Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Appendix C

APPENDIX C

PIER SCOUR COUNTERMEASURE SELECTION METHODOLOGY

C.1 Overview

This selection methodology provides a quantitative assessment of the suitability of six armoring-type countermeasures for pier scour based on selection factors that consider river environment, construction considerations, maintenance, performance, and estimated life-cycle cost (Lagasse et al. 2007). With the exception of life-cycle costs, the methodology analyzes the design factors by stepping the user through a series of decision branches, ultimately resulting in a site-specific numerical rating for each selection factor (see Section 3.2.5). The following countermeasures are evaluated by this methodology:

• Standard (loose) riprap
• Partially grouted riprap
• Articulating concrete blocks
• Gabion mattresses
• Grout-filled mattresses
• Grout-filled bags

To facilitate the decision-making process, the procedure was automated using a Microsoft^® Excel spreadsheet format. In the spreadsheet, the decision-making process can easily be modified to consider new situations or include additional information. Detailed directions are included in the program file, and automated features are incorporated in the program to step the user through the process.

C.2 Selection Index

Five factors are used to compute a Selection Index (SI0 for each countermeasure:

• S1: Bed Material size and transport
• S2: Severity of debris or ice loading
• S3: Constructability constraints
• S4: Inspection and maintenance requirements
• LCC: Life-cycle costs

The selection Index is calculated as:

SI = (S1 x S2 x S3 x S4)/LCC (C.1)

The countermeasure that has the highest value of SI is considered to be most appropriate for a given site, based not only on its suitability to the specific riverine and project site conditions, but also in consideration of its economy. The approach is sensitive to assumptions regarding initial construction cost, remaining service life, assumed frequency of maintenance events, and extent of maintenance required. Each of these factors requires experience and engineering judgment, as well as site- or region-specific information on the cost of materials and delivery, construction practices, and prevailing labor rates. It should be noted that the methodology can be used simply to rank the countermeasures in terms of suitability alone by assuming that the life-cycle costs are the same for all countermeasures.

In Section 3.2.5 the five factors that compose the methodology are described in detail. Flow charts illustrating selection factors S1 through S4 and reference to Excel spread sheet support follows.

C.3 Additional Considerations and Support

Federal or state regulations that preclude and use of a particular countermeasure because of environmental considerations and permitting issues are beyond the scope of NCHRP Project 24-07(2). The practitioner in any particular state must be aware of circumstances that may warrant the exclusion of a countermeasure for consideration at a specific site.

A feature allowing the user to easily include an additional design consideration, such as state-specific environmental concerns, to the computation of the Selection Index was added to the Excel-based selection methodology program. Inclusion of an additional selection criterion will require the user to assign values in the context of the selection factors for all countermeasures considered.

In addition, a feature was added to the selection methodology Excel spreadsheet capability to permit a user to introduce another countermeasure and generate selection factor values for that countermeasure. Inclusion of an additional countermeasure will require the user to assign values in the context of the design considerations and selection factors. The supplementary countermeasure feature and design consideration feature can be used independently or together, as described in the countermeasure selection Excel file available on the TRB website ( http://trb.org/news/blurb_ detail.asp?id=7998).