--- title: "Hydraulic design of tunnel; alignment criteria" subject: "Hydropower Engineering" chapter: 6 key: "semester_7/hydropower_engineering:tunnel-hydraulic-design-alignment-selection" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-hydraulic-design-alignment-selection site: BCT Engineering --- # Hydraulic design of tunnel; alignment criteria Hydropower Engineering · Chapter 6: Water Conveyance Structures - Asked in: 2070 Chaitra, 2+2 marks - Video: [HYDRAULIC DESIGN OF HYDROPOWER](https://www.youtube.com/watch?v=uQq_hdIgzCc) — 361° ENGINEERING BENCHMARK - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-hydraulic-design-alignment-selection - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6 ## Question What do you mean by hydraulic design of a tunnel? Explain the selection criteria of tunnel alignment. ## Short answer Hydraulic design means choosing tunnel shape, size, and slope for the design discharge with acceptable velocity (1.5-4 m/s for lined) and head loss, using Manning or Darcy-Weisbach; free-flow tunnels need air space, pressure tunnels need HGL above crown. Alignment selection favors the shortest straight route through good rock with adequate cover to resist internal pressure, stable portals, and crossing faults at right angles. ## Answer #### Hydraulic design of a tunnel It is the selection of the **shape, size and slope** of the tunnel so that the design discharge flows with an acceptable velocity and head loss, and so that the cost is the minimum. 1. Determine the design discharge $Q$ and select the type: free-flow (non-pressure) or pressure tunnel. 2. Choose the velocity: about 1.5-2.5 m/s for unlined and 2.5-4 m/s for lined tunnels (up to 5-6 m/s for pressure tunnels). 3. Size: $A=Q/V$; for circular, $D=\sqrt{4A/\pi}$. The economic diameter is found by optimisation (cost of the tunnel against the value of the energy lost). 4. Head loss: Manning $V=\frac{1}{n}R^{2/3}S^{1/2}$ ($n=0.013-0.015$ concrete, $0.03-0.04$ unlined rock) or Darcy-Weisbach; add the local losses (entry, bends, transitions). 5. For a free-flow tunnel, provide an air space of 15-20% of the area, a bed slope of 1:500-1:1000, and keep $Fr$ below 0.7 (not near 1) to avoid unstable flow. 6. For a pressure tunnel, the hydraulic grade line (including water hammer/surge) must stay above the crown, with a minimum rock cover. #### Selection of tunnel alignment - **Shortest and straightest** route possible (lowest cost and head loss), with gentle bends. - **Good geology**: avoid faults, shear zones, weak and water-bearing rock and landslide areas; cross the weak zones at right angles; avoid squeezing and swelling rock. - **Adequate rock cover** so that the internal pressure cannot burst the rock: the vertical rock cover must satisfy the Norwegian criterion $C_{RM}\ge\dfrac{h_s\gamma_w F}{\gamma_r\cos\beta}$ ($h_s$ = static head, $F\approx1.3$, $\beta$ = slope of the ground). - **Portals and adits**: stable slopes at the portals and adits at suitable intervals for faster construction; access roads. - **Ground water and inflow**: avoid high water pressure and heavy inflow zones. - **Connection with the intake, surge tank and powerhouse** so that the layout (surge shaft and penstock) is simple, with a sufficient head and low cost. - **Environment and safety**: avoid damage to settlements and existing structures. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the hydraulic design of a tunnel? A. Designing the powerhouse B. Choosing the explosives C. Selecting rock bolts D. Selecting shape, size and slope so Q flows with acceptable velocity and head loss at minimum cost Answer: D. It sizes the conduit hydraulically and economically. (Key point: Meaning) 2. How is the economic diameter found? A. By choosing the largest diameter B. By choosing the smallest diameter C. From rock cover D. By optimising tunnel cost against the value of energy lost Answer: D. Larger diameter costs more but loses less energy; the optimum balances them. (Key point: Economic diameter) 3. Which is a design provision for a free-flow tunnel? A. Full flow with Fr = 1 B. Air space of 15-20% of area and Fr below 0.7 C. Slope of 1:5 D. No air space Answer: B. Air space and Fr away from 1 prevent unstable flow. (Key point: Free-flow tunnel) 4. What does the Norwegian criterion check? A. Number of adits B. Tunnel velocity C. Vertical rock cover sufficient that internal pressure cannot burst the rock D. Shotcrete thickness Answer: C. C_RM >= hs gw F/(gr cos b) ensures rock cover resists internal water pressure. (Key point: Rock cover) 5. How should an alignment cross faults and shear zones? A. Parallel and close B. At right angles, minimising the length in weak rock C. Avoid any survey D. Along the fault Answer: B. Crossing at right angles shortens the exposure to weak ground. 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