Flood Control and stormwater quality are complex issues with several technical terms. The Regional Flood Control District is working to develop flood control and stormwater quality terminology and concepts that are easier to understand. Here are definitions to help explain many of the most common flood control and stormwater quality terms.

100-Year Storm – a storm with a magnitude that has a 1% chance of being equaled or exceeded each year.

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Acre-foot:  Volume of water that is 1-foot deep covering an area that is approximately the size of a football field, goal line to goal line without the endzones. For example, 10-acre-foot = the area of a football field with 10 feet of water above the field.

All-Weather Access: The ability to access a site during a major storm.
Aquifer: A layer of sand, gravel, and clay below the earth’s surface with enough water for people to withdraw for use (through wells and springs). Rainwater soaks into the ground and fills aquifers. 

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Backwater: Water upstream from an obstruction deeper than it would normally be without the obstruction. 

Baffle: A device, such as a concrete block, used to deflect, regulate, or check water. Typically used to slow storm water to reduce erosion.

Berm: An artificial ridge or embankment used to direct surface water along a desired path. Earthen berms, constructed compacted earth, are most common. 

Best Management Practices (BMPs): A Best Management Practice is a behavior or action that effectively prevents or reduces pollutants in stormwater runoff. 

Buffer Zone: A designated area separating hazardous areas, such as a river or channel, from publicly accessible areas. For example, a buffer zone could be a flat stretch of land between a hiking trail and a stream bank. A buffer zone may also be a designated area adjacent to or part of a stream or wetland that is an integral part of the stream or wetland ecosystem. 

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Caliche: A sedimentary rock, also called desert cement. Caliche is a hardened natural calcium carbonate cement that binds other materials – such as gravel, sand, clay, and silt. 

CCRFCD: Clark County Regional Flood Control District or Regional Flood Control District.

Channel: A long, narrow excavation or surface feature that conveys surface water and is open to the air. Channels may be artificial (constructed by man) or natural. 

Cistern: A waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns can be constructed either above or below ground. Underground cisterns are distinguished from wells by their waterproof linings.

Clean Water Act (CWA): A United States federal law intended to protect water pollution. The Clean Water Act establishes the basic structure for regulating discharges of pollutants into the waters of the United States and regulating quality standards for surface waters.

Amendments to the Clean Water Act of 1987 established requirements for stormwater discharges for each state in the United States under the National Pollutant Discharge Elimination System program. In response to those requirements, the Nevada Division of Environmental Protection (NDEP) issued a permit to the local municipalities (Henderson, Las Vegas, and North Las Vegas), Clark County and the Regional Flood Control District (collectively called the Permittees) for municipal stormwater discharges, commonly referred to as the Municipal Separate Storm Sewer System or "MS4."

Closed Depression: The low point in an area with no outlet that accumulates or receives runoff is part of a closed depression.

Community Rating System (CRS): The Community Rating System (CRS) is a voluntary incentive program that recognizes and encourages community floodplain management practices that exceed the minimum requirements of the National Flood Insurance Program (NFIP).

Concrete aprons: A concrete apron is a concrete slab placed at specific locations to control erosion.  For example, a concrete apron may be placed at the outlet of a storm drain pipe to a natural wash.  

Conditional Letter of Map Revision (CLOMR): A letter issued by the Federal Emergency Management Agency that confirms potential revisions to Special Flood Hazard Areas (flood zones) shown on the Flood Insurance Rate Maps should a proposed flood control project be built.

Constructed Conveyance System Facilities: Gutters, ditches, pipes, channels, and most stormwater  control and water quality treatment facilities. 

Conveyance System: Drainage facilities that collect, contain, and transfer stormwater from one point to another. Conveyance systems can be made up of natural elements, constructed facilities, or a combination of both. 

Crest: The top line of a mountain or hill. A crest may also refer to the top of a wave.

Critical Areas: These are areas where flooding and/or erosion conditions present an imminent likelihood of harm to the welfare and safety of the surrounding community. Critical Areas may also refer to sensitive environmental areas, such as Areas of Critical Environmental Concern (ACEC).

Cross Street Flow: Cross street flow occurs whenever stormwater runoff moves across a traffic lane, such as at an intersection.  

Cubic Foot per Second (CFS): The rate of discharge representing a volume of one cubic foot of water passing a given point during one second.   One cubic foot is approximately the size of a box a basketball would fit in. So, a CFS is the volume of water approximately equal to one basketball box, passing a point every second. Equivalent to 7.48 gallons per second or 448.8 gallons per minute.

Cubic Yards (CY): The approximate size of a standard washing machine. The standard dump truck typically carries 10 to 14 cubic yards of dirt.

Culvert: A closed conduit such as a pipe or concrete box structure designed to drain water under a roadway, embankment or other structure. Culverts are typically constructed perpendicular to the roadway alignment. 

Curb and Gutter: Pathway along streets that directs stormwater to a storm drain and into a pipe to be transported to a channel, wash, or river. Curbs and gutters are often referred to as gray infrastructure. 

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Dam: Structure built across a river or other waterway to impede the flow of water. Water stored behind a dam has many potential uses such as flood control, water supply, irrigation, and recreation. 

Detention: Capture and release of surface and stormwater runoff from the site at a slower rate than conveyed by the drainage facility system, the difference being held in temporary storage. 

Detention Basin: A facility that collects water from upstream areas, temporarily stores the water and releases it at lower, less damaging rates than it enters the collection system. The excess inflow over outflow is temporarily stored in an excavated hole and/or behind a dam structure, and is typically released over a few hours or days. In Clark County, detention basins generally range in size from 15 to 300 acres.

Direct Discharge: Undetained discharge from a proposed project to a receiving water. 

Discharge: Runoff leaving a proposed development through overland flow, built conveyance systems, or infiltration facilities. 

Ditch: A graded earthen channel. 

Diversion: Any change in the natural discharge path of stormwater runoff.

Downspout: The part of roof gutters that directs water away from a building. These are best directed onto planted areas to reduce the water that runs off a property and to make good use of the rainwater. 

Drainage: The collection, conveyance, containment, and/or discharge of surface and stormwater runoff. 

Drainage Facility: A natural or artificial feature that collects, conveys, stores, or treats surface and stormwater runoff. Artificial drainage facilities include but are not limited to pipelines, channels, ditches, gutters, closed depressions, pumps, flow control or water quality treatment facilities, erosion and sedimentation control facilities, and other drainage structures and appurtenances that provide for drainage. Natural drainage facilities include streams, rivers, lakes, wetlands, and natural washes.

Drinking Water: Water that is clean enough to be safe for ingestion (also called potable water). Most drinking water has been treated to remove pollutants. Conserving drinking water and reducing pollutants in natural water supplies helps municipalities save money. 

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Ecosystem: Community of living things (plants, animals, microorganisms) and nonliving things (water, air, soil) that interact to create habitats. When water is polluted, the ecosystem is impaired. 

Emergency Action Plan (EAP):  A plan developed for detention basins, dam structures and other facilities capable of storing large amounts of stormwater that provides important information to emergency managers and first responders regarding potential overtopping or failure of the structure.

Environmental Impact Statement (EIS): A technical report that describes the potential positive and negative effects of a proposed action, with alternatives and possible mitigation measures. It is required by the national and state environmental policy acts when projects are determined to have the potential for significant environmental impact. 

Embankment: A structure of compacted earth, gravel, or similar material raised to form a dam structure, levee, pond bank, or foundation for a road. 

Ephemeral Wash/Stream: A natural wash that is dry at all times of the year except for brief periods following precipitation events. 

Erosion: The process that moves material, especially soil, from one location to another. It is caused by the action of wind, water, ice, or other forces working on the Earth's surface. Fast-moving runoff water increases erosion. 

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FEMA-Federal Emergency Management Agency: FEMA is the federal government’s lead agency in response to and recovery from major disasters. FEMA is also responsible for managing the National Flood Insurance Program. 

Filtration: Passing a fluid through a medium to separate solids, such as when water passes through soil and solids are trapped by soil particles or plant roots. When water runs directly into a storm drain, it is not filtered.  Forcing surface water to pass through a vegetated area before entering a storm drain will help with filtration and produce cleaner water in the drainage system. 

First Flush: The initial runoff at the beginning of a storm collects the most pollutants, debris, oils, etc.

Flash Flood Watch: Issued by the National Weather Service that indicates conditions are favorable for flash flooding in a specific area. May also be called Flood Watch.

Flash Flood Warning: Issued by the National Weather Service that indicates a flood is already occurring or will occur soon in a specific area. May also be called Flood Warning.

Flood Control Facility: A channel, storm drain, or detention basin built to control stormwater flows to reduce the likelihood of property damage and/or loss of lives.

Flash Flood: A flood that happens within minutes or hours of heavy rainfall, or due to a failure of a dam or levee. Flash floods often occur with little or no warning for the affected areas. 

Floodplain Management: Provides the guidance, conditions, and restrictions for development in flood-prone areas while protecting the public’s health, safety, welfare, and property from damage due to flooding.

Flood Insurance: Special insurance is available to renters and homeowners that covers losses directly caused by flooding. Damage due to flooding is not included in most normal homeowners insurance policies. Flood insurance takes at least 30 days for a policy to take effect. Purchase through the National Flood Insurance Program (NFIP) at floodsmart.gov.

Flood Insurance Rate Map (FIRM or flood map): The official map of a community that defines both the special flood hazard areas and the flood zones applicable to the community. This map is used by the National Flood Insurance Program (NFIP) for floodplain management, mitigation, and insurance purposes.

Flood Mitigation: The community's measures to reduce flood risks before a damaging rain event. Efforts include constructing flood control infrastructure, special zoning, rules for building within floodplains, and special-purpose floodplain ordinances. Successful flood mitigation practices have proven to be much more cost effective than typical repose-recovery-preparedness disaster cycles without mitigation activities.

Flood Mitigation Assistance: A grant program funded by the National Flood Insurance Program that supports plans and projects for mitigating losses to insured buildings.

Flood Threat Recognition System (FTRS): Rain, water level, and weather gauges to measure the weather in near real-time. The system in Clark County includes more than 200 rainfall-measuring devices designed to report rainfall quantities and identify areas where flooding is occurring or imminent. Some sites also collect water level, temperature, humidity, dew point temperature and wind speed data.

Flow Restrictor: A device with a specifically sized orifice or design configuration used to control the volume of water flow. Flow restrictors are often used to control the release rates from detention basins. 

Funding: The Regional Flood Control District is funded by a ¼ percent sales tax Clark County voters approved in 1986. RFCD funded the design of the first flood control project in 1987. 

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Gauge: A measuring device, usually mechanical. RFCD uses rainfall and water level gauges to monitor system performance during storm events. 

Geographic information system (GIS): Computer-based map systems that allow the user view and interact with map elements to correlate geographic information with other data, such as property tax records or design specifications for completed flood control projects.

Groundwater: Underground water in the voids between soil particles, sand, and rock. Groundwater is usually stored in aquifers and originates from infiltration or rain and surface water. 

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Habitat: The specific area or environment where a particular plant or animal lives and grows. 

Headwaters: Source of a stream. 

Hydrologic Cycle: The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transpiration. 

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Impervious Surface: Any surface water cannot soak into, such as streets, sidewalks, driveways, rooftops, and compacted soils. Urban areas have many impervious surfaces, so there is a potential for increased stormwater runoff. 

Infiltration: Process of moving water into the soil from the surface In Clark County.
  
Infiltration Facility: A drainage facility designed to use the hydrologic process of water soaking into the ground (commonly referred to as percolation or infiltration) to dispose of surface and stormwater runoff. Types of infiltration facilities may include detention or retention basins, ponds, and perforated tanks. 

Illicit Discharge: Any discharge to a municipal separate storm sewer system (MS4) not composed entirely of stormwater. 

Illicit Discharge Detection and Elimination (IDDE): A program designed to find, fix, and prevent illicit discharges through various techniques and awareness campaigns. 

Inlet: Any structure designed to bring water into a drainage facility. Common inlet structures include: catch basins, drop inlets, culvert headwalls, or concrete chutes into detention basins. Catch basins and drop inlets located in streets collect runoff in curb openings and/or open grates and conveys that water to underground storm drains. 

Intermittent Wash/Stream: A stream that carries water during certain times of the year, such as after winter during snowmelt.

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L

Lateral Storm Drain Pipes: A lateral drainage system is a series of drain pipes that connect to the main storm drain line. Lateral pipes are designed to deliver stormwater to the main storm drain pipe at various points along the system. 

Letter of Map Revision (LOMR): A letter issued by the Federal Emergency Management Agency that confirms revisions to special flood hazard area (flood plain) boundaries on the Flood Insurance Rate Maps resulting from modifications to ground elevations or construction of drainage facilities.

Levee/Dam: A manmade structure to contain or prevent water from moving past a certain point.

Local Drainage: Smaller drainage facilities in neighborhoods that collect flows and route them to the regional system.

Local Flood Control Facilities: All facilities built and maintained by a municipality, unincorporated Clark County, or a private entity that collect and convey stormwater from a smaller local area into a Regional Flood Control Facility.

Local Off-Site (Public) Flood Control Facilities: All facilities dedicated to the responsible public entity that collect and convey stormwater from the Local Off-Site Flood Control Facilities to a Regional Flood Control Facility.

Local On-Site (Private) Flood Control Facilities: All privately owned and maintained facilities that collect and convey stormwater from a single development or parcel to the Local Off-Site Flood Control Facilities. These facilities serve only the development or parcel in question.

Low Impact Development (LID): Techniques and design considerations used in land development that help manage the rainwater that falls on the site by allowing some to evaporate back into the air, some to absorb into the ground, some to be captured and used later as needed, and the rest to slowly pass into the stormwater system and nearby streams with minimal impact to the environment. 

M
Major Storm: The design storm has a one-percent chance of occurring in a particular location in any given year. A major storm will cause a flood that statistically has a 1% chance of being equaled or exceeded in any given year. This storm event is commonly called the “100-year storm” or the “1% chance storm.”  

Master Planning (flood control): Establishing the layout and preliminary design of new 100-year flood control facilities and reconfiguring undersized facilities to provide an increased level of flood protection for residents, businesses, and tourists.

Master Plan Update (MPU): The Regional Flood Control District’s flood control Master Plan Update.

Minor Storm: Design storm with a ten-percent chance of occurring in a particular location in any given year. A minor storm will cause a flood that statistically has a 10% chance of being equaled or exceeded in any given year. This storm event is commonly called the “10-year storm.” 

Municipal Separate Storm Sewer System (MS4): A storm system that conveys stormwater runoff through dedicated pipes rather than combined with flow in the sanitary sewer system. 

N
National Flood Insurance Program (NFIP): The National Flood Insurance Program (NFIP) makes insurance against flood damages available for local jurisdictions that meet the minimum eligibility requirements for participation in the program.  The NFIP is managed by the federal government. Flood insurance policies can be obtained through insurance companies working with the NFIP. Call your local insurance agent or call 1-800-638-6620 or visit floodsmart.gov for more information.

National Pollutant Discharge Elimination System (NPDES): A permit program created in 1972 by the Clean Water Act (CWA) that helps address water pollution by regulating point sources that discharge pollutants to the Waters of the United States. 

National Weather Service (NWS): The National Weather Service is an agency of the United States federal government tasked with providing weather forecasts, watches and warnings of hazardous weather, and other weather-related products to organizations and the public for protection, safety, and general information.

Natural Conveyance System: Drainage systems not artificially created or altered by mankind, such as: washes, swales and small drainage courses, streams, rivers, lakes, and wetlands. 

Natural Onsite Drainage Feature: A natural swale, channel, stream, closed depression, wetland, or lake contained within a specific site. 

Network: A flood control network is a collection of drainage facilities connected such that stormwater runoff passes from one system to the next, combining with other portions of the network along the way.  Detention basins, channels, natural washes and storm drains work together to direct floodwater from the Las Vegas Valley’s west side to the east – eventually making it to Lake Mead.

Nevada Division of Environmental Protection (NDEP): A state agency charged with preserving and enhancing the environment of Nevada in order to protect public health and sustain healthy ecosystems by enforcing regulations to protect water and air quality.

Nonpoint Source Pollution: Pollution that cannot be easily traced to one source or property because small amounts come from many sources and properties. It occurs when rainfall, snowmelt, or irrigation runs over land or through the ground, picks up pollutants, deposits them into streams, rivers, lakes, and coastal waters, or introduces them into groundwater. These numerous small amounts of pollutants can eventually accumulate to become harmful to the environment.  

Nonstructural BMP: A preventative action to protect receiving water quality that does not require physical construction. Nonstructural BMPs rely predominantly on behavioral changes to be effective. Major categories of non-structural BMPs include education, recycling, maintenance practices, and source controls. 

O
Oil/Water Separator: A vault, usually underground, designed to separate oil from water for the sake of water quality. Floatable debris are also removed from the water with this device. 

Outfall: A point where collected and concentrated surface and stormwater runoff is discharged from a pipe system, channel, or other engineered structure. 

Overflow: Occurs at drainage structures when there is unanticipated blockage due to debris or during a storm that is producing runoff greater than the design event, allowing excess water to escape the structure.  

P
Peaking Basin: Similar to a detention basin, but designed to allow runoff from smaller storms to bypass the peaking basin and runoff from larger storms to be detained within the peaking basin.

Peak Flow: The highest flowrate of runoff at a single point of a drainage system during a storm event.

Pervious (porous) Surface: A surface that allows water to move into and through it (e.g., sandy soil, porous pavers) and reduces runoff amounts downstream. 

Point Discharge: The release of collected and/or concentrated surface and stormwater runoff from a pipe, culvert, or channel at a single point. 

Point Source Pollution: Pollution that can be traced to a specific source or property, such as a factory or pipe outlet. 

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R
Receiving Waters: Bodies of water or surface water systems receiving water from upstream man-made or natural systems. 

RFCD: Regional Flood Control District.

Regional Flood Control Facilities: All facilities included in any of the Regional Flood Control Master Plans as adopted by the Clark County Regional Flood Control District Board of Directors.

Regional Flood Control Significance: Facilities, land alteration, portions of the natural drainage system, and regulatory actions that impact the implementation of the most current update of the CCRFCD Flood Control Master Plan or lie within Special Flood Hazard Areas.

Retention: Collecting and holding surface and stormwater runoff with no outflow. 

Retention Basin: Drainage structure designed to collect surface and stormwater runoff during rain events and store the water with no release other than through infiltration or evaporation.

Riparian: Pertaining to the banks of rivers and streams, and sometimes also wetlands and lakes. 

Riprap: Riprap is a permanent layer of large, angular stone, cobbles, or boulders typically used to stabilize and protect the soil surface against erosion in a storm channel or other drainage facility.

Runoff: Water originating from rainfall and other precipitation that is not absorbed into the ground and ultimately flows overland into drainage facilities, rivers, streams, ponds, lakes, and wetlands, as well as shallow groundwater. 

S
Sanitary sewer: A sanitary sewer is an underground pipe or tunnel system for transporting wastewater (sewage, but not stormwater) from houses and commercial buildings to a sewage treatment plant. Sanitary sewers are normally a type of gravity sewer and are part of an overall system called a "sewage system" or sewerage.

Sediment: Primarily eroded soil, minerals, and rocks transported from one location to another. Sediment is often transported by stormwater runoff through the drainage system and is considered a pollutant. 

Septic System: An onsite wastewater collection system that stores and treats wastewater without connecting to the sanitary sewer system. 

Sheet flow: Sheet flow is characterized by water flowing over a plane surface as shallow unconcentrated flow over a wide area, resembling a thick film or sheet of water. 

Special Flood Hazard Area: An area that would be inundated by the one percent chance flood as defined on the Flood Insurance Rate Maps produced by the Federal Emergency Management Agency. 

Storm Drain Inlet: An open or grated opening built into a street curb or low point of a local depression that connects to a storm system directly or through a lateral pipe to carry stormwater. 

Storm Drain System: The system of gutters, pipes, ditches, and channels, used to carry surface and stormwater from surrounding lands to washes, streams, rivers, and lakes. Also called Storm Sewer System.

Stormwater or Stormwater Runoff: Water from precipitation (primarily rainfall in Clark County) that is not infiltrated into the soil or captured by local depressions and flows overland as concentrated or sheet flow. Stormwater includes water captured by drainage facilities, washes, rivers, streams, and lakes. Stormwater may originate from precipitation landing on any surface, including soft surfaces like lawns play fields, or natural desert surfaces, or hard surfaces like parking lots, sidewalks and roads. Stormwater in Clark County is not purified or cleaned in a treatment plant. Most stormwater in Clark County flows directly to Lake Mead.

Stormwater Facility: A facility that controls, conveys, or stores the discharge of stormwater. 

Stormwater Quality Management: The application of site design principles and construction techniques to reduce or eliminate sediments and other pollutants from stormwater runoff.  

Stormwater Management Program (SWMP or SMP): A comprehensive report describing community actions and activities to reduce stormwater pollution from urbanized areas. 

Stormwater Pollution: Anything in stormwater that makes it unclean. This can include contaminants like soil, pesticides, litter, oil, grass clippings, tree leaves, and bacteria collected by stormwater flowing over a surface and carried into receiving waters. 

Structural BMP: Constructed facilities or control measures to help protect receiving waters from pollution and improve stormwater quantity. Examples include storage, vegetation, infiltration, and filtration. 
Surface Water: Water found above the land, including oceans, estuaries, lakes, rivers, streams, and ponds. 

T
Total Maximum Daily Load (TMDL): A calculation of the maximum amount of a specific pollutant a waterbody can receive and still meet water quality standards. Water quality standards identify the uses for each water body. For example, there are different water quality standards for drinking, contact recreation (swimming), and aquatic life support (fishing). The Clean Water Act (CWA), section 303, establishes the water quality standards and TMDL programs. 

Toxic: Poisonous, carcinogenic, or otherwise directly harmful to life. 

U 
V 
Velocity: The speed of moving water, typically measured in feet per second. Fast-moving water is more dangerous for motorists and pedestrians and can also cause more erosion of streambanks and collect more pollutants.

W 
Wash: A natural stream channel common in the western United States. Washes are typically dry except during rainstorms.

Wastewater Treatment (Sewage) Plant: Municipal plants treat wastewater (water from toilets, washing machines, dishwashers, showers, etc.) before it is released into a stream or river. 

Water Cycle: The continuous movement of water through its liquid, gas, and solid phases above, on, and below the Earth. The water on Earth today is the same as when Earth was formed. New water is not added to the Earth when it rains. 

Watershed: The entire land area from which water drains into a particular surface water body such as a lake, stream, or river. 
Waters of the United States – areas determined to be jurisdictional and subject to protections under the Clean Water Act.

Weir: A low dam built across a river to raise the water level upstream or regulate its flow.

Wetland: An area inundated or saturated by ground or surface water at a frequency and duration sufficient to support a prevalence of vegetation typically adapted for life in saturated soil conditions.
Wetlands generally include swamps, marshes, bogs, and similar areas. 

Write Your Own: An insurance company that has agreed to sell flood insurance policies on behalf of the National Flood Insurance Program.
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