Studying the meander bend migration of the Arial Khan river

Abstract

Arial Khan is a meandering river, flowing south-eastward direction after bifurcating from the right bank of the Padma River. Incessant channel shifting and bend migration are very common phenomena for this river. This study investigates multiple time- and bend-scale morphological characteristics of the river. The river planform analysis and bend geometry estimation from remotely sensed satellite images have been conducted by GIS and CAD software. The time-series analysis is based on selected seven years of image processing from the long 49 years study period: 1972, 1980, 1988, 1999, 2007, 2017, and 2021. Among these seven years, 1972, 1980, 1999, 2007, and 2021 are represented the years when the offtake of the river arrives at the ultimate downward (southwest) and upward (northeast) points alternately and 1988 and 2017 are the interim years. Thus, the river takes on an average of 19 years to shift the offtake at the ultimate downward and 8 years to the ultimate upward position. The shifting of the offtake with time is correlated with the morphological changes of the offtake, i.e., alteration of the offtake bed elevation and in-front bar formation, which control the diversion of flow from the parent river into the Arial Khan. Thus, the offtake influences the dynamic behavior of the river morphology in terms of sinuosity, bend geometry, and bend migration. During downward shifting of the offtake, the sinuosity so as the length of the river increases, though 4-6 km linear path of the river gets subtracted due to this shifting. Again, the reduction of sinuosity and length occurs during upward shifting of the offtake, even though an extra 5-7 km linear distance of the river path is added due to this shifting. The average sinuosity of the river is 1.78, the highest and lowest values of it have been found as 1.62 and 1.95 in the years 2021 and 1980 respectively. The river is divided into the three reaches: upper, middle, and lower based on the spatial variability of floodplain, morphology, and bend migration pattern and direction. The upper reach has comparatively wide active floodplain and valley, having loosely compacted soil. The average water depth and thalweg elevation of the river increase towards downstream; currently the bend slope of the river is 0.037 m/km. The river width increases successively from the upper reach to the lower reach with the average value of 237 m, 277 m, and 320 m respectively. The sinuosities of the upper two reaches are lower compared to the lower reach, with values of 1.60, 1.55, and 2.11 respectively.A total of 37 characteristics bends from the three consecutive reaches have been selectedfrom different study periods. In the upper reach, downstream migration dominates where four out of six cutoffs occur during the study period. The lateral migration dominates in the middle and lower reaches; each of the reaches contains only one cutoff. The migration direction of the upper two reaches is not consistent but it is unidirectional in the lower reach. Among the six significant bend cutoffs, the chute and neck cutoffs are in an equal number of three. Before cutoff, the bends adopt a shape of curvature with an average radius of 501 m and all radii range between 218m and 817m. The average lifetime of this river bends before the occurrence of cutoff or channel straightening is 24 years, though reach-wise the lifetime successively increased towards downstream as 18 years, 25years, and 32 years respectively in the three reaches. During migration, all modes: expansion, rotation, translation, and extension persist in the river. In most cases, newly generated simple symmetrical bends are converted to asymmetrical, elongated, or compound shapes. The average radius of the bends of the Arial Khan River is 921m, though reach-wise the radii vary as 775m, 1127m, and 883m respectively for the upper, middle, and lower reaches. The upper reach is more migration prone; here the average migration rate is more than double of the other two reaches; which are 108 m/y, 42 m/y, and 46 m/y respectively. The higher migration rate of 70-80 m/y was found during the time period of 1981-1999, and for the rest of the periods, the rates were within the value of 45-50 m/y. The Arial Khan River sustains a particular relationship between migration rate and relative curvature; maximum migration rate prevails for the bends with relative curvature of 1.5- 3.5 and having a peak value around 3.0. The average values of relative curvature for the bends of the upper and lower reaches are 3.5 and 3.1 respectively, which fall within the estimated range. But for the middle reach, the value is 4.8, which is beyond the range. In time-scale, the relative curvatures are within the estimated range of 1.5- 3.5 during the time period of 1972-1999 and from 2000-2021 the values are higher than that range. Chronologically the change of morphology in terms of bend migration of the Arial Khan has been found as upper reach > lower reach > middle reach and in time-scale as 1972-19999 > 2000 -2021. The overall feature of the river from 1972 to 2021 reveals that a significant increase in sinuosity and reduction in river width and aspect ratio have occurred. The migration rate of the river fluctuates from time to time, but the trend has declined slightly. The frequency of bend cutoff reduces notably, the last cutoff occurred in 2004 in the Arial Khan River. All the states of the features indicate the adjustment of the river. The absence of sediment and local discharge data are the major limitations of this study. Otherwise, this study can be considered as a milestone of multiple time- and bend-scale analysis of a meander river of Bangladesh.