The purpose of this chapter is to lay the groundwork for application of the concepts of
open-channel flow, fluvial geomorphology, sediment transport, and river mechanics to the
design, maintenance, and environmental problems associated with highway crossings and
This manual is a basic reference for related Federal Highway Administration (FHWA) hydraulic
publications and National Highway Institute (NHI) Hydraulics Courses. Some of these
publications are: "Hydraulics of Bridge Waterways" (Bradley 1978), "Design of Riprap
Revetment" (Brown and Clyde 1989), "Evaluating Scour at Bridges" (Richardson and Davis
2001), "Stream Stability at Highway Structures" (Lagasse et al. 2001), "Bridge Scour and
Stream Instability Countermeasures - Experience, Selection and Design Guidance" (Lagasse
et al. 2001). Related NHI courses include: (1) River Engineering for Highway Encroachments,
(2) Stream Stability and Scour at Highway Bridges, and (3) Finite Element Surface Water
Modeling System (FESWMS).
Basic definitions of terms and notations adopted for use in this document have been presented
in the preceding section (Glossary) for rapid reference. Additionally, these important terms and
variables are defined and explained as they are encountered.
1.1 CLASSIFICATION OF RIVER CROSSINGS AND ENCROACHMENTS
The objective in this document is to consider the fluvial, hydraulic, geomorphic, sediment
transport, and environmental aspects of highway encroachments, including bridge locations,
alignments, longitudinal encroachments, stabilization works and road approaches.
Encroachment is any occupancy of the river and floodplain for highway use. Encroachments
usually present no problems during normal stages, but require special protection against
floods. Flood protection requirements vary from site to site.
Some bridges and culverts must accommodate the passage of livestock and farm equipment
during periods of low flow. Other bridges require low embankments for aesthetic appeal,
especially in populated areas. Still other bridges require short spans with long approaches and
numerous piers for economic reasons. All of these factors, and many more, contribute to the
difficulty in generalizing the design for all highway encroachments.
A classification of encroachments based on prominent features is helpful. Classifying the
regions requiring protection, the possible types of protection, the possible flow conditions, the
possible channel shapes, and the various geometric conditions aids the engineer in selecting
the design criteria for the conditions encountered.