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
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
|
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
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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