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Energy Dissipation Downstream of Labyrinth Weirs

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Hydraulic engineering is one of the most challenging fields in civil engineering and becomes more and more important in times of climate and demographic change. Particularly, the flow around structures is of a complex nature and flow phenomena are still not fully understood.
The 7th International Symposium on Hydraulic Structures took place in Aachen, Germany, 15 – 18 May 2018.
This symposium aimed to bring together experts working in the specialized field of hydraulic structure design from both, research and practice communities. It has provided a forum for presentation and discussion of recent advances in knowledge as well as future needs.

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Energy Dissipation Downstream of Labyrinth Weirs

  1. 1. Aachen, 15-18 May 2018 Jennifer Merkel Fabian Belzner Michael Gebhardt 7th International Symposium on Hydraulic Structures Energy Dissipation Downstream of Labyrinth Weirs Carsten Thorenz
  2. 2. www.baw.de |  Labyrinth Weirs are mainly used to increase the capacity of existing spillways  Energy dissipation takes place with the help of a - Ski-jump Aim of the Study (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 2
  3. 3. www.baw.de |  Labyrinth Weirs are mainly used to increase the capacity of existing spillways  Energy dissipation takes place with the help of a - Ski-jump - Stepped spillway Aim of the Study (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 3
  4. 4. www.baw.de |  Labyrinth Weirs are mainly used to increase the capacity of existing spillways  Energy dissipation takes place with the help of a - Ski-jump - Stepped spillway - Stilling basin  baffles,  end sills,  chute blocks or  lateral expansion Aim of the Study (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 4
  5. 5. www.baw.de |  Is there enough energy dissipation during weir overflow?  Is a stilling basin needed?  Which form of hydraulic jump occurs for specific discharges and downstream water levels?  Design of the stilling basin? Aim of the Study (II) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 5
  6. 6. www.baw.de | Flume  Length: 18.00 m  Width (W): 0.60 m  Height: 1.20 m  Maximum discharge: 420 l/s (q = 0.70 m²/s) Labyrinth Weir Model  Weir height (P): 0.255 m  Key width (Wi = Wo): 0.125 m  Length in flow direction (B): 0.50 m  Total developed length (L): 2.60 m  Crest: half rounded  L/W = 4.33 Laboratory Model 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 6
  7. 7. www.baw.de | Tested series  First: without stilling basin  Specific discharges: 0.02 m²/s ≤ q ≤ 0.30 m²/s  Downstream water levels: 0.05 m ≤ yt ≤ 0.30 m Classification of Hydraulic Jumps (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 7 Overflow length Supercritical section Surface roller Supercritical depth Conjugate depth
  8. 8. www.baw.de |17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 8 Classification of Hydraulic Jumps (II) No hydraulic jump (q = 0.02 m²/s ; y2 = 0.10 m) Hydraulic jump (q = 0.04 m²/s ; y2 = 0.05 m) Submerged hydraulic jump (q = 0.04 m²/s ; y2 = 0.15 m) Undular jump (q = 0.08 m²/s ; y2 = 0.05 m)
  9. 9. www.baw.de |  Energy losses during weir overflow by White Energy Losses during Weir Overflow (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 9 𝐻1 𝑦𝑐 = 2 𝑎 + ∆𝑧 𝑦𝑐 + 𝑏 + 𝑎 + ∆𝑧 𝑦𝑐 + 𝑏 2 4 White tested model a 1.06 -0.85 b 1.5 6.17 R - 0.88
  10. 10. www.baw.de |  Energy losses during weir overflow by White 𝐻1 𝑦𝑐 = 2 𝑎 + ∆𝑧 𝑦𝑐 + 𝑏 + 𝑎 + ∆𝑧 𝑦𝑐 + 𝑏 2 4 Energy Losses during Weir Overflow (I) 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 10 White tested model a 1.06 -0.85 b 1.5 6.17 R - 0.88 Hv,Labyrinth Weir >> Hv,linear weir
  11. 11. www.baw.de |  0.02 m²/s ≤ q ≤ 0.12 m²/s - Energy dissipation takes place in the outlet key section - Nappes are aligned partially laterally and partially in flow direction  0.14 m²/s ≤ q ≤ 0.30 m²/s - Local submergence effects (Crookston and Tullis 2011) in the outlet key section - Nappes are only aligned in flow direction 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 11 Energy Losses during Weir Overflow (II) q = 0.18 m²/s q = 0.08 m²/s
  12. 12. www.baw.de |  Depth:  Tailwater ratio:  Length: Preliminary Design of the Stilling Basin Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel 17.05.2018 Page 12 ε = 𝑦𝑡 + δ 𝑦2 δ = 1.05 ÷ 1.25 ∙ 𝑦2 −𝑦𝑡 𝐿 𝑠 = 6 ∙ (𝑦2 − 𝑦1) (a) (b) (c) δ = 0.5∙P Ls = 3.0∙P
  13. 13. www.baw.de |  Different lengths of the stilling basin were tested  Depth of the stilling basin: δ = 0.13 m  For all tested series, a submerged hydraulic jump was observed  Second flow transition as a result of the vertical sill, regardless of its length sloped sill Flow Characteristic in a Stilling Basin (I) Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel 17.05.2018 Page 13 q = 0.20 m²/s , Ls = 0.75 m q = 0.20 m²/s , Ls = 0.90 m q = 0.20 m²/s , Ls = 1.30 m
  14. 14. www.baw.de |  Wool threads represents the flow at the end of the stilling basin  Vertical Sill: - Flow seperates at the top of the sill  Turbulent fluidized areas  Sloped Sill: - Inclination of 1:3 allows a gradual transition - No second flow transition 17.05.2018 Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel Page 14 Flow Characteristic in a Stilling Basin (II) Sloped Sill, q = 0.16 m²/s, Ls = 0.75 m, δ = 0.13 m Vertical Sill, q = 0.16 m²/s, Ls = 0.75 m, δ = 0.13 m
  15. 15. www.baw.de |  Energy dissipation at Labyrinth Weirs differs from other weir types  Three dimensional flow with air entrainment and shockwaves  For the tested series no stilling basin would be necessary  However, due to erosion - Length is equal to 3.5 the weir height - Depth is equal to 0.5 the weir height  Further Questions: - Is a smaller basin depth possible? - Can the design parameters be generalized? Conclusion Energy Dissipation Downstream of Labyrinth Weirs | Jennifer Merkel 17.05.2018 Page 15
  16. 16. Bundesanstalt für Wasserbau 76187 Karlsruhe, Germany www.baw.de Thank you!

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