Accommodation facilities for this increasing population in city areas are becoming worse, alongside decreasing water bodies and frequent waterlogging incidents ( Fu and Butler, 2014 Sanyal et al., 2014 Zhang and Pan, 2014). With an almost 3% increase in urbanization rate, and land use changes in due course, inundation experiences rather increased in recent years in many parts of the city ( CWASA, 2015). The projected growth rates from 2011 to 20 to 2030 are 2% or above in some of the densely populated wards of Chattogram city, which include areas such as Boxirhat, Chawkbazar, West Bakalia, East Bakalia, West Sholokbahar, East Sholokbahar, North Agrabad, Kattoli and so on. These areas cover almost 41% of the total city, and are frequently flooded and waterlogged during the wet season ( Figure 1 ). From 1982 to 2016, a significant increase in precipitation was observed compared with historical values. Disappearing and narrowing tertiary drains or roadside channels, lack of adequate cross-section or maintenance of secondary drains (which connect and carry discharge from a tertiary system of residential or industrial developments to the primary Khals, i.e. natural channels or tributaries of a river), increased solid waste disposal and siltation in the primary drainage system are some of the principal causes of flooding in urban areas. Thus, in addition to heavy rainfall and urbanisation, three major issues are involved in increased urban flood experiences. ( a) There is a lack of detailed knowledge of the hydrological cycle due to the change in rainfall patterns as well as land uses. Water in flooded areas remains stagnant for 2 to 3 days ( Mohit et al., 2014). Thus, there is a knowledge gap regarding the rainfall–runoff relationships that are generating inundations in low-lying areas around the city. ( b) There is an absence of hydraulic assessment in terms of the conveyance capacity of the existing roadside drains, which have a constant width of 0.3 to 0.9 m ( Akter et al., 2017 Aysha and Md. Therefore, critical drain intersects remain unknown. ( c) A knowledge gap on flood hotspots around the city exists, owing to the absence of flood hazard maps. Hydrological modelling usually provides an excellent basis regarding the generation of surface runoff for drainage network planners, designers and engineers. Thus user-friendly tools – namely, the Hydrologic Engineering Center–hydrologic modelling system (HEC-HMS) developed by the US Army Corps of Engineers, and the personal computer storm water management model (PCSWMM) – have been adopted around the world to depict the generated surface runoff.
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