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

Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen
The maximum runoff intensity Q m [L s -1 ] is set into relation with the inflow intensity Q I [L s -
1 ], leading to Q f [without unit], which is the intensity factor. Some additional factors of the
flow duration are calculated as they are also indicators of the transformation of the input pulse
such as the begin and end times of the outflow at the end of the rill and the flow duration (t S
[s], t E [s] and T R [s] respectively). The changes in flow duration are indicated by the quotient
(d T ) [without unit] between runoff duration and inflow duration T I [s].
Velocity is recorded as an average value (v a ) [m s -1 ] and the maximum (v max ) [m s -1 ] value for
each of the 3 measurements in each experiment. The flow velocity at which each of the
samples was collected is estimated as follows: We assume linear changes between the
measured flow velocities (front, tracer 1 and tracer 2). Knowing the exact time of arrival of
the water front to the sampling point, we estimate the flow velocity for each of the samples
collected interpolating between the measured velocities.
Taking into account the rill’s morphology, the estimated velocity at the sampling location, the
points in time and the sediment concentration, we calculated the maximum hydraulic shear
stress τ [Pa] (eq. 3) according to Nearing et al. (1997).
τ = ρgRS
(eq. 3)
where ρ represents the fluid density [kg m -3 ] which was calculated taking into account the
sediment concentration in the samples, g the gravitational acceleration (9.81 m s -2 ), R the
hydraulic radius [m], and S the effective slope (sin(slope angle)). The hydraulic radius is
calculated as follow:
A
R (eq. 4)
WP
with A = flow cross section [m²] and WP = wetted perimeter [m]. Flow cross section is
calculated using the runoff and the flow velocity:
Q
A (eq. 5)
v
with Q = runoff [m³ s -1 ] and v = flow velocity [m s -1 ]. The runoff, needed to calculate the
hydraulic radius at the measuring points, was set equal to inflow (0.00015 m 3 s -1 ), so the
calculated values can be understood as the maximum possible. Using the known flow cross
section and the rill cross section, water level and wetted perimeter can be determined.
The sediment transport within the rill was characterized by the sediment concentration in the
samples. Additionally, we calculated the sediment transport per unit time and unit rill length
(D L [kg m -1 s -1 ]) and per unit rill bed area (D A [kg m -2 s -1 ]).
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