A practical method for developing future joint probabilities of riverine and coastal flood risk in complex tidal river systems – a case study

ABSTRACT The effects of sea level rise and extreme rainfall have drastically increased the risk of compound storm surge, tidal, and riverine flooding. This study addresses the complexity of assessing flood probability at a site faced with the complexity of a tidal river discharging to another tidal river, incorporating extreme riverine flows, tidal effects, and storm surges. It uses a copula-based joint probability analysis to assess compound current and future flood risk. It also includes a practical method to explore the significant impacts on future flood elevations of climate and hydrology projections and sea level rise. The urban site in Philadelphia, USA, was previously affected by severe flooding during Hurricane Ida. Utilizing historical data, future projections, and defined flood thresholds, the method yields actionable insights, including probabilistic water elevations under current and future scenarios. A current return period estimated using only a riverine flood model of a 50-year return interval is reduced to only 27 years when considering the effects of compound coastal and riverine flooding. The findings show that increases in current riverine water elevations range from 0.3 to 0.9 m, while sea level rise can add up to 1.2 m of water at the site.