Drainage Glossary A-D

100-Year Flood

A flood stage that, statistically has a 1% probability of occurring in any given year.


The sloping sides of valley that supports the ends of a dam.


  • The sand and gravel portion of concrete (65 to 75% by volume), the rest being cement and water
  • That which is installed for the purpose of changing drainage characteristics


A general term for all detrital material deposited or in transit by streams, including gravel, sand, silt, clay, and all variations and mixtures of these.


A pad of non-erosive material designed to prevent scour holes developing at the outlet ends of culverts, outlet pipes, grade stabilization structures, and other water control devices.


An underground porous, water-bearing geological formation.


The rise in water surface elevation caused by some obstruction such as a narrow bridge opening, buildings or fill material that limits the area through which the water shall flow.

Base Flood Elevation (BFE)

The water surface elevation corresponding to a flood having a one percent probability of being equaled or exceeded in a given year.

Base Flow

Stream discharge derived from groundwater sources as differentiated from surface runoff.


A marked point of known elevation from which other elevations may be established.


A narrow shelf or flat area that breaks the contiguity of a slope.

Best Management Practices (BMP)

The design, construction, and maintenance practices and criteria for storm water facilities that minimize the impact of storm water runoff rates and volumes, prevent erosion, and capture pollutants.

Capacity of a Storm Drainage Facility

The maximum flow that can be conveyed or stored by a storm drainage facility without causing damage to public or private property.

Catch Basin

A chamber usually built at the curb line of a street for the admission of surface water to a storm sewer of sub-drain, having at its base a sediment sump designed to retain grit and detritus below the point of overflow.

Chicken Wire

A woven wire fabric with an opening size of about 1½ inches.

Chute or Rock Chute

A high velocity, open channel (lined with rip-rap) for conveying water down a steep slope without erosion.

Combined Sewer

A sewerage system that carries both sanitary sewage and storm water runoff.


An imaginary line on the surface of the earth connecting points of the same elevation.

Contour Line

Line on a map which represents a contour or points of equal elevation.

County Surveyor

A constitutional officer of the county, elected to a 4-year term from the county at large. Primary duties of the surveyor include maintaining annexation descriptions, legal survey book, and section corner record book. The surveyor is also an ex-officio member of the County Drainage Board and the technical authority on the construction, reconstruction, and maintenance of all regulated drains or proposed regulated drains in the county. Other major responsibilities of the surveyor include administering filter strip programs, membership in the County Plan Commission, and certification to the Indiana Alcoholic Beverage Commission.


A graph or plot of ground elevation across a stream valley or a portion of it, usually along a line perpendicular to the stream or direction of flow.


A closed conduit used for the conveyance of surface drainage water under a roadway, railroad, canal, or other impediment.


The process of earth grading by excavating part of a higher area and using the excavated material for fill to raise the surface of an adjacent lower area.

Design Life

The period of time for which a facility is expected to perform its intended function.

Design Standards

Detailed engineering drawings and/or specifications promulgated by public or private organizations that leave little choice to design engineers and technicians (e.g. manhole, catch-basin, and inlet standards).

Design Storm

A selected storm event, described in terms of the probability of occurring once within a given number of years, for which drainage or flood control improvements are designed and built.


Managing storm water runoff by temporary holding and controlled release.


An embankment to confine or control water. Often built along the banks of a river to prevent overflow of lowlands: a levee.


Usually the rate of water flow. A volume of fluid passing a point per unit time commonly expressed as cubic feet per second, cubic meters per second, gallons per minute, or millions of gallons per day.


A man-made, open drainage-way in or into which excess surface water or groundwater drained from land, storm water runoff, or floodwaters flow either continuously or intermittently.


A buried slotted or perforated pipe or other conduit (subsurface drain) or a ditch (open drain) for carrying off surplus groundwater or surface water.


The removal of excess surface water or groundwater from land by means of ditches, or subsurface drains.

Drainage Area

The area draining into a stream at a given point. It may be of different sizes for surface runoff, subsurface flow and base flow, but generally the surface runoff area is considered as the drainage area.

Drainage Board

A board consisting of three to five persons including the county executive (commissioners) or members appointed by the executive body (at least one of the Board members must be a county executive). The County Surveyor serves on the Board as an ex-officio, non-voting member. The Board is responsible for adopting drain classifications and a long-range plan, and for making decisions regarding the design, construction, reconstruction, and/or maintenance of regulated drains in the county.

Drainage Improvement

An activity within or adjacent to a natural stream or a man-made drain primarily intended to improve the flow capacity, drainage, erosion and sedimentation control, or stability of the drainage-way.

Drainage Shed

See Watershed.


A natural or artificial stream, closed conduit, or depression that carries surface water. This term is usually applied to all types of drains and watercourses, whether man-made or natural.


A method for deepening streams, lakes, or reservoirs by scraping and removing solids from the bottom.