Skip to contentUnited States Department of Transportation - Federal Highway AdministrationFHWA HomeFeedback

Hydraulics Engineering


<< Previous Contents Next >>

Design For Fish Passage at Roadway - Stream Crossings: Synthesis Report


Active channel: A waterway of perceptible extent that periodically or continuously contains moving water. It has definite bed and banks, which serve to confine the water and includes stream channels, secondary channels, and braided channels. It is often determined by the "ordinary high water mark" which means that line on the shore established by the fluctuations of water and indicated by physical characteristics such as clear, natural line impressed on the bank, shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of litter and debris, or other appropriate means that consider the characteristics of the surrounding areas.

Aggradation: The geologic process by which a streambed is raised in elevation by the deposit of material transported from upstream. (Opposite of degradation.)

Apron: A flat or slightly inclined slab up- or downstream of culvert or weir that provides for erosion protection. A downstream apron may also produce hydraulic characteristics suitable for energy dissipation or fish exclusion.

Anadromous fish: Fish which mature and spend much of their adult life in the ocean, returning to inland waters to spawn. Examples include salmon and steelhead.

Armor: A surficial layer of course grained sediments, usually gravel or coarser, that are underlain by finer grained sediments.

Backwater: Water backed-up or retarded in its course as compared with its normal open channel flow condition. Water level is a function of some downstream hydraulic control.

Baffle: Wood, concrete or metal 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 discharge: The discharge corresponding to the state at which the floodplain of a particular stream reach begins to be flooded. The bankfull discharge is a morphological indicator that is related to the formation, maintenance, and dimensions of a stream channel, as it exists under modern climatic conditions. The bankfull discharge, often, has a flood frequency of approximately 1.5 years on the annual series, but the frequency can vary widely depending on the particular watershed and stream reach characteristics (FISRWG 1998).

Bankfull width: The point on a streambank at which overflow into the floodplain begins. The floodplain is a relatively flat area adjacent to the channel constructed by the stream. If the floodplain is absent or poorly defined, other indicators may identify bankfull. These include the height of depositional features, a change in vegetation, slope or topographic breaks along the bank, a change in the particle size of bank material, undercuts in the bank, and stain lines or the lower extent of lichens and moss on boulders. Field determination of bankfull should be calibrated to known stream flows or to regional relationships between bankfull flow and watershed drainage area (FISRWG 1998).

Bed: The land below the channel bed width.

Bedform: Elements of the stream channel that describe channel form (e.g. pools, riffles, steps, particle clusters).

Bedload: The part of sediment transport not in suspension consisting of coarse material moving on or near the channel bed.

Bed roughness: Irregularity of streambed material that contributes resistance to streamflow. Commonly characterized using Manning's roughness coefficient.

Bridge: A crossing structure with a combined span (width) greater than 20 ft.

Burst speed: See "Swimming speed."

Cascade: Tumbling flow with continuous jet-and-wake flow over and around individual large clasts (Montgomery and Buffington 1997). Cascades may be natural or constructed.

Channel: A natural or constructed waterway that has definite bed and banks that confine water.

Channel bed slope: Vertical change with respect to horizontal distance within the channel (Gradient).

Channel-bed width: The distance from the bottom of the left bank to the bottom of the right bank. The distinction between bed and bank are determined by examining channel geometry and the presence/absence of vegetation.

Channelization: Straightening or diverting a waterway into a new channel.

Countersink: Place culvert invert below stream grade.

Critical depth: The unique depth of flow in a channel that is characteristic only of discharge and slope. Often referred to as a flow control location.

Culvert: A conduit or passageway under a road, trail or other obstruction. A culvert differs from a bridge in that it usually consists of structural material around its entire perimeter and has a total span (width) of less than 6.1 m (20 ft).

Debris: Includes trees and other organic detritus scattered about or accumulated near a culvert by either natural processes or human influences.

Degradation: Erosional removal of streambed material that results in a lowering of the bed elevation throughout a reach. (Opposite of aggradation.)

Deposition: Settlement of material onto the channel bed.

Design flood: The probabilistic estimate of a flood whose magnitude is equaled or exceeded within a given frequency.

Dewatering: Removal of water from an area.

Embedded culvert: A culvert installation that is countersunk below the stream grade. It may or may not be filled with natural sediment or a design mix.

Entrainment: The process of sediment particle lifting by an agent of erosion.

Entrenchment: The vertical containment of a river and the degree to which it is incised in the valley floor.

Filter fabric: A natural or synthetic fabric used to block sediment and water from flowing to a subsurface or surface area such as through a revetment of riprap along channel beds.

Fish passage: The ability of fish to move both up and downstream through a bridge or culvert.

Fishway: A system that may include special attraction devices, entrances, collection and transportation channels, a fish ladder, exit and operation and maintenance standards to facilitate passage through bridges or culverts.

Fishway weir: A term frequently used to describe the partition between adjacent pools in a fishway.

Flood frequency: The frequency with which a flood of a given discharge has the probability of recurring. 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 over a very long time span or, more properly, has a 1 percent chance of being 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 occur several times.

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

Flow duration curve: A statistical summary of river flow information over a period of time that describe cumulative percent of time for which flow exceeds specific levels (exceedance flows), exhibited by a cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded. Flow duration curves are usually based on daily streamflow and describe the flow characteristics of a stream throughout a range of discharges without regard to the sequence of occurrence.

Fork length: The length of a fish measured from the most anterior part of the head to the deepest point of the notch in the tail fin.

Geomorphology: The study of physical features associated with landscapes and their evolution. Includes factors such as stream gradient, elevation, parent material, stream size, valley bottom width.

Geomorphic Simulation: Culvert design to replicate or maintain natural stream geomorphic elements including gradient, width, bedform, bed material and key features for approximately bankfull conditions. Fish passage requirements are assumed to be met when structures provide natural channel continuity.

Grade stabilization or Grade control: Stabilization of the streambed elevation against degradation. Usually a natural or constructed hard point in the channel that maintains a set elevation. In some cases it may require elevating or steepening a channel.

Head-cutting: Channel bottom erosion moving upstream through a basin, which may indicate a readjustment of the stream's flow regime (slope, hydraulic control, and/or sediment load characteristics).

Headwater: The water upstream from a structure or point on a stream.

Headwater depth: The depth of water at the inlet of a culvert.

High passage design flow: The maximum discharge used for fish passage design. Usually specified by agency policy.

Hydraulic Design: Design options utilizing natural or artificial flow control structures (including weirs, baffles, oversized substrate) to create hydraulic conditions passable for target fish species during specific periods of fish movement.

Hydraulic jump: Hydraulic phenomenon in open channel flow, where supercritical flow changes to sub-critical flow. This will result in an abrupt rise in the water surface elevation.

Hydraulic Simulation: Design techniques that attempt to closely match natural stream flow characteristics by using embedded culvert structures, avoiding most channel constriction, and utilizing natural and oversized sediment in the barrel.

Incision: The resulting change in channel cross-section from the process of degradation.

Interstitial flow: That portion of the surface water that infiltrates the streambed and moves through the substrate interstitial spaces.

Invert: The lowest point of the internal cross section of culvert or pipe arch.

Large Woody Debris (LWD): Any large piece of woody material such as root wads, logs and trees that intrude into a stream channel. LWD may occur naturally or be designed as part of a stream restoration project.

Low passage design flow: The minimum discharge used in fish passage design. Usually specified by agency policy.

Manning's n: Empirical coefficient for simulating the effect of wetted perimeter roughness used in determining water velocity in stream discharge calculations.

Mitigation: Actions to avoid or compensate for the impacts on fish resulting from a proposed activity.

Normal depth: The depth of flow in a channel or culvert when the slope of the water surface and channel bottom is the same and the water depth remains constant.

Ordinary High Water Mark (OHW): Generally, the lowest limit of perennial vegetation. There are also available definitions of OHW that include characteristics of erosion and sediment.

The OHW mark can usually be identified by physical scarring along the bank or shore, or by other distinctive signs. This scarring is the mark along the bank where the action of water is so common as to leave a natural line impressed on the bank. That line may be indicated by erosion, shelving, changes in soil characteristics, destruction of terrestrial vegetation, the presence of litter or debris or other distinctive physical characteristics.

Considerable judgment is required to identify representative OHW marks. It may be difficult to identify the mark on cut banks. In warm months grasses or hanging vegetation may obscure the OHW mark. Artificial structures (culverts, bridges or other constrictions) can affect the OHW mark by creating marks on the shore, which are consistent with OHW marks but above the elevation that is usually found in undisturbed river reaches.

Perching: The tendency to develop a scour hole at the outfall of a culvert due to erosion of the stream channel.

Pipe: A culvert that is circular (round) in cross section.

Pipe arch: A pipe that has been factory-deformed from a circular shape such that the span (width) is larger than the vertical dimension (rise).

Plunging flow: Flow over a weir or out of a perched culvert, which falls into a receiving pool.

Porosity: The percent of flow-through open area of a mesh, screen or streambed rack, relative to the entire gross area.

Reference reach: A stable section of stream beyond the influence of the crossing of interest, with channel characteristics and geomorphology representative of the channel that would exist in the absence of the culvert crossing. This reach provides a template for design of Geomorphic Simulation structures.

Regrade: The process of channel adjustment to attain a new "stable" bed slope. For example, following channelization, a stream bed will typically steepen upstream and flatten downstream.

Resident fish: Fish that migrate and complete their life cycle in fresh water.

Riffle: A reach of stream in which water flow is rapid and usually shallower than the reaches above and below. Natural streams often consist of a succession of pools and riffles.

Riparian: The area adjacent to flowing water (e.g., rivers, perennial or intermittent streams, seeps or springs) that contains elements of both aquatic and terrestrial ecosystems that mutually influence each other.

Riprap: Large, durable materials (usually rocks; sometimes broken concrete, etc.) used to protect a stream bank from erosion; may also refer to the materials used.

Scour: Localized erosion caused by flowing water.

Shear strength: The characteristic of soil, rock and root structure on a parallel submerged surface such as the channel bed or channel bank.

Shear stress: hydraulic force of water created by its movement on a parallel submerged surface such as the channel bed or channel bank.

Substrate: Mineral and organic material that forms the bed of a stream. In an armored channel, substrate refers to the material beneath the armor layer.

Supercritical flow: Occurs when normal depth is less than critical depth; rare for extended reaches in natural streams.

Swimming speeds: Fish swimming speeds can vary from essentially zero to over six meters per second, depending on species, size and activity. Three categories of performance are generally recognized:

Sustained speed: The speed a fish can maintain for an extended period for travel without fatigue. Metabolic activity in this mode is strictly aerobic and utilizes only red muscle tissues.

Prolonged speed: The speed that a fish can maintain for a prolonged period, but which ultimately results in fatigue. Metabolic activity in this mode is both anaerobic and aerobic and utilizes white and red muscle tissue.

Burst (Darting) speed: The speed a fish can maintain for a very short period, generally 5 to 7 seconds, without gross variation in performance. Burst speed is employed for feeding, escape and negotiating difficult hydraulic situations, and represents maximum swimming speed. Metabolic activity in this mode is strictly anaerobic and utilizes only white muscle tissue.

Tailwater: The water downstream from a structure or point on a stream.

Tailwater depth:Depth of water at a culvert outlet.

Thalweg: The longitudinal line of deepest water within a stream.

Toe: The break in slope at the foot of a bank where the bank meets the bed.

Upstream fish passage: Fish passage relating to upstream migration of adult and/or juvenile fish.

Upstream passage facility: A fishway system designed to pass fish upstream of a passage impediment, either by volitional or non-volitional passage.

Velocity: Time rate of motion; the distance traveled divided by the time required to travel that distance.

Average velocity: The discharge divided by the cross-sectional area of the flow in a culvert. Usually termed "average velocity."

Boundary layer velocity: Area of decreased velocity due to culvert boundary roughness. This region is restricted to only a few cm from the boundary.

Maximum velocity: The highest velocity within a cross-section of flow.

Weir: A short wall constructed on a stream channel that backs up water behind it and allows flow over or through it if notched. Weirs are used to control water depth and velocity.

Wetted perimeter: Across a channel section, the length of the channel surface in contact with water.

Glossary of Acronyms

ADFG Alaska Department of Fish and Game
ADOT Alaska Department of Transportation
AOP Aquatic Organism Passage
BMPs Best Management Practices
CALTRANS California Department of Transportation
CDFG California Department of Fish and Game
CMP Corrugated Metal Pipe
CWA Clean Water Act
CU Customary Units
DF&G Department of Fish and Game
DOT(s) Department(s) of Transportation
EDF Energy Dissipation Factor
EFH Essential Fish Habitat
EO Executive Order
ESA Endangered Species Act
FDC Flow Duration Curves
FHWA Federal Highway Administration
FWCA Fish and Wildlife Coordination Act
GAO General Accounting Office
HDS Hydraulic Design Series
HEC Hydraulic Engineering Circular
HSPF Hydrological Simulation Program: Fortran
MDOT Maine Department of Transportation
NEPA National Environmental Policy Act
NMFS National Marine Fisheries Service
NOAA National Oceanic-Atmospheric Administration
ODFW Oregon Department of Fish and Wildlife
OHW Ordinary High Water
QA/QC Quality Assurance and Quality Control
SI International System of Units
SPP Structural Plate Pipe
SPPA Structural Plate Pipe Arch
SWMM Storm Water Management Model
TMDL Total Daily Maximum Load
WDFW Washington Department of Fish and Wildlife
USFS United States Forest Service

<< Previous Contents Next >>

Updated: 04/07/2011


Bert Bergendahl

United States Department of Transportation - Federal Highway Administration