Abstract:
Assessment of suspended sediment concentration in running streams is crucial for a variety of applications, including environmental issues, hydraulic structure performance monitoring, and agricultural land use. Conventional methods for the measurement of suspended sediment load, both in-situ and in the lab, are expensive relative to the use of remotely sensed data. Both aerial photography and satellite imagery have been used to map suspended sediment.
This study investigated the applicability of open-access Landsat Enhanced Thematic Mapper (ETM+) images in estimating the SSC of the Jamuna. Multiple- temporal Landsat 7 ETM+ images were processed to extract Digital Numbers (DN) of pixels corresponding to Bahadurabad station of the Jamuna River. The DNs were converted to radiance and ultimately to top-of-atmosphere (ToA) reflectance. Since mostly clear scenes were used, in situ atmospheric correction was ignored. The ToA values for Landsat-7 bands 1-4, which sense electromagnetic radiation of 0.45-0.52, 0.52-0.60, 0.63-0.69 and 0.76-0.90 μm respectively,were combined with corresponding measured values of SSC, procured from historical data archives of BWDB, between the years 2000 to 2015 for determination of statistical relationship between them. R2 for bands 1, 2, 3 and 4 were 0.61, 0.59, 0.54 and 0.54 respectively. The results from analysis showed that Coefficient of Determination (R2) value of band 1 (Blue) presented the best relationship
- therefore chosen as the best SSC indicator. Scatter plot of predicted SSC values from a polynomial equation based on band 1 against in situ values of SSC generated strong positive coefficient of determination of 0.91 and Root Mean Square Error (RMSE) of
79.5 ppm.
Mann-Kendall and Sen’s slope test are applied on the water quality parameters from 2002-2021 for trend analysis. Further findings revealed that increasing trend in EC, Clˉ, salinity and TDS. In last 15 years chloride concentration increases about 2 times, EC increases about 4.5 times, TDS increases about 5 times and salinity increases about 3.5 times respectively. Higher amount of chloride, TDS, EC and salinity is found in pre- monsoon season. As in the pre-monsoon due to sea level rise salinity as well as chloride concentration increases whereas in the monsoon fresh water flow increases chloride concentration, salinity and EC reduce. There is also some relationship made between EC-Cl¯, TDS- Cl¯ and TDS-EC.
