Environmental changes in irrigation and flood control schemes: a case study of Gin Ganga regulation project

Gin River which is 113km long is situated in the South-West of Sri Lanka. Its lower basin has a flood history, which usually destroys about 6000ha of paddy and 3000ha of highland crops. The South-West of Sri Lanka receives an annual average rainfall of 2600mm. Gin River flood control project was completed in 1980. Lower basin of the catchment was protected by electric pumps in 3000ha and gravity drainage was newly provided for 1740ha of Holuwagoda Tract. The project occupied 250ha in between flood bunds in the flood corridor on each river bank. 78ha of paddy lands in the upper catchment were subjected to added flooding and backwater effect after the project. New 800ha of marshy lands were converted in to paddy lands. Normal drainage was provided during non-flood periods. The Project is presently maintained by Irrigation Department. Many paddy tracts were abandoned after the drought in 1992. Many environmental issues have contributed to present situation. The achievements and adverse effects are studied to identify the needs in planning and designs of flood control projects. The objectives of this study could be listed as: 1) Identification of environmental issues prior to the implementation of the irrigation and flood control project. 2) Identification of present environmental issues after the completion of the irrigation and flood control project. 3) Identifying the environmental issues, which require to be considered in case of irrigation and flood control projects. In order to carry out this as a case study, Gin Ganga Regulation Project (GRP) was selected as the project area. 1 Literature survey identified many environmental issues and initial site visits were used to compile all possible issues related to the project area. The issues were separated into processes and were summarised for analysis. Environmental changes due to the project were assessed using these issues. Assessment Site visits were carried out to assess the changes in selected locations. Significant issues were listed and micro analysis was done for every locality of the project using a questionnaire. Verification was done using collected data. The main achievements of GRP are, an increase in land value (150%) and infrastructure development (28%). The social processes are improved due to this project. The land use for rice was improved initially and two seasons per annum were properly cultivated. After 15 years some areas were abandoned due to low income. Some areas were rejected as planned by the project in the unprotected area. Some areas can be cultivated after desilting of canals. Soil subsidence has led to abandoning of some areas. Anticipated targets were reduced by abandoning of lower Holuwagoda tract. Soil subsidence has caused waterlogging in this area. Ganegama lower tract has lost cultivation due to subsidence. An overall assessment 84% of land use was retained for paddy under GRP. If the project was not effective this value will be reduced to 50% as in Bentara Dedduwa Scheme.