Quantifying the Sea Level and Estuary Contributions to Changing High Water Levels in Four Major Australian Estuaries

Abstract Coastal inundation occurs when the total height of mean sea level (MSL), tide, and surge, including subsequent propagation into estuaries, exceeds a given threshold. We developed a nonparametric Joint Probability of Maxima Method and applied to four major Australian estuaries, deriving frequency‐based exceedance thresholds and quantifying the respective contributions of sea level and estuary level components to changing high water (HW) levels. Our approach removes fluvial from coastally driven flooding drivers for determining HW exceedance thresholds at specific average recurrence intervals (ARIs) from joint probability distributions. Distributions of total HW level at estuary entrances are obtained from sea level components: (a) annual MSL, (b) HW tide levels, and (c) skew surges, accounting for seasonal co‐variation. Two estuary level components are introduced: (d) propagation and (e) deviation, representing the influences of estuary morphology and hydrology on HW levels. Changes in the heights of exceedance levels were determined at multiple locations within the Tweed, Hunter, Hawkesbury, and Swan estuaries (Australia) at 1, 5, and 10 years ARIs. Using tide gauge observations alone, we show that while MSL is the dominant driver of increasing HW levels between the two epochs compared (1986–2004 vs. 2005–2023), changes in tides and surges both add and subtract from MSL at the coast, and changes in estuary components (tidal propagation and deviation) can be at least as important as MSL. This approach provides an efficient way to quantify specific and localized changes in the drivers of coastally driven flooding in these estuaries.