STORM DRAINAGE SYSTEMS

Storm drainage systems are designed to drain all surface and sometimes subsurface water that may cause damage to Navy facilities, property, or adjoining land. They consist of pipe, inlets, catch basins, and other drainage structures to carry the surface runoff and subsurface water to a point of disposal.

Storm drainage systems should be separate from sanitary sewage systems wherever possible. Some Navy bases may have combination systems still in use. However, storm water should never be drained into sewers intended for sanitary sewage only.

EOs and BUs generally are responsible for building ditches, culverts, and other structures that are a part of storm sewers. Therefore, construction of these facilities is not covered in this chapter.

The UT is generally concerned with only the pipework itself. This involves laying storm drain lines both inside and outside buildings and other structures. This pipe material may be the same as that used for the sanitary system. Storm sewer systems, however, may include pipe of much larger sizes than are needed for sanitary sewers. Plain or reinforced concrete pipe (rather than clay, cast iron, or asbestos cement) is generally used for the larger lines. Also, it is not so important that the joints be watertight in storm sewer systems. In fact, the mortar is sometimes omitted from a portion of the joint and washed gravel is placed next to the opening; the storm drain thus serves also as an underdrain to pick up subsurface water.

Installation Considerations

Storm and sanitary systems may differ in the installation of the piping. Building storm drains should generally be graded at least 1/4 inch per foot whenever feasible. This amount of drop per foot provides an unobstructed and self-scouring flow. However, a greater drop per foot may be given as no fixture traps which might lose their seals are associated with it.

When a change of direction is necessary, long radius fittings are used and a cleanout need not be installed. This is especially true in and under buildings. But a manhole is used outside of buildings when a change of direction is necessary, or when two or more lines are connected together.

Sizing Building Storm Drains

To determine the size of building storm drains, a number of factors must be considered, such as rainfall intensity, roof size, and pitch of roof. Tables have been made for use in estimating the size of pipe to select. One example is table 7-7; it shows storm drain sizes. Remember that this table is to be used only as a guide when estimating for storm drainage, as different areas have different intensities of rainstorms.

Another method for sizing building storm drains is to provide 1 square inch of pipe crosssectional area for each 100 square feet of roof area. This method is easy to remember: 1 square inch for 100 square feet. (However, it is not as accurate as using table 7-7.) Using this method, you can prepare a table similar to table 7-8. Show the diameter in the first column; then the radius (which is one-half the diameter); then the square of the radius; then the cross-sectional area, which is pi (3.14) times the radius squared. Since each square inch may take 100 square feet of roof, move the decimal of the square inches over two places to the left (which is multiplying by 100) to get the area of the roof that may be drained to the pipe. As you can see by comparing table 7-7 with table 7-8, the second method is much more conservative.

Sizing Site Storm Sewers

While rules of thumb such as those just described are used to size building storm drains, different procedures are used to size the storm sewers that carry the runoff from the building site and surrounding land areas. The design and sizing of storm drains are provided by engineers. It is not necessary that the UT understand the factors that contribute to the design. Therefore, the information is not included here.

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