--- title: "Forebay design procedure" subject: "Hydropower Engineering" chapter: 6 key: "semester_7/hydropower_engineering:forebay-design-procedure" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/forebay-design-procedure site: BCT Engineering --- # Forebay design procedure Hydropower Engineering · Chapter 6: Water Conveyance Structures - Asked in: 2081 Baishakh, 4 marks; 2073 Shrawan, 3 marks (asked 2 times) - Video: [Design of Forebay || Hydropower Engineering Numerical](https://www.youtube.com/watch?v=wBR0XxeCHEw) — OPengineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/forebay-design-procedure - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6 ## Question Describe the design procedure of a forebay (write the procedure to compute the dimensions of the forebay and the equations used for such purpose) with a neat sketch. ## Short answer Volume V = Q·t (t≈2–3 min). Area A = Q/v with v ≤ 0.3 m/s. Depth H = S + D + clearance; submergence S = C·Vp·√D (C=0.5434 symmetrical, 0.7245 asymmetrical). Width B = A/H, length L = V/A (L/B~2–4). Spillway Q = C·L·h^(3/2) with C≈1.7 for full turbine flow. Freeboard and flushing gate. ## Answer A **forebay** is a small reservoir at the end of the headrace (canal or tunnel) from which the penstock takes off. It stores water to meet sudden load demand, provides the transition from open-channel flow to pressure flow, and provides a place for the trash rack, the spillway and sediment flushing. #### Design procedure 1. **Decide the discharge and retention time**: $Q$ = turbine discharge; the retention time $t$ is usually 2-3 minutes at full load. 2. **Volume of storage**: $V=Q\,t$ 3. **Cross-section**: limit the velocity in the forebay to $v\le0.2-0.3$ m/s, so that sediment settles and the flow is calm: $A=Q/v$ 4. **Water depth** from the submergence of the penstock intake: - velocity in the penstock, $V_p=\dfrac{Q}{n\,\frac{\pi}{4}D^2}$ ($n$ = number of penstocks) - Gordon's formula $S=C\,V_p\sqrt D$ ($C=0.5434$ for a symmetrical approach and 0.7245 for an asymmetrical approach) - depth $H=S+D+$ bottom clearance (0.3-0.5 m) 5. **Width and length**: $B=\dfrac{A}{H}$ and $L=\dfrac{V}{A}$ (check that $L/B$ is large enough, around 2-4, with a gradual expansion from the headrace). 6. **Spillway**: to pass the full design discharge when the turbines trip, $Q=C\,L_s\,h^{3/2}$ with $C\approx1.7$ for a broad crest and $h=0.3-0.5$ m. 7. **Free board** (0.5 m), a bottom slope towards a **flushing gate**, a trash rack at the intake and an air vent behind the penstock gate. 8. **Check** the entrance loss, and set the top of the forebay wall at full supply level + surge + free board. #### Sketch ```text PLAN headrace ====> \ forebay /|==> penstock transition| rack | ====> / |==> penstock spillway | flush gate v to river SECTION FSL ~~~~~~~~~~~~~~~~~~~~~~~~~~ | S |rack|gate | |####| ___ penstock crown bed |_____________|####|_/ D 0.5 m clearance ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the forebay storage volume determined? A. V = Q t with retention time of 2-3 minutes at full load B. V = Q/v C. V = H B L from the penstock diameter D. V = Q h^(3/2) Answer: A. The forebay must hold enough water to meet sudden load demand, so V = Q x t with t about 2-3 min. (Key point: Forebay storage volume) 2. Why is the forebay velocity limited to 0.2-0.3 m/s? A. To maximise head loss B. So flow is calm and sediment can settle; A = Q/v C. To keep the penstock full D. To prevent water hammer Answer: B. Low velocity gives calm flow, lets sediment settle, and A = Q/v sets the cross-sectional area. (Key point: Cross-section from velocity) 3. How is the forebay water depth fixed? A. H = S + D + bottom clearance, where S = C V_p sqrt(D) is the submergence of the penstock intake B. H = Q/(vB) C. H = S - D D. H = 3 D regardless Answer: A. Submergence S from Gordon's formula prevents vortex and air entry; adding the penstock diameter and 0.3-0.5 m clearance gives the depth. (Key point: Depth from Gordon's formula) 4. What is the forebay spillway designed to pass? A. 10% of the turbine discharge B. The full design discharge when the turbines trip, using Q = C L h^(3/2) C. Only the flood flow of the river D. Flushing discharge only Answer: B. If turbines trip, all incoming flow must be spilled safely at a small head h of 0.3-0.5 m. (Key point: Spillway sizing) ## Related in Hydropower Engineering - [Forebay design considerations; design for 12 m³/s](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-design-considerations-12cumec) — Hydropower Engineering, chapter 6 - [Forebay design with spillway length (14 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-design-14cumec-spillway-length) — Hydropower Engineering, chapter 6 - [Forebay design (14.5 m³/s, two penstocks)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-design-14-5cumec) — Hydropower Engineering, chapter 6 - [Forebay design (20 m³/s, 300 m penstock)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-design-20cumec-300m) — Hydropower Engineering, chapter 6 - [Forebay design (15 m³/s, 4 minutes)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-design-15cumec-4min) — Hydropower Engineering, chapter 6 - [Forebay dimension for 3 minutes storage](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aforebay-dimension-3min-storage-2-24m) — Hydropower Engineering, chapter 6 ## Related in other subjects - [Grit chamber: purpose, construction, design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-chamber-purpose-design) — Sanitary Engineering, chapter 7, asked 4× - [VIP latrine: working and design procedure](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-latrine-working-design) — Sanitary Engineering, chapter 9, asked 2× - [Design procedure of steel purlin](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Apurlin-design-procedure) — Design of Steel and Timber Structure, chapter 8, asked 2× - [Street lighting scheme design procedure](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Astreet-lighting-design-procedure) — Industrial Power Distribution and Illumination, chapter 8