Evolution of the Jamuna River

We have shown that the Brahmaputra avulsion was not the result of a single avulsion, as has been proposed in the preceding theories, but was the result of multi-channel multi-avulsion processes. We demonstrated that upstream avulsion occurred by incision, whereby new channels were created that subsequently intersected the existing rivers. We have found that initially the flow of the flow-enriched Konaie was switched into the Jenni through an unnamed channel near Nulshi close to Sirajganj town, and the combined flow rolled over the bed of the Jenni southwards to form the downstream part of the Jamuna subsequently. We specify the initiation and completion timelines as 1780–1800 and 1880s, respectively.  

In the pathway to analyze the causes of the Brahmaputra avulsion, we recognize the merits of those previous studies that provide the concept of triggering the avulsion by the high monsoonal discharges, avulsion of the Tista into the Brahmaputra and the high-magnitude floods. We have found all these factors were coupled to trigger the avulsion. We also recognize the merits of those previous studies that local sediment overload and consequent bank erosion were the cause of the avulsion. We have provided a more specific description of this avulsion scenario and have introduced the unstable bend migration theory, according to which a lower stability threshold for avulsion was established. Because of this lower threshold, the coupled switching mentioned above resulted in the eruptive creation of new channels to initiate the avulsion. We specifically focus on the bar orientation scenario in the unstable bend that played a vital role in the avulsion process.

Braiding Characteristics of the Jamuna River

An increasing trend in the value of the braiding index for all three Reaches of the river (Present study area covers a distance of about 231 km along the Brahmaputra-Jamuna the river system in Bangladesh and the study area have been divided into three reaches to understand the spatial variation of the presented geospatial information) over the study duration (1989-2022) is observed. Previously, there was no information regarding the Reach-based spatial variation in the values of the braiding index. Analysis shows that the braiding index diminishes with distance downstream. 

The increasing trend of braiding intensity in all three reaches is likely driven by a combination of factors: (i) continued high sediment supply from the upstream Brahmaputra and its tributaries (ii) persistent high stream gradient (particularly in the upstream reach), (iii) progressive stabilization of river banks through bank protection works, etc.

Noticeably significant increases in both erosion and accretion occurred in the Char lands in recent years.

Both erosion and accretion occurred in the Chars consistently having a dominancy of accretion. The dominancy of accretion in the Chars is in line with the increasing trend of braiding intensity mentioned above. It is observed that over the study years, the river banks have become more stable although the left bank of the study area has been more vulnerable to erosion than the right bank. The banks of the river have been under considerable protection activities in the last few decades. This, on one side, has made the banks stable and, on the other side, has made the Chars unstable through inward flow divergence. This is why in recent years significant increases in both erosion and accretion occurred in the Chars. 

Age-based analysis

 A weighted age index has been developed to determine the age of the Chars for the first time. A total of 315 Chars have been identified and the first-ever Char age map of the river has been presented. The number of Chars has an inverse relation and the mean Char area (area/Char) has a direct relation to the mean weighted age of the Chars. Higher accumulation of higher age Char downstream has been revealed which gives a new insight for the braided river. 

Stability Analysis of Chars using the 2D hydro-morphological model

From the simulation of the 2D hydro-morphological model, it is seen that in general, Chars with higher weighted ages are more stable (less erosion prone) than those with lower weighted ages. It is found that the area of Char is an important consideration for its stability. In general, a small Char is more erosion-prone than a large Char in an age class. Finally, we find the connectivity of a Char to flow magnitude dominantly governs its stability. Therefore, the analysis of the stability of Char should primarily be based on the category of flow magnitude connectivity of Chars.