Study of offtake morphology and conveyance characteristics of Arial Khan river

Abstract

In the adherent areas of Faridpur and Madaripur districts of Bangladesh the Arial Khan River is the catcher and player of major water resources activities. The river bifurcates from the Padma at right bank offtake situated near Chawdhurychar in the Faridpur district known as Arial Khan Upper and another narrow channel at Dubaldia known as Arial Khan Lower. The specific objective of the research is to study and evaluate the morphological characteristics in the form of assessing changes of cross sections in and around the offtake and remaining reach of the Arial Khan River. Analysis of hydrometric data of the period 1975-04 of both the Padma at Baruria and the Arial Khan at Chawdhurychar shows that water levels and discharges are in rising trend in both the Padma and the Arial Khan River. Fraction of Padma flows which is diverted to the Arial khan River is observed in terms of peak flow as 2% in 1975 and has been increased to about 3% in 2004. Water level slope of the Arial Khan offtake goes below zero in the of monsoon with the value around -3xlO-5 and gets higher than that of the Padma River with the value around 1.5xl0-4 in the post monsoon indicating flow hindrance due bed level rising in the offtake. Analysis of cross sections of the period 1972-03 shows that the Arial Khan Upper reach is observed morphologically active where the Arial Khan Lower reach is found in trend of becoming morphologically dead. But the offtake reach of the Arial Khan Upper River which is concerned for the study is observed in the trend of losing conveyance due to aggradations resulted from historic sedimentation where the value of conveyance factor is found not exceeding 1000. The rest reach of the Arial Khan Upper River has been experiencing average degradations as observed in the trend of gaining conveyance with the value exceeding 15000. Conveyance in the parent river reach in the Padma around the Arial Khan offtake have been found in losing trend revealing heavy sedimentation and char formation which has impacts over the distributary as well as dynamics of the Arial Khan offtake. Again, bed topography generated from bathymetric data of 2005 at the junction with the Padma clearly demonstrate that the main channel of the Padma is aligned far north which is opposite to the offtake and there is a char formed at the mouth of the Arial Khan hindering the flow and the channel has become narrowed. It is observed from the satellite images of 1974 to 2006 that char area in the vicinity of Arial Khan offtake was 60 sq Ian in 1974, 72 sq Ian in 1985,82 sq Ian in 1997 and 92 sq Ian in 2006. The yearly development rate of char area is 2%. The Arial Khan Upper offtake concerned for the research is found in the trend of abandoning stage and there must be a shifting offtake which has been dynamically under development to keep the river morphologically active. Recent bed topography of the offtake, conveyance, trend analysis of recent inflows and stages in the Arial Khan River support that the river is going to be the cause of sufferings in terms of flooding in the monsoon and navigability and other water resources activities will be hampered due to inadequate inflows in the river.