Route Surveys for Drainage
When man-made structures are erected in a certain area, it is necessary to plan, design, and construct an adequate drainage system. Generally, an underground drainage system is the most desirable way to remove surface water effectively from operating areas. An open drainage system, like a ditch, is economical; however, when not properly maintained, it is unsightly and unsafe. Sometimes, an open drainage system also causes erosion, thus resulting in failures to nearby structures. Flooding caused by an inadequate drainage system is the most prevalent cause leading to the rapid deterioration of roads and airfields. The construction and installation of drainage structures will bediscussed later in this chapter. At this point we are mainly interested in drainage systems and types of drainage.
DRAINAGE SYSTEM.—Sanitarysewers carry waste from buildings to points of disposal; storm sewers carry surface runoff water to natural water courses or basins. In either case the utility line must have a gradient; that is, a downward slope toward the disposal point, just steep enough to ensure a gravity flow of waste and water through the pipes. This gradient is supplied by the designing engineer.
Natural Drainage.—To understand the con-trolling considerations affecting the location and other design features of a storm sewer, you must know something about the mechanics of water drainage from the earth’s surface.
When rainwater falls on the earth’s surface, someof the water is absorbed into the ground. The amount absorbed will vary, of course, according to the physical characteristics of the surface. In sandy soil, for instance, a large amount will be absorbed; on a concrete surface, absorption will be negligible. Of the water not absorbed into the ground, some evaporates, and some, absorbed through the roots and exuded onto the leaves of plants, dissipates through a process called transpiration.
The water that remains after absorption, evaporation, and transpiration is technically known asrunoff. This term relates to the fact that this water, under the influence of gravity, makes its way (that is, runs off) through natural channels to the lowest point it can attain. To put this in terms of a general scientific principle, water, whenever it can, seeks its own level. The general, final level that unimpeded water on the earth’s surface seeks is sea level; and the rivers of the earth, most of which empty into the sea, are the earth’s principal drainage channels. However, not all of the earth’s runoff reaches the great oceans; some of it is caught in landlocked lakes, ponds, and other nonflowing inland bodies of water.
Let’s consider, now, a point high in the mountains somewhere. As rain falls in the area around this point, the runoff runs down the slopes of a small gully and forms a small stream, which finds a channel down-ward through the ravine between two ridges. As the stream proceeds on its course, it picks up more and more water draining in similar fashion from high points in the area through which the stream is passing. As a result of this continuing accumulation of runoff, the stream becomes larger until eventually it either becomes or joins a large river making its way to the sea—or it may finally empty into a lake or some other inland body of water.
In normal weather conditions, the naturalchannels through which this runoff passes can generally contain and dispose of all the runoff. However, during the winter in the high mountains, runoff is commonly interrupted by snow conditions; that is, instead of running off, the potential runoff accumulates in the form of snow. When this accumulated mass melts in the spring, the runoff often attains proportions that overwhelm the natural channels, causing flooding of surrounding areas. In the same fashion, unusually heavy rainfall may overtax the natural channels.
Artificial Drainage.—When artificial structures are introduced into an area, the natural drainage arrangements of the area are upset. When, for example, an area originally containing many hills and ridges is graded off flat, the previously existing natural drainage channels are removed, and much of the effect of gravity on runoff is lost. When an area of natural soil is covered by artificial paving, a quantity of water that previously could have been absorbed will now present drainage problems.
In short, when man-made structures, such asbridges, buildings, and so forth, are erected in an area, it is usually necessary to design and construct an artificial drainage system to offset the extent to which the natural drainage system has been upset. Storm sewers are usually the primary feature of an artificial
Figure 10-1.—Working drawing for a typical curb inlet.
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 asdischarge. 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 metalframe. 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 isinstalled, 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.
direction of the line gradient and often shaped withchannels for carrying the water across the box from the inflowing pipe to the outflowing pipe.