VA L L E M A G G I A destructive flood of 7th August 1978, which costed many lives, the river has been diked even more heavily. The flow at the time of this flood was about 5.000 m3/second, while the average rate flow of the river is 24 m3/second. In April and May the snowmelt provides for the highest flow rates, values are also high in October. The Maggia river can be subject to extreme flooding from time to time, due to heavy precipitation and the steep character of the upper part of the river. In terms of quality the river is amongst the cleanest in Switzerland. (A. Thorens, C. Mauch, 2002) RIPARIAN VEGETATION Nearly half the flora of Switzerland can be found in riparian zones (Gallandat et al., 1993). This richness is generated by large dynamics in flood plains. These dynamics are caused by floods events that rejuvenate riparian vegetation. When water level heightens, non-adapted species are eliminated and a population of pioneer species comes instead to colonize the area. (V. Favre, 2004) After the construction of the dams in the Maggia watershed in the 1950s, the Maggia river almost completely dried up. In 1969, the Ticino province decided that a minimal flow rate of 750 l/min had to be released, measured in Bignasco. In 1982 they increased this minimal flow rate to 1.200 l/min from October to 14th June and 1.800 l/min from 15th June to November (Pfammater, Zanetta, 2003) Still, the hydro-electrical exploitation has led to an important reduction of the flow rate of the Maggia river. The hydropower dams managed by OFIMA retain sediments, thus preventing their release downstream. Lowering of the ground water level and decrease of floods affect the riparian vegetation and soil quality. In a natural state, riparian vegetation is very young and dynamic due to floods, but as these floods are less intensive and frequent, riparian vegetation gets the chance to develop into mature forests. These mature forests are not original riparian vegetation, but can now be found in many riparian zones affected by dams.
Paola (A. Murray, C. Paola, 1994) This model gives a better understanding of how a braided river develops and which are the factors that play a role in this development. It offers the possibility to insert and change various parameters, like; topography, amount of water input, sediment input and hydraulic conductivity factor. The outcomes can be clearly visualized through short animations, this gives an a clear impression of the different scenarios. One of the drawbacks of this model is that the outcomes cannot be manipulated and inserted again in a continuous loop. Moreover, the model limits the user to general simulations, very site specific research is not possible. Also, the model runs simulations in several iterations, which are not related to a particular time lapse. Analyzing the outputs one needs to make assumptions about the tested period. MANIPULATING THE RIVER Through a Grasshopper definition which offers the possibility to define flooding zones in a Rhino model, we could generate various flooding scenarios. Input of different amounts of water, released at the dam, shows the areas which will be flooded, where lakes and (temporary) islands and channels would develop. We tested the
CLIMATE The climate in the Maggia Valley can be described as Mediterranean. Precipitation is very high in autumn, when the area is under influence of subpolar circulation. The combination of warm and humid air masses from the Mediterranean Sea with the deep pressure cyclones can create long-lasting heavy rainfall in this period. The summers are usually relatively dry under the subtropical influence. Winter precipitation is low. Because of the high elevation, most of the precipitation in winter falls as snow. STREAMFLOW AND FLUVIAL MORPHOLOGY The streamflow of the Maggia river is mainly dependent on precipitation and snowmelt. Because of the steep character of valley and thin soil layers with a bedrock of granite and gneiss, the catchment has a very fast response to precipitation. This leads to a very variable streamflow. The streamflow can very from 1m3/sec in very dry periods to hundreds of m3/sec during floods.
Figure 115-116. Maggia river close to Someo.
low flow scenario; 4 m3/sec partially based on simulations and information by MaVal Research group, ETH Zurich.
The main tributary of the Maggia river is the Rovana, which drains about 21% of the watershed. The Maggia river itself drains 52% of the watershed, the rest is drained by other small tributaries. A lot of the tributaries are currently diverted and therefore contain very little water. Because of this, the water often doesn’t even reach the Maggia river, but enters small side rivers through little waterfalls. The water infiltrates in the alluvial area of the Maggia before even reaching the Maggia river itself. Because of low turbidity, the Maggia river is very clear. Also, the amount of fine sediments in the Maggia river is relatively low, the floodplain in the main valley consists of coarse material. The Rovana river carries more sediments, but because it is cut off for hydropower purposes most of the year, these sediments don’t reach the Maggia river. Only in times of floods, the Rovana delivers large amounts of sediments into the Maggia river.
possible flood scenario; 1.000 m3/sec partially based on simulations and information by MaVal Research group, ETH Zurich.
AQUIFER The entire valley was formed by a glacier. The aquifer depth goes up to 150m and contains coarse material, sand and gravel in the upper 30m. The aquifer is less deep at the outlet of the watershed in Ponte Brolla, where granite and gneiss come to the surface and form a geological barrier. In the upper part of the main valley, infiltration to the aquifer from the river dominates, whilst in the lower part groundwater upwelling occurs due to the in-situ bedrock at the surface. In the central part both of these conditions can occur. INTERVENTION To research possible transformations in mountain rivers and riparian landscapes affected by human interventions, we used both software simulation tools as well as research (MaVal project) done by professors and students of the ETH Zurich. BRAIDING RIVERS BEHAVIOR To understand the behavior of a braided river, the configuration of streams and braid bars and the movement of water and sediment, we did several general tests through an Excel file (P&Mv4b.xls) This braided river model was coded by Dr Stuart McLelland of the University of Hull, United Kingdom. The model is based on a cellular model of braided rivers coded by A. Murray and C.
Figure 117. The central stretch of the Maggia valley close to Someo is a braided area over a length of 7-8 km. This braided area consists of many alternating sidestreams, braid bars with typical original riparian vegetation. This part of the river is very dynamic, changed a lot over time. Floods cause the vegetation to juvenile frequently, which gives the area its typical riparian character.
47 T H E R I PA R I A N L A N D-S H A P I N G M AC H I N E