FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

In addition to the types of scour mentioned above, lateral movement or shifting
of the stream may also erode the approach roadway to the bridge or change the
total scour by changing the angle of the flow in the waterway at the bridge
crossing. Factors that affect lateral movement and the stability'of a bridge
are the geomorphology of the stream,..location of the crossing on the stream,
flood characteristics, and the characteristics of the bed and bank materials.

The following paragraphs contain additional information on the types of scour
discussed above.

c. AGGRADATION AN\) DEGRADATION. LONG TERM STREAM BED ELEVATION CHAJl6ES

Long term bed elevation changes may be the natural trend of the stream or may
be the result of some modification to the stream or watershed condition.
The stream bed may be aggrading, degrading or not changing (in equilibrium) in
the bridge crossing reach. In this section we are considering long term
trends, not the cutting and filling of the bed of the stream that might occur
during a runoff event (general scour). 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 life time of the structure. What is the
current rate of change in the stream bed elevation? Is the stream bed
elevation in equilibrium? Is the stream bed degrading? Is it aggrading? 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 or the result
of man's activities. The engineer must assess the present state of the stream
and watershed and determine future changes in the river syst.. and from this
determine the long term stream bed elevation.

Factors that affect long term bed elevation changes are: dams and reservoirs
(upstream or downstream of the bridge), changes in watershed land use
(urbanization, deforestation, etc.), channelization, cutoffs of meander bends
(natural or man made), changes in the downstream base level (control) of the
bridge reach, gravel mining from the stream bed, diversion of water into or out
of the stream, natural lowering of the total system, movement of a bend, bridge
location in reference to stream platform and stream movement in relation to
the crossing.

Analysis of long term stream bed elevation changes must be made using the
principles of river mechanics. Such analysis requires the consideration of all
influences upon the bridge crossing; i.e. runoff from the watershed to the
channel (hydrology), the sediment delivery to the channel (erosion), the
sediment transport capacity of the channel (hydraulics) and the response of the
channel to these factors (geomorphology and river mechanics). Many of the
largest impacts are from man's activities. This analysis requires a study of
the past history of the river and man's activities on it; a study of present
water, land use, stream control activities, and finally contacting all agencies
involved with the river to determine future changes in the river.

4

'I
I
(f'
, I
I
I
I
I
I
I
I ( I
I
I
I
I
I
I
I
I
I