Numerous research studies on the effectiveness of
have been conducted, with
highly variable results. Some of the key observations common to multiple studies
flow conditions render
ineffective for sediment and nutrient
The effectiveness of a VFS
time as sediment accumulates unless
the vegetation within it can grow as fast
it is being buried.
are more effective. at removing suspended solids, than nutrients.
Numerous site-specific factors influence VFS efficiency, including filter length,
depth of flow, slope, cross slope, soil type, infiuent characteristics, clogging, and
hydraulic loading rate.
Much of the variabiiity in the reported values for VFS effectiveness was due to (1)
inconsistent methods in defining inputs, and (2) strip design (lengths from 15 feet to
2 miles, slopes from 2 percent to 16 percent).
et al. (1988) and Magette et
al. (I 987) used filter strips of sizes that are typical for cost-share practices (15 to 30
ft). Based on these two studies, Casman (1990) estimated that about 30 percent of
total nitrogen is retained by a VFS (surface and
For phosphorus, the only applicable study is Schwer and
presented a subsurface phosphorus retention value of 90 percent. Casman (1990)
noted that this value is probably too high and that a more appropriate estimate for
phosphorus removal is somewhere between 30 percent and 90 percent.
Effectiveness estimates are summarized in Table 2-21.
Table 2-21. Effectiveness data for
Field Borders, and Stripcropping
No effectiveness data available.