FLOOD06847

• County: Statewide
• Basin: Statewide
• Title: Bridge Scour and Stream Instability Countermeasures
• Date: 7/1/1997
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

. In most cases, the countermeasure does not "cure" the instability or scour
problem, and planning (funding) for continued maintenance of the
countermeasure will be required.

4.3 Environmental Considerations

The environmental permitting process can have a significant effect on the planning, design
and implementation of river engineering works. Often, permitting can become a lengthy
process for the implementation of bridge scour and stream instability countermeasures. To
expedite this process, a memorandum dated February 11, 1997, was prepared jointly by the
U.S. Army Gorps of Engineers (USAGE) Directorate of Givil Works and the Federal Highway
Administration (FHWA). The purpose of the memorandum is to facilitate timely decisions on
permit applications for work associated with measures to protect bridges determined to be at
risk as the result of scouring around their foundations. The USAGE and FHWA consider this
agreement essential to assure the safety of the traveling public while protecting the
environment. Since installing protective armoring is usually determined to be the most
feasible and economical method to protect bridge foundations, it is expected that USAGE
Districts may experience a significant increase in requests, from bridge owners, for permits
for the installation of this type of scour countermeasure.

1
i
i
l

Recognizing the importance of protecting the foundations of our Nation's scour critical
bridges with properly designed scour countermeasures and the need for environmentally
sound projects, the FHWA and the USAGE agree to work together with the bridge owners, in
a cooperative effort, to plan ahead for managing projects that will need a USAGE permit. A
strong cooperative effort will aid in advanced planning to avoid and minimize environmental
impacts, and in identifying locations where mitigation may be appropriate. If the bridge
foundation has been determined to be scour critical as part of the bridge owner's scour
evaluation program, the USAGE will give priority to the bridge owner's request for
authorization for the installation of scour countermeasures. Bridge owners must provide the
FHWA and USAGE Districts advance notice of the proposed countermeasure design and
construction schedule. The notice must include an evaluation of the environmental impacts
of the proposed scour countermeasure and appropriate mitigation of unavoidable impacts to
aquatic resources, including fisheries and wetlands. This will allow appropriate and timely
cooperation on project reviews. The USAGE will make the maximum use possible of forms
of expedited authorization, such as nationwide permits and regional permits, and Letters of
Permission and the use of FHWA's Gategorical Exclusion when the condition of the bridge
foundation meets the criteria for codes 0 through 4 for Item 113.

~

4.4 Other Design Considerations

4.4.1 Filter Requirements

Granular or geosynthetic filters are essential to the performance of hydraulic counter-
measures, especially armoring countermeasures. Filters prevent soil erosion beneath the
armoring material, prevent migration of fine soil particles through voids in the armoring
material, distribute the weight of the armor units to provide a more uniform settlement, and
permit relief of hydrostatic pressure within the soils. Experience has indicated that the
proper design of filters is critical to the stability of revetments. If openings in the filter
material are too large, excessive piping through the filter can result in erosion of the
subgrade beneath the armor. Conversely, if openings in the filter are too small, hydrostatic

10