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Debris. Any material including floating woody
materials and other trash, suspended sediment, or bed load moved
by a flowing stream.
Debris Barrier. A deflector placed at the en-trance
of a culvert upstream, which tends to deflect heavy floating debris
or boulders away from the culvert entrance during high-velocity
flow.
Debris Basin. Any area upstream from a drainage
structure utilized for the purpose of retaining debris in order
to prevent clogging of drainage structures downstream.
Debris Rack. A straight barrier that, when placed
across the stream channel, tends to separate light and medium floating
debris from stream flow and prevent the
debris from reaching the culvert entrance.
Degradation. General and progressive lowering
of the longitudinal profile of a channel by erosion.
Delta. System of channels through an alluvial
plain at the mouth of a stream.
Denuded. Land stripped of vegetation.
Deposit. An earth mass of particles settled
or stranded from moving water or wind.
Depth. Vertical distance, (1) from surface to
bed of a body of water. (2) From crest or crown to invert of a conduit.
Design Discharge. The quantity of flow that
is expected at a certain point as a result of a design storm.
Usually expressed as a rate of flow in cubic meters per second.
Design Flood. The peak discharge (when
appropriate, the volume, stage, or wave
crest elevation) of the flood associated with the probability of
exceedance selected for the design of an encroachment in a FEMA
flood plain.
Design Frequency. The recurrence interval for
hydrologic events used for design purposes. As an example,
a design frequency of 50 years means a storm of a magnitude that
would be expected to recur on the average of every 50 years.
(See Probability of Exceedance.)
Design High Water. The flood stage or tide crest
elevation adopted for design of drainage and bank protection structures.
(See Design Flood and High Water).
Design Storm. That particular storm which contributes
runoff which the drainage facilities were designed to handle.
This storm is selected for design on the basis of its probability
of exceedance or average recurrence interval (See Probability of
Exceedance.)
Detention. The process of temporarily collecting
and holding back storm water for later release to receiving waters.
Detention Storage. Surface water moving over
the land is in detention storage. Surface water allowed to
temporarily accumulate in ponds, basins, reservoirs or other types
of holding facility and which is ultimately re-turned to a watercourse
or other drainage system as runoff is in detention storage.
(See Retention Storage)
Detritus. Loose material such as; rock, sand,
silt, and organic particles.
Dike. (1) Usually an earthen bank alongside
and parallel with a river or open channel to restrict overflow (See
Levee). (2) An asphalt concrete berm along the edge of a shoulder.
Dike, Finger. Relatively short embankments constructed
normal to a larger embankment, such as an approach fill to a bridge.
Their purpose is to impede flow and direct it away from the major
embankment.
Dike, Spur. Relatively short embankments constructed
at the upstream side of a bridge end for the purpose of aligning
flow with the waterway opening and to move scour away from the bridge
abutment.
Dike, Toe. Embankment constructed
to prevent lateral flow from scouring
the corner of the downstream side of an abutment
embankment. Sometimes referred to as training dikes.
Dike, Training. Embankments
constructed to provide a transition from the natural
stream channel or floodplain, both to and from a constricting bridge
crossing.
Discharge. A volume of water flowing out of
a drainage structure or facility. Measured in cubic meters
per second.
Dissipate. Expend or scatter harmlessly, as
of energy of moving water.
Disturbed Areas. Areas that have been purposefully
cleared, grubbed, excavated, or graded by the contractor;
ground surface that has been disrupted by construction activities,
including construction access/roads, staging, and storage sites
producing significant areas of exposed soil and soil piles
Ditch. Small artificial channel, usually unlined.
Diversion. (1) The change
in character, location, direction, or
quantity of flow of a natural drainage
course (a deflection of flood water is not a diversion).
(2) Draft of water from one channel
to another. (3) Interception of runoff by works which
discharge it through unnatural channels.
D-Load (Cracking D-Load). A term used
in expressing the strength of concrete pipe. The
cracking D-load represents the test load
required to produce a 0.3 mm crack for a length of 300 mm.
Downdrain. A prefabricated drainage facility
assembled and installed in the field for the purpose of transporting
water down steep slopes.
Downdrift. The direction of predominant movement
of littoral materials.
Drain. Conduit intercepting and discharging
surplus ground or surface water.
Drainage. (1) The process of removing surplus
ground or surface water by artificial means. (2) The system
by which the waters of an area are removed. (3)
The area from which waters are drained; a drainage basin.
Drainage Area(Drainage Basin) (Basin). That
portion of the earth's surface upon which falling precipitation
flows to a given location.
Drainage Course. Any path along which water
flows when acted upon by gravitational forces.
Drainage Divide. The rim of a drainage basin.
A series of high points from which water flows in two directions,
to the basin and away from the basin.
Drainage Easement(See Easement).
Drainage System. Usually a system of underground
conduits and collector structures which flow to a single point of
discharge.
Drawdown. The difference in elevation between
the water surface elevation at a constriction in a stream or conduit
and the elevation that would exist if the constriction were absent.
Drawdown also occurs at changes from mild to steep channel slopes
and weirs or vertical spillways.
Drift. (1) Floating or non-mineral burden of
a stream. (2) Deviation from a normal course in a cross current,
as in littoral drift.
Drop. Controlled fall in a stream to dissipate
energy.
Dry Weather Flows. A small amount of water
which flows almost continually due to lawn watering, irrigation
or springs.
Dune. A sand wave of approximately triangular
cross section (in a vertical plane in the direction of flow) formed
by moving water or wind, with gentle upstream slope and steep
downstream slope and deposition on the downstream slope.
Easement. Right to use the land of others.
Ebb. Falling stage or outward flow, especially
of tides.
Eddy Loss. The energy lost (converted into heat)
by swirls, eddies, and impact, as distinguished from friction loss.
Eddy. Rotational flow around a vertical axis.
Embankment. Earth structure above natural ground.
Embayment. Indentation of bank or shore, particularly
by progressive erosion.
Encroachment. Extending beyond the original,
or customary limits, such as by occupancy of the river and/or flood
plain by earth fill embankment.
Endwall. A wall placed at the end of a culvert.
It may serve three purposes; one, to hold the embankment away
from the pipe and prevent sloughing into the pipe outlet
channel; two, to provide a wall which will prevent erosion of the
roadway fill; and three, to prevent flotation of the pipe.
Energy. Potential or kinetic, the latter being
expressed in the same unit (meters) as the former.
Energy Dissipator. A structure for the purpose
of slowing the flow of water and reducing the erosive forces present
in any rapidly flowing body of water.
Energy Grade Line. The line which represents
the total energy gradient along the channel. It is established
by adding together the potential energy expressed as the water
surface elevation referenced to a datum and the kinetic energy (usually
expressed as velocity head) at points along the stream bed or channel
floor.
Energy Head. The elevation of the hydraulic
grade line at any section plus the velocity head of the mean velocity
of the water in that section.
Entrance. The upstream approach transition to a constricted
waterway.
Entrance Head. The head required to cause flow
into a conduit or other structure; it includes both entrance loss
and velocity head.
Entrance Loss. The head lost in eddies and friction
at the inlet to a conduit or structure.
Environmental Protection Agency (EPA).
Agency that issued the regulations
to control pollutants in storm water runoff discharges
(The Clean Water Act and NPDES permit requirements).
Ephemeral. Of brief duration, as the flow of
a stream in an arid region.
Equalizer. A drainage structure similar to a
culvert but different in that it is not intended to pass a
design flow in a given direction. Instead it is often
placed level so as to permit passage of water in either direction.
It is used where there is no place for the water to go. Its
purpose is to maintain the same water surface elevation on both
sides of an embankment.
Erosion. The wearing away of natural (earth)
and unnatural (embankment, slope protection, structure, etc.) surfaces
by the action of external forces. In the case of drainage
terminology, this term generally refers to the wearing away of the
earth's surface by flowing water. It can also refer to the
wear on a structural surface by flowing water and the material carried
therein.
Erosion and Accretion. Loss and gain of land,
respectively, by the gradual action of a stream in shifting its
channel by cutting one bank while it builds on the opposite bank.
Property is lost by erosion and gained by accretion but not by avulsion
when the shift from one channel to another is sudden. Property
is gained by reliction when a lake recedes.
Erosion and Scour. The cutting or wearing away
by the forces of water of the banks and bed of a channel in horizontal
and vertical directions, respectively.
Erosion Control. Vegetation,
such as grasses and wildflowers, and other materials, such as
straw, fiber, stabilizing emulsion, protective blankets, etc., placed
to stabilize areas disturbed by grading operations, reduce loss
of soil due to the action of water or wind, and prevent water
pollution.
Estuary. That portion of a river channel occupied
at times or in part by both sea and river flow in appreciable quantities.
The water usually has brackish characteristics.
Evaporation. A process whereby water as a liquid is
changed into water vapor, typically through heat supplied from the
sun.
Excavation. The process of removing earth, stone,
or other materials.
Existing Vegetation. Any vegetated area that
has not already been cleared and grubbed.
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