1.1.1 Types of Encroachment
In the vicinity of rivers, highways generally must impose a degree of encroachment. In some
instances, particularly in mountainous regions or in river gorges and canyons, river crossings
can be accomplished with absolutely no encroachment on the river. The bridge and its
approaches are located far above and beyond any possible flood stage. More commonly, the
economics of crossings require substantial encroachment on the river and its floodplain, the
cost of a single span over the entire floodplain being prohibitive. The encroachment can be in
the form of earth fill embankments on the floodplain or into the main channel itself, reducing
the required bridge length; or in the form of piers and abutments or culverts in the main
channel of the river.
Longitudinal encroachments may exist that are not connected with river crossings. Floodplains
often appear to provide an attractive low cost alternative for highway location, even when the
extra cost of flood protection is included.
As a consequence, highways, including
interchanges, often encroach on a floodplain over long distances. In some regions, river
valleys provide the only feasible route for highways. This is true even in areas where a
floodplain does not exist. In many locations the highway must encroach on the main channel
itself and the channel is partly filled to allow room for the roadway. In some instances, this
encroachment becomes severe, particularly as older highways are upgraded and widened.
Often, a stretch of the river must be straightened to eliminate meanders to accommodate the
highway.
1.1.2 Geometry of Bridge Crossings
The bridge crossing is the most common type of river encroachment. The geometric
properties of bridge crossings illustrated in Figure 1.1 are commonly used depending on the
conditions at the site. The approaches may be skewed or normal (perpendicular) to the
direction of flow or one approach may be longer than the other, producing an eccentric
crossing. Abutments used for the overbank-flow case may be set back from the low-flow
channel banks to provide room to pass the flood flow or simply to allow passage of livestock
and machinery, or the abutments may extend up to the banks or even protrude over the banks,
constricting the low-flow channel. Piers, dual bridges for multi-lane freeways, channel bed
conditions, spurs and guide banks add to the list of geometric classifications.
The design procedures in this document have been derived from laboratory and field
observations of bridge crossings. The design procedures include allowances made for the
effects of skewness, eccentricity, scour, abutment setback, channel shape, submergence of
the superstructure, debris, spurs, wind waves, ice, piers, abutment types, and flow conditions.
These design procedures take advantage of the large volume of work that has been done by
many people in describing the hydraulics and scour characteristics of bridge crossings.
1.2 DYNAMICS OF NATURAL RIVERS AND THEIR TRIBUTARIES
Frequently, hydraulic engineers, and those involved in transportation, navigation, and flood
control mistakenly consider a river to be static; that is, unchanging in shape, dimensions, and
pattern. However, an alluvial river generally is continually changing its position and shape as a
1.2
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