Since modern barrier island morphology is the product of past and present coastal processes acting over preexisting morphologies, effective barrier island management requires a comprehensive knowledge of how an island has evolved to its current state in order to understand how it may change in the future. Continued sea level rise and future climatic uncertainty represent significant concerns about the resiliency of barrier islands and threats to many coastal communities (U.S. Environmental Protection Agency, 2016). Barrier island geomorphology can exhibit considerable variability alongshore, leading to varying responses to storm activity that ultimately determine the response of the island to sea level rise. Understanding the source of variability in beach and dune morphology can provide insight into how the barrier island is likely to change in response to future storms and sea level rise.

Storm waves interact with the variable morphology of the nearshore, beach, and dunes to determine how vulnerability varies along a barrier island. To some degree, variations in the nearshore, beach, and dune morphology are influenced by the framework geology (Hapke et al., 2010, 2016; Houser et al., 2008, 2018a; Houser, 2012; Riggs et al., 1995). In this paper, the term “framework geology” is defined as any subsurface variation in geologic structure, where variability in geologic structure can result from variations in sediment type (i.e., sand vs. silt), differences in compaction, or significant changes in the subsurface organic content or mineralogy. This term encompasses the subsurface and bathymetric geologic structure (onshore and offshore), which may include rhythmic bar and swale structures (Houser and Mathew, 2011; Houser, 2012), shoreface attached sand ridges (SASRs) overlying offshore glacial outwash headlands (Hapke et al., 2010; Schwab et al., 2013), or buried infilled paleo-channels (Anderson et al., 2016; Browder and McNinch, 2006; Fisk, 1959; McNinch, 2004; Schupp et al., 2006; Simms et al., 2010). Since the framework geology can provide insight into historical patterns of island transgression (Hapke et al., 2016; Houser, 2012; Houser et al., 2015; Lentz et al., 2013), it is vital to better understand how the framework geology influences variability in modern beach and dune morphology (Cooper et al., 2018). Despite its importance, framework geology remains absent from contemporary barrier island change models that treat the geology as being uniform alongshore (Goldstein and Moore, 2016; Goldstein et al., 2017; Gutierrez et al., 2015; Moore et al., 2010; Murray et al., 2015; Plant and Stockdon, 2012; Wilson et al., 2015). Sections of a barrier island that experience greater wash-over will experience a net loss of sediment landward and localized erosion, but the dissipative nature of shoreline change (see Lazarus et al., 2011) means that those losses are distributed alongshore. In this respect, the variation in beach and dune morphology alongshore forced by the framework geology can influence the rate of historical shoreline retreat and island transgression and needs to be considered in models of barrier island response to sea level rise.

The influence of framework geology on barrier island morphology is well documented by work along the New York, Florida, and North Carolina coasts. Submerged glacial outwash headlands along Fire Island, NY, are reflected in the nearshore bathymetry as a series of shore-oblique ridges and swales (Hapke et al., 2010; Schwab et al., 2013). The nearshore bathymetry impacts sediment transport gradients along the island, which has implications for beach and dune response and recovery following a storm (Brenner et al., 2018). Using sediment cores in conjunction with ground-penetrating radar (GPR) and seismic surveys, Houser (2012) demonstrated that variations in shoreline change patterns, beach width, and dune height corresponded to ridges and swales at Pensacola, FL. Shoreline position was more stable along the ridges, resulting in a wider beach which provided more sediment for onshore winds to create higher and more persistent dunes (Houser, 2012). Paleo-channels dissecting the southeastern US Atlantic coast also align with hotspots of shoreline change (Lazarus et al., 2011; Schupp et al., 2006). However, Lazarus et al. (2011; p.1) argued that “shoreline change at small spatial scales (less than kilometers) does not represent a peak in the shoreline change signal and that [shoreline] change at larger spatial scales dominates the [shoreline change] signal”. This implies that variations in the framework geology, such as paleo-channels, do not influence long-term shoreline change, but, as noted, shoreline change is influenced by the alongshore variation in beach and dune morphology. The dissipative behavior of shoreline change does not negate the importance of framework geology. While alongshore variation in dune morphology is also influenced by the distribution of vegetation in both space and time (Goldstein et al., 2017; Lazarus et al., 2011; Lazarus, 2016), the self-organized behavior of the dune morphology is ultimately set up by the framework geology (see Houser, 2012; Stallins and Parker, 2003; Weymer et al., 2015b).

The purpose of this paper is to test the hypothesis that relict infilled paleo-channels in the framework geology of a barrier island play a significant role and have an asymmetric influence on the alongshore variation in beach and dune morphology at a range of alongshore length scales. Based on the combination of a variable framework geology and a dominant alongshore current, it is feasible that the framework geology may influence barrier island geomorphology at discrete spatial scales and that this influence may be asymmetric. Central to this hypothesis is the idea that the modern island morphology itself is scale-dependent, which has been proposed and supported by previous studies (Houser, 2012; Houser et al., 2015; Lazarus et al., 2011; Lazarus and Armstrong, 2015; Lazarus, 2016). Padre Island National Seashore (PAIS) on North Padre Island, Texas, represents an ideal location to test this hypothesis because previous studies have documented significant variability in the subsurface framework geology (Fig. 1; Anderson et al., 2016; Fisk, 1959; Wernette et al., 2018; Weymer et al., 2018; Weymer, 2012, 2016), and there is substantial alongshore variation in beach and dune morphology. Given that the dominant current along the central Texas coast flowed from north to south during the Holocene (Sionneau et al., 2008), it follows that the dominant alongshore sediment transport gradient during that time also flowed from north to south. It is feasible that paleo-channels along PAIS would have had interacted with the southerly alongshore current and sediment transport to asymmetrically influence barrier island geomorphology during island transgression. In this scenario, areas up-drift of a paleo-channel would be distinctly different from areas down-drift of the paleo-channel because the channel acts as a unidirectional sediment sink in the coastal sediment budget during island development.

Subsurface apparent conductivity exhibits substantial LRD along the entire length of PAIS (Fig. 4a). Patterns in the subsurface framework geology LRD plot demonstrate that the framework geology is self-similar at broader scales and that this structure varies alongshore at finer alongshore length scales which correspond to the scale of the previously identified paleo-channels. The large LRD values at broad spatial scales (Fig. 4a) demonstrate that the paleo-topographic structure dominated by broad-scale coastal curvature over very broad spatial scales. Since the framework geology reflects the paleo-topography and the modern barrier island surface is dissipative at very broad scales, based on large LRD values at broad scales in the modern barrier island morphology, it follows that the framework geology is dissipative. The substantial LRD along much of the island supports previous work by Weymer (2016) and Weymer et al. (2018), which demonstrated that subsurface framework geology exhibits LRD at discrete locations and alongshore length scales.

The shoreline change LRD plot exhibits the greatest LRD values (i.e., highest LRD values across all broad spatial scales) along the length of PAIS, as indicated by the dominance of red hues in Fig. 4b. Most flares present in the shoreline change LRD are at relatively fine spatial scales, shorter than a few kilometers. Peaks in the shoreline change LRD plot are very narrow, which we interpret to mean that the long-term shoreline change is dominantly dissipative with only minor undulations due to localized coastal processes, consistent with the findings of Lazarus et al. (2011), who demonstrated that broad-scale and long-term shoreline change is dissipative. Waves impacting the coast can erode sediment from one area and transport it to another area, resulting in undulations in the shoreline orientation. Since long-term shoreline change is the result of cumulative daily wave processes eroding undulations in the shoreline shape and dissipating any short-term undulations, fine-scale variations in the nearshore bathymetry, such as nearshore bars and troughs, can affect patterns of erosion and deposition along the coast over longer periods of time (Hapke et al., 2016). Therefore, it follows that the long-term shoreline change LRD plot would exhibit a large amount of LRD.

Beach width LRD is more variable than shoreline change (Fig. 4c), with the least amount of variability concentrated in approximately the southern third of the island. These flares are likely present because transverse ridges in the nearshore bathymetry affect localized wave refraction patterns, thereby influencing fine-scale patterns in beach morphology. Patterns in the beach morphology in southern PAIS are likely more localized because the incoming wave energy is refracted around the transverse ridges, which impacts sediment transport gradients along this part of the island. Any variations in beach morphology are more locally influenced by relatively closely spaced transverse ridges (∼0.8 to 1.5 km alongshore spacing), resulting in broad-scale LRD along southern PAIS.

The central third of PAIS beach width is characterized by several significant flares in LRD, with many of the strongest flares adjacent to infilled paleo-channels previously identified by Fisk (1959) (Figs. 4c and 5a). The scale at which LRD transitions to SRD is at the broadest alongshore length scales proximal to Baffin Bay, and this threshold decreases in scale to the north (Figs. 4c and 6a). Given a dominant southerly alongshore current during island development in the Holocene (Anderson et al., 2016; Sionneau et al., 2008) and corresponding southerly sediment transport gradient, patterns in the beach morphology LRD plot suggest that the paleo-channels are asymmetrically influencing beach morphology. Simms et al. (2010) presented seismic profiles extending from north to south across the ancestral LOSP Creeks, which exhibit a series of onlapping reflectors on the northern edge of the seismic profiles. These onlapping reflectors are indicative of deposition on the northern edge of the paleo-channel and support the hypothesis that alongshore spit development occurred within the LOSP Creeks paleo-channel. The beach north of the large paleo-channel identified by Fisk (1959) would have been nourished by sediment discharged from the ancestral LOSP Creeks, now forming Baffin Bay. Similarly, the beach north of the ancestral LOSP Creeks paleo-channel may have been nourished by sediment from the ancestral Nueces River. In this way, beach morphology up-drift of the large paleo-channels would impact beach morphology within and south of the large paleo-channels.

Alongshore LRD in the dune crest elevation and dune height varies similarly to beach width LRD along PAIS (Figs. 4e, 4f, 5b, 5c, 6b, and 6c). The southern third of PAIS is characterized by LRD–SRD transitioning at finer alongshore length scales than the northern two-thirds of the island, as indicated by the flares in the dune height LRD plot (Fig. 4e and f). The most significant flares are proximal to the ancestral LOSP Creeks paleo-channels dissecting central PAIS and the ancestral Nueces River paleo-channel extending into Baffin Bay (Fig. 6). Given that the dominant alongshore sediment transport gradient is from north to south and that the beach morphology exhibits an asymmetric LRD to the north of the large paleo-channels, it follows that LRD and SRD patterns in dune morphology would exhibit similar asymmetry to beach morphology.

The transition from dune height LRD to SRD occurs at the largest scale, i.e., approximately at 35 km alongshore length scales (Figs. 4f and 6c). This maximum occurs at the southern edge of the ancestral LOSP Creeks paleo-channel, adjacent to Baffin Bay (Fig. 6c). The alongshore length scale can be interpreted as the alongshore distance that the paleo-channel affected wave refraction patterns and sediment distribution along the beach, ultimately affecting sediment supply to develop larger dunes. It follows that paleo-channel influence on dune crest elevation and dune height would be asymmetric, with greater LRD to the north of the paleo-channels, assuming paleo-channels inhibited southern alongshore sediment transport and starved the beach down-drift. The wide beach up-drift of a paleo-channel represents a larger sediment supply and greater fetch for aeolian transport and dune growth and is consistent with peaks in dune height identified by Wernette et al. (2018).

Island width exhibits the greatest alongshore variability in LRD of all island and framework geology morphometrics (Fig. 4g) and demonstrates that island width is dependent on broad- and fine-scale patterns of change. Areas of low dunes are likely to be overtopped during a storm, transporting sediment to the landward margin of the island. Waves and currents along the landward margin of the island erode the wash-over fans and redistribute sediment along the island. In this sense, the island width at one location is directly influenced by sedimentation patterns along the adjacent parts of the island. Undulations in the Gulf of Mexico shoreline are smoothed out over the long term, thereby reducing the likelihood that patterns in island width are solely caused by shoreline change patterns. This repeat wash-over, followed by sediment redistribution along the back-barrier shoreline, represents the mechanism by which barrier islands can transgress landward and keep up with sea level rise.

Bathymetric depth profiles at 2 and 4 km offshore exhibit substantial LRD at broad scales, but this breaks down at scales finer than ∼15 km alongshore (Fig. 4h and i). Long-range dependence breaks down at larger alongshore length scales in the 2 km bathymetry, compared to the 4 km bathymetry. Since modern coastal processes continue to affect alongshore sediment transport, large undulations in the bathymetry are smoothed out over time by sediment redistributed along the coast. Finer-scale variations in the modern nearshore bathymetry occur at similar spatial scales as previously identified at PAIS (Wernette et al., 2018). The 2 km bathymetric profile LRD breaks down at broader spatial scales than the 4 km bathymetry (Fig. 4h and i). This suggests that localized variations in coastal processes manifest themselves in the nearshore bathymetry closer to the shoreline. Wave shoaling and breaking will erode and deposit sediment along the coast, impacting bathymetric structures closer to the shoreline.

As noted, flares in the LRD plots are interpreted as areas where the morphometrics are more locally dependent on the adjacent values. Since flares in the LRD plots of surface morphometrics are most pronounced adjacent to the infilled paleo-channels and decrease to the north (Figs. 4, 5, and 6), this spatial correlation supports the hypothesis that the modern barrier island morphology was influenced by variations in the framework geology. Paleo-channels along PAIS range in scale, with the smallest channels only ∼13 m below the modern surface and the deepest and widest channels ∼50 to ∼64 m deep. Regardless of the paleo-channel dimensions, patterns in the LRD plots demonstrate that paleo-channels affect the nearshore bathymetry and modern island morphometrics asymmetrically and decrease in minimum alongshore scale to the north. Beach and dune morphology up-drift of a paleo-channel directly affects sediment available for areas of the beach down-drift. In this way, larger paleo-channels (depth and width) will have a greater accommodation space and influence beach–dune morphology along a greater stretch of coast, while smaller paleo-channels have a more limited accommodation space and, therefore, influence a smaller stretch of adjacent coastal morphology. Given that a paleo-channel would have acted as a sediment sink for excess sediment transported alongshore during sea level transgression, it follows that LRD values would remain high at fine spatial scales up-drift of the paleo-channel locations (Figs. 5 and 6).

The current paper is in agreement with previous research that demonstrates barrier island morphology is dissipative at broad spatial scales (Wernette et al., 2018; Lazarus et al., 2011). Long-range dependence is significant at very broad spatial scales in all island morphometrics except for island width. Previous research also demonstrates that rhythmic undulations and isolated paleo-channels can influence short-term shoreline change patterns (Lazarus et al., 2011; McNinch, 2004; Schupp et al., 2006) and beach and dune morphology (Houser et al., 2008; Houser and Barrett, 2010). This paper presents new information supporting the hypothesis that paleo-channels in the framework geology interact with alongshore currents to drive asymmetries in barrier island geomorphology and that the scale of influence is ultimately limited. This asymmetry is likely caused by paleo-channels acting as sediment sinks for sediment transported south by a prevailing southerly alongshore current during barrier island formation.

The alongshore distance that variations in the framework geology influence beach and dune morphology is dependent on paleo-channel scale and orientation, relative to the average shoreline orientation. Long-range dependence plots of beach and dune morphometrics suggest that beach and dune morphology within the largest paleo-channel dissecting the island, the ancestral LOSP Creeks, was influenced by beach and dune morphology up to 25 km north of the channel edge (Figs. 4c–f, 5, and 6). The large paleo-channel identified by Fisk (1959) is slightly smaller in scale than the paleo-channel forming Baffin Bay; however, the large Fisk (1959) channel intersects the coast at an oblique angle. Since the channel dissects PAIS at an oblique angle, the influence of this channel is more apparent on beach morphology than dune morphology. An oblique channel would have required more sediment and take longer to fill than a shore-normal channel. Subsequently, a wide beach and dunes would begin to form in the shore-normal paleo-channel before the oblique paleo-channel. For an oblique paleo-channel the volume of sediment required to fill the channel from alongshore sediment transport and fluvial deposition from the mainland would likely have been insufficient to build a wide beach to supply sediment for significant dune growth.

Paleo-currents during the Holocene were predominantly from north to south (Sionneau et al., 2008), which would have set up a southerly alongshore sediment transport gradient. Sediment transported from north to south along the coast would have nourished beaches up-drift (i.e., north) of the channel. Consequently, nourished beaches up-drift of the paleo-channel had a greater sediment supply and increased fetch for aeolian transport inland to promote large dune development (Bauer and Davidson-Arnott, 2002; Bauer et al., 2009). While beach nourishment and dune growth continued up-drift of the channel, excess sediment entering the channel was deposited along the up-drift edge of the channel (Fig. 7). Deposition on the up-drift edge was caused by the increased accommodation space within the channel. Increasing the area that the alongshore current flows through (i.e., transitioning from a confined alongshore current to an open channel), while maintaining the alongshore current discharge, resulted in a decreased flow along the northern edge. Reducing alongshore current velocity caused sands to be deposited along the northern edge of the channel (Fig. 7), while finer particles are transported farther into the channel and funneled offshore through the channel outlet. Given enough time and with continued sea level rise during the Holocene, this preferential deposition would have built a spit into the channel. Sediment trapped in the paleo-channel would be unavailable to the beach down-drift. The closest modern analogy to this alongshore sedimentation process is the formation and evolution of an alongshore spit eventually completely crossing the outflowing river channel, where the river is eventually cut off by the elongating spit. In this case, sediment is supplied to the up-drift beach and provides a sediment source for dunes to form.

Directional dependencies in beach and dune morphology, initially set up by the interaction of framework geology with a dominant southerly alongshore current, persist through time due to preferential wash-over reinforcing preexisting alongshore variation in dune height. Areas of the island with limited or no dune development are preferentially overtopped by elevated water levels during a storm. Conversely, areas with higher dunes resist storm wash-over/inundation and recover more rapidly following a storm. Alongshore variations in the barrier island morphometrics, such as dune height, persist through time because these patterns are reinforced by episodic wash-over of small dunes during storms.

The apparent disconnect between long-term shoreline change and framework geology is due to the cumulative influence of waves continuously interacting with the coast. This disconnect is further highlighted by the lack of storms impacting PAIS. Long-term shoreline change rate is the cumulative result of waves moving sediment on a daily basis, while short-term variations in shoreline position caused by storms are feasible. It is unlikely that short-term variations in PAIS shoreline position identified here are caused by storms because PAIS has not been significantly impacted by a storm since Hurricane Bret in 1999. Any short-term undulations in shoreline position are likely to disappear over longer timescales, especially since no storm has hit the island to cause significant localized shoreline erosion. Therefore, the long-term shoreline change rate LRD (Fig. 4b) is unlikely to exhibit substantial variation alongshore. Beach, dune, and island morphology do show significant variation in patterns of LRD along PAIS (Figs.  4c, 4d, 4e, 4f, 4g, 5, and 6) because the initial barrier island morphology was set up by the framework geology. If hurricanes had impacted PAIS more frequently, it is likely that the alongshore variations in dune morphology, which were initially set up by the paleo-channels, would have been reinforced. This is because areas set up as low dunes would be preferentially over-washed while areas of high dunes would be more resistant and resilient during and following a storm. Therefore, the impact of a hurricane would highlight alongshore variations in dune morphology set up by the paleo-channels. Predicting future changes to barrier island geomorphology requires a comprehensive knowledge of how the framework geology affected initial variation in the beach and dunes.

Understanding how the framework geology influences barrier island geomorphology has important implications for understanding how barrier islands are likely to respond to and recover following a storm or series of storms. While many models of barrier island recovery focus on spatiotemporal models of change, Parmentier et al. (2017) demonstrated that spatial autocorrelation outperformed temporal autocorrelation (e.g., “space beats time”, SBT) when predicting the recovery of vegetation following Hurricane Dean. Since vegetation recovery and dune geomorphic recovery are related (Houser et al., 2015), it follows that understanding spatial autocorrelation in beach and dune features is essential to predicting future changes to barrier island geomorphology. The current paper supports the conclusions of Parmentier et al. (2017) by demonstrating that spatial variations in the framework geology directly relate to alongshore variations in beach and dune morphology (Figs. 5 and 6). In the context of SBT theory, results of the current paper support the hypothesis that spatial variations in the framework geology (i.e., “space”) control barrier island evolution (i.e., “time”). Accurately predicting future barrier island change is predicated on comprehensively understanding what processes influenced its initial formation and what processes continue to influence island morphology. Predicting coastal change without accounting for all factors affecting formation and evolution, such as directional dependencies due to framework geology, is more prone to uncertainty, which can have important managerial applications.

Given that framework geology influences beach and dune morphology along the coast, the methods and results of this paper represent an opportunity for managers to improve coastal engineering projects, such as beach nourishment. Sediment budget imbalances set up by the framework geology dictate the long-term barrier island trajectory. Utilizing ARFIMA models to evaluate the alongshore beach and dune morphology can provide valuable insight into how the coast is likely to change naturally in the future. To reduce waste by coastal nourishment, future projects should seek to first comprehensively understand how the paleo-topography of an area continues to affect coastal processes and morphology. By understanding the long-term influence of framework geology, coastal nourishment projects can more effectively balance how a project focuses on the near-future coastal morphology with long-term natural changes.

This paper quantitatively demonstrates that variation in the framework geology influences patterns of beach and dune morphology along a barrier island. Understanding what controls beach and dune morphology and barrier island development is integral to predicting future changes to barrier island geomorphology and island transgression caused by storms and sea level rise. Storm impact and barrier island transgression patterns are controlled by beach slope, dune height, and wave run-up. Given a persistent alongshore sediment gradient during the Holocene, paleo-channels in the framework geology at PAIS likely acted as sediment sinks during island development. While wide beaches and, subsequently, large dunes are nourished with sediment up-drift of the channel, excess sediment can become trapped in the channel. These channels trap sediment, starving sediment from down-drift portions of the coast. The result of this asymmetry in sediment supply is that large dunes occur up-drift of the paleo-channel and small dunes occur down-drift of the paleo-channel. Effectively managing a barrier island underlain by a variable framework geology should seek to balance short-term societal pressures in the context of long-term natural change (i.e., framework geology).