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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Chapter 1 Introduction
The purpose of this document is to identify and provide design guidelines for bridge scour and stream instability countermeasures that have been implemented by various State departments of transportation (DOTs) in the United States. Countermeasure experience, selection, and design guidance are consolidated from other FHWA publications in this document to support a comprehensive analysis of scour and stream instability problems and provide a range of solutions to those problems. The results of recently completed National Cooperative Highway Research Program (NCHRP) projects are incorporated in the design guidance, including: countermeasures to protect bridge piers and abutments from scour; riprap design criteria, specifications, and quality control, and environmentally sensitive channel and bank protection measures. Selected innovative countermeasure concepts and guidance derived from practice outside the United States are introduced. In addition, guidance for the preparation of Plans of Action (POA) for scour critical bridges has been expanded to include a standard template for POA and instructions for its use.
Scour and stream instability problems have always threatened the safety of our nation's highway bridges. Countermeasures for these problems are defined as measures incorporated into a highway-stream crossing system to monitor, control, inhibit, change, delay, or minimize stream instability and bridge scour problems. A plan of action, which can include timely installation of stream instability and scour countermeasures, must be developed for each scour critical bridge. Monitoring structures during and/or after flood events as a part of a plan of action, can also be considered an appropriate countermeasure.
Numerous measures are available to counteract the actions of humans and nature which contribute to the instability of alluvial streams. These include measures installed in or near the stream to protect highways and bridges by stabilizing a local reach of the stream, and measures which can be incorporated into the highway design to ensure the structural integrity of the highway in an unstable stream environment. Countermeasures include river stabilizing works over a reach of the river up- and downstream of the crossing. Countermeasures may be installed at the time of highway construction or retrofitted to resolve scour and instability problems as they develop at existing crossings. The selection, location, and design of countermeasures are dependent on hydraulic and geomorphic factors that contribute to stream instability, as well as costs and construction and maintenance considerations.
While considerable research has been dedicated to design of countermeasures for scour and stream instability, many countermeasures have evolved through a trial and error process. In addition, some countermeasures have been applied successfully in one locale, state or region, but have failed when installations were attempted under different geomorphic or hydraulic conditions. In some cases, a countermeasure that has been used with success in one state or region is virtually unknown to highway design and maintenance personnel in another state or region. Thus, there is a significant need for information transfer regarding stream instability and bridge scour countermeasure design, installation, and maintenance.
This manual is presented in two volumes. Volume 1 is organized to:
Volume 2 presents detailed Design Guidelines for 16 stream instability and bridge scour countermeasures. Design Guidelines are presented for the following functional applications:
This manual is part of a set of Hydraulic Engineering Circulars (HEC) issued to provide guidance for bridge scour and stream stability analyses. The three manuals in this set are:
The Flow Chart shown in Figure 1.1 illustrates the interrelationship between these three documents and emphasizes that they should be used as a set. A comprehensive scour analysis or stability evaluation must be based on information presented in all three documents.
While the flow chart does not attempt to present every detail of a complete stream stability and scour evaluation, it has sufficient detail to show the major elements in a complete analysis, the logical flow of a typical analysis or evaluation, and the most common decision points and feedback loops. It clearly shows how the three documents tie together, and recognizes the differences between design of a new bridge and evaluation of an existing bridge.
The HEC-20 (Lagasse et al. 2001a) block of the flow chart outlines initial data collection and site reconnaissance activities leading to an understanding of the problem, evaluation of river system stability and potential future response. The HEC-20 procedures include both qualitative and quantitative geomorphic and engineering analysis techniques which help establish the level of analysis necessary to solve the stream instability and scour problem for design of a new bridge, or for the evaluation of an existing bridge that may require rehabilitation or countermeasures. The "Classify Stream," "Evaluate Stream Stability," and "Assess Stream Response" portions of the HEC-20 block are expanded in HEC-20 into a six-step Level 1 and an eight-step Level 2 analysis procedure. In some cases, the HEC-20 analysis may be sufficient to determine that stream instability and/or scour problems do not exist, i.e., the bridge has a "low risk of failure" regarding scour susceptibility.
In most cases, the analysis or evaluation will progress to the HEC-18 (Richardson and Davis 2001) block of the flow chart. Here more detailed hydrologic and hydraulic data are developed, with the specific approach determined by the level of complexity of the problem and waterway characteristics (e.g., tidal or riverine). The "Scour Analysis" portion of the HEC-18 block encompasses a seven-step specific design approach which includes evaluation of the components of total scour.
Since bridge scour evaluation requires multidisciplinary inputs, it is often advisable for the hydraulic engineer to involve structural and geotechnical engineers at this stage of the analysis. Once the total scour prism is plotted, then all three disciplines must be involved in a determination of the structural stability of the bridge foundation.
For a new bridge design, if the structure is stable the design process can proceed to consideration of environmental impacts, cost, constructability, and maintainability or if the bridge is unstable, revise the design and repeat the analysis. For an existing bridge, a finding of structural stability at this stage will result in a "low risk" evaluation, with no further action required. However, a Plan of Action must be developed for an unstable existing bridge (scour critical) to correct the problem as outlined in Sections 1.5 and 2.1.
The scour problem may be so serious that installing countermeasures would not provide a viable solution and a replacement or substantial bridge rehabilitation would be required. If countermeasures would correct the stream instability or scour problem at a reasonable cost and with acceptable environmental impacts, the analysis would progress to the HEC-23 block of the flow chart.
Hydraulic Engineering Circular 23 provides a range of resources to support bridge scour and stream instability countermeasure selection and design. A countermeasure matrix (Chapter 2) presents a variety of countermeasures that have been used to control scour and stream instability at bridges.
HEC-23 also includes specific Design Guidelines for the most common (and some uncommon) countermeasures used by DOTs, or references to sources of design guidance. Inherent in the design of any countermeasure are an evaluation of potential environmental impacts, permitting for countermeasure installation, and redesign, if necessary, to meet environmental requirements. As shown in the flow chart, to be effective most countermeasures will require a monitoring plan, inspection, and maintenance.
Each bridge identified as scour critical in Item 113 of the National Bridge Inventory must have a plan of action as required by the National Bridge Inspection Standards (NBIS) regulation 23 CFR 650.313(e) describing what will be done to address the scour problem. The plan of action should include a monitoring program and a schedule for the timely design and construction of hydraulic or structural countermeasures, if any are warranted. The purpose of the plan of action is to provide for the safety of the traveling public, and to minimize the potential for bridge failure, by prescribing site-specific actions that will be taken at the bridge to correct the scour problem. The actions (or countermeasures) taken can be categorized as hydraulic countermeasures, structural countermeasures, biotechnical countermeasures, or a monitoring program (see Chapter 2).
Hydraulic countermeasures are primarily designed to modify the stream flow or resist erosive forces. Examples of hydraulic countermeasures include the installation of river training structures and the placement of riprap at piers or abutments. Structural countermeasures usually involve modification of the bridge substructure to increase bridge stability. Typical structural countermeasures are underpinning and pier modification.
A scour monitoring program as part of the plan of action includes two primary components:
Note that a monitoring program involves more than just instrumentation. It must describe specific actions to be taken once a scour problem has been identified. In some cases, a properly designed scour monitoring program can be an acceptable countermeasure by itself. However, monitoring does not fix the scour problem, and therefore, does not allow changing the Item 113 coding on a scour-critical bridge. In other cases, a monitoring program allows time to implement hydraulic or structural countermeasures. Information in Section 2.1 outlines how to develop a plan of action for a scour critical bridge, and provides specific strategies for deciding when and how to implement a monitoring program.
This edition of HEC-23 uses dual units (English and SI metric). The "English" system of units as used throughout this manual refers to U.S. Customary units. In Appendix A, the metric (SI) unit of measurement is explained. The conversion factors, physical properties of water in the SI and English systems of units, sediment particle size grade scale, and some common equivalent hydraulic units are also given. This edition uses for the unit of length the meter (m) or foot (ft); of mass the kilogram (kg) or slug; of weight/force the newton (N) or pound (lb); of pressure the Pascal (Pa, N/m2) or (lb/ft2); and of temperature the degree Centigrade ( ° C) or Fahrenheit ( ° F). The unit of time is the same in SI as in English system (seconds, s). Sediment particle size is given in millimeters (mm), but in calculations the decimal equivalent of millimeters in meters is used (1 mm = 0.001 m) or for the English system feet (ft). The values of some hydraulic engineering terms used in the text in SI units and their equivalent English units are given in Table 1.1.