--- title: "Why RWH is necessary; methods" subject: "Building Technology" chapter: 12 key: "semester_6/building_technology:rwh-why-methods" source: https://www.bctengineer.com/q/semester_6/building_technology/rwh-why-methods site: BCT Engineering --- # Why RWH is necessary; methods Building Technology · Chapter 12: Other services in building - Asked in: 2070 Bhadra, 4+4 marks - Video: [Complete Rainwater Harvesting System Tour | PerfectWater Installation Explained](https://www.youtube.com/watch?v=tiOAsM-mIKE) — PerfectWater - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/rwh-why-methods - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12 ## Question Why is rain water harvesting necessary in a building? Explain any method for harvesting rain waters. ## Short answer RWH is necessary due to water scarcity, irregular supply, runoff reduction, and its provision of soft, low-cost water. A common method is rooftop storage: rain from a clean roof is collected via gutters and down pipes, passed through a first-flush diverter and a gravel/sand/charcoal filter, and stored in a tank with overflow to a recharge pit. ## Answer #### Why necessary - Water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley. - Poor or irregular piped water supply; RWH reduces dependence on tankers and groundwater pumping. - Reduces surface runoff, urban flooding and soil erosion. - Rainwater is soft and nearly free of salts; it is cheap and simple to collect. - Saves the cost of water and energy; helps groundwater recharge. #### Method: rooftop harvesting with storage Rain falling on the roof is collected and stored for use. Steps: 1. The roof (catchment) is kept clean; gutters are fixed at the eaves with a slope of about 1:100. 2. Water flows through down pipes (75-100 mm). 3. A first-flush diverter discards the initial dirty water. 4. Water passes through a filter (gravel, sand, charcoal). 5. Clean water is stored in a tank (underground or overhead; RCC, plastic or ferrocement), with overflow to a recharge pit. 6. Disinfection (chlorine) if the water is used for drinking. ```text roof -> gutter -> down pipe -> first flush -> filter -> storage tank -> use ; overflow -> recharge pit ``` **Water harvested per year** = annual rainfall x roof area x runoff coefficient (0.8-0.9). The tank is sized from the daily demand and the length of the dry period. **Other method (groundwater recharge):** filtered roof runoff is led to a recharge pit or trench (1-2 m wide, 2-3 m deep) filled with boulders, gravel and sand, or to a recharge well, so that it soaks into the aquifer. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is RWH necessary in a building? A. To raise the surface runoff from the plot B. To make rain water hard so it lasts longer C. To deal with scarce or irregular supply and falling groundwater while cutting runoff, flooding and water cost D. To avoid ever needing a sump or tank Answer: C. RWH supplements scarce or irregular supply, helps recharge groundwater, reduces runoff and flooding, and saves water and energy cost. (Key point: Why RWH is necessary) 2. In the rooftop storage method, why is a first-flush diverter placed before the filter? A. To discard the initial dirty water that washes dust, droppings and leaves off the roof B. To increase water pressure at the tank C. To disinfect the water with chlorine D. To store water for the recharge pit Answer: A. The first rain washes the roof, so the diverter discards that dirty water before it reaches the filter and tank. (Key point: Role of first flush) 3. How is storage capacity of a rooftop RWH system estimated? A. Rainfall x roof area x runoff coefficient (0.8-0.9) B. Rainfall / roof area C. Roof area x pipe diameter D. Rainfall x runoff coefficient / roof area Answer: A. Capacity = rainfall x roof area x runoff coefficient, with a coefficient of about 0.8-0.9 for roofs. (Key point: Storage capacity formula) 4. In the groundwater-recharge method, what is the recharge pit or trench filled with? A. Clay and silt B. Cement concrete C. Only clean water from the tank D. Boulders, gravel and sand so water soaks into the aquifer Answer: D. The pit (1-2 m wide, 2-3 m deep) is filled with boulders, gravel and sand and filtered runoff soaks into the aquifer. (Key point: Recharge method) ## Related in Building Technology - [RWH: reasons and basic components](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-reasons-components) — Building Technology, chapter 12 - [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3× - [Components of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-components) — Building Technology, chapter 12, asked 2× - [Types of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-types) — Building Technology, chapter 12 - [Fire safety features; first flush in RWH](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afirst-flush-fire-safety) — 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× ## Related in other subjects - [Is reservoir necessary: village of 800, spring 0.5 L/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-b81) — Water Supply Engineering, chapter 7 - [Is balancing tank necessary: 20000 L/day spring](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-k72) — Water Supply Engineering, chapter 7 - [Is treatment necessary? 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