Environmental Engineering Reference
In-Depth Information
located in watersheds with the highest levels of row crop agricultural land use
and impervious landscapes, these same wetlands typically had the most produc-
tive biota measured by numbers of individual macroinvertebrates and birds. We
found the converse also true: the highest quality wetlands had more species of
birds, macroinvertebrates, and plants; higher diversity; but lower abundances of
the species present.
DIVERSITY, HYDRAULIC PERFORMANCE, AND OUTLET
DESIGN CHARACTERISTICS WERE ALL RELATED
In many instances, watershed and site characteristics had a much greater influence
on wetland quality and the frequency and duration of bounce events than the outlet
design. In settings where the watershed ratio is low and upland perennial vegetation
can be planted to buffer the watershed, wetland restorations may be higher quality.
Operable outlets may be used in this setting primarily for vegetation management,
and possibly for controlled pre-spring drawdown during wet climatic cycles. Outlet
design wetlands with large watershed ratios tended to be two-stage, with relatively
higher outflow capacity than for the low-watershed-ratio wetlands. Higher and more
frequent bounce is more probable and natural seasonal drawdown less probable for
these wetlands, due to their larger watershed ratios.
High-quality wetlands were believed difficult to achieve in watersheds with larger
watershed ratios and where land use is primarily agricultural. Wetlands in water-
sheds with higher watershed ratios showed declines in biodiversity.
Bounce relationships examined the contributors to bounce maximum amplitude,
duration, and number of bounce events in > 0.5 foot, > 1.0 foot, > 2.0 foot, and
> 3.0 foot categories. This analysis showed very strong multilinear relationships
between bounce metrics in wetlands and land use within watersheds High-quality
wetlands were found to have lower amplitude and lower frequency bounce events,
and medium- and low-quality wetlands to have durations generally meeting a ten-
day bounce drawdown and less than 2.0 foot bounce amplitude. Where high-quality
wetland biodiversity is a desired outcome in restorations, lower bounce levels, fre-
quencies, and durations during the growing season are encouraged.
An interesting discovery of the research was the effects of wetland water level
drawdown rates on biodiversity. We found a polynomic relationship where slowest
and fastest drawdown rates were routinely correlated with the lowest biodiversity.
Intermediate rates correlated with the highest biodiversity for all biotic groups mea-
sured. Example rates are provided in Table 9.2.
A basin-wide tributary water management framework is needed to ensure a
coordinated approach to achieve outcomes that will be effective at the local, water-
shed, and basin levels. This framework should encourage the implementation of
measures that reduce local, watershed, and main stem flood damage potential.
Development of a basin-wide framework requires an understanding of critical fac-
tors that affect flooding, including runoff timing and volume. When implementing
individual restoration projects, it is necessary to know how water from any given
area will arrive.
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