Assessment of sedimentation and watershed prioritization using remote sensing and GIS techniques

Abstract

Sedimentation is considered one of the major significant issues for all water development projects, which leads to the reduction of the water storage capacity, hinders the water supply of channels, irrigation, navigation, and damages streams geomorphology, and also limits the reservoir for the fulfillment of the purpose for which it is designed. To ensure the long-term sustainability of any reservoir or dam, it is necessary to scientifically manage soil, water, and vegetation with integrated approaches at the watershed or sub-watershed scale where watersheds need to be considered as planning units for the administrative purposes to conserve land and water resources. The severity of the sedimentation problem can be evaluated by priority delineation of the sub-watershed through the estimation of sediment yield index (SYI), land cover/ land use (LULC), and morphometric parameters analysis. In Bangladesh, the efficient use of water resources in agricultural sectors faces different challenges due to different natural calamities, diversion of river water on the upstream sides, and continuous silting up of different rivers. Approximately 550,000 hectares of land are currently irrigated under 15 large irrigation programs. The Muhuri Irrigation Project (MIP) located in the southeastern region of Bangladesh was designed to serve as supplemental irrigation in the dry season as well as increase agricultural production in this region. But over time, it has lost productivity in terms of both irrigated area and crop intensity due to limited access to the surface and groundwater and poor drainage due to the sedimentation in the rivers and their associate khals. The present study assessed the extent and rate of sedimentation in the reservoir of MIP and investigated the spatio-temporal variations of water storage capacity using Remote Sensing and Geographic Information System techniques. The prioritization map based on the integration of SYI, LULC, and morphometric analysis is also developed for this region. To analyze the sedimentation, water depth, and extract the LULC data, Landsat satellite imageries of 1988, 1998, 2008, and 2020 have been chosen based on the availability of the average dry season maximum water level of five gauging stations during the passage of the satellite over the study area. For the estimation of spatial and temporal variations of water depth, the log-ratio algorithm has been used and the analysis of the result revealed that the areas of bathymetry depths within the range of ≤ 6m, ≤ 9m, and ≥ 9m are1.66 km2,12.93 km2, 4.54 km2 for the 1988 year 0.58 km2,11.24 km2, 4.19 km2 for the 1998 year 2.38 km2, 7.56 km2, 1.27 km2 for the 2008 year and 1.66 km2, 5.76 km2, 3.76 km2 for the 2020 year and the total waterbody decreased to 2.91 km2,4.79 km2,0.036 km2 respectively during1988-1998,1998-2008,2008-2020 period. To estimate the incremental volume of the reservoir, the primordial formula has been used and the results showed that about 5.216 Mm3 volume of sediment is deposited from 1988 to 2020 between 4.44-3.93 m water level. Finally, to estimate the sediment yield between the 1988-2020 period for all Upazilas covered by the study area, an empirical equation of SYI has also been effectively used. According to SYI analysis, Fulgazi and Porshuram Upazila fall under low priority, while Chagalniya and Sonagazi Upazila fall under medium and Feni Sadar and Mirsharai Upazila are under the high priority category. However, morphometric parameters analysis revealed that Chagalniya Upazila falls under low priority and all other Upazilas i.e., Feni Sadar, Fulgazi, Porshuram, Mirsharai, and Sonagazi Upazila fall under the high priority category. Based on the analysis of LULC data, Feni Sadar, Fulgazi, and Mirsharai Upazilas fall in high priority areas, Chagalniya and Parshuram Upazilas fall in medium priority areas, and the rest of Sonagazi Upazila falls in the low priority area. Based on the integration of morphometric, LULC, and SYI parameters, the common sub-watersheds that fall under high priority are Feni Sadar and Mirsharai Upazila whereas Chagalniya, Parshuram, and Sonagazi Upazilas are under low priority, and only Fulgazi Upazila falls under medium priority. However, the results of this study may be useful for the watershed manager, decision-makers, and planners in framing an appropriate plan for the intervention of sustainable management of sediment issues in the region.