TR(BR)-I-2013 Study of reservoir sedimentation, impact assessment and development of catchment area treatment plan for Kodar reservoir in Chhattisgarh state

Abstract

The soil erosion, movement and deposition are part of natural hydrological processes, but the rate of sedimentation is accelerated due to environmental degradation, lack of conservation measures, change in land use, deforestation etc. The catchment area which is the source of endowment is generally overlooked in most of the water resources projects cause reduction of useful storages, loss of nutrients, overtopping of reservoir and life of reservoirs. Reservoir surveys are necessary to get more realistic data/estimate regarding the rate of siltation and to provide reliable criteria for studying the implication of annual loss of storage over a definite period of time with particular reference to reduction of intended benefits in the form of irrigation potential, hydropower, flood absorption capacity and water supply for domestic and industrial uses etc; and periodic reallocation of available storage for various pool levels. The fast growing development and ignorance of catchment area treatment plan during and after implementation of project accelerating the rate of sedimentation and disturbing the ecological balance in reservoirs. While it is not possible to totally avoid or stop siltation, one way to reduce the siltation of the reservoirs is to implement scientifically designed catchment area treatment (CAT) plan that may enhance the life and efficiency of the project as well as the availability of water in command area. Chhattisgarh is one of the States which is included in the Hydrology Project Phase II. For the livelihood of the state, the irrigation projects and their working with the designed efficiencies, periodic assessment of reservoirs, conservation and treatment plan for catchment and modification in reservoir operation plan are essential. The Kodar reservoir which is constructed on river Kodar, a tributary of river Mahanadi has been selected for the systematic and scientific study of reservoir sedimentation, sediment yield from catchment areas, prioritization of catchment for soil conservation measures, sediment modeling in the inflowing rivers and impact assessment analysis of conservation measures on sedimentation under the PDS. The identification of environmentally stressed areas, development of catchment area treatment (CAT) plan and rainfall-runoff- sediment modeling require to handle spatial data. Therefore, GIS based data base of the study area have been generated using ILWIS and Arc GIS software. The sedimentation study for the Kodar reservoir has been carried out using digital image processing technique of remote sensing data. The normalized deviation water index (NDWI), normalized deviation vegetation index (NDVI), image ratio (IR) and false color composite (FCC) have been used for identification of water pixels from rest of images. The sedimentation study showed that 24.94 Mm3 of gross storages and 4.89 Mm3 of dead storage have been lost in 32 years (1976-77 to 2008-09). The analysis of rainfall and other meteorological data have been carried out and data base have been created for sediment modeling. The land use map of Kodar catchment has been generated using supervised classification technique of LISS IV data. The Kodar catchment is primarily an agriculture watershed with dense forest on ridges only. The soil testing on 11 sites in the study area considering

variation of soil type and land uses have been carried out for in-situ tests including infiltration rate, saturated hydraulic conductivity, field density and laboratory tests for textural analysis, nutrient analysis and sp. gravity etc. The soils in the study area are silty loam and sandy loam type with saturated hydraulic conductivity varies from 0.10 cm/hr to 88.95 cm/hr. The Kostikov and modified Kostikov models are found the most suitable to display the infiltration characteristics of soils in the catchment. The results of soil analysis in Kodar catchment have been used in prioritization, development of CAT plan and sediment modeling.

For identification of environmentally stressed sub-watersheds in Kodar catchment, Saaty's analytical hierarchical process (AHP) has been employed with participation of nine environmental hazardous parameters (EHPs) including soil loss using USLE/RUSLE model (SL), sediment production rate (SPR), sediment yield (SY), sediment transport index or sediment power index (STI or SPI), slope (Si), drainage density (Dd), channel frequency (CO, form factor (RI) and circulatory ratio (Rc). The Saaty's AHP is a structured technique for dealing with complex decisions involves building a hierarchy (Ranking) of decision elements and then making comparisons between each possible pair in each cluster (as a matrix). For prioritization and development of CAT plan, the Kodar catchment has been divided into sixty seven sub-watersheds. The soil loss in the Kodar catchment using RUSLE model indicated that sub-watershed SW-27 may be the least vulnerable produces minimum average soil loss of 0.51t/ha, while sub-watershed SW-44 should be considered on top priority with 73.21 t/ha annually. For estimation sediment yield (SY) from sub-watersheds of Kodar catchment, a simple regression model based on rainfall, slope, land use and geomorphological parameters suggested by Kumar, 1985 and Rao & Mahabaleswara, 1990 has been used. The sediment transport index, sediment power index, average slope, drainage density (Dd), channel frequency (Cf), form factor (RO and circulatory ratio (Re) for all sub-watersheds have been computed with the help of sub-routine and other applications of ILWIS 3.7 software. The weights obtained from Saaty's AHP and normalized values (varies between 0 and 1) of EHPs have been used to compute final priority of each sub-watershed in Kodar catchment. On the basis of final priority, all sub-watersheds of Kodar catchment has been grouped in five classes of priority namely very high, high, moderate, low and very low on the basis of priority ranking. From the Saaty's AHP analysis, the normalized priority for SW-44 has been computed as 0.74 and identified as the top most priority watershed. Similarly, SW-41 may be considered at the last in conservation works. The AHP analysis suggested that more than 21 sub-watersheds covering 117 sq. km area of Kodar reservoir catchment comes under very high and high priority and hence a scientifically developed CAT plan consisting mechanical, biological and agronomic measures should be implemented in these sub-watersheds on priority basis.

The scientifically developed catchment area treatment plan identifies environmentally stressed areas, necessity and intensity of mechanical and biological measures to arrest further soil erosion and conserve water with in the watershed. In the study, weighted overlay technique of various thematic maps including geology, geomorphology, soil, land use and drainage with some guiding principles have been used to identify suitable sites for mechanical measures and areas which can be brought under agronomic and biological measures of soil conservation. The CAT plan for Kodar catchment consists of 37 gully plugs, 22 nala plugs, 21 boulder bunds and 6 check darns under mechanical measures with 101.61 ha land for afforestation, 114.86 ha for agro-forestry and 11.41 ha land for development of grazing land under agronomic and biological measures. The proposed CAT plans consisting gram panchayat wise areas for specific soil conservation measure may be beneficial for local administration to take these works on priority basis The Soil and Water Analysis Tool (SWAT) model has been employed for rainfall-runoff- sediment model and impact assessment of soil conservation measures under CAT plan on runoff and sediment regime in Kodar catchment. The SWAT model is a distributed parameter and continuous time simulation model designed to predict the impact of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions over long periods of time. The SWAT model was first setup on river Kodar up to Koma G/D site where runoff and sediment data were collected during the period of PDS (2010- 1012) for calibration and validation later extended to whole Kodar catchment. From the analysis of sensitivity simulation, it has been observed that the threshold depth of water in shallow aquifer (GWQMN) and Manning's N for main channel (CH N2) is the most important from runoff and sediment modeling respectively. After manual changes in the sensitive parameters, rewritten of files and simulation run were carried out to determine computed runoff, sediment etc. from different subwatersheds. The observed and computed values of runoff/sediment were compared using goodness of fit parameters including root mean absolute error (RMAE), integral square error (ISE), Nash-Sutcliffe efficiency, scatter plot and graphical representation. After successful validation, the model parameters with suitable modification wherever required were implemented on whole Kodar catchment. The impact assessment analyses on runoff and sediment have been carried out by generating two different scenarios prior and after application of soil conservation measures as Pre-BMP and Post-BMP. The results indicated that maximum sediment load found in the month of Sept 2011 which was 2.97 t/ha under monthly rainfall of 743 mm in Kodar reservoir catchment during the period of implementation of model (2010 to 2012). If suitable soil conservation measures and BMP applied in the catchment, the sediment entry in the reservoir can be reduced to 1.63 t/ha under same rainfall condition. The BMP and CAT plan have little impact on runoff pattern from the catchments of Koma and Kodar reservoir, but able to reduce significantly the

sediment transported through channels which otherwise deposited in Kodar reservoir if no measures were taken. The results of the study and methodology suggested in the PDS can beneficially be used in other water resources projects for reduction of useful storages, increase in water availability, social and economical development of weaker section of society and generation of employment through conservation measures. The proposed methodology can be used as guidelines for assessment of expected soil loss and suitable conservation measures for sustainable development in design of new water resource projects. During the course of PDS, extensive field visits were made and two knowledge dissemination workshops were organized to get feedback from stakeholders, government department, technocrats etc. Overwhelming response have been received during the interaction and need of regular estimation of reservoir revised capacities, development and implementation of scientifically designed CAT plan and awareness for soil and water conservation measures in mass were identified as the key issues for sustainable development of water resources.