Evaluating the adequacy of rain water harvesting structures

CASE STUDY OF ONE RESIDENTIAL STREET IN MYLAPORE

-Sneha Raghuram

Project Background

Chennai has been facing a water crisis for years now and people are forced to either buy water or dig their own wells. The government is unable to provide adequate water supply to cope with the needs of the growing population. One of the measures taken by the state government is to make rainwater harvesting mandatory in all buildings.
While this is a well-intentioned policy, there are limitations in implementation. Saving rainwater is one of the crucial aspects to addressing our water security.

Research question

1.To assess whether existing RWH structures are adequate to capture the rainfall of the region.
2.Are the volumes of these structures built to satisfy the norms prescribed?

TAKSRA

TAKSRA (Thiruveedhiamman Koil Street Resident Association) is a very active community of residents committed to being environmentally responsible and model citizens of Chennai.
The goal of my report is to assess whether the efforts taken by TAKSRA is adequate to capture rainfall as per existing norms. The study will also determine what further steps the residents of TAKSRA can take to improve the groundwater conditions in their street.

Method

I will study each building on the street and document the RWH facilities provided. Under the guidance of a Professor from Civil Engineering department in IIT Madras, I will assess the efficiency of the structures. Using calculations based on existing CPWD norms, I have devised a calculation tool that will allow anyone to determine the adequacy of the RWH structures by inputting few key data.

Data required for calculating capacity of rainwater harvesting structures
1.Rooftop area of each building
2.Area provided for gardening in each building
3.Area provided for driveways / pathways in each building

Formula for calculating capacity
Q = C I A*
Where Q stands for total runoff volume in cubic meters per day.
C is the runoff coefficient, which depends on the surface
(Eg, for tiled roofs it ranges from 0.8 to 0.9)
I is the rainfall intensity in m/day for annual average rainfall and mm/hour for peak intensity rainfall
A is the area of catchment (area where rainwater gets collected)

*Refer annexure A for detailed calculations

Based on the above formula the following table illustrates the existing RWH structures in the buildings on Thiruveedhi Amman Koil street

Observation

Based on my study of the street and the existing RWH structures provided there are three main observations:
1.A few building have adequate RWH structures within the site boundary.
2.For some buildings that have adequate structures, they still need to be clean for them to be efficient.
3.Half of the buildings require additional structures for capturing rain water
4.The street provides a lot of opportunity for storing rainwater. The existing two RWH are inadequate for collecting rainwater. An additional X collection pits bring the facility provided upto the norms prescribed by the CPWD

Limitations of the study

Since the RWH structures are not completely accessible it is difficult to ascertain the actual capacity. Some assumptions have been made to arrive at the calculations.
The rainfall data has seen a lot of variations in recent years. I am using the information from 2002 to arrive at the method of calculations.
There were 3 independent houses on the street. These are not considered for the purpose of the study.
The RWH structures at the entrance of each house have not been factored into the calculations

Conclusion

Additional scope for study

Based on my study for rain water harvesting, I observed that by recycling grey water we can possibly generate adequate water for maintaining larger area of landscape. This could be the next step for those building which hve already provided adequate rwh structures. Preliminary steps have already been taken in one of the buildings. Using plants to treat this grey water is a sustainable way to further increase the recharge of the water into the soil.

References

CPWD norms :

http://cpheeo.gov.in/upload/uploadfiles/files/Rainwater%20Harvesting%20Manual-CPWD.pdf

Tools used to calculate area:

https://www.daftlogic.com/projects-google-maps-area-calculator-tool.htm#

Google maps link of location: https://www.google.com/maps/search/sringeri+mutt/@13.0256787,80.259866,19z

ANNEXURE A

Detailed Calculations

Formula for calculating capacity

Q = C I A

Where Q stands for total runoff volume in cubic meters per day
C is the runoff coefficient, which depends on the surface
(Eg, for tiled roofs it ranges from 0.8 to 0.9)
I is the rainfall intensity in m/day for annual average rainfall and mm/hour for peak intensity rainfall
A is the area of catchment (area where rainwater gets collected)
Run-off Coefficients:Rooftop :1
Landscape/Garden : 0.3
Driveway :0.6

Terrace Area Run-Off:

I              =            Intensity of rainfall for Chennai is assumed to be 30 mm per day.
A            =            Total Rooftop Area
C            =            Coefficient of Runoff which is 1
Q            =            CIA

Total Roof top rainwater sump capacity must be a little more than the value of Q
Total Run-off from Driveway:
I              =            Intensity of rainfall for 160mm per hour for Chennai
A            =            Driveway Area
C            =            Coefficient of run-off which is 0.6
Q            =            Total run-off for driveway
=            CIA

Let Total run-off for driveway be X.
Total run –off from Landscape Area:
I              =            Intensity of rainfall for 160mm per hour for Chennai
A            =            Landscape Area
C            =            Coefficient of run-off which is 0.3
Q            =            Total run-off for Landscape
=            CIA
Let Total run-off for Landscape be Y.
Total amount of Run-off for Driveway and Landscape
Considering 20 mins of retention time,
Total Run-off, Z   =   (X+Y)* 20/60
Recharge pit Calculation: c
Assumed data:
Infiltration rate is 25mm/hr =   0.025m/hr
Recharge pit of Diameter    m
Depth of pit      m
Area of bottom of pit = 3.14*Diameter/4
Area of wetted perimeter = 3.14*Depth*Diameter
Total Area   = Area of bottom + Area of perimeter
Total water permeability/pit/hr =  Total area*0.025 m³/hr
Assuming 6 hours of percolation,
Total amount of water percolated in one day, W
=            Total water permeability/pit/hr * 6 m³
Total number of percolation pits required    =            Z/W