DIY Design of Rooftop Rainwater Harvesting Structure

There are hundreds of parts in a typical-sized rainwater harvesting system and with time and research most aspects of a rain water harvesting system can be figured out.

Rain water harvesting (RWH) is an excellent technique of water conservation for future needs and also to recharge groundwater. Due to the alarming population burden, climate change, uneven distribution of rainfall and abrupt variation of meteorological parameters, the surface and ground water resources are continuously depleting in India. Hence adoption of different water conservation techniques at individual, institute and community level has become imperative to cater to the needs.

The instructions on this blog help you get started. This is by no means a complete list but it will provide you a great starting place.

Determine how much you can capture –

The rooftop surface area is the catchment area that receives the incident rainfall. The rooftop area and heights of the selected building in consideration is noted

Estimation of water harvesting potential

The quantity of water that is received from rainfall over an area is called the rainwater potential of that area. And the quantity that can be effectively harvested is called the rain water harvesting potential. Rain water harvesting potential can be calculated using the following formula.

Rainwater Harvesting potential (m³) = Area of Catchment (m²) X Amount of rainfall (mm) X Runoff coefficient

Runoff coefficient

Runoff coefficient value was taken from the manual of artificial recharge of ground water, Government of Kenya Ministry of Water Resource design manual on runoff coefficient values to be adopted for design purpose.

SURFACE	RUNOFF COEFFICIENT (K)
Roof Conventional	0.7-0.8
Roof Inclined	0.85-0.95

Estimation of water demand

The Total water demand for a household is estimated considering the per capita consumption of water for domestic use as per the Kenya water design manual Per capita consumption of water for domestic use

Activities	Liters/Person
Drinking	3
Cooking	4
Bathing	20
Flushing	40
Washing Cloths	25
Washing Utensils	20
Gardening	23
Total demand of water needed	135 liters/person/day

Calculation of discharge

 To find out the required diameter of the pipe to be used for draining the rainwater down from the roof, first we need to calculate the discharge Q

i.e. given by:- Q = CIA (1)

Where,

 Q= Discharge from roofs due to rainfall in (m3 /s)

 C= Coefficient of runoff by rational method taken as 0.8 for this case

I= Intensity of rainfall i.e.20mm/hr.

A= Area of catchment,

.Calculation of number of rainwater pipes (R.W.P)

Assuming the diameter of pipe as 10 cm, the total number of required pipes was calculated in this blog. Q = C×I×A

=

/Where;

Q=Discharge

 I=Intensity of rainfall

A=Area of catchment

n=Minimum no. of pipes

d=Diameter of rainwater pipe i.e. R.W.P

 v=Velocity of water on the roof when it is at the verge of entering in the pipe due to the slope available at the roof. As the roofs are flat or having 0-2% slope so; v=0.1m/s (as per CGWB guidelines) So, no. of pipes are calculated as: n=Q / (0.785 ×v)