PRELIMINARY ASSESSMENT DON SAHONG SEDIMENT AND RIVER MORPHOLOGY

1. Information and Knowledge Management Programme Mekong River Commission Secretariat Technical Consultation for Don Sahong Hydropower Project 07 March 2014, Vientiane, Lao PDR PRELIMINARY ASSESSMENT DON SAHONGSEDIMENT AND RIVER MORPHOLOGY

2. Scope and Objectives of the Review • References • Environmental Impact Assessment Report (January 2013) • Engineering Status Report (September 2011) • Objectives • Evaluation of sediment data collection • Methodology to estimate Suspended Sediments and Bed Load • Sedimentation of headpond – Changes in river morphology • Evaluation of Sediment Management – Sediment Flushing • Assessment of Monitoring Effort

3. Sediment Data • Site specific field data collection is very limited • Suspended Sediments: nil • Bed Load: nil • Grain Size Distribution: 3 samples (sand from sandbars) • Petrography: 3 samples (but results not included in the report). • MRC data • Discharge at Pakse 1982-2009 (source not clearly referenced) • Suspended Sediments at Pakse 1982-2009 • Water Quality Data – Total Suspended Sediments 1985-2009 • Engineering approach for design • Estimated Suspended Sediments load: based on MRC data • Bed Load: approx 15 % of suspended sediment (sensitivity analysis) • Grain Size Distribution: derived from other rivers

4. Grain Size Distribution Suspended Sed. • Based on 3 samples from a sand bar at Hou Sahong • Literature review

5. Sediment Flushing through Turbines • About 20 % settle in the Headpond (i.e. 1.8 Mt/year ) • About 80 % will be flushed/sluiced through the Turbines (i.e. 7.5 Mt/year ) • Regular sediment flushing through turbines by increasing the flow by 100 to300 m3/s (above the 1,600 m3/s design discharge) + 150 m3/s + 300 m3/s Not understandable for steady state

6. Sediment Flushing • Sediment Flushing Methods • Sediment flushing through the turbines(possibly concentrated at night times not to influence the flow over the Khone Falls during the day) • Flushing through Low Level Outlets considered not feasible or needed • Mechanical Removal (Dredging) if proposed flushing does not work • Proposed measures at DSHHP to facilitate sediment management • Excavation/lowering of an island at the inflow section of the headpond • Skimming wall at the upstream of the headpond to keep the heavier and coarser bed load material in the main river branch (Khone Falls) • Proposed/acceptable deposition of about 2 to 3 Mt of sediment within the headpond (total volume approximately 10 Mm3) • Regular flushing through the turbines (mostly in the wet season) • Excavation / deepening of the tailrace to facilitate sediment transport away from the HPP and mixing with the main stream flow • Preparation for mechanical dredging mentioned

7. Conclusions and Recommendations • The Environmental Impact Assessment Report does not address the topics of Sediment Transport and River Morphology; only the Engineering Status Report • The chosen engineering approaches to estimate sediment related parameters are reasonable. However, actual measured data on suspended sediments and bed load would assist to confirm the design criteria. • The calculated sedimentation rates & volumes in the headpond seem reasonable. • The proposed headpond flushing method through the turbines needs further investigation and would need to be included in the plant operation procedure. • Criteria for the flushing procedure need to be established. • The height of the skimming wall at the entrance to Hou Sahong could be optimized based on actual sediment measurements. • Provisions for mechanical dredging could be included in the HPP design. • A sediment management plan proposed for the tailrace section would be useful. • A sediments and river morphology monitoring programme would need to be included in the report.

8. THANK YOU….