Assessment of impact of different implementation schemes for the proposed Sandwip-urir char-Noakhali cross-dam on regional hydro morphological regime

Abstract

The Meghna estuary houses one of the most dynamic and morphologically active coastal regions in the world. Land formation through natural accretion takes place every year at a steady but slow and diminishing rate. This combined with the decreasing rate of sediment influx poses a challenge in formulating long-term land reclamation strategies. The proposed Sandwip-Urir Char-Noakhali cross-dam region is dominated by tidal and wave forcing with minimal effect of river discharge and seasonal wind variations. Morpho-dynamics in the region is influenced by residual current and tidal asymmetry features.

A two-dimensional hydrodynamic model was applied to assess the impact of the Sandwip-Urir Char cross-dam on the different physical factors that contribute to the erosion-accretion processes. The model used global bathymetry data that has been refined for the shallower near to coast portions with data from MES studies. The model input included harmonic tidal constituent as downstream seaward boundary data and river flow and sediment discharge as upstream boundary data representing forcing mimicking that of a high monsoon discharge. Calibration and validation of model was done using observed water level data from the MES studies. The model was simulated for 17 days with a high-resolution grid for flow analyses and with a coarser grid resolution for 47 days for sediment analyses.

A two cross-dam setup was used with four different dam implementation cases. Results show strong upward residual current through the Sandwip channel circulating Urir Char from north and south ends and exiting through Jahajir Char-Sandwip and Jahajir Char-Noakhali. Natural accretion takes place in the southern tips of Jahajir Char and Sandwip. Impact from implementation of both cross-dams alter this flow and sediment regime and induces accelerated sediment deposition in the northern, western and southern sides of Urir Char. Flow analyses reveal the region to be highly flood dominated and asymmetry values as well as slack water durations indicate accelerated sediment transport and deposition in the vicinity of dams surrounding Urir Char and the northern ends of Jahajir Char and Sandwip. The proposed cross-dams will yield best results if used in conjunction. For a single cross-dam setup, the dam from Urir Char to Noakhali is to be used and to be built first in case of a two-dam setup. The proposed cross-dams will facilitate land formation through accelerated accretion in key locations and thus aid in land reclamation in the region.