Compound events · Coastal flooding
Storm surge and heavy rain arrive together far more often than independence assumes
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Short answer
Compound coastal flood drivers co-occur about twenty times more often than independence implies today, and become more frequent along most coastlines under RCP8.5.
What happened
Using ERA-Interim reanalysis for 1980–2014 and six CMIP5 models, the authors fitted copulas to three-day precipitation and meteorological tide (surge plus 0.2 times wave height) along the global coastline, then compared 1970–2004 with 2070–2099 under RCP8.5. The present-day median joint return period was 17 years, roughly twenty times more frequent than the 365 years an independence assumption implies. Under RCP8.5 the drivers co-occur more often along 60% of coasts, the global median return period falls about 20% and events become 26% more probable. North of 40°N frequency averages 2.6 times present.
Why it matters
Coastal defences are designed against each hazard separately, which is equivalent to assuming they are independent. The 17-versus-365-year comparison prices that assumption: it is not a small conservatism but a twentyfold error in how often the damaging combination occurs.
Evidence
- Study type
- Copula-based dependence analysis of reanalysis and CMIP5 projections along the global coastline
- Sample
- ERA-Interim 1980–2014 with six CMIP5 models; ~100 km coastal spacing
- Journal
- Communications Earth & Environment · peer reviewed
- Replication
- Not assessed in this corpus; compound-event dependence is an active and growing literature
- Limitations
- Large river catchments and mean sea-level rise are excluded, only six models are used, and meteorological tide is a proxy that omits full inundation modelling.
What this connects to
Sources
The one study this explanation is built from, by the role each plays. Every source links to PaperFren’s explanation of it and to the original paper.
Primary study
- Compound coastal storms become more common by 2100
Heavy rain and storm-driven high seas already coincide far more than by chance, and that pairing is projected to increase along most coasts.
What it does not showLimitations
The analysis excludes large river catchments and mean sea-level rise. Only six CMIP5 models and ~100 km coastal spacing. Meteorological tide omits full inundation modelling. Dependence change is small compared with marginal precipitation change.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Before
Coastal flood risk was commonly assessed by treating rainfall-driven and surge-driven flooding as separate hazards with separate return periods.
Now
Their dependence is strong now and strengthens under warming across most of the global coastline. The analysis excludes large river catchments and mean sea-level rise, uses six CMIP5 models at roughly 100 km coastal spacing, and stops short of full inundation modelling.