Evaluation of storms through the lens of erosion potential along the New Jersey, USA coast

Highlights

• SEI is capable of classifying and ranking major storms that have impacted the New Jersey coast based on erosion potential.

• Approximately 130 unique storms are identified at each of the thirteen shoreline segments in New Jersey between 1980 2013.

• The 34-year climatology is used to identify spatial and temporal patterns in erosion potential for the state.

• The coastline towards the middle of the state experiences more storms and more intense storms than the northernmost sections.

• By month, October has the highest cumulative SEI due to the number of high intensity storms.

Abstract

Coastal erosion is driven by both a storm's erosion potential and by an area's vulnerability. Therefore, the problem of estimating impacts can be approached in two-step. The first includes an assessment of erosion potential based on readily available storm parameters, while the second combines this information with highly localized parameters, describing vulnerability, to more directly predict local impacts. The work presented in this paper focuses on this first step, where a storm erosion potential climatology is developed by analyzing historical storms and is then utilized to identify historic patterns. Specifically, storms which have impacted the New Jersey coast over the past several decades are reevaluated using the Storm Erosion Index (SEI), developed by Miller and Livermont (2008), which considers the three primary storm-related drivers of coastal erosion (wave height, water level, and storm duration). These storms are assessed at thirteen shoreline segments defined along New Jersey's Atlantic coast from a 34 year-period (1980–2013) when concurrent wave and water level data is available. Approximately 130 unique storms are identified with the top three being the December 1992 nor'easter, the November 2009 Veteran's Day Storm, and Hurricane Sandy in October 2012, each having an estimated return period of greater than 15 years. The resulting climatology is found to exhibit several interesting spatial and temporal trends. Both portions of the state as well as months of the year that have historically experienced more storms and/or higher cumulative SEI (over the 34 years) are identified. While analysis of the climatology has also identified periods of reduced storm activity and those of intensified conditions, future monitoring is suggested to assess whether these patterns are persistent or related to climatic variations with cycles longer than can be captured by the thirty years included in this study.

Introduction

While the impact of tropical storms (e.g. Hurricane Ike, Sandy, Michael, etc.) are well documented, the impacts of extratropical storms (i.e. nor'easters), especially the cumulative effects of smaller magnitude, lower return period storms, are less well documented and understood. Exemplifying the potential hazards, four sequential storms in March 2018 cumulatively resulted in 10 fatalities and approximately $3 billion (USD) in damages in southern New Jersey (Aon Benfield, 2018). To mitigate such damages, coastal communities require reliable information about an oncoming storm in order to anticipate its impacts and make preparatory decisions. Beach scraping (McNinch and Wells, 1992) is an example of a practice commonly used by communities to mitigate the effects of oncoming storms; however this preparation diverts resources from other potentially more valuable efforts. Thus, there exists a need to properly estimate a storm's severity in terms of erosion potential prior to its impact.

Traditionally, storm intensity has been quantified by one or more of a storm's properties. For tropical storms this includes meteorological properties such as barometric pressure, maximum wind speed, or storm surge potential. The most well-known example of this in the United States is the Saffir-Simpson Hurricane Wind Scale (SSHWS) which classifies hurricanes on a scale from 1 through 5 based on their 1-min maximum sustained wind speed (Schott et al., 2012). The categories are often used, sometimes inappropriately, to infer potential property damage and flooding due to storm surge. There have been several criticisms of this scale as it relies solely on wind speed and does not consider other parameters such as measured storm surge, waves, or precipitation (Kantha, 2006). Several recent storms that have highlighted shortcomings of the SSHWS include Tropical Storm Debby and Hurricane Sandy in 2012. Neither Tropical Storm Debby nor Hurricane Sandy were classified as a hurricane according to the SSHWS at landfall, yet both caused extreme erosion and significant damage (Blake et al., 2013, Wehof et al., 2014). Over the years, there have been several suggestions to modify the SSHWS scale, including the addition of parameters such as storm size and forward speed (Done et al., 2015, Hebert et al., 2010, Kantha, 2006, Powell and Reinhold, 2007).

Extratropical storms are fundamentally different from tropical storms in terms of how they form and draw their energy (Davis and Dolan, 1993) and therefore cannot be categorized by wind speed the same way tropical storms typically are (Herrington and Miller, 2010). Understanding the intensity of these storms is important however, particularly in the mid-Atlantic region, where they routinely cause significant beach erosion and property damage along the coast. Although these storms are often less intense than their tropical counterparts, they impact this area of the coast more frequently, resulting in extreme cumulative damage. Traditionally, a stage frequency analysis of either water level or wave height has been used to estimate storm severity of extratropical storms. However, this approach, while suitable for predicting or assessing flood potential, does not adequately estimate the erosion potential of a storm, as a combination of parameters are involved in this process.

Beach erosion, which is one of the common impacts associated with both tropical and extratropical storms is driven by a combination of storm and beach state parameters. The three primary storm parameters which include wave conditions, total water level, and storm duration, essentially define the erosion potential of the storm, while the beach state parameters impact how much of that potential is realized. The wave conditions relate to how much energy is available to generate sediment movement and to what direction that sediment will move. The total water level, which includes the tide level and storm surge, determines how high on the beach the water rises and thereby what portion of the beach is subject to wave action. Storm duration describes how long the beach is subjected to intensified wave conditions and elevated water levels. The storm duration is therefore associated with how much erosion is accumulated over the duration of the storm.

The Veteran's Day Storm which impacted New Jersey in November 2009 demonstrates how stage frequency analysis of water levels does not adequately indicate severity in terms of erosion. The Veteran's Day storm resulted in severe beach erosion along much of the New Jersey coast and is considered one of the most damaging storms in New Jersey in recent history (Herrington and Miller, 2010) despite barely exceeding the moderate flood threshold set by the National Weather Service (2.13 m above MLLW at Atlantic City). This can be attributed to the large waves (H_s > 8m at NDBC buoy 44009) generated by the storm that persisted over several tidal cycles. The importance of storm duration has also been acknowledged by Dohner et al. (2016) who found that Hurricane Sandy and Hurricane Joaquin (October 2015) resulted in more erosion at Lewes, Delaware than Winter Storm Jonas (January 2016) despite Jonas having larger wave heights. This was attributed to the relatively short duration of Jonas in comparison to the other two storms.

The aforementioned storms emphasize the difficulty of assessing a storm based on a single parameter. Several more robust indices have been developed which take multiple parameters into account. Dolan and Davis (1992) derived an index specifically for extratropical storms based on maximum wave energy and storm duration. This index places a storm into one of five classes with expected storm impacts (e.g. beach and dune erosion, overwash, and property damage) associated with each class based on the researcher's experience in the mid-Atlantic. Mendoza et al. (2011) modified this scale where storms were classified by wave energy content integrated over the duration of the storm. This modification aims to eliminate possible overestimation in the Dolan and Davis scale which arises by describing the storm by a single wave energy value. Kriebel et al. (1996) also developed an index aimed to classify extratropical storms considering wave height, storm duration, and storm surge which was missing from the previously described index. Although the study's focus was on Delaware, it is noted that the methodology can be applied to other locations. Additional parameters that have been use to represent storm intensity focus on storm surge and tide (Balsillie, 1986, Zhang et al., 2001), cumulative wave energy (Splinter et al., 2014), total horizontal momentum (Basco and Mahmoudpour, 2012, Basco and Walker, 2010), and wave run-up (Kraus and Wise, 1993). A table of several of the parameters used as a measure of storm intensity is provided to serve as a source of comparison (Table 1).

Miller and Livermont (2008) developed the Storm Erosion Index (SEI) which evaluates storms based on their erosion potential and includes the three primary drivers of coastal erosion: wave height, total water level, and storm duration. This index makes no distinction between tropical and extratropical storms. It has been successfully applied to a number of locations including but not limited to the Gulf of Mexico and Atlantic coasts of Florida (Janssen et al., 2019, Miller and Wehof, 2013, Wehof et al., 2014), North and South Carolina (Miller, 2015), New Jersey (Miller and Livermont, 2008), and Spain (Villatoro et al., 2014) and has been shown to be more closely related to observed erosion than more traditional indices (Miller and Livermont, 2008).

The purpose of the research presented here is to reevaluate storms that have impacted the coast of New Jersey over the past several decades in terms of their erosion potential. The intent is to utilize the resulting erosion potential climatology to evaluate historic trends (presented here) and provide input to a more comprehensive coastal storm impacts model (ongoing work) that takes into account localized beach state parameters. To assess the erosion potential of the historical storms, the Storm Erosion Index (SEI), developed by Miller and Livermont (2008) is applied. SEI is chosen because it considers the three primary storm-based drivers of beach erosion (wave height, water level, and duration) and is applicable to both tropical and extratropical storms, both of which impact the New Jersey coast. The following sections review the Storm Erosion Index, describe the development of the climatology, and highlight some of the more interesting trends contained within the climatology.