Geoscience Reference
In-Depth Information
lined bed
8.00
rough, block stoned bed
7.82
55
31
55
40
45
69
55
69
3
.30
3.30
2.40
2.40
0.00
0.00
Fig. 2 Cross section of the lined bed and the rough, block stoned bed in the Alpbach
capacity of the channels in the populated area will be additionally enhanced by the
following modifications:
Increase of the flow section by increasing the channel width and banks and
dismounting bridges with low freeboard
l
Increase of the bed slope of Milibach by moving the confluence further upstream
(Fig.
1
)
l
At the same time, the existing lined channel bed of the Alpbach is considered to
get changed to a rough, block stoned bed in order to gain an ecologically more
valuable channel (Fig.
2
). The drawback, however, is the smaller transport capacity
in comparison to the lined channel.
As the catchment areas of Milibach and Alpbach are small, the discharges react
immediately to rainfalls and two different types of flood events can be distin-
guished: long-lasting floods (30 h), as in the year 2005, arising from persisting
but weak precipitation with a lot of mobilized material due to the long wetting, and
short floods (3-5 h) arising from thunderstorms with a high discharge but little total
sediment input. The discharge and sediment hydrographs of floods arising from
persistent rainfalls are rather stationary, whereas floods of thunderstorms exhibit a
single peak. Table
1
depicts the peak loads of the design floods HQ
100
and the
extreme floods EHQ examined in the model tests.
2 The Physical Model
The aim of the physical model tests was to prove and optimize the planned
measures in terms of increased flood safety for the design flood HQ
100
and extreme
flood EHQ as well as in terms of sediment transport. In case of EHQ, defined as
1.5
HQ
100
, certain areas are accepted to be flooded. The evaluation of the rough
and lined Alpbach channel in terms of flood safety and maintenance expenses for
bed load excavation after a flood was of particular significance. Furthermore, the
confluence (Fig.
1
(3)), the influence of the backwater of the river Aare (Fig.
1
(5)),
and the construction of the sediment retention basin dam (Fig.
3
(7) and (8)) were
