Exploring impact of street layout on urban flood risk of people and vehicles under extreme rainfall based on numerical experiments

Urban street layout is an important factor in the formation process, characteristics, and risk level of urban flooding;therefore, this study numerically investigates the impact of street layout on urban flood risk to people and vehicles. Four typical street-layout scenarios with areas of 3 km × 3 km are established based on a block-scale investigation. The layout types are regular grid,irregular grid, radial, and annular. Urban inundation models are then constructed for these typical street layouts based on the twodimensional(2D) hydrodynamic method. Two historic, extreme rainfall events, which occurred in Beijing on July 21, 2012 and in Zhengzhou on July 20, 2021, are used as rainstorm scenarios for urban inundation modelling. The flood risks to people and vehicles are then calculated. Results show that, for an extreme rainstorm on the block scale, the street layout impacts the spatial and temporal distributions of the inundation variables, which include the water depth, flow velocity, flood volume, and inundated area. Moreover, for the same extreme-rainfall scenario, the greatest differences in the total flood volume, maximum street-water depth, and maximum street-flow velocity caused by street-layout differences are 17.22%, 60.25%, and 61.50%, respectively.Among the four street layouts considered in this study, the annular street layout exhibits the lowest degrees of inundation and flood risk. For the same extreme-rainfall scenario, the proportions of high-risk road sections for adults and children in this layout are 58.89% and 62.28% smaller than those for the layout with the largest proportion of high-risk road sections, respectively;the proportions of high-risk road sections for the Honda Accord and Audi Q7 were 55.31% and 53.04% smaller, respectively. The findings of this study may aid scientific understanding and development of “flood-sensitive” block-scale street layouts and urban planning in the context of the changing environment.

OPTIMIZING LAYOUT OF URBAN STREET CANYON USING NUMERICAL SIMULATION COUPLING WITH MATHEMATICAL OPTIMIZATION

Optimizing the layout of the urban street canyon to achieve the maximum environmental benefits should become a new idea for modem urban street design and planning. This paper aims to find out the optimized street canyon from a viewpoint of environmental protection by using the two-dimensional numerical simulation model with the κ-ω turbulence model, coupling with the mathematical optimization method. The total pollutant concentration within and at top of the specific street canyons was taken as the objective function, and the height of one side of the canyon as the constrained condition. A nonlinearly improved constrained variable metric solver was used. The effect of the height of the leeward building and windward building on the integrated pollutant dispersion was studied to achieve the most beneficial configuration of the urban geometry. The optimization of layout for an asymmetrical street canyon was obtained. It is further found that the step-down street canyon with a large height difference is generally a good layout favoring to reduce the concentration accumulation in the street canyon.

Effects of urban square entry layouts on spatial ventilation under different surrounding building conditions

Effective urban square ventilation is important for the living comfort and health of residents.This research aims to establish some principles of street entry layouts for urban square ventilation optimization.Using computational fluid dynamics(CFD)simulation techniques,two types of street entry squares(two-intersection and four-intersection)were investigated under the effect of surrounding buildings.Three indices—the spatial mean velocity(<V^(*)>),air change rate(ACH)and mean flow rates()through the street entry and square roof—were calculated to quantify the square ventilation performance.The simulation results indicate that the surrounding buildings could influence the square space ventilation.When the surrounding building conditions change to a building coverage ratio of 0.25,theentering through the street entry can be reduced by 35%,and the<V*>within the square decreases by more than 45%.The optimal street entry layout design depends greatly on the wind direction.When the wind direction is perpendicular to the square street entry,the corner oblique-entry layout square shows a better ventilation performance than the other designs.When the wind direction is oblique to the square street entry,the<V^(*)>declines greatly(up to 68%),and the lateral-entry layout design shows the best ventilation performance.