Page 88 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 83

4.6.11 Alluvial Terraces
Terraces are formed when a mature river is rejuvenated, that is, when it again
becomes youthful as a result of lowering of its base level. This can occur when the
base level of harder rock is cut through or a dam breaks. Then instead of cutting
laterally, the river cuts downward, or entrenches. Meanders upstream then cut
deep scrolls in the former floodplain.

River entrenchment was world-wide during the Pleistocene glaciation as sea level
was lowered over 100 m as water was held as ice. Then as glaciers melted and sea
level rose, the deeply entrenched river valleys were drowned to become estuaries.
Rivers carrying glacial outwash filled their valleys with alluvial gravel and sand.
Glacial sand and gravel outwash deposits extend over 100 m (300 ft) below the
modern floodplain, indicating the extent to which sea level was lowered.

Deposits in alluvial terraces reflect their origin: for example, terraces of rivers that
did not carry glacial outwash may have approximately the same composition
as the modern floodplain, whereas those associated with glacial outwash mainly
contain sand and gravel. Older terraces that were formed prior to loess deposition
may be covered by wind-blown loess, discussed in a later section.

The youngest terraces may be so low that they are best seen from the ground by
observing slight differences in ground elevation. Low terraces also may be subject
to flooding, then being referred to as ‘‘second bottoms.’’

4.6.12 Meandering and Tributary Entrenchment
The position of river meanders on a floodplain also affects the base level of
tributary streams crossing the floodplain to enter the river. As meanders swing
from one side to the other, tributary base levels are alternately raised and lowered,
causing erosion and problems with bridges.

4.6.13 Braided Streams

Some streams are so loaded with sediment that there is no energy left over
for pattern meandering. They nevertheless can aggressively and randomly erode
their banks as channels are plugged and diverted by sediment. The channels
of a braided stream divide and recombine to enclose almond-shaped sand bars.
The channels are ever-changing because of local encounters with tree stumps
or boulders. There usually is one dominant channel that can shift during periods
of high water.
Two common occurrences of braided streams are: (1) in arid/semiarid areas where
there is a shortage of water, and (2) as previously indicated, as glacial outwash in
which case there is an excess of sediment, Fig. 4.9. In arid areas streams are
intermittent and may flow only briefly after a rain, but the rare heavy rain can

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