Drainage Glossary S-Z


In soils, the point at which a soil or aquifer will no longer absorb any amount of water without losing an equal amount.


Solid material (both mineral and organic) that is in suspension, is being transported, or has been moved from its site of origin by air, water, gravity, or ice and has come to rest on the earth’s surface.


The process that deposits soils, debris and other materials either on the ground surfaces or in bodies of water or watercourses.


Soil fraction consisting of particles between 02 an 0.05 mm in diameter. A soil textural class indicating more than 80% silt.

Silt Fence

A fence constructed of wood or steel supports and either natural (e.g. burlap) or synthetic fabric stretched across area of non-concentrated flow during site development to trap and retain on-site sediment due to rainfall runoff.

Storm Water Runoff

The water derived from rains falling within a tributary basin, flowing over the surface of the ground or collected in channels or conduits.

Storm Sewer

A sewer that carries storm water, surface drainage, street wash, and other wash waters but excludes sewage and industrial wastes. Also called storm drain.

Subsurface Drain (SSD)

A pervious backfield trench, usually containing stone and perforated pipe, for intercepting groundwater or seepage.

Surface Runoff

Precipitation that flows onto the surfaces of roofs, streets, the ground, etc., and is not absorbed or retained by that surface but collects and runs off.


An elongated depression in the land surface that is at least seasonally wet, is usually heavily vegetated, and is normally without flowing water. Swales conduct storm water into primary drainage channels and may provide some groundwater recharge.


The water surface elevation at the downstream side of a hydraulic structure (i.e. culvert, bridge).

Tile Drain

Pipe made of perforated plastic, burned clay, concrete, or similar material, laid to a designed grade and depth, to collect and carry excess water from the soil.

Tile Drainage

Land drainage by means of a series of tile lines laid at a specified depth, grade, and spacing.

Toe of Slope

The base or bottom of a slope at the point where the ground surface abruptly changes to a significantly flatter grade.

Top of Casting

The elevation of the top of pipe.

Topographic Map

Graphical portrayal of the topographic featured of a land area, showing both the horizontal distances between the features and their elevations above a given datum.


The representation of a portion of the earth’s surface showing natural and man-made features of a give locality such as rivers, streams, ditches, lakes, roads, buildings and most importantly, variations in ground elevations for the terrain of the area.

Trash Rack

A structural device used to prevent debris from entering a pipe spillway or other hydraulic structure.


A small diameter perforated pipe that allows the bottom of a detention basin, channel or swale to drain.

Water Table

The free surface of the groundwater. That surface subject to atmospheric pressure under the ground, generally rising and falling with the season or from other conditions such as water withdrawal.


The region drained by or contributing water to a specific point that could be along a stream, lake or other storm water facilities. Watersheds are often broken down into sub-areas for the purpose of hydrologic modeling.

Watershed Area

All land and water within the confines of a drainage divide.


Areas that are inundated or saturated by surface water or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.