Abstract
Even if urban catchments are adequately drained by sewer infrastructures, flooding hotspots develop where ongoing development and poor coordination among utilities conspire with land use and land cover, drainage, and rainfall. We combined spatially explicit land use/land cover data from Luohe City (central China) with soil hydrology (as measured, green space hydraulic conductivity), topography, and observed chronic flooding to analyze the relationships between spatial patterns in pervious surface and flooding. When compared to spatial-structural metrics of land use/cover where flooding was commonly observed, we found that some areas expected to remain dry (given soil and elevation characteristics) still experienced localized flooding, indicating hotspots with overwhelmed sewer infrastructure and a lack of pervious surfaces to effectively infiltrate and drain rainfall. Next, we used curve numbers to represent the composite hydrology of different land use/covers within both chronic flooding and dry (non-flooding) circles of 750 m diameter, and local design storms to determine the anticipated average proportion of runoff. We found that dry circles were more permeable (curve number (mean ± std. error) = 74 ± 2, n = 25) than wetter, flooded circles (curve number = 87 ± 1). Given design storm forcing (20, 50, 100 years' recurrence interval, and maximum anticipated storm depths), dry points would produce runoff of 26 to 35 percent rainfall, and wet points of 52 to 61 percent of applied rainfall. However, we estimate by simulation that runoff reduction benefits would decline once infiltration-excess (Hortonian) runoff mechanisms activate for storms with precipitation rates in excess of an average of 21 mm/h, contingent on antecedent moisture conditions. Our spatial metrics indicate that larger amounts and patches of dispersed green space mitigate flooding risk, while aggregating buildings (roofs) and green space into larger, separate areas exacerbates risk.
Highlights
A dramatic increase in the global frequency and intensity of flooding disasters has been attributed to the interaction between urbanization and climate change [1,2,3,4,5]
We identified 25 areas where chronic flooding was recorded by the local government and 25 random areas where flooding was absent, and compared their land use/cover characteristics to understand which of these factors may contribute to vulnerability of flooding
The measured green space saturated hydraulic conductivity translates to an “A” type hydrologic soil group, from which we tabulated the corresponding curve number (CN) value of our general Luohe P3 cover, and the other land use classes (Table 1) [32,33]
Summary
A dramatic increase in the global frequency and intensity of flooding disasters has been attributed to the interaction between urbanization and climate change [1,2,3,4,5]. The increase in impervious surfaces, resulting from rapid development of new urban core areas, has contributed to chronic flooding during the rainy season (May to October) [7,8]. Recent statistics indicate that major storm floods are becoming more common, and directly impact an estimated 2.55 million people per year [9]. These trends prompted the Chinese government to initiate a nation-wide “Sponge Cities” program in 2015 to address urban flooding issues [10]
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