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<br />Public Roads, sponson'll a project at Colora.do
<br />State r niversity to study means of reducing scour
<br />und,'r a bridge by the use of spur dikes (19, 25)
<br />(elliptical shaped earth embankments placed at the
<br />upstrel\m end of a bridgc abutment).,
<br />The above laboratory studies, in which hydraulic
<br />models served as the principl\1 research tool, have
<br />been completed, Since then considerable progress
<br />has been made in the collection of field data by the
<br />U,s. Geological Survey to substantiate the model
<br />results and extend the range of application. There
<br />is still much to be learned froIll field observations
<br />and it is recommended that this phase of investiga-
<br />tion be continued for sometime to come.
<br />1.3 Bridge backwater. An account of the
<br />testing procedure, a record of b""ic data, and an
<br />analysis of results on the bridge backwater studies
<br />are contained in the comprehensive report (18) is-
<br />sued by Colora.do State University, Results of re-
<br />search described in that report were drawn upon
<br />for this publication, which deal8 with that part of
<br />the waterway problem that pertains to the natnre
<br />and magnitude of backwater produced by bridges
<br />constricting streams, This publication is prepared
<br />specifically for the designer and contains practical
<br />design charts, procedures, examples, and a text
<br />limited principally to describing the proper use of
<br />the information,
<br />1.4 Nature of bridge backwater. It is seldom
<br />economically feasible or necessary to bridge the
<br />entire width of a stream"" it occurs at flood flow,
<br />Where conditions permit, approach embankments
<br />are extended out onto the flood plain to reduce costs,
<br />recognizing that, in so doing, the embankments will
<br />constrict the flow of the stream during flood stages.
<br />This is acceptable practice so long "" it is done
<br />within reason,
<br />The manner in which flow is contracted in passing
<br />through a channel constriction is illustrated in
<br />figure L The flow bounded by each a.dj acent pair of
<br />streamlines is the same (1,000 c.Ls,), Note that the
<br />channel constrietion appears to produce practically
<br />no alteration in the shape of the streamlines near
<br />the center of the channel. A very marked change is
<br />evideneed near the abutments, however, since the
<br />momentum of the flow from both sides (or flood
<br />plains) must foree the advancing central portion
<br />of the stream over to gain entry to the constriction,
<br />Upon leaving the constriction the flow gra.dually
<br />expands (5 to 6 degrees per side) until normal condi-
<br />tions in the stream are again reestablished,
<br />Constriction of the flow causes a loss of energy,
<br />the greater portion occurring in the reexpansion
<br />
<br />downstream, This loss of energy is reflected in a
<br />rise in the water surface and in the energy line up-
<br />stream from the bridge. This is best illustrated by a
<br />profile along the center of the stream, M shown in
<br />figures 2A and 3A, The normal stage of the stream
<br />for a given discharge, before constricting the chan-
<br />nel, is represented by the d""h line labeled "normal
<br />water surface," (Water surface is abbreviated as
<br />"W.s." in the figures.) The nature of the water
<br />surface after constriction of the channel is repre-
<br />sented by the solid line, "actual water surface."
<br />Note that the water surface starts out above normal
<br />stage at section 1, pa.sses through normal stage close
<br />to section 2, reaches minimum depth in the vicinity
<br />of section 3, and then returns to normal stage a con-
<br />siderable distance downstream, at section 4. Deter-
<br />mination of the rise in water surface at section 1,
<br />denoted by the symbol h,. and refeITed to as the
<br />bridge backwater, is the primary objective of this
<br />publication. Attention is called to a common mis-
<br />understanding that the drop in water surface across
<br />the embankment, bh, is the backwater caused by a
<br />bridge, This is not correct as an in8pection of figure
<br />2A or 3A ,,~ll show, The backwater is represented
<br />by the symbol hl. on both figures and is always less
<br />than l1h,
<br />The Colora.do laboratory model represented the
<br />ideal case since the testing w"" done principally in
<br />a rectangular, fixed bed, a.djustable sloping flume,
<br />8 ieet wide by 75 feet long. Roughness of the bed
<br />w"" changed periodically but for any particular set
<br />of tests, it w"" uniform throughout the flume, Except
<br />for roughness of the bed, the flow was in no way
<br />restrained from contracting and expanding, The
<br />model data would apply to relatively straight
<br />reaches of a stream having approximately uniform
<br />slope and no restraint to lateral movement of the
<br />flow, Field measurements indicate that a stream
<br />cross section can vary considerably without cau8ing
<br />serious error in the computation of backwater, The
<br />very real problem of scour was avoided in the initial
<br />tests by the use of rigid boundaries, Ignoring scour
<br />in computations will give generou8 backwater
<br />values but 8cour must be considered in Msessing the
<br />safety of abutments and piers, The inereMe in water
<br />area in the constriction caused by scour will in turn
<br />produce a reduetion in backwater over that for a
<br />rigid bed, On the other hand, unusually heavy vege-
<br />tation on the flood plain downstream can interfere
<br />with the natural reexpansion proeess to such an
<br />extent M to increase the bridge backwater over
<br />normal conditions.
<br />1.5 Types of flow encouutered. There are
<br />
<br />2
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