IECA Resources

Definitions T – Z

Talus

Loose rocks and debris disintegrated from a steep hill or cliff standing at repose along the toe.

Tapered Inlet

A transition to direct the flow of water into a channel or culvert. A smooth transition to increase hydraulic efficiency of an inlet structure.

Temporary Construction Site BMPs

BMPs that are required only temporarily to address a short-term storm water contamination threat. For example, silt fences are located near the base of newly graded slopes that have a substantial area of exposed soil. Then, during rainfall, the silt fences filter and collect sediment from runoff flowing off the slope.

Terrace

Berm or bench-like earth embankment, with a nearly level plain bounded by rising and falling slopes.

Tetrahedron

Bank protection element, basically composed of 6 steel or concrete struts joined like the edges of a triangular pyramid, together with subdividing struts and tie wires or cables.

Tetrapod

Bank protection element, precast of concrete, consisting of 4 legs joined at a central block, each leg making an angle of 109.5 degrees with the other three, like rays from the center of a tetrahedron to the center of each face.

Texture

The arrangement and interconnection of surface and near-surface particles of terrain or channel perimeter.

Thalweg

The line following the lowest part of a valley, whether under water or not. Usually the line following the deepest part of the bed or channel of a river.

Thread

The central element of a current, continuous along a stream.

Tide

The periodic rising and falling of the ocean and connecting bodies of water that results from gravitational attraction of the moon and sun acting on the rotating earth.

Time of Concentration

The time required for storm runoff to flow from the most remote point, in flow time, of a drainage area to the point under consideration. It is usually associated with the design storm.

Topography

The physical features of a surface area including relative elevations and the position of natural and man-made features.

Topping

The top layer on horizontal revetments or rock structures; also capping or cap stones.

Total Maximum Daily Load (TMDL)

A process established by the Clean Water Act to guide the application of state water quality standards to individual water bodies and watersheds by defining the amount of a particular pollutant that a water body can absorb on a daily basis without violating applicable water quality standards. Once this load is determined, the regulatory agency allocates a portion to each source of that pollutant within a particular watershed.

Total Suspended Solids (TSS)

The weight of particles that are suspended in water. Suspended solids in water reduce light penetration in the water column, can clog the gills of fish and invertebrates, and are often associated with toxic contaminants because organics and metals tend to bind to particles.

Training

Control of direction of currents.

Transition

A relatively short reach or conduit leading from one waterway section to another of different width, shape, or slope.

Transport

To carry solid material in a stream in solution, suspension, saltation, or entrainment.

Trash Rack

A grid or screen across a stream designed to catch floating debris.

Tributary

A river or stream which flows into a larger river or stream.

Trough

Space between wave crests and the water surface below it.

Trunk (or Trunk Line)

In a drainage system, the main conduit for transporting the storm waters. This main line is generally quite deep in the ground so that laterals coming from fairly long distances can drain by gravity into the trunk line.

Tsunami

A gravity wave caused by an underwater seismic disturbance (such as sudden faulting, landsliding or volcanic activity).

Turbidity

A measure of the amount of material suspended in the water. Increasing the turbidity of the water decreases the amount of light that penetrates the water column. High levels of turbidity are harmful to aquatic life.

Turbulence

The state of flow wherein the water is agitated by cross-currents and eddies, as opposed to a condition of flow that is quiet and laminar.

Turbulent Flow

That type of flow in which any particle may move in any direction with respect to any other particle, and in which the head loss is approximately proportional to the square of the velocity.

Undercut

Erosion of the low part of a steep bank so as to compromise stability of the upper part.

Underflow

The downstream flow of water through the permeable deposits that underlie a stream.

1. Movement of water through a pervious subsurface stratum, the flow of percolating water; or water under ice, or under a structure.
2. The rate of flow or discharge of subsurface water.

Undertow

Current outward from a wave-swept shore carrying solid particles swept or scoured from the beach or foreshore.

Unsteady Flow

A flow in which the velocity changes with respect to space and time.

Updrift

The direction opposite that of the predominant movement of littoral materials.

Uplift

Upward hydrostatic pressure on the base of an impervious structure.

Urban Runoff

A substance, such as rain, that runs off of surfaces in a watershed in excess of the amount absorbed by the surfaces (usually the ground). Urban runoff can contain sediments and contaminants (non-point source pollution) that can add to water quality degradation in the watershed. Increases in impervious surface usually result in increased urban runoff.

Velocity

The rate of motion of objects or particles, or of a stream of particles.

Velocity Head

A term used in hydraulics to represent the kinetic energy of flowing water. This "head" is represented by a column of standing water equivalent in potential energy to the kinetic energy of the moving water, calculated as (V2/2g), where the "V" represents the velocity in meters per second and "g" represents the potential acceleration due to gravity, in meters per second per second.

Vernal Pools

Vernal pools are seasonally flooded landscape depressions that support distinctive (and many times rare) plant and animal species adapted to periodi or continuous inundation during the wet season, and the absence of either ponded water or wet soil during the dry season.

Wash

Flood plain or active channel of an ephemeral stream, usually in recent alluvium.

Water Table

The surface of the groundwater below which the void spaces are completely saturated.

Watercourse

A definite channel with bed and banks within which water flows, either continuously or in season. A watercourse is continuous in the direction of flow and may extend laterally beyond the definite banks to include overflow channels contiguous to the ordinary channel. The term does not include artificial channels such as canals and drains, except natural channels trained or restrained by the works of man. Neither does it include depressions or swales through which surface or errant waters pass.

Waters of the United States

(a) All waters, which are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerce, including all waters which are subject to the ebb and flow of the tide; (b) All interstate waters, including interstate wetlands; (c) All other waters such as intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds, the use, degradation, or destruction of which would affect or could affect interstate or foreign commerce including any such waters: (1) which are or could be used by interstate or foreign travelers for recreational or other purposes; (2) from which fish or shellfish are or could be taken and sold in interstate or foreign commerce; or (3) which are used or could be used for industrial purposes by industries in interstate commerce; (d) All impoundments of waters identified in paragraphs (a) through (d) of this definition; (f) The territorial sea; and (g) Wetlands adjacent to waters (other than waters that are themselves wetlands) identified in paragraphs (a) through (f) of this definition. Waste treatment systems, including treatment ponds or lagoons designed to meet the requirements of CWA (other than cooling ponds as defined in 40 CFR 423.11 (m) which also meet the criteria of this definition) are not waters of the United States. This exclusion applies only to manmade bodies of water which neither were originally created in waters of the United States (such as disposal area in wetlands) nor resulted from the impoundment of waters of the United States.

Watershed

The area that contributes surfacewater runoff into a tributary system or water course.

Waterway

1. That portion of a watercourse that is actually occupied by water.
2. A navigable inland body of water.,/li>

Wave

1. An oscillatory movement of water on or near the surface of standing water in which a succession of crests and troughs advance while particles of water follow cyclic paths without advancing.
2. Motion of water in a flowing stream so as to develop the superficial appearance of a wave.

Wave Height

The vertical distance between a wave crest and the preceding trough.

Wave Length

The horizontal distance between similar points on two successive waves (for example, crest to crest or trough to trough), measured in the direction of wave travel.

Wave Period

The time in which a wave crest travels a distance equal to one wave length. Can be measured as the time for two successive wave crests to pass a fixed point.

Weephole

A hole in a wall, invert, apron, lining, or other solid structure to relieve the pressure of groundwater.

Weir

A low overflow dam or sill for measuring, diverting, or checking flow.

Well

1. An artificial excavation for withdrawal of water from underground storage.
2. The upward component of velocity in a stream.

Wetland

Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.

Wet Weather Flow

Rainfall (storm water) runoff.

Windbreak

1. A barrier fence or line of trees to break or deflect the velocity of wind.
2. Any device designed to block wind flow and intended for protection against any ill effects of wind, particularly wind erosion.

Windwave

A wave generated and propelled by wind blowing along the water surface.

Young

Immature, said of a stream on a steep gradient actively scouring its bed toward a more stable grade.