Bars are distinctive features of the braided rivers, and have long been the subject of research in fluvial geomorphology, with analysis of data from both flume studies and field observation [1,2]. The bar (Char) is the deposited (sand, silt, gravel) islands on the riverbed that enforce the river to flow in different channels forming braided river plan form (Figure 1). It is recognized that the formation of bars that divide channels and divert flow is responsible for the initiation and maintenance of the braided pattern [2,3-11]. The common prerequisites for the initiation, growth and development of braid bars are high-energy fluvial environments with steep valley gradients, large and variable discharges, high bedload transport, and non-cohesive bank materials [1-3,5,6,12]. The Padma has the cycle process from meandering to braid and/or straight, and therefore, it is important to locate the bars (chars) on the riverbed to know the morphodynamic nature of the river. The processes of bar formation and their impact on channel dynamics are inferred from comparatively few studies [2,3,12-16]. The nature of bar dynamics is critical to understanding channel stability as bank erosion and growth and multiplication of braid bars are interrelated, and thus, it has important implications for the explanation of the process-response linkage between bank erosion and braid bar formation [2,10].

Figure 1: The study reach of the Padma river in Bangladesh.

The plan form properties of braided rivers have received considerable attention, especially of their braiding intensity [2,20,17-27]. Usage of a suitable braiding parameter is an important measure towards better interpretation of braided rivers [20] as the intensity of braiding varies greatly from river to river [25], and even from reach to reach along the course of a particular river under different flow stages [2,27,28]. Yeasmin and Islam [27] have merely quantified the sinuosity ratio and braiding index of the Ganges-Padma River (the up stretch). This study has integrated the char incidence map and braiding intensity of the down stretch i.e., the Padma is referred to as the river stretch (located 22038/ _ 23044/ north latitudes and 89050/ _ 91000/east longitudes) from Aricha to the confluence with the upper Meghna River at Chandpur in Bangladesh (Figure 1). The length of the study reach is about 100 km and is a very large sand-bed river. The bankfull discharge varies from 75,000 - 80,000 m³/s and the annual average discharge is 30,000 m³/s. The flood water slope of the river is about 5 cm/km, transports about one billion tons of sediment every year. The average median size of the bed material at Mawa is 0.12 mm [29-34]. The down stretch is more braided and significance as the reach carries combined flow of the Jamuna and Padma Rivers [27]. The Padma is loading more sediment and the drastic erosion nature has characterized its more braiding intensity [35]. The bar depositional modeling in the braided plan form stages of the Padma River is important for the Padma Bridge construction. Therefore, the purpose of this study is to locate the bars (chars) and their duration (age) and braiding intensity on the Padma riverbed.

Digital satellite image data has been used to detect and map change and used as important tools for measuring the physical variables which promote, sustain and control change of the fluvial environment [36-40]. This study has used satellite images and GIS technology of Center for Environmental and Geographic Information Services (CEGIS). The available satellite images of different characteristics (Table 1) between 1967 and 2009 have used for bars (chars) incidence map and char ages on the Padma riverbed.

Table 1: Characteristics of used satellite images.

The dry season time-series satellite images have visualized the braiding intensity by integrating the quantification of the braiding index. Brice [17] has measured the braiding intensity by braiding index (BI) is defined as follows:

BI=2(ΣLi)/Lr (1)

Where, ΣLi is the total length of bars and (or) chars in the reach, and Lr is the length of the reach measured mid-way between the banks of the river. A total braiding index of 1.50 was selected by Brice to differentiate braided from non-braided reaches [2,27]. A conceptual braiding intensity model has been developed as the sediment input in the Padma River and other morphological changes of the river system.

Char incidence and char ages

The stability of chars and a char incidence map of the Padma River have been prepared by superimposing seven classified images (water and land) acquired between 1973 and 1992. Figure 2 has shown that the chars have consistently appeared in the reaches at the south of Harirampur, northeast of Faridpur district and upstream of the Padma bridge crossing. The other reaches show frequent changes of the channels and chars. There are a few areas where chars had never existed such as at the downstream of Paturia and close to the left bank at Mawa. Stability of the chars has also assessed through the determining the age of chars by superimposing the 23 classified images acquired between 1973 and 2009. GIS technology has integrated the spatial and attributes data (Table 2) and assessed the char ages (Figure 3). Table 2 has shown that there is predominance of young chars/bars on the riverbed of below eight years (48%). Most of the short duration bars are probably related to high channel slope and magnitude of flood and severe bank erosion [2,41,42]. The longstanding chars, 8 – below 20 years by definition are almost the same (43%) that consistently appear in the char incidence map (Figure 3). But the very long duration chars are few (9%) on the river bed that have been vegetated cover and a good number of displacees have migrated in the chars [43].

Table 2: Age of chars as visible in 2009 satellite images.

Figure 2: Char incidence maps for the period 1973 to 1992.

Figure 3: The char ages on the Padma river bed.

Braiding intensity

The braiding intensity is a measure of the number of anabranches in a section. Applying the equation 1 (data and methodology section) the braiding parameters and braiding intensity of the Padma River have been measured in Table 3.

Table 3: Measured braiding parameters and braiding index of the Padma River.

The braiding intensity generally varies with the stage of the river. In the late 1960s and early 1970s, the braiding intensity of the Padma River was 1.63 and 1.75 that is very close to each other (Figure 4). During 1976 to 1984, the braiding intensity dropped to a minimum, the range of which varied from 1.38 to 1.50. In this period the river was almost a single threaded straight channel. The braiding intensity started to rise very rapidly from the early 1990s and reached its peak value of 2.13 in 1997. Abandonment of the right anabranch in the middle reach upstream of Mawa caused a reduction of the braiding intensity to 2.00 in 2004. The variations in the braiding intensity of the Padma River illustrate the changes in the river’s planform pattern from the end of the 1960s. The minimum braided intensity remained as one indicating that at some reaches the river maintained a single threaded planform while on the other hand the maximum braiding intensity varied between 2 and 3. Sediment wave and braiding intensity.

Figure 4: Changes in braiding intensity of the Padma river over time.

Recent researches of Sarker and Thorne [32] and Sarker [44] showed that during the second half of twentieth century there were huge changes in width and braiding intensity of the Jamuna, Padma and Lower Meghna Rivers. They found a downstream phase lag of the occurring of the changes and concluded that these changes were due to the propagation of sediment wave generated by the Assam earthquake of 1950. The sediment wave took several decades to propagate through the Brahmaputra-Jamuna-Padma-Lower Meghna River systems and finally to reach the Bay of Bengal. Sarker and Thorne [32] and Sarker [44] also developed a conceptual model to elucidate the processes of the sediment wave and the subsequent morphological changes of the river system. They developed validated model for the Padma River is given in Figure 5.

Figure 5: Validated conceptual model showing the changes in the morphological parameters and plan form with the changes in sediment input of the Padma river.

The conceptual model shows how changes in sediment cause changes in bed level, width and braiding intensity of the river. Figure 6 presents the variations in sediment input into the Padma river at the Baruria Transect from the mid-1960s to the end of the 1980s. Rapid changes in the morphology of the river during the last few decades might not only be due to the effect of the Assam earthquake of 1950 [44]. There might be several other factors are active in this system, such as, the long-term northeast migration of the river courses and its interaction with the cohesive bank along the left bank, or the apparent cyclic behavior of the river.

Figure 6: Changes in bed material transport in the Padma river at Baruria transect.

Width of the Padma river

Maps of 1860 and 1925 showed that the river had increased its length-averaged width gradually. The gradual increase in width can be attributed to the gradual diversion of discharge of the Brahmaputra River through the Jamuna [44,45]. The length-averaged width of the Padma River was about 5.4 km in 1860. It increased by a few hundred meters in the next 65 years and reached 5.8 km (Figure 7). The range of variation between maximum and minimum widths had decreased significantly during this period and the river had attained a relatively uniform width throughout its length. From 1925 to 1952, the length-averaged width had increased to 8.4 km and the main contributor of this increment was the increase of width of the river at the Mawa section to 17 km in 1943 and 1952. During the following three decades, the width of the river reduced to 6.8 km in 1980 and the maximum width was also reduced to 12.8 km. During the last three decades the river has widened at a very high rate with the length averaged width reaching 10.5 km in 2009. More importantly, the maximum width of the river reached 21.2 km. Sarker and Thorne [32] related the recent changes in width of the river to the propagation of sediment wave generated by the 1950 Assam earthquakes. Analysis of the variation of width does not show any cyclic processes.

Figure 7: Changes in length-averaged, maximum and minimum width of the Padma river.

Observations of braiding intensity and bar morphodynamics have provided the characteristics of the recent transformed braided river of the Padma. It is observed that channel-braiding initiates with the formation of bar which are depositional in nature. At the same time the bars of the Padma River are extremely unstable and they change their position and behavior through localized erosion and accretion process. The bars are dynamic in nature and that their mobility varies spatially and temporally. Under different river stages sediment is mobilized, and accordingly shape and size are modified laterally and longitudinally. The intensity and prolongation of bar migration and/or disappear depends on the periodicity and duration of monsoon floods and flow directions. The braiding intensity oscillated over the last several decadal time scales. The increasing or decreasing of the braiding intensity changes planform pattern of the river that is important in fluvial geomorphology as well as in river training work.