Page 89 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
84 Geotechnical Engineering

Figure 4.9
A braided
stream carrying
glacial outwash
and a source for
loess. A dust
cloud is
silhouetted against
the mountain.
Matanuska River,
Alaska.

create a ‘‘flash flood’’ and channel instability that washes out roads and bridges.
Heavy rains therefore can create extremely hazardous driving conditions in the
desert, particularly at night when vision is obscured.

4.6.14 Deposits from Braided Streams
Braided stream gradients and flow rates generally are considerably higher than
those for meandering streams of a comparable size, which is consistent with their
higher load-carrying capability. The high gradient and flow rate keeps small
particles in suspension, so deposits are mainly coarse-grained, sand and gravel.
The erratic shifting of channels creates uneven lenses and beds that are
cross-bedded as one channel cuts across another that has been filled with
sediment, as illustrated in Fig. 4.10.

Each cycle of deposition following a decline of high water involves a gradually
decreasing flow rate, so large particles are deposited first, followed by progres-
sively finer particles. A single bed therefore may contain gravel at the bottom,
grading upward into finer gravel, sand, and finally silt, clay being washed out. The
bottom-to-top, coarse-to-fine transition is called graded bedding. Gravel layers can
indicate how deep the river has scoured during past periods of high water, which is
important for the design of bridge pier foundations, as scour can extend even
deeper because of diversion of current and a concentrated flow.

4.6.15 Silt and Wind Erosion
During waning river stages the water velocity may slow down sufficiently to
deposit silt on top of sand bars of both braided and meandering streams. In
braided streams the lack of protective vegetation and large exposed areas are

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