Storm surge hazard over Bengal delta: A probabilistic-deterministic modelling approach

Storm-surge induced coastal inundation constitutes a substantial threat to lives and properties along the vast coastline of the Bengal delta. Some of the deadliest cyclones in history made landfall in the Bengal delta region claiming more than half a million lives over the last five decades. Complex hydrodynamics and observational constraints have hindered the understanding of the risk of storm surge flooding of this low-lying (less than 5 m above mean sea level), densely populated (> 150M) mega-delta. Here, we generated and analysed a storm surge database derived from a large ensemble of 3600 statistically and physically consistent synthetic storm events and a high-resolution storm surge modelling system. The storm surge modelling system is developed based on a custom high-accuracy regional bathymetry enabling us to estimate the surges with high-confidence. From the storm surge dataset, we performed a robust probabilistic estimate of the storm surge extremes. Our ensemble estimate shows that there is a diverse range of water level extremes along the coast and the estuaries of the Bengal delta, with well-defined regional patterns. We confirm that the risk of inland storm surge flooding at a given return period is firmly controlled by the presence of coastal embankments and their height. We also conclude that about 10 % of the coastal population is living under the exposure of a 50-year return period inundation under current climate scenarios. In the face of ongoing climate change, which is likely to worsen the future storm surge hazard, we expect our flood maps to provide relevant information for coastal infrastructure engineering, risk zoning, resource allocation, as well as future research planning.

Climate Precursors of Satellite Water Marker Index for Spring Cholera Outbreak in Northern Bay of Bengal Coastal Regions

Cholera is a water-borne infectious disease that affects 1.3 to 4 million people, with 21,000 to 143,000 reported fatalities each year worldwide. Outbreaks are devastating to affected communities and their prospects for development. The key to support preparedness and public health response is the ability to forecast cholera outbreaks with sufficient lead time. How Vibrio cholerae survives in the environment outside a human host is an important route of disease transmission (see a review of Racault et al. 2019). Thus, identifying the environmental and climate drivers of these pathogens is highly desirable. Here, we elucidate for the first time a mechanistic link between climate variability and cholera (Satellite Water Marker; SWM) index in the Bengal Delta, which allows us to predict cholera outbreaks up to two seasons earlier. High values of the SWM index in fall were associated with above-normal summer monsoon rainfalls over northern India. In turn, these correlated with the La Niña climate pattern that was traced back to the summer monsoon and previous spring seasons. We present a new multi-linear regression model that can explain 50% of the SWM variability over the Bengal Delta based on the relationship with climatic indices of the El Niño Southern Oscillation, Indian Ocean Dipole, and summer monsoon rainfall during the decades 1997–2016. Interestingly, we further found that these relationships were non-stationary over the multi-decadal period 1948–2018. These results bear novel implications for developing outbreak-risk forecasts, demonstrating a crucial need to account for multi-decadal variations in climate interactions and underscoring to better understand how the south Asian summer monsoon responds to climate variability.

Influence of chemical fertilizers on arsenic mobilization in the alluvial Bengal delta plain: a critical review

Arsenic contamination of alluvial aquifers of the Bengal delta plain causes a serious threat to human health for over 75 million people. The study aimed to explore the impacts of chemical fertilizer on arsenic mobilization in the sedimentary deposition of the alluvial Bengal delta plain. It selected ten comparatively higher affected Districts and the least affected two Divisions as a referral study site. The countrywide pooled concentration of arsenic in groundwater was 109.75 μg/L (52.59, 166.91) at a 95% confidence interval, which was double the national guideline value (50 μg/L). The analysis results showed a strong positive correlation (r ≥ 0.5) of arsenic with NO3, NH4, PO4, SO4, Ca, and K, where a portion of those species originated from fertilizer leaching into groundwater. The results showed that PO4 played a significant influence in arsenic mobilization, but the role of NO3, SO4, and NH4 was not clear at certain lithological conditions. It also showed that clay, peat, silt-clay, and rich microbial community with sufficiently organic carbon loaded soils could lead to an increase in arsenic mobilization. Finally, the study observed that the overall lithological conditions are the main reason for the high arsenic load in the study area.

Time-Series Remote Sensing Study to Detect Surface Water Seasonality and Local Water Management at Upper Reaches of Southwestern Bengal Delta from 1972 to 2020

Bengal delta experiences immense seasonality of surface water due to its geographical position. This study aims to explore the extent and seasonality of surface water in the southwestern part of Bangladesh (SWB) where human intervention has been rapidly changing the land use for several decades. This explorative study relied on a total of 312 high-resolution Landsat images from 1972 to 2020 and interviews to present crucial months, seasons, and periods for surface water in SWB. The study uses a valid threshold point ‘0′ for Normalized Difference Water Index (NDWI) to extract water pixels and confirms that the NIR band has better efficacy to separate water pixels. On average, the SWB has faced around 5.5% of surface water between 1972–2001, which increased to 12.8% between 2002 and 2020. Based on the median value, around 6% of surface water was observed in the 1990s, which increased to 16% in the 2010s. The average surface water was detected around 6% and 7% in December and January between 1972 and 2001, which expanded to 18% and 19% between 2002 and 2020, mainly because of human interventions such as mix-cropping. The study strongly suggests considering December and January months for further land use and land class studies which focus on the southwestern part of Bangladesh.

A report groundwater arsenic contamination assay in the delta area of West Bengal

Groundwater contaminated with arsenic adds an extra percentage of arsenic load at the time of the different irrigated processes. In an arsenic-contaminated locality, an experimental drinking water contamination study was done on tap water, pump water used for irrigation, hand pump water used for cooking, and drinking in the delta area in West Bengal, Nadia, 24-Parganas (north and south) Hooghly, Bardhaman, suspicious villages and in Kolkata, in Bengal delta.

Towards an efficient storm surge and inundation forecasting system over the Bengal delta: chasing the Supercyclone Amphan

The Bay of Bengal is a well-known breeding ground to some of the deadliest cyclones in history. Despite recent advancements, the complex morphology and hydrodynamics of this large delta and the associated modelling complexity impede accurate storm surge forecasting in this highly vulnerable region. Here we present a proof of concept of a physically consistent and computationally efficient storm surge forecasting system tractable in real time with limited resources. With a state-of-the-art wave-coupled hydrodynamic numerical modelling system, we forecast the recent Supercyclone Amphan in real time. From the available observations, we assessed the quality of our modelling framework. We affirmed the evidence of the key ingredients needed for an efficient, real-time surge and inundation forecast along this active and complex coastal region. This article shows the proof of the maturity of our framework for operational implementation, which can particularly improve the quality of localized forecast for effective decision-making over the Bengal delta shorelines as well as over other similar cyclone-prone regions.