LONG-TERM BED ELEVATION CHANGES

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with a plane bed configuration. With antidunes occurring upstream and in the bridge crossing;

the maximum depth of scour, from the limited research of Jain and Fisher (1979), is from 10 to

20 percent greater than the equilibrium depth of scour. In general, with sand bed streams a

dune bed changes to plane bed or antidune flow during flood flow.

7.4 ARMORING

Armoring occurs on a stream or in a scour hole when the forces of the water during a particular

flood are unable to move the larger sizes of the bed material. This protects the underlying

material from movement. Contraction scour or local scour around an abutment or pier may

initially occur, but as the scour depth increases the coarser bed material moves down in the

bridge cross-section or local scour hole and protects the bed so that the full scour potential is

not reached.

When armoring occurs, the coarser bed material will tend to remain in place or quickly

redeposit so as to form a layer of riprap-like armor in the cross-section or local scour hole and,

thus, limit further scour for a particular discharge. When larger flows occur the armor layer can

be broken and the scour depth deepened either until a new armor layer is developed or the

maximum scour is reached.

7.5 LONG-TERM BED ELEVATION CHANGES

Long-term bed elevation changes (aggradation or degradation) may be the natural trend of the

stream or may be the result of some modification to the stream or watershed condition. In

scour analyses, only long-term degradation is considered. Aggradation, if it occurs, is not used

to decrease total scour estimate.

The streambed may be aggrading, degrading or not changing (equilibrium) in the bridge

crossing reach. When the bed of the stream is neither aggrading or degrading, it is in

equilibrium with the sediment discharge supplied to the bridge reach and the elevation of the

bed does not change. In this section, we consider long-term trends, not the cutting and filling

of the bed of the stream that might occur during a runoff event. A stream may cut and fill

during a runoff event and also have a long-term trend of an increase or decrease in bed

elevation. The problem for the engineer is to determine what the long-term bed elevation

changes will be during the lifetime of the structure. What is the current rate of change in the

stream bed elevation? Is the stream bed elevation in equilibrium? Is the streambed

degrading? Is it aggrading? Is there a head cut or nickpoint moving upstream? What is the

future trend in the stream bed elevation?

During the life of the bridge the present trend may change. These long-term changes are the

result of modifications of the state of the stream or watershed. Such changes may be the

result of natural processes of the result of human activities. The engineer must assess the

present state of the stream and watershed and determine future changes in the river system,

and from this assessment determine the long-term stream bed elevation.

Factors that affect long-term bed elevation changes are: dams and reservoirs (up or

downstream of the bridge), changes in watershed land use (urbanization, deforestation, etc.),

channelization, cutoff of a meander bend (natural or human induced), changes in the

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