Ramachandra T V
ENVIS[RP], Environmental Information System, Energy and Wetlands Research Group,
Centre for Ecological Sciences, Indian Institute of Science - 560012
envis.ces@iisc.ac.in    tvr@iisc.ac.in      Phone: 080 22933099/22933503

3.0 STUDY AREA

Indian Institute of Science (IISc) campus has been considered for the present study. The campus limits are enclosed within 13^o00'38''N to 13^o01'15''N latitude and 77^o33'34''E to 77^o34'26''E longitude. The area falls in the 57G/12 of SOI topo-sheet of scale 1:50000, 57G of scale 1:250,000. IISc is located in northern part of Bangalore city and Figure 1 gives the map of IISc campus with important landmarks. It is endowed with lush green vegetation and has a green canopy of trees covering the buildings in the campus.

The campus, consisting of five bounds, covers an area of 180 ha. The main campus covers 158 ha. The altitude of campus varies from 914m (from Mean Sea Level) to 942 m. The natural terrain of the campus is rolling and provides good natural drainage. The campus slopes towards three sides (north, east and west) from a central ridge along the middle and along the main road of the campus (Gulmohar Marg). Historical precipitation data of 54 years reveal that, the major portion of rainfall being May-June and thunderstorms during September-October. Bangalore receives mean annual rainfall of 860 mm (Radhakrishnan et al., 1996). Sankey Lake located to the Southeast of the campus. A large drain running all along the eastern side of the campus forms the main feeder of the Sankey Lake.

4.0 METHODOLOGY

4.1 Development of database (spatial and attribute data) for GIS

The Survey of India (SOI) toposheets of scale 1:1000 of IISc campus in 10 sheets and GIS software Geomedia Professional 5.1 and Geomedia Grid were used to build the vector layers of IISc boundary, boundary with landmarks, buildings, roads, surrounding lakes of IISc, IISc zones based on utilities, tree distribution, canopy cover in IISc based on NDVI ranges, drainage and contours.

A vector layer of vegetation showing all the trees (species wise) in the IISc was prepared through an extensive field survey carried out simultaneously. Contour maps (generated using Geomedia Professional 5.1 and Geomedia Grid) are used to generate slope maps of the area and DEM.

The SOI toposheets (scale 1:1000) were scanned and georegistered in geomedia professional. For georegistration a feature class definition of point feature was made and the original corner points from the topsheets were inserted for that point feature class. Then the scanned toposheet was brought to the geoworkspace working window with the help of insert interactive images. The image is selected through the select tool and source control point and target control points are inserted through the image registration dialog. For each feature class, projection system is defined and the datum and projection is set as per the geographical location of the region and through digitization of the raster image, individual features (like roads, contours etc.) are transformed to vector format.

Different feature class (layers) of IISc was created. This includes boundary, boundary with landmarks, buildings, surrounding lakes, zones based on utilities, tree distribution, canopy cover and contour layer (1m interval) by digitizing toposheets SOI (scale 1:1000).

4.2 DEM generation in GEOMEDIA grid

The contour feature (layer) of one m interval is brought to the geoworkspace working window and new study area is defined and cell resolution and output unit is set for the new study area. The contour features are selected from the legend and are rasterized. Through the edit window the non-void cells values are combined and the values are noted and one third of the noted value is calculated for random sampling. The output unit is set from the information dialog, same as that of the output unit of study area defined. Interpolation of the random sample result layer is done and the interpolated layer is smoothed. The depression (in DEM) is filled for the smoothed layer. The filled depression layer's name with color assignment to the layer was done through the edit window (by selecting the first and last cell in the edit window and right clicking on the selected cells to assign color's through color sequence dialog) of filled depression layer. For DEM, the shaded relief model is made with the filled depression layer and is viewed in the map window.

4.3 Field Investigation

A detailed field survey was undertaken to map trees (with diameter >10 cm) in the campus. Figure: 2 shows tree distribution in IISc campus. The field investigation gave an idea of the terrain, drainage pattern, vegetation cover and other constraints. Apart from this, a detailed field survey was under taken to map trees in various sub catchments. The drainage network of the campus was also studied to find out the possible problems of channeling the rainwater. The slope of the drains was also noted to delineate the catchment for the Pond. The local problems like blockage, clogging of the drains were also noted, so that remedial actions could be taken.

4.3.1 Storm-water pond

Economic and ecological considerations highly favor the creation of a water body inside the campus and divert the storm water generated in the campus to the pond. The water stored in the pond may be used for various purposes such as gardening and for meeting the non-drinking water needs of the adjacent buildings. A small filtration plant near the pond would help in treating the stored water usable for various purposes. A garden may be developed around the pond to enhance the aesthetic appeal. Creation of the water body, apart from providing usable water, is also advantageous from ecological considerations by providing a water source for the number of bird species (about 110 species) that the campus is proud to be a habitat.

The need to conserve water assumes more importance than ever before in the present scenario of water table depletion due to increased demand and over exploitation of groundwater resources. Surface water harvesting has multiple advantages, viz., recharge of ground water and efficient use of rainwater, which otherwise gets wasted or contaminated. This study through spatial and temporal data highlights the parameters to be considered for designing harvesting pond at an appropriate suitable location in the campus. The harvesting pond is designed with a detailed investigation of land use pattern, hydrological analyses, catchment delineation and identification of potential problem areas. The optimal design was arrived at taking into account ecological, economical, social and technical aspects.

4.4 Data analyses

Data analyses involved:

Calculations of the area of sub-catchments and land use analysis,

Computation of water yield in respective sub-catchments, Suitable location of harvesting structures, Optimal capacity of the pond.

The sub-catchments were delineated from the main catchment area and coefficients of runoff were given to each according to their catchment characteristics. The daily precipitation data at Bangalore city observatory from 1990 to 1997 were used to arrive at a water yield of the pond. Daily yield was calculated using the rational formula, which was computed for the catchment for all these years. Daily yield was cumulated to obtain the monthly and yearly yields.

Topographical analyses with the help of DEM suggest suitable location of the harvesting structure in the eastern part of the campus, which is currently a marshy region. About 1.5 hectare land is available in this region to construct harvesting structure.

During the early 1960's, there were many lakes around the campus that acted as water storage structures and aided in groundwater recharge mechanism. Anthropogenic stresses have led to conversion of lakes to residential, commercial layouts and public utilities. Recent study shows that about 35 % lakes have disappeared during the last twenty years (Deepa, et al. 1998).

Aswathnarayana-kere and Nagashettihalli-kere in the vicinity of IISc are now non-existent, and have been converted into residential layouts. Figure 3 shows the IISc campus bounded by the existing lakes. Consequently, during peak rainfall season, water from the catchment gets into the low-lying areas adjacent to the IISc campus (like ISRO Head quarters, Vigyanapura, etc.).

The three-dimensional elevation image generated using Geomedia Grid was used to identify suitable location for this rainwater-harvesting pond. Figure 4 shows the DEM of the IISc campus. With an identification of lowest positions in a region, the possibility of channeling water was explored considering the following aspects.

Social : Acceptance of pond and usage of its water by people in the vicinity.

Proper managerial solution to avoid breeding of mosquitoes.

Technical : Remedial measures to prevent water seepage into the soil taking into account the stability aspects of structures in the vicinity.

Ecological : Environmental impact assessment to ensure minimum damage to the ecosystem from biodiversity point of view.

Economical : To ensure cost effective structure.

All possible locations were identified in the campus. The best option was selected considering above criteria and based on the simplicity of construction and channeling of the storm water. The pond was located at a point where the storm water drained can be channeled without much alterations of the existing drainage network.

After identification of suitable location for constructing harvesting structures (ponds), region was explored to find out the extent of land available. Location and area were decided based on ecologically sound strategies. From the land use map, Figure 5 gives the land use map of IISc campus based on utility (vector layer) of the spatial coverage of possible harvesting structure (pond / reservoir) was prepared. The area available ranges from 1 to 1.5 ha. One and half hectare spatial coverage entails removal of about 15 trees belonging to Acacia nilotica species. Considering these two areas and for different depths, capacity of the pond was computed. Volumes were compared with the water yield of the catchment of the campus considering the continuity equation i.e.

Storage volume = Inflow - Outflow - Losses (evaporation, seepage, etc.)

The losses considered were the evaporation from the catchment using the monthly mean of daily evaporation data for Bangalore (Muthreja, 1995). The demand for the swimming pool, gardening was ascertained from the volume of the swimming pool and the volume of the storage tank for gardening respectively.

The average rainfall of Bangalore was computed taking 55 years (from 1940 to 1997- 1948, 1967 were not available) of daily rainfall data from Bangalore city observatory.

Harvesting structure in an available area of 1.5 ha and a depth of about 3 m can hold about 22000 m³ of rainwater. This is sufficient to meet the requirement of swimming pool, gardening and toilet usage in the campus. The campus swimming pool approximately requires 1000 m³ of water per refilling. The pool faces scarcity of water during the lean season (i.e. during month of January, February and March). Apart from this, storage structures helps in recharging of the groundwater sources.

The maximum depth available at the proposed site is about 1.5 m. It is desirable to increase this to about 3m, so that the storage may be increased. All round excavation may be taken up to achieve a maximum depth of about 3 m, and to give aesthetic appearance.

Inlet to the pond: The existing main drain that runs in the eastern parts of the campus collects a major portion of the storm water. The lowest point on this drain occurs opposite to the proposed pond location, which could be easily connected to the pond.

Outlet from the pond: An outlet drain of size 1.8 m width and 1.2 m height must be provided at a slope of 1 in 1000, and joined to the existing drain leading to the municipal drain along the eastern part. The invert level of this outlet drain must be the same as that of the inlet drain.

Maintenance of water quality: Adequate care and attention must be focussed on maintenance of clear water in the pond, so that it does not become a source for unhygienic and unpleasant surrounding. Under no circumstances, sewage water and other waste water must be permitted inside the pond. For prevention of mosquito breeding, periodic treatment with lime and/or biological control is required.