Flood, levee, and erosion control glossary - L
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Lacustrine Flooding
Flooding produced by a lake or pond.
LAG
Abbreviation for Lowest Adjacent Grade (to a structure).
Lag Time
The interval between the center of mass of the
storm precipitation and the peak flow of the resultant runoff.
Lagoon
Water impoundment in which organic wastes are
stored or stabilized or both. A shallow, artificial treatment pond where
sunlight, bacterial action, and oxygen work to purify wastewater; a
stabilization pond. An aerated lagoon is a treatment pond that uses oxygen to
speed up the natural process of biological decomposition of organic wastes.
Lake
A large body of water, typically freshwater, which
can be formed by glaciers, river drainage, surface water runoff, or ground
water seepage. Lakes provide an area for recreational activity (e.g., boating,
water skiing, and fishing) and a habitat for wildlife. They are particularly
important to migrating wildlife.
Land Application
Refers to the application of wastewater
or storm water to agricultural land.
Land Capability
The suitability of land for use without
permanent damage. The risks of land damage from erosion and the difficulties in
land use because of the physical land characteristics.
Land Cover
Anything that exists on, and is visible from
above the Earth's surface. Examples include vegetation, exposed or barren land,
water, snow, and ice.
Land Disturbing Activity
Any activity that results in a
change in the existing soil cover (both vegetative and non-vegetative) and/or
the existing soil topography. Land disturbing activities include, but are not
limited to demolition, construction, clearing, grading, filling and excavation.
Land Treatment Measures
Measures used to reduce runoff of
water to streams or other areas; techniques include: maintenance of trees,
shrubbery and vegetative cover; terracing; slope stabilization; grass
waterways; contour plowing; and strip farming.
Land Use
The way land is developed and used in terms of
the kinds of anthropogenic activities that occur (e.g., agriculture,
residential areas, industrial areas).
Landings
Logging decks; areas where logs are skidded for
concentration.
Landscape
The spatial pattern or structure of a geographic
area (including its biological composition, its physical environment, and its
anthropogenic or social patterns) and is designed to identify repeating
patterns associated with dominant land uses in an area. The relative proportion
of forest, agriculture, and urban land cover contained in the area also defines
a landscape as does the interrelationships between them.
Landscape Conditions
The apparent status or
characteristics of a landscape unit as measured by one or more landscape
indicators.
Landscape Ecology
The study of the distribution patterns
of communities and ecosystems, the processes that affect those patterns, and
changes in pattern and process over time.
Landscape Indicator
A characteristic of the environment
that is measured to provide evidence of the biological condition of one or more
resources at the ecosystem level.
Landscape Unit
Designed to identify repeating patterns
associated with dominant land uses in an area, and defined by the relative
proportions of forest, agriculture, and developed (urban) land cover contained
in that area.
Landslide
An extreme form of erosion which is a natural
process of the earth's surface and occurs when external forces exceed internal
forces within the soil and rock of a hillside.
Photo courtesy of US Geological Service. |
Photo of a Landslide. |
Larvae (Larva)
Early form of an animal that is unlike its
adult form and must metamorphose before assuming adult characteristics.
Latent Heat
The heat (energy) absorbed or released as
water changes between the gas (water vapor), the liquid (water droplets), and
the solid (ice) states.
Launching
Stockpiling stone along the top bank so that the
stone is undercut and slides down the slope, protecting the bank against future
erosion.
Layering, Soil
An undesirable stratification of a soil.
LD50 (Median Lethal Dose)
The statistically derived single
dosage of a toxic substance required to kill 50% of the test organisms under
study in a given period of time.
Leachate
Water that collects contaminants as it trickles
through wastes, pesticides or fertilizers. Leaching may occur in farming areas,
feedlots, and landfills, and may result in hazardous substances entering
surface water, ground water, or soil.
Leaching
Removal of the more soluble materials from the
soil by percolating waters. The downward transport of dissolved or suspended
minerals, fertilizers, and leaks, waste disposal, or drainage from rain
material storage.
Legume
A member of the Leguminosae family, one of the most
important and widely distributed plant families. Leaves are alternate, have
stipples and are usually compound. Most legumes are nitrogen-fixing plants.
Legume inoculation
The addition of nitrogen-fixing
bacteria to legume seed or to the soil in which the seed is to be planted.
Length of Wave
The horizontal distance between similar points on two successive waves measured perpendicularly to the crest.
Lethal Dose (LD)
The amount of a toxic substance required
to cause death of an organism under study, in a given period of time.
Letter of Map Amendment (LOMA)
A Letter of Map Amendment
(LOMA) is an official amendment, by letter, to an effective National Flood
Insurance Program map. A LOMA establishes a property's location in relation to
the Special Flood Hazard Area. There is no appeal period. The letter becomes
effective on the date sent.
Letter of Map Change (LOMC)
A LOMC is a letter which
reflects an official revision to an effective NFIP map. LOMCs are issued
in place of the physical revision and republication of the effective map.
Letter of Map Revision (LOMR)
A Letter of Map Revision
(LOMR) is an official revision, by letter, to an effective National Flood
Insurance Program map. A LOMR may change flood insurance risk zones, floodplain
and/or floodway boundary delineations, planimetric features, and/or Base Flood
Elevations. See the LOMR Effective Chart to determine when a LOMR becomes
effective.
Levee
1. An embankment or a barrier of compacted soil constructed to prevent a river
or stream from flooding adjacent lands. A structure of earth or stone built
parallel to a river to protect land from flooding. 2. An earthen embankment, floodwall, or structure along a watercourse whose purpose is flood risk reduction or water conveyance.
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Photo of a Levee Breach. |
Level (Degree) Of Protection
The greatest flood level
against which a protective measure is designed to be fully effective; often
expressed as a recurrence interval (e.g., 100-year level of protection) or as
an exceedance frequency (e.g., one-percent chance of exceedance).
Levee Accreditation
Often referred to as levee certification, this status is given to a levee when it is accepted by FEMA as meeting all current FEMA standards, including the 100-year-protection level.
Levee Board
A board composed of elected or appointed officials whose job is to oversee the operation of a local, or larger, levee district.
Levee Certification
The certification of levees for FEMA’s National Flood Insurance Program performed by the local levee owner or sponsor.
Levee Certification Defined
Levee certification is a technical finding for floodplain mapping purposes as part of the National Flood Insurance Program (NFIP), that concludes there is reasonable certainty that the levee protecting the area will contain the base (1% annual chance exceedance, sometimes referred to as the 100-year) regulatory flood. The certification finding must be accomplished by either a registered professional engineer or a Federal agency with levee design and construction qualifications such as USACE.
Levee District/Reclamation District
Local levee or reclamation districts are government entities responsible for the day-to-day operation, management and maintenance of levees. This legal entity may receive (tax and assess) and expend moneys for the purposes of repair and maintenance of the public levee district's levee and related appurtenances operating within its jurisdiction.
Levee Inventory and Risk Assessment Initiative
The objective of this program is to provide a living database repository of information relative to the status and safety of the nation’s levee system and together with assessment methods under development, provide a consistent risk based framework to evaluate levees nationally. The potential risk of all levee projects and the consequences due to unsatisfactory performance that are the outcome of the assessments will be communicated to the general public and the owner of each levee system.
Levee Oversight
Oversight of levee performance by designated local, state, and federal authorities. The Army Corps of Engineers has specific and limited responsibilities for approximately 2,000 levees nationwide. There is no single agency with responsibility for levee oversight nationwide.
Levee Owner
The governmental, or private organization or individual who owns a specific levee. The owner may be the U.S. Army Corps of Engineers, a state government, a local levee district, a corporation, or an individual.
Levee Saturation
Soil saturation that has occurred in an earthen levee because of floodwaters remaining above flood stage for extremely long periods of time. This condition can lead to catastrophic failure of the levee.
Level Spreader
A temporary BMP used to spread stormwater runoff uniformly over the ground surface as sheet flow. The purpose of level
spreaders are to prevent concentrated, erosive flows from occurring. Level
spreaders will commonly be used at the upstream end of wider biofilters to
ensure sheet flow into the biofilter.
Levee System
Levee systems are designed to provide a specific level of flood protection via a number of natural or man-made levees. Some level of flood risk exists in these levee-impacted areas.
Levee Toe
The outer edge of the levee base where it meets the levee grade.
LFD
Abbreviation for Letter of Final Determination.
Lifetime Health Advisory
This health advisory is defined
as the concentration of a chemical in drinking water that is not expected to
cause any adverse effects over a lifetime of exposure, with a margin of safety.
These health advisories are calculated for a 70-kilogram (154-pound) adult and
assumes the consumption of 2 liters of water per day.
Liquefaction
A type of ground failure triggered by
earthquakes and which occurs when seismic shock waves pass through
unconsolidated and saturated soil.
Littoral
Of or pertaining to the shore, especially of the sea.
Littoral Drift
The movement of sand by littoral (longshore) currents in a
direction generally parallel to the beach along the shore. Jetties and groins
often stop littoral drift, leading to sand reduction on downstream beaches.
Littoral Transport
The movement of littoral drift along
the shoreline by waves and currents. Includes movement parallel (longshore
transport) and perpindicular (on-offshore transport) to the shore.
Littoral Zone
1. In beach terminology, an indefinite zone extending seaward from the shoreline to just beyond the breaker zone. 2. In coastal engineering, the area from the shoreline to just beyond the breaker zone. In biological oceanography, it is that part of the benthic division extending from the high water line out to a depth of about 200 meters. The littoral system is divided into a eulittoral and sublittoral zone, separated at a depth of about 50 meters. Also, frequently used interchangeably with intertidal zone.
Lithosphere
A general term for the outer layer of the
Earth.
Live Staking
A bioengineering technique in which cuttings
from living branches are inserted and tamped into the ground, where they
eventually take root.
LMMP
Abbreviation for Limited Map Maintenance Program.
Loads
Forces imposed
on a surface such as a wall or floor, an entire structure, or on the ground.
Loam
An easily crumbled soil consisting of a mixture of
clay, silt, and sand.
Local Flooding
Flood conditions which occur over a relatively limited area.
Local Responsibility
The responsibilities of local levee owner or sponsor. These may include levee safety; land use planning and development; building codes; and operations, maintenance, repair, rehabilitation and replacement of the levee.
Lock
A structure adjacent to a dam or in a canal to allow
passage of vessels from one water level to another. The lock consists of a
chamber, with gates at either end, in which water is raised or lowered. Navigation locks and dams normally do
not normally store flood waters.
LODR
Abbreviation for Letter of Determination Review.
Loess
Soil material transported and deposited by wind and
consisting of predominantly silt-sized particles. A wind blown deposit of silty
soil having little or no stratification.
Log Boom
A device used to prevent large objects floating on the water surface from entering an area like an intake or spillway.
LOMR-F
Abbreviation for Letter of Map Revision based on Fill. A Letter of
Map Revision Based on Fill (LOMR-F) is an official revision, by letter, to an
effective National Flood Insurance Program map. A LOMR-F provides FEMA's
determination concerning whether a structure or parcel has been elevated on
fill above the Base Flood Elevation and excluded from the Special Flood Hazard
Area. The letter becomes effective on the date sent.
Longer-Term Health Advisory (Child)
This health advisory
is defined as the concentration of a chemical in drinking water that is not
expected to cause any adverse effects up to approximately 7 years of exposure,
with a margin of safety. These health advisories are calculated for a
10-kilogram (22-pound) child and assumes the consumption of 1 liter of water
per day.
Longshore
Parallel to and near the shoreline.
Longshore Drift
Movement of (beach) sediments approximately parallel to the coastline.
Losing Stream
A stream or reach of stream from which some water flows from the stream bed into the ground.
Low Flow Channel
An incised or paved channel from inlet to
outlet in a dry basin which is designed to carry low runoff flows and/or
baseflow, directly to the outlet without detention. That section of stream
which carries the more frequent, periodic streamflows.
Lower Area
The area that exists between the elevated floor
and "grade" of an elevated structure.
Lower Bank
Portion of the streambank below the elevation
of the average water level of the stream.
Lowland Flooding
Inundation of the very lowest portions of floodplain areas near a river, stream or lake, which are normally subject to frequent flooding; usually considered nuisance flooding.
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