Study on hydraulic geometry of selected minor rivers of North -West Bangladedh

Abstract

The study was conducted to determine the hydraulic geometry of the Mohananda river and the Karatoa-Atrai System. The analysis has been carried out to find the variation of the cross-sectional area, section factor, thalweg position, mean bed level and erosion/accretion of the above minor rivers over the study years. The analysis was carried out mainly based on the available cross-sections data from 76-77 to 2000-01 for the Mohananda river and from 93-94 to 2001-02 for the Karatoa-Atrai System. Twelve (12) available cross-sections of the Mohananda river and Twentynine (29) available cross-sections of the Karatoa-Atrai has been used in this study. It may be seen from the analysis of the cross-sectional data that there is no remarkable consistency in variation of cross-sections over the years. However, the analysis has shown certain trend in the variation of the different hydrogeometric parameters. The study of cross-sectional area at different locations over the years has shown that the variation of cross-sectional area is high of the Mohananda river as compared to the Karatoa-Atrai system over the years. Cross-sectional area at most of the sections of Mohananda river increases almost steadily with increase of water level from low to high. On the other hand, most of the sections of the Karatoa-Atrai system, the area at first increases slowly at low water level, then, increases at a faster rate with increase of high water level. The mean bed level of the Mohananda river is very dynamic as compared to Karatoa-Atrai system. The variation of mean bed level over the study years of Mohananda river is above 0.90 m (with maximum of3.26 m) from kIn 15.000 to 46.890 and approximately S; 0.50 m from km 50.844 to 71.325. On the other hand the variation of the mean bed level of the Karatoa-Atrai system is less than that of the Mohananda river which is less than 0.90 m for the whole length of the river system. An abnormal scour hole is found at km 59.133 of the Mohananda river. From the field visit it appears that at the upstream of this location the left bank is composed of stiff clayey soil and there exists a large beel adjacent to the left bank. At this location the soil is silty clay with loose consistency. So initially there was a deep scour hole at this point. Once this scour level touched the underlying aquifer layer there exist a continuous sand boiling and materials are being picked up from the river bed easily which may be a reason for the abnormal scour hole at this location. Huge seepage from the beel may also be a reason for such abnormal scour hole. It reveals from the analysis that the river at the extreme downstream reach seems to be more stable than the other reach of the river. From the analysis of the section factors it appears that the section factor varies with time as well as with location. From the analysis of correlation between water level and section factors over the years, it appears that there is a good relationship between water level and section factor. For Mohananda river, most of the equations of average graphs fit well giving the value ofR2 above 0.80. On the other hand, for Karatoa-Atrai system, most of the equations of average graphs fit well giving the value of R2 above 0.85. This indicates that the sections of the Mohananda river as well as the Karatoa-Atrai System are in balance to water conveyance. The movement of the thalweg of Mohananda river in the vertical direction is very dynamic over the study years. On the other hand, the movement of the thalweg of the Karatoa-Atrai system is not significant as compared to Mohananda. The shifting of thalweg line from the left bank is not significant for the case of Mohananda river. But for the Karatoa-Atrai system, at the upstream reach the thalweg line is gradually shifting towards the left bank while at the downstream reach it is gradually shifting towards the right bank over the years.