Journal of Hydro-environment Research xxx (2016) xxx–xxx Contents lists available at ScienceDirect Journal of Hydro-environment Research journal homepage: www.elsevier.com/locate/JHER Research papers Potential impacts of the Sunderban mangrove degradation on future coastal flooding in Bangladesh ⇑ Mithun Deb a, , Celso M. Ferreira a a Department of Civil, Environmental & Infrastructure Engineering, George Mason University, Fairfax, VA 22030, USA article info abstract Article history: The coastal areas of Bangladesh are recognized by the United Nations (UN) as the most vulnerable areas Received 30 June 2015 in the world to tropical cyclones and also the sixth most vulnerable country to floods around the world. Revised 27 July 2016 Cyclone Sidr (2007) was one of the most catastrophic natural disasters in Bangladesh causing nearly Accepted 14 November 2016 10,000 deaths and $1.7 billion damage. During cyclone Sidr, mangrove forests in coastal areas played a Available online xxxx crucial role in the mitigation of these deadly effects. Sunderban mangrove, the world’s largest mangrove ecosystem with 7900 sq. miles, forms the seaward frontier of the bay and is now facing significant degra- Keywords: dation. The Sunderban mangrove ecosystem is increasingly being degraded for a variety of purposes such Bangladesh as agriculture, fishing, farming and settlement. In this study, we evaluate the potential impacts from the Sunderban mangrove Cyclone degradation of the Sunderban mangrove on storm surge flooding. We evaluate two hypothetical and Storm surge extreme scenarios: 1) the conversion of the entire mangrove land cover to an estuarine forested wetland; Coastal hazard and 2) by considering a full degradation scenario where the entire mangrove is converted to grassland. To ADCIRC quantify the benefits of the mangrove forests to attenuate storm surge in this area, we applied a frame- work combining a spatial characterization of mangroves vegetation with numerical simulations. Storms surge was calculated using a hydrodynamic model (ADCIRC) coupled with wave model (SWAN) under a High Performance Computing environment. An unstructured numerical mesh with 200,000 nodes was developed and validated along with the coupled SWAN + ADCIRC model at six separate locations in the Bangladesh coast using Cyclone Sidr (2007) meteorological inputs. Twenty-seven model simulations were performed considering nine cyclones of different categories to quantify the effects of mangrove degradation on spatial flood inundation and storm surge magnitude. Simulation results showed that, on average, the mangroves degradation to grassland could raise the surge elevation as high as 57% and had a significant impact on increasing the velocity of the flood wave by up to 2730% for category 3 cyclones. In addition, the inundation inland penetration and total flooded area would increase almost 10 km and 18% respectively for low intensity cyclones. Furthermore, these hypothetical scenarios support the importance of the existing Sunderban mangrove in the reduction of surge elevation, velocity, inunda- tion penetration and flooded area. More importantly, it also demonstrates how the continuous degrada- tion of this important ecosystem has the potential to adversely impact the future cyclone induced hazards in the region. Ó 2016 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved. 1. Introduction the low-lying heavily inhabited areas and a continental shelf with shallow bathymetry leaves the entire coastal region of Bangladesh Cyclone surges in Bangladesh have caused deaths of over very vulnerable to catastrophic inundations along the coast (Murty 700,000 people since 1960 (Chowdhury and Karim, 1996) and it et al., 1986; Dube et al., 1997; Madsen and Jakobsen, 2004). Almost is still considered as one of the most damaging meteorological phe- one sixth of tropical cyclones that developed in the Bay of Bengal nomena in the region. The exposure to storm surge combined with made landfall on the Bangladesh coast (Islam et al., 2011); however, the death toll observed to be 80% of the global record (Debsharma, 2007). Numerous historic super cyclones, such as ⇑ Corresponding author at: Center for Applied Coastal Research, University of the 1970 Bhola cyclone, the 1991 Bangladesh cyclone and the Sidr Delaware, Newark, DE 19716, USA. 2007 cyclone had catastrophic effects in the coastal areas of E-mail addresses: [email protected], [email protected] (M. Deb), [email protected] (C.M. Ferreira). Bangladesh (Islam et al., 2011; Madsen and Jakobsen, 2004). http://dx.doi.org/10.1016/j.jher.2016.11.005 1570-6443/Ó 2016 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved. Please cite this article in press as: Deb, M., Ferreira, C.M. Potential impacts of the Sunderban mangrove degradation on future coastal flooding in Bangla- desh. J Hydro-environ Res (2016), http://dx.doi.org/10.1016/j.jher.2016.11.005 2 M. Deb, C.M. Ferreira / Journal of Hydro-environment Research xxx (2016) xxx–xxx The Sundarban mangrove offers coastal protection to millions of characterized as mangroves to an estuarine forested wetland and people in Bangladesh and lies in a zone facing cyclonic storms and subsequently to grassland. These hypothetical scenarios are based tidal bores from the Bay of Bengal periodically. The population on current trends of deforestation and situations already observed density surrounding the Sunderban mangrove is a significant fac- elsewhere in the world, for example, Colombia’s Caribbean coast tor, which made it a unique location for vulnerability related mangroves were converted into agricultural lands and shrimp assessments. During cyclone Sidr, mangrove forests in coastal aquaculture ponds, and in Quintana Roo (México), deforestation areas played a crucial role in the mitigation of the deadly effects of mangroves changed the land cover to low-density human settle- from coastal flooding. It has been well demonstrated that the man- ments and roads (Blanco et al., 2012). Afterwards, we used cate- groves play an important role in coastal defense and disaster risk gory 5 cyclone (Sidr 2007) modifying from its original track, and reduction as they reduce storm surge water levels by slowing the hypothesized about two additional cyclone categories 3 and 4 to flow of water, and reducing surface waves and wind stress observe the influence of storm attributes on inundation over the (McIvor et al., 2012). Several studies in recent past were conducted altered land covers. Then, the simulation results such as maximum to document the role of mangrove forests in the dissipation of water level, velocity, inundation distance and flood area were com- storm surges or tsunami inundation (e.g. Zhang et al., 2012; pared at different locations. Hiraishi and Harada, 2003; Yanagisawa et al., 2010). McIvor et al. As the coastal regions of Bangladesh are low-lying areas, the (2012) suggested that storm surge reduction through mangroves impact from mangrove degradation would certainly trigger wide- range from 5 to 50 cm of water level reduction per kilometer of spread damages. Surge elevation, velocity and inundation area mangrove width. However, the surge reduction rate depends on would increase largely and affect the existing dense population many significant contributing factors such as storm surge eleva- around the coast. The main goal of the analysis is to demonstrate tion, topography, distance to the shoreline and size of mangrove and quantify the impact that such drastic land cover changes could forests (Liu et al., 2013). Krauss et al. (2009) recorded water level potentially have on amplifying surge related hazards in the Sun- data in the mangrove zone of the South Florida coast during Hurri- derban mangrove region. canes Charley (2004) and Wilma (2005) and reported that man- groves reduce storm surge heights approximately 9.4 cm/km. 2. Study area However, the world’s largest mangrove forest, Sunderban, is facing a significant degradation. It occupied approximately twice The Sunderban mangrove lies on the delta of Ganges, Brahma- its current extent at the beginning of the colonial era in India putra and Meghna Rivers on the Bay of Bengal (Fig. 1). It is inter- (Islam et al., 1997). Globally, the area of mangroves has reduced connected by a complex network of tidal waterways, mudflats by almost 20% between 1980 and 2005 (Barros and Albernaz, and small islands of mangrove forests and the floor varies from 2014) and continues to decrease at a rate of 1–2% annually (Duke 1.5 to 3.0 m from the mean sea level (e.g. Karim and Mimura, et al., 2007). More than 50% of the country’s forests were extin- 2008). The Sundarbans was formerly (200 years ago) measured to guished in the last 30 years (MOEF, 2008; Roy et al., 2012). Other be of about 16,700 km2, and currently has reduced to an area of studies showed that 50% of the total tree cover has vanished over 10000 km2. Sixty-two percent of the total mangrove falls within the past 20 years (e.g. Kabir and Hossain, 2008). Population density the territory of Bangladesh and western boundary borders with close to Sundarban mangroves is among the highest in the world the Indian Sunderban (Islam and Wahab, 2005). The forest meets (Giri et al., 2007). Nearly 10 million people of the coastal regions the Bay of Bengal on the south side; to the east it is surrounded are relying on the Sunderban mangrove directly or indirectly, for by Baleswar River and agricultural lands on the north side (e.g. a variety of purposes such as agriculture, fishing, farming, human Wahid et al., 2007). The Sundarban has a unique biodiversity with settlement, collection of housing materials and human foods and a wide range of flora and fauna and the dominant mangrove Heri- employment opportunities in forestry practices (e.g. Islam, 2005). tiera fomes, locally known as Sundri or Sundari (e.g. Bhowmik and Shrimp farming is one of the major and perhaps the most detri- Cabral, 2013).