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Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9

photo of a failed bridge with debris around it

Third Edition

Publication No. FHWA-IF-04-016
September 2005

PDF Version (4.4 mb)

Technical Report Documentation Page
Table of Contents
List of Figures
List of Tables
List of Symbols
Acknowledgments
Glossary
Chapter 1 - Introduction
1.1 Purpose
1.2 Background
1.3 Document Organization
1.4 Dual System of Units
Chapter 2 - Debris Characterization
2.1 Debris Classification
2.1.1 Types of Debris
2.2 Flow Behavior of Floating Debris
2.2.1 Debris Transport and Transport Mechanisms
2.2.2 Debris Jams and Debris Dams
2.2.3 Debris Accumulation
2.3 Problems Associated with Debris
Chapter 3 - Estimating Debris Quantities
3.1 Introduction
3.2 Debris Volumes
3.3 Potential for Debris Delivery
3.3.1 Task A: Estimating Potential for Debris Delivery to Site
3.3.2 Task B: Estimating the Largest Debris Size Delivered to Site
3.3.3 Task C: Assigning Location Categories to All Parts of the Highway Crossing
3.4 Potential for Debris Accumulation
3.4.1 Task A: Assigning Bridge Characteristics to All Immersed Parts of the Bridge
3.4.2 Task B: Determining Accumulation Potential for Each Bridge Component
3.5 Size of Debris Accumulation at Structures
3.6 Factors that Affect Debris Production
Chapter 4 - Analyzing and Modeling Debris Impacts to Structures
4.1 Introduction
4.2 Hydraulic Analyses of Debris
4.3 One-Dimensional Debris Analysis Modeling
4.3.1 Data Requirements
4.3.2 Background of Modeling Methods and Approaches
4.3.2.1 Low Flow Conditions
4.3.2.2 High Flow Conditions
4.3.3 Scenarios for Hydraulic Modeling of Debris Accumulation
4.3.3.1 Bridge Debris Scenarios
4.3.3.2 Culvert Debris Scenarios
4.4 Advanced Modeling
4.5 Local Pier Scour Associated with Debris Accumulation
4.6 Hydraulic Loading Associated With Debris Accumulation
4.7 Hydraulic Loading Example Problems
4.7.1 Example 1 - Hydraulic Loading on a Single Pier (SI)
4.7.2 Example 2 - Hydraulic Loading on Two Adjacent Piers, Case 1 (SI)
4.7.3 Example 3 - Hydraulic Loading on a Superstructure (SI)
4.7.4 Example 4 - Hydraulic Loading on a Single Pier (CU)
4.7.5 Example 5 - Hydraulic Loading on Two Adjacent Piers, Case 1 (CU)
4.7.6 Example 6 - Hydraulic Loading on a Superstructure (CU)
Chapter 5 - Debris Countermeasures
5.1 Background
5.2 Countermeasures for Culverts
5.2.1 Structural Measures
5.2.2 Non-structural Measures
5.3 Countermeasures for Bridges
5.3.1 Structural Measures
5.3.2 Non-Structural Measures
5.4 Countermeasures for Fire Damaged / Deforested Areas
5.4.1 Fire Damaged Areas
5.4.2 Deforestation
Chapter 6 - Design Procedures for Debris Countermeasures
6.1 General Procedures and Considerations
6.1.1 Field Investigations
6.1.2 Selecting the Type of Countermeasures
6.1.3 Design for Bridges versus Culverts
6.1.4 Existing Structures Versus Proposed Structures
6.1.5 Maintenance Accessibility of the Countermeasure
6.2 Design Guidelines for Culverts
6.2.1 Debris Deflectors for Culverts
6.2.1.1 Debris Deflector Example (Culvert)
6.2.2 Debris Racks for Culverts
6.2.2.1 Debris Rack Example (Culvert)
6.2.3 Debris Risers for Culverts
6.2.4 Debris Cribs for Culverts
6.2.5 Debris Fins for Culverts
6.2.6 Debris Dams / Basins for Culverts
6.2.7 Combined Debris Controls for Culverts
6.3 Design Guidelines for Bridges
6.3.1 Debris Deflectors for Bridges
6.3.2 Debris Fins for Bridges
6.3.3 Crib Structures for Bridges
6.3.4 River Training Structures for Bridges
6.3.5 In-Channel Debris Basin for Bridges
6.3.6 Flood Relief Structures for Bridges
6.3.7 Debris Sweepers for Bridges
6.3.8 Design Features for Bridges
6.4 Non-Structural Guidelines (Debris Management)
Chapter 7 - Maintenance
Chapter 8 - References
Appendix A - Metric System, Conversions, and Water Properties
Appendix B - State Survey

List of Figures

Figure 2.1. Hypothetical Patterns of secondary flow in straight and curved channels
Figure 2.2. Single pier debris accumulation (led to pier scour failure)
Figure 2.3. Span blockage accumulation bridge failure. (Louisiana)
Figure 2.4. Missouri Highway 113 bridge over Florida Creek near Skidmore, Missouri
Figure 2.5. Debris accumulation failure at bridge located in Oklahoma
Figure 2.6. Debris accumulation at a bridge structure.(17)
Figure 2.7. Debris induced bridge and bank failure structure. (Louisiana)
Figure 2.8. Effects of debris accumulation at a bridge structure. (New York)
Figure 3.1. Schematic Illustrating Estimate of Maximum Design Log Length
Figure 4.1. Sketch of the sluice gate type of pressure flow
Figure 4.2. Sketch of fully submerged pressure flow
Figure 4.3. Sketch of pressure and weir flow
Figure 4.4. Changing downstream expansion cross section location
Figure 4.5. Adding downstream ineffective flow locations
Figure 4.6. Adding upstream ineffective flow locations
Figure 4.7. "Dummy" bridge pier used to simulate an asymmetrical debris accumulation
Figure 4.8. Upstream face of the bridge for Example 1
Figure 4.9. Water surface profile for Example 1
Figure 4.10. Upstream face of the bridge for Example 2
Figure 4.11. Water surface profile for Example 2
Figure 4.12. Upstream face of the bridge for Example 3
Figure 4.13. Water surface profile for Example 3
Figure 4.14. Upstream face of the bridge for Example 4
Figure 4.15. Water surface profile for Example 4
Figure 4.16. Upstream face of the bridge for Example 5
Figure 4.17. Water surface profile for Example 5
Figure 4.18. Upstream face of the bridge for Example 6
Figure 4.19. Water surface profile for Example 6
Figure 5.1. Steel rail debris deflector for large rock (looking upstream of culvert)
Figure 5.2. Steel rail debris deflector (looking downstream)
Figure 5.3. Steel rail and cable debris deflector. In boulder areas, cable is more desirable for its flexibility than a rigid rail (looking towards entrance)
Figure 5.4. Steel debris deflectors installed at entrances to a battery of culverts
Figure 5.5. Steel rail debris deflector for battery of culverts (see Figure 5.6)
Figure 5.6. Installation of Figure 5.5 during flood; functions well under heavy debris flow
Figure 5.7. Steel rail debris deflector in area of heavy flowing debris (looking upstream)
Figure 5.8. Timber pile debris deflector for boulders and large floating debris
Figure 5.9. Timber pile debris deflector protected culvert during heavy floods. Nearby culverts without deflectors were plugged
Figure 5.10. Rail debris rack over sloping inlet. Heavy debris and boulders ride over rack and leave flow to culvert unimpeded
Figure 5.11. Post and rail debris rack, in place for 35 years, for light to medium floating debris installed 100 ft upstream of culvert
Figure 5.12. Rail debris rack
Figure 5.13. Timber debris rack (note how suspended by cables)
Figure 5.14. Hinged steel debris rack in urban area. Due to nature of debris and possible entry by children, bar spacing is close
Figure 5.15. Steel debris rack in urban area
Figure 5.16. Debris rack used in State of Washington
Figure 5.17. Rail debris rack in arid region (see Figure 5.18)
Figure 5.18. Installation in Figure 5.17 after several years of fine silt deposition at entrance
Figure 5.19. Steel rail debris rack. Note amount of debris accumulation in upstream channel
Figure 5.20. Steel debris rack probably saved the culvert from plugging
Figure 5.21. Steel grill debris rack with provision for cleanout afforded by concrete paved area in foreground
Figure 5.22. Steel grill debris rack on slope mitered culvert entrance
Figure 5.23. Metal pipe debris riser in basin (note anti-vortex device on top)
Figure 5.24. Metal pipe debris riser placed during initial construction of culvert provides relief in case the culvert entrance becomes plugged (see Figure 5.25)
Figure 5.25. Installation shown in Figure 5.24 after flood. Riser conveyed large flows during flood. Fence partially surrounding riser was of no value for debris control
Figure 5.26. Debris crib of precast concrete sections and metal dowels. Height increased by extending dowels and adding more sections
Figure 5.27. Arid region debris crib of precast concrete sections and metal dowels
Figure 5.28. Redwood debris crib with spacing to prevent passage of fine material. Basin had buildup of 30 feet
Figure 5.29. Concrete debris fins with sloping leading edge as extension of culvert walls
Figure 5.30. Concrete debris fin with sloping leading edge as extension of center wall
Figure 5.31. Concrete debris fin with rounded vertical leading edge as extension of culvert center wall
Figure 5.32. Combined installation of concrete debris fin and metal pipe debris riser with single corrugated metal pipe culvert (looking downstream)
Figure 5.33. Concrete debris fin for single culvert (Prefer more area between wingwalls and fin)
Figure 5.34. Debris fin and metal pipe debris riser in conjunction with single barrel culvert
Figure 5.35. Timber debris fins with sloping leading edge
Figure 5.36. Metal bin type debris dam
Figure 5.37. Gabion debris dam
Figure 5.38. Debris dam of precast concrete sections fabricated to enable placement in interlocking fashion
Figure 5.39. Debris dam of precast concrete sections fabricated to enable placement in interlocking fashion
Figure 5.40. Debris dam and basin along with steel debris rack over culvert entrance in foreground. A metal pipe riser is visible over the spillway
Figure 5.41. Bendway weirs on outer bank of Hatchie River looking upstream (TDOT)
Figure 5.42. Kellner jacks used for redirecting the flow patterns
Figure 5.43. Debris deflectors installed at State Route 59 south crossing of the Eel River in central Indiana
Figure 5.44. Debris sweeper being installed on a bridge over Staunton River in Altavista, Virginia
Figure 5.45. Close up of a debris sweeper installed on the Cedar Creek in Washington
Figure 5.46. Close up of a debris sweeper installed on the South Fork Obion River in Tennessee
Figure 5.47. Close up of double-stacked installation debris sweeper on Interstate 24 over the Mississippi River
Figure 6.1. Debris deflector designed in example

List of Tables

Table 1.1. Commonly Used Engineering Terms in SI and CU Units
Table 3.1. Major Phases and Tasks in Evaluating Debris Accumulation Potential at a Bridge
Table 3.2. Determining Potential for Debris Accumulation across a Span or Vertical Gap
Table 3.3. Determining Potential for Debris Accumulation on a Single Pier
Table 3.4. Maximum Extent of Debris Accumulation
Table 4.1. Drag Coefficient for Debris on Piers
Table 4.2. Drag Coefficient for Debris on Superstructure
Table 4.3. Results of Hydraulic Calculations for Example 1
Table 4.4. Results of Hydraulic Calculations for Example 2
Table 4.5. Results of Hydraulic Calculations for Example 3
Table 4.6. Results of Hydraulic Calculations for Example 4
Table 4.7. Results of Hydraulic Calculations for Example 5
Table 4.8. Results of Hydraulic Calculations for Example 6
Table 6.1. Debris-Control Countermeasures Matrix
Table 6.2. Management Plan for the Large Woody Debris Formations

List of Symbols

A = Cross sectional area of flow, m2 (ft2)
AB = Net area of the bridge opening, m2 (ft2)
Abu = Net area of the bridge opening at the upstream face of the bridge, m2 (ft2)
Abd = Net area of the bridge opening at the downstream face of the bridge, m2 (ft2)
Ac = Unobstructed cross-sectional flow area in the contracted section, m2 (ft2)
Ad = Cross-sectional flow area blocked by debris in the contracted bridge section, m2 (ft2)
AD = Area of wetted debris based on the upstream water surface elevation projected normal to the flow direction, m2 (ft2)
Ahd = Area of the vertically projected, submerged portion of the debris accumulation below the downstream water surface, m2 (ft2)
Ahu = Area of the vertically projected, submerged portion of the debris accumulation below the upstream water surface, m2 (ft2)
Ao, Ai = Outlet and inlet storm drain cross-sectional areas, m2 (ft2)
Ao = Orifice area, m2 (ft2)
Arack = Area of debris rack, m2 (ft2)
B = Blockage ratio
C = Expansion and contraction loss coefficients
Cg = Discharge coefficient for sluice gate type of pressure flow
Cd = Discharge coefficient for fully submerged pressure flow
CD = Drag coefficient
Cw = Discharge coefficient for weir flow
DBEL = Bottom elevation of the debris accumulation, m (ft)
D = Culvert diameter, m (ft)
DHW = Design high water elevation, m (ft)
Do = Outlet pipe diameter, m (ft)
D50 = Mean riprap size, m (ft)
Et = Total energy, m (ft)
F = Total segment force on the bridge structure, N (lbs)
FD = Drag force, N (lbs)
FDEL = Elevation of drag force on the bridge structure, m (ft)
FDST = Station of drag force on the bridge structure, m (ft)
FEL = Elevation of total segment force on the bridge structure, m (ft)
Ff = External force due to friction, N (lbs)
Fh = Total hydrostatic force on the bridge structure, N (lbs)
Fhd = Hydrostatic force on downstream side of the bridge structure, N (lbs)
FhEL = Elevation of hydrostatic force on the bridge structure, m (ft)
Fhu = Hydrostatic force on upstream side of the bridge structure, N (lbs)
FhST = Station of hydrostatic force on the bridge structure, m (ft)
Fr = Froude number
FST = Station of total force on the bridge structure, m (ft)
g = Acceleration due to gravity, 9.81 m/s2 (32.2 ft/s2)
h = Vertical distance from water surface to center of gravity of flow area, m (ft)
hf = Friction loss, m (ft)
hv = Velocity head, m (ft)
H = Increase in water surface elevation from the downstream side to the upstream side of the bridge, m (ft)
H = The difference in the upstream energy gradient elevation and the downstream water surface elevation, m (ft)
H = Height of debris-control structure, m (ft)
hcu = Vertical distance from the upstream water surface to the centroid of area Ahu, m (ft)
hcd = Vertical distance from the downstream water surface to the centroid of area Ahd, m (ft)
HGLi = Hydraulic grade line elevation at the inflow pipe, m (ft)
HGLo = Hydraulic grade line elevation relative to the outlet pipe invert, m (ft)
hL = Energy head loss, m (ft)
Hw = Difference between the upstream energy and the road crest, m (ft)
INV = Inlet invert elevation, m (ft)
K = Conveyance, m3/s (ft3/s)
K = Yarnell's pier shape coefficient
Kc = Units conversion factor or coefficient
Ke = Expansion coefficient
L = Horizontal length of curve, flow length, length of basin at base length of pipe, or length of culvert, m (ft)
Lw = Effective length of the weir, m (ft)
n = Manning's roughness coefficient
P = Wetted perimeter, m (ft)
P = Hydrostatic pressure force, N (lbs)
Q = Flow, m3/s (ft3/s)
Q = Total discharge through the bridge opening, m3/s (ft3/s)
Qw = Total discharge over the roadway approaches and the bridge, m3/s (ft3/s)
R = Hydraulic radius (flow area divided by the wetted perimeter), m (ft)
s = Spacing between the bars of a debris-control structure, m (ft)
S = Surface slope, m/m (ft/ft)
Sf = Friction slope, m/m (ft/ft)
SL = Longitudinal slope, m/m (ft/ft)
So = Energy grade line slope, m/m (ft/ft)
SL = Main channel slope, m/km (ft/mi)
t = Bar thickness, m (ft)
T = Surface width of open channel flow, m (ft)
V = Mean velocity, m/s (ft/s)
V = Storage volume, m3 (ft3)
Vr = Reference velocity, m/s (ft/s)
V2 = Mean velocity for the cross-section at the downstream side of the bridge, m/s (ft/s)
V3 = Average flow velocity at the cross section immediately upstream of the bridge, m/s (ft/s)
y = Flow depth, m (ft)
yr = Average flow depth corresponding with the reference velocity, m (ft)
Y3 = Hydraulic depth at the cross section immediately upstream of the bridge, m (ft)
w = Width of debris-control structure, m (ft)
W = Force due to weight of water in the direction of flow, N (lbs)
WD = Width of debris accumulation defined by design log length, m (ft)
WSDS = Water surface elevation downstream of the bridge, m (ft)
Wmin = Minimum width of debris rack, m (ft)
WSUS = Water surface elevation upstream of the bridge, m (ft)
Z = Elevation above a given datum, m (ft)
z = Horizontal distance for side slope of trapezoidal channel, m (ft)
γ = Specific weight of water, 9810 N/m3 (62.4 lb/ft3) at 15.6 EC (60 EF)
γs = Specific weight of sediment particle, N/m3 (lb/ft3)
τ = Average shear stress, Pa (lb/ft2)
ρ = Fluid density, kg/m3 (slugs/ft3)
ω = Ratio of velocity head to depth for the cross-section at the downstream side of the bridge
α = Obstructed area of the piers divided by the total unobstructed area for the cross section at the downstream side of the bridge
α = Apex angle for a culvert debris deflector, degrees

Acknowledgments

First Edition

Mr. G. Reihsen, Highway Engineer with the Federal Highway Administration wrote the first edition of this Hydraulic Engineering Circular. The manual was dated February 1964.

Second Edition

Messrs. G. Reihsen and L.J. Harrison, FHWA revised the first edition in March 1971. In that second edition, the authors incorporated comments from practitioners and added additional remarks on safety.

Third Edition

This third edition substantially revises these earlier HEC-9 editions, adding new information, while keeping important portions of the original documents. The authors of this third edition wish to acknowledge the contributions made by the authors of earlier editions.

The writers wish to also acknowledge the technical assistance of Dr. Arthur Parola, Professor at University of Louisville. Finally, the authors wish to thank everyone at FHWA who was involved in the preparation of this manual.

Glossary

abutment: The structural support at either end of a bridge usually classified as spill-through or vertical.
aggradation: General and progressive buildup of the longitudinal profile of a channel bed due to sediment deposition.
alluvium: Unconsolidated material deposited by a stream in a channel, floodplain, alluvial fan, or delta.
average velocity: Velocity at a given cross section determined by dividing discharge by cross-sectional area.
backwater: The increase in water surface elevation relative to the elevation occurring under natural channel and floodplain conditions. It is induced by a bridge or other structure that obstructs or constricts the otherwise unobstructed flow of water in a channel.
backwater area: The low-lying lands adjacent to a stream that may become flooded due to backwater.
bank: The side slopes of a channel between which the flow is normally confined.
bank, left (right): The side of a channel as viewed in a downstream direction.
bankfull discharge: Discharge that, on the average, fills a channel to the point of overflow.
bar: An elongated deposit of alluvium within a channel, not permanently vegetated.
bed: The bottom of a channel bounded by banks.
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.
bed material: Material found on the bed of a stream (May be transported as bed load or in suspension).
boulder: A rock whose diameter is greater than 250 mm.
bridge opening: The cross sectional area beneath a bridge that is available for conveyance of water.
bridge waterway: The area of a bridge opening available for flow, as measured below a specified stage and normal to the principal direction of flow.
channel: The bed and banks that confine the 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 a man-made channel.
clay: A particle whose diameter is in the range of 0.00024 to 0.004 mm.
cobble: A rock whose diameter is in the range of 64 to 250 mm.
constriction: A 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.
contraction: The effect of a natural or man-made channel constriction on flow streamlines.
countermeasure: A measure intended to prevent, delay or reduce the severity of stream or river problems.
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.
cross section: A section normal to the trend of a channel or flow.
culvert: A drainage conduit that conveys flow from one side of an embankment to the other.
dam jam: A type of debris jam that extends entirely across the channel as a result of the debris length being approximately equal to the channel width.
debris: Floating or submerged material, such as logs, vegetation, or trash, transported by a stream.
debris accumulation: The collection of debris material on a fixed object.
debris cribs: Open crib-type structures placed vertically over the culvert inlet in log-cabin fashion to prevent inflow of coarse bedload and light floating debris.
debris dams and basins: Structures placed across well-defined channels to form basins that impede the streamflow and provide storage space for deposits of detritus and debris.
debris deflectors: Structures placed at the culvert inlet to deflect the major portion of the debris away from the culvert entrance.
debris fins: Walls built in the stream channel upstream of a culvert or bridge. Their purpose is to align debris, such as logs, with the axis of the culvert or bridge so that the debris will move through the culvert or bridge opening.
debris jam: Accumulation of floating or neutrally buoyant debris material formed around large, whole trees that may be anchored to the bed or banks at one or both ends, once in the stream system.
debris racks: Structures placed across the stream channel to collect the debris before it reaches the culvert entrance. Debris racks are usually vertical and at right angles to the streamflow, but they may be skewed with the flow or inclined with the vertical.
debris risers: A closed-type structure placed directly over the culvert inlet to cause deposition of lowing debris and fine detritus before it reaches the culvert inlet.
deflector jam: A type of debris jam that redirects the flows to one or both of the banks. These types of jams usually occur when the channel width is slightly greater than the average tree height.
degradation (bed): A general and progressive (long term) lowering of the channel bed due to erosion over a relatively long channel length.
design log length: A length above which logs are insufficiently abundant and insufficiently strong throughout their full length to produce an accumulation equal to their length. This length does not represent the absolute maximum length of trees within the watershed upstream of the site.
detritus: Non-debris sediment or bed load characterized as fine or course. Fine detritus is a fairly uniform bed load of silt, sand, gravel more or less devoid of floating debris, tending to deposit upon diminution of velocity. Coarse detritus consists of coarse gravel or rock fragments.
dike: An impermeable or semi-permeable 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): A 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).
drift: Alternative term for "debris" that is floating on or through a river.
eddy current: A vortex-type motion of a fluid flowing contrary to the main current, such as the rotational water movement that occurs when the main flow becomes separated from the bank.
effective length of debris: The length of the debris element that can support the load of the debris accumulation.
ephemeral stream: A 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.
erosion: Displacement of soil particles on the land surface due to water or wind action.
floodplain: A nearly flat, alluvial lowland bordering a stream, that is subject to frequent inundation by floods.
flow-control structure: A structure either within or outside a channel that acts as a countermeasure by controlling the direction, depth, or velocity of flowing water.
Froude number: A dimensionless number that represents the ratio of inertial to gravitational fluid forces. High Froude numbers can be indicative of high flow velocity and the potential for scour.
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.
gravel: A rock fragment with a diameter ranging from 2 to 64 mm.
groin: A structure extending from the bank of a stream in a direction transverse to the current. 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.
guide bank: An embankment extending 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 see spur dike).
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 the point bar.
hydraulic problem: An effect of stream flow, tidal flow, or wave action such that the integrity of the highway facility is destroyed, damaged, or endangered.
hydraulic radius: The ratio of a channel's cross sectional area to its wetted perimeter.
ice debris: Accumulation or transport of ice floes in the waterway.
inlet: Entrance of the culvert at the upstream end.
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 a minor tributary with a larger stream.
large floating debris: Type of debris consisting of trees, logs, and other organic matter with a length greater than 3.5 feet. Also referred to as Large Woody Debris (LWD)
lateral erosion: Erosion in which the removal of material is progressing primarily in a lateral direction, as contrasted with degradation and scour that progress primarily in a vertical direction.
light floating debris: Type of debris consisting of small limbs or sticks, orchard prunings, tules, and refuse.
levee: An embankment, generally landward of top bank, that confines flow during high-water periods, thus preventing flooding into lowlands.
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.
medium floating debris: Type of debris consisting of tree limbs, and large sticks.
mid-channel bar: A bar lacking permanent vegetal cover that divides the flow in a channel at normal stage.
migration: Change in position of a channel by lateral erosion of one bank and simultaneous accretion of the opposite bank.
outlet The downstream end of a culvert.
overbank flow: Water movement that overtops the bank either due to stream stage or to overland surface water runoff.
parallel jam: A type of debris jam that is oriented parallel to the flow. These types of jams usually occur when the channel width is significantly greater than the maximum debris length.
perennial stream: A stream or reach of a stream that flows continuously for all or most of the year.
pressure flow scour: The increase in local scour at a pier subjected to pressure (or orifice) flow as a result of flow being directed downward towards the bed by the superstructure (vertical contraction of the flow) and by increasing the intensity of the horseshoe vortex. The vertical contraction of the flow can be the more significant cause of the increased scour depth.
reach: A segment of stream length that is arbitrarily bounded for purposes of study.
revetment: Rigid or flexible armor placed to inhibit scour and lateral erosion (see bank revetment).
riprap: In the restricted sense, layer or facing of rock or concrete dumped or placed to protect a structure or embankment from erosion; also the broken rock or concrete suitable for such use. Riprap has also been applied to almost all kinds of armor, including wire-enclosed riprap, 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.
roughness coefficient: Numerical measure of the frictional resistance to flow in a channel, as in the Manning's or Chezy's formulas.
sand: A 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).
scoured depth: Total depth of the water from water surface to a scoured bed level (compare with "depth of scour").
sediment: Fragmental material transported, suspended, or deposited by water.
sediment yield: The 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.
single pier accumulation: Debris accumulation that occurs only on a single bridge pier as a result of the maximum effective length of the floating debris being less than the effective opening between the bridge piers.
slope (of channel or stream): Fall per unit length along the channel centerline.
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.
span accumulation: Debris accumulation that accumulates across an entire span of a bridge structure as a result the length of floating debris exceeding the effective opening between piers.
spill-through abutment: A bridge abutment having a fill slope on the streamward side.
spur: A 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.
stable channel: A 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.
stream: A 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 flow in the channel. 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 due to removal of material at the toe of the bank by scour.
streambank protection: Any technique used to prevent erosion or failure of a streambank.
structural countermeasure: A structural component used to prevent, delay or reduce the severity of stream or river problems.
substructure: The components of a bridge which includes all elements supporting the superstructure. Its purpose is to transfer the loads from the superstructure to the foundation soil or rock.
superstructure: The entire portion of a bridge structure which primarily receives and supports traffic loads and in turn transfers these loads to the bridge substructure.
subcritical, supercritical flow: Open channel flow conditions with Froude Number less than and greater than unity, respectively.
thalweg: The line extending down a channel that follows the lowest elevation of the bed.
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.
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.
underflow jam: A type of debris jam that exists near the bankfull level. These types of jams usually occur in small watersheds where the tree height is greater than the channel width.
uniform flow: Flow of constant cross section and velocity through a reach of channel at a given instant. Both the energy slope and the water slope are equal to the bed slope under conditions of uniform flow.
velocity: The rate of motion of a fluid in a stream or of the objects or particles transported therein, usually expressed in m/s (ft/s). The average velocity at a given cross section is determined by dividing discharge by cross-sectional area.
Waterway opening width (area): Width (area) of bridge opening at (below) a specified stage, measured normal to the principal direction of flow.

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