---
title: "Short note: Rainwater harvesting"
subject: "Building Technology"
chapter: 12
key: "semester_6/building_technology:rainwater-harvesting-note"
source: https://www.bctengineer.com/q/semester_6/building_technology/rainwater-harvesting-note
site: BCT Engineering
---

# Short note: Rainwater harvesting

Building Technology · Chapter 12: Other services in building

- Asked in: 2079 Asoj, 5 marks; 2079 Jestha, 4 marks; 2076 Bhadra, 4 marks; 2075 Baisakh, 3 marks; 2074 Bhadra, 4 marks (asked 5 times)
- Video: [Rooftop Rainwater Harvesting (English)](https://www.youtube.com/watch?v=8Ls0rM2j8iQ) — Pranis Creations
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/rainwater-harvesting-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12

## Question

Write a note on rainwater harvesting (what do you mean by rainwater harvesting?).

## Short answer

Rainwater harvesting collects and stores rooftop rain for use or groundwater recharge, easing water scarcity and urban flooding. Key components: clean roof catchment, gutters, first-flush diverter, sand-gravel-charcoal filter, storage tank, and optional recharge pit; harvested water can be disinfected for drinking.

## Answer

**Rainwater harvesting (RWH)** is the collection and storage of rain falling on roofs or the 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; 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.

#### Methods
- **Rooftop harvesting:** roof water is led through gutters and down pipes, a first-flush device and a filter to a storage tank for use.
- **Surface runoff harvesting:** collection in ponds or tanks.
- **Groundwater recharge:** through pits, trenches and wells.

#### Components of rooftop RWH
1. **Catchment:** the roof (RCC, tiles, GI sheet) that receives the rain; it should be a clean, non-toxic surface.
2. **Gutters:** half-round channels at the roof edge (GI, PVC, aluminium) with a slope of 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 the gutters to storage.
4. **First-flush device:** diverts the first dirty rain (first 10-15 minutes, or about 0.5-1 mm of rain), which carries dust, bird droppings and leaves, to waste.
5. **Filter unit:** layers of coarse sand, gravel and charcoal (or a 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 provision for cleaning.
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
```

#### Benefits
- Supplements the water supply and reduces the water bill.
- Raises the groundwater table and improves its quality (dilutes fluoride and salts).
- Reduces urban flooding and erosion.
- An easy, low-cost technology that can be used in any building; the water is suitable for toilets, gardening, washing and, after treatment, drinking.

**Design:** volume that can be harvested per year = annual rainfall (m) x roof area (m^2) x runoff coefficient (0.8-0.9 for roofs).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is rainwater harvesting?
   A. Draining rain into sewers
   B. Collecting and storing rain from roofs/ground for later use or groundwater recharge
   C. Pumping groundwater
   D. Treating sewage
   Answer: B. Instead of running off, rain is collected and stored or recharged. (Key point: Definition)

2. What does a first-flush device do?
   A. Diverts the first dirty rain to waste
   B. Stores clean water
   C. Chlorinates water
   D. Increases roof area
   Answer: A. The first rain carries dust and droppings. (Key point: First flush)

3. Gutters should have a slope of about:
   A. 1 in 2
   B. 1 in 10
   C. 1 in 100
   D. Zero slope
   Answer: C. A gentle slope of about 1 in 100 leads water to the down pipe. (Key point: Gutter)

4. How is harvestable volume calculated?
   A. Rainfall / roof area
   B. Roof area + rainfall
   C. Rainfall x coefficient only
   D. Rainfall x roof area x runoff coefficient (0.8-0.9)
   Answer: D. Volume = rainfall (m) x roof area (m^2) x runoff coefficient. (Key point: Design volume)

## Related in Building Technology

- [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3×
- [Benefits of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-benefits) — Building Technology, chapter 12
- [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