--- title: "RWH: reasons and basic components" subject: "Building Technology" chapter: 12 key: "semester_6/building_technology:rwh-reasons-components" source: https://www.bctengineer.com/q/semester_6/building_technology/rwh-reasons-components site: BCT Engineering --- # RWH: reasons and basic components Building Technology · Chapter 12: Other services in building - Asked in: 2075 Bhadra, 1+1+2+2 marks - Video: [RAIN WATER HARVESTING AND RECHARGE STRUCTURES ||BUILDING SERVICES](https://www.youtube.com/watch?v=eChHKD5I0sI) — BHAVIKA PATEL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/rwh-reasons-components - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12 ## Question Explain rainwater harvesting, write down its reasons behind it and list out its basic components. Illustrate components of the rooftop rain water harvesting system. ## Short answer Rainwater harvesting (RWH) collects roof rain for use or recharge, addresses water scarcity, irregular supply, runoff, and provides soft water cheaply. Basic components: roof catchment, gutters, down pipes, first-flush diverter, filter, storage tank, recharge pit. A rooftop system includes a clean roof, meshed gutters, PVC down pipes, first-flush device, gravel/sand/charcoal filter, and a tank sized for demand with overflow to recharge. ## Answer #### Rainwater harvesting **Rainwater harvesting (RWH)** is the collection and storage of rain falling on roofs or ground for later use, or for recharging groundwater, instead of letting it run off. #### Reasons - Water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley. - Poor or irregular piped water supply; reduces dependence on tankers and groundwater pumping. - Reduces surface runoff, urban flooding and soil erosion. - Rain water is soft and nearly free of salts; cheap and simple to collect. - Saves cost of water and energy; helps groundwater recharge. #### Basic components Catchment (roof), gutters, down pipes, first-flush device, filter, storage tank (and recharge pit for overflow). #### Components of the rooftop system 1. **Catchment:** the roof (RCC, tiles, GI sheet) that receives rain; clean and non-toxic surface. 2. **Gutters:** half-round channels at roof edge (GI, PVC, aluminium) with slope about 1 in 100 leading water to the down pipe; fitted with mesh to stop leaves. 3. **Down-take (conveyance) pipes:** PVC pipes (75-100 mm) taking water from gutters to storage. 4. **First-flush device:** diverts the first dirty rain (first 10-15 minutes or about 0.5-1 mm of rain) containing dust, bird droppings and leaves to waste. 5. **Filter unit:** layers of coarse sand, gravel and charcoal (or mesh/sand filter) to remove suspended matter. 6. **Storage tank:** underground or overhead tank (RCC, ferrocement, plastic) sized for demand and rainfall; with inlet, overflow, outlet, vent, and cleaning provision. 7. **Recharge structure (optional):** recharge pit, trench, or well for the overflow. 8. **Disinfection / delivery:** chlorination or UV before drinking use; pump and distribution. ```text \ roof (catchment) / ===+====================+=== |gutter | v | down pipe | | | [first flush]--> waste | | [ filter: gravel/sand/charcoal ] | [ storage tank ]--overflow--> recharge pit | pump -> use ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does rainwater harvesting (RWH) mean? A. Treating sewage so it can be reused for flushing B. Collecting and storing rain from roofs or ground for later use or groundwater recharge instead of letting it run off C. Pumping groundwater to the roof tank during the rainy season D. Draining rain quickly away from the building through storm drains Answer: B. RWH is the collection and storage of rain for later use or for recharging groundwater, rather than letting it run off. (Key point: Definition of RWH) 2. Which is a valid reason for adopting RWH in urban areas such as the Kathmandu valley? A. It increases surface runoff so drains stay clean B. Rain water is hard and rich in salts, so it is ideal for boilers C. It removes the need for any storage tank D. Water scarcity and falling groundwater levels, and it reduces urban flooding and soil erosion Answer: D. RWH answers scarcity and falling water tables, and by reducing runoff it also lessens flooding and erosion. (Key point: Reasons for RWH) 3. Which sequence lists the basic RWH components in the order water flows? A. Catchment, gutters, down pipes, first-flush device, filter, storage tank B. Storage tank, filter, gutters, catchment, first-flush device C. Gutters, catchment, filter, first-flush device, down pipes D. Catchment, filter, first-flush device, gutters, tank Answer: A. Water falls on the catchment, runs along gutters and down pipes, passes the first-flush device and filter, then is stored in the tank. (Key point: Components in sequence) 4. What is the recommended arrangement of roof gutters? A. Level channels with no slope so water stands in them B. Half-round channels sloping about 1 in 100 towards the down pipe, with mesh to stop leaves C. Closed pipes with a slope of 1 in 2 D. Flat concrete drains sloping away from the down pipe Answer: B. Gutters are half-round channels with about 1 in 100 slope leading to the down pipe, fitted with mesh against leaves. (Key point: Gutter details) ## Related in Building Technology - [Components of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-components) — Building Technology, chapter 12, asked 2× - [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3× - [Types of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-types) — Building Technology, chapter 12 - [Short note: Rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arainwater-harvesting-note) — Building Technology, chapter 12, asked 5× - [Benefits of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-benefits) — Building Technology, chapter 12 - [Principles of laying out water supply system](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-principles) — Building Technology, chapter 12 ## Related in other subjects - [Function of basic components of a digital communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adcs-basic-components) — Communication Systems, chapter 1 - [Typical water supply system and components](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atypical-ws-system-components) — Water Supply Engineering, chapter 1, asked 4× - [Feed water heating combined circuit and reasons](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-reasons) — Power Plant Equipment, chapter 8 - [Objectives of water supply and components](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aobjectives-ws-rural-diagram) — Water Supply Engineering, chapter 1, asked 3×