In September 2017, A Dam under construction in Laos, which is located 90 km upstream of the Nam Nigep 1 Hydropower Project (NNP1), was breached, and around 500,000 m³ water in the reservoir under impounding was flown to the downstream river course. The highly turbid water impacted on the ecology of the downstream river course and around 700,000 m³ sediment consisting of clay and silt was accumulated in the re-regulation dam reservoir of the NNP1, which has just completed at that time. It was feared that when the NNP1 starts the initial impounding of the main dam reservoir which is located upstream of the re-regulation dam while we'll keep environmental flow during the impounding, the anaerobic deteriorated clay and silt are expected to be released with the environmental flow and may impact on the ecology of downstream river course. In order to mitigate this harmful impact, the NNP1 decided to implement sediment flushing in early dry season of 2017, when the NNP1 River has a certain volume of the flow, before starting the initial impounding to be scheduled in May 2018. This paper presents the dam break impact on the downstream river course and resultant sediment management at the downstream NNP1 dam.