Estelar CHAPTER-IV. Rain Water Harvesting

Transcription

1 CHAPTER-IV Rain Water Harvesting 4.1 General: Rain water is the main source of fresh water which is available annually through the hydrological cycle. The principle of water harvesting is collection and storage of the rainwater when and where it falls. Major portion of available rainwater goes unutilized in the Himalayan region mainly due to its distribution pattern and topography of the area. About 80% of annual rainfall occurs in three months during monsoon. The only option is to harvest this rainwater and to store it for lean season. In the areas where slopes are steeper, overland flow gets flushed of very swiftly resulting in non-availability of water in peak water stress period even through the average rainfall is very high. In such areas efforts should be made to harvest maximum rainwater, possible by constructing suitable water harvesting structures to artificially recharge the ground water and store water for domestic use by constructing roof water harvesting structures and water harvesting structures. Integration of Remote Sensing and Geographical Information System (GIS) techniques provides reliable, accurate and updated database to identify suitable sites for water harvesting such as farm ponds, ground water recharge zones, check dams, percolation tanks, nala bundh and shaugel pond etc. The population of the Dabka watershed is completely dependent on spring water for their daily needs. The increased population pressure and declining trends of spring discharge has caused severe water shortage in the area. For the optimum use of the available water resources in the catchment, it is necessary for conservation of the water resources in the area. The area, Dabka watershed comprises fifteen villages out of which, seven villages namely Bansi, Ghughukhan, Sigri, Saur, Dola, Baghni, Jalna have been found to face severe water scarcity during water stress periods of the year. Beside these villages some parts of other villages and grazing lands located at comparatively head ward region of the area have been found to suffer from this 60

2 issue. Therefore, to make the availability of water throughout the year for domestic, animal husbandry and local agricultural use, an attempt has been made to conserve water thorough rainwater harvesting methods. In this regard an attempt has been made by conducting pilot studies in few selective villages, to assess the conservation of water. Irrigation is very essential for the survival of agricultural crops in rainfed areas where slopes are steep and dry. The excess rainwater has to be conserved or stored in different storage structures, which can be used later. Water harvesting systems have largely focused on the technology of roof top in catchment areas. Usually the roof is made of tiles (roof stones), iron or asbestos sheets. It is connected by gutters and down pipes to one or more storage containers ranging from simple ponds to large cement tanks. 4.2 Methodology for Rainwater Harvesting:-Site selection for water harvesting structure, the technical guidelines of Integrated Mission for Sustainable Development (IMSD), prepared by National Remote Sensing Centre (NRSC), Indian National Committee of Hydrology (INCOH) are used in the present study. On the basis of structure, lithological, geomorphological, socio-economical conditions, check dams and shaugel pond are proposed in the study area. For the site selection of Check dams lower order streams up to third order (Fig. 4.2) with medium slopes are taken. Check dams are proposed where water table fluctuations are very high. The slope of the region was considered flat to gentle, so that the maximum quantity of water could be stored. Farm ponds can be proposed within individual farms. The main aim for the construction of Farm ponds is, i) to provide water storage for life saving irrigation in a limited area, ii) to provide drinking water for livestock and human beings in arid areas, iii) to serve as water storage for providing critical irrigation to a limited number of fruit plants for establishment purpose. 61

3 Nala bundhs and percolation tanks are structures constructed across nallah or streams for checking velocity of runoff, increasing water percolation etc. Nala bundhs are less expensive which is smaller in dimension and constructed usually by local material. The percolation tanks are large and more expensive. The main aim for construction of Nala bundhs and Percolation tanks are i) to impound surface runoff coming from the catchment and to facilitate percolation of stored water into the strata to raise the groundwater level and ii) to hold the site flow. 4.3 Site selection for harvesting:-the area under investigation comprises of very rugged topography which includes hillocks with steep slopes, escarpments with cliffs and long narrow ridges. Apart from the area comprises of tightly folded and faulted rock terrain with closely spaced fractures and shears. The available formation exhibits their respective attitudes vary in different directions. Hence, the configuration of formations in the area is found to be dominantly manifested with the fracture porosity and bedding planes in different directions. However, the intensity of the distortion of such directions is found increasing towards the main thrusts. So, construction of recharge wells may not feed recharge to the right desired directions. The area receives an annual average rainfall of the order of cm (Table 3.2). The monsoons are found to contribute the major portions of the rainfall. The socio economical condition of the inhabitants is observed to be based on the subsistence agro-economy of the area, animal husbandry etc. Thus, the inhabitants are forced to lead a difficult life. Therefore, keeping in consideration the geology, topography and social economic conditions of the inhabitants an attempt has been made to propose the construction of selective water harvesting structures in the Dabka watershed so that their living condition could be improved (Fig. 4.2). 62

4 Three types of structures may be recommended for the area (i) Shaugel ponds (ii) Check dams (iii) (i) Farm ponds Shaugel Ponds: - Shaugel ponds for rainwater harvesting and overland flow interception have been proposed in the area. The different feasible locations are Taliya (1 no.), Dhanak (1 no.), Jalna (2 nos.), Chhara (1 no.), Salba (1 no.), Sigri (1 no.), Ghughukhan (1 no.), Saur (3 nos.), Banshi (4 nos.), Baghni (5 nos.) are proposed (Fig.4.2). The shape of such ponds is governed by lithology, slope and structures of the bed rock. The area in general comprises of hard rock terrain in upper reaches with slopes in the range of more than 20 0 with dominance of fracture porosity will be introduced with such ponds of trench shape. Cube or cuboid shaped ponds will suit to the parts of the area having slopes in the range of and considerable unconsolidated regolith cover. Whereas the bowel shaped ponds are best suited in the areas having gentler slopes in the range of , with thick cover of unconsolidated regolith. The construction of Shaugel pond (Fig. 4.1) is efficiently low cost and easy to maintain. To stop infiltration of water thin plastic lining is used with clay covering. It is calculated that for a pond having 2.50 m width, 1 m height and 2 m length, about 16 m 2 plastic sheets would be required. For maintenance point of view it is very easy and low cost effective. 63

5 Fig. 4.1: A sample of Shaugel pond (ii) Check Dams: -The proposed sites selected for construction of Check dams (Fig. 4.2) on the comparatively higher reaches will arrest the part of the sediment load being shifted from their still respective higher reaches and overland flow during rainy spells thus diffusing the potential of the sliding material and diffuse runoff to minimize its erosional impact farther down slopes. Such check dams are to be constructed by dry stone masonry work. The arrested regolith will behave like aquifer skeleton and will absorb part of the rainwater or overland flow. The interception of rainfall or overland flow in this way would definitely lead to prolonged seepage of water down slopes which otherwise has flushed out spontaneously in absence of such structures. The construction of such Check dams will be cost effective and will help in maintaining sustainable flow in respective nallahs for longer periods after an event of rainfall. Whereas proposed sites for such structures on the perennial nallahs of the area have to be constructed in accordance with the design by engineers so as to ensure optimum availability of water all the year round. Check Dams are proposed at Baghni (1 no.), Bansi (1 no.), Saur (3 nos.), Sigri (1 no.), Ghughukhan (1 no.), Jalna (3 nos.) and Taliya (1 no.). (iii) Farm pond: - Farm pounds are purposed at farms in the catchment. Farm ponds are proposed at Baghni (4 nos.), Bansi (2 no.), Saur (6 nos.), Jalna (2 nos.), Devipura (1 no.) to support horticulture, floriculture and seasonal vegetable farming in the area (Fig. 4.2). 64

6 Fig. 4.2: Proposed locations for water harvesting structures in Dabka watershed 65

7 4.4 Roof top rain water harvesting:-domestic rain water harvesting or Roof top rain water harvesting is the technique through which rain water is captured from roof catchments (Fig. 4.3) and stored in tanks, ground water aquifers. It consists of conservation of roof top rain water and to increase ground water storage by artificial recharge. It requires connecting the outlet pipe from roof top to divert collected water to existing tanks, wells, ground water aquifers etc Component of Roof Top Rain Water Harvesting: (Rainwater Harvesting and conservation manual, 2002) The system comprises the following basic components:- i) Catchment area/roof: surface upon which rain falls. ii) Gutters: transport channels from catchment surface to storage. iii) Cisterns or storage tanks: where collected rain water is stored. iv) Conveying: the delivery system for treated rain water, either by gravity or pumps. v) Water Treatment: filters and equipment and additives to settle, filter and purify. Fig. 4.3: Sample of roof top rain water harvesting for hilly area 66

8 4.4.2 Methodology for Roof top rainwater harvesting:-roof top rain water harvesting methodology has been taken from manual of Consultancy Services Organization, Central Public Work Department, New Delhi. For this pilot project seven sample village namely- Sigri, Bansi, Ghughukhan, Saur, Dola, Baghni, Jalna were selected. A few houses are selected to calculate the Rainwater harvesting potential and effective harvesting of water by following the Central Public Work Department, New Delhi manual. To compute runoff coefficient of corrugated metal sheets coefficient is taken (Table 4.1). For calculating Effective rainwater harvesting potential the average annual rainfall 172.8cm is taken. The calculated availability of rainwater through roof top is compared with the calculated values (Table 4.2). The amount of rain fall which could be harvested in different villages is given in (Table 4.3 to Table 4.9) Sample calculation for effectively harvested water from total rainfall (Bansi village) The total amount of water i.e. received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this the amount that can be effectively harvested is called the rainwater harvesting potential. Rain water harvesting potential Annual rain fall of the area Height of the rainfall Area of Roof Catchment = Rain fall (mm) X collection Efficiency = 1728 mm = m = m = 40 sq m Vol. of rain fall over the plot = Area plot X height of rain fall Rain water endowment of that area = 40 Sqm X m = cum = 69,120 liters Considering roof catchment is having Corrugated metal sheet, so coefficient for roof surface can be taken as 0.80 (Table 4.1) 67

9 Another constant coefficient for evaporation, spillage and first flush wastage can be considered as 0.80 (for all situations). Effectively harvested water = Rain Water endowment of that area (A) X 0.80 X Surface efficient = 69,120 X 0.80 X 0.80 = liters The rain water falling over an area cannot be completely harvested because of evaporation, spillage etc. First spell of rain is flushed out, evaporation and spillage cannot calculate factors like run off coefficient as shown various types of roof and land surfaces etc. (Table 4.1). So, a constant co-efficient of 0.80 may be assumed for all situations. This is because of the first spell of rain carries with larger amount of pollutants from the air and catchment surface area. Table 4.1- Runoff co-efficient of various surfaces (Source - Rainwater harvesting and conservation manual, 2002) 1. Roof catchment Co-efficient 1.1 Tiles Corrugated metal sheets Ground surface covering 2.1 Untreated ground catchments Soil on slope less than 10% Rocky Business area Down town Neighborhood Residential complexes in urban area Single family

10 2.2.2 Multiunit, detached Multiunit, attached Residential complexes in Suburban area apartments Industrial Light Heavy Parks, cemeteries Play grounds Railroad yard Unimproved land areas Asphaltic or concrete pavement Brick pavement Lawns, sandy soil having slopes Flat 2% Average 2 to 7% Steep 7% Lawns, clayey soil having slopes Flat 2% Average 2 to 7% Steep 7% General Driveways and walls

11 Table 4.2 Availability of Rainwater through Roof Top Rain water Harvesting Rainfall (mm) Roof top area (Sqm) Harvested water from Roof Top (cum) Source: Rainwater harvesting and conservation manual,

12 Table 4.3: Roof top rain water harvesting in Bansi village Total No of house 50 No. of houses Length of one Breadth of the Surface area Rain fall in Rain water harvesting the roof (m) one roof (m) of the roof ( m 2 Effective harvesting water (lit) ) year 2002 (m) potential (cubic mt) Source: Census data and Field survey. 71

13 Table 4.4: Roof top rain water harvesting in Ghughukhan village Total No 22 of houses Average Effective Length of one Breadth of the Surface area of Rain water harvesting the roof (m) one roof (m) the roof ( m 2 annual rain harvesting water ) potential (cubic mt) fall (lit) Source: Census data and Field survey. 72

14 Table 4.5: Roof top rain water harvesting in Sigri village Total No of house 15 Length of one the roof (m) Breadth of the one roof (m) Surface area of the roof ( m 2 ) Average annual fall rain Rain fall in (mt cube) Rain water harvesting potential (cubic mt) Effective harvesting water (lit) Source: Census data and Field survey. 73

15 Table 4.6: Roof top rain water harvesting in Saur village Total No of houses 70 Average Effective Length of one Breadth of the Surface area of Rain water harvesting the roof (m) one roof (m) the roof ( m 2 annual rain harvesting ) potential(cubic mt) fall (lit) water Source: Census data and Field survey. 74

16 Table 4.7: Roof top rain water harvesting in Dola village Total No of house 15 No. of houses Length of Breadth of Surface Average Rain water harvesting Effective harvesting water one the the one roof area of the annual rain roof (m) (m) roof ( m 2 potential (cubic mt) (lit) ) fall Source: Census data and Field survey. 75

17 Table 4.8: Roof top rain water harvesting in Baghani village Total No of house 10 Average No. of Length of one Breadth of the Surface area of Rain water harvesting houses the roof (m) one roof (m) the roof ( m 2 annual rain ) potential (cubic mt) fall Source: Census data and Field survey. Table 4.9: Roof top rain water harvesting in Jalna village 76 Effective harvesting water (lit) Total No of house 10 Average Effective No. of Length of one Breadth of the Surface area of Rain water harvesting houses the roof (m) one roof (m) the roof ( m 2 annual rain harvesting water ) potential (cubic mt) fall (lit) Source: Census data and Field survey.

18 A common practice is to use average coefficients for various types of areas and assumed that the coefficients will be constant throughout the duration of the storm. Village name The approximate volume of water available for harvesting with respect to roof top area and annual rain fall has been shown in (Table 4.2) for designing the Rain Water Harvesting Structures. Table 4.10: Village wise analysis for Roof Top Rain water Harvesting No. of houses Population in 2001 (NIC, Nainital) data ) Rain water harvesting potential (cubic mt) Effective harvesting water (lit) Per day total water availability (lit) Bansi Ghughukhan Sigri Saur Dola Baghni Jalna Total Source: National Information Centre (NIC) and Census data, Nainital No. of persons that could be provided water at the rate of 40 lit/day 77

19 4.5 Observation and discussion: In the upper reached water scarcity problem has been observed. To reduce the water scarcity problem three rainwater harvesting structures Shaugel pond, Check dam and Farm ponds are proposed at different locations by taking consideration of geology, geomorphology, geomorphology, socio-economic conditions. It is observed that constructions of Shaugel pond are low cost effective and easy to maintain. Check dams are proposed to harvest rainwater for local people and for recharge the ground water. At some places form ponds are also proposed in the individual farm to support farms horticulture, floriculture and seasonal vegetable farming. Careful perusal roof top rainwater harvesting of the available rain water, for harvesting in the seven pilot villages indicate that the area has immense potential for harvesting rain water in Dabka watershed. It is calculated that 15 houses of Bansi, 22 houses of Ghughukhan, 15 houses of Sigri, 70 houses of Saur, 15 houses of Dola, 10 houses of Baghni and 30 houses of Jalna, can harvest lits water per day (Table 4.10). In which 823 (Table 4.10) person at the rate of 40 lit/day from the seven selected villages can be fed throughout the year. In area which is facing acute shortage of water, such water harvesting and spring water conservation measures could be of immense help to the community. 78