 |
| Titel |
Aeolian sediment transport and spatial patterns of sand deposition in vegetation canopies: Observations from wind tunnel experiments using coloured sand |
| VerfasserIn |
Katrin Burri, Christof Gromke, Frank Graf |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056321
|
|
|
|
|
|
| Zusammenfassung |
| Desertification is a major environmental hazard directly affecting about 250 million people
through the loss of soil resulting in a reduction in the land’s productivity and potentially
concerns about 2.5 billion people living in drylands worldwide. Additionally, the importance
in view of global change is increasingly recognised. Shifts in vegetation structure and albedo
as a result of desertification can significantly affect regional climate, with feedbacks to
ecosystem dynamics.
Today, re-vegetation is accepted as the most efficient and promising strategy to combat
wind erosion and desertification in the long term. Vegetation plays an important role in
reducing soil erosion by wind in arid and semi-arid environments. Plants protect the soil from
wind erosion by (i) covering a proportion of the surface and thereby sheltering the soil from
the erosive force of the wind, (ii) extracting momentum from the wind, and (iii) trapping soil
particles in transport. Furthermore, plants reduce wind erosion by altering soil and
atmospheric characteristics, such as soil structural stability and near-surface air and soil
moisture.
The objective of this study was to compare the total sediment mass flux, the fine dust
concentration and the vertical profile of sediment mass flux in three different canopy densities
of Perennial Ryegrass (Lolium perenne) and on a bare sand surface. In our wind tunnel
study we examined aeolian sediment transport in live plant canopies, instead of
employing model plants as in most previous studies. The results show that both total
sediment mass flux Q and PM10 concentration in the air decreased exponentially
with increasing canopy density. In the high-density canopy (frontal area index λ =
0.58), Q and PM10 concentration were reduced to 0.01% and 0.4% of the unplanted
bare surface configuration. In the medium-density canopy (λ = 0.16), Q and PM10
concentration were reduced to 6.6% and 48.5%. In the low-density canopy (λ = 0.03),
however, Q and PM10 concentration were increased to 117.5% and 145.6%. This
is attributed to elevated shear stress on the sand bed caused by flow acceleration
around the tussocks and vortical structures in their lee. Furthermore, the grasses
were observed to trigger erosion by oscillating movements at the ground surface. It
was also found that the vertical profiles of sediment mass flux in the medium- and
high-density canopy strongly deviated from the exponential decay curve of the
unplanted configuration, showing a local maximum at approximately twice the canopy
height.
Furthermore, we used coloured quartz sand to visualise spatial patterns of sediment
deposition within grass canopies and to identify areas of net deposition. In the low- and
medium-density canopy, the wake areas downstream of the tussocks were the main locations
of sediment deposition. In the medium-density canopy, these wedge-shaped wake
deposits overlapped with the adjacent downstream tussocks, while in the low-density
canopy they did not, indicating a wake-interference flow and an isolated roughness
flow, respectively. In the high-density canopy, only few sand grains were entrained
by the wind and they deposited mainly within reach of the grass tussocks. The
deposited sand grains were evenly distributed around the tussocks, without pronounced
accumulations on their upstream, downstream or lateral sides, suggesting a skimming
flow regime. The fraction of the non plant covered (nc) sand surface which was
potentially exposed to erosion (EPnc) was notably smaller than the area which was not
covered by grasses. It accounted for 67-78 % of the non-covered surface in the
low-density canopy, and for 44-77 % in the medium-density canopy. This finding
indicates that wind erosion models overestimate the sediment source area if they
approximate the erodible area with the total surface not covered by roughness elements. |
|
|
|
|
|
|