Page 230 - Biosystems Engineering

208 Cha pte r S i x

divert the water before it enters the field. If the water is present on
most of the land surface because of snowmelt or rainfall, reshaping
the land may be necessary to improve surface water runoff. The fol-
lowing sections describe different drainage methods in detail.
Land Forming
When puddles of water exist in many parts of a field, it might be more
appropriate to reshape the land surface by land forming. Land forming
is the process where the undulating land surface is reshaped by moving
soils from higher areas into depressions. First, a contour survey is car-
ried out on a grid, based on which contour maps are drawn for the area.
Based on the general slope of the land, two design slopes in orthogonal
directions are selected to minimize the cut and fill. Once the cut and fill
at each location on the grid is determined, an assessment of the depth of
the cut and fill is made. It is important to preserve the topsoil during
land forming because it takes about 25 to 50 years to form a 25-mm layer
of topsoil in a temperate climate. Generally, topsoil is scraped up and
piled before the land forming begins. The land is reshaped according to
the design cut/fill requirements. Land forming is done in two passes
running perpendicular to each other. Because cutting the soil releases
the overburden pressure, the soil fluffs up in areas where the soil has
been cut. Similarly, the soil will settle over time in areas where it has
been filled. Therefore, the two passes are done over two seasons to allow
time for the soil to settle. Once the reshaping is completed, the topsoil is
spread over the land in preparation for planting.
Although land forming is more expensive, in the end it will
improve machine efficiency by making it easier to navigate the field
without having to cross drainage ditches. Unlike drainage ditches,
well-shaped land requires hardly any maintenance, thus saving a
considerable amount of money in drainage maintenance costs.
Surface Drainage
If only a few areas are waterlogged in the field, it might be easier and
cost effective to construct drainage ditches just to drain the low spots.
Water has to be drained toward an outlet, which could be a project
drainage ditch, a lake, or a river. The selection of ditch route should
also take into account field operations and machinery efficiency. A
few ditch routes are chosen and a profile survey is conducted to
determine the land elevation along the alternative ditch routes. The
preliminary ditch design is done within depth and slope constraints.
Generally, the maximum depth of a ditch is 1 m to allow for machin-
ery crossing. The minimum depth of cut is 0.15 m to allow for a suf-
ficient cross-sectional area for water to flow. Over time, the ditches
get silted up and the cross-sectional area decreases, leading to a
reduction in drainage capacity.
The steeper the slope, the greater the velocity of flow and the
lesser the cross-sectional area required to transmit a given flow rate.