Restoration of the stilling basin of the Drtijs']jscica dam

The Drtijscica dam and the reservoir represent the most important part of the flood protection system in the motorway section to the northeast of the city of Ljubljana. The reservoir is designed as a relief system for retention of peak discharges of flood waves, enabling higher flood safety for the entire area along the motorway. The dam is an earth dam with a central clay core, having a height of 18 m and a crest length of 256 m. The evacuation of water through the dam cross-section is through a shaft overspill and the bottom outlet into the channel of the Drtijscica stream. In September, 2010 a flood event occurred, when the high waters were in the range of the designed maximum flow. During the evacuation of flood water extensive erosion-induced damage to the stilling basin of the bottom outlet and the right part of the toe of the dam occurred. After the conditions steadied, the damaged structures were rehabilitated in the matter of priorities, together with the proposal of minor improvements of the original design, where this proved justified. After the event, an analysis of the conditions during the flood event was performed, where it was found that the damages were the result of several factors: firstly, there were exceptional operational conditions that exceeded the normal operational conditions, secondly, the end-part of the bottom outlet was inadequately designed, and, thirdly, the hydraulic conditions downstream of the dam were inadequate. For determination of the final solution a comprehensive redesign of the evacuation structures on the dam will be needed, taking into account the conclusions of the analysis of the 2010 event.