Environmental Engineering Reference
In-Depth Information
In the experiments, no step-pools could develop if there is no gravel larger than 1/8 times of the
channel width. Sediment with size distribution 1,2, or 3 in Fig. 3.31 were used to study the mechanism of
step-pool development and results show that only several step-pools developed with size distribution 3,
and continuous step-pool systems developed well with size distributions 1 and 2.
Incoming sediment load from the upstream entrance negatively affects the development of step-pools.
In a few experiments sediment was supplied to the flow from the entrance of the flume. The sediment
load reduced or even stopped the development of step-pools. In fact, the step-pool system develops as a
result of bed erosion. If the incoming sediment load is equal to or greater than the sediment-carrying
capacity of the flow, there is no bed erosion, and, hence, step-pools cannot develop. The same conclusion
has been obtained from field investigation in the Xiaojiang watershed on the Yunnan Plateau, China.
From the laboratory experiments and field investigations it has been learned that the development of
step-pools requires:
ķ
high channel bed slope, generally in the range of 3% 20%;
ĸ
small width/depth
ratio;
Ĺ
non-uniform sediment composition; and
ĺ
sediment-starved flow and channel bed erosion.
Analyzing the data the following empirical formula has been obtained to estimate the degree of
development of the step-pool system:
m
D
§
D
· §
g
·
S
S
max
b
1
bin
c
(3.7)
¨
¸ ¨
¸
P
D
g
©
¹ ©
¹
50
b
in which
D
max
is the maximum diameter of the bed materials
ˈ
g
bin
and
g
b
are the incoming bed load from
upstream and bed-load carrying capacity of the flow. The coefficient D the constant
b
and
c
, and the
exponent
m
may have different values in different streams and should be determined with data.
Experiments and field investigations have proved that the development of step-pool systems increases
the flow resistance, consumes the flow energy, and protects the streambed from erosion. The results of
flume experiments have extended this role in suggesting that step-pool systems not only increase flow
resistance but also maximize it (Whittaker and Jaeggi Martin, 1982; Abrahams et al., 1995). Their
innovative experiments and field observations led Abrahams et al. (1995) to conclude that step-pool
systems, evolve towards a state of maximum resistance because that implies maximum stability. Thus, an
explanation of why step-pool systems develop and why they have a particular morphology can be
couched in terms of their effect on energy dissipation. Moreover, step-pools may provide high diversity
of habitats for aquatic bio-communities. The flows over the steps adsorb oxygen from the air and
increase the concentration of dissolved oxygen, which is important for the aquatic ecosystem.The
experiments indicated that the resistance of step-pools is high for low flow depths and low for high flow
depths. If the flow depth is less than the height of the steps, the flow over the steps jumps into the pools
and causes turbulent eddies and a hydraulic jump. If the flow depth is high, the steps are submerged
under water, the flow skims over the steps, no hydraulic jumps result, and the energy dissipation is less
(Curran and Wohl, 2003). Following the increase in the relative depth the function of energy dissipation
of step-pools reduces and the resistance then becomes smaller and smaller. In most cases the flow depth
is less than the step height and hydraulic jumps occur in the pool sections. The function of energy
dissipation of step-pools is closely related to the hydraulic jumps. The flow is supercritical in the step
sections and becomes subcritical in the pool sections. The hydraulic jump makes the flow change from
supercritical to subcritical and during the jump the flow energy is dissipated. A lot of turbulent eddies are
produced and air bubbles are mixed with water in the jumps.
In summary, a step-pool system develops when the streambed undergoes erosion. If the flow power is
small, fine particles are removed and coarse particles remain and form an armor layer, which creates,
high grain resistance. If the flow power is high and all particles can be removed by the flow, the moving
particles form dunes, which generate form drag. If there are cobbles and large gravel, the largest particles
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