Hydro-morphological changes of the ganges river in Bangladesh

Abstract

The present study has been carried out to investigate the hydro-morphological changes of Ganges River lying within Bangladesh for the period from 1970 to 2017. The Ganges River is meandering one, carrying flashy flood from the Ganges River basin. In this study, Ganges River has been investigated to find out the current status after the construction of Farakka Barrage in India. The Ganges River covering a length of 108 km in Bangladesh has been studied. To carry out the study discharge, water level data, and cross sectional data were collected from BWDB and WARPO for the last 46 years from 1970 to 2016. For Image Analysis the data was collected from USGS website for the last 45 years from 1972 to 2017.

In this study, hydrological and morphological analyses have been carried out to evaluate the nature of the Ganges River after the construction of Farakka Barrage. Three hydrological and three morphological analyses have been carried out in this study. Three hydrological analyses include frequency analysis, trend analysis and RVA analysis. Three morphological analyses include image analysis, cross-sectional analysis and sediment transport rate analysis. Under frequency analysis, six types of distribution is considered. They are Extreme Value I Distribution, Normal Distribution, Log-normal Distribution, Log-pearson type III, Foster’s Distribution and Ven te chow’s Distribution. RVA parameter analysis has been carried out to investigate the situation of the study area after the construction of the Farakka Barrage. In this analysis, twenty-two RVA parameters were used. ArcGIS 10.2.2 and ArcMap were used for analyzing the images. The cross-sectional data of each year were used to calculate top width, cross-sectional area, wetted perimeter, average depth and hydraulic radius by using Flowmaster. Observed Top width and average depth were compared with the methods given by i) Julien and Wardagalam (1995), ii) Singh et al. (2003), iii) Lee and Julien (2006), iv) Lacey’s regime depth (1929), and v) Julien-Wardagalam-Lee(2006). Sediment transport rate analysis has been assessed by six sediment transport equations, namely Ackers & White, Van Rijn, Yang, Shen-Hung, Engelund-Hansen and Hossain's equation to calculate sediment transport rate.

At Hardinge Bridge station, for Maximum discharge, Normal, Log-Normal and Gumble’s distribution are fitted for the Chi-Square test. For Minimum water level, Normal and Log-Normal distribution are fiited. For Maximum water level, Normal distribution is best fitted for. On the other hand, in case of Baruria transit station, for minimum water level, Normal and Log-Normal distribution are best fitted. For Maximum water level, only Normal distribution is best fitted. Particularly in dry periods (especially in February, March, April and May), more than 90% of the post-Farakka periods (1975–2016) would have failed to meet the thresholds. Similarly, all of the years of the post-Farakka period failed to meet the lower threshold of the minimum flow parameters. The maximum erosion rate about was 4902.248 m/year in the periods of 1993-2003 along the right bank. The very high flood of 1998 was responsible for such large-scale erosion. In 1972 (wet season), the area within the banklines of the study reach was 21,158.90 ha. By 2017, this flow area had increased to 26,490.74 ha, representing a loss of 5,331.84 ha of the floodplain.