Estuaries are potentially exposed to compound flooding where weather-driven extreme sea levels can occur synchronously with extreme fluvial discharge to amplify the hazard. The likelihood of compound flooding is difficult to determine due to multiple interacting physical processes operating at sub-daily scales, and poor observation records within estuaries with which to determine potential future probabilistic scenarios. We hypothesize that fluvial extremes can occur within the peak of the surge in small/steep catchments because of rapid runoff times, whilst the length-scale in larger/flatter catchments will result in fluvial and marine extremes being out-of-phase. Data (15 min river flow and hourly sea level) spanning 40 years were analyzed to assesses the behaviour and timings of fluvial and sea level extremes in two contrasting estuaries: Humber and Dyfi (United Kingdom). Compound events were common in the Dyfi, a small/steep catchment on Britain’s west coast with fast fluvial response times. Almost half of the 937 skew-surge events (95th-percentile) occurred within a few hours of an extreme fluvial peak, suggesting that flood risk is sensitive to the storm timing relative to high tide—especially since flows persisted above the 95th-percentile typically for less than 12 h. Compound events were more frequent during autumn/winter than spring/summer. For the Humber, a larger/flatter catchment on the east coast with slower fluvial response times, extreme fluvial and skew-surge peaks were less frequent (half as many as the Dyfi) and compound events were less common (15% of events co-occurred). Although flows in the Humber persisted above the 95th-percentile for typically between one and 4 days, hence overlapping several high tides and possibly other surges. Analysis of 56 flooding events across both estuaries revealed: 1) flooding is more common in the Dyfi than Humber; 2) Dyfi flooding is driven by 99th-percentile flows lasting hours and co-occurring with a 95th percentile skew-surge; 3) Humber flooding was driven by 95th-percentile flows lasting days, or surge-driven—but rarely co-occurring. Our results suggest that compound flooding studies require at least hourly data (previous analyses have often used daily-means), especially for smaller systems and considering the potential intensification of rainfall patterns into the future.

<p>Coastal flooding worldwide causes the vast majority of natural disasters; for the UK costing £2.2 billion/year. Fluvial and surge-tide extremes can occur synchronously resulting in combination flooding hazards in estuaries, intensifying the flood risk beyond fluvial-only or surge-only events. Worse, this flood risk has the potential to increase further in the future as the frequency and/or intensity of these drivers change, combined with projected sea-level rise. Yet, the sensitivity of contrasting estuaries to combination and compound flooding hazards at sub-daily scales – now and in the future – is unclear. Here, we investigate the dependence between fluvial and surge interactions at sub-daily scales for contrasting catchment and estuary types (Humber vs. Dyfi, UK), using 50+ years of data: 15-min fluvial flows and hourly sea levels. Additionally, we simulate intra-estuary (<50 m resolution) sensitivities to combination flooding hazards based on: (1) realistic extreme events (worst-on-record); (2) realistic events with shifted timings of the drivers to maximise flooding; and (3) modified drivers representing projected climate change.</p><p>For well-documented flooding events, we show significant correlation between skew surge and peak fluvial flow, for the Dyfi (small catchment and estuary with a fast fluvial response on the west coast of Britain), with a higher dependence during autumn/winter months. In contrast, we show no dependence for the Humber (large catchment and estuary with a slow fluvial response on the east coast of Britain). Cross-correlation results, however, did show correlation with a time lag (~10 hours). For the Dyfi, flood extent was sensitive to the relative timing of the fluvial and surge-tide drivers. In contrast, the relative timing of these drivers did not affect flooding in the Humber. However, extreme fluvial flows in the Humber actually reduced water levels in the outer estuary, compared with a surge-only event. Projected future changes in these drivers by 2100 are likely to increase combination flooding hazards: sea-level rise scenarios predicted substantial and widespread flooding in both estuaries. However, similar increases in storm surge resulted in a greater seawater influx, altering the character of the flooding. Projected changes in fluvial volumes were the weakest driver of estuarine flooding. On the west coast of Britain containing many small/steep catchments, combination flooding hazards from fluvial and surges extremes occurring together is likely. Moreover, high-resolution data and hydrodynamic modelling are necessary to resolve the impact and inform flood mitigation methodology.</p>