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

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13. Erosion history of the stream.

14. Development history (past, present and future) of the stream and
watershed. Collect maps;' photographs, area photographs; interview
local residences; check for water research projects planned or
contlllllPlated.

15. Sand and gravel mining for stream bed up or downstream fOnD site.

16. Other factors that could affect the bridge.

17. Make a qualitative evaluation of the site with an estimate of the
potential of stre.. movement and its effect on the bridge.

STEP 2. ANALYSIS OF LONG TEAM BED ELEVATIOI atM6E

A. Using the information collected in Step 1 above, determine qualitatively
the long term trend in the stream elevation over the long term. Where
conditions indicate that significant aggradation or degradation is likely,
estimate the change in bed elevation over the next 100 years using one or
more of the following:

1. available computer programs such as The Corps of Engineers MEt 6;

2. straight line extrapolation of present trends;

3. engineering judgment; and

4. The worst case scenarios, i.e., in the case of the confluence to
another stream just downstream of the bridge, assume the design flood
would occur with a low the downstream water surface elevation through a
qualitative assessment of the joint probability of flood magnitudes and
river conditions on the main stream and its tributary.

B. If the stream is aggrading and this condition can be expected to affect the
crossing, consider countermeasures including a relocation of the bridge.
This should be done taking into account contraction scour.

t. If the stream is degrading then use the change in elevation in the
calculations of total scour.

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