PROGRAMS AND PLANS--Guidelines for the Transmittal of Bridge-Scour Information to Cooperators

In Reply Refer To:                           November 18, 1991
Mail Stop 415


Subject:  PROGRAMS AND PLANS--Guidelines for the Transmittal of
                              Bridge-Scour Information to

This memorandum provides guidelines for the transmittal of bridge-
scour data and information to cooperators.  Many districts are
conducting investigations of channel-bed scour at bridges to
provide information and understanding to cooperators to promote
public transportation safety.  The specific objectives of these
investigations are diverse; however, they may generally be grouped
into those that improve scour-prediction techniques and those that
evaluate scour potential for existing structures.

Investigations to improve scour-prediction techniques and to
improve definitions of scour processes include collection of
channel-geometry data during normal and flood-flow conditions, as
well as hydraulic, bed-material, and bridge-geometry data.  After
appropriate review, data from these investigations may be
transmitted under a cover letter to cooperators.  Of course,
cooperators may be notified immediately of potentially hazardous
bridge-scour conditions.  Interpretive results from these studies
will be transmitted to cooperators in Director-approved reports.

Investigations regarding scour potential and channel instability
at selected bridges provide that information at three levels of
detail.  The first level of detail consists of qualitative and
quantitative data collected during a site visit and summarized to
provide an index to assess scour potential.  The data are fluvial-
geomorphic information and bridge characteristics related to
bridge-scour potential.  The data may be summarized into an
observed scour index (for observed scour conditions) and/or a
potential scour index.  It is important to note that the potential
scour index is not for a boundless range of flow conditions, but
is typically limited to bank-full flood conditions, which may be
envisioned by the assessor without the benefit of hydraulic
analysis.  Over 15,000 of these channel instability assessments
are being done in ongoing studies.  Many existing studies have
used the assessment form and index developed for West Tennessee
and described in a Director-approved 1989 proceedings article by
Simon, Outlaw, and Thomas.  Accuracy, transferability, and other
issues related to scour assessment procedures are being
investigated by a work group composed of scour assessment project
chiefs and organized by the Office of Surface Water.

Basic data from the level-one assessments may be transmitted under
a cover letter to cooperators provided that the data are reviewed
for adequacy and accuracy.  Developing a procedure to summarize
these data into an index requires judgment and interpretation.
Thus, the procedure must be documented in a Director-approved
report before transmittal of scour indices for selected bridges.
Modifications to an existing procedure must also be documented in
a Director-approved report.

The second level of detail includes information collected in the
level-one analysis, estimation of flood magnitude for specified
recurrence intervals, a step-backwater computation to obtain
hydraulic characteristics for these floods, and use of specific
empirical equations to estimate local scour.  Cooperators use this
information and their bridge foundation data to analyze the
susceptibility of bridges to scour and to determine where scour
countermeasures are needed.  About 600 of these scour evaluations
will be done in ongoing investigations.  The information from this
level scour evaluation is rather analogous to the results of
indirect measurements of peak discharge.  These scour evaluations
follow a specified procedure which usually does not require
hydrologic interpretation.  Following regional review, information
from these evaluations may be transmitted under a cover letter.
If undocumented procedures are used in a level-two evaluation, it
should receive further review and be transmitted in a Director-
approved report.

The level-three evaluations are based upon sediment transport
modeling of the stream through the bridge site.  About 30 of these
analyses are planned in ongoing investigations.  Results from
these analyses will be transmitted to cooperators in Director-
approved reports.

                                  Ernest F. Hubbard
                                  Acting Chief
                                  Office of Surface Water