extent that a bridge will be in danger unless the substructures are founded deep enough.
Cohesive bed and bank material such as clays, silty clays, silts and silty sands or even coarser
bed material such as glacial tills, which are cemented by chemical action or compression, will
erode. The erosion of cohesive and other cemented material is slower than sand bed material,
but their ultimate scour will be as deep if not deeper than the scour depth in a non-cohesive
sandbed stream. It might take the erosive action of several major floods, but the effects of
scour can be cumulative in both cohesive and non-cohesive materials.
Scour at bridge crossings is a sediment transport process. Long-term degradation, general
scour, and local scour at piers and abutments result when more sediment is removed from
these areas than is transported into them. If there is no transport of bed material into the
bridge crossing, clear-water scour exists. Transport of appreciable bed material into the
crossing results in live-bed scour. In this latter case the transport of the bed material limits
the scour depth. Whereas, with clear-water scour the scour depths are limited by the critical
velocity or critical shear stress of a dominant size in the bed material at the crossing.
Chapter 7 presents the basic definitions of scour at bridges, and develops the basic equations
for determining the scour components. However, a more detailed analysis of scour at highway
bridges and detailed example problems in SI and English systems of measurement are
presented in HEC-18 (Richardson and Davis 2001).
1.7.7 Chapter 8 Data Need and Data Sources
The purpose of Chapter 8 is to identify data needed for calculations and analyses for
highway crossings and encroachments of rivers. The types and amounts of data needed for
planning and designing river crossings and lateral encroachments depend upon the class of
the proposed highway. In addition to identifying data needed, Chapter 8 identifies sources of
1.7.8 Chapter 9 - Design Considerations for Highway Encroachment
and River Crossings
Chapter 9 presents applications of the fundamentals of hydraulics, hydrology, fluvial
geomorphology, sediment transport, 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 analyses. 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.
Chapter 9 contains a series of conceptual examples (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 practical examples (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.