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DITCHING AND DEWATERING

                                                                               DITCHING AND DEWATERING  5.53

                                    Flatter slopes increase the danger of silting.

                                  Tile Size.  Figure 5.43 shows the number of acres that will be drained by tile of various sizes, at
                                  slopes of up to 1 percent. Some of the sizes listed may not be generally available, and these fig-
                                  ures are for average conditions.
                                    It is good practice to use tile of larger than minimum diameter for a line so that effectiveness
                                  will not be lost as readily by silting or misalignment.
                                  Surface Inlets. A tile drainage system may include one or more places where surface water can
                                  drain into it. The flow of water should be calculated and the tile increased in size to accommodate it.
                                    Such inlets require careful design. They must be arranged so that dirt cannot be washed in, so that ani-
                                  mals cannot enter, and so that hydraulic pressure cannot be exerted on the tile underneath the opening.
                                    For example, if a 6-inch inlet to a 6-inch line is used, water ponding over the inlet due to exces-
                                  sive rainfall may put hydraulic pressure on the underground pipe, causing too fast a flow and
                                  probable erosion.
                                    If the inlet is choked down to 3 or 4 inches, or provisions are made for surface overflow, this
                                  difficulty should not occur.

                                  Outlets.  The tile line or system may terminate in a drainage ditch; a large drain system; a river,
                                  lake, or sea; or a sump. If a sump, an automatic electric pump should be used to keep the water
                                  level below the bottom of the tile to ensure free drainage.
                                    A projecting metal pipe may be used, as in Fig. 5.44, or a masonry spillway, as in Fig. 5.45, to
                                  avoid bank erosion and undermining. Since outfall lines often lie under surface channels, it may
                                  be necessary to make provision for surface flow also, as shown.
                                    The outlet should be protected against entrance of small animals by coarse-mesh wire, a grat-
                                  ing, or an automatic gate.



































                                          FIGURE 5.43  Area-tile table.
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