Flooding From the Ground Up: An Analysis of Rain and River Behavior on the North Shore of Kaua‘i

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2023

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University of Hawaii at Manoa

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The windward side of Kauaʻi is prone to heavy rainfall events due to its topographical features and geographical location. Persistent northeasterly trade winds, coupled with steep changes in elevation, create an ideal environment for orographic precipitation on the north shore. In addition, Kaua‘i is located further north than the other main Hawaiian Islands, which makes it more likely to intercept midlatitude features, such as Kona lows, upper-level lows, and cold fronts, that frequently result in high rainfall and river discharge conditions. These cases of high river discharge can redistribute sediment along the rivers in Halele‘a, Kaua‘i, increasing uncertainty in watershed behavior and making flooding events more unpredictable. As such, it is important to update knowledge on the baselines of river gauge information and conduct studies that contextualize the observational data. This work focuses on using data from rain and river gauges in Halele‘a to understand the past and present characteristics of the Hanalei River and the Wainiha River. Three primary studies were conducted using these data: a statistical analysis of heavy rainfall and streamflow patterns, a case study of flooding events in Hanalei, and an analysis of atmospheric disturbances. The statistical analysis calculated the return levels of Halele‘a rainfall and streamflow, then used heavy and very heavy rainfall/streamflow definitions to highlight the seasonal and annual water patterns. The case study used rainfall data and streamflow time series from recent flooding events to understand the roles that antecedent conditions play in river behavior. The final study identified the atmospheric conditions that were responsible for the highest streamflow cases in Halele‘a. The findings that come from this project can improve the predictability of flooding events, shed light on the impacts of different atmospheric disturbances, and help community members to estimate when floodwaters may reach the town centers.

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Atmospheric sciences, Atmospheric Disturbance, Flooding, Rain and rainfall

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