HyMoCARES

Alpine Rivers are working for Us

The study site

Figure 1: Location of the study area (Maggia Valley) and the concerned river section Figure 1: Location of the study area (Maggia Valley) and the concerned river section

The river Maggia originates in the north-western Alps and flows in Canton Ticino in Switzerland.

This area contains a remnant glacier and the geology of the catchment is predominantly composed of gneiss and granite (part of the Penninic basement). The climate is Mediterranean with a long-term mean annual rainfall of approximately 1730 mm. Summers are relatively dry and heavy rainfall typically occurs during autumn. Winter precipitation is low, occurring mainly as snow due to the high altitude.

The natural streamflow regime of the Maggia River can be described as pluvio-nival with a typical snowmelt peak in June. Largest flood peaks may reach close to 1000 m3 (Ruf, 2007).

The riverine corridor in the main valley, having approximately a length of 22 km and a width of 500 m, is the concerned river section in this project. The river bed is characterized by strong dynamics and high sediment transport rates. As a result, a braided river morphology with gravel bars and vegetated islands is observed over a length of around 7.5 km.



Figure 2: A view of the main valley with its steep hillslopes (©Wolfgang Ruf). In the chart: mean monthly precipitation and temperature over the period 1929-2003

Table 1: Main physical features of the pilot site

Pilot Site

Bignasco – Ponte Brolla

Drainage area [km2]

592

Location of the study reach

Bignasco (46.3382° N, 8.6089° E) to

Ponte Brolla (46.1853° N, 8.7528° E)

Length of the study reach [km]

22

Active channel width [m]

500

Channel slope [m/m]

0.005 (braided area) – 0.02

Planform morphology

braided/alternate bars (in the central part)

 

 

Figure 3: The braided area of the Maggia Valley (©Wolfgang Ruf)

This braided area is a very active floodplain with riverine vegetation undergoing its natural dynamics of erosion, seedling, succession, and rejuvenation. The braided area has an average slope ranging from 0.5-1%, while in the upper and lower parts channel slopes reach 2%.

 

Restoration/Management

 

Flow alterations

The Maggia Valley with a large hydropower system (OFIMA) and many reservoirs in the upstream part of the catchment is an example of an alpine environment highly affected by streamflow regulation due to hydropower operation. Its riverine corridor has undergone a strong reduction in flow magnitude and variability as well as a drop in groundwater levels and sediment delivery. The seasonality of streamflow also changed significantly. However the upstream reservoirs are not large in size and are not built for flood protection. The very highest discharges in the affected reach are not significantly reduced. Thereby the geomorphic reworking of the gravel bed by large floods continues to take place.

The effects of human impacts on the floodplain aquatic ecosystem in the Maggia have different main dimensions, but the general tendency occurs despite the fact that the river system is still able to rework the bed and to erode pioneering vegetation (mostly Salicaceae) significantly after large flood events.

For managers of river systems the question then is whether hydropower operation has a significant effect on floodplain processes due to the altered surface water flow regime or changes in groundwater levels.

Sediment alterations

The second dimension concerns sediment budgets. It is expected that reservoir construction in the headwaters of the river may have significantly reduced sediment input into the affected river reach and/or shifted the grain size distribution of the incoming sediment towards coarse material. This may have significant effects on floodplain soil water storage and the erodibility of the river bed.

For managers of river systems there are opportunities for supplying sediment to starved river reaches and/or allowing the river system to access natural sediment sources during floods. Although these actions are not envisioned for the Maggia.

The TiRiLab which is an environmental computing laboratory at ETH Zurich within the HyMoCARES project will use advanced hydrodynamic modelling in the Maggia to provide a DEMO of options focuses around two fundamental questions:

(a) can we quantify the erosion/deposition rates in the river bed as well as the required flood level for a significant geomorphic work to take place?;

(b) can we quantify alterations of the physical habitat, transportability of sediment, and habitat suitability for fish with respect to the pre- and post-dam period, with a view towards full ecosystem services analysis?

Monitoring activities

 

Monitoring activities in the Maggia River consist of networks covering climate and hydrology, river sediment, floodplain vegetation, and river ecology. In these networks there are both continuous as well as intermittent (snapshot) type surveys have been (are being) conducted.

  1. Climate and Hydrology
  2. River Sediment
  3. Floodplain Vegetation
  4. River Ecology

 

 

PROJECTS:

 

TiRiLab: Ticino Rivers Lab