Study on bar characteristic of the Jamuna river

Abstract

This study has been carried out to determine the bar characteristic of the River Jamuna for examining the morphology and dynamics of bar growth using overlay technique. The methoid has been applied in two segments for different years. To carry out the study 14 satellite (LANDSAT TM and MSS) images are used covering a period of 1973 to 1997. The images covering the reach length of about 240 lOll out of which 100km reach length has been taken for study purpose. First segment is located at the upstream of the Bangabandhu Multipurpose Jamuna Bridge near Sirajganj extending from BTM coordinate Northing 695000m to 745000m, Easting 460000m to 485000m and second segment near Bahdurabad extending from BTM co-ordinate Northing 750000m to 800000m, Easting 450000m to 480000m.The images are collected from Environmental and Geographic Information System Support Project for Water Sector Planning (EGIS). Related data of water levels, discharge & cross-section are used and those are collected from Bangladesh Water Development Board (BWDB). To determine the bar characteristics, different parameters i,e average length, average width, area, age, distribution of land cover and flood depth map of selected bars have been analyzed. Processing and analysis of the satellite images have been performed using the computer facilities of EGIS. The study segments are identified from the image(s) and then traced on the transparent sheet from the hard copy. Then the traced sheet(s) are digitized with proper geo-refereneing procedure and digital maps for channels and bars/islands have been made using pc ARC/INFO. ArcView of GIS version has been applied for spatial analysis and graphical presentation of the findings. The bars/islands and channels of these digital maps are identified by different identification number(s).In this study for easy identification of bar/island and channel a general approach has been applied, for any image, such as bars/islands of which the length exceeds 12km starts from 100, identification number for bars less than 12km starts from 200 and for the whole channel identification number is 300. The alignment of bars/islands and channels of the identified digital map(s) have been checked by displaying on the respective image. Any missing and miss leveled were corrected, once the digital map is finalized then the topology has been built to get the arc attribute table (AA T) and polygon attribute table (PAT). These maps and attributed tables were used to examine the time series morphology and dynamics of bar growth. The study reveals that the area, average width and average length of the selected bar in segment-I has a overall falling trend during the study period 1973-1997, but in segment-2 the bar has the rising trend. Based on the analysis, it seems that the area of the selected bar in segment-1 from the year 1994 to 1997 in fact docs not have any change, may be the morphological impact due to construction or Bangabandhll Multipurpose .Jamllna Bridge at the down stream but 011 the other hand the bar area or the selected bar ill segmellt-2 has the opposite trend. From the analysis of bar age and common bar area in 1997 with respect to area of 1973 in both the study segments it has been found that in 24 years (1997 -1973) the existing bar area in 1973 comes to about 2.5%, which indicates that if this trend continues the bar may disappear in about 30 years. From the land cover analysis it has been found that during the study period 1973 to 1997, in segment-1 and segment-2 areas of water and bar have increased 12% both combinedly and separately. In fact in terms of morphological point of view. the overall increasing trend of land cover in these segments is insignificant. It is revealed from the Flood-Depth map analysis that in segment-1 the selected bar has been inundated from 271 ha to 418 ha due to the change in water level from 7.33m to 9.05m. But due to the increase in water level ranged between 9.05m to 12.07m bar has been inundated from 418ha to 2218ha and at the water level 17.5 the whole bar has gone under water which is very much consistent with the 1988 and 1998 flood level. Similarly, in segment-2 the selected bar has also gone under water at water level 20.50m which is also consistent with the 1988 and 1998 flood level. From the above analysis it is proved that overlay technique is a very good tool for studying the morphology and dynamics of bar growth for the braided rivers and might also be useful for Bar/Island development and settlement aspects.