Hybrid Coast Protection Approach in Bangladesh: A Case Study on Effectiveness of Small-Scale Forest in Reducing Surge Induced Inundation and Supporting Local Livelihoods

Abstract

Polders are subject to significant threats and often fail to protect the hinterland during cyclone induced storm surges. This study investigated the impact of mangrove afforestation in attenuating storm surge impacts on the embankment, as well as the social benefits of adapting this measure. Hydrodynamic model Delft-3D and ArcGIS 10.3 have been used to simulate, calculate and visualize coastal flood inundation condition. Primary and secondary source information were used to validate the model and assess social benefits of afforestation. For polder 47/4, the presence of 1.61 km² afforestation on the foreshore, protected additional 3 km² land from inundation during cyclone Sidr. While increasing polder height to 7 m slightly influenced inundation extent by reducing it 0.3 km², additional afforestation of the same roughness as exists now, helped to further reduce inundation depth in the hinterland. Social benefits provided by the existing forest were also considerable as local people collect fuelwood and Nipa palm regularly. While fuelwood was used for non-commercial uses, Nipa palm, on the other hand, was used by poor village women for preparing mats to sell at the local market. From the collected data, net benefit of this forest was calculated to be Tk. 5990.68 ($70.20)/hectare/year.

Appendix

1.1 Model Domain

Fig. 10

Model domain along with upstream and downstream boundary of the model. (Al Azad et al. 2018)

1.2 Validation of Model for Cyclone Sidr

From storm surge validation, trial runs are executed in Delft-Dashboard. The simulations also include tidal constituents from open boundary for both events. For the Sidr 2007 cyclone, Indian Meteorological Department (IMD) estimated maximum wind speeds of about 60 m/s during landfall and the lowest central pressure of the cyclone to be 944 hPa with a pressure drop of 66 hPa during landfall. The couple model run was performed for the entire cyclone period of 5 days from its generation at 0000 UTC, 11 November 2007, to its dissipation on 0000 UTC, 16 November 2007.

The model results were observed to perform well while calculating the inundation extent for S 2007 (Fig. 11). High water-level values were also extracted for the same locations (Table 3) from previous studies conducted by JSCE, 2008b, Lewis et al. 2013 and Deb and Ferreira, 2017 (Table 3). The computed storm surge was found to vary between 2.0 and 6.0 m along the coastal districts and estuaries where Sidr 2007 made landfall, and previous studies also demonstrated similar results on the same locations (Table 3).

Fig. 11

Inundation map of cyclone Sidr. (Haque et al., 2018)

Table 3 Comparison of model outcomes with high water levels (HWLs) extracted from existing studies on storm surge modelling in Bangladesh coast for cyclone Sidr

1.3 Sample Area for Manning’s Roughness Calculation

Fig. 12

Manning’s roughness calculation sample area (a) Section-1: less dense area (represented by orange box) and another one is (b) Section-2: comparatively denser area (represented by the blue box)

1.4 Polder Height Measurement

Table 4 Measured dimensions of polder at selected locations

Fig. 13

Photos of locals collecting and using firewood and nipa plam from the forest (Source Author)