Digeshwar Prasad Sahu*

National Institute of Technology, Raipur 492010, Chhattisgarh, India

*Corresponding  author: digeshwarsahu@gmail.com

[Received 21 January 2019; Revised version 06 March 2019; Accepted 14 March 2019]

Abstract. Water is crucial and it has always been considered throughout history as a natural resource for the survival of humanity and other living beings therefore globally the shortage of quality water and its availability is an important issue. Groundwater is the only source of water of the Swami Vivekananda Airport Raipur, and it faces water crisis when the groundwater level decreases. In this scenario, Rain Water Harvesting (RWH) can contribute considerably to tackle these problems. Rainwater treatment systems can be installed at different potential locations in study area based on their physical site conditions such as slope, elevation etc. The airport has 38955.62 Square meters rooftop area out of 2.108 Square Kilometres total area, water collected from the rooftop will fulfil approximately 30% of the total daily demand of the airport. Apart from this airport is properly designed drainage system storm water is collected and separated into three potential zones. Zone I, II and III having area 0.2393, 0.4307 and 1.4382 sq. Kms respectively, which produce a different runoff amount based on maximum daily rainfall. Storm water is treated by Slow Sand Filter (SSF) of rate of 200 litres per hour per square meter and stored under design capacity, size and number of treatment unit depend on the maximum discharge at their respective zones. Treated rainwater is utilized for recharge purpose through the recharge trench when excess water comes from the rainwater tank. The cost-benefit analysis also done for before and after implementation of the project.

Keywords: Drainage design, Rainwater harvesting, Rainwater treatment, Groundwater recharge.

Introduction

The rapid growing demand of fresh water across the globe arises as a disquieting issue for both developing and developed countries, so that the demand of drinking water with proper quality also increases. In highly populated areas like urban area, the availability of fresh water even for daily use is not within reach and need external processes to get the water to the inhabitants of the area. Assessing and managing justified of water resources can help for preservation and sustainable use becomes a vital issue in person's life, mainly in an area where the ground water level is very low and had less rainfall. In this sequence to complete the freshwater demand of inhabitants, use of rainwater is becoming an extensively influential instrument. For harvested water to remove microbial contamination and other chemical substances, rainwater needs some treatment system prior utilize that water. The type of treatment to be provided depends on the purpose of intended use and characteristics of collected water from the ground surfaces or roofs. A low-cost traditional treatment method like slow sand filter and disinfection by chlorination can be used for the region like an airport where treatment should be done only for rainy days and SSF is a highly efficient filter that removes 98 to 99% of bacterial contamination from water.

India has approximately 17% of the world's population and has only 4% of water resource in the world. India gets fresh water from precipitation about 4000 trillion liters in the form of snowfall and rain and the maximum of this water reaches to the ocean and seas through the so many rivers which are flowing across the country. The United Nation (UN) apprise that the shortage of freshwater is the most serious problem to produce enough amount of food for a growing population of the world. Declination in per capita availability of water in 1947 to 2017 is 5200 m3 to 1500 m3 per year respectively (Report of Indian Institute of Remote Sensing, Dehradun). In different parts of India water is being used at a much faster rate than can be only refilled by rainwater. Therefore, we can say that India is not far away from water crises so we need to adopt some other water resource.

The locality such as airport required water in significant amount to manage their operational routine and infrastructural demand. The main purpose of this study is to reduce flash flood and use rainwater for the non-potable demand of airport. Swami Vivekananda airport Raipur has a source of water only in the form of groundwater, during summer airport faces water crises because of lower down the groundwater table. Although in monsoon because of undulation in the ground level (slope variation) storm water create a flooding problem. Therefore, to resolve all these problems Rainwater Harvesting is one of the exclusively important methods to satisfy the demand of the airport. Proper drainage system also required for collection of storm water at different potential zones where water gets treated. After knowing all the significance of fresh water to our growing inhabitants and thriving industries, to compensate these highly increasing demands RWH techniques can be adopted.

Methodology

Study area

Raipur, the capital city of Chhattisgarh and Swami Vivekananda Airport is near to the city and its boundaries spread in the range between 21˚10ʹ15ʺ to 21˚12ʹ00ʺ North latitudes and 81˚43ʹ27ʺ to 81˚46ʹ20ʺEast longitudes. Airport bounds are limited to toposheet no. 64G/12 and 64G/11 which is provided by Survey of India (SOI). Thetotalplotareaoftheairportisapproximately520.89 acre (2.108 sq. Kms.). Semi-arid tropical climatic conditions prevail in Raipur urban area.

The area experiences a very warm summer of longer duration of March to the middle of June and after this monsoon season will be started, which lasts for almost four months from the middle of June to September. December to the end of February study area faces the winter season. Temperature varying from 10°C to 46°C, humidity ranges from 30% to 85% and it receives 1185 mm an average rainfall.

Figure 1. Location map of the study area

Physiographical study area is situated in the South-Central part of Chhattisgarh basin having gentle undulating topography. Raipur is situated on the Proterozoic Chandi Formation of Raipur group (Chhattisgarh Super Group), comprising of limestone, shale and sandstone. In the airport area, there is the occurrence of limestone and shale. Rainfall run off modelling

The model was created using ArcGIS extension known as HEC-GeoHMS, which includes various steps in a series collectively term called as terrain preprocessing. The data needed for modeling such as DEM, LULC, soil maps are imported and merged with the proper projection system. Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) of version 4.1 is used for rainfall-runoff modeling. The HMS model allowed parameterization of various infiltration losses could choose by the modeler. For spatially dispensed calculation of infiltration enables by the Soil Conservation Service (SCS) curve number (CN) method. The SCS-CN method is used for quantifying storm runoff of a particular area on the basis of their soil, land use land cover type and hydrological soil group. The ability of infiltration of any soil to decide that soil fall under which hydrological soil group. To differentiate the infiltration and runoff from the rainfall some important equation is used which are empirical and derived by the infiltration loss method.

Where P is effective rainfall depth, Q is event discharge or Surface runoff (mm), S is the potential maximum soil retention, and Ia is Initialabstraction.

Where value of λ vary according to soil type and Antecedent Moisture Content (AMC) and CN also varies in the range of 0 (no runoff produces) to 100 (produce all rain as runoff) which depend on LULC and soil condition.

Figure 2. Flow chart of rainfall runoff model	Figure 3. Flow chart of rainwater treatment

Drainage design

The Storm Water Management Model (SWMM) is developed by EPA, which is extensively used for urban runoff simulation. The SWMM is widely operating for analysis, design, and planning related to the urban drainage system. SWMM of version 5.1 is used for designing of the drainage network, input data editing, simulation run and showing the results in the suitable form of tables, thematic maps, graphs and reports in statistical format. IDF curve has generated to determine the intensity of the rainfall event at various time durations for different return periods. These curves have generated for the return periods of 2, 5, 10, 20, 25, 30, 50 and 100 years. The design of drainage system was done for the 20-year return period. The cross-section of the barrel is taken as a rectangle and the size (depth & width) of the barrel is depending on the water accumulated at a particular point. The study area has 14 sub-catchments, 27 junctions which collect and divert storm water towards the outfall. The outfall is located at the lowest elevated point of respective zones, and water is forwarded to the treatment unit where it gets treated and stored.

Rainwater treatment and storage

Storm water contains sediment particle, to remove the sediment settling tank can be constructed so that the filter will not be chocked during filtration. The Size of the settling tank and SSF is based on the maximum discharge at the outlet point. The rate of filtration (ROF) is must be high so that filter system can complete the demands and to meet this filtration rate thickness of the layers in the filter will be specified. After filtration clear water forwarded the rainwater tank, water can be stored in the tank under its design capacity and excess water used for groundwater recharge when it reaches to recharge structure. In recharge pit, 2 perforated borewells are installed to recharge ground water because of the geological formation in the form of limestone and shale available in the study area. Therefore, some fractures are present in the ground, which can utilize for water recharge and the depth of the borewells is depends on the availability of fracture below the recharge structure.

Result and discussion

Figure 4. Water consumption profile of studyarea

Models calibration

Rainfall runoff of relation in monthly wise is calibrated through the HEC-HMS modelling. These relations are essential for managing the drainage design and dimensions of barrel and junctions are provided for 20-year return period.

Figure 5. Monthly Rainfall - Runoff relation

In the HMS, Monthly variation of rainfall-runoff is calculated for 34 years (1980 to 2013) which shows the scenario of runoff. The drainage system is designed using SWMM, a detailed network system is shown in figure 6 through the barrels stormwater passes and goes to the treatment unit. Some essential flow checks can be provided inside the storm water drains to retain the debris.

Table 1. Runoff volume of the Study area

Figure 6. Drainage map of the study area

Sedimentation tank

The design of sedimentation tank is based on the maximum daily runoff on the study area. Some important flow checks should be installed within the drains which contains storm water so that settling of the silt is restricted. A mesh can be provided near the inlet of tank to remove the debris entering inside the settling tank. Size of the sedimentation tank is varied according to the water availability at outlet points.

	Freeboard 0.3 m Supernatant water 1.5 m Filter medium(sand) 0.9 m Gravel bed 0.4 m Brick filter bottom 0.16 m Total depth 3.26 m
Figure 7. Sedimentation tank	Table 2.  Depth of layers in SSF
Figure 8. Section view of SSF	Figure 9. Section view of Rainwater tank

Slow sand filter

Clean river sand of effective size 0.2 to 0.35 mm and thickness of sand bed is 0.9 m. Gravel effective size 3 to 60 mm and thickness of gravel bed is 0.4 m. Rate of filtration 200 liter per hour per square meter and width of wall is taken as 0.23 m of the filter. SSF should be installed parallel near the settling tank and minimum 2 filters must be provided as per guidelines.

Rain water tank

Rainwater tank is design for 80% capacity of total runoff accumulated in 48 hours because of the limited area and 10% extra volume is provided for airspace in the tank (shown in figure 9). Excess water goes to the recharge trench to raise the ground water level. The separate rainwater tank is also constructed for rooftop water which is collected near existing treatment unit of the airport. The rainwater tank should be clean before the rainy season so that the tank is free from any contamination.

Recharge structure

The artificial recharge systems are engineering technique, where available surface water is injected in or on the ground for process like infiltration and some other subsequent movement to aquifers to increase groundwater level. Some other purposes of artificial recharge are to improve the quality of the water through soil-aquifer treatment or geo-purification, to make groundwater out of the surface water where groundwater is traditionally preferred over surface water for drinking purpose, to store surface water, and to use the aquifers as a water conveyance system, The geological formation of the airport is situated on the Proterozoic Chandi Formation, which is having low infiltration capacity. Therefore, recharge well can be constructed at significant depth based on the geological survey by instrument such as resistivity meter which can show the availability of fracture below the ground.

Conclusion

In Swami Vivekananda Airport the application of rainwater harvesting is needed to meet the increasing water consumption demands such as cooling system, domestic use etc. The water supply at the airport depends on groundwaters which are taken from five borewells located within the study area. The wastewater from the terminal building is treated at the sewage treatment plant and the treated wastewater is used for irrigation of gardens. The model performance criteria with different parameter showed that the models result was excellent in runoff estimation, drainage design, rainwater treatment, and recharge of ground water in the study area. The airport has a large open area which collects a huge amount of stormwater at different zone of the study area that will help to save about 126.34×103 m3 of water per year and excess water is used for groundwater recharge. Filter system such as slow sand filter can provide economical and sustainable water reuse facility for large areas like the airport, and it is used where rate of filtration demand is less. Construction of the recharge wells can be done at the pre-determined locations. The depth of the recharge wells may vary as per specific site conditions and depth is to be restricted up to the depth where the well starts giving discharge. This will ensure intake of runoff water so that water get recharge at a rapid rate.

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