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<br />Chapter I.-INTRODUCTION
<br />
<br />1.1 General, There was a time, now past,
<br />when backwater caused by the presence of bridges
<br />during flood periods was considered a necessary
<br />nuisance-first, because the public clamored for
<br />bridges to replace ferries and fords; and second,
<br />because there was no accurate means of determining
<br />the amount of backwater a bridge would produce
<br />after it was in place, With the spread of urbanization;
<br />with indefinite, unenforceable restrictions on the
<br />construction of housing and bU8iness establishments
<br />on flood plains of rivers and streams throughout
<br />the country; with new highway bridges being con-
<br />structed at an ever-increasing rate; and with prop-
<br />erty values increasing at an unprecedented rate in
<br />the past two decades, it is now imperative that the
<br />backwater produced by new bridge8 be kept within
<br />very knowledgeable and reasonable limits, This
<br />places demands on the hydraulic engineer, who has
<br />not been consulted too often in the past, to promote
<br />and develop a more scientific approach to the bridge
<br />waterway problem, ProgTess in structural design
<br />has kept pace with the times, Structural engineers
<br />are well aware of the economies which can be at-
<br />tained in the proper type, selection and design of a
<br />bridge of a given overall length and height. The role
<br />of the hydraulic engineer in establishing what the
<br />length and vertical clearance should be and where
<br />the bridge should be placed is less well understood
<br />due principally to the lack of hydrological and
<br />hydraulic information on the waterways,
<br />In fact, until recently, bridge lengths and clear-
<br />ances have been proportioned principally on rough
<br />calculations, individual judgment, and intuition,
<br />This may still be true in some cases, Today traffic
<br />volumes have become so great on primary roads
<br />that bridge failures or bridges out of service for any
<br />length of time can cause severe economic loss and
<br />inconvenience; even closing one lane of an arterial
<br />highway for repairs creates pandemonium,
<br />Confining flood waters unduly by bridges can
<br />cause excessive backwater resulting in flooding of
<br />upstream property, backwater damage suits, over-
<br />topping of roadways, excessive scour under the
<br />bridge, costly maintenance, or even loss of a bridge,
<br />
<br />On the other hand, overdesign or making bridges
<br />longer than necessary for the sake of safety, can
<br />add materially to the initial cost, especially when
<br />dual or multiple lane bridges are involved, Both
<br />extremes in design have been experienced, Some-
<br />where between the two extremes is the bridge which
<br />will prove not only safe but the most economical
<br />to the public over a long period of time, Finding
<br />that design is of great concern to the Bureau of
<br />Public Roads, which has 8ponsored and financed
<br />research on related projects for the past decade and
<br />a half,
<br />Recent improvements in methods of dealing
<br />with the magnitude and frequency of floods, experi-
<br />mental information on scour, and the determination
<br />of expected backwater all are providing stepping
<br />stones to a more scientific approach to the bridge
<br />waterway problem, This publication is intended to
<br />provide, within the limitations discussed in chapter
<br />XIII, a means of determining the effect of a given
<br />bridge upon the flow in a stream, It does not pre-
<br />scribe criteria as to the allowable amount of back-
<br />water or frequency of the design flood; these are
<br />policy matters that must take into account class of
<br />highway, density of traffic, seriousness of flood
<br />damage, foundation conditions and other factors,
<br />1.2 Waterway .tudie.. In recognition of the
<br />need for dependable hydraulic information, the
<br />Bureau of Public Roads initiated a cooperative
<br />research project with Colorado State University in
<br />1954 which culminated in the investigation of
<br />several features of the waterway problem, These
<br />included a study of bridge backwater (18) , ·
<br />scour at abutments and piers, and the effect of scour
<br />on backwater, Concurrently with this work, the
<br />Iowa State Highway Commission and the Bureau
<br />of Public Roads sponsored studies of scour at bridge
<br />piers (:83) and scour at abutments (t!4l at the Iowa
<br />Institute of Hydraulic Research at Iowa City. In
<br />1957 the State Highway Departments of Mississippi
<br />and Alabama, in cooperation with the Bureau of
<br />
<br />* Italic numbel'8 in parentheses refer &0 puhlicatioP8 listed in the &elected
<br />bibliography.
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