Lyddon et al. (2025) Climate Change Likely to Intensify Storm‐Driven Compound Flooding in an Exemplar UK Estuary

Identification

• Journal: Earth s Future
• Year: 2025
• Date: 2025-12-01
• Authors: Charlotte Lyddon, Laura Devitt, Mirko Barada, Gemma Coxon, Jonathan Tinker, Tom Coulthard, Andrew Barkwith, Peter Robins
• DOI: 10.1029/2024ef005822

Research Groups

• Department of Geography and Planning, University of Liverpool, Liverpool, UK
• School of Geographical Sciences, University of Bristol, Bristol, UK
• School of Ocean Sciences, Bangor University, Bangor, UK
• Met Office, Exeter, UK
• Department of Natural Sciences, Manchester Metropolitan University, Manchester, UK
• British Geological Survey, Nottingham, UK

Short Summary

This study uses novel high-resolution, physically consistent climate projections to assess future storm-driven compound flooding in the Dyfi estuary, UK, under a high-emissions scenario (RCP8.5). It finds that by 2080, river discharge extremes will intensify, and compound flood events will become more frequent and concurrent, leading to increased inundation.

Objective

• To assess how the timing and magnitude of extreme storm-driven compound flood events, resulting from the interaction of storm surge and river discharge, will change in the Dyfi estuary, UK, under a high-emissions climate change scenario (RCP8.5), providing insights for future flood management strategies.

Study Configuration

• Spatial Scale: Dyfi Estuary, west Wales, UK (catchment area 470 km²). Climate projections at 7 km (sea level) and 2.2 km (precipitation) resolution. Hydrodynamic model (LISFLOOD-FP) at 20 m grid cell size.
• Temporal Scale: Hourly resolution for climate projections and model simulations. Three 20-year time slices: baseline (1980–2000), near future (2020–2040), and far future (2060–2080). Idealized hydrodynamic simulations for 3-day events.

Methodology and Data

• Models used:
  • Global Climate Model (GCM): HadGEM3-GC3.0 (60 km resolution).
  • Regional Climate Model (RCM): UKCP18 RCM-PPE (12 km resolution).
  • Convection-Permitting Model (CPM): UKCP18 Local (2.2 km resolution for atmospheric processes/precipitation).
  • Regional Ocean Model: NEMO 4.04 (Coastal Ocean version 9, CO9) on Atlantic Margin Model 7 km grid (AMM7) (7 km resolution for marine processes/sea level).
  • Hydrological Model: DECIPHeR.
  • Hydrodynamic Model: LISFLOOD-FP (2D).
• Data sources:
  • Climate Projections: 12 perturbed parameter ensembles (PPEs) from HadGEM3-GC3.0 GCM under RCP8.5 scenario, downscaled to UKCP18 Local (2.2 km precipitation) and NWSPPE (7 km sea level).
  • Observation Tide Gauge Data: National Tidal and Sea Level Facility (NTSLF) Class A tide gauge at Barmouth (15-min resolution, 1991–2021).
  • Observation River Gauge Data: National River Flow Archive (NRFA) at Dyfi Bridge (gauge ID 64001) (15-min resolution, 1970–2021).
  • Observational Rainfall: CEH-GEAR 1 hr (1 km resolution, 1990–2014).
  • Potential Evapotranspiration: CEH McGuinness-Bordne.
  • Sea-Level Rise (SLR) Projections: UKCP18 21st century time-mean sea level projections (for RCP2.6 and RCP8.5).
  • Topographic Data: Digital Elevation Model (DEM) from multiple sources, corrected to Ordnance Datum Newlyn (ODN).

Main Results

• River Discharge: Under the RCP8.5 scenario, mean river discharge is projected to decrease by 27% by the far future (2060-2080), but extreme (75th and 95th percentile) discharges are projected to increase by 24.5% and 30.8% respectively. 1/20-year and 1/50-year river discharge events are projected to increase by 29% and 28% respectively by 2080.
• Skew Surge: Skew surge magnitudes show only marginal increases (approximately 3 cm) by the far future, with 1/20-year and 1/50-year events increasing by 4% and 3.8% respectively.
• Compound Events:
  • The Kendall rank correlation coefficient between peak river discharge and associated skew surge shows a weak but increasing relationship (from 0.10 to 0.16) by the far future.
  • The mean frequency of extreme compound events (both drivers >95th percentile) is projected to increase from 3 to 3.57 per storm season.
  • The average time lag between peak skew surge and peak discharge is projected to decrease by up to 1 hour by the far future, indicating more concurrent events.
  • Extremal dependence (χ values) shows a modest rise in the likelihood of joint extremes in the upper tail (e.g., 0.02 to 0.06 increase for 70th-99th percentiles).
• Flood Inundation: Hydrodynamic modeling projects up to a 38% increase in flood inundation area by the far future (2060-2080) for a 1/50-year event under RCP8.5, relative to the baseline 1/20-year scenario. In the near future, flood extent is primarily driven by event severity, while in the far future, SLR dominates the flood hazard.

Contributions

• First study to utilize multiple ensembles of hourly and sub-mesoscale, physically consistent, hydrological and marine projections to diagnose future changes in compounding flood conditions.
• Provides novel insights into how the timing, magnitude, and dependence of storm-driven compound events will change under climate change.
• Demonstrates a significant advancement in the capacity to estimate future compound flood behavior in estuaries by integrating downscaled marine and fluvial projections into a hydrodynamic estuary impact model.
• Highlights the critical need for appropriately downscaled, coastal, and fluvial projections for future flood management strategies, emphasizing the importance of joint probability and dependence in hazard assessments.

Funding

• Natural Environment Research Council (NE/V004239/1)
• Met Office Hadley Centre Climate Programme (funded by DSIT and Defra)
• Water Informatics: Science and Engineering (WISE) Centre for Doctoral Training (CDT)
• UKRI Future Leaders Fellowship (MR/V022857/1)
• FUTURE-FLOOD (NE/X014134/1)

Citation

@article{Lyddon2025Climate,
  author = {Lyddon, Charlotte and Devitt, Laura and Barada, Mirko and Coxon, Gemma and Tinker, Jonathan and Coulthard, Tom and Barkwith, Andrew and Robins, Peter},
  title = {Climate Change Likely to Intensify Storm‐Driven Compound Flooding in an Exemplar UK Estuary},
  journal = {Earth s Future},
  year = {2025},
  doi = {10.1029/2024ef005822},
  url = {https://doi.org/10.1029/2024ef005822}
}