Table 2-14. Effectiveness and Cost-Effectiveness of Water Control Structures
Using the cost data from Table 2-13 and the effectiveness data as summarized in
Evans et al.
we have calculated cost-effectiveness values for water
control structures in eastern North Carolina (Table 2-14). The relatively high unit
cost for phosphorusis due to the fact that controlled drainage is not highly
decrease
effective for phosphorus removal. Controlled drainage tends
phosphorus concentrations in systems where the majority of the outflow is via
surface runoff. However, on systems where the outflow is primarily through the
soil profile, controlled drainage has the opposite effect (Evans et al., 1991)
Grassed
and Diversions
2.3.3
Little data exist on the nutrient reduction effectiveness of grassed waterways and
diversions. Consequently, we can only present a narrative discussion of the
potential nutrient reduction effectiveness for these practices. However, there is
some evidence that, taken together as part of a "farm plan," these practices can
afford a level of nutrient load reduction to the stream. Camacho (1990) presented
a summary of literature values for surface water reduction efficiencies. Practices
with farm plan measures showed an increased reduction efficiency over practices
without farm-plan components for both nitrogen and phosphorus. However, these
results were for surface water studies only and included other practices in
to grass waterways and diversions. Due to the lack of data on both surface and
subsurface losses, it was not possible to calculate cost-effectiveness values for
these practices.
Grassed Waterways
Grassed waterways are used to provide a stable outlet for field water and to
Research studies have indicated that,
stabilize ditches and reduce
under conditions of channelized flow, which are prevalent in grassed waterways,
nutrient removal efficiencies will approach zero. In addition, sediment-bound
nutrients deposited in the waterway tend to wash back into the water (Casman,
1990).
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