Calculating Monsoon Rain: Rainwater Harvesting in Chennai.

an ech2o cpd 2016 www.ech2o.co.uk
Calculating Monsoon Rain
Rainwater Harvesting in Chennai
Westminster Uni Design studio
September 29th 2016
Cath Hassell
cath.hassell@ech2o.co.uk
@CathHassell

Who are ech2o and what do we do?
• We are environmental consultants specialising in
sustainable water strategies and low carbon or
renewable technologies
• We work in new build or existing buildings
• We work with occupants of the building to change their
behaviour as well as identifying the best technological
solutions
• Our clients include local authorities, housing
associations, schools and colleges, water companies,
architects, community groups, and householders

The carbon load of water
• In the UK it takes 1.2 kWh of
(mostly) electrical energy to
supply and treat 1m3 of
mains water.
• This results in 0.7 kg of CO2
emissions per m3 of mains
water used (UK electricity
has a carbon content of 0.54
kg per kWh generated)
• Or 1.0 kgCO2e
• Electricity in India has a
carbon content of greater
than 0.9 kg per kWh
generated

Group work
Why rainwater harvesting?

The main reasons to harvest rainwater
• To prevent overburdening the sewer system,
whether separate or combined
• To prevent localised flooding – streets and
watercourses
• As a supply of water to offset drought or water wars
• To protect the Pallikaranai wetlands?

Kaveri River, India (Karnataka v Tamil Nadu)
• Ongoing since 1883
• An agreement drawn up
in 1924 for 50 years
• A Supreme Court
Judgement to release
extra water per year led
to attacks on Tamils living
in Karnataka

We must decouple rainwater from the
sewer system

Combined sewers design

The result of a CSO (Combined Sewer
Overflow) event

Storm drainage channels are often used
instead of drains and sewers
• Chennai's
original storm
drains
designed to
6.35mm/hour
rainfall
• Now all Indian
cities design to
12mm or
greater/hour
• In overcrowded cities storm
drainage channels are useful
land and encroachment is rife

Group work
What information do you need to
know to harvest rainwater?

How much? - Rainfall in India
• Average yearly UK rainfall is
1000 mm/year with a spread of
550 – 4000 mm
• India’s spread is far greater (less
than 200 to over 10,000mm).
State averages are given as
opposed to nationwide
• Average rainfall In Tamil Nadu is
998mm, 1500mm in Chennai
• Mawsynram, Meghalay has
11,873 mm of rainfall a year

When? -
Summer
monsoon

When? -
Winter
monsoon

Rainfall in Chennai
• 1,267 (Indian
Meteorological
Dept. 2002)
• 1,541 (Chennai
Climate
temps.com)
• Receives both
summer and
winter monsoon
0
500
1000
1500
2000
2500
3000
1 2 3 4 5 6 7
Average rainfall in Chennai 2004 ‐ 2010

Rainfall Chennai 2004 -2010
0
100
200
300
400
500
600
700
800
900
1000
January February March April May June July August September October November December
Rainfall Chennai 2004 ‐2010
Series1 Series2 Series3 Series4 Series5 Series6 Series7

Slowing down the movement of
rainwater for recharge and to
prevent flooding

Rainwater harvesting as part of WSUD

Greening the urban infrastructure

Sustainable drainage systems (SuDS) 1
• Increasing development exacerbates
stormwater run off problems.
Increasing intensity of storms
exacerbates stormwater run off
problems
• Sustainable drainage systems
attenuate, treat and dispose of
stormwater to reduce pollution and
local flooding
• SUDS can provide biodiversity as well
as controlling stormwater run off

Sustainable drainage
systems (SuDS) 2
• Green roofs and rainwater
harvesting (as source control
mechanisms), soakaways, filter
drains, permeable surfaces, rain
gardens, swales, basins and
ponds are all part of SuDS

Rain gardens and tree pits instead of
road gullies
• www.raingardens.info for
a UK rain gardens guide

SUDS for front gardens
– a UK example
• Don’t pave over front gardens for off-street
parking
• Alternatives are gravel, permeable paving
or just leave as a garden
• Thames Water to pay a subsidy for
householders in London to do this? A
mass retrofit of paved gardens throughout
London could have a major effect re the
size and cost of the new interceptor
tunnel, currently £36b.

Rainwater harvesting in Chennai –
Design issues

Run-off
• Recharge happens when
rainwater is held in one place
• Urbanisation increase amount of
runoff and rate of run-off and
reduces the space for recharge
• Drainage channels in Chennai
designed and built when lots
more permeable areas in the city

Use check dams to
capture monsoon rains
• Check dams prevent
monsoon rains run-off.
• Create temporary water
stores that can be used
for irrigation
• Recharge ground water
supplies

Use sand dams to capture seasonal rains
• Sand dams store water in large
grains of sand held up behind a
retaining wall built across
sandy river beds
• Large sand grains held up
behind the wall. Water retained
in the space between grains
(Volume up to 40% water).
• Less evaporation, sand filters
and cleans water

Rainwater harvesting and
recharge - small scale

Rainwater harvesting and
recharge - large scale

Green roofs and green walls

Cooling effect of vegetation

Green Roofs as part of a SuDS strategy
• Potsdamer Plazt, Berlin
• Camden, London

Green Roofs as part of
a SuDS strategy 2
• Plants provide biodiversity
• Sub strata provides the
attenuation. In Germany substrata
of between 80-240mm are
accepted as reducing run off
compared to a hard roof surface
by up to 60%
• Roofs can be intensive, extensive
(sedum mats) or bio-diverse
• Well suited to commercial or
industrial developments

Goldsmiths Place, Camden
• Part of greening Camden’s housing
estates. Mixture of bio-diverse and
sedum roofs.
• Upgrading the roof insulation to the
blocks.
• Substrate of the green roof acts as a
ballast for the insulation.
• No water points. Installed at the start
of the drought in summer 2006.
• No access to roofs for tenants but
roofs are overlooked.

• Some green walls
require a lot of watering
and feeding
• Consider simpler
alternatives for Chennai
Green Walls

Green roofs and green walls – issues
for Chennai
• Need to ensure no requirement for
watering
• Green roofs potential on lower roofs
only and store rainwater on higher
adjacent roofs for irrigation
• Green walls, plant in ground and
grow up

Rainwater harvesting for water
supply

Rainwater harvesting as a water supply
• In the UK rainwater is classified as fit for purpose
for WC and urinal flushing, washing machines,
grounds watering.
• In India will be used for all purposes (drinking and
cooking with probable extra filtration)
• Generally, rainwater harvesting is better suited to
larger buildings, agricultural, educational
commercial, industrial.

Wide variety of
storage solutions

Rainwater Harvesting System Layout

The importance of prioritising water
efficiency measures
• 18 flats, Boatemah Walk
• Original specification RWH and 6
litre flush WCs
• Upgraded WC spec to 4/2.5 litre
dual flush
• 220m3 savings from WC
specification (£1,800)
• 176m3 from rainwater
specification. (£18,000 plus)
• Rainwater supplies 80% of
flushing demand

Group work
What should you look out for when
in Chennai?

Potential for rainwater harvesting and
greening the city in Chennai
• Ways the city currently collects rainwater – gutters and
down pipes
• If no gutters how/where does the rainwater discharge
• Fall of existing roofs may have to be changed
• Space for check dams
• Wells for recharge
• What water efficiency measures are currently used?
• Walls for green walls
• Roofs for green roofs

DESIGN CALCULATIONS

Yield calculations
Roof catchment (m2) x yearly rainfall (mm)
x drainage factor x filter efficiency
= yearly yield in litres
(Divide by number of days building is occupied for
daily yield)
Yearly yield in litres/1000
= m3 of mains water offset per year
(if storage sized correctly)

Drainage factor
• The drainage factor is how much of the rainwater that
falls on the roof drains off into the gutters. It is a
combination of the pitch of the roof and roof build-up
• The drainage factors we use in the UK are: a metal or
plastic pitched roof has a drainage factor of 0.9 (i.e.
90% of the rain falling on the roof drains off);.a slate
pitched roof has a drainage factor of 0.8; a flat roof has
a drainage factor of 0.6; a green roof has a drainage
factor of as low as 0.4
• Due to the intensity of rain during a monsoon I would
expect the drainage factor off a flat roof in Chennai to
be greater than 0.6

Storage calculations
• In the UK storage is sized at 5% of yearly yield, or
5% of yearly demand, if used on a daily basis
• If collecting winter rainfall for grounds watering in
the summer, then calculate watering demand for
the summer months and size storage to meet
demand for June, July and August, when watering
demand is highest and rainfall is lowest.
• In Chennai you will need to consider that most rain
falls in October, November and December. So a
large amount of storage is needed if rainwater is
going to be the only source of water for the
intervening months

Design calculations – Group work 1
• A new apartment block in Chennai
• 400m2 of hard roof area and 100m2 of green roof
• 1,450mm of rainfall a year.
• Design a system that collects from the roof of a
block of flats
• Work out if enough rainwater to supply all flats

1. What would be the yearly yield of rainwater?
2. What storage capacity would you recommend?
3. How much rainwater is available per flat per day?
4. What appliances will you feed with the rainwater?

• If I was looking at a solution to keep as much
rainwater out of the drainage system as possible in
Chennai I would concentrate on calculating for
rainfall levels during the winter monsoon.

Calculating Monsoon Rain: Rainwater Harvesting in Chennai.

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