Rainwater Harvesting

RAINWATER HARVESTING AND ITS TECHNIQUES FOR URBAN AND RURAL AREAS

Parveen Gulia, Student M. Tech 3^rd Sem. (Infrastructure Planning)

G.R.D. School of Planning, Amritsar

INTRODUCTION

 “Water is driver of life” Leonardo-Da-Vinci has once said, this line is still true as water is basic to the human health, welfare and economic development. Settlements have developed around the resources of water and these resources have affected form, shape, and structure of these settlements. Fresh water is essential element for survival of human beings. In ancient times water requirement was mainly for drinking bathing and cooking etc but now with the advancement of civilization the utility of water, has increased enormously. Water being a finite   and limited resource so to use it more efficiently for sustainable economic and social development public water supply network/system was developed.

India is one of the few countries in the world endowed with abundant land and water resources it is the 7th largest country in the world and Asia’s second largest country with an area of 3,287,590 Km². Most of the Indian landmass is semi-arid tropical belt characterized by seasonal rainfall lasting over a period of three to four months. Total population of India is over one billion in 2001. The water resources of India are enormous but they are unevenly distributed in space, time and quantity. There are three major resources of water-surface water, ground water and rainfall. But due to lack of proper urban planning with respect to water resource and water budgeting some resources are over exploited while others are being wasted.

The water which reaches to us through rain is absolutely free of cost, sweet, pure and full of minerals. The method through which rain water is saved to recharge underground aquifers and strode for use later on, is known as “Rainwater Harvesting”. Rainwater harvesting is a way to capture the rainwater when it rains and store that water about or charge the underground for its later on use. The rainwater harvesting happence naturally in open rural area, but in construction, mettalled and over paved urban areas metropolitan cities, there is a need to harvests rain water through modern techniques. The decision for storing and recharging water depends on the rainfall pattern of a particular region. The formulation of underground strata and geology of the area also play an important role for changing rainwater. At places where sub-strata is impermeable, recharging is not a feasible idea.

NEED OF RAINWATER HARVESTING:

            Rainwater harvesting is necessary to overcome the inadequate of surface water to meet our demand and it is very necessary to arrest decline in ground water levels. It is also essential to enhance availability of ground water at specific place and time and utilize rain water for sustainable development. With the help of rainwater harvesting we can increase infiltration of rain water in the subsoil this has decreased drastically in urban areas due to paving of open area. It is also essential to improve ground water quality by dilution and to improve ecology of the area by increase in vegetation cover etc.

BENEFITS OF RAINWATER HARVESTING

The main benefits of rainwater harvesting are discussed below:

v  It can bring down consumer utility (Water supply) bills. This is of great value especially to industries, institutions, and commercial establishment that spend considerable sums on water.

v  Rainwater recharging into the ground will have a positive impact on round quality through dilution of fluorides, nitrates and salinity. It would also arrest decline in groundwater levels, provided there is sustainable extraction.

v  Rainwater has a nearly neutral pH and zero hardness. This property makes it very favorable for use in a variety of applications in homes, institutions and commercial establishment and industry. It stored for future use, rainwater can function as a useful supplementary supply thus reducing the stress on public water supply source.

v  Urban flooding can be controlled if the residents of the city harvest rainwater from their roof top for future use or take steps to effectively recharge groundwater within their premises.

v  Using harvested rainwater reduces water demand in the municipality, which in turn reduces energy consumption into water distribution network.

v  The stored water collected from the road is enough for watering the plants in lawns of schools, colleges, hospitals and institutional areas. Rainwater is good for plants because it is free of salts and there minerals that harm root growth. As rainwater percolates into the soil, it forces salts down and away from root zone, allowing roots to grow better and making plants more drought tolerant.

TECHNIQUE OF RAINWATER HARVESTING IN URBAN AREA:-

In urban areas, the changing of underground water in national way like rural areas is not taking place due to large sprawl of urban settlement. Moreover, due to non-changing of rain water during rainy season, the urban areas are causing heavy flood. These floods destroy urban infrastructure and facilities and crops of suburban areas every year. Hence, there is necessity to propagate housing of rain water in urban areas. In urban areas the following technique is useful for Rain water harvesting.

Roof Top Rainwater Harvesting:

The roof top rainwater harvesting requires four elements i.e.

i)                    A catchment area

ii)                  Conduits

iii)                Settlements Tanks

iv)                Recharge or storage facilities

For rainwater harvesting system, a dried bore well, an underground well, storage tank or sock pit are required along with conduits or the pipes carrying rain water from the catchment or roof top harvesting/storage system. The typical model of rainwater harvesting is shown in fig. no. 1.

Fig. No. 1 Typical Rainwater Harvesting Installation

a)      Roof Catchment              

 The roof of the house is used as the catchment for collecting the rain water. The style, construction and material of the roof affect its suitability as catchment. Roof made of corrugated iron sheet, asbestos sheet, tiles or concrete can be utilized as such for harvesting the rainwater.                                                                                                           

                                                                                                   Fig No.2 Roof Catchment    

b)      Gutters

            Gutters are channels fixed to the edges of roof all around to collect and transport the rainwater from the roof to the storage tank.

                                                                                                                                                                                                                             Fig No. 3 Gutter                  

c)      Down pipe

            Down pipe is the pipe, which carries the rainwater from the gutter to the storage tank. Down pipe is joined with the gutters at one end and other and is connected to the filter with the storage tank.

                      

                                          Fig No. 4 Down Pipe

The sizes of down pipes are given in table.

S. No.	Diameter of pipe (mm)	Average rate of Rain Fall (mm per hour)
		25	50	75	100	125	150	200
		Roof Area (Sq m)
I	50	26.4	13.4	8.9	6.6	5.3	4.4	3.3
II	65	48.2	24.1	16.0	12.0	9.6	8.0	6.0
III	75	81.6	40.8	27.0	20.4	16.3	13.6	10.2
IV	100	170.8	85.4	57.0	42.7	34.2	28.5	21.3
V	125	300	-	-	80.5	64.3	53.5	40.0
VI	150	-	-	-	-	-	83.6	62.7

Source: - Manual, consultancy services organization, central public works department.

d)      First flush Pipe 

            Debris, dirt and dust collect on the roofs during non-rainy periods. When the first rains arrives this unwanted material will be washed in to the storage tank this caused contamination of water collected in the storage tank thereby rendering it unfit for drinking and cooking purpose. Therefore a first flush system is incorporated in the roof top Rainwater harvesting system to dispose of the ‘First flush’ water so that it does not enter in the tank.                                                                                   

         Fig No. 5 First Flush Diverter

e)      Filter Unit

            It is preferable to filter the rainwater before storing and recharging the groundwater. Leaves and other organic material in storage tanks decompose and support bacterial growth in the tanks and it contaminate the ground water.

v  Primary Filter/Leaf Guards:  The first level of filtration could be a grating at the outlet of the catchment or the inlet of the gutters or down take pipes to prevent large coarse debris like leaves from entering the rainwater transportation and storage network. For open gutters leaf guards, which are usually ¼ inch mesh screens in wire frames, can be installed along the length of the gutter.

v  Secondary Filter: The second level of filtration is required to remove finer particles and even bacteria, so that the collection rainwater is free from suspended solid and organic contaminates.

i)                    Sand Gravel Filter:

 The Sand gravel filter unit is a contains on chamber filled with filter media such as coarse sand, charcoal, coconut fiber, pebbles and gravels to remove the debris and dust from water that enters the tank.

                                            Fig No. 6 Sand gravel filter

ii)                  Charcoal Water Filter

This is almost similar to sand filter except that a 10-15 cm thick charcoal layer placed above the sand layer. Charcoal layer inside the filter result into better filtration and purification of water.

                                                                  Fig No. 7Charcoal Filter

f)       Storage Facility

            Storage tank is used to store the water that is collected from the roof top. There are various options available for the construction of these tanks with respect to the size shape and the material of construction.

v  Shape: The storage tank may be in various shapes like-cylindrical, rectangular and square.

v  Material of Construction: Rain forced cement concrete (RCC), Ferro cement, Masonry, plastic (polyethylene) or metal (galvanized iron) sheets are commonly used.

v  Position of Tank: Depending on space availability these tanks could be constructed above ground, partly underground or fully underground. Some maintenance measures like cleaning and disinfection are required to ensure the quality of water stored in the contain.

g)      Settlement Tank:

Settlement tanks are used to remove silt and other floating impurities from rainwater. A settlements tank is like an ordinary storage container having provision for inflow (bringing water from the catchment), out flow carrying water to the recharge well and overflow. A settlement tank can have an unpaved bottom surface to allow standing water to percolate into the soil.                                     Fig No. 8 Settlement Tank

h) Recharge Structure:

Rainwater may be charged into the ground water aquifers through any suitable structures like drywell, bore well, recharge trenches and recharge pits.

v  Recharge Pits:

This method is suitable where permeable strata is available at shallow depth. It is adopted for buildings having roof area up to 100 sqm. Recharge pit of any shape is constructed generally 1-2 m wide and 2-3 m deep. The pit is filled with boulders, gravel and sand for filtration of rain water. Water entering in to RWH structure should be silt free. Top layer of sand of filter should be cleaned periodically for better ingression of rain water in to the sub soil.

Fig No.9 Recharge Pit

v  Percolation Pits:

Percolation pits one of the easiest and most effective means of rainwater harvesting. It is generally not more than 60 x 60 x 60 cm pits (designed on the basis of expected run off filled with pebbles or brick) jelly and river sand, covered with performed concrete slobe wherever necessary.                                                        Fig No. 10 Percolation Pit

v  Recharge Trenches:

A recharge trench is a continuous trench excavated in the ground and refilled with porous media like pebbles, boulders or broken bricks. A recharge trench can be 0.5 m to 1 m wide and 1 m to 1.5 m deep. The length of the recharge trench is decided as per the amount of runoff expected. For recharging through recharge trenches, fewer precautions have to the rainfall runoff. Runoff from both paved and unpaved catchments can be tapped.

This structure is capable of harvesting only a limited amount of runoff because of the limited with regard for size.    

                 Fig No. 11 Recharge Trench

v  Modified Injection Well:

In this method water is not pumped into the aquifer but allowed to percolate through a filter bed, which comprises sand and gravel. A modified injection well is generally a bore well which is drilled to the desired depth depending upon the geological conditions, preferably 2 to 3 meter below the water table in the area. Inside a slotted casing pipe of 200 meter diameter is instated. The annual space between the borehole and the pipe is filled with gravel and developed with a compressor till it gives clear water. To stop the suspended solid from entering the recharge tubewell, a filter mechanism is provide at the top.   

Fig No. 12 Modified Injection Well

v  Recharging Through Dugwell and Abandoned Tubewell:

            In alluvial and hard rock areas, there are thousands of wells which have either gone dry or where water level has declining considerably. There can be recharge directly with roof top runoff. Rainwater that is collected on the roof top of the building is diverted by drain pipes to a settlements or filtration tank, from which it flows into recharge well.

            If a dugwell is used for recharge the well lining should have opening (weep-holes) at regular intervals to allow seepage of water through the sides. Dugwells should be covered to prevent mosquito breeding and entry of leaves and debris. The bottom of recharge well should be desilted annually to maintain the intake capacity.

Fig No. 13 Recharging Through Dugwell and Abandoned Tubewell

WATER HARVESTING POTENTIAL

The total amount of water that is received in the form of rainfall over an area is called the ‘rainwater endowment’ of the area out of this, the amount that can be effectively harvested is called the water harvesting potential.

Water Harvesting Potential = Rainfall (mm) x Collection efficiency

            The collection efficiency accounts for the fact that all the rainwater falling over an area cannot be effectively harvested, because of evaporation, spillage etc. Factors like runoff coefficient and the first flush wastage are taken into account when estimated the collection efficiency.

Example:

            Assume in an area there is 100 sq m flat terrace and the average annual rainfall is approximately 600 m (24 inches). In simple term this means that if the terrace floor is assumed to be impermeable and all the rain that falls on its retained without evaporation, then in one year there will be rain water on the terrace floor to a height of 600 mm.

Area of plot =100 sq m (120 sq yards)

Height of the rainfalls 0.6 m (600 mm or 24 inches)

Volume of rainfall over the plot = Area of plot x height of rainfall

Assuming that only 60% of the total rainfall is effectively harvested.

Volume of water harvested = 36, 000 liter. (6000 liters x 0.6)

            This volume is about twice the annual drinking water requirement of a 5 member family. The average daily drinking water requirement per person is 10 liters.

Rooftop area Vs Rainfall wise Availability of Water for Rainwater Harvesting

Rooftop Area	Rainfall (in mm)
(sq.m.)	100	200	300	400	500	600	800	1000	1200	1400	1600	1800	2000
	Volume of Water (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

Source: - Manual, consultancy services organization, central public works department.

RAINWATER HARVESTING IN RURAL AREAS

Alluvial tracts of North India are breadbaskets of the country and agriculture is the main stay.  This part of the country is topographically flat having very little ground slope due to which the flow of rainwater is in the form of sheet.  In spite of vast network of irrigation canals, ground water development is very high. As per the studies carried out in large areas of northern India ground water levels are declining due to over exploitation. The rate of water level decline is more than one meter/year in the certain parts. In order to augment the ground water recharge for sustainable development and management of ground water, artificial recharge to ground water using the surplus rainwater runoff and canal water should be adopted. Otherwise existing ground water abstraction structures will become defunct and/or land under irrigation will reduce. Basic requirements of the artificial recharge are availability of surplus water and space for sub-surface storage.

During the rainy season copious volume of water is available in the surface drains and also in irrigation canals which can be utilized for artificial recharge, apart from the water which is stagnated in the depression, village tanks etc. Sub-surface hydrogeology plays an important role to harness the surplus surface water in limited time and also to recharge to ground water.  The sub-surface hydrogeology is key factor for selection of appropriate artificial recharge techniques. It is observed that if the vadoze zone comprising of silty clay with thick bands of clay the natural recharge is restricted.  In these areas direct techniques are more beneficial as the availability of land is very less and the evaporation from surface water bodies is more.

In rural areas, rain water harvesting is taken up considering watershed as a unit. Surface spreading techniques are common since space for such systems is available in plenty and quantity of recharged water is also large. Following techniques may be adopted to save water going waste through slopes, rivers, rivulets and nalas.

TECHNIQUES OF RAINWATER HARVESTING RURAL AREAS

i)                    Rain water harvesting through gully plug

ii)                  Rain water harvesting through contour bund

iii)                Rain water harvesting through gabion structure

iv)                Rain water harvesting through percolation tank

v)                  Rain water harvesting through check dams / cement plugs / nala bunds

vi)                Rain water harvesting through recharge shaft

vii)              Ground water dams or sub-surface dykes

i)                    RAIN WATER HARVESTING THROUGH GULLY PLUG

Gully Plugs are built using local stones, clay and bushes across small gullies and streams running down the hill slopes carrying drainage to tiny catchments during rainy season. Gully Plugs help in conservation of soil and moisture. The sites for gully plugs may be chosen whenever there is a local break in slope to permit accumulation of adequate water behind the bunds.

ii)                  RAIN WATER HARVESTING THROUGH CONTOUR BUND

Contour Bunds are effective methods to conserve soil moisture in watershed for long duration. These are suitable in low rain fall areas where monsoon run off can be impounded by constructing bunds on the sloping ground all along the contour of equal elevation. Flowing water is intercepted before it attains the erosive velocity by keeping suitable spacing between bunds. Spacing between two contour bunds depends on the slope the area as the permeability of the soil. Lesser the permeability of soil, the close should be spacing of bunds. Contour bunding is suitable on lands with moderate slopes without involving terracing.

Fig No. 14 Rainwater Harvesting Through Gully Plug and Contour Bund

iii)                RAIN WATER HARVESTING THROUGH GABION STRUCTURE

This is a kind of check dam commonly constructed across small streams to conserve stream flows with practically no submergence beyond stream course. A small bund across the stream is made by putting locally available boulders in a mesh of steel wires and anchored to the stream banks. The height of such structures is around 0.5 m and is normally used in the streams with width of less than 10 m. The excess water over flows this structure storing some water to serve as source of recharge. The silt content of stream water in due course is deposited in the interstices of the boulders in due course and with growth of vegetation, the bund becomes quite impermeable and helps in retaining surface water runoff for sufficient time after rains to recharge the ground water body.                                                                              Fig No. 15 Gabion Structure

iv)                RAIN WATER HARVESTING THROUGH PERCOLATION TANK

These are the most prevalent structures in India as a measure to recharge the ground water reservoir both in alluvial as well as hard rock formations.  The efficacy and feasibility of these structures is more in hard rock formation where the rocks are highly fractured and weathered.  In the States of Maharashtra, Andhra Pradesh, Madhya Pradesh, Karnataka and Gujarat, the percolation tanks have been constructed in plenty in basaltic lava flows and crystalline rocks. The percolation tanks are however also feasible in mountain fronts occupied by talus scree deposits.  These are found to be very effective in Satpura Mountain front area in Maharashtra. The percolation tanks can also be constructed in the Bhabar zone. Percolation tanks with wells and shafts Percolation tanks are also constructed to recharge deeper aquifers where shallow or superficial formations are highly impermeable or clayey with certain modification.                                                                        Fig No. 16 Percolation Tank

v)                  RAIN WATER HARVESTING THROUGH CHECK DAMS / CEMENT PLUGS / NALA BUNDS

Check dams are constructed across small streams having gentle slope and are feasible both in hard rock as well as alluvial formation.  The site selected for check dam should have sufficient thickness of permeable bed or weathered formation to facilitate recharge of stored water within short span of time.  The water stored in these structures is mostly confined to stream course and the height is normally less than 2 m.  These are designed based on stream width and excess water is allowed to flow over the wall.  In order to avoid scouring from excess run off, water cushions are provided at down streamside.  To harness the maximum run off in the stream, series of such check dams can be constructed to have recharge on regional scale.

A series of small bunds or weirs are made across selected nala sections such that the flow of surface water in the stream channel is impeded and water is retained on pervious soil/tock surface for longer body. Nala bunds are constructed across bigger nalas of second order streams in areas having gentler slopes.  A nala bund acts like a mini percolation tank.

Check Dams                                                Cement Plugs

Fig No. 17

vi)                RAIN WATER HARVESTING THROUGH RECHARGE SHAFT

This is the most efficient and cost effective technique to recharge unconfined aquifer overlain by poorly permeable strata. Recharge shaft may be dug manually if the strata is of non-caving nature. The diameter of shaft is normally more than 2 m. The shaft should end in more permeable strata below the top impermeable strata. It may not touch water table. The unlined shaft should be backfilled, initially with boulders/ cobbles followed by gravel and coarse sand. In case of lined shaft the recharge water may be fed through a smaller conductor pipe reaching up to the filter pack. These recharge structures are very useful for village ponds where shallow clay layer impedes the infiltration of water to the aquifer. It is seen that in rainy season village tanks are fully filled up but water from these tanks does not percolate down due to siltation and tubewell and dugwells located nearby remains dried up. The water from village tanks get evaporated and is not available for the beneficial use. By constructing recharge shaft in tanks, surplus water can be recharged to ground water. Recharge shafts of 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed depending upon availability of quantum of water. The top of shaft is kept above the tank bed level preferably at half of full supply level. These are back filled with boulders, gravels and coarse sand. In upper portion of 1 or 2 m depth, the brick masonry work is carried out for the stability of the structure. Through this technique all the accumulated water in village tank above 50% full supply level would be recharged to ground water. Sufficient water will continue to remain in tank for domestic use after recharge.                                                                 Fig No. 18 Recharge Shaft

vii)              RAIN WATER HARVESTING THROUGH GROUND WATER DAMS OR SUB-SURFACE DYKES

Sub surface dyke or under-ground dam is a subsurface barrier across stream which retards the base flow and stores water upstream below ground surface. By doing so, the water levels in upstream part of ground water dam rises saturating otherwise dry part of aquifer. The site where sub-surface dyke is proposed should have shallow impervious layer with wide valley and narrow out let. After selection of suitable site, a trench of 1-2 m wide is dug across the breadth of stream down to impermeable bed. The trench may be filled with clay or brick/ concrete wall upto 0.5m. below the ground level. For ensuring total imperviousness, PVC sheets of 3000 PSI tearing strength at 400 to 600 gauge or low-density polythene film of 200 gauges can also be used to cover the cut out dyke faces. Since the water is stored within the aquifer, submergence of land can be avoided and land above the reservoir can be utilized even after the construction of the dam. No evaporation loss from the reservoir and no siltation in the reservoir takes place. The potential disaster like collapse of the dams can also be avoided.                                    Fig No. 19 Ground Water Dams or Sub-Surface Dykes

CONCLUSIONS

            Rain water harvesting is the techniques of collection and storage of rain water at surface or in sub surface aquifer, before it is lost as surface run off. The augmented resource can be harvested in the time of need. Artificial recharge to ground water is a process by, which the ground water reservoir is augmented at a rate exceeding that under natural condition of replenishment.

            Rainwater harvesting is the need of present scenario. Because shortage of portable water supply in urban and rural areas is one of the main problem which faced by all the countries. The underground water is depleting day by day where as population is increasing at very fast rate because of it is necessary to capture every drope of rain and rain water harvesting is very suitable technique to avoid water crisis.

BIBLIOGRAPHY

1.      Ministry of Water Resource, Central Ground Water Board, Faridabad, Rainwater Harvesting Techniques to augment ground water.

2.       Rainwater Harvesting Conservation Manual, Government of India, New Delhi.

3.      www.cse.com (Date 15/12/2009)

4.      www.rainwaterharvesting.org (Date 16/12/2009)