Abstract
Urban floods pose a societal and economical risk. This study evaluated the risk and hydro-meteorological conditions that cause pluvial flooding in coastal cities in a cold climate. Twenty years of insurance claims data and up to 97 years of meteorological data were analyzed for Reykjavík, Iceland (64.15°N; < 100 m above sea level). One third of the city’s wastewater collection system is combined, and pipe grades vary from 0.5-10%. Results highlight semi-intensive rain (<7 mm/hr; ≤3-year return period) in conjunction with snow and frozen ground as the main cause for urban flood risk in a climate which undergoes frequent snow and frost cycles (avg. 13 and 19 per season, respectively). Floods in winter were more common, more severe and affected a greater number of neighborhoods than during summer. High runoff volumes together with debris remobilized with high winds challenged the capacity of wastewater systems regardless of their age or type (combined vs. separate). The two key determinants for the number of insurance claims were antecedent frost depth and total precipitation volume per event. Two pluvial regimes were particularly problematic: long duration (13-25 hours), late peaking rain on snow (RoS), where snowmelt enhanced the runoff intensity, elongated and connected independent rainfall into a singular, more voluminous (20-76 mm) event; shorter duration (7-9 hours), more intensive precipitation that evolved from snow to rain. Closely timed RoS and cooling were believed to trigger frost formation. Average seasonal snow depth and volume of rain and snowmelt during RoS events were found to be increasing due to climate change. More emphasis, therefore, needs to be placed on designing and operating urban drainage infrastructure with regard to RoS co-acting with frozen ground. Furthermore, more detailed, routine monitoring of snow and soil conditions is important to predict RoS flood events.
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