Abstract:
The study was conducted to assess the hydrodynamic changes during construction of a closure in a 4 km wide tidal channel between Subornachar and Swarnadip island located in the Meghna Estuary. Two construction methods have been considered-vertical and horizontal closing method. A developed hydrodynamic model was used for the simulations of different construction stages of these two closing techniques using MIKE-21FM model. For the horizontal closing method the channel was narrowed down from the sides and for vertical closing method it was raised from the bottom of the bed at regular intervals by varying sill levels.
For the horizontal closing method, nine construction stages were consideredand and for the vertical closing method five construction stages were considered. The construction stages for horizontal closing method were H1(3650 m opening), H3(2850 m opening), H5(2050 m opening), H6(two 250 m openings and one 1050 m opening), H7(two 250 m openings and two 300 m openings), H8 (two 300 m openings) and H9 (two 250 m openings). The five construction stages for vertical closing method were V1 at sill level -5 mPWD, V2 at sill level -3 mPWD, V3 at sill level -1 mPWD, V4 at sill level +1 mPWD and V5 at sill level +5mPWD. For the horizontal closure the model was run by keeping the sill level fixed at +7.5 m PWD and the channel was narrowed down from the sides at regular intervals. Model simulation shows that the maximum flow velocities during the construction of the closure were obtained during flood tideas 1.43 m/s (H1), 1.7 m/s (H3), 2.28 m/s(H5), 2.56 m/s (H6), 4.36 m/s (H7), 4.58 m/s (H8) and 4.76 m/s (H8). It is suggested that the channel should be closed at once after H7 stage, because as the opening becomes smaller (H8 or H9) the velocities are seen to be increasing.
For the vertical closing method, theflow velocity over the sill was calculated using the Weir and Villemonte Formula. The maximumflow velocity initially increasesfor V1 (2.76 m/s), V2(3.07 m/s) and V3 (3.88 m/s) and then decreased for V4 (3.14 m/s) and V5 (2.9 m/s). These flow velocities were found during flood tide. This gradual drop in flow velocities occur as the flow becomes supercritical. As the channel was closed theflow velocityat north of the closure were found to be decreasing and that south of the closure were found to be increasing. Since the channel has a width of about 4000m, constructing the tidal closure poses severe challenges. The construction period is desired to be during neap tide of a dry season(preferably December which has the lowest tidal range of all times based on the water level data for Swarnadip). However, the construction would require a series of neap tidal cycles with spring tides coming in between and pausing the construction period. Thereforeflow velocities for spring tide have been also determined, so that sufficient material could be used to bear the increasedflow velocity due to narrowing down of the channel. For such a long channel having a width of 4000m, the vertical closing method seems more feasible as it has lower flow velocities during construction than horizontal closing method and narrowing down sucha wide channel from the sides is practically difficult than clsoing it from the bottom.
