9.2.10 Site Selection
Most of the factors cited in the preceding sections have a bearing on the final site selection.
In summary, such factors as the form of the river, the alignment of the river, variations of the
river form over time, the type of bed and bank material, the hydrologic and hydraulic
characteristics of the river, and past, present and future watershed conditions are all
important inputs to the site selection. In addition, it is necessary to consider the requirements
of the area to be served and the economic and environmental factors that relate to the
crossing. Having made a detailed study of possible alternate sites, and having determined
the best site considering these important factors, one can then proceed with the
determination of the geometry and length of the approaches to the crossing, the type and
location of the abutments, the number and location of the piers, and the depth of structural
support for the piers to insure against danger from local scour. The location of the
longitudinal encroachment in the floodplain, the amount of allowable longitudinal
encroachment into the main channel, and the requirement for river training works all need to
be considered.
9.2.11 Channel Stability Investigations
In conjunction with the background information discussed in the preceding paragraphs, it is
essential to determine the need for bank stabilization. The location, selection, and design of
various types of river training works must be considered. The selection of training works is
significantly affected by the characteristics of the river and the river system itself. The
magnitude of local scour at the training structure must be considered. The possible
necessity of holding the river in a selected alignment must also be adequately explored. With
regard to these particular issues, the principles of Chapter 6 can be applied to develop
suitable designs for stabilizing the approaches and banks of the main channel, and the design
of training works that assist in controlling the alignment of the river relative to the crossing or
longitudinal encroachment. Hydraulic Engineering Circular (HEC) No. 23 (Lagasse et al.
2001) also provides experience, selection, and design guidance for a variety of river training
works and their applicability under a range of characteristics of the river environment.
9.2.12 Short-Term Response
Having completed the tentative design of the crossing or the encroachment based on river
form, channel geometry, hydrologic and hydraulic data; it is essential to consider the short-
term response of the river system to construction. Similarly, existing or proposed
developments up- and downstream of the site and at the site itself should also be considered.
The techniques that may be utilized to investigate the short-term response at the site or in
the vicinity of the crossing or encroachment involve the utilization of qualitative geomorphic
relationships followed by the application of more sophisticated analyses using the principles
presented in the chapters on open channel flow, sediment transport and river mechanics. As
discussed in Chapter 5, it is possible to establish a mathematical model designed to route
both water and sediment through the system. If this model is appropriately designed and
utilized, it is possible to evaluate the response of the river system to both the construction of
the crossing or encroachment and to other river development projects in the immediate area.
For example, it may be important to establish the pattern of clear-water releases from a dam
upstream of a crossing. Knowing the type of flow the channel would be subjected to and that
the water being released is clear, one can make an estimate of the extent of degradation in
9.5
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