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Floods by nature are complex events caused by climate variability and a range of human activities, including inappropriate development planning. There are different ways of classifying and categorising floods according Fto geographic and geophysical characteristics. In Bangladesh, floods are most commonly classified as:
Flash floods
Which are caused by heavy or excessive rainfall in a short period over a relatively small area. During flash flooding, water levels rise and fall rapidly with little or no advance warning. Typically, they occur in areas where the upstream basin topography is relatively steep and the time needed for the water to flow from the most remote point in a watershed to the watershed outlet is relatively short. The most affected areas are in the Haor Basin of the northern belt of Bangladesh, which is made up of Sylhet, Sunamganj, Moulvibazar, Habiganj, and Netrakona Districts, as well as the southeast in Chittagong, Cox’s Bazar and Bandarban Districts (BWDB 2014; WMO 2003). Flash floods are most common from April to July and from September to October (WMO 2003). Flash floods carry sediment that has eroded from hilly catchment areas. During heavy rainfall in the hilly regions, massive erosion occurs on exposed surfaces of the hills. When there is high rainfall, coarser sediment erodes and moves along the rivers. During a flash flood, sediment transport rates increase significantly resulting in a disproportionate distribution of sediment and changes in channel sizes, shapes, and even location (BWDB 2014).
RIVER/SEA FLOODING
Also known as monsoon flooding (June to September), is the most common type of flooding in Bangladesh. It refers to, both, “normal” inundation of up to 25% of the country as well as extreme flooding which can inundate up to 70% of the country (WMO 2003). Bangladesh has four major river systems, with many tributaries and distributaries. Districts that have rivers running through them, or are in close proximity to the sea, and are prone to severe flooding are Brahmanbaria, Barisal, Bogra, Chandpur, Chapai Nawabganj, Comilla, Dhaka, Faridpur, Gaibandha, Gazipur, Jamalpur, Kishoreganj, Kurigram, Kushtia, Lakshmipur, Madaripur, Manikganj, Munshiganj, Narayanganj, Narsingdi, Natore, Pabna, Rajbari, Rajshahi, Rangpur, Shariatpur, Sherpur, Sirajganj, and Tangail. The effects of river flooding on Northern Bangladesh, is often most severe, due to the pre-existing underlying vulnerabilities.
WATERLOGGING
Refers to stagnant river floodwaters that are unable to recede. The southwest costal belt across Satkhira, Jessore, Jhenaidah and Khulna Districts are primarily affected due to unabated encroachment of canals and a lack of proper drainage. Silting in nearby rivers, like the Chitra and the Madhumati, as well as canals, have significantly reduced the water flow (The Daily Star 01/05/2017). It has also become an increasingly concerning issue in urban areas like Dhaka and Chittagong Districts due to rapid urbanization and poor waste management. Waterlogging can be particularly problematic at the end of the monsoon period (August/ September).