The Abbeville press and banner. [volume] (Abbeville, S.C.) 1869-1924, 1917-08-29
BRUSH DAMS BUILT FOR CHECKING EROSION.


FOR CHECKING EROSION.
channel to carry the same amount Of
water on a light grade as on, a heavy
one. With a variable-graded terrace
there is less likelihood Of overlapping
because the grade is increased at
short intervals along the terrace. ,
Again, breaks in graded terraces are
very frequent where gullies and de-
pressions are crossed and at abrupt
bends. Such breaks are due to. sud-
den changes in the direction Of flow
or to q change in grade, and often to
both. The usual practice Of crossing
depressions at a low elevation to avoid
abrupt bends, results in an increase
Of grade to the middle of the 6epres-
sion and a decrease beyond the mid
die. In order to avoid a break due to
this diminution in grade it becomes
necessary to maintain the top Of the
terrace at a uniform grade. This ne
cessitates the building Of a high and
broad embankment across the Gepres-
sion similar to the one described for
level terraces. Wherever it can be
done without increasing the grade to
such an extent as to cause serious ero-
sion, it is advisable to make the grade
greater for that portion Of the ter
race leading away from the middle
Of the depression than for the por-
ton leading to the middle.
Use of Graded Terrace.
The graded terrace is adapted par
ticularly for use on impervious and
WORKOUT soils, and on shallow open
soils with an fmperlneable subsoil
fou~ndation-in general, soils that are
incapable Of absorbing much water.
Since the object Of %terracing Is to pre-
vent erosion, and as this Is accom-
plished best by securing the least
movement Of the surface water, it can
be seen readily that, within limits,
the efficiency Of a graded terrace va
ries inversely with the amount Of fall
given to it. The greater the fall, the
greater the velocity and, hence, the
greater the erosive power Of the NOV
ing water.
The embankment Of a graded ter
race, being subjected to the erosive
action Of the water on its upper side,
Is often washed considerably, particu-
larly at bends.
The deposit of soil in the terrace
channel reduces both the grade and
the cross-sectional area Of the chan
mel and renders the terrace extremely
susceptible to overlapping during the
next rain. Also the finer, lighter, and
more fertile particles Of soil remain
suspended in the moving water and
are carried off the field. In such cases,
by the use of excessive grades, the
very cream Of the safl is lost. Where
erosion Of q terrace takes place no
attempt should be made to cultivate
the terrace. 1t should be seeded to
grass.
The result that should be attained
by q system of terraces and proper
farming methods has been expressed
in this way:
The primary object is conservation
Of both solid and fluid parts of the soil
through q balanced distribution of the
water supply. The ideal distribution
is attained when all the rainfall or
melting snow is absorbed by the
ground or it's cover, leaving one to
run off over the surface Of the field or
pasture; in which case the water so
assorted is retained in the soil and
subsoil until utilized largely or wholly
in the making Of useful crops, while
any excess either remains in the deep
er subsoil and rocks as ground water
or through seepage feeds the perma-
nent streams.
These conditions are fulfilled most
nearly by the horizontal bench terrace
and the broad-base level-ridge terrace,
since the movement Of the water is
reduced to a minimum by both. The
graded terrace lacks much in meet
ing the requirements.
In general it Is recommended that
the broad-base levcl-ridge terrace be
used wherever conditions of soil and
topography will permit-that is, where
the soil absorbs a lortion, of the rain
fall and the slopes are not too steep.
The b%road-base ltvel-rtdge terrace
supplemented by efficient tile drains
suitably located would afford the most
ideal method for preventing soil ero-
sion on any type of soil. Often the
yields obtained and the saving result
, ing from the absence Of soil erosion
would justify, in a financial way, the
installation of tile.