Flood, levee, and erosion control glossary - D
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D/S
Abbreviation for "downstream of".
Dam
A structure built across a waterway. Dams are used to control depths upstream
for navigation; or to create space to store water for flood control,
irrigation, water supply, hydropower or other purposes. A structure formed to
hold water back, generally built near uncontaminated water collection sources
in order to provide a drinking water supply to the surrounding communities.
DDT
A group of colorless chemicals used as insecticides.
DDTs are toxic to man and animals when swallowed or absorbed through the skin.
DDT in water is spread over large areas during flooding.
De-Accredited Levee
A levee that was once shown on the flood map as providing protection from the 1%-annual-chance flood; however, because sufficient documentation has not been provided to verify that the levee continues to meet NFIP requirements, the area landward is now shown as an Special Flood Hazard Area (SFHA) on FEMA’s flood map. If a levee cannot be certified as providing protection from the one-percent-annual-chance flood, the levee will not be accredited by FEMA. Decertified or uncertified levees will not be depicted on flood maps as providing the required level of protection. The areas behind these levees will be mapped as a high-risk areas and flood insurance will be required for buildings behind the levee with a federally backed mortgage.
Deadman
A log or block of concrete, or other material
buried in a streambank that is used to tie in a revetment
with cable, chain, or steel rods
Debris
Any combination of soil, rock, mud, trees, or vegetation usually transported
by "debris flow". The loose material
arising from the disintegration of rocks and vegetative material and
transportable by streams, ice, or floods. Objects such as vegetation, building
wreckage, vehicles, shopping carts or dead animals carried by water in a flood
(or by wind, as in a hurricane or tornado).
Photo courtesy of USDA NRCS.
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Debris from a flood. |
Debris Basin
A basin constructed to trap sediment or debris that would clog or damage a flood channel.
Debris
Flows
Consist of any soil, rocks, boulders, trees, or brush being
moved by storm waters and containing sufficient strength to destroy or move
objects such as cars and buildings in their path.
Debris Impact
See Mudflow.
Sudden loads induced on a structure by debris carried by floodwater.
Decay of Waves
The change waves undergo after they leave a generating area (FETCH) and pass through a calm, or region of lighter winds. In the process of decay, the significant wave height decreases and the significant wavelength increases.
Deep Percolation
Water that moves downward through the
soil profile below the root zone and cannot be used by plants.
Deepwater Habitats
Permanently flooded areas having a
depth of greater than two meters.
Deflector
Structural barrier (groin, jetty) projecting into a stream to divert flow away
from eroding sections of streambank.
Degradation
Process of a channel
lowering its elevation through increased erosion, channel bed scour, or down-cutting. A type of fluvial geomorphic
instability.
Delmarva Peninsula
The land separating Chesapeake Bay from
the Atlantic Ocean. The Delmarva Peninsula falls within the states of Delaware,
Maryland, and Virginia, from which it gets its name - Delmarva.
Depression Storage
The amount of precipitation that is
trapped in depressions on the surface of the ground. Water stored in surface
depressions and therefore not contributing to surface runoff.
Depth of Flow
The depth of flow is the vertical distance from the bed of a stream to the water surface.
Depurate
To cleanse. For example, shellfish contaminated
with coliform bacteria can be placed in clean seawater to depurate. Clean water
flowing through the organism will remove the bacteria over a period of time.
Note that this process does not apply to all contaminants (e.g., chlorinated
pesticides).
Design Capacity
An engineering term used to describe the
amount of water that a modified channel was
designed to convey. Generally, the design capacity for improved District
facilities is to accommodate the 1 percent or 100-year
flood. This is the level of protection. Capacity is in CFS or Q. See
"Flood Capacity."
Design
Flood
Commonly used to mean the magnitude of flood used for design and
operation of flood control structures or other protective measures. It is
sometimes used to denote the magnitude of flood used in floodplain regulations.
The maximum amount of water for which a flood control project will offer
protection. Selection is based on engineering, economic and environmental
considerations.
Design Flow
The magnitude of stream flow that is used in
design of channel improvements and structures
across the channels.
Design Storm
A prescribed hyetograph and total
precipitation amount (for a specific duration recurrence frequency) used to
estimate runoff for a hypothetical storm of interest or concern for the
purposes of analyzing existing drainage, designing new drainage facilities or
assessing other impacts of a proposed project on the flow of surface water.
Designated Floodway
The channel
of a stream and that portion of the adjoining floodplain designated by a
regulatory agency to be kept free of further development to provide for
unobstructed passage of flood flows.
Detention
Water management practice or system that delays the downstream progress of storm water by the use of temporary storage or metered outlets.
Detention Facility
An above or below ground facility, such
as a pond or tank, that temporarily stores stormwater
runoff and subsequently releases it at a slower rate than it is collected by
the drainage facility system. There is little or no infiltration of stored
stormwater.
Detention
Pond
A structure built to divert part or all of the runoff water from
a land area and to release the water under a controlled condition.
Detention Time
The theoretical time required to displace
the contents of a stormwater treatment facility
at a given rate of discharge (volume divided by rate of discharge).
Detritus
Non-living
organic matter (e.g., dead organisms or leaves) in water.
Dew Point
The temperature to which air must be cooled to
cause condensation of the water vapor it contains. The higher the dew point,
the higher the moisture content of the air.
Dewater
To remove water from wastes, soils or chemicals.
DFIRM
Digital Flood Insurance Rate Map
Dike
A structure designed either to reduce the water velocity as streamflow passes
through the dike so that sediment deposition
occurs instead of erosion (permeable dike), or to deflect erosive currents away
from the streambank (impermeable dike); also groin, palisade, spur, jetty, or deflector. In most areas of the U.S., an
earthen structure built partway across a river for the purpose of maintaining a
navigation channel. In other areas the term is
used synonymously with levee.
Photo courtesy of USDA NRCS.
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Photo of a dike. |
Diluent
Any liquid or solid material used to dilute or
carry an active ingredient.
Dilution Ratio
The relationship between the volume of
water in a stream and the volume of incoming water. It affects the ability of
the stream to assimilate waste.
Direct Runoff
Water that flows over the ground surface or
through the ground directly into streams, rivers, and lakes.
Discharge
Rate at which a volume of water passes a given
point (see CFS). Rate of stream flow. Usually measured as the volume of water
flowing past a cross section of stream per unit of time (m3 or ft3/s).
The release of storm water or the other substance from a conveyance system or storage container.
Discharger
The person(s) and/or authority discharging
storm water from a conveyance system or storage
container.
Dissolved Oxygen
Oxygen that is dissolved in water and
therefore available for use by plants (phytoplankton), shellfish, fish, and
other animals. If the amount of oxygen is too low, aquatic plants and animals
may die. In addition, aquatic populations exposed to low dissolved oxygen
concentration may be more susceptible to adverse effects of other stressors
(e.g., disease, toxic substances). Wastewater and naturally occurring organic
matter contain oxygen-demanding substances that, when decomposing, consume
dissolved oxygen.
Distressed Streambank
A bank experiencing erosion or
failure.
Ditch
A channel to convey water for irrigation or drainage.
Diversion
A structural flood control measure that
intercepts flood flows upstream of a damage-prone or constricted area and
routes flood flows around the area through an artificial channel or designated flow-way.
Diversion
Channel
A vegetated channel constructed across the slope of a field to catch water and carry it off a
field.
Photo courtesy of USDA NRCS.
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Diversion dam and channel. |
DLG
Abbreviation for Digital Line Graph.
Dormant Seeding
The application of cool season seed during
late fall or early winter for germination the following spring.
DOT
Abbreviation for Department of Transportation (federal or state).
Downdrift
The longshore direction of predominant movement
of littoral materials.
Downgradient
The direction in which groundwater flows.
Drain
A buried pipe or other conduit (closed drain). A
ditch (open drain) for carrying off surplus surface water or ground water.
Drain Inlet
Street exits points for stormwater and other related water runoff.
Drainage
Refers to the collection, conveyance, containment, and/or discharge of surface and storm
water runoff. The removal of excess surface water or ground water from land.
Drainage Area
Total land area from which water drains into
a point on a river or its tributaries. The Mississippi River drainage area comprises 41% of the land area of the
48 contiguous states.
Drainage Basin
A geographic and hydrologic sub-unit of a
watershed.
Drainage Channel
A drainage pathway with a well-defined
bed and banks indication frequent conveyance of
surface and stormwater runoff.
Drainage Course
A pathway for watershed drainage
characterized by wet soil vegetation; often intermittent in flow.
Drainage Divide
The boundary between one drainage basin
and another.
Drainage Easement
A legal encumbrance that is placed against
a property's title to reserve specified privileges for the users and
beneficiaries of the drainage facilities contained within the boundaries of the
easement.
Drainage Patterns
The drainage paths storm water runoff
usually or historically takes through a given area.
Drainage Well
(1) A well pumped in order to lower the
water table; (2) vertical shaft to a permeable substratum into which surface
and subsurface drainage is channeled which is now illegal.
Drainage, Soil
As a natural condition of the soil, soil
drainage refers to the frequency and duration of periods when the soil is free
of saturation. In well-drained soils, the water is removed readily but not
rapidly; in poorly drained soils, the root zone is waterlogged for long periods
unless artificially drained, and the roots of ordinary crop plants cannot get
oxygen. In excessively drained soils, the water is removed so completely that
most crop plants suffer from lack of water. Excessively drained soils are a
result of excessive runoff due to steep slopes or low available water holding
capacity due to small amounts of silt, clay, and organic matter in the soil
material.
Drawdown
(1) Lowering of the water table, surface water,
or piezometric surface resulting from the withdrawal of water from a well or
drain; (2) the elevation of the static water level (at the well) at a given
discharge.
Dredge Material
Soil that is excavated from a stream channel, lake, or other body of water.
Dredged Channel
An artificially maintained sea lane extending from an inland water body into the marginal sea to accommodate vessel traffic through coastal shallows.
Drilled Well
A well of varying depth usually 10 inches or less
in diameter, drilled with a drilling rig and cased with steel or plastic pipe.
Drinking Water Equivalent Level
Protective level of
exposure related to potentially non-carcinogenic effects of chemicals that are
also known to cause cancer.
Drop Structure
A structure designed to convey flows over a
vertical distance from a higher to a lower elevation.
Dry Flood Proofing
A
method used in areas of low-level flooding to completely seal a
structure against water by making the structure substantially impermeable to
the passage of water.
Dry Pond
A facility that provides stormwater
quantity control by containing excess runoff in a detention basin, then
releasing the runoff at allowable levels.
Dry Vault/Tank
A facility that treats stormwater for water quantity control by detaining
runoff in underground storage units and then releases reduced flows at
established standards.
Dug Well
A large diameter well dug by hand, usually old
and often cased by concrete or hand-laid bricks. Such wells typically reach
less than 50 feet in depth and are easily and frequently contaminated.
Dune Stabilization
The most frequently used type of land
treatment in coastal areas, including protection or establishment of plant
cover on existing sand dunes and/or construction of replacement dunes.
Duration Of Flood
The interval of time in which a tidal current is flooding, determined from the middle of slack waters.
Duration Of Flood And Duration Of Ebb
Duration of flood is the interval of time in which a tidal current is flooding, and duration of ebb is the interval in which it is ebbing, these intervals being reckoned from the middle of the intervening slack waters or minimum currents. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tidal current or a period of 24.84 hours for a diurnal current. In a normal semidiurnal tidal current, the duration of flood and duration of ebb each will be approximately equal to 6.21 hours, but the times may be modified greatly by the presence of nontidal flow. In a river the duration of ebb is usually longer than the duration of flood because of fresh water discharge, especially during spring months when snow and ice melt are predominant influences.
Dunes
Ridges or mounds of loose, wind-blown material,
usually sand.
Dystrophic
Low in nutrients and highly colored with
dissolved humic organic material (not necessarily a part of the natural trophic
progression).
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