ABSTRACT
The evaluation of potential dam sites is usually performed in three phases. During the first phase all the potential sites and their comparable characteristics are determined. The second phase comprises of evaluation of these potential sites for prioritization. In the third phase the detailed design is taken up for the selected ones and these are taken up for development. Thus during the first two phases satellite data and the Geographic Information System (GIS) can be of immense value. The map scales for the entire area under construction should be 1:250,000 for Phase I study and 1:25,000 to 1:50,000 for Phase II study. The third phase has very large map scale requirements wherein aerial remote sensing and field surveys are needed. Using the satellite data and collateral data from SOI toposheets the geologic maps of surficial deposits, slope map, slope stability map, lithology map could be prepared. These are used as vector/raster layers in a GIS environment for selection of suitable hydroelectric sites. The information on construction materials, foundation conditions, internal drainage conditions, weak planes and mass-wasting process which facilitate good planning and safer/cheaper construction could also be derived using satellite and ancillary data. Geologic maps on the topographic base, together with accompanying hydrologic information, provide just such knowledge, which are basic to any rational understanding of the ground conditions that the planner must have.
Digital Terrain Models (DTM) may be created by either digitising contours and spot heights from existing topographic maps, collecting elevations with field survey or as a product of photogrammetric stereo-compilation. The spatial frequency of the sample elevations and the precision of the data are two other factors that must be considered with respect to DTM. The ability of IRS-1C to “point” on-board sensor upon command at anywhere upto ±26 degrees off-nadir provides the possibility for acquiring high resolution (5.8m) stereoscopic data in panchromatic mode over large geographic areas (swath of 70km) and revisit frequency of five days. Thus the DTM which can be generated using either topomaps or satellite data can be used to get computer derived slope, exposition and height or relief for each pixel. Further there is the methodology of DTM analysis in generating three types of data sets, i.e., original DEM with depressions filled, a data set indicating the flow direction for each cell, and a flow accumulation data set for each cell which define the topographic structure. Thus the digital representation of topographic surfaces after suitable accuracy assessments help in estimating peak discharge values in map units chosen for small hydroelectric sites. Hence suitable sites for hydroelectric power can be arrived in a reconnaissance way.
208Seismotectonic analysis and predictive earthwork cost estimates can be carried out using satellite data or aero-space data at a targeting level in the selection of site for small-hydro. Knowledge based selection of geotechnically favourable attributes for hydel site can be carried out using GIS package.
The ERDAS and EASI/PACE image processing software, ARC/INFO GIS software and micro photogrammetry equipment with Geocomp software are available at the Regional Remote Sensing Service Centre (RRSSC) of ISRO in Bangalore. Geocomp Version-8 of DTM can handle data sets upto 32670-3D points and a modeling speed of over 30,000 points/minute. These can be put to use in arriving at such predictive models for peak flow, estimation of earthwork, seismicity and landslides near a small hydro site.
