FLOOD06421

• County: Statewide
• Basin: Statewide
• Title: Highways in the River Environment
• Date: 2/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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1.1.1 Tvpes 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 cornrnonly,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
fir earth fill embankments over the floodplain or into the main chanm~l itself..
reducinq the required bridqe lenqth; or in the form of piers and abutments or
culverts in the main channel of the river.
There are also longitudinal encroachments not connected with river crossings.
Floodplains often appear to provide an attractive low cost alternative for highway
location, even when the cxtra cost uf flood protection is included. As a
consequence, hiqhwavs, including interchanges, yften encroach on ~. floodplain over
IOlN.-r:Jistances. In some regions, river valleys provide the only feasible rouLe 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. 1 here is
also often the need to strailJhten a stretch of the river, eliminating meanders, to
accommodate the highway.

1.1.2 Geometry Of Bridqe Crossinqs
The bridge crossing is the most common type uf river encroachment. 1 he
geometric properties of bridge crossings illustrated in Fig. 1.1.1 are commonly used
depending on the conditiol15 at the site. The approaches may be skewed or normal
([>erpenrJicular) to the direction of flow, or one approach may be lonqer than the
other. producinq an eccentric crossinq. Abutments used fur 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 brid'Jes for multi -lane freeways, channel bed conditiuns, spur
dikes and guide banks add to the list of geometric classifications.

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