Behavior Of Meander Loops. If the proposed bridge or roadway is located near a meander
loop, it is useful to have some insight into the probable way in which the loop will migrate or
develop, as well as its rate of growth. No two meanders will behave in exactly the same way,
but the meanders on a particular stream reach tend to conform to one of the several modes of
behavior illustrated in Figure 5.29.
Mode A (Figure 5.29) represents the typical development of a loop of low amplitude, which
decreases in radius as it extends slightly in a downstream direction. Mode B rarely occurs
unless meanders are confined by valley sides on a narrow floodplain, or are confined by
artificial levees. Well developed meanders on streams that have moderately unstable banks
are likely to follow Mode C. Mode D applies mainly to large loops on meandering or highly
meandering streams. The meander has become too large in relation to stream size and flow,
and secondary meanders develop along it, converting it to a compound loop. Mode E also
applies to meandering or highly meandering streams, usually of the equiwidth point-bar type.
The banks have been sufficiently stable for an elongated loop to form (without being cut off),
but the neck of the loop is gradually being closed and cutoff will eventually occur at the neck.
Modes F and G apply mainly to locally braided sinuous or meandering streams having unstable
banks. Loops are cut off by chutes that break diagonally or directly across the neck.
Effects Of Meander Cutoff. If cutoffs seem imminent at any meanders in the vicinity of a
proposed bridge crossing the probable effects of cutoff need to be considered. The local
increase in channel slope due to cutoff usually results in an increase in the growth rate of
adjoining meanders, and an increase in channel width at the point of cutoff. On a typical
wide-bend point-bar stream the effects of cutoff do not extend very far upstream or
downstream.
Figure 5.29. Modes of meander loop development. A, Extension. B, Translation. C, Rotation.
D, Conversion to a compound loop. E, Neck cutoff by closure. F, Diagonal cutoff
by chute. G, Neck cutoff by chute.
5.62
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