drainage system; however, there are other features,such as drainage ditches. Both storm sewers andditches carry surface runoff. The only real differencebetween a drainage ditch and a storm sewer is the factthat the ditch lies on the surface and the storm sewerlies below the surface.Similarly, there is no essential difference inmechanical principle between an artificial and anatural drainage system. Like a natural channel, anartificial channel must slope downward and mustbecome progressively larger as it proceeds along itscourse, picking up more runoff as it goes. Like anatural system, an artificial system must reach adisposal point—usually a stream whose ultimatedestination is the sea or a standing inland body ofwater. At the terminal point of the system where theaccumulated runoff discharges into the disposal point,the runoff itself is technically known as discharge.The discharge point in the system is called the outfall.Ditches.— A surface drainage system consistsprincipally of ditches that form the drainage channels.A ditch may consist simply of a depression formed inthe natural soil, or it may be a paved ditch. Where aditch must pass under a structure (such as a highwayembankment, for example), an opening called aculvert is constructed. A pipe culvert has a circularopening; a box culvert has a rectangular opening.Walls constructed at the ends of a culvert are calledend walls. An end wall, running perpendicular to theline through the culvert, may have extensions calledwings (or wing walls), running at an oblique angle tothe line through the culvert.Storm Sewers. —An underground drainagesystem (that is, a storm sewer) consists, broadlyspeaking, of a buried pipeline called the trunk ormain, and a series of storm water inlets, which admitsurface runoff into the pipeline. An inlet consists of asurface opening that admits the surface water runoffand an inner chamber called a box (sometimes calleda catch basin). A box is usually rectangular but maybe cylindrical. An inlet with a surface opening in theside of a curb is called a curb inlet. A working drawingof a curb inlet is shown in figure 10-1. An inlet with ahorizontal surface opening covered by a grating iscalled a grate (sometimes a drop) inlet. A generalterm applied in some areas to an inlet that is neither acurb nor a grate inlet is yard inlet.Appurtenances. —Technically speaking, theterm storm sewer applies to the pipeline; the inlets arecalled appurtenances. There are other appurtenances,the most common of which are manholes andjunction boxes. A manhole is a box that is installed,of necessity, at a point where the trunk changes direc-tion, gradient, or both. The term manhole originallyrelated to the access opening at one of these points;however, a curb inlet and a junction box nearly alwayshave a similar access opening for cleaning, inspection,and maintenance purposes. One of these openings isoften called a manhole, regardless of where it islocated. However, strictly speaking, the accessopening on a curb inlet should be called a curb-inletopening; and on a junction box, a junction-boxopening. Distances between manholes are normally300 feet, but this distance may be extended to amaximum of 500 feet when specified.The access opening for a manhole, curb inlet, orjunction box consists of the cover and a supportingmetal frame. A frame for a circular cover is shown infigure 10-2. Some covers are rectangular. The frameusually rests on one or more courses of adjustingblocks so that the rim elevation of the cover can bevaried slightly to fit the surface grade elevation byvarying the vertical dimensions, or the number ofcourses, of the adjusting blocks.A junction box is similar to a manhole but isinstalled, of necessity, at a point where two or moretrunk lines converge. The walls of an inlet, manhole,or junction box maybe constructed of special concretemasonry units or of cast-in-place concrete. Thebottom consists of a formed slab, sloped in theFigure 10-2.—Frame for an access opening.10-5
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