The Policy Research Working Paper Series disseminates the findings of work in progress to encourage the exchange of ideas about development issues. An objective of the series is to get the findings out quickly, even if the presentations are less than fully polished. The papers carry the names of the authors and should be cited accordingly. The findings, interpretations, and conclusions expressed in this paper are entirely those of the authors. They do not necessarily represent the views of the International Bank for Reconstruction and Development/World Bank and its affiliated organizations, or those of the Executive Directors of the World Bank or the governments they represent.

and participants of the Economics of Adaptation to Climate Change country consultations for their useful comments and suggestions. We are also grateful to Roufa Khanam for her valuable help with the GIS and Norma Adams for editorial help. The views expressed here are the authors', and do not necessarily reflect those of the World Bank, its Executive Directors, or the countries they represent.

No abstract

Mangroves are an important ecosystem-based protection against cyclonic storm surge. As the surge moves through the mangrove forest, the tree roots, trunks, and leaves obstruct the flow of water. Damage to adjacent coastal lands is attenuated mainly by reducing (i) surge height, which determines the area and depth of inundation and (ii) water flow velocity. But the extent of mangrove protection depends on the density of tree plantings and the diameter of trunks and roots, along with an array of other forest characteristics (e.g., floor shape, bathymetry, spectral features of waves, and tidal stage at which waves enter the forest). Making efficient use of mangroves’ protective capacity has been hindered by a lack of location-specific information. This study helps to fill that gap by estimating reduction in storm surge height and water flow velocity from mangroves at selected sites in cyclone-prone, coastal Bangladesh. A hydrodynamic model for the Bay of Bengal, based on the MIKE21FM system, was run multiple times to simulate the surge of cyclone Sidr (2007) at the Barisal coast. Estimates of surge height and water flow velocity were recorded first without mangroves and then with mangroves of various forest widths and planting densities, including specific information on local topography, bathymetry, and Manning’s coefficients estimated from species’ root and trunk systems. The results show a significant reduction in water flow velocity (29–92%) and a modest reduction in surge height (4–16.5 cm). These findings suggest that healthy mangroves can contribute to significant savings in rehabilitation and maintenance costs by protecting embankments from breaching, toe-erosion, and other damage.

Bangladesh is one of the most flood prone countries in the world. Two thirds of the country is less than 5 m above sea level. Past monsoon flood records indicate that about 21% of the country is subject to annual flooding and an additional 42% is at risk of floods with varied intensity. Although annual regular flooding has traditionally been beneficial, providing nutrient-laden sediments and recharging groundwater aquifers, the country often experiences severe flooding during a monsoon that causes significant damage to crops and properties with adverse impacts on rural livelihoods and production. The 1998 flood inundated two thirds of the land area, resulting in damages and losses of over US$2 billion, or 4.8% of GDP. Climate models suggest increased precipitation, higher transboundary water flows, and sea-level rise will all increase the destructive power of monsoon floods. Using climate change scenarios out to 2050, hydrological and hydrodynamic models, this article estimates an incremental cost to climate-proof roads and railways, river embankments protecting productive agricultural lands, and drainage systems and erosion control measures for major towns of US$2,671 million initially and US$54 million in annual recurrent costs.