Flood, levee, and erosion control glossary - E
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Easement
Right of way granted by a property owner to the Santa Clara Valley Water
District for an expressed purpose of either constructing flood and erosion
control improvements or maintaining the channel.
The property owner retains the ownership rights on the area under the easement
but his use of the area is restricted only by those uses which are compatible
with the District uses.
Ecological
Relating to the interrelationships of organisms
and their environment.
Ecological Indicator
A characteristic of the environment
that is measured to provide evidence of the biological condition of a resource.
Ecological indicators can be measured at different levels such as organism,
population, community, or ecosystem.
Ecological Risk Assessment
A process that evaluates the
likelihood that adverse ecological effects may occur or are occurring as a
result of exposure to one or more pollutants. See Risk
Assessment.
Ecology
The study of interrelationships of organisms to
one another and to their environment.
Ecoregion
A relatively homogeneous geographic area
perceived by simultaneously analyzing a combination of causal and integrative
factors including land surface form, soils, land uses, and potential natural
vegetation. Ecoregions are generally considered to be the regions of relative
homogeneity in ecological systems or in relationships between organisms and
their environments.
Ecosystem
The most complex level of organization is the ecosystem. An ecosystem
includes the plant and animal communities in an area together with the
non-living physical environment that supports them. Ecosystems have physically
defined boundaries, but they are also dynamic - their boundaries and
constituents can change over time. They can import and export materials and energy
and thus can interact with and influence other ecosystems. They can also vary
widely in size.
Eddy Current
A circular water movement that develops when
the main flow becomes separated from the bank. The eddy current may then be set
up between the main flow and the bank.
Edge
The transitional zone where one cover type ends and
another begins.
Edge Habitat
The outermost belt (ranging from a few to
several hundred feet) encompassing a patch that has an environment very
different from the interior of a patch. See Interior
Habitat.
Effluent
The discharge to a body of water from a defined
or point source, generally consisting of a mixture of waste and water from
industrial or municipal facilities. Wastewater, treated or untreated, that
flows out of a treatment plant, sewer or industrial outfall. Generally refers
to wastes discharged into surface water. Resource: DTSC.
Elevation
The raising
of a structure to place the lowest floor at or above the flood
protection elevation on an extended support structure.
EMAP
Abbreviation for Environmental Monitoring and Assessment Program, an
Environmental Protection Agency Office of Research and Development research
program (http://epa.gov/emap/).
EMAP Benthic Index (BI)
An index developed to reduce
individual measures of the benthic community into a single number. Parameters
in the BI include a measure of species diversity and measures of pollution
intolerant organisms. A positive BI indicates good conditions, while a negative
BI indicates impacted benthic community.
Embankment
The part of the soil next to a stream, lake, or
body of water where the soil elevation adjacent to the water is higher than the
water level; usually referred to as the bank.
Emergency
Any instance for which, in the determination of
the President, federal assistance is needed to supplement state and local
efforts and capabilities to save lives and protect property and public health
and safety or to lessen or avert the threat of a disaster in any part of the
United States.
Emergency (Temporary) Flood Proofing
Emergency measures
would be, upon receipt of a warning or forecast, either improvised just prior
to or during an actual flood or carried out according to an established
emergency plan of action.
Emergency Program
The interim program of the National Flood Insurance Program as implemented on an
emergency basis to provide a first layer of subsidized insurance before the
detailed risk studies from which actuarial rates are computed have been completed.
Emergency Spillway
A channel
used to safely convey flood discharges in excess of the capacity of the
principal outlet.
Emergent Wetlands
Wetlands, commonly called marshes and
wet meadows that are dominated by grasses, sedges, and other herbaceous
(non-woody) plants.
Encroachment Lines
The lateral limits or line drawn along
each side and generally parallel to a watercourse or body of water, to preserve
the flood carrying capacity of the stream or other body of water and its floodplain, and to assure attainment of the basic
objective of improvement plans that may be considered or proposed. Their
location should be such that the floodway between them will effectively carry
and discharge the base flood, or 100-year
frequency flood.
Encroachments
Activities or construction within the
floodway (including fill, new construction, substantial improvements, and other
development) that result in an increase in flood levels.
Environmental Assessment
An examination of the positive and adverse impacts
on the environment of a proposed water resources solution and alternative
solutions.
Environmental Impact Statement
A
detailed environmental analysis and documentation of a proposed water resources
solution when the proposed solution is expected to have a significant effect on
the quality of the human environment or the area's ecology.
Endangered Species
Any species in danger of extinction
throughout all or a significant portion of its range/habitat.
Energy Dissipater
Any means by which the total energy of
flowing water is reduced. In stormwater design,
they are usually mechanisms that reduce velocity prior to, or at, discharge
from an outfall in order to prevent erosion. They include rock splash pads,
drop manholes, concrete stilling basins or baffles, and check dams.
Engineered Concrete Block Walls
Walls engineered to
withstand loads caused by water and debris. These walls are considered to be
permanent, and do not require yearly replacement.
Engineering
The application of science to the design, creation,
and function of machines, construction etc.
Environment
The sum total of all external conditions that
may act upon an organism or community to influence its development or
existence.
Environmental Impact Report (EIR)
A detailed statement
describing and analyzing the significant environmental impacts of a project and
discussing ways to mitigate or avoid the effects.
Epilimnion
The upper layer of water in a thermally
stratified lake or reservoir. This layer consists of the warmest water and has
a fairly uniform (constant) temperature. The layer is readily mixed by wind
action.
Equal Degree of Encroachment
A rule, used in determining
permissible floodplain encroachments, that the floodplain on each side of a
stream must be capable of conveying a proportionate share of the design flood flow.
ERM
Abbreviation for Elevation Reference Mark.
Erodability
The relative amount of energy required to excavate particles, or aggregations of particles, by erosion, or scouring, with a moving fluid.
Erodible Slopes
(EM 1110-2-5026) Slopes of more than three feet per 100 feet that, when bare of vegetative cover, are highly erosive, especially those primarily made of silt and silty clay soils.
Erosion
The wearing away of the land surface by running water, wind, ice, or other
geological agents, including such processes as gravitational creep. Also,
detachment and movement of soil or rock fragments by water, wind, ice, or
gravity.
Erosion (Rainfall Impact)
The impact of raindrops on bare
soil causes erosion. Because the rainfall impact has a low velocity, this type
of erosion will normally result in minimum surface erosion on undisturbed land.
Even in areas of California with a semi-arid climate and minimal vegetative
cover, natural desert soil conditions (including compacted hardpan) provide
protection against erosion for all but the most intense rainfalls. Construction
activities remove the protective cover of vegetation and natural soil
resistance to impact erosion.
Erosion (Rill and Gully)
As runoff accumulates, it
concentrates in rivulets cutting grooves into the soil surface. If the flow is
sufficient, rills may develop into gullies.
Rills/gullies form sooner on exposed soils than vegetated soils.
Erosion (Sheet)
After rainfall strikes the ground, it
flows in a thin layer, called sheet flow for a short distance. The distance of
sheet flow depends on slope, type of soil vegetative cover, and rainfall
intensity. Sheet flow has a low velocity and causes little erosion on
undisturbed soils. However, clearing the soil during construction makes the
soil more susceptible to erosion, increases velocity, and causes the flow to
concentrate in rivulets.
Erosion (Soil)
Soil erosion is the process by which soil
particles are removed from the land surface, by wind, water or gravity. Most
natural erosion occurs at slow rates; however, the rate of erosion increases
when land is cleared or altered and left disturbed.
Erosion (Stream and Channel)
The banks and bottoms of
natural drainage channels are commonly eroded by three factors which occur
during construction Clearing the soil during construction increases the runoff
flows, velocities and volumes which reach natural streams Covering the soil
with buildings and pavement further increase runoff; and Site landscaping and
improperly designed desiltation basins may disrupt the natural balance of
erosion and sedimentation.
Erosion and Sediment Control (ESC)
Any temporary or
permanent measures taken to reduce erosion, control siltation and
sedimentation, and ensure that sediment-laden water does not leave a site.
Erosion and Sediment Control Facility
A type of drainage
facility designed to bold water for a period of time to allow sediment
contained in the surface and stormwater runoff
directed to the facility to settle out so as to improve the quality of the
runoff.
Erosion Control Blanket
Blanket made from straw, coir,
excelsior, or synthetic material and enveloped in plastic or biodegradable
netting. Used to stabilize disturbed or highly erosive soils while vegetation
is established. Temporary blankets made from biodegradable or photodegradable
components last several months to a year, and permanent blankets (also called
turf reinforcement mats) can last for several years.
Erosion Flood Plain
A flood plain that has been created by the lateral erosion and the gradual retreat of the valley walls.
Escarpment
A steep ace or a ridge of high land.
Essential element (plant nutrition)
A chemical element required
for the normal growth and reproduction of plants.
ESDP
Abbreviation for Engineering Study Data Package.
Estuary
A confined coastal water body with an open connection to the sea and a
measurable quantity of salt in its waters.
Photo courtesy of USDA NRCS.
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Satellite photo of Schelde estuary. |
Estuary (Estuaries)
Regions of interaction between rivers
and near-shore ocean waters, where tidal action and river flow mix fresh and
salt water. Such areas include bays, mouths of rivers, salt marshes, and
lagoons. These brackish water ecosystems
shelter and feed marine life, birds, and wildlife.
Estuary (Estuarine)
Areas where fresh water from rivers
and salt water from near shore ocean waters are mixed. These areas may include
bays, mouths of rivers, salt marshes and lagoons. These water ecosystems shelter and feed marine life, birds and
wildlife.
Eutrophication
A condition in an aquatic ecosystem where high nutrient concentrations stimulate blooms of algae (e.g., phytoplankton). These excess nutrients can lead to a condition in which prolonged blooms of algae deprive light and oxygen from other organisms while turning waterways green and foul smelling. Algal decomposition may lower dissolved oxygen concentrations. Although eutrophication is a natural process in the aging of lakes and some estuaries, it can be accelerated by both point and nonpoint sources of nutrients.
Evaporation
The process of returning moisture to the
atmosphere. Water on any surface, especially the surfaces of mudholes, ponds,
streams, rivers, lakes, and oceans, is warmed by the sun's heat until it
reaches the point at which water turns into the vapor, or gaseous, form. The
water vapor then rises into the atmosphere.
Evapotranspiration
The combined loss of water to the
atmosphere from land and water surfaces by evaporation and from plants by
transpiration.
Event
An occurrence of flooding.
Exceedance Frequency
The frequency that a flood of a certain discharge will
be equaled or exceeded in any given year; equal to the inverse of the
recurrence interval.
Exceedance Probability
The average frequency with which a
flood of a particular magnitude will be exceeded; expressed as the probability
that a flood will be exceeded in any year (the annual exceedance probability)
or as the average recurrence interval (the n-year flood).
Executive Order 11988
The "Floodplain Management" Executive Order
issued by the President and which specifies the responsibilities of federal
agencies in floodplain management. E.O. 11988 directed federal agencies to
evaluate and reflect the potential effects of their actions on floodplains and to include the evaluation and
consideration of flood hazards in agency permitting and licensing procedures. See http://www.usace.army.mil/cw/cecwo/reg/eo11988.htm.
Exposure
Direction of slope with respect to points of a
compass.
Extended Foundation Wall
The construction of an additional
wall to gain height above the existing foundation walls in order to elevate a
structure to or above the design flood
elevation.
Extinct
A species of plant or animal that is no longer
living.
Extraction Wells
Wells that are used primarily to remove
groundwater from the ground. Water level measurements and water samples can be
collected from extraction wells.
Eye
In meteorology, usually the "eye of the storm" (hurricane): the roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone.
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