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

2. Guide Banks (Spur Dikes). Methods for designing guidebanks or spur dikes are
contained in the FHWA publication, "Hydraulic Design Series No.1, Hydraulics
of Bridge Waterways." The hydraulic effect of guidebanks can be ~deled
through the use of the FHWA/USGS Computer Program .WSPRO" (8).
The purpose of the guide bank is to provide for a smooth transition for flows
on the floodplain returning to the main channel at the bridge. The guidebank
serves to move the point of maximum scour upstream and away from the
abutment, and should be considered for'protecting bridge abutments whenever
there is a significant amount of flow on the floodplain that .ust return to
the main channel at the bridge.

3. Channel Improvements. A wide variety of countermeasure. are available for
stabilizing and controlling flow patterns in streams. Reference 6 contains
methods for designing channel improvements.

a, Countermeasures for aggrading streams include:

o contracting the stream upstream and through the bridge to cause it to
scour;

o construction of upstream sediment dams;

o periodic cleaning of the channel; and

o raising the grade of the bridge and its approaches.

b. Countermeasures for degrading streams include the construction of sills
and the strengthening of foundations as discussed below.

c. Countermeasures for controlling lateral movement of a stream due to
stream meanders include placement of dikes along the stream banks to
redirect the flow through the bridge along a favorable path that
minimizes the angle of attack of the current on the bridge foundations.
The FHWA Manual HEC No. 20 (7) will address these type of countermeasures
in detail. Another useful reference is the Transportation Research
Record 950 of the Transportation Research Board dated 1984 (4).

4. Structural Scour Countermeasures. The use of structural designs to
underpin existing foundations is discussed in the AASHTO Manual for Bridge
Maintenance, 1987 edition (15). While structural measures may be more
costly, they generally provide more positive protection against scour than
countermeasures such as riprap since they can be designed for the worst
conditions anticipated.

5. Constructing Sills or Drop Structures. The use of sills and drop
structures at bridges to stabilize the stream bed and counteract the
effects of degradation is discussed in the FHWA publication, HEC 20 (7).

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