The 400-year history of Dhaka says that the city once had several well-known natural canals (khals) that drained stormwater and graywater. In addition to city’s combined sewer system, these water bodies offered an essential natural drainage system that allowed to manage the monsoon rainfall effectively. However, over the past three decades, due to rapid urbanization, these khals have significantly depleted to the point where they are no longer capable of draining the city’s monsoon runoff. Using past, present, and future Land Use and Land Cover (LULC) and urban drainage modeling, this study identified the effects of such LULC change on pluvial flooding of the northern part of the city. Analysis shows that the rapid and extensive changes in LULC over the past decades have resulted in significant shrinkage of these khals, consequently leading to escalated rates of urban flooding in this region. The western part of Turag thana, low-lying areas close to the Baunia Khal depression, and the upstream region of Abdullahpur Khal are highly vulnerable to future urban floods. The projected LULC change indicates an increase of 8.47%, 8.11%, and 4.05% in the total inundation area by 2042 for rainfall events with return periods of 50 years, 25 years, and 2.33 years, respectively. The findings also indicate that 11% more area is likely to experience long-duration flooding due to LULC change.

This is the second article of a two-part series where the model validated in part 1 is extended to study the hydrodynamic response and group effect of elevated pile cap substructures to impulsive surge impact. Two conventional grids (2×2, 2×3 array-layout) and a hexagonal pile arrangement under three submersion levels are considered in studying the pile group effect on hydrodynamic forcing mechanism, flow characteristics, and factors governing pressure variability. The uplift thrust on the pile cap was found to be dependent on the surge fluid mass, incident velocity, and vertical distance between the surge breaking point and cap bottom. At impulsive impact, gridded pile arrangement offered greater resistance to incident surge flow. In reference to free flow condition, hexagonal layout can reduce the average flow velocity by 8.22-19.76%, while for a gridded system, it is between 14.00%-23.52% depending on the submersion level. However, the reversed under quasi-steady state, where the hexagonal system showed greater flow resistance. The study also compared the numerical results to accepted offshore design standards, which were found to underestimate the pile’s group effect, specifically for complex hexagonal layouts. . Later, it proposes rationalized force diagrams to be applied in design of complex elevated structures.