Geoscience Reference
In-Depth Information
4 Results of the Model Tests
The comparison of the model tests of a reduced, not extended hydrograph (Fig.
7b
)
and an adapted, extended hydrograph (Fig.
7c
)ofaHQ
100
long-lasting event with
an Aare water level of HQ
30
showed the same behavior of deposition and erosion
both in time and space (Fig.
8
). Deposition took only place in the section where the
Alpbach was influenced by the backwater effect of the Aare. Upstream of it, the
sediment got transported in the channel.
As the hydrograph of a HQ
100
long-lasting event is rather stationary, the
equilibrium slope of both model tests is reached at the same time, independent of
the extension of the hydrograph. In comparison to unsteady hydrographs of thun-
derstorms, the extension by the extension factor g (
5
) does not significantly influ-
ence the recorded water level, but influences the total bed load balance.
In Alpbach and Milibach, the design flood events HQ
100
of both rainfall types
(Table
1
) do not lead to an overtopping. Local depositions take place in the channels
but do not endanger flood safety. Concerning the extreme flood events EHQ, the
maximum transport capacity is exceeded in both channels and deposition reduces
the flow section such that water and sediments overtop the banks and overflow the
area between the channels and the flood walls.
The rough bed (Fig.
2
) behaves stable under all examined flow conditions. In
relation to the lined channel, the water level and the deposition height are increased.
Flow is supercritical in both configurations. However, in the lined channel, the
energy level is higher. Thus, the hydraulic jump at the backwater of the river Aare is
stronger exhibited. In both bed configurations, the sediment which is transported
through the channels deposits at the hydraulic jump and initiates a forward and
backward migration of deposition. In comparison to the rough channel, stationary
waves are generated at the deposition front in the lined channel, as depicted in
Fig.
9
. The stationary waves migrate with the deposition front up- or downstream,
depending if deposition or erosion is taking place. Due to the interaction of the
waves with the bed, antidunes are generated (Kennedy
1963
). The differences
608
604
+
+
+
600
max. water level b)
deposition b)
right bank
left bank
bed level
+
max. water level c)
deposition c)
water level Aare (
HQ
30)
596
200
100
0
distance to Aare [m]
Fig. 8 Results of the model tests of a not extended (b) and an extended (c) hydrograph of a HQ
100
long-lasting event with an Aare water level of HQ
30
: bed level at the end of the event (deposition)
and maximum measured water level during the flood. Depicted is the lowest section of the
Alpbach, from 0 to 200 km
