Assessment of environmental impacts of water development projects using overlay technique

Abstract

Environmental Impact Assessment (EIA) has been made mandatory for all water development projects in Bangladesh, to be carried out at project planning stage. The main objective of EIA is to identify the probable effects, both beneficial and adverse, resulting from the project. Then through EIA, suggestions are given to the planners so as to minimize and mitigate the negative or adverse impacts and enhance the positive or beneficial impacts. EIA then prepares an Environmental Management Plan (EMP) for future monitoring of the project, both during and after its implementation so that sustainability is ensured. There are a number of techniques for carrying out EIA namely: Checklist, Matrices, Networks, Environmental Evaluation System, Overlay Technique, Environmental Impact Indices and Costbenefit Analysis. Besides, many institutions have developed their own guidelines for EIA. Bangladesh has adopted the Weighted Matrix method and a guidelines for carrying out EIA of water development projects has already been developed and adopted. Every method has some advantages and disadvantages in their application. As for example, Network method has been found suitable for identifying secondary and as well as tertiary impacts. Overlay technique is considered suitable for presentation of spatial variation in impacts. This study was taken up to carryout a comparative study of the various methods of EIA and evaluate whether some elements of Overlay technique can be adopted while presenting findings through Weighted Matrix method in accordance with the approved guidelines. To carry out these objectives, data available from post evaluation of the System Rehabilitation Project (SRP) were used. SRP, during 1990-95, rehabilitated 35 water development projects of the Bangladesh Water Development Board (BWDB). A multidisciplinary Evaluation Team assessed seven SRP projects, using EIA methodology. In this study, data of two projects were used. GIS based mapping technique was used to produce the Overlays. For comparison with conventional method, some important environmental components (lECs) were chosen, namely: Flood Control, Drainage, Irrigation, Crop Damage and Capture Fisheries. Other IECs, relevant to water resources development and management projects have been identified on the basis of various studies available on evaluation of completed projects. Preliminary assessment of suitability of overlay techniques and availability of data led to selection of the aforesaid five parameters. For this type of projects, modification in flooding characteristics and changes in cropping are most significant parameters. It has been found that EIA results can be better presented through Overlays for the selected IECs. Impact of probable floods, corresponding to various return periods, and matching with any river water level, on the project area, can be clearly represented by Overlay. Similarly improvement in drainage in post-monsoon condition can be better represented with spatial variations shown on the maps. Impact of the project on potentials as well as actual performance of irrigation were clearly visible in Overlay maps. Impacts on capture fisheries, studied with overlays, clearly demarked the areas of potential conflict between agriculture and fisheries. It was observed that in the projects studied, such conflict was prevalent over small areas. On the other hand flooding pattern has significantly adverse impact on agriculture. These analysis would be of great value in future water management practices. Finally it can be said that, overlay mapping can have very important role in the EIA process. The conventional method can not assess the impact and quantify the spatial variation. The method is also very flexible and the importance of the time function can be well addressed.