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Artificial Drainage - 14070_202
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Engineering Aid 1 - Advanced Structural engineering guide book
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drainage system; however, there are other features, such  as  drainage  ditches.  Both  storm  sewers  and ditches carry surface runoff. The only real difference between a drainage ditch and a storm sewer is the fact that the ditch lies on the surface and the storm sewer lies  below  the  surface. Similarly,  there  is  no  essential  difference  in mechanical  principle  between  an  artificial  and  a natural drainage system. Like a natural channel, an artificial  channel  must  slope  downward  and  must become progressively larger as it proceeds along its course,  picking  up  more  runoff  as  it  goes.  Like  a natural  system,  an  artificial  system  must  reach  a disposal  point—usually  a  stream  whose  ultimate destination  is  the  sea  or  a  standing  inland  body  of water.  At  the  terminal  point  of  the  system  where  the accumulated 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  consists principally of ditches that form the drainage channels. A ditch may consist simply of a depression formed in the natural soil, or it may be a paved ditch. Where a ditch must pass under a structure (such as a highway embankment,   for   example),   an   opening   called   a culvert is constructed. A pipe culvert has a circular opening; a  box culvert  has  a  rectangular  opening. Walls constructed at the ends of a culvert are called end walls. An end wall, running perpendicular to the line through the culvert, may have extensions called wings (or wing walls), running at an oblique angle to the line through the culvert. Storm  Sewers. —An   underground   drainage system  (that  is,  a  storm  sewer)  consists,  broadly speaking,  of  a  buried  pipeline  called  the  trunk  or main, and a series of storm water inlets, which  admit surface  runoff  into  the  pipeline.  An  inlet  consists  of  a surface  opening  that  admits  the  surface  water  runoff and an inner chamber called a  box (sometimes   called a catch basin). A box is usually rectangular but may be cylindrical. An inlet with a surface opening in the side of a curb is called a  curb inlet.  A  working  drawing of a curb inlet is shown in figure 10-1. An inlet with a horizontal  surface  opening  covered  by  a  grating  is called a grate (sometimes a  drop) inlet. A general term applied in some areas to an inlet that is neither a curb nor a grate inlet is yard inlet. Appurtenances. —Technically   speaking,   the term storm sewer applies to the pipeline; the inlets are called appurtenances. There are other appurtenances, the  most  common  of  which  are   manholes  and junction 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 originally related to the access opening at one of these points; however, a curb inlet and a junction box nearly always have a similar access opening for cleaning, inspection, and maintenance purposes. One of these openings is often  called  a  manhole,  regardless  of  where  it  is located.  However,  strictly  speaking,  the  access opening on a curb inlet should be called a curb-inlet opening;  and  on  a  junction  box,  a  junction-box opening. Distances between manholes are normally 300  feet,  but  this  distance  may  be  extended  to  a maximum of 500 feet when specified. The access opening for a manhole, curb inlet, or junction box consists of the cover and a supporting metal frame. A frame for a circular cover is shown in figure 10-2. Some covers are rectangular. The frame usually rests on one or more courses of  adjusting blocks so that the rim elevation of the cover can be varied slightly to fit the surface grade elevation by varying  the  vertical  dimensions,  or  the  number  of courses, of the adjusting blocks. A  junction  box  is  similar  to  a  manhole  but  is installed, of necessity, at a point where two or more trunk lines converge. The walls of an inlet, manhole, or  junction  box  maybe  constructed  of  special  concrete masonry   units   or   of   cast-in-place   concrete.   The bottom  consists  of  a  formed  slab,  sloped  in  the Figure 10-2.—Frame for an access opening. 10-5







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