Fundamentals of Engineering Design
cohesive layers is usually distinct and well defined (Thorne, 1981, p. 460). The number of layers present
should be noted as well as the thickness of the layers.
Tension cracks and crack depth note the presence of tension cracks. Generally, tension cracks
develop vertically down the bank face on steep banks and significantly reduce the stability of a bank with
respect to mass failure. The width and depth of the crack should be recorded.
Structure type describes the presence and type of any structure in the study reach.
Structure condition describes the current condition of the structure. A stable designation indicates
a structure that is functioning as designed and is not being undermined or destroyed by unstable fluvial
processes. Marginal describes a structure that is not completely functioning as designed or is degrading or
near failure. Failed characterizes a structure that is not performing as designed or has failed due to some
adjustment in the fluvial system.
Observed problems identify the type of problems related to the structure and the failure to perform
acceptably.
Bank vegetation describes the type, condition, and location of vegetation. The general types of
vegetation prevalent in and along the streams are important for determining the overall bank stability, the
rate of bank shifting, the erodibility of the banks, and the resistance to flows. The existence of vegetation
on the bank can serve as an indicator of bank stability.
Vegetation broadly classifies the types of vegetation along the bank.
Tree types describe the different types of trees along the bank. Different tree types affect the bank
stability in different ways. Conifers are shallow rooted and lack a thick vegetative cover compared to
deciduous trees. Leaning trees are an excellent indication of an upcoming mass bank failure and the angle
should be noted. High water level can be estimated as the level at which tree growth begins.
Health describes the condition of the vegetation. Dead or dying vegetation can be a serious liability
to bank stability.
Roots describe the relationship between the vegetation roots and the bank surface. If the bank
surface is relatively stable, the roots are normally found just below the surface. If sediment is accumulating
on the bank, vegetation produces adventitious roots into the new sediment. If the bank is eroding, roots
are exposed. If the erosion is rapid, the roots are standing straight out of the bank face, while if the erosion
is gradual, the roots often turn and grow back into the soil.
Height is an important factor in determining the effects of vegetation on impeding near bank flows.
Tall vegetation encourages sedimentation while it reduces conveyance. Note the height of the average
vegetation.
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