CHAPTER 9
DESIGN CONSIDERATIONS FOR HIGHWAY ENCROACHMENTS
AND RIVER CROSSINGS
9.1 INTRODUCTION
The objective of this chapter is to present applications of the fundamentals of hydraulics,
hydrology, fluvial geomorphology, and river mechanics to the hydraulic and environmental
design of river crossings and highway encroachments. The principal factors to be
considered in design are presented, followed by a discussion of the procedures
recommended for the evaluation, analysis and design of river crossings and encroachments.
The design of most complex problems in river engineering can be facilitated by a qualitative
evaluation combined with a quantitative analysis. In most cases, the systematic approach of
a qualitative assessment of channel response, followed by a quantitative estimate, is
necessary for a meaningful analysis of complex river response problems.
This chapter contains several hypothetical cases of river environments and their response to
crossings and encroachments based upon geomorphic principles given in Chapter 5. These
cases indicate the trend of change in river morphology for given initial conditions. The
hypothetical cases are followed by actual case histories for river crossings in the United
States. These histories document river response to highway crossings and encroachments
and illustrate river response qualitatively.
This chapter uses two types of examples (conceptual and actual examples) related to river
crossings and highway encroachments. Applications of the basic principles developed in
Chapters 1 through 8 are illustrated by these conceptual examples and specific case
histories related to the subject matter of this manual.
9.2 PRINCIPAL FACTORS TO BE CONSIDERED IN DESIGN
Identification of the principal factors to be considered in design of river crossings and
encroachments is useful. These factors are generally interrelated, and fundamental
mechanisms and relationships of the governing physical processes must be clearly
understood prior to design.
9.2.1 Types of Rivers
In selecting the site for a crossing or an encroachment on a river it is necessary to give
detailed consideration and study to the type of river or rivers involved. A sandbed river may be
meandering, it may be essentially straight, or it may be braided. In addition, a meandering
river may be small, medium, or large. The same channel can be classified as youthful,
mature, or old. Each of these different river types requires different design procedures. For
example, in designing training works for large sandbed channels (braided or meandering) it is
unlikely that Kellner jetties alone will be useful to stabilize the bank alignment (see Chapter 6).
It may be necessary to stabilize the banks with rock riprap and to control the overbank flows
using jetties to achieve a set of specific purposes. Gravel and cobble bed channels are
9.1
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