High tide flooding in near future projections for popular travel destinations

Abstract: High tide flooding (HTF) is becoming more common in coastal regions of the United States, although this phenomenon remains largely undocumented in published literature from other parts of the world. No matter the emissions scenario over the next decades, sea level rise (SLR) is projected to continue worldwide until at least the end of the 21st century, continually pushing the frequency and severity of HTF. Despite the expectation of continued SLR long-term and the resulting implications on future HTF, there is a lack of continuous near future, high granularity projections of HTF useful to decision-makers, urban planners, residents and travelers to cities with high coastal flooding risks. Accordingly, this thesis presents a novel HTF projection framework and early warning system specialized to model localized HTF frequency into the near future (over the next ~19-year nodal cycle), designed with the specific and unique ability to identify years and months/seasons of the greatest predicted HTF frequencies. The capabilities of the HTF projection framework presented in this thesis were shown by conducting monthly granularity predictions of minor and moderate HTF and presenting seasonal event patterns among the world’s top ten popular travel destinations projected to be at the most risk from potential SLR. The data input was drawn exclusively from higher frequency tide gauge records. Novel features of the projection framework included the characterization of intra-annual sea level change due to earlier melting periods of sea and land ice, less multi-year ice, delayed freeze-up in the winter, and cascading effects on thermohaline circulation. Another novel method implemented took into account the correlation of astronomical tides with intra-annual sea level variability. Additionally, the methods implemented considered the combined effects of seasonal sea level change (SLC) and modulations by precessions in the Moon’s orbital path around the Earth affecting the heights of the highest astronomical tides, while a representation of seasonal extremes in mean sea levels in a second projection of SLC was also included to quantify the upper bounds of projected HTF frequencies. Deterministic seasonal predictions of surge, wave setup, and swash were also implemented. Large increases in the frequency of HTF were projected during the 2020’s in Bangkok and Manila. The following decade, New York City and Jacksonville were projected to suffer increasing HTF frequencies as well. Furthermore, HTF events were projected to be concentrated during particular month(s) or season(s). In Bangkok, 89% of HTF days were projected to occur between October and February. In Manila, HTF frequency was projected to be nearly four times higher between June and August than during any other month. HTF in Jacksonville was projected to be clustered between October and December, while HTF in New York City was projected to be most common in October and December. Furthermore, surprising differences in intra-annual SLC trends were found mainly in Europe, while effects of lunar precession on high tide levels and thus HTF frequency were found to be widespread; especially noticeable in Bangkok, Cardiff, and Manila. Results were discussed in the context of published existing and planned coastal flooding defense adaptation and mitigation strategies and infrastructure. Results gave insight into years and months of the greatest HTF frequencies in major metropolises over the near future: HTF days in Bangkok were projected to exceed eighty every year between 2025 and 2028, and again in 2036, before reaching one hundred in 2037. December through February were projected to be the most affected months with the majority of days in December experiencing at least minor HTF. Bangkok was found to be the most vulnerable city to HTF in the near future due to unprecedented increases in HTF frequency combined with unfinished flood barriers, drainage capacity already being consistently overwhelmed by HTF, as well as exponential urban growth and unenforced planning laws. Manila was also projected to suffer unprecedented increases in HTF frequencies during the 2020’s, peaking in 2027 and 2028. Summer months were projected to bring the highest frequency in HTF, with up to 10 HTF days per month. Recent local initiatives such as the Manila Bay Sustainable Development Masterplan and the North Manila Bay Flood Protection Strategy were found to be positioned to relocate those living in flooding-prone areas while improving the city’s resilience to HTF. In light of the results found here, this dissertation includes a discussion of further potential in diversifying methods related to the prediction of seasonal surge, wave setup, and swash in the face of existing climatic shifts and changes in thermohaline circulation.