Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
scheme was that we could not collect the whole water at a MP, so we had to take aliquots. At
the beginning we expected the highest fluctuations, hence the distance between sample 1 and
2 was lower than between sample 2 and 3 and sample 3 and 4. Depending on the flow length
to the MP it was possible that after 3 minutes the runoff stopped at this point, we had to take
the samples before this occurred.
At each MP, rill cross section was measured. With thin metal sticks, the distance between
ground level and rill bottom was measured in 0.002 m steps. This allowed an accurate
calculation of the rill’s cross section area and an estimation of the rill’s volume. Additionally,
the water depth at a certain point in the rill was measured several times using a yardstick. By
combining the rill cross section and the water depth it was possible to calculate flow area,
wetted perimeter and hydraulic radius using the georeferencing tool of a GIS.
2.3 Descriptors for soil detachment
The relationship between detachment rate and detachment capacity and respectively the
relationship between transport rate and transport capacity are important values for assessing
different processes acting in the rill. These variables were calculated as follow:
DC = K
r
(τ τcr
) (Foster et al., 1995) Eq. (1)
T
1.5
C
= K
t
R τ
(modified from Wagenbrenner et al., 2010) Eq. (2)
SC
V A
DR =
Eq. (3)
L WP
TR = SC
V A
Eq. (4)
with D C = detachment capacity [kg m -2 s -1 ], K r = rill erodibility factor [s m -1 ], τ = shear stress
[Pa], τ cr = critical shear stress [Pa], T C = transport capacity [kg s -1 ], K t = transport coefficient
[s² m 0.5 kg -0.5 ], R = hydraulic radius [m], D R = detachment rate [kg m -2 s -1 ], S C = sediment
concentration [g L -1 ], V = flow velocity [m s -1 ], A = flow area [m²], L = flow length [m], WP
= wetted perimeter [m], T R = transport rate [kg s -1 ].
The rill erodibility factor K r can be calculated for cropland or rangeland; in this case, we used
the equation of Flanagan and Livingston (1995) for cropland. Parameters are very fine sand
content (vfs) and organic material content (org.Mat.).
K
1.84(org.
Mat.
)
r
= 0.00197 + 0.0003 (vfs) + 0.03863
Eq. (5)
Depending on land use, the parameters for the critical shear stress are clay content and very
fine sand content (Flanagan and Livingston, 1995).
τ c
= 2.67 + 0.065 (clay) 0.058 (vfs)
Eq. (6)
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