Seite - 590 - in Book of Full Papers - Symposium Hydro Engineering

maximum block volume which does not lead to overtopping. The size of the points is proportional to the volume of the blocks (maximum is B6-v1 with 526,500 m3 (upper green dot)). Furthermore, prevention of overtopping is impossible for blocks with an impact momentum higher than 45-55 GNs by reducing the reservoir water level. This is at least true for blocks with an impact direction rather straight towards the dam. SUMMARY Global warming based on climate change lead to a considerable increase of glacier melting in the sub-tropical region. Most notable the glaciers in the Cordillera Blanca of Peru showed significant reduction of the glacier masses up to 95 % (see [8]). This retreat generates the risk of GLOF due to big ice blocks, which are isolated and fall into the lagoons located at the toe of the glaciers. In the given case 18 ice blocks of the Huantsán glacier with a potential risk of falling into the downstream lagoon were investigated prior identification by the means of site inspections and satellite images. Based on [10] it was shown, that the identified blocks represent a risk of detachment due to the very steep slopes and the high impact heights (up to 1800 m) as well as the mean annual temperature in the area. Based on a pre-assessment applying the excel tool from [12] six ice blocks with the most import potential risk regarding a GLOF event were selected for a detailed 2D simulation. Impact heights between 300 and 1’600 m cause an overtopping of the existing dam with wave heights between 1 and 19 m. For many cases, only a quite drastic reduction of the operational water level, which also would mean to abandon the reservoir, could prevent an overtopping. The technically feasible reduction of the FSL turns out to be approximately 18 m as higher reductions would reach levels below the intake. Even with the initial conditions of the lagoon (without dam), there would be a risk of overflow (wave heights approx. 8 m), as it would be necessary to reduce the level of Lagoon by 35.0 m, far below the level of the former moraine. As there are many other lagoons in the Cordillera Blanca with similar conditions, the risk of a GLOF event in the sub-tropical glaciers becomes more evident taking into consideration the effect of climate change, which already can be observed in the Andean Glaciers in the last decades. To insure the future operation of these reservoirs permanent observation of the glacier by the means of images (satellite and direct photography) might be realized. This also would improve the situation of the still poor database, which nowadays exist in the Andean region with respect of GLOF risk assessment. 590