--- title: "Dry and wet river intakes; unstable bank" subject: "Water Supply Engineering" chapter: 5 key: "semester_5/water_supply_engineering:wet-dry-intake-unstable-bank" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/wet-dry-intake-unstable-bank site: BCT Engineering --- # Dry and wet river intakes; unstable bank Water Supply Engineering · Chapter 5: Intakes - Asked in: 2073 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/wet-dry-intake-unstable-bank - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-5 ## Question Describe the characteristics of dry and wet river intakes with neat sketch. What modification will you make if the river bank is unstable? ## Short answer Wet and dry river intakes differ in that wet intakes have flooded interiors with underwater gates operated from a bridge, while dry intakes have dry chambers with accessible valves. For unstable banks, the intake is moved into the river as an isolated tower connected by a footbridge, with deep foundations, bank protection like pitching and groynes, river training works to fix the channel, or a canal intake built away from the migrating bank. ## Answer #### Wet river intake A **wet intake** is a tower (or well) built in the river, with its interior filled with water up to river level. Ports at different levels admit water, and the intake pipe leaves from the bottom of the well. The valves are operated from the top, since they are underwater. ```text Foot bridge =====#===== Stem (control rod) | HFL ~~~~~~~~~~~~~~~~~~~~ | NWL ~~~~~~[port + screen]| <- water inside LWL ~~~~~~[port + screen]| well = river level bed ///////[port]--------| | intake pipe +---------------> to pump well / plant ``` **Operation and maintenance**: The tower is filled with water and the pipe is always flooded. Gates at each port are operated from the bridge by a stem. Screens must be raked and cleaned from above, and divers or stop-logs are used when the tower has to be examined. It is cheaper than a dry intake, but harder to maintain. #### Dry river intake A **dry intake** is a tower built on the river bank (or just inside it) whose interior is kept dry. Water enters through ports in the wall, flows into a **separate sump** by a connecting pipe, and only the valves and pipes are in the dry chamber. The operator can work inside it at any time. ```text Foot bridge ===+ | operating floor HFL ~~~~~~~~~ || gate valves ||[=]---+ NWL ~~~~~~~~ ||[=]---+--> pipe to ||[=]---+ sump/pump well LWL ~~~~~~~ || dry chamber ///////// ||____________ screens at ports river bed | (water-tight walls) ``` **Operation** - Water flows from the river through the screened ports, then through the valves in the dry chamber, to the sump or directly to the pump. - The operator selects the port at the best level and closes the others. - Valves and screens can be inspected, cleaned or repaired from inside, in dry conditions. **Merits and demerits**: Operation and maintenance are easy, but it needs stronger water-tight walls and a heavier, deeper foundation (to resist floatation when it is empty), so it costs more than a wet intake. #### Modification if the river bank is unstable - Move the intake into the river away from the bank, so it stands as an **isolated tower** (a wet or dry tower) connected to the bank by a **foot bridge**, with the intake pipe laid in a protected trench under the bed. - Provide **bank protection**: stone pitching or gabion, groynes, spurs or retaining walls upstream and downstream of the intake. - Use deep foundations (well, pile or caisson) for the tower so that scour cannot undermine it. - Fix the river course near the intake with river-training works (guide banks, training walls, spurs), or choose a straight reach where the bank is firmer; avoid the eroding face of an outer bend unless it is protected. - Use a **canal or channel intake** (a cut leading water to the intake) if the river migrates, and build the structure well away from the moving bank. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What basic modification if the bank is unstable? A. Isolated tower in the river joined by foot bridge, pipe in protected trench B. Use a spring box C. Remove ports D. Build on the bank itself Answer: A. Moving into the river avoids the unstable bank. (Key point: Unstable bank) 2. Which foundation resists undermining by scour? A. Timber mat B. Deep well, pile or caisson C. No foundation D. Shallow footing Answer: B. Deep foundations resist scour. (Key point: Foundations) 3. Which works protect the bank? A. Vent pipes B. Fence C. Stone pitching, gabion, groynes, spurs D. Screens Answer: C. They stabilise the bank near the intake. (Key point: Bank protection) 4. What distinguishes a dry intake? A. No valves B. Full of water C. Underwater valves D. Interior dry, separate sump Answer: D. Valves and pipes are in a dry chamber. (Key point: Dry intake) ## Related in Water Supply Engineering - [Wet and dry river intake](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-dry-river-intake) — Water Supply Engineering, chapter 5 - [Dry river intake with sketch](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adry-river-intake) — Water Supply Engineering, chapter 5, asked 2× ## Related in other subjects - [Peaking RoR: max power and wet/dry energy ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apeaking-ror-reservoir-250m-q25) — Hydropower Engineering, chapter 3 - [Canal discharge for dry and wet crops](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-20000-30000) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [Canal discharge for dry and wet crops (30000/40000 ha)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-30000-40000) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [When wet process is better than dry](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Awet-process-better-than-dry) — Civil Engineering Materials, chapter 5