the channel, the amount of sediment derived from the bed and bank, the instability of the
banks and even the types of lateral shifting that may be induced in the river system as it
affects the crossing or encroachment.
9.2.13 Long-Term Response
The long-term river response at a crossing or a longitudinal encroachment and in the river
system itself should be considered based on all river development projects including the
highway. This type of treatment is, in general, beyond the scope of this particular manual.
Nevertheless, significant advances have been made pertaining to the mathematical modeling
of river systems, considering both their short and long-term response. Mathematical
modeling can be time consuming and expensive, requiring a substantial amount of additional
data for calibration of the model. However, this approach is worth considering on important
projects where determining long-term response may be critical to project success (see
Section 5.6).
9.3 PROCEDURE FOR EVALUATION AND DESIGN OF RIVER CROSSINGS AND
ENCROACHMENTS
This section presents a summary of a general procedure to evaluate and design river
crossings and encroachments. Due to the multi-disciplined complexity of these problems, it
is difficult to develop a procedure which is applicable to all situations that may be
encountered. A generalized approach can be described; however, modification to this
procedure must be made to tailor the procedure to an individual project.
9.3.1 Approach to River Engineering Projects
The evaluation and design of river crossings should proceed from a broad evaluation of the
characteristics of the river and the principles to be considered in design (described in Section
9.2) to detailed computations and analysis. The evaluation should begin with a qualitative
assessment of the river. As the analysis progresses, the analysis becomes more and more
detailed and subsequently more quantitative. At all stages of the investigation and design,
qualitative evaluation is important to determine, if possible, the interrelationship of all aspects
of the project.
The three-level procedure outlined in HEC-20 (Lagasse et al. 2001) and illustrated in Figure
9.1 is the recommended approach for evaluating projects in the river environment. The
method begins with broad considerations and proceeds through a series of steps of
increasing complexity to narrow down to the finer points of the project. Additionally, this
approach provides for back checking or "feedback" loops to insure that the interdependence
of all the variables is continually adjusted (see HEC-20, Chapters 1 and 3) (see also Figure
7.1).
In Level 1, the analysis consists of: (1) identifying the goals of the project; (2) developing
several options to achieve those goals; (3) determining the problems and possible solutions
to problems associated with each option; and (4) performing a qualitative assessment of all
aspects of the project.
9.6
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