Dynamic equilibrium of sandbar position and height along a low wave energy micro-tidal coast

Highlights

• Equilibrium sandbar height and position are identified along a 15-km stretch of coast in west-central Florida.

• Alongshore variations of equilibrium sandbar height and position are related to wave height distribution along the shoreline.

• Storms introduce perturbations to the equilibrium of sandbar, which are typically restored within 4–6 months after the storm.

• Alongshore variations of onshore and offshore bar migration during a single storm are controlled by pre-storm bar-crest depth.

Abstract

Nearshore sandbars play an essential role in dissipating incident wave energy and protecting the beach landward. Thus, understanding the dynamic equilibrium of nearshore bars is valuable to beach management and shore protection. This study examines the sandbar equilibrium in terms of bar height and cross-shore bar location, in order to assess how the dynamic equilibrium is maintained and influenced by storms along a low wave energy micro-tidal coast. The bar height and bar position were extracted from 51 beach profiles surveyed every two months, spaced at 300 m along a 15-km stretch of beach from October 2010 to August 2014. For the studied coast, alongshore variation in equilibrium bar position measured from the shoreline ranges between 40 and 80 m and equilibrium bar height between 0.20 and 0.70 m. Greater equilibrium sandbar height tends to occur around a headland, where waves are higher. Alongshore variations of bar behavior were observed during storms, with both onshore and offshore bar migration observed during one storm. Water depth over the pre-storm sandbar crest is a major factor controlling the storm-induced onshore or offshore bar migration. On average, the depth over the onshore migrating sandbar is found to be 0.20 m deeper than that over the offshore migrating bar during both summer and winter storms. There is no significant correlation between incident wave angle and sandbar height changes, while significant correlation exists between wave angle and sandbar movement under certain wave conditions, with more oblique waves being associated with further offshore movement of the sandbar. Energetic storm conditions tend to make the bar higher than the equilibrium height, while post-storm adjustment would restore the equilibrium height within 4–6 months. Although the exact values may vary at different locations, the concept of dynamic equilibrium of bar height and distance to shoreline could apply at many locations.

Introduction

The sandbar and trough features are important parts of a nearshore equilibrium profile (Wang and Davis, 1998), and they have important implications on the performance of beach and nearshore nourishments (Kroon et al., 1994, Van Duin et al., 2004, Roberts and Wang, 2012 Brutsché et al., 2014). By inducing wave breaking, the nearshore bars reduce the incident wave energy arriving at the shoreline and therefore provide protection against beach erosion. Due to their control on wave breaking, sandbars influence the spatial distribution of turbulent kinetic energy generated by breaking waves as they propagate onshore (Longo et al., 2002, Cheng and Wang, 2015A). Nearshore water quality can also be influenced by the existence of sandbars (Feng et al., 2013). Thus, understanding and quantifying the sandbar behavior play an important role in coastal management and shore protection. Sandbar morphodynamics remains a challenging research topic due to complicated interaction between breaking waves and sediment transport in the energetic surf zone (Voulgaris. and Collins, 2000, Ruessink and Kuriyama, 2008).

Time-series of beach profile surveys along a significant stretch of coast, e.g., on the order of tens of kilometers, are essential to quantifying the temporal and spatial behavior of beach-sandbar system (Browder and Dean, 2000, Roberts and Wang, 2012). However, long-term field measurements of sandbar configurations (e.g. bar height and bar location) and their response to incident wave conditions are limited to a few locations (Ruggiero et al., 2009). Well known examples include Duck, North Carolina, USA (Holman and Sallenger, 1993, Larson et al., 2000, Plant et al., 2001), Egmond, Netherlands (Ruessink et al., 2000, Pape et al., 2010), Hasaki, Kashima Coast, Japan (Kuriyama et al., 2008), and Gold Coast, Australia (Castelle et al., 2007). Most of these well studied coasts have multiple nearshore sandbars in the surf zone and cyclic cross-shore bar migration occurs. Such a cycle is typically characterized by a net offshore bar migration (NOM) comprising sandbar generation near the shoreline by a storm, followed by a period of offshore bar migration, and eventual bar decay at the seaward edge of the breaker zone (Kuriyama et al., 2008, Ruessink et al., 2000). Field studies on nearshore sandbar behavior along low-energy coasts are scarce and the applicability of findings from multibarred and high-energy coasts along low-energy micro-tidal coasts is not clear. A few existing examples include research conducted at the fetch limited Mediterranean Sea (Guillén and Palanques, 1993, Certain and Barusseau, 2005, Ojeda et al., 2011). General cyclic morphological behavior with interruption from severe storm events was documented along those coasts. Compared to previous studies at high-energy coasts, the morphological response of low-energy micro-tidal coasts is more sensitive to changing wave conditions (Aleman et al., 2015).

Several numerical modeling studies have revealed some insights on the mechanisms of sandbar height change and its cross-shore movement (Almar et al., 2010, Dubarbier et al., 2015, Fernández-Mora et al., 2015, Hoefel and Elgar, 2003, Splinter et al., 2011, Walstra et al., 2012). Both Walstra et al. (2012) and Dubarbier et al. (2015) concluded that more oblique incident waves tend to cause increased offshore sandbar migration under certain wave climates. Regarding the effect of wave angle on bar height changes, Walstra et al. (2012) also found that the oblique incident waves induce bar growth, whereas shore-normal waves tend to cause bar decay. On the other hand, Dubarbier et al. (2015) did not find any significant correlation between wave angle and bar height changes. All the existing beach profile models are site specific and are controlled by a number of calibration parameters that vary from one site to another (Fernández-Mora et al., 2015). These considerable variations in the empirical parameters from one site to another may indicate an attempt of applying constraining parameters to compensate model limitations that primarily arise from simplifications of the physics (Walstra et al., 2012, Dubarbier et al., 2015). Systematic field measurements of sandbar characteristics at a regional scale are therefore crucial to improve our modeling capability.

In this study, beach profiles surveyed every two months along a 15-km stretch of west-central Florida coast over a 4-year period are analyzed to answer the following questions. What are the morphodynamic characteristics of nearshore bars under low wave energy and micro-tidal conditions? More specifically, what is the morphodynamic equilibrium of the sandbar height and cross-shore bar location? How do storm events impact such sandbar equilibrium? The studied coast extends around a broad headland with a shoreline orientation change of 65 degrees. The curved shoreline provides an opportunity to investigate the longshore variations of sandbar responses to incident wave conditions. The sandbar morphodynamics are examined under various incident wave climates including calm summer seasons, relative energetic winter seasons, and tropical storms. Fifty-one beach profiles surveyed every two months along the coast of Sand Key barrier island over a 4-year period from October 2010 to August 2014 are obtained. Data from two of the 4 years, October 2010 to August 2011 and October 2013 to August 2014 are analyzed to investigate equilibrium bar morphology, represented here by sandbar height and cross-shore bar location. Deviations from the sandbar equilibrium state induced by energetic storms, exemplified by a series of strong winter storms in early 2011 and the Tropical Storm (TS) Debby in June 2012, are subsequently studied. The study area is described in Section 2. The methodologies applied in the study are discussed in Section 3. The results are presented in Section 4 and discussed in Section 5, respectively, and conclusions are given in Section 6.