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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STEP 3 Conduct field and office scour evaluations of the bridges on the
prioritized list, step 2, using an interdisciplinary te.. of
structural, hydraulic, and geotechnical engineers: (See MNDOT
Tentative Procedures, Appendix 8.)

(a) The recommended evaluation procedure is to estimate scour for a
superflood, a flood exceeding the 100 year flood, and then
analyze the foundations for vertical and lateral stability for
this condition of scour. This evaluation approach ;s similar in
concept to the check procedure set forth in paragraph 6, step 8
of the design procedure in Chapter 111, and FHWA recommends using
the 500 year flood or a flow 1.7 times the 100 year flood for
this purpose. The difference between designing a new bridge and
assessing ~n old bridge is simply that the location and geometry
of a new bridge and its foundation are not fixed as they are for
an old bridge. Thus, the steps for predicting scour at the piers
and abutments should be carried out for an existing bridge.
Then, instead of designing the foundations, the maximum scour
depths that can be withstood by the existing foundation are
compared with the scour for the superflood. An engineering
assessment must then be made as to whether the bridge should be
classified as a scour critical bridge; that is, whether the
bridge foundations can withstand the scour from the superflood
without failing.

(b) Enter the results of the scour evaluation study in the bridge
inventory in accordance with the instructions in the (pending
revision of the) FHWA Bridge Recording and Coding Guide. (See
Reference 5 and Appendix C.) Make a list of the scour critical
bridges.

STEP 4 For bridges identified as scour critical, (Step 3) determine a plan of
action (See Chapter 7) for monitoring and correcting the scour problem
i nc 1 udi ng:

(a) Timely installation of scour countermeasures.

(b) Plans for inspecting scour critical bridges during and after
flood events and for blocking traffic if need be until scour
countermeasures are installed.

STEP 5 After completing the scour evaluations for the list of potential
problems compiled in step 1, the remaining waterway bridges including
in the State's bridge inventory should be evaluated. In order to
provide a logical sequence for accomplishing the remaining bridge
scour evaluations another bridge list should be established, giving
priority status to the following:

(a) The functional classification of the highway on which the bridge
is located with highest priorities assigned to arterial highways
and lowest priorities to local roads and streets.

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