Considering underground structure for 1D/2D urban flood modeling

Due to climate change and ongoing urbanisation, flood risks are expected to significantly rise in the future [1]. Therefore, reliable flood modelling becomes increasingly important. Most models consider buildings as obstructions without any openings. In reality, the water enters buildings [2] and floods underground structures such as basements or underground parking areas. This additional storage can influence the conditions on the surface [3].  

To increase the understanding of urban flooding such water entries must be considered in models. Therefore, we set up a 1D/2D model in PCSWMM2D and implement different types of water entries in a specific study site in Graz (Austria) to analyse three scenarios: i) without underground structure (scenario 0); ii) considering underground structure (all entries are permanently open) (scenario 1); iii) considering underground structure (all entries are controlled) (scenario 2). Underground structures are modeled based on the storages and orifices elements of SWMM5.1. Light wells and gravel beds are considered using additional storage elements. Since windows and doors are usually not constantly open (scenario 1) control rules for the realistic scenario (2) were included to simulate water entering after exceeding the conditions of water depth over 0.5m or velocity higher than 0.2m/s. The positioning of underground structures was done by using Google Earth, a field study and a list of basements provided by the city government. Due to a lack of information, we assumed basement surface area equal to building area.  

All scenarios were simulated for three real heavy storm events in Graz (50.7mm [8-4-2020], 45.3mm [8-13-2020], 104.9mm [7-30-2021]). A comparison of all three scenario suggest that the developed control rules resulted in realistic water conditions. Considering the underground structure leads to a small reduction in surface water volume and a decrease of the water level around the area of the flooded underground structure. This knowledge can be used for better understanding of urban floods for risk management.  

References 

[1] C. Field et al., “Climate change 2014: impacts, adaptation, and vulnerability – IPCC WGII AR5 summary for policymakers,” 2014, pp. 1–32. 

[2] H. Starl, “‘Klimawandel - Aktuelle Entwicklungen- Anpassungen der Siedlungswasserwirstschaft’ (in english: ‘Climate change - Current developments - Adaptations of urban water management’),” presented at the ÖWAV Tagung (in english: Austrian Water and Waste Management Association conference), online, Nov. 25, 2021. 

[3] B. Jamali, P. M. Bach, and A. Deletic, “Rainwater harvesting for urban flood management – An integrated modelling framework,” Water Res., vol. 171, p. 115372, Mar. 2020, doi: 10.1016/j.watres.2019.115372. 

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