Glossary of Drainage Terms
by C. Drew Brown

C. Drew Brown, Manager of Public Education for PWD, was my first point of contact with the Department (see my 1997 "Down Under!" article at the link to the left). Years later, he is still my direct supervisor as well as my collaborator on various PWD public education projects.

In April 2008, Drew created this glossary for our annual Wingohocking Mystery Tour. To be honest, I didn't read this when Drew first created it--not because I think I "know-it-all," but because I was busy creating the voluminous picture and map packets we provide for tour participants. Now that I have finally read Drew's "drainage dictionary," I think it's too valuable to simply let lie on some computer hard drive, so got his permission to post it here.

I present this glossary as Drew created it. The terms are not in alphabetical
order, but rather follow a logical hierarchy, beginning with the proper definition of a watershed and working to smaller elements of both natural and man-made drainage systems.

The History of Philadelphia's Watersheds and Sewers

Compiled by Adam Levine
Historical Consultant
Philadelphia Water Department
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Watershed: The watershed of a stream (or of any body of water) is all of the land that sheds water to that stream when it rains (or when snow melts).

Watershed (continued): A watershed may also be called a basin (yes, think of your sink or bathtub) or a catchment area. The watershed of a large stream, such as the Delaware River, is commensurately large (the Delaware River Watershed is more than 13,000 square miles in area, covering parts of four states); conversely, small streams have relatively small watersheds. The watershed of a stream contains within it all of the watersheds of all of the tributaries to that stream (also called the subwatersheds of the larger watershed). For example, the Delaware River Watershed includes the entire watershed of its largest tributary, the Schuylkill River, as well as the watersheds of its many small tributaries, such as the Frankford Creek, and also the watersheds of every size in between. Another way of thinking about this is that the water in the Delaware River comes from all of its tributaries, including the Schuylkill River and Frankford Creek, so the watersheds of the tributaries must be part of the Delaware River Watershed.

Drainage Divide: the high land (usually a ridge) between two watersheds which serves as the boundary between them. Stormwater runoff on one side of the ridge will flow by gravity to the stream on that side of the ridge, while stormwater runoff on the other side of the ridge will flow by gravity to the stream on that side. [Most of the dictionaries I've consulted over the years have mistakenly used this definition of a drainage divide as the definition of a watershed, usually in terms of a "watershed event" (i.e. a life-altering event, or a turning point) in a person's life.]

Stream: a body of flowing water. A natural stream is usually called, from smallest to largest in size, a run, brook, creek, or river. A millstream is a man-made stream, as are the gutter at the curb in a street, the gutter at the edge of a roof, the downspout on a house, and the sewer under a street.

Tributary: a stream which flows into another, larger stream. The term may be applied to natural as well as to man-made streams.

Stormwater: rain that has reached the ground, and melted snow.

Runoff, or stormwater runoff: the portion of stormwater that flows downhill by gravity over the earth's surface because it can't, for any number of reasons, soak into the ground by infiltration. Compacted soil (such as that under a lawn), soil with a dried crust, soil saturated with water, frozen soil, soil buried under impervious cover (such as the concrete or macadam paving of a street, sidewalk, or parking lot), and the roof of a building will all result in runoff essentially equal in volume to rainfall. The uncompacted soil of a forest may produce essentially no runoff. Evaporation may reduce the volume of runoff; the rate of evaporation varies with weather conditions.

Sewer: a man-made, underground conduit which conveys, by gravity, sewage, stormwater runoff, or a combination of the two (sewage and stormwater runoff) away from a populated area to a nearby stream, often via a sewage treatment plant.

Sanitary Sewer: a separate sewer designed to carry, to a nearby stream, only sewage from the interior drains of houses and other buildings - drains which capture the wastes of, for example, sinks, bathtubs, dishwashers, washing machines, and toilets.

Storm Sewer: a separate sewer designed to carry, to a nearby stream, only stormwater runoff collected by: 1) outdoor storm drains in streets and parking lots, and 2) roof gutters.

Combined Sewer: a sewer designed to carry sewage on dry days, and a mixture of sewage and stormwater runoff on rainy days. [Historical note: It appears that from his arrival in 1682, William Penn instructed the new citizens of Philadelphia to appoint surveyors, or "street superintendents," who would design the City's streets such that they shed stormwater efficiently, and to build storm sewers of wood or brick under the unpaved streets which would carry the runoff to the Delaware River. Penn hoped that the sewers would promote commerce by facilitating the movement of people and goods over the streets during wet weather. One can assume that the settlers of Germantown and Frankford were encouraged to provide a similar amenity. The first, de facto, combined sewer was born sometime after 1801, when an unknown but enterprising plumber first hooked a house drain into one of the extant storm sewers -- operation of the City's water distribution system made running water available in buildings, and with it was created the City's first sewage, for which the City's water-system engineers appear to have made no provision.] Today, roughly 60% of Philadelphia, including the Wingohocking Creek Watershed, is served by combined sewers. Only the areas of the City developed since about 1880 have separate storm and sanitary sewers, including Northwest Philadelphia beyond Germantown, Northeast Philadelphia beyond Frankford, Southwest Philadelphia around Philadelphia International Airport, and the Overbrook Park area on the northwestern fringe of West Philadelphia.

Intercepting Sewer (or "interceptor"): a sewer constructed parallel to a stream, often right in the bank of the stream. An interceptor is designed to intercept the flow from all of the tributary sanitary and combined sewers approaching the stream. The intercepting sewer protects the stream from pollution by the sewage in the tributary sewers, which would discharge to the stream were their flows not intercepted by the intercepting sewer.

Combined Sewer Overflow (CSO): an unusual discharge to a natural stream, from a combined sewer, of sewage diluted with stormwater runoff. A CSO can occur during a rain event once an intercepting sewer is filled to capacity, even as a combined sewer which is a tributary to that interceptor still has flow in it. The flow in the combined sewer, which contains a variety of pollutants, including pathogens, is allowed to overflow to the stream to avoid backing-up diluted sewage into the streets and into the basements of homes in the watershed.

Point-source pollution: pollution in a stream originating at a point, such as a blocked sanitary sewer or a CSO.

Non-point-source pollution: also called stormwater runoff pollution, non-point-source pollution in a stream originates from a source or sources that can not be traced to a point. The polluted water discharged from a storm sewer into a stream, for example, is the result of a wide variety of human activities within the watershed of that storm sewer - activities as innocuous as driving a car (oil slowly dripping from the
engine onto the street where the car is parked; particulate matter in the exhaust that has settled to the street; tiny bits of rubber worn from the tires), applying fertilizer to a lawn, littering, leaving a pile of bare earth at a construction site, and failing to pick-up the dog's feces on a lawn or sidewalk, to name just a few. Stormwater runoff washes pollutants off a watershed and carries them to the stream, where they pollute the stream as non-point-source pollution.

Pathogen: a microscopic organism which can cause disease. For example, two common water-borne diseases of late-19th- and early-20th-century America, typhoid fever and cholera, are caused by pathogenic bacteria which are discharged from the intestines of infected individuals. The bacteria live very well in untreated water supplies. Today, the diseases are controlled through a combination of careful collection and treatment of sewage and vigilant treatment of drinking water.

Topography: the shape of the land. William Penn's colonists began altering the topography of the original City of Philadelphia, which was naturally fairly flat (thanks to its location on the Coastal Plain), soon after they began arriving in the late 17th and early 18th centuries. With the objective of facilitating the movement of goods and people in the City, the Surveyors and Street Superintendents sought to provide smooth slopes for the grid of streets, and to restrict the flow of stormwater runoff to gutters in, and sewers beneath, the streets. The smooth slopes were obtained by cutting down hills and filling depressions. Small stream channels were eliminated in the course of this leveling, either by encapsulating the streams in sewers and filling over them, or by diverting the streams to other channels. Landowners were responsible for filling or cutting their properties to meet the grades of the streets established by the City's surveyors. There is evidence that the same strategies were employed throughout the County of Philadelphia by its other municipalities and by the many railroads traversing it.

Topography (continued): By the late 19th century, larger stream channels were being filled with solid waste - principally the ash from coal burned in the furnaces of homes and in the boilers of factories' steam engines. The City's leaders must have considered this strategy a
win-win proposition - a nearby and thus cheap solution to the largest waste disposal problem of the day, and simultaneous creation of a smooth, dry landscape for building homes for the thousands of workers needed for the factories of the new industrial economy. The many railroads operating in the City, faced with disposing of the ash from their steam locomotives, and required by legislation to eliminate "at grade" crossings between the rails and the City's streets, embarked on programs to build long embankments of ash and soil for their newly elevated lines, and to raise the elevations of the lowlands among their many property holdings, by landfilling.

Spring: a point on the earth's surface at which the underground water stored in the Earth's underground reservoir, called groundwater, contacts the Earth's surface and seeps from the ground, resulting in a small stream; thus, one or more springs are considered the "headwaters," or starting points, of a stream. The best known springs in Philadelphia today are probably those that form icicles on the rocky cliffs above Lincoln Drive near its intersection with Kelly Drive.

Groundwater: water stored in the Earth's vast, natural, underground "reservoirs" in soil and rock; the underground surface of the groundwater is called the groundwater table.

Floodplain: the natural, generally narrow plain adjacent to one or the other or both sides of a stream which serves as the supplemental channel for flood flows caused by the runoff from an exceptionally large rainfall event in the watershed. A stream may overflow its banks onto the floodplain more frequently, and for smaller rainfall events, as impervious cover in the watershed of the stream replaces woodlands.

Impervious Cover: a man-made surface covering, such as concrete or macadam, which is impenetrable by water.

Erosion: the wearing away of soil or rock on the Earth's surface, usually caused by water flowing by gravity over the soil or rock. Damaging erosion within streams is a consequence of human development in a watershed, because the conversion of woodlands to shopping malls, parking lots, roads, and roofs, etc, increases the impervious cover within a watershed - after development, more of the rainfall becomes
runoff than before the development, resulting in increased stream flows and erosion of the stream channel in that watershed.

Storm Drain (or "Sewer Inlet," among Water Department employees): an outdoor drain in a street or parking lot which conveys stormwater runoff to a storm sewer or to a combined sewer beneath the street. A storm drain collects stormwater runoff through a heavy iron grate, through a "mouth" cut in the curb, or a combination; in any case, the stormwater falls beneath the street into a rectangular, brick catch basin (to catch grit and trash) with a trap (to prevent noxious gases from escaping from the sewer), drained by a pipe leading
to the sewer.

Confluence: the point at which two streams flow together to form one stream; for example, the confluence of the Schuylkill and Delaware Rivers is at the Philadelphia Navy Yard.


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