Flood mitigation at the downstream areas of a transboundary river

Floods in the basin of Ardas river, a transboundary river that crosses Bulgaria and has its outlet in Greece, have often created havoc and caused millions of damage, especially in downstream Greek areas, which also repeatedly receive unregulated flow from upstream dams. More specifically, Ardas River, a tributary of Evros river, flows for 214 km in Bulgaria and for only 39 km in Greece and its catchment stretches for 5 250 km2 (94% of the total area) in Bulgaria and for 350 km2 (6% of the total area) in Greece. Three large dams along the river have been constructed in Bulgaria (Kardzhaly, Studen Kladenets and Ivaylovgrad), the last one, Ivaylovgrad dam, in short distance (approx. 15 km) from the transnational borders. During heavy rain, excessive flow from the Ivaylovgrad dam is often released downstream, in order to relieve the reservoir that is kept at maximum level for energy production reasons. As a result, the downstream areas, also affected by the same heavy rain events, need to regulate large flows, often with inadequate response time and relevant means. The present study presents an approach to estimate flood water levels in the Greek territory, caused by both rain events and releases from the upstream dam. For this purpose the study area was divided into three subbasins and the corresponding flood volumes were calculated using several methodologies. The paper concludes with a series of structural and non-structural measures that are suggested to be taken to confront and mitigate flood effects.