Environmental Engineering Reference
In-Depth Information
measurements is available near the gaging station (Rantz 1982 :5). It is usually
impossible to meet all the above conditions, and judgement must be exercised to
select an adequate location despite some shortcomings. For measuring outflow from
a wetland, conditions 6 and 7 can easily be satisfied by using the wetland itself as a
pool and measuring stage in the wetland near the outlet. However, controlling the
areal extent and density of vegetation in the outlet channel is often a challenge in
wetland settings.
Some of the conditions above can be improved by modifying a channel; for
example, reinforcing the bank or removing flow obstacles, without significantly
altering the habitat characteristics for aquatic life. The most accurate data are
obtained by installing an artificial control structure, but installation may require an
environmental impact assessment if the required channel modifications are extensive.
3.6.2 Characteristics of Flow Control
Some stream reaches are relatively straight for a long distance with constant slope,
channel geometry, and bed roughness. This situation, where the control on the
upstream stage-discharge relation is the channel itself, is called channel control.
Other flow-control settings include a place where the stream flows across bedrock, a
reach where the channel narrows, or the point beyond which the downstream stream
reach steepens (i.e., the upstream end of a riffle). Some artificial structures that are not
designed specifically to be a flow control, such as a bridge or a culvert under a road,
may serve as a good flow control. Two attributes of a satisfactory flow control are
stability and sensitivity (Rantz 1982 :11). If a control is stable, then the stage-discharge
rating curve does not require frequent adjustment. For example, a constriction
provided by rock-ledge outcrop is not affected by flood events (stable), but upstream
boulders and gravel may move during floods (unstable). Regarding sensitivity, a
control section ideally should have a relatively narrow width at low discharge condi-
tion so that a small change in discharge is reflected by a significant change in stage.
If natural conditions do not provide adequate stability and sensitivity, artificial
control should be considered. In natural streams having a wide range of discharge
conditions, it is common to use broad-crested weirs (Fig. 3.11a ) that conform to the
general shape and height of the streambed (Rantz 1982 :12). It is generally impractical
to build the control high enough to avoid submergence at high discharges. Therefore,
broad-crested weirs are effective for low to medium discharge only. In canals and
drains where the range of discharge is limited, thin-plate weirs (Fig. 3.11b )orflumes
(Fig. 3.11c ) may be used to cover the complete range of discharge. Thin-plate weirs are
suitable for channels in which the flow has relatively low sediment load and the banks
are high enough to accommodate the increase in stage (backwater) upstream of the
weir. Flumes are largely self-cleaning and can be used in channels with high sediment
load, and do not cause significant backwater. However, flumes are generally more
costly to build than weirs (Rantz 1982 :13). Other types of control structures include
weirs with moving gates installed in canals, commonly referred to as “head gates”.
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