Nibedita Das

Qualitative Assessment of Bank Erosion Hazard in a Part of the Haora River, West Tripura District

In West Tripura District Haora is the only major river which is suffering from severe bank erosion due to which it is also gradually shifting. Out of its 46.88 km length, only 19 km stretch has been represented in this paper, within which 4 sites namely, Champaknagar, Rabicharan Thakur Para, Golak Thakur Para and Mekhlipara have been identified as vulnerable to bank erosion through overlay of temporal datasets. This study aims to analyse the temporal change of the bank line of the Haora River and to calculate the Bank Erosion Hazard Index (BEHI) of these vulnerable sites. Overlay of two temporal datasets show the variability of the River Haora during 1932–2009. Then for estimating BEHI intensive field survey has been carried out for qualitative ranking of erosion risk. The variables selected for BEHI are bank height ratio, root depth ratio, weighted root density, bank angle, surface protection, bank material etc. The study shows that the bank erosion hazard rating varies from low to high. The Haora River has shifted about 3–4 km southwards during last 22 years (1.75 m/year) and there is positive correlation between bank erosion hazard rating and shifting.

Sedimentation-Induced Depositional Lands of the Gumti River of Tripura and Its Land Use Pattern

Sedimentation is the process through which sediment carried by the running water is deposited in suitable places. When the energy of a river to carry load gradually decreases, sediment gets deposited and gives rise to bar formation. Due to gradual siltation, area of bars gradually increases and it eventually turns into stable land which uses to be very fertile. With the passage of time, various human activities started to develop on this fertile land and give rise to diverse land use patterns. Gumti is the largest and longest river of Tripura. Lower course of the river is highly prone to sedimentation. Lots of bar formation can be identified in this part of the river, the area of which gradually increases and gives rise to new depositional land. Thus, the objective of the study is to identify the change in channel plan form (sinuosity index and radius of curvature) during the period 1932 to 2016, to measure the area of depositional land generated during this period and to analyse the present land use pattern of this land. For this purpose, the study area has been categorized into several reaches and the layers of different years have been superimposed. Finally, the land use map of the area has been prepared using Global Mapper Software. The results indicate that the River Gumti is gradually changing its channel pattern from meandering to sinuous by increasing meander wavelength due to combined effect of erosion and sedimentation. Besides, with the passage of time diverse land use pattern has been developed in the sedimentation-induced depositional land, the forms of which eventually change due to increasing human habitation.

Spatio-Temporal Shift of Right Bank of the Gumti River, Amarpur Town, Tripura and Its Impact

Gumti River is the largest river of Tripura which flows for 47 km (35 % of the total length) in a meandering course through the synclinal valley between Baramura and Atharamura hill ranges. The Amarpur town (23°31′ 42.6761″N latitude and 91°39′ 52.1358″ E longitude) is situated to its right bank which has observed the channel’s variability of shift in space and time during last 74 years. Fluctuations of different hydrological variables namely, discharge velocity etc. of the Gumti River for both pre and post monsoon seasons have been analysed on the basis of the hydrological dataset from 1995 to 2010. The rate of shift (%) of the right bank of the River Gumti at Amarpur town during both pre and post monsoon seasons have been calculated. The average annual shifting of the meander at Amarpur town is 3.03 m and average eroded area is 767.14 m2/year. Thus the study aims to quantify different hydrological parameters and river course change using GIS technique. The work has been done on the basis of SOI topographical map of 1932–1933 and 1974–1975 (No. 79M/10) and satellite images for the year (1989, 2006). Overlaying of these four temporal dataset has been carried out to indentify the direction of shift and to mark the amount of area under loss (erosion) and gain (deposition). Due to gradual shift of the channel people lost their valuable property every year and the Amarpur town becomes more and more vulnerable to erosion. So it is highly significant to study the hydrological behaviour, channel characteristics, as well as, the bank material characteristics of the River Gumti to get an idea about propensity to vulnerability of Amarpur town.