Surface
drainage is the removal of water that collects on the land surface. A surface
drainage system consists of shallow ditches and should include land smoothing
or land grading. This type of system is suitable for all slowly permeable soils
and for soils with fragipans or clay subsoils.
The
rate at which water is removed by surface drainage depends on several
interrelated factors, including rainfall, soil properties, and cropping
patterns. For most row crops, a surface drainage system should remove excess
water within 24 to 48 hours. More rapid removal may be necessary for higher
value truck crops.
Before
designing a surface drainage system, you should make a topographic survey and
develop a contour map of the area, using grid surveys, laser techniques,
photogrammetric methods, or some combination of these. Keep copies of the
contour maps, as-built plans, and profiles as a record of permanent
improvements. These resources will be invaluable later when the ditches have to
be re-shaped or the channel re-graded.
A
surface drainage system consists of an outlet channel, lateral ditches, and
field ditches. Water is carried to the outlet channel by lateral ditches, which
receive water from field ditches or sometimes from the surface of the field.
Plan
a minimum number of field ditches located, where possible, at right angles to
the lateral ditch and crop rows. It is essential that lateral ditches be deep
enough to drain the field ditches completely enough to permit crossing by farm
machinery. The minimum depth of lateral ditches is 1.0 foot. At points where
lateral ditches enter the outlet channel, grade back small overfalls on a
nonerosive grade (Figure
6a). If the outlet is too deep or some other problem makes it difficult to
grade the overfall, install a chute, drop spillway, or pipe (Figure
6b).
Two
common types of field ditches are the single ditch and the W ditch (also called
the twin or double ditch). The single ditch is used where spoil can be moved
and spread in low areas of the field without obstructing flow into the ditch.
The double or W ditch (illustrated in Figure
7) is used where the land drains towards the ditch from both directions,
where the land is very flat and row drainage will enter from each side, and
where the excavated material is not needed to fill depressions.
The
cross section of a single ditch is usually trapezoidal or V-shaped, as shown in
Figure 8. Its minimum depth should be 6 inches for trapezoidal and 9 inches for V
sections, each having a minimum cross sectional area of 5 square feet. Field
ditches should ordinarily not be deeper than 1 foot where they are to be
crossed frequently by farm machinery. Side slopes should be 8 to 1 or flatter
for a trapezoidal section and 10 to 1 for V sections.
The
cross section of the W ditch system is similar to that of the single ditch,
except that if farm machinery is not expected to cross the ditch the side
slopes toward the field should be 8 to 1 and those on the crowned section 4 to
1. The excavated earth should be shaped into a crowned section between the two
ditches. The crowned section should be large enough to accommodate the spoil
that has been removed. The minimum distance between ditch centers should be 30
feet. Determine the capacity of the ditches according to curve B or C in Figure
2. This figure is implemented
interactively in the Ditch Sizing program. The minimum recommended design grade is 0.1
foot per 100 feet (0.1 percent). Where the channel grade is less than 0.05 foot
per 100 feet, ponding and siltation may occur.
Diversions
may be included in a drainage system to prevent surface runoff from sloping
land from reaching a flat or depressional area. Diversion ditches are located
at the base of a slope to intercept and carry surface flow to an outlet. Their
side slopes range between 2 to 1 and 4 to 1, and they should be kept in sod. To
minimize overtopping of the diversion, design the ditch cross section to carry
the runoff from a 10-year frequency storm of 24-hour duration.
The
two main types of surface drainage patterns are random and parallel. Each
includes lateral ditches that permit water to flow from the drainage system to a
suitable outlet. The pattern you choose depends upon the soil type and
topography of the land.
Random
The random ditch pattern is adapted to slowly
permeable soils having depressional areas that are too large to be eliminated
by land smoothing or grading. Field ditches connect the major low spots and
remove excess surface water from them. They are generally shallow enough to
permit frequent crossing by farm machinery. Soil from the ditches can be used
to fill minor low spots in the field.
Field ditches should extend through most of the
depressions, as shown in Figure
9, to assure complete drainage, and they should follow the natural slope of
the land in accordance with
Parallel
The parallel ditch pattern is suitable for flatter,
poorly drained soils that have numerous shallow depressions (Figure
10). In fields that can be cultivated up and down slope, parallel field
ditches are installed across the slope to break the field into shorter units of
length and make it less susceptible to erosion. The field should be farmed in
the direction of the greatest slope. Dead furrows are neither desirable nor
necessary.
Although the ditches must be parallel, they need not
be equi-distant. The spacing between them depends upon the permissible length
of row drainage for the soil type and upon the amount of earth and the distance
it must be moved to provide complete row drainage. The maximum length of the
grade draining to a ditch should be 660 feet.
The success of a surface system using a parallel
pattern depends largely upon proper spacing of the parallel ditches and the
smoothing or grading between them. During the grading operation, fill all
depressions and remove all barriers. Excavated material from ditches can also
be used as fill for establishing grades.
Grading
Land
grading (also termed precision land forming) is the reshaping of the surface of
land with tractors and scrapers to planned grades. Its purpose is to provide
excellent surface drainage although the amount of grading will depend upon the
soil and costs. To do a good job of land grading, you need a detailed
engineering survey and construction layout.
To
assure adequate surface drainage, eliminate all reverse surface grades that
form depressions. The recommended surface grades range from 0.1 to 0.5 percent
and may be uniform or variable. The cross slopes normally should not exceed 0.5
percent. Minimum grade limits should include a construction tolerance that will
permit the elimination of all depressions either in original construction or in
post-construction touch up. Reverse grades can be eliminated with relative ease
in a field that has minimum grades of 0.2 percent. Unusual precision in
construction is required to eliminate reverse surface grades in fields that
have 0.1 percent and flatter grades.
Land
grading is hampered by trash and vegetation. This material should be destroyed
or removed before construction and kept under control while the work is being
done. The fields should be chiseled before construction if there are hard pans.
The field surface should be firm when it is surveyed so that rod readings taken
at stakes will reflect the true elevation. Do not grade fields when they are
wet because working wet soil impairs the physical condition of the soil.
Smoothing
Land
smoothing removes irregularities on the land surface and should be done after
land grading and may be useful in other situations. Special equipment such as a
land plane or land leveler should be used. The purpose of land smoothing is to
improve surface drainage. The smoothing operation may ordinarily be directed in
the field without detailed surveys or plans, although grid surveys may be
needed for some critical parts of the field.
A
smoothing operation consists of a minimum of three passes with a land leveler.
Make the first two passes on opposite diagonals as noted in Figure
11 and the last pass in the direction of cultivation. Either before or
after the final land smoothing operation, chisel fields to loosen the cut
surfaces and to blend the fill material with the underlying soil. The finished
surface should be free from minor depressions so that runoff will flow
unobstructed to field or lateral ditches.
The
outlet channel, lateral ditches, and field ditches should be cleaned as needed
to keep them functioning properly. Small deposits of silt often greatly reduce
the capacity of a surface drainage system and cause partial or complete failure
of it. After each heavy rain, the outlet channel and ditches should be
inspected, and silt deposits or other obstructions removed. Brushy types of
vegetation, such as cattails, willows, and cottonwoods, are a menace to surface
ditches and should be cut or sprayed once or twice each year as needed.
Maintenance of areas that have been graded is critical
during the first year or two after construction. Settlement of the fill areas
may make several annual land smoothing operations necessary. In some cases,
particularly where deep fill has been made, it may be necessary to cut and fill
again, using tractors and scrapers to eliminate depressions and reverse surface
grades. It is suggested that after each plowing a land plane be operated over
the area at least twice, making one pass along each diagonal (Figure
11). This operation will not only take care of settlement in the field
areas and erase all scars to the land surface caused by field operations but
also provide a good seed bed.