel most soil movement occurred at depths of 3 to 7 m (10 to 24 ft) rather than at the surface. All failure depths were less than the gully depth, while soil movement exceeded gully depth. This indicated that access tubes should be installed as much as 5 m (16 ft) below the channel bottom to record complete movement. Results indicate that this procedure can be used to quantify bank conditions leading to faillure.