TECHNICAL GUIDELINES ON ROOFTOP RAINWATER HARVESTING RECHARGE STRUCTURES

GROUNDWATER RECHARGE 

In places where the withdrawal of water is more than the rate of recharge, an imbalance in the groundwater reserves are created. Recharging of aquifers are undertaken with the following objectives:
  • To maintain or augment natural groundwater as an economic resource
  • To conserve excess surface water underground
  • To combat progressive depletion of groundwater levels
  • To combat unfavourable salt balance and saline water intrusion


COMPONENTS OF ROOFTOP RAINWATER FOR GROUNDWATER RECHARGE


1. ROOF CATCHMENT

The roof of the house is used as the catchment for collecting the rainwater. The style, construction and material of the roof determine its suitability as a catchment. Roofs made of corrugated iron sheet, asbestos sheet, tiles or concrete can be utilized as such for harvesting rainwater. Thatched roofs, on the other hand, are not suitable as pieces of roof material may be carried by water and may also impart some colour to water.

AVAILABILITY OF RAINWATER THOUGH ROOFTOP RAINWATER HARVESTING
Rainfall(mm)

Rooftop Area (Sq.m)
100
200
300
400
500
600
800
1000
1200
1400
1600
1800
2000

Harvested Water From Rooftop (In Cum)
20
1.6
3.2
4.8
6.4
8
9.6
12.8
16
19.2
22.4
25.6
28.8
32
30
2.4
4.8
7.2
9.6
12
14.4
19.2
24
28.8
33.6
38.4
43.2
48
40
3.2
6.4
9.6
12.8
16
19.2
25.6
32
38.4
44.8
51.2
57.6
64
50
4
8
12
16
20
24
32
40
48
56
64
72
80
60
4.8
9.6
14.4
19.2
24
28.8
38.4
48
57.6
67.2
76.8
86.4
96
70
5.6
11.2
16.8
22.4
28
33.6
44.8
56
67.2
78.4
89.6
100.8
112
80
6.4
12.8
19.2
25.6
32
38.4
51.2
64
76.8
89.6
102.4
115.2
128
90
7.2
14.4
21.6
28.8
36
43.2
57.6
72
86.4
100.8
115.2
129.6
144
100
8
16
24
32
40
48
64
80
96
112
128
144
160
150
12
24
36
48
60
72
96
120
144
168
192
216
240
200
16
32
48
64
80
96
128
160
192
224
256
288
320
250
20
40
60
80
100
120
160
200
240
280
320
360
400
300
24
48
72
96
120
144
192
240
288
336
384
432
480
400
32
64
96
128
160
192
256
320
384
448
512
576
640
500
40
80
120
160
200
240
320
400
480
560
640
720
800
1000
80
160
240
320
400
480
640
800
960
1120
1280
1440
1600
2000
160
320
480
640
800
960
1280
1600
1920
2240
2560
2880
3200
3000
240
480
720
960
1200
1440
1920
2400
2880
3360
3840
4320
4800

2. DRAIN PIPES

The drain pipes of suitable size, made of PVC / Stoneware are provided in RCC buildings to drain off the rooftop water to the storm drains. They are provided as per the building code requirements.

SIZING OF WATER PIPE FOR ROOF DRAINAGE
Sr.No
Pipe Diameter  (mm)
The average rate of rainfall (mm per hr)
50
75
100
125
150
200
Roof Area Sq.m
1
50
13.4
8.9
6.6
5.3
4.4
3.3
2
65
24.1
16.0
12.0
9.6
8.0
6.0
3
75
40.8
27.0
20.4
16.3
13.6
10.2
4
100
85.4
57.0
42.7
34.2
28.5
21.3
5
125
-
-
80.5
64.3
53.5
40.0
6
150
-
-
-
-
83.6
62.7

3. FIRST FLUSH

First flush diversion protects your rainwater quality by minimizing the volume of suspended and dissolved fine particles that end up in the water you harvest. Generally speaking, the more water is diverted, the better the quality of the rainwater that ends up in the recharge zone. However, diverting more rainwater than necessary can lead to volume shortages, so it’s important to calculate diversion needs and divert water accordingly. All the recharge structures should have the first flush assembly for a better life and efficiency.

FIRST FLUSH SIZING
Sr.No
Roof Top Area (Sq.mt)
First Flush Volume (L)
1
50
100
2
100
200
3
200
400
4
300
600
5
400
800
6
500
1000
7
1000
1500
8
2000
2000
9
3000
2500

FIRST FLUSH DIVERTER

4. FILTRATION UNIT
Filtration forms the most important process in the purification of water. It usually involves allowing water to pass through a filter media e.g. sand and gravel. Filtration essentially involves the removal of suspended and colloidal impurities present in water. Depending on the type of filtration, the chemical characteristics of water may be altered and the bacterial content may be considerably reduced. These effects take place due to various processes such as mechanical straining, sedimentation, biological metabolism and electrolytic changes. Filtered unit is essential when the water is directly recharged into the ground through borewell, hand-pump or recharge shaft.

5. RECHARGE STRUCTURES

A) Recharge Pit

i. Recharge pit is constructed for recharging the shallow aquifer

ii. It is suitable for the small building having a roof area up to 100 sqm

iii. Recharge pits are generally 2-3 m wide and 1-2 meter deep, they can be of any
     shape i.e circular square or rectangular

iv. After excavation pits are refilled with pebbles and boulders

v. Recharge water should be silt free

vi. Cleaning of the pit should be done annually preferably.

vii. First flush arrangement is mandatory

viii. Recharge water should be silt free

SIZING OF RECHARGE PIT
Sr.No
Roof Top Area (sq.m)
The average rate of rainfall (mm per hr)
50
75
100
125
150
200
Recharge pit Volume (Liters)
1
50
750
1000
1500
2000
2500
3000
2
75
1000
1500
2000
2500
3000
4000
3
100
1500
2000
2500
3500
4000
5000

TYPICAL RECHARGE PIT
B) Abandoned/ Running Hand Pump/Shallow Borewell

i. An Abandoned/ Running/ Hand Pump/shallow borewell can be used for recharge

ii. The structure is suitable for the small building having a roof area up to 300 sqm

iii. The water diverted from rooftop to hand pump though pipe of 50 to 100 mm diameter

iv. For running hand pump closing valve is fitted in convenience system near the hand pump to avoid       entry of air in the suction pipe

v. Recharge water should be silt free

vi. First flush arrangement is mandatory


SIZING OF FILTRATION TANK (HAND PUMP RECHARGE)

Sr.No
Roof Top Area (sq.m)
The average rate of rainfall (mm per hr)
50
75
100
125
150
200
Filtration Tank Volume (Liters)
1
50
750
1000
1500
2000
2500
3000
2
75
1000
1500
2000
2500
3000
4000
3
100
1500
2000
2500
3500
4000
5000
4
200
2500
4000
5000
6500
7500
10000
5
300
4000
6000
7500
10000
12500
15000


RECHARGE TO HAND PUMP

C) Gravity Head Recharge Well

i. Deep borewell/tube wells can be used as recharge structure

ii. This technology is suitable for (a) Land availability is limited (b) When the aquifer is deep and over the impermeable strata

iii. The roof water should be silt free and filtered though the media filtration

iv. The well can also be used for pumping but it’s always a good idea to install the separate recharge well near to the working well and vice versa.

v. First flush arrangement is mandatory


SIZING OF FILTRATION TANK (BOREWELL RECHARGE)

Sr.No
Roof Top Area (sq.m)
The average rate of rainfall (mm per hr)
50
75
100
125
150
200
The volume of Filtration Tank (Liters)
1
50
750
1000
1500
2000
2500
3000
2
100
1500
2000
2500
3500
4000
5000
3
200
2500
4000
5000
6500
7500
10000
4
300
4000
6000
7500
10000
12500
15000
5
400
5500
7500
10000
12500
15000
20000
6
500
7000
9000
12500
15000
18000
25000
7
1000
14000
18000
25000
30000
40000
50000


RECHARGE TO BOREWELL

D) Recharge Shaft

i. Recharge Shaft is dug manually or drilled by the reverse/direct rotary method

ii. The diameter of recharge shaft varies from 0.5m to 3 m depending upon the availability of the water to be recharged

iii. It is constructed where the shallow aquifer is located below the clayey surface

iv. Recharge shaft is backfilled with boulders gravel and coarse sand

v. It should end in more permeable strata (sand)

vi. Depth of recharge shaft varies from 10-15 meter below ground level

vii. It should be constructed 10-15 meter away from the building

viii. It should be cleaned annually preferably a scarping top layer of sand and refilling it.

SIZING OF RECHARGE SHAFT
Sr.No
Roof Top Area (sq.m)
The average rate of rainfall (mm per hr)
50
75
100
125
150
200
The volume of Recharge Shaft (Liters)
1
1000
12500
18750
25000
31250
37500
50000
2
2000
25000
37500
50000
62500
75000
100000

Sr.No
Volume
Diameter (m)
Depth (m)
1
12500
1.21 (4ft)
11
2
20000
1.52 (5ft)
11
3
30000
1.82 (6ft)
12
4
40000
2.13 (7ft)
12
5
50000
2.44 (8ft)
12
6
100000
3.04 (10 ft)
15

RECHARGE SHAFT

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