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

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pressures can build up in the underlying soil and result in failure of the countermeasure.
Guidelines for the selection, design, and specifications of filter material can be found in
Brown and Clyde (HEC-11) (1989), and detailed information on the use of geosynthetic
filters can be found in Holtz et al. (FHWA HI-95-038) (1995) (see Section 6.2 Supplemental
References). The State of California Department of Transportation also provides guidance
on the use of geotextile filters with slope protection measures (see Section 6.2 Supplemental
References).

4.4.2 Edge Treatment

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Undermining of the edges of armoring countermeasures is one of the primary mechanisms
of failure. The edges of the armoring material (head, toe, and flanks) should be designed so
that undermining will not occur. For channel bed armoring, this is accomplished by keying
the edges into the subgrade to a depth which extends below the combined expected
contraction scour and long-term degradation depth. For side slope protection, this is
achieved by trenching the toe of the revetment below the channel bed to a depth which
extends below the combined expected contraction scour and long-term degradation depth.
When excavation to the contraction scour and degradation depth is impractical, a launching
apron can be used to provide enough volume of rock to launch into the channel while
maintaining sufficient protection of the exposed portion of the bank. Continuous systems,
such articulating concrete block systems and grout filled mattresses applied on side slopes,
should be designed with an apron or toe trench so that the system provides protection
below the combined expected contraction scour and long-term degradation depth. Tension
anchors may be used to increase stability at the edges of these continuous systems.
Additional guidelines on edge treatment for armoring countermeasures can be found in
Brown and Clyde (HEC-11) (1989).

5.0 DESIGN GUIDELINES

5.1 Overview

Following the countermeasures matrix, design guidelines are provided for several
countermeasures which have been applied successfully on a state or regional basis, but for
which only limited design references are available in published handbooks, manuals, or
reports. No attempt has been made to include in this document design guidelines for all the
countermeasures listed in the matrix. There are, however, references in the matrix to
publications that contain at least a sketch or photograph of a particular countermeasure, and
in many cases contain more detailed design guidelines. FHWA currently has four
pUblications dealing with stream instability and bridge scour countermeasures. HEC-18
("Evaluating Scour at Bridges"), HEC-20 ("Stream Stability at Highway Structures"),
"Highways in the River Environment" (HIRE), and HEC-11 "Design of Riprap Revetment"
contain detailed design procedures for the following countermeasures:

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. Impermeable and permeable spurs
. Drop structures (hydraulic design only)
. Guide Banks
. Riprap stability factor design
. Sizing rock riprap at abutments
. Sizing rock riprap at piers
. General revetment design

- HEC-20, HIRE
- HEC-20
- HEC-20
- HIRE
- HEC-18
- HEC-18
- HEC-11

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