Coastal Flooding and Inundation and Inland Flooding due to Downstream Blocking

Leonard Pietrafesa,Hongyuan Zhang,Paul Gayes,Shaowu Bao,Jason Hallstrom

doi:10.3390/jmse7100336

Abstract

Extreme atmospheric wind and precipitation events have created extensive multiscale coastal, inland, and upland flooding in United States (U.S.) coastal states over recent decades, some of which takes days to hours to develop, while others can take only several tens of minutes and inundate a large area within a short period of time, thus being laterally explosive. However, their existence has not yet been fully recognized, and the fluid dynamics and the wide spectrum of spatial and temporal scales of these types of events are not yet well understood nor have they been mathematically modeled. If present-day outlooks of more frequent and intense precipitation events in the future are accurate, these coastal, inland and upland flood events, such as those due to Hurricanes Joaquin (2015), Matthew (2016), Harvey (2017) and Irma (2017), will continue to increase in the future. However, the question arises as to whether there has been a well-documented example of this kind of coastal, inland and upland flooding in the past? In addition, if so, are any lessons learned for the future? The short answer is “no”. Fortunately, there are data from a pair of events, several decades ago—Hurricanes Dennis and Floyd in 1999—that we can turn to for guidance in how the nonlinear, multiscale fluid physics of these types of compound hazard events manifested in the past and what they portend for the future. It is of note that fifty-six lives were lost in coastal North Carolina alone from this pair of storms. In this study, the 1999 rapid coastal and inland flooding event attributed to those two consecutive hurricanes is documented and the series of physical processes and their mechanisms are analyzed. A diagnostic assessment using data and numerical models reveals the physical mechanisms of downstream blocking that occurred.

Highlights

Compound hazards are those events that occur simultaneously or successively whose combination and interaction with underlying conditions amplify the hazardous impacts from individual events [1], creating storm surge and seawater inundation
We show below that this set up in the SW corner had the effect of blocking the flows at the mouths of both the Tar-Pamlico and the Neuse Rivers, so water levels had to rise upstream filling the water basins to near capacity
Hurricane Dennis wobbled somewhat off the coast, so at any location the relative wind field changed in intensity with time, while about 280 mm of rain was deposited over the coastal region and the water levels fluctuated

Summary

Introduction

Compound hazards are those events that occur simultaneously or successively whose combination and interaction with underlying conditions amplify the hazardous impacts from individual events [1], creating storm surge and seawater inundation. A higher downstream coastal water level changes the river’s downstream boundary conditions, and affects upstream river flow dynamics and inland freshwater flooding [3,4,5,6,7]. This study documents the events that preceded, were present during, and followed the passages of Dennis and Floyd and offers the possibility of an improved model prediction scheme for inland and upland flooding in coastal states. Hurricane Dennis wobbled somewhat off the coast, so at any location the relative wind field changed in intensity with time, while about 280 mm of rain was deposited over the coastal region and the water levels fluctuated. As Dennis moved across and departed the state on September 06, offshore coastal and upstream river water levels began to return to their prior state

Water Transport through the Inlets

Findings

The Water Level Blocking Effect and Subsequent Flooding