Exploring the hydrologic response to the urban building coverage ratio by model simulation

The increasing building coverage ratio accompanied by the increase in impervious surfaces during the urbanization process may change the flow concentration conditions, but little attention has been given to this issue. This study aims to explore the hydrologic response to the building coverage ratio in small catchments. A representative urban region in Nankai District, Tianjin city, was selected as the study area. Different building coverage ratio scenarios were designed and then simulated using the cellular automata dual-drainage simulation model (CADDIES-caflood). Maximum water depth (WD) and peak time (PT) were used as the metrics to assess the surface flow response. Three drainage capacities were set up to check the influence of drainage facilities. Moreover, the area inundation characteristics were analysed through the distribution of WD. The results showed that the WDs at the water collection points decreased and the PTs increased with increasing building coverage ratio for rainfall with a relatively lower return period, but the WD may increase for rainfall with a higher return period. When the drainage capacity increased, the WD showed a significant decreasing trend for rainfall with a higher return period. The proportion of inundated area had an obvious uptrend with 1 year return period rainfall but a downtrend trend with rainfall return periods greater than 2 years. The mean value of the WDs of the water points increased with increasing building coverage ratio, while the skewness and kurtosis all declined. These findings may offer a new perspective on urban runoff risk mitigation.