Multivariate statistical modelling of the drivers of compound flood events in south Florida

Robert Jane,Thomas Wahl,Luis Cadavid,Jayantha Obeysekera

doi:10.5194/nhess-20-2681-2020

Abstract

Abstract. Miami-Dade County (south-east Florida) is among the most vulnerable regions to sea level rise in the United States, due to a variety of natural and human factors. The co-occurrence of multiple, often statistically dependent flooding drivers – termed compound events – typically exacerbates impacts compared with their isolated occurrence. Ignoring dependencies between the drivers will potentially lead to underestimation of flood risk and under-design of flood defence structures. In Miami-Dade County water control structures were designed assuming full dependence between rainfall and Ocean-side Water Level (O-sWL), a conservative assumption inducing large safety factors. Here, an analysis of the dependence between the principal flooding drivers over a range of lags at three locations across the county is carried out. A two-dimensional analysis of rainfall and O-sWL showed that the magnitude of the conservative assumption in the original design is highly sensitive to the regional sea level rise projection considered. Finally, the vine copula and Heffernan and Tawn (2004) models are shown to outperform five standard higher-dimensional copulas in capturing the dependence between the principal drivers of compound flooding: rainfall, O-sWL, and groundwater level. The work represents a first step towards the development of a new framework capable of capturing dependencies between different flood drivers that could potentially be incorporated into future Flood Protection Level of Service (FPLOS) assessments for coastal water control structures.

Highlights

Florida is more vulnerable to sea level rise (SLR) in terms of housing and population relative to local mean high-tide levels than any other state in the country (Strauss et al, 2012)
The strength of the correlation of rainfall and Ocean-side Water Level (O-sWL) with groundwater level decreases with distance north across study sites
Applying a lag to the groundwater level will account for its maximum correlation with O-sWL and rainfall at sites S22 and S28

Summary

Introduction

Florida is more vulnerable to sea level rise (SLR) in terms of housing and population relative to local mean high-tide levels than any other state in the country (Strauss et al, 2012). Miami-Dade County, located in the south-east of Florida, is vulnerable due to its gently sloped low-lying topography, densely populated coastal areas, and economic importance (Zhang, 2011). The county’s principal metropolitan area, is consistently ranked among the world’s most exposed and vulnerable cities to coastal flooding While debate surrounds the region’s vertical land motion (Parkinson and Donoghue, 2010), the contribution of SLR to nuisance or tidal flooding (Wdowinski et al, 2016) as well as its role in escalating socio-economic impacts such as climate gentrification is becoming increasingly apparent (Keenan et al, 2018). Higher baseline ocean levels allow storm surges to propagate further inland whilst reducing pressure gradients in rivers hampering efficient drainage; SLR increases the fluvial flood potential (Schedel et al, 2018)

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