Modeling the wave-tide hydrodynamics and assessment of the effectiveness of erosion protection structures along Teknaf coast

Abstract

The coastal zone of Bangladesh is one of the most important areas of the country because of its contribution to the economy and biodiversity.Coastal erosion is one of the major challenges at some places of Bangladesh’s coast including the Sabrang coast of TeknafUpazila of Cox’s Bazardistrict. As the Naf river passes beside the Sabrang area and the other side of this area is connected with the Bay of Bengal, the government of Bangladesh is turningthe Sabrang area into a tourist economic zone and eco-tourism park. But the ongoing erosion along the Sabrang coast eliminated a significant portion of the beach. Historical image analysis shows that this area has been experiencing coastal erosion and over a kilometer of the Sabrang beach was lost over the last two decades. If shore protection measures like revetment or seawalls are constructed, it may protect the shore but the beaches may be lost over time. Measures like groynes, breakwater, or beach nourishment may alleviate this problem. Therefore, this research has been taken to study the nearshore hydrodynamics of the Sabrang area and to devise the best possible beach protective measures. For that, a Delft 3D flow modelwas developed for the Bay of Bengal. The model was then downscaled for the Teknaf region and coupled with a wave model using the SWAN wave module. Using the coupled model, the nearshore wave-tide hydrodynamics was simulated.The model simulation results with the existing condition (having no protection measures) show that the magnitude of the flow velocity reaches 1.19 m/s near the Naf estuary. Also, the bed shear stress exceeds 3.0 N/m2 at some locations which are higher than the critical shear stress of those locations. The results also revealed that the longshore currents are dominant during both flood and ebb tides during all seasons. Therefore, the groyne was selected to arrest the longshore current and reduce the longshore sediment transport at the study area and the effectiveness of series of groynes having different spacing was simulated using the developed model. Based on the severity of the erosion along the Teknaf coast, an area of around 5.5 km along the coastline of Sabrang was analyzed with series of groyne as a case study. Based on the location of the surf zone, the length of the groynes was determined as 200 m. Model simulated results reveal that the series of groynes with 300m of spacing will be most effective in reducing the ongoing erosion at the study area and protect the beach. A total of 19 groynes with a length of 200 m and spacing of 300 m is expected to be sufficient to combat the beach erosion along the coastline of the study area.