Environmental Engineering Reference
In-Depth Information
FIGURE 5.11
Cross-sectional diagram of a stream showing the riffle pool sequence, accu-
mulation of fine sediments, and water flow through the shallow subsurface (hyporheic).
exhibit the typical riffle and pool sequence. The riffle and pool sequences
can be tied to stream flow and meandering patterns.
A cross section of the straight portion of a stream shows that the wa-
ter velocity is often maximum in the center (termed the
thalweg
) and lower
near the sides and bottom (Fig. 5.10). In areas where the stream or river
is curved, the maximum velocity occurs nearer to the outside of the bend.
Furthermore, because the water on top is moving more rapidly than the
water on the bottom of the bend, the direction of flow tends to move
downward and cut into the outside bank. This higher velocity and down
cutting lead to erosion on the outside bend. The slower water velocity on
the inside of the bend allows deposition on the inside of the curve and for-
mation of a point bar (Fig. 5.10).
Streams
meander
(wander in “s” shaped patterns) unless they are con-
strained by outcrops or bedrock. Water flowing across the surfaces of glac-
iers, currents flowing in oceans, and rivers flowing into reservoirs or oceans
can all meander. The process is a self-organizing procedure that can be
characterized by the relatively new mathematical tools of fractal geometry
(Stølum, 1996). The process of meandering is characterized by erosion and
deposition, which exaggerate the meander over time (Fig. 5.10). Eventu-
ally, when the meander cuts itself off, an oxbow lake forms. Meander for-
mation occurs in similar fashion in rivers of all sizes; the wavelength (the
distance to meander out, back, out the other way, and back again) aver-
ages about 11 times the channel width, and the radius of curvature of a
channel bend is generally about one-fifth of the wavelength (Leopold,
1994). Thus, meander size is a function of discharge (Fig. 5.12).
In addition to meandering, some rivers also flow in a
braided
fashion.
This pattern generally occurs when water flows in a broad sheet across
noncohesive sediments. These channels are common in river deltas that en-
ter oceans and lakes and in sandy or gravel-filled valleys with relatively low
slope. Individual channels combine and split, form and disappear, some-
times over relatively short periods of time. Braiding is a basic physical
process that can be modeled using multiple cells where sediment is trans-
ported from one cell to the next (Murray and Paola, 1994).
Meandering over time and deposition of materials by a river leads
to a
floodplain
that is relatively flat across the river valley (Fig. 5.13A).
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