Effectiveness of pilot dredging in the Gorai river

Abstract

The effectiveness of pilot dredging of Gorai has been studied on the basis of dredge and post dredge observed bathymetry. The dredge bathymetry stands for the bathymetry measured immediately after the dredge and the post dredge bathymetry stands for the bathymetry after the passage of first year flood flows over the dredged Gorai. It is found Crom the analyses of these bathymetry that the n01111alisedbed levels of the dredge bathymetry has been lowered all along the dredged Gorai while the bottom bed levels is found to have been raised. This implies that the pilot dredging has increased the high level conveying capacity of the Gorai while the low level conveying capacity has been decreased after the passage of first year flood flows over the dredged Gorai. Evidently this change in decreasing the low level conveying capacity of the dredged Gorai is due to the sedimentation in the deeper channel and therefore, the first year pilot dredge is not meaningfully effective in the sense of self-sustaining development for low flow augmentation. It is found that the bottom bed level of dredge channel has been raised close to the level of pre-dredge (Post monsoon 1997) state after the passage of first year flood event over the dredged channel. The study considers a 30 km reach of Gorai which is studied to estimate changes in dredged bed levels and alignments at different discharge scenarios. The changes are computed using the MIKE21 hydro-morphological software which allows the study to estimate effective discharge responsible for low flow bed formation. In this study, dominant discharges of the Gorai river at Gorai Railway Bridge (GRB) is estimated at 4125 m3/s by Schafernak's method (sediment concept) as it offers the most infornlation about channel dynamics and the interaction of the channel to watershed hydrology. In addition, this effective discharge inherently allows for the optimisation of channel size for specific sediment transport concern. The changes in dredged bed levels of Gorai has been studied for three effective discharge scenarios, e.g.; 100%, 75% and 50% of dominant discharges. The study finds that the effective discharge for low flow bed formation and transp0l1 capacity can be estimated at 50% of dominant discharge as it closely estimates the bottom bed levels of pre-dredge (post monsoon 1997) and post dredge (post monsoon 1999) state of bathymetry The bottom bed level of the regime state of bathymetry corresponding to 50% dominant discharge (which represents low flow bed formation) is found much higher than the dredged bed level, but closer to the bottom bed level of low flow pre-dredge bathymetry and the post dredge bathymetry as well. This indicates that the bottom bed level of the dredged Gorai will go back to its low flow pre-dredge state in course of time and therefore, dredging will not be effective for self sustaining low flow augmentation. Regime state of bathymetry corresponding to the dominant discharge shows that the left bank entrance chmmel shifts toward west and the deeper channel at Kushtia becomes more close to the groynes. The Gorai mouth gets sedimented and the deeper entrance channel is si Ited up. This phenomenon was also observed during the monitoring of passage of 1999 nood over the dredged bathymetry. Regime bathymetry upstream of GRB shows that the deeper channel moves more towards eastern bank while this simulation pattern is sUPP0l1ed by the second year dredge alignments. On the basis of this study, it can be inferred that the first year dredge level will not be effectively maintained as the Gorai tends to go back to its low flow pre-dredge state. The study also gives an idea about the changes in alignment of deeper channel, for instance, shifting of deeper channel upstream of Kushtia more towards west and shifting of deeper channel upstream of GRB more towards the eastern bank.