Flood
Bangladesh has an extensive flood history that is entwined with the geophysical and social development of the country. Flooding in Bangladesh normally covers approximately 20% of the land which can increase depending on the scale of the flooding, however, higher return period events (i.e., 100 years) such as the 1988 flood results in a relatively higher amount of flooded area. Approximately 80% of the country is floodplain that drains water from big rivers like the Brahmaputra, Padma, and Meghna to the Bay of Bengal in the south- the floodplain communities are highly exposed to frequent hazards and low per capita income only increases their risk (Brouwer et al., 2007).
The FOREWARN flood expert group, in addition to the Flood Impact Model created by JBA, has made substantial contributions to the foundational risk analytics component of our Flood Disaster Risk Financing (DRF) system. The availability of impact information and lead time has significantly enhanced our ability to prepare effectively and take early actions.
Currently, the Flood Expert Group is actively exploring strategies for ensuring the accessibility of the Early Warning System (EWS) for floods to the most remote communities and pioneering innovative approaches to establish localized triggers for flood events. Presently, this expert group is providing crucial support to the Flood DRF system within the four districts of the Jamuna basin, namely Kurigram, Gaibandha, Jamalpur, and Sirajganj.
Forecast
Flood Forecasting and Warning Centre (FFWC) generates and provides flood forecast and warning information based on scientific principles, real-time data, weather forecast information, and mathematical models. Based on its findings, FFWC disseminates information about the water level in absolute terms as well in references to the danger level and flood prediction. There are four main observation stations in Bangladesh: the Bhairab Bridge, Bahadurabad, Hardinge Bridge and Aricha are representing the Meghna, Brahmaputra-Jamuna, Ganges, and Padma rivers respectively. These four river systems are the major sources of flooding in Bangladesh. Except the Bhairab bridge, other stations measure the water level at the entry point of the basin. The Bahadurabad hydrological station of Bangladesh Water Development Board (BWDB) is in the upstream boundary of the Jamuna river and maintains long observation records of water levels, discharge, and flooding in the Brahmaputra- Jamuna basin. Hence, the Bahadurabad station has been always considered and used as a threshold to represent the Brahmaputra-Jamuna basin flooding and starting point to predict flood impacts. On the other hand, Bangladesh faces another type of flood, called the ‘flash flood’, when continuous rain falls over hilly areas within a short duration causing surface water runoff or mudflows. Bangladesh Metrological Department (BMD) weather radars now observe such rainfall over the hills which can trigger flash floods. The FFWC issues five days’ deterministic forecast that indicates the water level in reference to the predetermined danger level. Also, the FFWC produces a medium-range (1-10 days) probabilistic forecast. The Global Flood Awareness System (GloFAS) is produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) as part of the Copernicus Emergency Management Service (CEMS) that provides operational extended-range ensemble flood forecasts with a 30-day lead-time for major world river basins including the Brahmaputra in Bangladesh (Alfieri et al., 2013). The GloFAS team from the University of Reading and ECMWF are working together with the Bangladesh Flood Forecasting and Warning Centre (FFWC) and humanitarian partners to improve flood early warning in Bangladesh so that issued forecasts to the government and stakeholders are as skillful and useful as possible. GloFAS is freely available through a dedicated web interface (https://www.globalfloods.eu/). The type of flood that happens can vary significantly depending on the monsoon rainfall and basin hydrological characteristics. GLoFAS flood forecasters consider three very important questions: when will flooding commence during the monsoon period, how long it will last, and will there only be one flood wave or many flood waves? The flood impact for Bangladesh combines the 2-, 5- and 20-year exceedance probabilities into category-based information. It shows where the ENS-max (maximum of the ensemble mean discharge) for forecast days 11-30 is >20 year (purple) / 5-20 year (red) / 2-5 year (dark yellow). Light yellow indicates where the ENS-max is below the 2-year return period value. The probability is considered 0-100 for 5 years and 20 years return period. For better understanding the impact-based forecast, the following risk matrix considers the GLoFAS and FFWC forecast information: LIKELIHOOD (5 years and 20 Years) HIGH >70% MEDIUM 50%-70% LOW 30%-50% X VERY LOW >30% Risk Seriousness <2 Years (70,000 to 75,000 m'3) 2-5 Years (75,000 to 90,000 m'3) 5-20 Years (90,000 to 105,000 m'3) >20 Years (>105,000 m'3) VERY LOW (MINIMAL) LOW (MINOR) MEDIUM (SIGNIFICANT) HIGH (SEVERE) IMPACT (at Bahadurabad Station) Table: Risk matrix for flood Based on the above risk matrix and continuous monitoring of GLoFAS and FFWC forecast from June to September, an agency could determine its own trigger model for anticipatory actions in monsoon floods. Central Emergency Response Fund Monsoon Flood Trigger Model in 2021 The model makes use of available forecasts with a two-step trigger system to predict severe monsoon floods in Bangladesh: Stage I: Readiness trigger is reached when the water discharge at the Bahadurabad gauging station over a period of three consecutive days is forecasted by the GloFAS model with a maximum 15-day lead time to be more than 50% likely to cross the 1-in-5-year return period. Stage II: Action trigger is reached when the water level at Bahadurabad is forecasted by the FFWC 5-day lead time model to cross the government-defined “Danger Level” + 0.85 meters, and probabilistic forecasts with longer lead times (GloFAS/RIMES) show a sustained or increasing trend of the water discharge at the Bahadurabad gauging station for at least three consecutive days beginning from the day when the danger level is forecast to be crossed.
Impact
Bangladesh, located within the Ganges-Brahmaputra-Meghna (GBM) river basin, faces significant flooding due to its geographical position. The country comprises about 8% of the GBM catchment, while the remaining 92% lies in India, Nepal, and Bhutan, draining through Bangladesh. The nation experiences floods covering around 20% of its land, with variations based on flood scale. Historical events like the 1988 flood (60% area flooded) caused $2 billion in damage, affecting 45 million people, while the 1998 flood (68% area flooded) incurred $2.8 billion in losses. Monsoon and flash floods are common, with a risk index indicating varying impact levels across districts: very high in 4 (10.53%), high in 4 (10.53%), medium in 14 (36.84%), low in 12 (31.58%), and very low in 4 (10.53%) districts. The relationship between flooded area and economic loss is linear, but factors like event duration, location, and floodwater altitude influence the impact.