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Feasibility and Sustainability of Rain Water Harvesting for Ground Water Recharging as Against Storage and Recycling: A Case Study of
Sloping Roof Structure
1Archana Bele, 2Pradeep Padgilwar, 3Kishor Rewatkar, 4Rashmi Dande
1,2,34
Priyadarshini Institute of Architecture and Design studies, Nagpur
Abstract: Water management is very critical for the growth and development of any economy. This vital resource is now under stress, because of excessive groundwater abstraction in the course of socioeconomic development and meeting increasing needs of growing population [1]. Therefore, we need to use and conserve the rain water wisely. There are two ways of utilizing the rain water either for storage and recycling it for various purposes or for recharging of ground water table.
The paper aims to find out the feasibility and sustainability of rain water harvesting for ground water recharging as against storage and recycling of rain water for domestic purpose especially for sloping roof structures. For this purpose, the campus of Priyadarshini Institute of Architecture and Design Studies (PIADS), Nagpur has been taken up as case study. Two proposals have been worked out. One proposal emphasizes on storage and recycling of rainwater for domestic purposes like cleaning, flushing and landscaping. Other proposal focuses on percolation of rain water for recharging of ground water table. Consideration has been given to carrying out the harvesting in the most indigenous way so as to make the project more sustainable and cost effective which has also become the basis for selection criteria of either of the two methods.
It has been found that the expenditure for storage and recycling of rain water for sloping roof structure like PIADS is enormously high (about 10 times) as compared to recharging of rain water into the ground. Moreover, only about 22% of water which is obtained from rain in the city like Nagpur with annual rainfall of about 1000mm could be stored annually for domestic purpose as the quality of water over the time gets declined. If we increase the storage capacity it further adds to the cost of harvesting scheme as construction of underground sump for higher capacity storage is again a very costly matter. This clearly indicates that rest of the water obtained from rain i.e. about 78%
cannot be stored economically hence gets wasted. So instead of storing the water for recycling it for various proposes it is found more feasible to discharge the water into the ground for recharging.
Keywords: Sloping roof structure, rain water storage, ground water recharging, feasibility and sustainability
I. INTRODUCTION:
Water management is very critical for the growth and development of any economy. This vital resource is now under stress, because of excessive groundwater abstraction in the course of socioeconomic development and meeting increasing needs of growing population [1].
Water scarcity is caused by low water storage capacity, low infiltration, larger inter annual and annual fluctuations of precipitation (due to monsoonal rains) and high evaporation demand. The term water harvesting was probably used first by Geddes of the University of Sydney. He defined it as the collection and storage of any form of water either runoff or creek flow for irrigation use [2]. The Global Development Research Centre defines rainwater harvesting as a technology used for collecting and storing rainwater from rooftops, the land surface or rock catchments using simple techniques such as jars and pots as well as more complex techniques such as underground check dams. The techniques usually found in Asia and Africa arise from practices employed by ancient civilizations within these regions and still serve as a major source of drinking water supply in rural areas. Rain water harvesting is being practiced as early as 4500 B.C. by the people of Ur (present Iraq) and also latest by the Nabateans and other people of the Middle East. While the early water harvesting techniques used natural materials, 20th century technology has made it possible to use artificial means for increasing runoff from precipitation [2].
II. HISTORICAL OVERVIEW OF RAIN WATER HARVESTING IN INDIA:
Indian history reflects the ingenuity and wisdom of our forefathers who made harvesting of water and its management an integral part of the native culture and community life. This meant that these practices were perceived by the common man as his sacred duty and by the communities as part of good local self-governance and social responsibility. This Water-Wisdom at all levels of society ensured adequate availability of water for all, which in turn, formed the basis for all round development and prosperity.
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In 1615, during the Mughal rule, Abdul Rahim Khan built a unique water supply system of the Burhanpur town (Madhya Pradesh). The system involved construction of long lines of underground tunnels with vertical airshafts to tap the underground water flow from the nearby Satpura hill ranges to the Tapi river lower down. The system is still functioning well and is adequate to meet the entire water requirements of the town.
Fig 1: Engineering Marvel of Burhanpur The Panhala Fort of Maharaj Shivaji built on a hillock near Kolhapur in Maharashtra had Baolis and wells to tap underground springs originating in nearby higher hill slopes.
Fig 2: Baolis to tap underground springs in Panhala Fort Most of the old temples in south India built centuries ago have large tanks in their premises. These tanks are either fed by harvested rain water or by tapping underground springs. In Tamil Nadu alone there are 39 temple tanks with areas varying from 0.25 to 3 hectares.
These are all fed by rain water. Though these were used mainly for bathing and religious purposes, these also recharged the drinking water wells [3].
A khadin, also called a dhora, is an ingenious construction designed to harvest surface runoff water for agriculture. Its main feature is a very long (100-300 m) earthen embankment built across the lower hill slopes lying below gravelly uplands. Sluices and spillways allow excess water to drain off. The khadin system is based on the principle of harvesting rainwater on farmland and subsequent use of this water-saturated land for crop production. First designed by the Paliwal Brahmins of Jaisalmer, western Rajasthan in the 15th century, this system has great similarity with the irrigation methods of the people of Ur (present Iraq) around 4500 BC [4].
Fig 3: Khadin to harvest surface runoff
III. METHODS OF RAIN WATER HARVESTING:
There are various ways of utilizing the rain water which includes storage and recycling it for the purpose of drinking (after treatment) or other daily applications like cleaning, flushing tanks or landscaping or could even be utilized in large scale industries. This results basically in saving the precious resource like water thereby saving a lot of funds on treatment of it to make it usable. It also reduces the cost for pumping of ground water.
There is one more way of utilizing the rain water which is percolation of it into the ground for recharging of aquifers. There are various methods of recharging of ground water which includes recharging of bore wells, recharging of dug wells, recharge pits, recharge trench, recharge shaft and percolation tank. There could be innumerable benefits of recharging the rainwater though not seen instantly but in near future most important of which is increase in level of ground water table. It also enhances ground water quality by dilution. It improves soil moisture and reduces soil erosion by minimizing run-off water. Selection of either of the two could be done on the basis of benefits they provide. Compared to storage and recycling of rain water for short term benefits, recharging of ground water for future long term benefits is more advantageous. Factor of economics is also of great significance especially when we are referring to an institutional building. So in this paper detailed scheme of both the types of rain water harvesting i.e. storage and recycling and rain water percolation for ground water recharging is worked out.
Estimates for both are also carried out and by comparing the economical, functional as well as environmental benefits of both the methods final assumption is made.
IV. RAIN WATER HARVESTING FOR STORAGE AND RECYCLING PURPOSE
AT PIADS:
The PIADS building comprises of four wings placed in four directions with courtyard in the centre. All the wings are provided with gable roofs with slopes in two directions out of which one side sloping towards the courtyard and another outside along the periphery of the building.
Fig 4: Front (south wing) façade of the building
Fig 5: West facade of the building
Fig 6: Central Open courtyard
Fig 7: Eaves level
Fig 8: Sloping roof of passage
Since one side slopes inside, water is collected in the open courtyard in the centre. For all four wings and the central courtyard, G.I sheet gutters (300 x 150mm) are necessary on the eaves line along outer and inner
periphery of the roof from where water is taken down through P.V.C. Rain water down take pipes of 160mm diameter. Constructing an underground sump for storage of water would have become a still more costly matter.
Hence rain water from P.V.C. pipes is proposed to be discharged into P.V.C. water tanks placed on concrete platform on the surface of ground. From this tank, water could be made available for domestic purpose.
V. CALCULATIONS FOR RAIN WATER HARVESTING POTENTIAL:
To design rain water harvesting storage and recycling scheme, it is necessary to find out annual rain water harvesting potential. Annual Rain Water Harvesting Potential = Roof top area (sqm) x Annual Rain fall of the city (m) x Runoff Coefficient x Constant Coefficient.
The total area of roof top to be considered for rain water harvesting= 507.09 sqm (west wing) + 2052.92 sqm (north wing + south wing + central corridors) + 477.37 sqm (east wing) + 192.91 (Foyer) +935.453 (Central courtyard) = 4165.743 sqm. Annual Rain fall of Nagpur City = 1.112 m [5], Runoff Coefficient= 0.85 [5], Constant Coefficient = 0.80 [5]. Hence Annual Rain Water Harvesting Potential = 4165.743 x 1.112 x 0.85 x 0.80 = 3149.968 cum = 3149.968 x 1000 = 31,49,968 liters.
Say rain water harvest potential from roof top and central courtyard is about 31,50,000 liters.
VI. CALCULATIONS FOR WATER DEMAND FOR DOMESTIC PURPOSE:
Water demand is calculated for domestic purpose only i.e. for cleaning, flushing and landscaping. Water demand for drinking water is not considered as treatment of water for making it potable again adds to the cost. Hence drinking water demand is not considered. Average requirement of water for cleaning, flushing and landscaping purpose at PIADS is approx.
8000 liters/day. Substantial rain is received only for about 3 month i.e. 90 days. Even water cannot be stored for more than 3 months as quality of water gets deteriorated with time. For 90 days, water demand will be 90 x 8000 =7,20,000 liters. So quantity of rain water which could be actually utilized for domestic purpose is only about 7,20,000 liters as against 31,50,000 liters which is received annually. Making arrangement for storage of 7,20,000 liters of rain water is not a practical solution. Hence considering that provision for storage of rain water is done for one week, the storage capacity required would be 8000 x 7 = 56000 liters. Provision should be made for 56,000 liters of storage capacity because rains come intermittently and when it comes it might have the capacity to fulfill all the 56,000 liters of storage capacity.
VII. DETAIL SCHEME OF RAIN WATER STORAGE AND RECYCLING:
The scheme is designed with G.I. Rain water gutters provided at eaves from where rain water is taken down
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through P.V.C. rain water pipes. The rain water pipes are connected with the R.C.C. drains with inspection chambers wherever required from where water is
ultimately discharged into the P.V.C. tank. Detailed drawings for this proposal are worked out and based on them estimate is prepared.
Fig 9: Rain water storage and recycling scheme
VIII. ESTIMATE FOR THE PROPOSED RAIN WATER STORAGE AND RECYCLING SCHEME:
The estimate includes cost required for G.I. rain water gutters with brackets, P.V.C. down take pipes, drains made out of R.C.C, water tanks along with the cost of preparation of concrete base and labour charges. While estimating the labour charges, it is noticed that the cost required for scaffolding is going too high because of the sloping roof since there is no provision for reaching to the eaves from the roof top which is covered with Mangalore tiles. This results in shooting up of the overall estimate of rain water harvesting for storage and recycling purpose. The cost for rain water gutters and scaffolding could have been saved in case of a flat roofed structure which brings major difference in cost as against sloping roof structures.
Estimate for the Proposed Rainwater Storage and Recycling Scheme at PIADS, Nagpur
No Item Quantity Rate Unit Amount (Rs.)
1 Providing and fixing z shaped GI gutter at the eaves end 300mm x 150mm
478.554 160 Rmt 76568.64
2 Providing and fixing MS brackets at 2.5 distance centre
to centre for holding the rainwater gutter in position 192 280 nos 53760.00 3 Providing and fixing on wall face unplasticised Rigid
PVC rain water pipes 160mm diameter conforming to IS:13592 Type A including jointing with seal ring conforming to IS : 5382 leaving 10 mm gap for thermal expansion, Single socketed pipes.
157.5 726 rmt 114345.00
4 Providing & fixing PVC elbow Providing & fixing PVC elbow to the rain water down take pipe to small chamber.
Job includes supplying of required material, removing of existing shoe and fixing new elbow.of 4" dia / 6" dia
15 700 nos 10500.00
5 Earth work in excavation by mechanical means (Hydraulic excavator) / manual means in foundation trenches or drains (not exceeding 1.5 m in width or 10 sqm on plan) including dressing of sides & ramming of bottoms, lift upto 1.5 m, including getting out the excavated soil & disposal of surplus excavated soil as directed within or outside the site at an unobjectionable
221.994 182 sqm 40402.91
place.
6 Construction of 150mm thick brick Inspection Chamber 0.60Mtr x0.60Mtr with light (Dhapa cover) of brand BHARAT/BN/KK brand 12mm internal plaster in single coat, with floor concrete in M:20, making channel, restoring the ground etc. complete
19 8000 nos 152000.00
7 Providing and laying non-pressure NP2 class (light duty) 300 mm dia. R.C.C. pipes with collars jointed with stiff mixture of cement mortar in the proportion of 1:2 (1 cement : 2 fine sand) including testing of joints etc.
complete.
411.1 585 rmt 240493.50
8 Providing and fixing PVC Water Tank for collection of
rain water 56000 8 liters 448000.00
9 Providing and erecting a cement concrete platform for the base of the PVC Tank
Lumpsum 125000.00
10 Providing and erecting scaffolding for installation of
gutters 5000 160 sqm 800000.00
TOTAL 2061070.05
Add miscellaneous expenses 15 % of 2061070 309160.51
GRAND TOTAL 2370230.56
Say 23,70,000/- It is found that the cost required for rain water
harvesting for recycling purpose was about Rs.
23,70,000/- which is considerably high because of the sloping roof structure. So the water which could be stored and recycled for domestic purpose is only about 22% of annual rain water received (31,50,000 liters) which is very less that too carried out at such a higher cost. Apart from this, the embodied energy of materials used for storage and recycling purpose like P.V.C. pipes and water tanks, G.I. sheet gutters, steel brackets, R.C.C.
pipes, etc. is very high which goes against eco-friendly approach. Hence it cannot be considered as a much sustainable solution.
VIII. RAINWATER HARVESTING FOR RECHARGING OF GROUND WATER
TABLE:
Since the cost of storage and recycling of rain water is enormously high with considerable environmental cost as well, next option worked out is percolation of rain water into the ground for recharging of ground water table. While doing this, consideration has been given to carrying out the recharging in the most indigenous way so as to make the project more sustainable and cost effective. One of the alternatives could be recharging of bore well or dug well. But drilling a bore well or dug well is again a costly affair. So it is decided to go with the alternative of recharge trench which is the most sustainable and cost effective way of percolating the rain water for ground water recharging.
IX. DETAIL SCHEME FOR RAINWATER HARVESTING FOR RECHARGING OF
GROUND WATER:
A trench on surface of ground is proposed along the periphery of roof below the eaves. The width of trench is decided taking into consideration the maximum area
below the eaves where water falls down and splashes at the time of raining. By observation it is found that most intense rain falls below the eaves within 600mm width.
Adding 200mm on both sides for splashing of water, width of trench is considered to be 1000 mm at the top and 600mm at the bottom and depth is taken 1000mm.
The inclined walls of the trench prevent the soil from falling down. The sides of trench are provided with fly ash brick lining. The fly ash brick layer on surface of ground may act as pathway also.
Fig 10: Effective area under the eaves where water falls The trench is designed by making certain modifications in the standard recharge trench given in CPWD rain water harvesting and conservation manual.
Fig 11: Details of Recharge Trench as per CPWD rain water harvesting and conservation manual The trench is proposed to be filled with brick bats, boulders and pebbles. The first layer of brick bats of
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about 400mm depth is given followed by second layer of boulders of 400mm depth topped with the layer of pebbles 200mm.
Fig 12: River Rock Pebbles Fig 13: Pea Gravels
Fig 14: Crushed Granite Gravels
The topmost layer could be made with different options like river rock pebbles, pea gravels, crushed granite gravels, etc. matching with the theme of the landscape which will add to the aesthetic value also. Efforts have been made to use as much natural materials as possible so as to reduce the embodied energy involved.
Fig 15: Rain water harvesting scheme for recharging of ground water
Fig 16: Proposed design of Recharge Trench
X. ESTIMATE FOR THE PROPOSED RAIN WATER HARVESTING FOR GROUND WATER RECHARGE:
The detailed estimate for recharging of rain water into the ground has been worked out which includes cost of excavation and backfilling with brick bats, boulders and pebbles along with laying of fly ash bricks.
Estimate for the Proposed Rainwater Harvesting for Ground Water Recharge at PIADS, Nagpur
No Item Quantity Rate Unit Amount (Rs.)
1 Earth work in excavation by mechanical means (Hydraulic excavator) / manual means in foundation trenches or drains (not exceeding 1.5 m in width or 10 sqm on plan) including dressing of sides & ramming of bottoms, lift upto 1.5 m, including getting out the excavated soil & disposal of surplus excavated soil as directed within or outside the site at an unobjectionable place.
285.68 182 cum 51993.76
2 Supplying, filling, spreading & leveling brick bats, stone boulders of size range 5 cm to 20 cm and pebbles, in recharge pit, in the required thickness, for all leads & lifts, all complete as per direction of Engineer-in-charge.
285.68 580 cum 165694.40
3 Providing Fly ash lime Brick masonry (Bricks conforming to I.S.12894 : 1990 and of size 19 cm x 9 cm x 9 cm) at the periphery of rain water recharge trench complete.
114.8 5657 cum 649423.60
TOTAL 217688.16
Add miscellaneous expenses 15 % of 217688.16 21768.82
GRAND TOTAL 239456.98
Say 2,40,000/-
The estimated cost of recharge trench is worked out to be Rs. 2,40,000/- which is very less as compared to the
total expenditure required for storage and recycling of rain water for sloping roof structure.
XI. CONCLUSION:
It has been found that the expenditure for recycling of rain water for sloping roof structure like PIADS is enormously high (about 10 times) as compared to recharging of rain water into the ground. Moreover, only about 22% of water which is obtained from rain in the city like Nagpur with annual rainfall of about 1000mm could be stored annually for domestic purpose as the quality of water over the time gets declined. Increase in storage capacity further adds to the cost of recycling scheme as construction of underground sump for higher capacity storage is again a very costly matter. This clearly indicates that rest of the water obtained from rain i.e. about 78% cannot be stored economically for sloping roof structures as it requires lot of investment in constructing the rain water drains, gutters, erection of scaffolding as well as construction of storage tank.
Hence it gets wasted. Apart from this the high amount of embodied energy of various materials used for storage and recycling of rain water does not prove it to be a sustainable solution.
Whereas the cost required for percolation of rain water for recharging of ground water is very less as compared to the storage and recycling of rainwater. In addition to this, rainwater harvesting for recharging of ground water is furthermore essential now a days since surface water
is inadequate to meet our demand and we have to depend on ground water. Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished [6]. So instead of storing the rain water for recycling it is found more cost-effective and appropriate to discharge the water into the ground for recharging ground water table which proves to be more economical and most importantly sustainable solution. Hence it could be concluded that the feasibility and sustainability of rainwater percolation for ground water recharging is more than rain water storage and recycling for sloping roof structures.
Storage and recycling of rain water could be sought as a short term benefit but when we look at the bigger picture, rain water percolation for ground water recharging makes more sense looking to the ever increasing remand of water and fast depletion of ground water resources. The Water-Wisdom used by our ancestors at all levels of society which ensured adequate availability of water for all formed the basis for all round development and prosperity. By practicing ground water recharging we can actually revitalize and expand this old wisdom for the benefit of the society.
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REFERENCES:
[1] Giridhar V S S Mittapalli, P.N.Rao, Sowmya Puram, Roof top Rain Water Harvesting for ground water Sustainability- A case Study from Jawaharlal Nehru Technological University Hyderabad Conference: Workshop on Challenges and perspectives in conservation of water in urban Areas, At Hyderabad March 2014
[2] Dr. R. K. Sivanappan, Rain Water Harvesting, Conservation and Management Strategies for Urban and Rural Sectors, National Seminar on Rainwater Harvesting and Water Management 11-12 Nov. 2006, Nagpur
[3] Rain water harvesting manual, Public Health Engineering Department, Meghalaya http://megphed.gov.in
[4]
http://www.rainwaterharvesting.org/rural/traditi onal2.html
[5] CPWD rain water harvesting and conservation manual, CPWD, India
[6] Kabir Das Rajbhandari, WaterAid in Nepal (2011) Report - Rainwater harvesting for recharging shallow groundwater, WaterAid in Nepal Publication- www.nepal.wateraid.org
