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Backwater Effects on Waterway Openings

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2.10.2 Backwater Effects on Waterway Openings
It is necessary to distinguish between the following types of backwater effects.
Backwater on a floodplain resulting from construction of a long, skewed or curved road
embankment as sketched in Figure 2.38a, where the bridge opening is, in effect, located
up-valley from one end of the embankment. The backwater effect along the embankment
arises from ponding of water along a line running obliquely down-valley. In the case of steep
rivers with wide floodplains this effect can be very large, since a large pond is created. This
type of effect can be prevented by choosing a suitable location and alignment, or by providing
dikes (shown on the figure) to close off the affected part of the floodplain from flood waters,
or possibly by providing a relief span.
Backwater in an incised river channel without substantial overbank flow, resulting in part from
constriction of flow through an opening somewhat smaller than the natural cross section, and
in part from obstructive effects of piers (Figure 2.38b). The backwater effect arising from this
type is seldom large, but may be significant in developed areas.
Backwater in a river with floodplain where the road crossing is more or less normal to the
valley but the road approaches block off overbank flow (Figure 2.38c). In these cases the
backwater may be significantly greater than in type b. The effect of guide banks shown in
Figure 2.38c is to reduce the backwater effects by improving the hydraulic efficiency of the
opening.
It is advisable to be aware of other unusual backwater effects that might occur in special
circumstances, although they might never arise in ordinary bridge design practice.
2.10.3 Effects of a Submerged Superstructure
If the high-water level reaches the bottom of the superstructure, the bridge will act as a short
culvert. For bridges which are designed to be submersible under certain conditions, it is
advisable to provide a rounded nosing on the leading edge of the girder, in order to improve the
hydraulic efficiency and to reduce the tendency to catch driftwood and ice as illustrated in Figure
2.39. Also, the superstructure must be anchored to counter buoyancy.
2.10.4 Effects of Supercritical Flow
In contrast to the usual drop at a constriction in subcritical flow, in supercritical flow water levels
may rise suddenly at the contracted section. The phenomenon of "choking" is particularly likely if
the Froude number only slightly exceeds 1.0. "Choking" may occur even in subcritical flow if the
constriction is severe enough. Wider or additional openings should be designed if choking
effects are expected to occur.
2.10.5 Types of Flow in Bridge Openings
Three types of flow, I through III, illustrated in Figure 2.40 are often encountered in bridge
waterway design. As the scale of the normal depth is the same for all flow profiles, the
discharge, boundary roughness and slope of the channel must increase from Type I to Type II
and to Type III.
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