FLOOD06514 - Laserfiche WebLink

Home Browse Search

Mechanllma 01 Slreambank Failure tit 3.4.1 Failures In COmposite Banks. Failure of a composite bank: depends on the nature and thickness of the individual layers and their relative locations. Nonnally, the sand/gravel in the lower sections is subjected to more npid surface erosion, causing the upper cohesive bank to be undercutln high composite banks, with a thick cohesive upper bank overlying a weaker layer. this may result in a rotational failure with the movement occurring on the interface between the two laYer!. Sometimes the thickness of individual layers is sufficient to cause part of the bank: to fail in a slab-like manner, in which case nonnal methods of analyses of plane failures m ilY be used. In low composite banks (< 2m high), the normal methods of analysis are inappropriate. The unde~utting can create a cantilever overhang. Three modes of failure of this cantilever block are possible. Of these, the shear-type failure is uncommon, but the other two, namely tension and beam-type failures, are frequently observed in such banks. Note that desiccation cracks propagate upwanls from the base of the cantilever section ailing the fissures, and the type of possible failure (i.e tension or beam) depends solely on the cantilever geometry, and not on the soil parameteIS. Methods have been developed (Chowdhury 1978) which consider tensile effects in the soil, to estimate the apparentlactor of safety of low composite banks against different failure modes. Such methods probably have more relevance to mmphological studies than engineering design, as any over-steep, undercutting bank: is inherently unstable, and tensile effects in the soil cannot be relied upon in the long tenn. 3.4.2 Removal 01 Debris From Mass Failure . As a result of mass failure, large volumes of materials are deposited on the lower part of the bank.:! the block or mass of material is broken up by the failure, then the individual particles !:an be entrained by the river. Cohesive material, especially if hound by a vegetative root mat, can remain intact In time the blocks themselves will be broken up by hoth w:athering and river flow. While the debris remains at the foot of the failure, the bank will be protected from further collapse. Subsequently, the material may be removed hy the river and the failure cycle can then recur. Alternatively, the debris may remain to form a cohesive layer in which vegetation may become established. In the latter case, the flow regime in the river will be affected, and this may trigger erosion elsewhere, leaving the fonner failure area relatively stable. 3.5 Other MeChanical Action. During the winter months, the banks of the river in northern regions are subjected to freeze-thaw cycles and frost heaving. This is a process where the uppermost layer of soil freezes, including the water in the pores. The ice layer grows with time pushing the overlayinlllayer of soil upward. As the soil thaws, it settles back toward its original position in a loosened state that is easily eroded. On an inclined streambank, when thawing occurs, the loosened soil can stump, slide, or fall down the bank into the water (Simons J 979). Groundwater, soil type, and bank aspect alt influence the rate of bank loss from this mechanism. e 22 Colored,) Erosion Control Manual