Adequacy assessment of an urban drainage system considering future land use and climate change scenario

Dhaka, the capital of Bangladesh, has been experiencing severe water-logging and urban flooding in the last few decades. In this paper, we estimate the peak storm runoff of Hatirjheel-Begunbari canal – the largest drainage system of the city – under different operational, land use and climate scenarios (2013, 2025 and 2040). Our method includes digital elevation model (DEM) reconditioning, watershed delineation, and development of future land use scenario. We apply HEC-RAS to check the adequacy of Begunbari canal cross-sections to carry peak runoff for the scenarios considered here. The Hatirjheel-Begunbari system is found to drain stormwater from ∼25% of the city. Within the system, built-up areas are increasing linearly by 0.8 Km/year, whereas water body and wetlands are decreasing exponentially, which might increase the runoff coefficient by 11% in 2040 relative to 2013. Climate-induced change in rainfall intensity along with land-use change show three times higher runoff in 2040 than in 2013. Around 58% of canal cross-sections appear to be overflown at both banks while carrying a 5-year return period peak runoff under the 2013 scenario. For future scenarios, all sections seem to cause an overflow, which is alarming.

Mowla & Islam ). This stormwater becomes polluted as it mixes with solid waste, clinical waste, silt, contaminants, and domestic wastes and turns into a health hazard.
The stagnant stormwater leads to the increase of waterborne diseases (e.g. diarrhea, malaria, dengue), respiratory problems, and eye and skin diseases by creating the breeding sites for disease vectors (Mowla & Islam ). For example, in 2019, the city struggled with the worst outbreak of dengue fever, a mosquito-borne viral infection, affecting nearly 60,000 people (Mone et al. ).
The cities problems with water logging and urban flooding problem are created as the existing storm sewers are not fully operational owing to the occurrence of natural siltation, blockage by leaves and wastes including household and non-perishable plastic wastes and lack of proper maintenance (Alom & Khan ). In addition, the internal and peripheral canals of Dhaka, an important part of the overall drainage system of the city, are being illegally encroached (Ishtiaque et al. ). Thus, assessment of the capacity of the existing drainage system of Dhaka, especially the Hatirjheel-Begunbari (HB) canal drainage systemthe largest stormwater drainage system -has become imperative for better understanding and improving the drainage capacity of the city. However, few studies have been conducted so far to assess the capacity of different components of the HB system (the HB system is described in detail in the for different storm events without considering its aftereffect on Begunbari canal. In contrast, Matin et al. () and BRTC () assessed the adequacy of Begunbari canal along with all canals connecting to Balu river without considering Hatirjheel. A recent study (Ahammad et al. ) modeled the catchment of Begunbari canal using GeoSWMM and suggested the establishment of an embankment along the Balu river with a pump regulation at the canal outlet to prevent backwater flow from Balu River to Begunbari canal. To our knowledge, no study assessed the capacity of Begunbari canal considering the contribution of the Hatirjheel lake flow to it. In addition, none of the studies considered future stormwater runoff due to climate and land-use change.
Changing landscape is one of the anthropogenic stressors for the variation in the future stormwater runoff of a city (  In this paper, we estimate the future stormwater runoff of the combined Hatirjheel Begunbari drainage system considering future land-use scenarios, and future precipitation variability due to climate change in addition to the 2013 runoff of the system. We then check the adequacy of the existing geometry of the Begunbari canal to carry the future and 2013 stormwater runoff for different operating conditions of Hatirjheel Lake. A basic diagram showing the workflow of this paper is presented in Figure S1 in Supplementary Materials.

Land-use analysis and runoff coefficient calculation
For land-use analysis, we used the historical satellite images (1989, 1999, and 2009) with respect to their spectral and spatial profiles. We used a chosen color composite   'Sutivola', 'Gozaria', and 'Nasirabad-Nandipara') directly contribute to Begunbari canal flow; in contrast, western catchments (e.g. 'Hatirjheel' and 'Gulshan-Banani' lake) primarily contribute to Hatirjheel flow, except for SSDS 6 and SSDS 11 catchments. The flow from SSDS 6 and SSDS11 discharges directly into Bengunbari canal beyond Hatirjheel gate. Hence, for the ease of calculating the inflow to Begunbari canal, we treat the catchments of SSDS 6 and SSDS 11 separately from the 'Hatirjheel' and 'Gulshan-Banani' lake catchments and present them as separate groups (Figure 3).
Overall, nearly 61% of the total HB watershed directly contributes to the inflow of Begunbari canal and stormwater from the rest of the watershed routes towards Hatirjheel lake. The calculated catchment area of Begunbari canal (summation of eastern catchments, and SSDS 6 and 11) is 43.2 km 2 , which is comparable to the catchment area of 47.67 km 2 found in a previous study (Ahammad et al. ), where a hydrologic model was applied for catchment delineation.

Determination of maximum flow (Q)
The 2013 and future maximum rainfall intensities of the seven major catchments of HB drainage system are estimated for different return periods. Rainfall intensities considering a 5-year return period suggest that maximum rainfall intensity would be around three times higher in future years than that of 2013 for all the catchments (Table S4), which is due to the likelihood of frequent occurrence of extreme rainfall events by climate change (Rajib et al. ). We then determine the 2013 as well as future peak stormwater runoff for the seven major catchments of the HB drainage system considering a constant catchment area (Table S4). Overall, 'Nasirabad-Nandipara' catchment has the largest contribution to the inflow of Begunbari canal followed by Hatirjheel lake and Sutivola canal. We observe higher contributions from eastern catchments (Sutivola, Nasiranad-Nandipara, and Gozaria) relative to western catchments to Begunbari canal flow. For example, total peak runoff from the eastern catchments is 57% higher than the western catchments for the 2013 scenario. Note that the contribution of western catchments (except SSDS 6 and 11) to Begunbari flow depends on the operating condition of Hatirjheel the (see Table S1).
Among the two future runoff scenarios, the discharge in 2040 is higher for all catchments due to the higher runoff coefficient in 2040. canals merge into the Begunbari canal respectively (see Table S5). The peak 2013 and future storm runoffs for each of these 19 stations are then calculated from the estimated maximum runoff of the contributing catchments for both operating conditions of Hatirjheel (Table S5). The discharge data of Table S5 depict that Begunbari needs to carry successively higher runoff as it flows towards the Balu river due to the merging of eastern catchments.
We also see a considerably high flow in Begunbari after  . The condition will be more critical for the higher runoff in future scenarios (Table 1)    findings of the study suggest that future stormwater runoff is likely to increase significantly. The land-use trend investigation shows that built-up areas, within the HB watershed boundary, have been increasing linearly over the last three decades. Consequently, wetland and water body, and open and cultivated lands have been decreasing. Thus, overall land cover is changing and so is the runoff coefficient.
Only the change in land use types shows an 11% increase in runoff in 2040 relative to 2013. When climate change impact is added, this study indicates around three times higher peak runoff in 2040 than that of the 2013 scenario, given that land use and precipitation will continue to follow the observed trend until 2040.
The adequacy analysis using 1D HEC-RAS model indicates that some of the downstream sections of the Begunbari canal are inadequate to carry a 5-year return period peak runoff for the 2013 precipitation scenario even without the contribution from the western catchments. These sections are mostly located after the eastern catchments (e.g. Sutivola and Nasirabad canal) merge Note that the monsoon backwater flow into the canal from Balu River was not considered in the model due to lack of bathymetric and discharge data of Balu river. However, the inadequacy of existing cross-sections of Begunbari even without considering backwater flow suggests that gravity drainage is not capable of draining water from Begunbari to Balu river without an overflow. Our results The results from this study suggest that change in landuse and climate-induced change in extreme rainfall events might intensify the risk of severe waterlogging and flood vulnerability in future years in Dhaka. The goal of this paper was to assess whether the existing cross-section of Begunbari canal, an important part of the drainage system of the city, overflows under maximum stormwater runoff. The estimation of the magnitude of overflow at the banks caused by this overflow was out of the scope of this paper. Future study is recommended to determine the depth of overflowed water throughout the flood plain and develop flood inundation maps under the considered scenarios. Backwater flow effects can also be considered in future upon data availability. Overall, this study has made substantial attempts to enhance our understanding of how this HB system is working, and how important this system is in draining stormwater from a major part of the capital. Hence, the outcomes of this research will be useful for the policymakers to undertake development projects (e.g. pump drainage provision and excavation of specific segments of Begunbari) to enhance the performance of the HB system with the ultimate goal of improving the drainage situation of Dhaka city. In addition, this research will serve as a base for assessing the effectiveness of future development initiatives. This study also indicates the link of waterbody and marshy lands with stormwater drainage and management, and thus restates the importance of conserving water body and marshy lands. The results of this study can be helpful to the city planners while initiating appropriate plans to make Dhaka a more livable and planned city in the future.
Thus, the current study makes several noteworthy contributions to the current literature by addressing various knowledge gaps and bridging several aspects of stormwater drainage of Dhaka aiming to reduce waterlogging problems faced by the city dwellers.