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. Hydrological modelling usually provides an excellent basis regarding the generation of surface runoff for drainage network planners, designers and engineers. ( c) A knowledge gap on flood hotspots around the city exists, owing to the absence of flood hazard maps. Therefore, critical drain intersects remain unknown. ( 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. Thus, there is a knowledge gap regarding the rainfall–runoff relationships that are generating inundations in low-lying areas around the city. Water in flooded areas remains stagnant for 2 to 3 days ( Mohit et al., 2014). ( a) There is a lack of detailed knowledge of the hydrological cycle due to the change in rainfall patterns as well as land uses. Thus, in addition to heavy rainfall and urbanisation, three major issues are involved in increased urban flood experiences. 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. 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. From 1982 to 2016, a significant increase in precipitation was observed compared with historical values. These areas cover almost 41% of the total city, and are frequently flooded and waterlogged during the wet season ( Figure 1 ).
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. 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). 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). In the 2006 drainage network model, only 60% of the drains that had been realized from the 1995 master plan still remained in existence. Increased urban flooding has been experienced in Chattogram city in Bangladesh in recent years and so the city has several locations that have been inundated. ‘Urban flooding’ is experienced in some major cities in Bangladesh due to urbanisation and change in rainfall patterns. It is envisaged that development authority and city planners would benefit from this coupled model. Thus, the validated model was used for flood hazard mapping. Three major outlets were considered for model calibration for 2014, the model simulated runoff showed a reasonable match with field records – that is, R 2 = 0.782, R 2 = 0.719 and R 2 = 0.768 for the reaches Chaktai Khal, Rajakhali Khal and Mahesh Khal, respectively. The use of HEC-HMS generated the runoff the PCSWMM model simulation was carried out for 760 conduits and 693 junctions in the city that was studied. To feature flooding conditions, both hydrological and hydraulic model studies were carried out using 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). The field survey comprises cross-sectional data for tertiary drains these are then transferred for setting up the numerical model. This aim of this study was to develop a flood hazard map through a field survey and numerical modelling. Inadequate drainage capacity often fails to offer the desired response in urban flooding.