2.3.6
Terraces
Terraces are a series of regularly spaced embankments across a slope that form
side of the embankment. In North Carolina, terraces are
a channel on the
used
with contour rows to act as a check for contour row failure (R.
Hansard, 1994). Although they have been shown to be very
in reducing
nutrient and sediment runoff, terraces
quite expensive to install and
maintain. Also, terraces can fail during large storms if the water in a terrace
overtops the embankment. A resulting cascade effect can lead to serious
downslope erosion (Heatwole et al., 1991). Terraces reduce nutrient runoff by
temporarily storing water, allowing sediment deposition and water infiltration.
Table 2-l 8 presents cost data for terraces in the Tar-Pamlico basin.
Table 2-18. Cost Data for Terracing
,709
9,369
5% of base
10 years
505 ha
cost
of terrace per acre (USDA Soil Conservation Service 1991)
Assuming 400
Regional effectiveness data for terraces are extremely scant.
et al.
(1985) studied phosphorus losses from conventional till corn (winter fallow)
converted to terraces in the southern Piedmont. He reported an annual total
phosphorus reduction efficiency of 66 percent. However, this was for surface
runoff only. Ellis et al. (1985) studied partitioning of nutrients between surface
and subsurface flow. Casman (1990) used Ellis' partitioning results to estimate
net (surface and subsurface) reductions based on the results of Langdale. The
results are presented in Table 2-19. Casman noted that, although no studies of
total nitrogen reduction by terracing were found, it is reasonable to assume that
soils with low moisture content, it would be reasonable to assume that nearly all of
delivered to the stream. In organic soils
the subsurface nitrogen load
with sufficient moisture to create
conditions, denitrification could
substantially reduce nitrogen loads
1991). However, in the Tar-Pamlico
basin, terracing is used primarily in the Piedmont, which is dominated by mineral
soils. Consequently, we assume that terracing in the Tar-Pamlico basin is not
effective in reducing nitrogen loads
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