*Article* **Coastal Flooding and Inundation and Inland Flooding due to Downstream Blocking**

#### **Leonard J. Pietrafesa 1,2,\*, Hongyuan Zhang 1, Shaowu Bao 1, Paul T. Gayes 1 and Jason O. Hallstrom 3,4**


Received: 16 May 2019; Accepted: 20 September 2019; Published: 26 September 2019

**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 ye<sup>t</sup> been fully recognized, and the fluid dynamics and the wide spectrum of spatial and temporal scales of these types of events are not ye<sup>t</sup> 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.

**Keywords:** downstream blocking; compound flooding; coastal storm surge and inundation; explosive lateral flooding; hurricane inland and upland flooding
