--- title: "Spillway need; chute, shaft; ski-jump conditions" subject: "Hydropower Engineering" chapter: 4 key: "semester_7/hydropower_engineering:spillway-chute-shaft-skijump-conditions" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-chute-shaft-skijump-conditions site: BCT Engineering --- # Spillway need; chute, shaft; ski-jump conditions Hydropower Engineering · Chapter 4: Headworks of Storage Plants - Asked in: 2079 Bhadra, 1+4+1 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/spillway-chute-shaft-skijump-conditions - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4 ## Question Why is a spillway provided in a dam? Mention with neat sketches the conditions of providing a chute and shaft spillway. In which conditions is a ski-jump type energy dissipater provided below a spillway? ## Short answer A spillway prevents dam overtopping by passing excess flood. A chute spillway suits earth/rockfill dams on side saddles with good foundation. A shaft spillway fits narrow gorges with rock abutments and a reusable diversion tunnel. A ski-jump dissipator is used when tailwater is low, the dam is high, and downstream rock is sound. ## Answer #### Why a spillway is provided A reservoir cannot store unlimited flood water. A spillway passes the excess flood safely to the river downstream, so the dam is not overtopped or damaged (this matters most for an earth or rockfill dam, which would be breached by overtopping). #### Conditions for a chute (trough) spillway - The dam is of earth or rockfill type, so the spillway cannot be built over the dam body; it is a separate structure on a side abutment. - The terrain has a gentle saddle or side-hill, so a long open channel on a firm foundation is economical. - The foundation is good (rock or stiff soil), and the chute slope is steeper than the critical slope so that the flow stays supercritical. - Large floods and wide valley sections, where the channel can easily be widened. ```text reservoir control weir ~~~~~~~~~~\___ \___ chute (steep open channel) \___ stilling basin \______ river ``` #### Conditions for a shaft spillway - Narrow valley with steep rock abutments, so there is no room for a chute or an overflow section. - A diversion tunnel (built during construction) can be reused as the discharge tunnel, saving cost. - Moderate floods, sound rock for the shaft and tunnel, and little floating debris (or trash booms are provided). ```text ~~~~\____/~~~~ | | vertical shaft | | \__|____ horizontal tunnel to the river ``` #### Ski-jump energy dissipator A ski-jump (flip) bucket is provided below a spillway when: - the **tail water depth is too low** (much less than the sequent depth), so a hydraulic jump cannot form at the toe without a very deep and costly stilling basin; - the dam is **high** and the water arrives with a **large head and velocity**, so the jet can be thrown well clear of the dam toe; - the **rock downstream is sound and erosion-resistant** (massive, not heavily jointed), so the scour hole formed by the jet does not endanger the dam; - the jet can land far enough from the dam, in a deep plunge pool or a wide river, where its energy is dissipated in the air and in the pool. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a spillway especially essential for earth and rockfill dams? A. Overtopping would breach them B. They leak more C. They cannot store water D. They have no foundation Answer: A. Earth and rockfill dams are eroded and breached by overtopping, so floods must be passed by a spillway. (Key point: Why a spillway) 2. Which condition favours a chute spillway? A. Earth or rockfill dam with a side saddle and good foundation B. Narrow gorge with no side space C. Very small floods in a deep valley D. Soft clay foundation Answer: A. A chute is a separate side-channel structure on firm ground, used where the dam body cannot carry the spillway. (Key point: Chute conditions) 3. Which condition favours a shaft spillway? A. Narrow valley with steep rock abutments and a reusable diversion tunnel B. Wide valley with gentle slopes C. Large debris load with no trash booms D. Soft foundation with no rock Answer: A. A shaft needs little space and the diversion tunnel can serve as outlet, saving cost. (Key point: Shaft conditions) 4. When is a ski-jump dissipator provided? A. High dam, large head, deep plunge pool or channel and sound rock foundation B. Low dam with small head and weak soil C. Where tail water is very high above the crest D. Canal falls with small discharge Answer: A. The jet is thrown clear of the toe and dissipates energy in air and the plunge pool, needing sound rock to resist scour. (Key point: Ski-jump use) ## Related in Hydropower Engineering - [Discharge through chute spillway ogee crest](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-chute-discharge-ogee-250m) — Hydropower Engineering, chapter 4 - [Spillway definition; siphon and shaft spillway](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-define-siphon-shaft) — Hydropower Engineering, chapter 4 - [Hydraulic jump stilling basin at spillway toe](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astilling-basin-hydraulic-jump-80cumec-2070) — Hydropower Engineering, chapter 4 - [Advantages and suitability of spillway types](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-types-advantages-suitability) — Hydropower Engineering, chapter 4 - [Discharge from hydraulic jump depths 1.5 m to 4 m](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astilling-basin-hydraulic-jump-depth-change-120m) — Hydropower Engineering, chapter 4 - [Is a hydraulic jump stilling basin required?](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astilling-basin-required-check-100cumec) — Hydropower Engineering, chapter 4 ## Related in other subjects - [Jump and chute block with hump control](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ajump-chute-block-hump-critical-5m-channel) — Hydraulics, chapter 9 - [Jump at spillway toe; repelled and submerged jump](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ajump-at-toe-of-spillway-repelled-submerged) — Hydraulics, chapter 9 - [Superelevation and conditions for providing it](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asuper-define-conditions) — Transportation Engineering I, chapter 3 - [Jump upstream of a low dam](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ajump-low-dam-uniform-depth-0-5m) — Hydraulics, chapter 9