Hydro-morphological study for fixation of the alignment of coastal closure-a case study over Mainka-Montaz channel in the coastal area

Abstract

The present study has been conducted to hydrological analysis and find out the zone of tidal meeting points for fixation of alignment of the closure over Mainka and Montaz channel. The Mainka and Montaz channel is connected channel of Bura Gauranga River (branch of Tetulia Channel) and kukri-Mukri Channel (branch of Shabazpur Channel) which both finally flows to the Bay of Bengal. Analysis of cross-section of Mainka and Montaz channel and selection of the closure sites (16 no. cross section of Mainka Channel and 25no. cross section of Montaz channel). Those stretches of both Channels have a regular shape with very constant width and depth. The closure over Montaz channel already constructed last monsoon by BWDB. The maximum and minimum design water level was calculated and got 3.5m and -0.80m. The tidal range varies from less than 1.0 meter during neap to 2.5 meter during spring. It is envisaged to execute final closures in three days from 13-15 February and from 27 February- 1March. The maximum discharge of Montaz channel is 425 m3/see on 28/12/09 and 700m3/see on 2/1/10. Similarly the maximum discharge of Mainka channel is 775 m3/see on 29/12/09 and 600m3/see on 3/1/10. The critical velocities during final closure (gap considered 100m) are 1.2 m/sec in the Mainka Channel and 1.4 m/sec in the Montaz Channel. The conclusions on the main morphological developments observed in the period from 1973 to 2008 are presented this study. The planform of the chars and channels in the cross dam area has changed significantly during the period from 1984 to 1996. The area of south Bhola has been accreting rapidly in the last 25 years. At present a number of channels between Bhola and the chars to the south of Bhola have more or less stabilized. The construction of closure in the coastal area, the selection of timing during final closure is the most vital issue. In this study it has been find out the closing time by analysis of water level. A suitable period around neap has been selected; in this case the first neap in February seems adequate. Innovative techniques and new construction materials have been developed in the past decades for constructing dams, embankments and slope protection structures. Geo-textile tube (shortly called geo-tube) technology is one of these new features that have been successfully used to create sustainable sea defense structures, retaining structures, slope reinforcement and erosion protection. They are also cost effective and time saving when compared to traditional construction methods.