Study on the braiding indices of the Brahmaputra-Jamuna river

Abstract

A study has been conducted to determine the braiding indices of the lower Brahmaputra-Jamuna River lying within Bangladesh for the different years using different approaches. To carry out the study 13 satellite (LANDSAT TM and MSS) images covering the period 1973 to 1996 have been used. The images covering a reach length of about 270 Ian were collected from the Environment and Geographic Information System Support Project for Water Sector Planning (EGIS). To calculate the time series of the braiding indices of the Brahmaputra-Jamuna river three different methods are used, they are braiding intensity 'El' and total sinuosity 'E2' proposed by Howard et al.(1970) and braided channel ratio 'E3' proposed by Friend and Sinha (1993). For classification of the channel order, Bristow's method for classification of anabranches and sub-channels was followed. Second and third order channels were used to calculate the braiding indices in the present investigation. For this study the whole river reaches within Bangladesh are sub-divided into several reaches based on different characteristics. For the reach type-I, the entire river reach is divided into eight sub-reaches according to various characteristics forms of the river e.g., relatively straight reaches, relatively stable island reaches etc. This reach was similar to the one used by Environment and Geographic Information System Support Project for Water Sector Planning (EGIS) of the Government of Bangladesh. In reach type-2 seven island sub-reaches was selected and those reaches were identified by the Consultant of Flood Action Plan-l i.e., for Brahmaputra river training studies (FPCO, FAP-I, 1991). Reach type-3 was selected on the basis of identification of nodal points on satellite images and six sub-reaches was identified and these sub-reaches were identified by EGIS (WARPO, 1997). The study revealed that the Braiding Intensity 'El' varies from 1.33 to 8.00 for the third order channel and from 1.00 to 4.33 for second order channel. Again Total Sinuosity 'E2' was found to vary from 1.46 to 7.90 for the third order channel and from 1.17 to 3.72 for second order channel. Braided Channel Ratio 'E3' on the same basis was found to vary from 1.77 to 7.17 for third order channel and from I.00 to 3.33 for second order channel. Close scrutiny on the results showed that the braiding indices for sub-reaches at upstream of Sirajganj appears to have been relatively steady during the 1970s, and then to have increased markedly during 1980s and 1990s. But braiding indices downstream of Sirajganj has a different trend. In the 1970s the degree of braiding downstream of Sirajganj decreased notably and by 1980, braiding indices reached its lowest values. These values are consistent with the predominantly meandering course of the river that can be identified in the satellite image during this period. During the 1980s braiding intensity remained low, but total sinuosity and total braided channel ratio increased due to meandering growth in a very large second-order channel. The study further indicated that the degree of braiding was constant or declined slightly until the mid-1980s for the Brahmaputra-Jamuna river as a whole. But afterwards its started increasing and in 1996 became the highest during the period of record. Based on the current trends, it seems likely that the degree of braiding of the Brahmaputra-Jamuna river will continue to increase in the immediate future. This reflects that any large scale water resources development projects on the Brahmaputra-Jamuna deserve the highest technological considerations and remedial measures for arresting such trends. The Braiding indices calculated in this study are broadly consistent with those found in the pervious studies.