Hazard assessment and outburst flood estimation of naturally dammed lakes in Central Asia

The studies on naturally dammed lakes in Central Asia involved studies in selected key locations in the Russian Altai Mountains and mountain ranges in Kazakhstan and Uzbekistan in addition to compilations of inventories of potentially hazardous lakes in the region. Hazard assessments considering modeled potential outburst floods magnitudes were carried out and forwarded to local authorities. For most of the key locations, water level observations throughout the year, analysis of dam conditions and conditions of percolating water passing through the dams by measurements of electric conductivity and temperature were investigated. At different locations, various indicators of instability like incised channel downstream of springs within the dam or over-steepened dam-slopes were found. Geo-electric surveys confirm previous estimations of permafrost distribution within several dams. The inventories of naturally dammed lakes are based on new data from remote sensing analysis and compilation of literature as well as personal observation reports. Based on first assessments of dam stability, potentially hazardous lakes were identified for further detailed studies in the near future. During the modeling approaches for the estimation potential outburst flood magnitudes, the natural range of dam and lake parameters like grain-size distribution, permafrost-content, geometry and lake level limits the application of process based approaches. Consequently, a new approach based on normalized hydrograph patterns and their extrapolation considering individual lake and dam dimension was developed. While the outburst floods themselves provide no significant danger for the inundation of settlements due to relatively low water levels, local infrastructure like bridges and roads are in danger to be destroyed. Also the formation of mudflows initiated by outburst floods is should be considered in risk assessment beyond focusing on the water flow only. Engineering approaches by artificial spillway cutting or enlargement to limit the volume of the lakes is found to be counter-productive. As consequence of such measures, the permafrost as main element of dam stability becomes exposed and its decay might lead to a collapse of entire dam structures. In addition, dendro-chronological and –ecological studies at key locations in the Altai Mountains were carried out. The first data reveal a promising potential for the estimation of the age and lateral growth of the lakes by their age and recent death due to inundation.