The Canadian province of British Columbia (BC) is subjected to large-scale, destructive floods. The most dramatic was a mid-November 2021 event when atmospheric rivers (ARs) linked to high-intensity storms caused heavy rainfall in southwestern BC, triggering catastrophic flooding. This study examines 37 floods from 2000 to 2021 using information from over 250 climatological stations and compares events with the mid-November 2021 flood. The dates of the floods showed a bi-modal pattern: a primary season (spring to early summer, 16 floods) and a secondary season (fall to early winter, 21 floods). Five mechanisms controlled these floods: heavy rainfall, rapid snowmelt, severe ice jam, rain-on-snow, and a mixture of snowmelt and ice jam; the mid-November 2021 flood was mainly driven by heavy rainfall. Of the 37 floods, those affected by either heavy rainfall (18 floods) or rain-on-snow (10 floods) were used to derive a relationship between the average daily precipitation amount over the duration of an event and the associated integrated water vapour transport . Flood events showed a strong linear relationship between these variables with < 0.05, and values of these parameters were significantly higher for the mid-November 2021 flood than for > 90% of the others, although they were not the highest. The mid-November 2021 flood was also one of the four rainfall-related floods that occurred in the secondary season with > 400 kg m s. The frequency of flood events over the last five years of the study period has slightly decreased when considering flood events with unknown insured cost. In contrast, insured costs of these events have increased, suggesting that present-day floods are becoming more impactful and may require changes to flood management strategies to reduce costs.

中文翻译：

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加拿大不列颠哥伦比亚省 2021 年 11 月中旬洪水的气候背景


加拿大不列颠哥伦比亚省 (BC) 遭受大规模破坏性洪水。最引人注目的是 2021 年 11 月中旬的一次事件，当时与高强度风暴相关的大气河流 (AR) 在 BC 西南部造成了强降雨，引发了灾难性洪水。这项研究利用来自 250 多个气候站的信息检查了 2000 年至 2021 年期间的 37 次洪水，并将事件与 2021 年 11 月中旬的洪水进行了比较。洪水的日期呈现出双峰模式：主季节（春季至初夏，16 次洪水）和次季节（秋季至初冬，21 次洪水）。控制洪水的机制有五种：强降雨、快速融雪、严重冰塞、雨雪结合、融雪冰塞混合； 2021年11月中旬的洪水主要是由强降雨造成的。在 37 次洪水中，利用受强降雨（18 次洪水）或雪雨（10 次洪水）影响的洪水来推导事件持续时间内的平均日降水量与相关的综合水汽输送之间的关系。洪水事件显示这些变量与 < 0.05 之间存在很强的线性关系，并且 2021 年 11 月中旬洪水的这些参数值明显高于其他 > 90% 的洪水，尽管它们不是最高的。 2021 年 11 月中旬的洪水也是第二季发生的 4 场与降雨相关的洪水之一，降雨量为 > 400 kg·m·s。当考虑保险成本未知的洪水事件时，研究期间过去五年的洪水事件发生频率略有下降。 相比之下，这些事件的保险成本有所增加，这表明当前洪水的影响力越来越大，可能需要改变洪水管理策略以降低成本。