QUANTIFYING RATES OF EROSION USING THE OCCURRENCE AND MAGNITUDE OF FLOOD EVENTS IN THE LEPONTINE DOME, SWITZERLAND

Uplift of the Swiss Alps caused by the collision of the Adriatic and European plates may also be affected by climate. Areas with higher precipitation may experience higher erosion rates which would drive isostatic uplift. To test this hypothesis, a comparison of the rates of erosion and uplift between areas with varying amounts of precipitation must be conducted. In this study, we estimate erosion rates inside the Lepontine Dome, which receives the highest annual precipitation in the Swiss Alps. We focused on three drainages; the Bosco Gurin, the Verzasca Valley, and the Maggia river systems in the Vizzara Valley. For each river, we estimated the erosional force of major flood events by determining the size and occurrence of past floods.

We estimated the competence, shear stress, and capacity of past floods by measuring the b-axis of the largest boulders within a 100-meter reach of each river. We used these data in combination with the shear stress equation derived by Leopold, Wolman and Miller (1964) and the Parker equation (Mueller and Pitlick, 2005) to estimate the shear stress and capacity of the flood required to move each boulder. We then dated the last movement of each boulder using lichenometry and a lichen growth curve developed for this area. Analysis of the boulder size and age data in the Bosco Gurin suggests that major floods occurred at 52, 76 and 106 years ago. We saw no correlation between boulder size and date of last movement in the other rivers, which prevented us from assessing flood events in those drainages. For the Bosco Gurin, our estimates of the shear stress and capacity for the 52, 76 and 106 year old floods are 464N/m2 and 1.03 m3/s, 633 N/m2 and 1.83 m3/s, 734 N/m2 and 2.38m3/s respectively. Assuming that peak flow lasted 1 hour, an extrapolation of these results suggests that 0.24 km3 of material would be removed by repeated occurrence of these floods over 1 m.y., corresponding to an average erosion of ~10 cm over the 50 km2 drainage.

Taken at face value, these estimates suggest that fluvial erosion is not a major factor in uplift of the Lepontine Dome. However, we may underestimate fluvial erosion because we do not account for annual sediment transport and our flood record only extends for ~100 years. Alternatively, fluvial erosion may be low and mass movements and glaciers may account for the majority of erosion in the region.