FAQ – Dam Safety
What will be the impact downstream in the event of a dam breach at Don Sahong?
Is the foundation for the DSH power station sufficient, noting that the rock mass appears to have ???cracks????
The risk posed by a theoretical breach of the DSH embankments is low, as the embankments are relatively low (about 20m maximum height). During a significant flood event the downstream water level will be less than 10m lower than the headpond level, as water levels in the Mekong River downstream of the embankments are >10m more than dry season water levels, and the potential outflow from the Hou Sahong headpond in the event of an embankment breach will be small in comparison to the total river flows downstream during a flood event.
Owing to the relatively small impounded height and volume of the headpond, the potential outflow from the Hou Sahong headpond in the event of an embankment breach will be small in comparison to the total river flows downstream during a flood event. Any surge resulting from dam failure would be significantly dampened by the time it reaches significant population centres downstream.
The powerhouse site has had an extensive geotechnical investigation programme carried out, including 31 machine drill holes to a maximum depth of 70m totalling 810.7m, in-situ testing for compressive strength and permeability, and laboratory testing (undertaken in Hanoi by PECC1). The results demonstrated good compressive strengths averaging 50-55 MPa, and very low permeability averaging 1.5 Lugeons. The main findings were that at the powerhouse foundation level the joints sets/defects would be tight and that the foundation is very suitable for a stable powerhouse structure. Some rockbolting is expected to be required on the right bank side near the top of the excavation to maintain excavation stability. Please refer to Section 2 (Geotechnical Investigations) of the DSH Engineering Status Report provided on the MRCS website for more details.
What seismic design coefficient has been used for the DSH project?
Seismic design criteria for the project were developed from data available from the USGS NEIC database and from other records of other available project-specific investigations from other nearby hydro projects in Laos and Vietnam. The project area was found to be in an area of low seismicity. For structures critical to dam safety integral with the ground, maximum design horizontal acceleration of 0.15g is specified, and for structures above ground with moderate ductility, the horizontal design coefficient is 0.38g.