Page 328 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

7.22 THE WORK

If the vegetation is removed by plowing, disking, close grazing, or fire, the water comes in
direct contact with the soil and tends to remove the surface particles. This effect is usually rather
uniformly distributed at first, and is called sheet erosion. It can be reduced to slight proportions
by proper farming, including contour plowing and cultivating, and terracing or returning to sod
when necessary.
Erosion is most active where the amount or velocity of water is greatest, or the soil is least
resistant. Such places tend to wash out more than the surrounding area, and then, being lower as
a result, will catch the runoff from a larger area, increasing the quantity and velocity of water, and
its eroding effect. The deepening of the channel therefore tends to build up forces which will make
it deepen more rapidly.
In its early stages such a gully may be destroyed by plowing or harrowing, so that it is choked by clods
and some of its water is diverted elsewhere. However, unless close-growing vegetation is planted,
or weeds are allowed to grow, new storms will reform the channel or create new ones nearby, and
they may eventually become too deep to be choked by plowing or even to be crossed by a plow.
Once a gully is formed, it enlarges by three separate processes. One of these is channel erosion—
the scouring action of the water deepening the bottom. This is accompanied by the falling in of
the sides as they are undermined.
The upper ends (heads) of gullies advance into the land by waterfall erosion, both along the main
drainage line and branches which are acquired. Subsoil is often less resistant to erosion than topsoil.
Water pouring into the gully will cut it into steep banks, undermining the topsoil and causing it to
fall. The impact of the waterfall on the bottom gouges holes which accelerate channel erosion.
Waterfall erosion usually produces a gully with a U cross-section. It becomes less active as it
approaches the head of drainage and the quantity of water is reduced.
If the subsoil is equal or superior to the topsoil in resistance, waterfalls will not develop, but
the gully can progress by extensions of channel erosion.
The third major factor in extending gullies is sloughing off of soil due to alternate freezing and
thawing, or following saturation by heavy rains. This process is most active on southern and eastern
slopes, and will eat through a field with little regard to slopes or drainage lines.
Continued progress of either waterfall or sloughing erosion depends on sufficiently active channel
erosion to remove the loosened dirt.
Once well established, a gully will continue to enlarge even if the surrounding land is planted
in erosion-resistant vegetation, as the head and side slopes will undermine the surface.
A gully may advance downstream by channel erosion. More often, it deposits debris in a delta
fan at or near its mouth, so that the land is built up. This process is destructive also as it buries
topsoil under subsoil.

Damage. The damage from gullies includes actual destruction of farmland, cutting up fields so that
they cannot be worked economically, lowering the water table so that crops dry up, undermining
buildings, roads, and bridges, burial of lower lands under barren subsoil, and choking of streams
with silt.
An individual gully can do damage amounting to many thousands of dollars, and the national
loss from them is in the hundreds of millions. Their control is therefore of great importance.

CONTROL

Control measures after gullying has started may include diverting water to other drainageways,
planting, breaking down walls, building check dams, and proper use of the affected land.
Diversion. Water entering the gully can sometimes be diverted by plowing an arc around its head,
as in Fig. 7.16(A). The slice should be turned toward the gully to make a dam to back up the furrow.
More often it is necessary to build dams or to dig ditches. Dams are safer, as a new ditch or
even a plow furrow may start a new gully unless watched and controlled.