Study on the geometric aspects of the Arial Khan River

Abstract

A study was carried out to investigate the geometric aspects of the Arial Khan for a reach length of about 200 km from its offtake. The study includes analysis of cross-sectional geometry, development of non-dimensional correlation, thalweg and bankline shifting, mean bed level variation, specific gauge analysis and determination of meander parameters. The study reach was divided into various sections approximately at an interval of 6.0 km. The study showed that the cross-sectional area varied widely and randomly at various sections. Cross-sectional area at sections 1, 2, 4 to 7 and 10 had an increasing trend while at sections 3, 8, 9, 11 and 12 had an decreasing trend indicating erosion and deposition at these sections respectively. The average cross-sectional area of the study reach was found to increase by 21.48% in 1987-88 compared to 1970-71. The study revealed that WID decreases with the increase of V/~ and Q/ ( ~ D) while V/.{gD increases with the increase of Q/ (~ D) • The thalweg for the study reach in 1987-88 was found to shift randomly compared to that in 1970-71. At sections 1,5 to 8, 11 and 12, the thalweg moved leftward while at sections 2 to 4, 9 and 10, the thalweg moved rightward. The average thalweg depth in 1987-88 was found to be 5.86% higher than that of 1970-71. i Mean bed .level (MBL) of 1987-88 compared to 1970-71 was found to rise at the lower reaches while fall at the upper reaches. This caused decrease in bed slope by 85.477%. Over the study reach a net deposition was found to occur by about 20.0lX106 m3 during 1970-71 to 1987-88 indicating about 3.96 cm sediment deposition per year. Specific gauge analysis at Chowdhury Char for the selected discharge levels showed a decreasing trend of stage and area-elevation relationships indicated the effect of erosion or deposition at various sections over the study period. Analysis of landsat imageries for the years 1972 and 1991 showed an average sinuosity of 2.164 for the study reach in 1972 while the same as 1.876 in 1991. This apparently gives an indication that the river is getting less sinuous. Moreover, the shifting of the bankline between 1972 and 1991 determined by superimposing the traced maps of these years showed maximum lateral and maximum longitudinal shifting at Bagmara and at Chandra by 3.5 kID left and 2.5 km downstream respectively.