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Active Channel Stage: The active channel or ordinary high water level is an elevation delineating the highest water level that has been maintained for a sufficient period of time to leave evidence on the landscape, such as the point where the natural vegetation changes from predominantly aquatic to predominantly terrestrial or the bank elevation at which the cleanly scoured substrate of the stream ends and terrestrial vegetation begins.

Adfluvial: Produced by river action; occasionally used in reference to fish that mature in lakes and migrate upstream into tributaries to spawn.

Alviens: Newly hatched fish with the yoke sack still attached.

Anadromous Fish: Fish such as salmon and some trout that are born in fresh water rivers and tributaries, migrate downstream, mature in the ocean, and return to fresh water to spawn.

Apron: A hardened surface (usually concrete or grouted riprap) placed at either the invert of the culvert inlet or outlet to protect structure from scour and storm damage. Aprons often produce shallow depths with high velocities, creating barriers to upstream fish movement.

Aquatic Ecosystem: The total community of living species and its interrelated physical and chemical environment that is directly related to the functions of a particular water drainage.

Arch: An open bottomed road stream crossing structure usually formed of bolted structural plates.

Armoring: Protective covering, such as rock, vegetation, or engineered materials used to protect stream banks, fill or cut slopes, or drainage structure outflows from flowing water energy and erosion.

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Baffles: Wood, concrete or metal panels mounted in a series on the floor and/or wall of a culvert to increase boundary roughness and thereby reduce the average water velocity in the culvert.

Bankfull Stage: Corresponds to the stage at which channel maintenance is most effective, that is, the discharge at which the stream is moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphologic characteristics of channels. The bankfull stage is most effective or is the dominate channel forming flow, and in many streams has an approximate recurrence interval of 1.5 years (Dunne & Leopold 1978).

Bedload: Sand, silt, and gravel, or soil and rock debris rolled along the bottom of a stream by the moving water. The particles of this material have a density or grain size which prevents movement far above or for a long distance out of contact with the streambed under natural flow conditions.

Bottomless-arch: See Open Bottom Arch

Breaks-in-slope: Steeper sections within a culvert. As culverts age they often sag as the road fills settles.

Bridge: A structure, including supports, erected over a depression or an obstruction, such as water, a channel, road, trail, or railway, and having a deck for carrying traffic or other moving loads.

Bridge Load Rating: The bridge capacity measured in tons of Gross Vehicular Weight (GVW). Bridge capacities are rated at two load levels.

Burst Speed: The highest speed a fish can swim for a short time.

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CFS: Cubic feet per second.

CSP: Corrugated steel pipe. Pipe barrel is comprised of a single sheet of material.

CMP: Corrugated metal pipe. Pipe barrel is comprised of a single sheet of material.

Cofferdam: Temporary enclosure built in a water course and pumped dry to permit work on a structure by separating the work from the water.

Corrugations: Refers to the undulations present in CSP and SSP culvert material. Corrugations provide surface roughness which increases over the width and depth of standard dimensions.

Critical Depth: Depth of flow at which specific energy is a minimum; depth in a conduit at which maximum flow will occur if the conduit is at critical slope, the water is flowing at critical velocity, and an adequate supply of water exists.

Critical Flow: A condition existing at critical depth where the sum of the velocity head and static head is a minimum.

Critical Slope: The slope at which maximum flow will occur at minimum velocity; the slope equal to loss of head per foot resulting from flow at a depth giving uniform flow at critical depth.

Cross Drain: A ditch relief culvert or other structure or shaping of the traveled way designed to capture and remove surface water from the traveled way or other road surfaces.

Crown: Surface shaping of the roadway with the high point in the middle causing surface runoff to flow both towards the uphill shoulder or ditch and the downhill shoulder.

Cruising Speed: The speed a fish can swim for an extended time.

Culvert: A specific type of stream crossing, used generally to convey water flow through theroad prism base. Typically constructed of either steel, aluminum, plastic, or concrete. Shapes include circular, oval, squashed-pipe (flat floor), open bottom arch, square,or rectangular.

Culvert Entrance: The downstream end of a culvert through which fish enter to pass upstream.

Culvert Exit: The upstream end of a culvert through which a fish exit to pass upstream.

Culvert Inlet: The upstream end of a culvert through which stream flow enters.

Culvert Outlet: The downstream end of a culvert through which stream flow discharges.

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Design Discharge: Flow quantity expected at a point in a channel resulting from the design storm.

Design Frequency: The recurrence interval for a hydrologic event used for structure design purposes.

Design Life: Length of time of service for a facility without major repair.

Debris Plugging: Reduction in flow capacity of a road stream crossing drainage structure or ditch relief pipe due to blockage by woody materials.

Diversion Potential: The possibility, caused by a road, for streamflow to leave its established channel.

Drawings: The documents, including plan and profile sheets, cross sections, diagrams, layouts, schematics, descriptive literature, illustrations, schedules, performance and test data, and similar materials showing details for construction of a transportation facility.

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Embedment: The depth to which a culvert bottom is buried into the streambed. It is usually expressed as a percentage of the culvert height or diameter.

Erosion: The detachment and subsequent transport of soil particles by water, wind, or ice.

Exceedance Flow: n percent exceedance flow is the flow that is equaled or exceeded n percent of the time.

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Fish Habitat: Conditions essential for fish life including sufficient water quality and quantity, spawning, nursery, and rearing areas, and food supply.

Fish Passage: The ability of both adult and juvenile fish to move both up and down stream.

Fishway: A structure for passing fish over vertical impediments. It may include special attraction devices, entrances, collection and transportation channels, a fish ladder, and exit.

FishXing: (pronounced “Fish Crossing”)A computer software program developed by the Forest Service and cooperators. FishXing models culvert hydraulics (including open-bottom structures) and compares the predicted values with data regarding swimming and leaping abilities and minimum water depth requirements for numerous fish species.

Flood Frequency: The frequency with which a flow has the probability of being equaled or exceeded. For example, a "100-year" frequency flood refers to a flood discharge of a magnitude likely to occur on the average of once every 100 years or, more properly, has a one-percent chance of being equaled or exceeded in any year. Although calculation of possible recurrence is often based on historical records, there is no guarantee that a "100-year" flood will occur at all within the 100-year period or that it will not recur several times.

Flood Frequency Analysis: A procedure for identifying the magnitude of flow, i.e., the N year precipitation event, that will be equaled on an average of every N years. In the case of a 20-year event, there is a 5% chance that it will be equaled during any given year.

Floodplain: The area adjacent to the stream built by the river in the present climate and inundated during periods of high flow.

Flood Prone Zone: The area corresponding to the modern floodplain, but can also include river terraces subject to significant bank erosion. For delineation, see definition for floodplain.

Flow Duration (or Annual Exceedance Flow): A flow duration curve is a cumulative frequency curve that shows the percentage of time that specified flows are equaled or exceeded. Describes the natural flow characteristics of a stream by showing the percentage of time that a flow is equal to or greater than a given value during a specified period. Flow exceedance values are important for describing the flow conditions under which fish passage is required as applied annually, monthly, or to the migration period.

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Gradient Control Weirs: Stabilizing weirs constructed in the streambed to prevent lowering of the channel bottom.

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HW/D: Headwater to Diameter Ratio is a measure of the depth of water at the inlet of the culvert.  HW/D < 1 describes a culvert with a headwater depth below the top of the culvert inlet, HW/D = 1 is the condition in which the headwater at the inlet is equal to the culvert diameter, and HW/D >1 is when the headwater depth exceeds the culvert diameter.  In this condition, flow entering the culvert is under pressure and water is “ponding” at the inlet. D represents diameter for circular pipes and rise or height for arch or box culverts.

Head: The force per unit area exerted by a column of liquid at a height above a depth (and pressure) of interest. Fluids flow down a hydraulic gradient, from points of higher to lower hydraulic head.

Headcutting: Erosional process moving upstream from the location of initial downcutting.

Headwater elevation: The depth of water above the upstream side of the culvert. This depth represents the amount of potential energy available to convey water through the culvert.

Hydraulic Capacity: The maximum amount of flow  that a stream crossing can convey at a specific headwater depth.

Hydraulic Controls: A location within the channel that controls the water depth and velocity upstream.  For example, the tailwater control below a culvert may be a hydraulic control influencing hydraulic conditions within the culvert.  

Hydraulic Gradient: Pressure gradient, or a line representing pressure or piezometrichead in a pipe flowing full, or the water surface in open channel flow.

Hydraulic Jump: An abrupt transition in streamflow from shallow and fast (supercritical flow) to deep and slow (subcritical flow).

Hydraulic Radius: The ratio of area of flow to wetted perimeter.

Hydrograph: Plot depicting discharge of water versus time for a stream, including surface, subsurface, and base flows.

Hydrology: The water of the earth and air; its flow, distribution, characteristics, and actions.

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Invert: The part of a culvert below the spring line that represents the lowest point in the internal cross section. Also the stream bed or floor within a structure or channel.

Inlet: Upstream entrance to a culvert.

Inlet bottom: Measured in the center of the culvert inlet for both standard and embedded (sunken) culverts.  If the culvert is embedded, this should be the elevation of the natural bottom at the inlet and not the elevation of the culvert inlet invert.

Inlet Control: Culvert configuration for which the cross sectional area of the barrel and headwater depth are the primary controls on culvert capacity. Generally occurs in steeper culverts that are not backwatered.

Inlet Invert: Location at inlet, on the culvert floor where an elevation is measured to calculate culvert slope.

Invert: The part of a culvert below the spring line that represents the lowest point in the internal cross section. Also the stream bed or floor within a structure or channel.

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Juvenile: A young fish.

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Manning’s Formula: An equation for determining flow quantity given hydraulic radius, cross sectional area of flow, slope (for uniform flow), and a coefficient of roughness.

Maximum Average Water Velocity in Culvert: The highest average water velocity for any cross-section along the length of the culvert, excluding the effects of water surface drawdown at the culvert outlet.

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Open Bottom Arch: A type of culvert with rounded sides and top attached to concrete or steel footings set below stream grade. The natural stream channel and substrate run through the length of the culvert, providing streambed conditions similar to the actual stream channel.

Ordinary High Water Mark: The mark along the bank or shore up to which the presence and action of the water are common and usual, and so long continued in all ordinary years, as to leave a natural line impressed on the bank or shore and indicated by erosion, shelving, changes in soil characteristics, destruction of terrestrial vegetation, or other distinctive physical characteristics.

Outfall: The outlet end of a culvert.

Outlet: Downstream opening of a culvert.

Outlet Bottom: Measured in the center of the culvert outlet for both standard and embedded (sunken) culverts. If the culvert is embedded, this would be the elevation of the natural bottom at the outlet and not the elevation of the culvert outlet invert.

Outlet Control: Culvert flow in which the cross sectional area of the barrel, inlet configuration, amount of headwater or ponding, tailwater in the outlet channel, and slope, roughness, and length of barrel are of controlling importance to hydraulics of flow.

Outlet Invert: Location at outlet, on the culvert floor, where an elevation is measured to calculate culvert slope.

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Passage Flow: Range of flows for fish migration. The passage flows define the range of flows analyzed by FishXing. See also Qhp and Qlp.

Peak Flow: The greatest discharge in a given channel from a given precipitation event.

Perching: The development of falls or a cascade at a culvert outfall because of downstream erosion.

Perched Outlet: A condition in which a culvert outlet is suspended over the immediate downstream pool, requiring a migrating fish to leap into culvert.

Pipe-arch: A type of culvert with a flat bottom and rounded sides and top, usually created by shaping or squashing a circular CSP or SSP pipe.

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Qhp: Stream discharge (in cfs) at high passage flow. For adult salmonids, in California defined as the 1 percent exceedance flow (the flow equaled or exceeded 1 percent of the time) during the period of expected migration.

Qlp: Stream discharge (in cfs) at low passage flow. For adult salmonids, in California defined as the 90 percent exceedance flow for the migration period.

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Recurrence Interval: Also referred to as flood frequency, or return period. It is the average time interval between occurrences of a hydrological event of a given or greater magnitude. For example, a flood event with a two-year recurrence interval has a 50 percent chance of being equaled or exceeded in any given year.

Resident Fish: Fish that spend their entire life in a limited range of habitats, such as fresh water.

Reynolds Number: A nondimensional coefficient used as a dynamic scale of flow. Interpreted as the ratio of inertial forces to viscous forces in a fluid (inertial forces/viscous forces).

Riffle Crest: See "tailwater control".

Riparian Area: The area containing moist soils and hydric vegetation along and interacting with a stream comprised of two ecosystems, riparian and aquatic, sometimes depicted by a measured width.

Risk: The chance of failure.

Roads: For purposes of these guidelines, roads include all sites of intentional surface disturbance for the purpose of vehicular or rail traffic and equipment use, including all surfaced and unsurfaced roads, temporary roads, closed and inoperable roads, legacy roads, skid trails, tractor roads, layouts, landings, turnouts, seasonal roads, fire lines, and staging areas.

Rust line: A line defined by the top of the corroded metal that forms in the bottom of steel culverts.  The rust line is useful as an indicator of base high flows.

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SSP: Structural steel plate. Pipe diameter is comprised of multiple sheets of material which are usually bolted together.

Salmonid: Any fish belonging of the family Salmonidae including whitefish, grayling, salmon, and trout.

Scour: Underwater erosion of a stream bottom or bank or at a drainage structure outflow.

Section 10 and 404 Regulatory Programs: The principal federal regulatory programs, carried out by the US Army Corps of Engineers, affecting structures and other work below mean high water. The Corps, under Section 10 of the River and Harbor Act of 1899, regulates structures in, or affecting, navigable waters of the US as well as excavation or deposition of materials (e.g., dredging or filling) in navigable waters. Under Section 404 of the Federal Water Pollution Control Act Amendments (Clean Water Act of 1977), the Corps is also responsible for evaluating application for Department of the Army permits for any activities that involve the placement of dredged or fill material into waters of the United States, including adjacent wetlands.

Slope:  The rise divided by the run in stream channels and culverts.

Soffit: The bottom of the top of a pipe, the uppermost point on the inside of a pipe. The crown is the uppermost point on the outside of the pipe wall.

Spawning Bed: A habitat used by fish for producing or depositing eggs.

Spring line: The line of the outer most points on the side of a culvert. For circular pipes it the line one half the diameter above the invert. Also, the maximum horizontal dimension of culvert or conduit.

Stream Crossing: Any human-made structure generally used for transportation purposes that crosses over or through a stream channel including a paved road, unpaved road, railroad track, biking or hiking trail, golf-cart path, or low-water ford. A stream crossing encompasses the structure employed to pass stream flow as well as associated fill material within the crossing prism.

Subcritical Flow:  Slower and deeper flowing water in which gravitational forces (potential energy) dominate, and are greater than inertial forces.

Supercritical Flow: Faster and shallower flowing water that is usually associated with a hydraulically steep, smooth surface. In supercritical flow, inertial forces exceed gravitational forces. Sometimes referred to as rapid or “shooting flow.”

Sustaining Speed: The swimming speed a fish can maintain for several minutes.

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Tailwater Control: The channel feature that determines the water surface elevation immediately downstream of the culvert outlet. The location controlling the tailwater elevation is often at the riffle crest immediately below the outlet pool. Tailwater control is also the channel elevation that determines residual pool depth.

Tailwater: Natural stream or channel just downstream of a culvert or hydraulic structure. The hydraulic response of the downstream water level to discharge from the culvert affects the capacity of the culvert system.

Tailwater depth: The water depth immediately downstream of the culvert as measured from the invert of the outlet.

Thalweg: The line connecting the lowest or deepest points along a streambed.

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Vented Ford: A crossing where the road grade is above the stream channel bottom and all of the water passes through the structure during periods of low flow. During floods, most of the flow overtops the structure. The openings through the structure may be corrugated metal pipe, concrete pipe, concrete box culverts, or treated timber.

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Waters of the United States: Currently defined by regulation to include all navigable and interstate waters, their tributaries and adjacent wetlands, as well as isolated wetlands and lakes and intermittent streams.

Waterbar: Combination of ditch and berm installed perpendicular or skew to road centerline to facilitate drainage of surface water, sometimes non-driveable and used to close the road.

Watershed: An area or region bounded peripherally by ridges or divides such that all precipitation falling in the area contributes to its watercourse or water body.

Weir: a) A notch or depression in a levee, dam, embankment, or other barrier across or bordering a stream, through which the flow of water is measured or regulated; b) A barrier constructed across a stream to divert fish into a trap; c) A dam (usually small) in a stream to raise the water level or divert its flow.  Active Channel Stage: The active channel or ordinary high water level is an elevation delineating the highest water level that has been maintained for a sufficient period of time to leave evidence on the landscape, such as the point where the natural vegetation changes from predominantly aquatic to predominantly terrestrial or the bank elevation at which the cleanly scoured substrate of the stream ends and terrestrial vegetation begins.

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