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

Publication No. FHWA-NHI-09-111 HEC-23
September 2009

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

DESIGN GUIDELINES (Volume 2)

ACKNOWLEDGMENTS AND DISCLAIMER

GLOSSARY

CHAPTER 1. INTRODUCTION

1.1 PURPOSE

1.2 BACKGROUND

1.3 MANUAL ORGANIZATION

1.4 COMPREHENSIVE ANALYSIS

1.5 PLAN OF ACTION

1.6 DUAL SYSTEM OF UNITS

CHAPTER 2. PLAN OF ACTION AND THE COUNTERMEASURES MATRIX
2.1 STRATEGIES FOR PROTECTING SCOUR CRITICAL BRIDGES

2.1.1 Technical Advisories

2.1.2 Additional Guidance and Requirements

2.1.3 Management Strategies for a Plan of Action

2.1.4 Inspection Strategies in a Plan of Action

2.1.5 Closure Instructions

2.1.6 Countermeasure Alternatives and Schedule

2.1.7 Other Information Necessary in a Plan of Action

2.1.8 Development and Implementation of a POA

2.2 STANDARD TEMPLATE FOR A PLAN OF ACTION

2.2.1 Overview

2.2.2 Executive Summary

2.3 THE COUNTERMEASURE MATRIX
2.4 COUNTERMEASURE GROUPS

2.4.1 Group 1. Hydraulic Countermeasures

2.4.2 Group 2. Structural Countermeasures

2.4.3 Group 3. Biotechnical Countermeasures

2.4.4 Group 4. Monitoring

2.5 COUNTERMEASURE CHARACTERISTICS

2.5.1 Functional Applications

2.5.2 Suitable River Environment

2.5.3 Maintenance

2.5.4 Installation/Experience by State Departments of Transportation

2.5.5 Design Guideline Reference

2.5.6 Summary

CHAPTER 3. CONSIDERATIONS FOR SELECTING COUNTERMEASURES
3.1 INTRODUCTION
3.2 CRITERIA FOR THE SELECTION OF COUNTERMEASURES

3.2.1 Erosion Mechanism

3.2.2 Stream Characteristics

3.2.3 Construction and Maintenance Requirements

3.2.4 Vandalism

3.2.5 Countermeasure Selection Based on Cost

3.2.6 Countermeasure Selection Based on Risk

3.3 COUNTERMEASURES FOR MEANDER MIGRATION
3.4 COUNTERMEASURES FOR CHANNEL BRAIDING AND ANABRANCHING
3.5 COUNTERMEASURES FOR DEGRADATION AND AGGRADATION

3.5.1 Countermeasures to Control Degradation

3.5.2 Countermeasures to Control Aggradation

3.6 SELECTION OF COUNTERMEASURES FOR SCOUR AT BRIDGES

3.6.1 Countermeasures for Contraction Scour

3.6.2 Countermeasures for Local Scour

3.6.3 Monitoring

CHAPTER 4. COUNTERMEASURE DESIGN CONCEPTS
4.1 COUNTERMEASURE DESIGN APPROACH

4.1.1 Investment in Countermeasures

4.1.2 Service Life and Safety

4.1.3 Design Approach

4.2 ENVIRONMENTAL PERMITTING
4.3 HYDRAULIC ANALYSIS

4.3.1 Overview

4.3.2 Physical Models

4.3.3 Scour at Transverse Structures

4.3.4 Scour at Longitudinal Structures

4.3.5 Scour at Protected Bendways

4.3.6 Hydraulic Stress on a Bendway

CHAPTER 5. RIPRAP DESIGN, FILTERS, FAILURE MODES, AND ALTERNATIVES
5.1 OVERVIEW
5.2 RIPRAP DESIGN

5.2.1 Introduction

5.2.2 Riprap Revetment

5.2.3 Riprap for Bridge Piers

5.2.4 Riprap for Bridge Abutments

5.2.5 Riprap Protection for Countermeasures

5.2.6 Riprap for Special Applications

5.2.7 Termination Details

5.2.8 Riprap Size, Shape, and Gradation

5.3 FILTER REQUIREMENTS

5.3.1 Overview

5.3.2 Placing Geotextiles Under Water

5.4 RIPRAP FAILURE MODES

5.4.1 Riprap Revetment Failure Modes

5.4.2 Pier Riprap Failure Modes

5.4.3 Pier Riprap Failure Modes - Schoharie Creek Case Study

5.5 RIPRAP INSPECTION GUIDANCE

5.5.1 General

5.5.2 Guidance for Recording Riprap Condition

5.5.3 Performance Evaluation

5.6 GROUTED AND PARTIALLY GROUTED RIPRAP
5.7 CONCRETE ARMOR UNITS
CHAPTER 6. BIOTECHNICAL ENGINEERING
6.1 OVERVIEW
6.2 CURRENT PRACTICE
6.3 GENERAL CONCEPTS
6.4 ADVANTAGES AND LIMITATIONS OF BIOTECHNICAL ENGINEERING
6.5 DESIGN CONSIDERATIONS FOR BIOTECHNICAL COUNTERMEASURES
6.6 COMMONLY USED VEGETATIVE METHODS
6.7 ENVIRONMENTAL CONSIDERATIONS AND BENEFITS
6.8 APPLICATION GUIDANCE FOR BIOTECHNICAL COUNTERMEASURES

6.8.1 Streambank Zones

6.8.2 Biotechnical Engineering Treatments

6.9 SUMMARY
CHAPTER 7. COUNTERMEASURE DESIGN GUIDELINES
7.1 INTRODUCTION
7.2 DESIGN GUIDELINES

7.2.1 Countermeasures for Stream Instability

7.2.2 Countermeasures for Streambank and Roadway Embankment Protection

7.2.3 Countermeasures for Bridge Pier Protection

7.2.4 Countermeasures for Abutment Protection

7.2.5 Filter Design

7.2.6 Special Applications

CHAPTER 8. OTHER COUNTERMEASURES AND CASE HISTORIES OF PERFORMANCE
8.1 INTRODUCTION
8.2 HARDPOINTS
8.3 RETARDER STRUCTURES

8.3.1 Jacks and Tetrahedrons

8.3.2 Fence Retarder Structures

8.3.3 Timber Pile

8.3.4 Wood Fence

8.4 LONGITUDINAL DIKES

8.4.1 Earth or Rock Embankments

8.4.2 Rock Toe-Dikes

8.4.3 Crib Dikes

8.4.4 Bulkheads

8.5 CHANNEL RELOCATION
8.6 CASE HISTORIES OF COUNTERMEASURE PERFORMANCE

8.6.1 Flexible Revetment

8.6.2 Rigid Revetments

8.6.3 Bulkheads

8.6.4 Spurs

8.6.5 Retardance Structures

8.6.6 Dikes

8.6.7 Guide Banks

8.6.8 Check Dams

8.6.9 Jack or Tetrahedron Fields

8.6.10 Special Devices for Protection of Piers

8.6.11 Channel Alterations

8.6.12 Modification of Bridge Length and Relief Structures

8.6.13 Investment in Countermeasures

CHAPTER 9. SCOUR MONITORING AND INSTRUMENTATION
9.1 INTRODUCTION
9.2 PORTABLE INSTRUMENTATION

9.2.1 Components of a Portable Instrument System

9.2.2 Instrument for Making the Measurement

9.2.3 System for Deploying the Instrument

9.2.4 Positioning Information

9.2.5 Data Storage Devices

9.3 FIXED INSTRUMENTATION

9.3.1 NCHRP Project 21-3

9.3.2 Scour Measurement

9.3.3 Summary of NCHRP Project 21-3 Results

9.3.4 Operational Fixed Instrument Systems

9.3.5 NCHRP Project 20-5

9.3.6 Application Guidelines

9.4 SELECTING INSTRUMENTATION

9.4.1 Portable Instruments

9.4.2 Fixed Instruments

9.5 FIXED INSTRUMENT CASE HISTORIES

9.5.1 Introduction

9.5.2 Typical Field Installations

CHAPTER 10. REFERENCES
APPENDIX A - Metric System, Conversion Factors, and Water Properties
APPENDIX B - Standard Template for a Plan of Action
APPENDIX C - Pier Scour Countermeasure Selection Methodology
APPENDIX D - Riprap Inspection Recording Guidance
LIST OF FIGURES

Figure 1.1 Flow chart for scour and stream stability analysis and evaluation

Figure 3.1 Risk-based approach using NBI data

Figure 3.2 Comparison of channel bend cross sections for natural conditions and for stabilized bend

Figure 3.3 Meander migration in a natural channel and a channel with stabilized bend

Figure 3.4 Typical guide bank layout

Figure 4.1 BAW laboratory, Karlsruhe, Germany, pier scour model of railway bridge over Rhine River near Mannheim

Figure 4.2 Scour along a vertical wall as a function of unconstrained valley width

Figure 4.3 Definition sketch of width and mean water depth at the crossing upstream of the bend and maximum water depth in the bend

Figure 4.4 Shear stress multiplier Kb for bends

Figure 5.1 Riprap shape described by three axes

Figure 5.2 Close-up photo of SandMatTM geocomposite blanket

Figure 5.3 SandMatTM geocomposite blanket being unrolled

Figure 5.4 Sand-filled geotextile containers

Figure 5.5 Filling 1.0 metric tonne geotextile container with sand

Figure 5.6 Handling a 1.0 metric tonne sand-filled geotextile container

Figure 5.7 Two hundred lb (91 kg) sand-filled geotextile containers

Figure 5.8 Riprap failure by particle erosion

Figure 5.9 Damaged riprap on left bank of Pinole Creek at Pinole, CA

Figure 5.10 Riprap failure by translational slide

Figure 5.11 Riprap on Cosumnes River at Site 2 near Sloughhouse, CA

Figure 5.12 Riprap failure by modified slump

Figure 5.13 Riprap on Cosumnes River at Site 3 near Sloughhouse, CA

Figure 5.14 Riprap failure due to slump

Figure 5.15 Riprap on left bank Consumnes River at Site 1 near Sloughhouse, CA

Figure 5.16 South elevation - Schoharie Creek Bridge showing key structural features and a schematic geological section

Figure 5.17 Pier scour holes at Schoharie Creek Bridge in 1987

Figure 5.18 Photograph of riprap at pier 2, October 1956

Figure 5.19 Photograph of riprap at pier 2, August 1977

Figure 5.20 Close-up view of partially grouted riprap

Figure 5.21 "Conglomerate" of partially grouted riprap

Figure 5.22 Concrete armor units

Figure 5.23 Laboratory study of Toskanes for pier scour protection

Figure 5.24 Installation of A-Jacks ® modular units installed by Kentucky DOT

Figure 6.1 Details of brush mattress technique with stone toe protection

Figure 6.2 Details of root wad and boulder revetment technique

Figure 6.3 Vegetated riprap - willow bundle method

Figure 6.4 Vegetated riprap - bent pole method

Figure 6.5 Vegetated riprap - brush layering with pole planting

Figure 6.6 Vegetated riprap - construction techniques

Figure 6.7 Vegetated riprap with joint planting

Figure 6.8 Bank zones defined for slope protection

Figure 8.1 Perspective view of hardpoint installation with section detail

Figure 8.2 Typical tetrahedron design

Figure 8.3 Typical jack unit

Figure 8.4 Retarder field schematic

Figure 8.5 Timber pile bent retarder structure

Figure 8.6 Typical wood fence retarder structure

Figure 8.7 Light double row wire fence retarder structure

Figure 8.8 Heavy timber-pile and wire fence retarder structures

Figure 8.9 Typical longitudinal rock toe-dike geometries

Figure 8.10 Longitudinal rock toe-dike tiebacks

Figure 8.11 Timber pile, wire mesh crib dike with tiebacks

Figure 8.12 Anchorage schemes for a sheetpile bulkhead

Figure 8.13 Encroachments on meandering streams

Figure 9.1 Sounding pole measurement

Figure 9.2 Lead-line sounding weight

Figure 9.3 Portable sonar in use

Figure 9.4 Kneeboard float

Figure 9.5 Pontoon float

Figure 9.6 AIDI system

Figure 9.7 FHWA articulated arm in use

Figure 9.8 NCHRP 21-3 articulated arm truck

Figure 9.9 Sonar instrument deployed from articulated arm truck

Figure 9.10 Unmanned, remote control boat

Figure 9.11 Master station with data logger for use with any of the fixed instruments

Figure 9.12 Above-water serviceable low-cost fathometer system

Figure 9.13 Sonar scour monitor installation including remote station

Figure 9.14 Manual read out magnetic sliding collar device

Figure 9.15 Automated read out magnetic sliding collar system

Figure 9.16 Detail of magnetic sliding collar on the streambed

Figure 9.17 Float-out devices prior to installation

Figure 9.18 Tilt meter on California bridge

Figure 9.19 Detail of tilt meter

Figure 9.20 Time domain reflectometry probe

Figure 9.21 Brisco Monitor sounding rods installed at a bridge pier in New York

Figure 9.22 Active streambed scour monitoring locations in Alaska

Figure 9.23 Oblique aerial photograph of the Knik River Old Glenn Highway

Figure 9.24 Conduit to sonar scour monitor at Wantagh Parkway

Figure 9.25 Solar panel, remote station and (inset) conduit to sonar monitor

Figure 9.26 Adjustable stainless steel sonar mounting bracket

Figure 9.27 Installation of a sonar scour monitor on Salinas River Bridge

Figure 9.28 Close-up of sonar scour monitor on Salinas River Bridge

Figure 9.29 CALTRANS drilling with hollow stem auger for installation of float out devices at Salinas River Bridge

Figure 9.30 Installation of float out device on Salinas River Bridge

Figure 9.31 Typical instrument shelter with data logger, cell-phone telemetry

Figure 9.32 Installation of a float out device by Nevada DOT to monitor riprap

LIST OF TABLES

Table 1.1 Commonly Used Engineering Terms in SI and English Units

Table 2.1 Stream Instability and Bridge Scour Countermeasures Matrix

Table 5.1 Minimum and Maximum Allowable Particle Size in Inches

Table 5.2 Minimum and Maximum Allowable Particle Weight in Pounds

Table 9.1 Instrumentation Summary by Category

Table 9.2 Portable Instrumentation Summary

Table 9.3 Positioning System Summary

Table 9.4 Estimated Cost Information for Portable Instruments

Table 9.5 Fixed Instrumentation Summary

Table 9.6 Fixed Instrumentation Selection Matrix

Table 9.7 Estimated Cost Information for Fixed Instruments

DESIGN GUIDELINES
SECTION 1 - COUNTERMEASURES FOR STREAM INSTABILITY

Design Guideline 1 - Bendway Weirs/Stream Barbs

Design Guideline 2 - Spurs

Design Guideline 3 - Check Dams/Drop Structures

SECTION 2 - COUNTERMEASURES FOR STREAMBANK AND ROADWAY EMBANKMENT PROTECTION

Design Guideline 4 - Riprap Revetment

Design Guideline 5 - Riprap Design for Embankment Overtopping

Design Guideline 6 - Wire Enclosed Riprap Mattress

Design Guideline 7 - Soil Cement

Design Guideline 8 - Articulating Concrete Block Systems

Design Guideline 9 - Grout-Filled Mattresses

Design Guideline 10 - Gabion Mattresses

SECTION 3 - COUNTERMEASURES FOR BRIDGE PIER PROTECTION

Design Guideline 8 - Articulating Concrete Block Systems at Bridge Piers

Design Guideline 9 - Grout-Filled Mattresses at Bridge Piers

Design Guideline 10 - Gabion Mattresses at Bridge Piers

Design Guideline 11 - Rock Riprap at Bridge Piers

Design Guideline 12 - Partially Grouted Riprap at Bridge Piers

SECTION 4 - COUNTERMEASURES FOR ABUTMENT PROTECTION

Design Guideline 13 - Grout/Cement Filled Bags

Design Guideline 14 - Rock Riprap at Bridge Abutments

Design Guideline 15 - Guide Bank

SECTION 5 - FILTER DESIGN

Design Guideline 16 - Filter Design

SECTION 6 - SPECIAL APPLICATIONS

Design Guideline 17 - Riprap Design for Wave Attack

Design Guideline 18 - Riprap Protection for Bottomless Culverts

Design Guideline 19 - Concrete Armor Units

Technical Report Documentation Page

1. Report No. FHWA NHI
                    HEC-23
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle

BRIDGE SCOUR AND STREAM INSTABILITY
COUNTERMEASURES
Experience, Selection and Design Guidance
Volumes 1 and 2
Third Edition
5. Report Date

September 2009
6. Performing Organization Code
7. Author(s)

P.F. Lagasse, P.E. Clopper, J.E. Pagán-Ortiz, L.W. Zevenbergen,
L.A. Arneson, J.D. Schall, L.G. Girard
8. Performing Organization Report No.
9. Performing Organization Name and Address

Ayres Associates
3665 JFK Parkway
Building 2, Suite 200
Fort Collins, Colorado 80525
10. Work Unit No. (TRAIS)
11. Contract or Grant No.

DTFH61-06-D-00010/T-06-001
12. Sponsoring Agency Name and Address

Office of Bridge Technology
FHWA, HIBT-20
1200 New Jersey Ave.,
Washington, D.C. 20590
        National Highway Institute
4600 North Fairfax Dr.,
Suite 800 SE
Arlington, Virginia 22203
13. Type of Report and Period Covered
14. Sponsoring Agency Code
15. Supplementary Notes

Project Managers: Dr. Larry A. Arneson and Mr. Jorge E. Pagán-Ortiz, FHWA
Technical Assistants: Scott Anderson, Kornel Kerenyi, Joe Krolak, Barry Siel, FHWA; S. Mishra, Ayres Associates;
B. Hunt, STV Inc.
16. This document identifies and provides 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. Selected innovative countermeasure concepts and guidance derived from practice outside the United States are introduced. Management strategies and guidance for developing a Plan of Action for scour critical bridges are outlined, and guidance is provided for scour monitoring using portable and fixed instrumentation.

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. This additional material required expanding HEC-23 to two volumes. Volume 1 now contains a complete chapter on riprap design, specifications, and quality control as well as an expanded chapter on biotechnical countermeasures. The guidance on scour monitoring instrumentation has been updated and now includes additional installation case studies. Volume 2 contains 19 detailed design guidelines grouped into six categories, including countermeasures for: (1) stream instability (2) streambank and roadway embankment protection, (3) bridge pier protection, (4) abutment protection, (5) filter design, and (6) special applications.
17. Key Words

stream stability, scour, countermeasures, plan of action, bendway weirs, soil cement, wire enclosed riprap, articulating concrete block systems, concrete armor units, gabion mattresses, grout filled mattresses, grout bags, rock riprap, partially grouted riprap, spurs, guide banks, check dams, revetments, scour monitoring instrumentation
18. Distribution Statement

This document is available to the public through the National Technical Information Service, Springfield, VA 22161 (703) 487-4650
19. Security Classif. (of this report)

Unclassified
20. Security Classif. (of this page)

Unclassified
21. No. of Pages

256
22. Price

Form DOT F 1700.7 (8-72)
Reproduction of completed page authorized

ACKNOWLEDGMENTS

This manual is a major revision of the second edition of HEC-23 which was published in 2001. The writers wish to acknowledge the contributions made by Morgan S. Byars (formerly Ayres Associates) as a co-author of the first edition (1997). Technical assistance for the second edition was provided by J. Sterling Jones (FHWA) and A. Firenzi, J.L. Morris, E.V. Richardson, W.J. Spitz, and A. Waddoups (Ayres Associates).

DISCLAIMER

Mention of a manufacturer, registered or trade name does not constitute a guarantee or warranty of the product by the U.S. Department of Transportation or the Federal Highway Administration and does not imply their approval and/or endorsement to the exclusion of other products and/or manufacturers that may also be suitable.

GLOSSARY

abrasion: Removal of streambank material due to entrained sediment, ice, or debris rubbing against the bank.
aggradation: General and progressive buildup of the longitudinal profile of a channel bed due to sediment deposition.
alluvial channel Channel wholly in alluvium; no bedrock is exposed in channel at low flow or likely to be exposed by erosion.
alluvial fan: Fan‑shaped deposit of material at the place where a stream issues from a narrow valley of high slope onto a plain or broad valley of low slope. An alluvial cone is made up of the finer materials suspended in flow while a debris cone is a mixture of all sizes and kinds of materials.
alluvial stream: Stream which has formed its channel in cohesive or noncohesive materials that have been and can be transported by the stream.
alluvium: Unconsolidated material deposited by a stream in a channel, floodplain, alluvial fan, or delta.
alternating bars: Elongated deposits found alternately near the right and left banks of a channel.
ambient bed elevation: Initial (unscoured) bed elevation.
anabranch: Individual channel of an anabranched stream.
anabranched stream: Stream whose flow is divided at normal and lower stages by large islands or, more rarely, by large bars; individual islands or bars are wider than about three times water width; channels are more widely and distinctly separated than in a braided stream.
anastomosing stream: An anabranched stream.
angle of repose: Maximum angle (as measured from the horizontal) at which gravel or sand particles can stand.
annual flood: Maximum flow in 1 year (may be daily or instantaneous).
apron: Protective material placed on a streambed to resist scour.
apron, launching: Apron designed to settle and protect the side slopes of a scour hole after settlement.
armor (armoring): Surfacing of channel bed, banks, or embankment slope to resist erosion and scour. (A) Natural process whereby an erosion‑ resistant layer of relatively large particles is formed on a streambed due to the removal of finer particles by streamflow; (B) placement of a covering to resist erosion.
articulated concrete mattress: Rigid concrete slabs which can move without separating as scour occurs; usually hinged together with corrosion-resistant cable fasteners; primarily placed for lower bank protection.
average velocity: Velocity at a given cross section determined by dividing discharge by cross sectional area.
avulsion: Sudden change in the channel course that usually occurs when a stream breaks through its banks; usually associated with a flood or a catastrophic event.
backfill: Material used to refill a ditch or other excavation, or the process of doing so.
backwater: Increase in water surface elevation relative to elevation occurring under natural channel and floodplain conditions. It is induced by a bridge or other structure that obstructs or constricts the free flow of water in a channel.
backwater area: Low‑lying lands adjacent to a stream that may become flooded due to backwater.
bank: Sides of a channel between which the flow is normally confined.
bank, left (right): Sides of a channel as viewed in a downstream direction.
bankfull discharge: Discharge that, on average, fills a channel to the point of overflowing.
bank protection: Engineering works for the purpose of protecting streambanks from erosion.
bank revetment: Erosion‑resistant materials placed directly on a streambank to protect the bank from erosion.
bar: Elongated deposit of alluvium within a channel, not permanently vegetated.
base floodplain: Floodplain associated with the flood with a 100-year recurrence interval.
bed: Bottom of a channel bounded by banks.
bed form: Recognizable relief feature on the bed of a channel, such as a ripple, dune, plane bed, antidune, or bar. Bed forms are a consequence of the interaction between hydraulic forces (boundary shear stress) and the bed sediment.
bed layer: Flow layer, several grain diameters thick (usually two) immediately above the bed.
bed load: Sediment that is transported in a stream by rolling, sliding, or skipping along the bed or very close to it; considered to be within the bed layer (contact load).
bed load discharge (or bed load): Quantity of bed load passing a cross section of a stream in a unit of time.
bed material: Material found in and on the bed of a stream (May be transported as bed load or in suspension).
bedrock: Solid rock exposed at the surface of the earth or overlain by soils and unconsolidated material.
bed sediment discharge: Part of the total sediment discharge that is composed of grain sizes found in the bed and is equal to the transport capability of the flow.
bed shear stationary (tractive force): Force per unit area exerted by a fluid flowing past a boundary.
bed slope: Inclination of the channel bottom.
biotechnical engineering: Countermeasure techniques that combine the use of vegetation with structural (hard) elements.
blanket: Material covering all or a portion of a streambank to prevent erosion.
boulder: Rock fragment whose diameter is greater than 250 mm.
braid: Subordinate channel of a braided stream.
braided stream: Stream whose flow is divided at normal stage by small mid‑channel bars or small islands; the individual width of bars and islands is less than about three times water width; a braided stream has the aspect of a single large channel within which are subordinate channels.
bridge opening: Cross‑sectional area beneath a bridge that is available for conveyance of water.
bridge owner: Any Federal, State, Local agency, or other entity responsible for a structure defined as a highway bridge by the National Bridge Inspection Standards (NBIS).
bridge waterway Area of a bridge opening available for flow, as measured below a specified stage and normal to the principal direction of flow.
bulk density: Density of the water sediment mixture (mass per unit volume), including both water and sediment.
bulkhead: Vertical, or near vertical, wall that supports a bank or an embankment; also may serve to protect against erosion.
bulking: Increasing the water discharge to account for high concentrations of sediment in the flow.
catchment: See drainage basin.
causeway: Rock or earth embankment carrying a roadway across water.
caving: Collapse of a bank caused by undermining due to the action of flowing water.
cellular-block mattress: Interconnected concrete blocks with regular cavities placed directly on a streambank or filter to resist erosion. The cavities can permit bank drainage and the growth of vegetation where synthetic filter fabric is not used between the bank and mattress.
channel: Bed and banks that confine surface flow of a stream.
channelization: Straightening or deepening of a natural channel by artificial cutoffs, grading, flow‑control measures, or diversion of flow into an engineered channel.
channel diversion: Removal of flows by natural or artificial means from a natural length of channel.
channel pattern: Aspect of a stream channel in plan view, with particular reference to the degree of sinuosity, braiding, and anabranching.
channel process: Behavior of a channel with respect to shifting, erosion and sedimentation.
check dam: Low dam or weir across a channel used to control stage or degradation.
choking (of flow): Excessive constriction of flow which may cause severe backwater effect.
clay (mineral): Particle whose diameter is in the range of 0.00024 to 0.004 mm.
clay plug: Cutoff meander bend filled with fine grained cohesive sediments.
clear-water scour: Scour at a pier or abutment (or contraction scour) when there is no movement of the bed material upstream of the bridge crossing at the flow causing bridge scour.
cobble: Fragment of rock whose diameter is in the range of 64 to 250 mm.
concrete revetment: Unreinforced or reinforced concrete slabs placed on the channel bed or banks to protect it from erosion.
confluence: Junction of two or more streams.
constriction: Natural or artificial control section, such as a bridge crossing, channel reach or dam, with limited flow capacity in which the upstream water surface elevation is related to discharge.
contact load: Sediment particles that roll or slide along in almost continuous contact with the streambed (bed load).
contraction: Effect of channel or bridge constriction on flow streamlines.
contraction scour: Contraction scour, in a natural channel or at a bridge crossing, involves the removal of material from the bed and banks across all or most of the channel width. This component of scour results from a contraction of the flow area at the bridge which causes an increase in velocity and shear stress on the bed at the bridge. The contraction can be caused by the bridge or from a natural narrowing of the stream channel.
Coriolis force: Inertial force caused by the Earth's rotation that deflects a moving body to the right in the Northern Hemisphere.
countermeasure: Measure intended to prevent, delay or reduce the severity of hydraulic problems.
crib: Frame structure filled with earth or stone ballast, designed to reduce energy and to deflect streamflow away from a bank or embankment.
critical shear stress: Minimum amount of shear stress required to initiate soil particle motion.
crossing: Relatively short and shallow reach of a stream between bends; also crossover or riffle.
cross section: Section normal to the trend of a channel or flow.
current: Water flowing through a channel.
current meter: Instrument used to measure flow velocity.
cut bank: Concave wall of a meandering stream.
cutoff: (A) Direct channel, either natural or artificial, connecting two points on a stream, thereby shortening the original length of the channel and increasing its slope; (B) natural or artificial channel which develops across the neck of a meander loop (neck cutoff) or across a point bar (chute cutoff).
cutoff wall: Wall, usually of sheet piling or concrete, that extends down to scour‑resistant material or below the expected scour depth.
daily discharge: Discharge averaged over 1 day (24 hours).
debris: Floating or submerged material, such as logs, vegetation, or trash, transported by a stream.
degradation (bed): General and progressive (long-term) lowering of the channel bed due to erosion, over a relatively long channel length.
depth of scour: Vertical distance a streambed is lowered by scour below a reference elevation.
design flow (design flood): Discharge that is selected as the basis for the design or evaluation of a hydraulic structure.
dike: An impermeable linear structure for the control or containment of overbank flow. A dike-trending parallel with a streambank differs from a levee in that it extends for a much shorter distance along the bank, and it may be surrounded by water during floods.
dike (groin, spur, jetty): Structure extending from a bank into a channel that is designed to: (A) reduce the stream velocity as the current passes through the dike, thus encouraging sediment deposition along the bank (permeable dike); or (B) deflect erosive current away from the streambank (impermeable dike).
discharge: Volume of water passing through a channel during a given time.
dominant discharge: (A) Discharge of water which is of sufficient magnitude and frequency to have a dominating effect in determining the characteristics and size of the stream course, channel, and bed; (B) discharge which determines the principal dimensions and characteristics of a natural channel. Dominant formative discharge depends on the maximum and mean discharge, duration of flow, and flood frequency. For hydraulic geometry relationships, it is taken to be the bankfull discharge which has a return period of approximately 1.5 years in many natural channels.
drainage basin: Area confined by drainage divides, often having only one outlet for discharge (catchment, watershed).
drift: Alternate term for vegetative "debris."
eddy current: Vortex‑type motion of a fluid flowing contrary to the main current, such as the circular water movement that occurs when the main flow becomes separated from the bank.
entrenched stream: Stream cut into bedrock or consolidated deposits.
ephemeral stream: Stream or reach of stream that does not flow for parts of the year. As used here, the term includes intermittent streams with flow less than perennial.
equilibrium scour: Scour depth in sand-bed stream with dune bed about which live bed pier scour level fluctuates due to variability in bed material transport in the approach flow.
erosion: Displacement of soil particles due to water or wind action.
erosion control matting: Fibrous matting (e.g., jute, paper, etc.) placed or sprayed on a streambank for the purpose of resisting erosion or providing temporary stabilization until vegetation is established.
fabric mattress: Grout-filled mattress used for streambank protection.
fall velocity: Velocity at which a sediment particle falls through a column of still water.
fascine: Matrix of willow or other natural material woven in bundles and used as a filter. Also, a streambank protection technique consisting of wire mesh or timber attached to a series of posts, sometimes in double rows; the space between the rows may be filled with rock, brush, or other materials.
fetch: Area in which waves are generated by wind having a rather constant direction and speed; sometimes used synonymously with fetch length.
fetch length: Horizontal distance (in the direction of the wind) over which wind generates waves and wind setup.
fill slope: Side or end slope of an earth‑fill embankment. Where a fill‑slope forms the streamward face of a spill-through abutment, it is regarded as part of the abutment.
filter: Layer of fabric (geotextile) or granular material (sand, gravel, or graded rock) placed between bank revetment (or bed protection) and soil for the following purposes: (1) to prevent the soil from moving through the revetment by piping, extrusion, or erosion; (2) to prevent the revetment from sinking into the soil; and (3) to permit natural seepage from the streambank, thus preventing the buildup of excessive hydrostatic pressure.
filter blanket: Layer of graded sand and gravel laid between fine‑grained material and riprap to serve as a filter.
filter fabric (cloth): Geosynthetic fabric that serves the same purpose as a granular filter blanket.
fine sediment load: That part of the total sediment load that is composed of particle sizes finer than those represented in the bed (wash load). Normally, the fine‑sediment load is finer than 0.062 mm for sand‑bed channels. Silts, clays and sand could be considered wash load in coarse gravel and cobble-bed channels.
flanking: Erosion around the landward end of a stream stabilization countermeasure.
flashy stream: Stream characterized by rapidly rising and falling stages, as indicated by a sharply peaked hydrograph. Typically associated with mountain streams or highly disturbed urbanized catchments. Most flashy streams are ephemeral, but some are perennial.
flood-frequency curve: Graph indicating the probability that the annual flood discharge will exceed a given magnitude, or the recurrence interval corresponding to a given magnitude.
floodplain: Nearly flat, alluvial lowland bordering a stream, that is subject to frequent inundation by floods.
flow-control structure: Structure either within or outside a channel that acts as a countermeasure by controlling the direction, depth, or velocity of flowing water.
flow hazard: Flow characteristics (discharge, stage, velocity, or duration) that are associated with a hydraulic problem or that can reasonably be considered of sufficient magnitude to cause a hydraulic problem or to test the effectiveness of a countermeasure.
flow slide: Saturated soil materials which behave more like a liquid than a solid. A flow slide on a channel bank can result in a bank failure.
fluvial geomorphology: Science dealing with morphology (form) and dynamics of streams and rivers.
fluvial system: Natural river system consisting of (1) the drainage basin, watershed, or sediment source area, (2) tributary and mainstem river channels or sediment transfer zone, and (3) alluvial fans, valley fills and deltas, or the sediment deposition zone.
freeboard: Vertical distance above a design stage that is allowed for waves, surges, drift, and other contingencies.
Froude Number: Dimensionless number that represents the ratio of inertial to gravitational forces in open channel flow.
gabion: Basket or compartmented rectangular container made of wire mesh. When filled with cobbles or other rock of suitable size, the gabion becomes a flexible and permeable unit with which flow‑ and erosion-control structures can be built.
general scour: General scour is a lowering of the streambed across the stream or waterway at the bridge. This lowering may be uniform across the bed or non-uniform. That is, the depth of scour may be deeper in some parts of the cross section. General scour may result from contraction of the flow or other general scour conditions such as flow around a bend.
geomorphology/morphology: That science that deals with the form of the Earth, the general configuration of its surface, and the changes that take place due to erosion and deposition.
grade-control structure (sill, check dam): Structure placed bank to bank across a stream channel (usually with its central axis perpendicular to flow) for the purpose of controlling bed slope and preventing scour or headcutting.
graded stream: Geomorphic term used for streams that have apparently achieved a state of equilibrium between the rate of sediment transport and the rate of sediment supply throughout long reaches.
gravel: Rock fragment whose diameter ranges from 2 to 64 mm.
groin: Structure built from the bank of a stream in a direction transverse to the current to redirect the flow or reduce flow velocity. Many names are given to this structure, the most common being "spur," "spur dike," "transverse dike," "jetty," etc. Groins may be permeable, semi-permeable, or impermeable.
grout: Fluid mixture of cement and water or of cement, sand, and water used to fill joints and voids.
guide bank: Dike extending upstream from the approach embankment at either or both sides of the bridge opening to direct the flow through the opening. Some guide banks extend downstream from the bridge (also spur dike).
hardpoint: Streambank protection structure whereby "soft" or erodible materials are removed from a bank and replaced by stone or compacted clay. Some hard points protrude a short distance into the channel to direct erosive currents away from the bank. Hard points also occur naturally along streambanks as passing currents remove erodible materials leaving nonerodible materials exposed.
headcutting: Channel degradation associated with abrupt changes in the bed elevation (headcut) that generally migrates in an upstream direction.
helical flow: Three‑dimensional movement of water particles along a spiral path in the general direction of flow. These secondary‑type currents are of most significance as flow passes through a bend; their net effect is to remove soil particles from the cut bank and deposit this material on a point bar.
hydraulics: Applied science concerned with behavior and flow of liquids, especially in pipes, channels, structures, and the ground.
hydraulic model: Small‑scale physical or mathematical representation of a flow situation.
hydraulic problem: An effect of streamflow, tidal flow, or wave action such that the integrity of the highway facility is destroyed, damaged, or endangered.
hydraulic radius: Cross‑sectional area of a stream divided by its wetted perimeter.
hydraulic structures: Facilities used to impound, accommodate, convey or control the flow of water, such as dams, weirs, intakes, culverts, channels, and bridges.
hydrograph: The graph of stage or discharge against time.
hydrology: Science concerned with the occurrence, distribution, and circulation of water on the earth.
imbricated: Reference to stream bed sediment particles, having an overlapping or shingled pattern.
icing: Masses or sheets of ice formed on the frozen surface of a river or floodplain. When shoals in the river are frozen to the bottom or otherwise dammed, water under hydrostatic pressure is forced to the surface where it freezes.
incised reach: A stretch of stream with an incised channel that only rarely overflows its banks.
incised stream: Stream which has deepened its channel through the bed of the valley floor, so that the floodplain is a terrace.
invert: Lowest point in the channel cross section or at flow control devices such as weirs, culverts, or dams.
island: A permanently vegetated area, emergent at normal stage, that divides the flow of a stream. Islands originate by establishment of vegetation on a bar, by channel avulsion, or at the junction of minor tributary with a larger stream.
jack: Device for flow control and protection of banks against lateral erosion consisting of three mutually perpendicular arms rigidly fixed at the center. Kellner jacks are made of steel struts strung with wire, and concrete jacks are made of reinforced concrete beams.
jack field: Rows of jacks tied together with cables, some rows generally parallel with the banks and some perpendicular thereto or at an angle. Jack fields may be placed outside or within a channel.
jetty: (A) Obstruction built of piles, rock, or other material extending from a bank into a stream, so placed as to induce bank building, or to protect against erosion; (B) similar obstruction to influence stream, lake, or tidal currents, or to protect a harbor (also spur).
lateral erosion: Erosion in which the removal of material is extended horizontally as contrasted with degradation and scour in a vertical direction.
launching: Release of undercut material (stone riprap, rubble, slag, etc.) downslope or into a scoured area.
levee: Embankment, generally landward of top bank, that confines flow during high-water periods, thus preventing overflow into lowlands.
live-bed scour: Scour at a pier or abutment (or contraction scour) when the bed material in the channel upstream of the bridge is moving at the flow causing bridge scour.
load (or sediment load): Amount of sediment being moved by a stream.
local scour: Removal of material from around piers, abutments, spurs, and embankments caused by an acceleration of flow and resulting vortices induced by obstructions to the flow.
longitudinal profile: Profile of a stream or channel drawn along the length of its centerline. In drawing the profile, elevations of the water surface or the thalweg are plotted against distance as measured from the mouth or from an arbitrary initial point.
lower bank: That portion of a streambank having an elevation less than the mean water level of the stream.
mathematical model: Numerical representation of a flow situation using mathematical equations (also computer model).
mattress: Blanket or revetment of materials interwoven or otherwise lashed together and placed to cover an area subject to scour.
meander or full meander: Meander in a river consists of two consecutive loops, one flowing clockwise and the other counter‑clockwise.
meander amplitude: Distance between points of maximum curvature of successive meanders of opposite phase in a direction normal to the general course of the meander belt, measured between center lines of channels.
meander belt: Distance between lines drawn tangent to the extreme limits of successive fully developed meanders.
meander length: Distance along a stream between corresponding points of successive meanders.
meander loop: Individual loop of a meandering or sinuous stream lying between inflection points with adjoining loops.
meander ratio: Ratio of meander width to meander length.
meander radius of curvature: Radius of a circle inscribed on the centerline of a meander loop.
meander scrolls: Low, concentric ridges and swales on a floodplain, marking the successive positions of former meander loops.
meander width: Amplitude of a fully developed meander measured from midstream to midstream.
meandering stream: Stream having a sinuosity greater than some arbitrary value. The term also implies a moderate degree of pattern symmetry, imparted by regularity of size and repetition of meander loops. The channel generally exhibits a characteristic process of bank erosion and point bar deposition associated with systematically shifting meanders.
median diameter: Particle diameter of the 50th percentile point on a size distribution curve such that half of the particles (by weight, number, or volume) are larger and half are smaller (D50).
mid-channel bar: Bar lacking permanent vegetal cover that divides the flow in a channel at normal stage.
middle bank: Portion of a streambank having an elevation approximately the same as that of the mean water level of the stream.
migration: Change in position of a channel by lateral erosion of one bank and simultaneous accretion of the opposite bank.
mud: A soft, saturated mixture mainly of silt and clay.
natural levee: Low ridge that slopes gently away from the channel banks that is formed along streambanks during floods by deposition.
nominal diameter: Equivalent spherical diameter of a hypothetical sphere of the same volume as a given sediment particle.
nonalluvial channel: Channel whose boundary is in bedrock or non-erodible material.
normal stage: Water stage prevailing during the greater part of the year.
overbank flow: Water movement that overtops the bank either due to stream stage or to overland surface water runoff.
oxbow: Abandoned former meander loop that remains after a stream cuts a new, shorter channel across the narrow neck of a meander. Often bow‑shaped or horseshoe‑shaped.
pavement: Streambank surface covering, usually impermeable, designed to serve as protection against erosion. Common pavements used on streambanks are concrete, compacted asphalt, and soil‑cement.
paving: Covering of stones on a channel bed or bank (used with reference to natural covering).
peaked stone dike: Riprap placed parallel to the toe of a streambank (at the natural angle of repose of the stone) to prevent erosion of the toe and induce sediment deposition behind the dike.
perennial stream: Stream or reach of a stream that flows continuously for all or most of the year.
phreatic line: Upper boundary of the seepage water surface landward of a streambank.
pile: Elongated member, usually made of timber, concrete, or steel, that serves as a structural component of a river‑training structure.
pile dike: Type of permeable structure for the protection of banks against caving; consists of a cluster of piles driven into the stream, braced and lashed together.
piping: Removal of soil material through subsurface flow of seepage water that develops channels or "pipes" within the soil bank.
point bar: Alluvial deposit of sand or gravel lacking permanent vegetal cover occurring in a channel at the inside of a meander loop, usually somewhat downstream from the apex of the loop.
poised stream: Stream which, as a whole, maintains its slope, depths, and channel dimensions without any noticeable raising or lowering of its bed (stable stream). Such condition may be temporary from a geological point of view, but for practical engineering purposes, the stream may be considered stable.
probable maximum flood: Very rare flood discharge value computed by hydro-meteorological methods, usually in connection with major hydraulic structures.
quarry-run stone: Stone as received from a quarry without regard to gradation requirements.
railbank protection: Type of countermeasure composed of rock‑filled wire fabric supported by steel rails or posts driven into streambed.
rapid drawdown: Lowering the water against a bank more quickly than the bank can drain without becoming unstable.
reach: Segment of stream length that is arbitrarily bounded for purposes of study.
recurrence interval: Reciprocal of the annual probability of exceedance of a hydrologic event (also return period, exceedance interval).
regime: Condition of a stream or its channel with regard to stability. A stream is in regime if its channel has reached an equilibrium form as a result of its flow characteristics. Also, the general pattern of variation around a mean condition, as in flow regime, tidal regime, channel regime, sediment regime, etc. (used also to mean a set of physical characteristics of a river).
regime change: Change in channel characteristics resulting from such things as changes in imposed flows, sediment loads, or slope.
regime channel: Alluvial channel that has attained, more or less, a state of equilibrium with respect to erosion and deposition.
regime formula: Formula relating stable alluvial channel dimensions or slope to discharge and sediment characteristics.
reinforced-earth bulkhead: Retaining structure consisting of vertical panels and attached to reinforcing elements embedded in compacted backfill for supporting a streambank.
reinforced revetment: Streambank protection method consisting of a continuous stone toe‑fill along the base of a bank slope with intermittent fillets of stone placed perpendicular to the toe and extending back into the natural bank.
relief bridge: An opening in an embankment on a floodplain to permit passage of overbank flow.
retard (retarder structure): Permeable or impermeable linear structure in a channel parallel with the bank and usually at the toe of the bank, intended to reduce flow velocity, induce deposition, or deflect flow from the bank.
revetment: Rigid or flexible armor placed to inhibit scour and lateral erosion. (See bank revetment).
riffle: Natural, shallow flow area extending across a streambed in which the surface of flowing water is broken by waves or ripples. Typically, riffles alternate with pools along the length of a stream channel.
riparian: Pertaining to anything connected with or adjacent to the banks of a stream (corridor, vegetation, zone, etc.).
riprap: Layer or facing of rock or broken concrete dumped or placed to protect a structure or embankment from erosion; also the rock or broken concrete suitable for such use. Riprap has also been applied to almost all kinds of armor, including wire‑enclosed riprap, grouted riprap, partially grouted riprap, sacked concrete, and concrete slabs.
river training: Engineering works with or without the construction of embankment, built along a stream or reach of stream to direct or to lead the flow into a prescribed channel. Also, any structure configuration constructed in a stream or placed on, adjacent to, or in the vicinity of a streambank that is intended to deflect currents, induce sediment deposition, induce scour, or in some other way alter the flow and sediment regimes of the stream.
rock-and-wire mattress: Flat wire cage or basket filled with stone or other suitable material and placed as protection against erosion.
roughness coefficient: Numerical measure of the frictional resistance to flow in a channel, as in the Manning's or Chezy's formulas.
rubble: Rough, irregular fragments of materials of random size used to retard erosion. The fragments may consist of broken concrete slabs, masonry, or other suitable refuse.
runoff: That part of precipitation which appears in surface streams of either perennial or intermittent form.
sack revetment: Sacks (e.g., burlap, paper, or nylon) filled with mortar, concrete, sand, stone or other available material used as protection against erosion.
saltation load: Sediment bounced along the streambed by energy and turbulence of flow, and by other moving particles.
sand: Rock fragment whose diameter is in the range of 0.062 to 2.0 mm.
scour: Erosion of streambed or bank material due to flowing water; often considered as being localized (see local scour, contraction scour, total scour).
sediment or fluvial sediment: Fragmental material transported, suspended, or deposited by water.
sediment concentration: Weight or volume of sediment relative to the quantity of transporting (or suspending) fluid.
sediment discharge: Quantity of sediment that is carried past any cross section of a stream in a unit of time. Discharge may be limited to certain sizes of sediment or to a specific part of the cross section.
sediment load: Amount of sediment being moved by a stream.
sediment yield: Total sediment outflow from a watershed or a drainage area at a point of reference and in a specified time period. This outflow is equal to the sediment discharge from the drainage area.
seepage: Slow movement of water through small cracks and pores of the bank material.
shear stress: See unit shear force.
shoal: A relatively shallow submerged bank or bar in a body of water.
sill: (A) Structure built under water, across the deep pools of a stream with the aim of changing the depth of the stream; (B) low structure built across an effluent stream, diversion channel or outlet to reduce flow or prevent flow until the main stream stage reaches the crest of the structure.
silt: Particle whose diameter is in the range of 0.004 to 0.062 mm.
sinuosity: Ratio between the thalweg length and the valley length of a stream.
slope (of channel or stream): Fall per unit length along the channel centerline or thalweg.
slope protection: Any measure such as riprap, paving, vegetation, revetment, brush or other material intended to protect a slope from erosion, slipping or caving, or to withstand external hydraulic pressure.
sloughing: Sliding or collapse of overlying material; same ultimate effect as caving, but usually occurs when a bank or an underlying stratum is saturated.
slope-area method: Method of estimating unmeasured flood discharges in a uniform channel reach using observed high‑water levels.
slump: Sudden slip or collapse of a bank, generally in the vertical direction and confined to a short distance, probably due to the substratum being washed out or having become unable to bear the weight above it.
soil-cement: A designed mixture of soil and Portland cement compacted at a proper water content to form a blanket or structure that can resist erosion.
sorting: Progressive reduction of size (or weight) of particles of the sediment load carried down a stream.
spill-through abutment: Bridge abutment having a fill slope on the streamward side. The term originally referred to the "spill-through" of fill at an open abutment but is now applied to any abutment having such a slope.
spread footing: Pier or abutment footing that transfers load directly to the earth.
spur: Permeable or impermeable linear structure that projects into a channel from the bank to alter flow direction, induce deposition, or reduce flow velocity along the bank.
spur dike: See guide bank.
stability: Condition of a channel when, though it may change slightly at different times of the year as the result of varying conditions of flow and sediment charge, there is no appreciable change from year to year; that is, accretion balances erosion over the years.
stable channel: Condition that exists when a stream has a bed slope and cross section which allows its channel to transport the water and sediment delivered from the upstream watershed without aggradation, degradation, or bank erosion (a graded stream).
stage: Water‑surface elevation of a stream with respect to a reference elevation.
stone riprap: Natural cobbles, boulders, or rock dumped or placed as protection against erosion.
stream: Body of water that may range in size from a large river to a small rill flowing in a channel. By extension, the term is sometimes applied to a natural channel or drainage course formed by flowing water whether it is occupied by water or not.
streambank erosion: Removal of soil particles or a mass of particles from a bank surface due primarily to water action. Other factors such as weathering, ice and debris abrasion, chemical reactions, and land use changes may also directly or indirectly lead to bank erosion.
streambank failure: Sudden collapse of a bank due to an unstable condition such as removal of material at the toe of the bank by scour.
streambank protection: Any technique used to prevent erosion or failure of a streambank.
suspended sediment discharge: Quantity of sediment passing through a stream cross section above the bed layer in a unit of time suspended by the turbulence of flow (suspended load).
sub-bed material: Material underlying that portion of the streambed which is subject to direct action of the flow. Also, substrate.
subcritical, supercritical flow: Open channel flow conditions with Froude Number less than and greater than unity, respectively.
tetrahedron: Component of river‑training works made of six steel or concrete struts fabricated in the shape of a pyramid.
tetrapod: Bank protection component of precast concrete consisting of four legs joined at a central joint, with each leg making an angle of 109.5 ° with the other three.
thalweg: Line extending down a channel that follows the lowest elevation of the bed.
tieback: Structure placed between revetment and bank to prevent flanking.
timber or brush mattress: Revetment made of brush, poles, logs, or lumber interwoven or otherwise lashed together. The completed mattress is then placed on the bank of a stream and weighted with ballast.
toe of bank: That portion of a stream cross section where the lower bank terminates and the channel bottom or the opposite lower bank begins.
toe protection: Loose stones laid or dumped at the toe of an embankment, groin, etc., or masonry or concrete wall built at the junction of the bank and the bed in channels or at extremities of hydraulic structures to counteract erosion.
total scour: Sum of long-term degradation, general (contraction) scour, and local scour.
total sediment load: Sum of suspended load and bed load or the sum of bed material load and wash load of a stream (total load).
tractive force: Drag or shear on a streambed or bank caused by passing water which tends to move soil particles along with the streamflow.
trench-fill revetment: Stone, concrete, or masonry material placed in a trench dug behind and parallel to an eroding streambank. When the erosive action of the stream reaches the trench, the material placed in the trench armors the bank and thus retards further erosion.
turbulence: Motion of fluids in which local velocities and pressures fluctuate irregularly in a random manner as opposed to laminar flow where all particles of the fluid move in distinct and separate lines.
ultimate scour: Maximum depth of scour attained for a given flow condition. May require multiple flow events and in cemented or cohesive soils may be achieved over a long time period.
uniform flow: Flow of constant cross section and velocity through a reach of channel at a given time. Both the energy slope and the water slope are equal to the bed slope under conditions of uniform flow.
unit discharge: Discharge per unit width (may be average over a cross section, or local at a point).
unit shear force (shear stress): Force or drag developed at the channel bed by flowing water. For uniform flow, this force is equal to a component of the gravity force acting in a direction parallel to the channel bed on a unit wetted area. Usually in units of stress, Pa (N/m2) or (lb/ft2).
unsteady flow: Flow of variable discharge and velocity through a cross section with respect to time.
upper bank: Portion of a streambank having an elevation greater than the average water level of the stream.
velocity: Time rate of flow usually expressed in m/s (ft/sec). Average velocity is the velocity at a given cross section determined by dividing discharge by cross-sectional area.
vertical abutment: An abutment, usually with wingwalls, that has no fill slope on its streamward side.
vortex: Turbulent eddy in the flow generally caused by an obstruction such as a bridge pier or abutment (e.g., horseshoe vortex).
wandering channel: Channel exhibiting a more or less non‑systematic process of channel shifting, erosion and deposition, with no definite meanders or braided pattern.
wandering thalweg: Thalweg whose position in the channel shifts during floods and typically serves as an inset channel that conveys all or most of the stream flow at normal or lower stages.
wash load: Suspended material of very small size (generally clays and colloids) originating primarily from erosion on the land slopes of the drainage area and present to a negligible degree in the bed itself.
watershed: See drainage basin.
waterway opening width (area): Width (area) of bridge opening at (below) a specified stage, measured normal to the principal direction of flow.
weephole: A hole in an impermeable wall or revetment to relieve the neutral stress or pore pressure in the soil.
windrow revetment: Row of stone placed landward of the top of an eroding streambank. As the windrow is undercut, the stone is launched downslope, thus armoring the bank.
wire mesh: Wire woven to form a mesh; where used as an integral part of a countermeasure, openings are of suitable size and shape to enclose rock or broken concrete or to function on fence‑like spurs and retards.
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Updated: 11/22/2011

FHWA
United States Department of Transportation - Federal Highway Administration