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CASE STUDY OF WADI LABA AND WADI MAI ULE (ELWDP – ERITREA)

CASE STUDY OF WADI LABA AND WADI MAI ULE (ELWDP – ERITREA). Lesson 5. ianmcanderson@aol.com. Wadi Laba. Initial Areas for examination/concern: Sediment Basin/Gravel Trap She’eb Khethin Culvert Breaching Bund Capacity of Main Intake Supply to She’eb Khethin Area Supply to Id Abay.

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CASE STUDY OF WADI LABA AND WADI MAI ULE (ELWDP – ERITREA)

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  1. CASE STUDY OF WADI LABA AND WADI MAI ULE (ELWDP – ERITREA) Lesson 5 ianmcanderson@aol.com

  2. Wadi Laba • Initial Areas for examination/concern: • Sediment Basin/Gravel Trap • She’eb Khethin Culvert • Breaching Bund • Capacity of Main Intake • Supply to She’eb Khethin Area • Supply to Id Abay

  3. Wadi Labi Gravel Trap

  4. Gravel Trap • Planned to clean after every big flood and about 10 times per year • Floods in 2004 = 15; 2005 = 39 • Cleaned 3 times since 2003 • Cost this year Nfa 600,000 or Nfa 170/ SFA member for one cleaning • Farmers want no cleaning & silt to field • Prepared for extra canal cleaning and farm levelling

  5. She’eb Kethin Culvert - Both Barrels blocked at outlet after cleaning of d/s canal

  6. She’eb Kethin Culvert • Designed for 7.5 cumecs • Only works for medium floods and above • Farmers not getting enough water – max 4 cumecs & for less period • Downstream weir levels too high – only 0.3 m below culvert invert compared with design of 1.03 m = 0.7 m too high • Silted up due as velocity drops • One barrel completely blocked; other seasonally blocked • Farmers built temporary Agim to get enough water

  7. Temporary Agim for She’eb Khethin

  8. Main Intake • Designed for 35 cumecs • Takes full flow with zero flow over weir • Flood flow to be restricted to 50 cumecs • In practice no gate operation so Q rises to 83 cumecs • Thus can get enough water into intake for command area

  9. Breaching Bund Breaks about once a year

  10. Impact of Breaching of Bund • Reduced surcharge on intake • Force of impact breaks She’eb Khethin temporary Agim • Water available to Ide Abay downstream • Increased O & M cost • Greatest impact on She’eb Khethin

  11. What do farmers Want • Solution to temporary Agim • Reliable supply for She’eb Khethin • New intake for She’eb Khethin on right bank by breaching bund (will replace culvert) • Will overcome lack of capacity of intake as supplies large area separately • Do not clean sediment basin

  12. Proposals - Immediate • Sediment Basin/Gravel Trap • Check Weir Structure in sedimentation basin • She’eb Khethin Culvert • Breaching Bund • Flow Recording

  13. Proposals – Next 12 Months • Re-examine Supply to She’eb Khethin Command area with FULL farmer involvement. • New Right bank intake • Monitor Gravel Trap and Check Weir Structure • Record flow data and monitor against flood occurrences

  14. Layout in Staff Appraisal Report 1995

  15. WADI LABA Farmers Discuss way forward and options in the immediate and longer term

  16. Wadi Mai Ule • Areas for examination/concern: • New Right Bank intake of Farmers • Breaching Bund • Capacity of Main Intake • Supply to Tiluk Area • Flood diversion canals and embankments • Current design and capacity to contain flood peaks • Liability and risk associated with the design

  17. Floods Experienced • 2004 = 6 (small: 2; Medium: 1; High; 3) • 2005 = 13 (small: 4; Medium: 6; High; 3) • Small – not over weir • Medium – over weir before breaching • High – Leads to breaching • Duration medium & large : 3 hrs

  18. Problems • Floods of short duration and reach peak quickly • Flows passing through breaching bund, scour sluice or over weir lost to all system • Capacity of gates not sufficient for short peak before breach about 1 hour • Gibathr area 300 ha – largest and not getting water • Farmers built own offtake d/stream of weir

  19. Farmers new Intake

  20. Concerns with new intake • Cannot take design flood flow • Approach flow encouraged to go to left and right bank • New channel completely blocked • Left bank of intake will not withstand large flood

  21. Inadequate left bank protection – liable to overturn and scour

  22. What Farmers Want • Reliable supply to all areas to use excess flow over weir and through bund • Increased opening downstream to 100 metres • Not to use new flood channel

  23. Proposals – Short term • Improvement of New Right Bank Farmers Offtake to withstand floods • Allow extra flood to pass down new flood channel • Flow Recording

  24. Proposals – Next 12 Months • Re-examine Supply to all Command area with FULL farmer involvement. • Monitor performance of improved Farmers offtake this flood season • Second Right bank offtake similar to farmers • Record flow data and monitor against flood occurrences

  25. WADI MAI ULE Farmers Discuss way forward and options in the immediate and longer term

  26. Next Steps - Immediate • Prepare programme for immediate works • Prepare cost estimates for works • MOA to mobilise resources to complete in one month • Force account • Hire extra machinery • Involve SFA where possible

  27. Next Steps – next 12 months • Complete design modifications with full farmer involvement • Request funding • Start construction next season

  28. END RESULT • Farmers content with final result • Can irrigate more area in good years • Suffering stream bank erosion and damage • Content to accept this • Process made Farmers Association more cohesive • Solution not ideal from Investors or Engineers viewpoint • Many lessons learnt…………

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