Geoscience Reference
In-Depth Information
Spatial Variability, Mean Drag Forces, and Drag
Coefficients in an Array of Rigid Cylinders
T. Schoneboom, J. Aberle, and A. Dittrich
1
Introduction
Floodplain vegetation is an integral part of riparian ecosystems covering a wide
range of conditions from highly flexible low grass to bushes and trees with rigid
stems. However, vegetation also increases flow resistance, changes backwater
profiles, and modifies sediment transport and deposition (e.g., Yen
2002
). Thus,
to meet both ecological and flood protection needs, it is crucial to understand the
interplay of biological, physical, and chemical processes and how vegetation type,
density, and pattern influence flow resistance and the turbulent flow field. In fact,
the area of conflict between ecology and water resources has initiated an abundance
of studies toward the hydrodynamics of aquatic and riparian ecosystems (e.g.,
Thorp et al.
2008
; Nikora
2010
and references therein).
Various studies have shown that the resistance behavior of flexible vegetation
elements differs substantially from the resistance behavior of rigid elements (e.g.,
Oplatka
1998
;J
a
2002
,
2004
; Armanini et al.
2005
; Wilson et al.
2008
; Aberle
et al.
2010
). However, until today, there exists no universal approach for the
determination of
C
D
for natural vegetation and species-specific resistance laws
and parameters are still under development (e.g., Jarvela
2004
). Therefore, most
of the existing computational approaches for the determination of flow resistance
due to vegetation are based on the assumption that riparian vegetation can be
modeled by rigid cylinders (e.g., Baptist et al.
2007
; Huthoff et al.
2007
). This is
reflected by the fact that most of the existing approaches require only plant density
(expressed by the ratio of stem diameter
d
and averaged longitudinal and transverse
plant spacing
a
x
and
a
y
, respectively) and the drag coefficient
C
D
as input para-
meters for the description of vegetation characteristics (e.g., Fischer-Antze et al.
2001
; Beffa and Connell
2001
). Simulating vegetation by rigid cylinders has the
advantage that the drag coefficient
C
D
can be estimated relatively straightforward
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