channel. For most rivers, if the appropriate hydraulic and hydrologic data are available, it is
possible to develop simple relations showing how width and depth vary with discharge.
9.2.5 Hydrologic Data
It is necessary to gather all of the hydrologic data pertinent to the behavior of the river and to
the design of the river crossing or encroachment. As pointed out in Chapter 8, records of the
flood flows are essential. From such information, flow duration curves can be developed,
seasonal variations in the river system can be considered, and design discharge values can
be established depending upon the discharge frequency criteria used in the design.
Also, it is important to consider the low flows that the river channel will be subjected to and the
possible changes in flow conditions that may be imposed on the river system as a
consequence of water resources development in the area. Sometimes low flows may lead to
a more severe local scour situation at bridge piers and footings. Finally, in terms of
hydrologic data it is usually necessary to synthesize some of the required data. Conventional
techniques may be used to fill in missing records or it may be essential to synthesize records
where limited hydrologic data exist. In synthesizing data it is very important to compare the
particular watershed with other watersheds having similar characteristics. With this
information, reasonably good estimates of what can be anticipated at the site can be
established.
9.2.6 Hydraulic Data
At the site of a crossing or a longitudinal encroachment, it is essential to know the discharge
and its variation over time. Coupled with this, it is necessary to know the velocity distribution
across the river cross-section and its variation in the river system. This involves determining
the type of velocity distribution across the channel as well as in the vertical. Knowledge of the
distribution of velocities should be coupled with a study of changes in position of the thalweg
to estimate the severity of attack that may occur along the river banks and in the vicinity of the
crossing. Furthermore, it is essential to develop stage-discharge relations since these
relations fix key elevations of the structure in design and serve as bench-mark data when
considering channel protection measures that may alter the stage of the river. Large changes
in velocity can occur in a river system with changing discharge and stage. In a sandbed river,
as flow conditions bring about a transition from lower regime to upper regime, the average
velocity in the cross section may actually double. From another viewpoint, changes induced
in the river system, such as those due to artificial cutoffs or channelization, may sufficiently
steepen the gradient so the river operates in upper regime over its whole range of discharge.
These possibilities must be considered in the detailed design.
9.2.7 Characteristics of the Watershed
The water flowing in the river system and the sediment transported with the flow are usually
intimately related to the watershed feeding the river system. Consequently, one needs to
study the watershed considering its geology, geometry and land use. In the case of
development, land uses include recreation, industrial development, urbanization, flood control,
agriculture, and grazing. Similarly, one needs to consider the vegetation cover on the
watershed and the watershed response to changes in vegetation cover by human activities or
by climatic changes.
9.3
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