The following sections present data evaluating the natural and human-induced dynamics of shoreline and morphologic change on Big Hickory Island in southwest Florida. Near-term changes (1944 through 2012) are analyzed through shoreline change using historic aerial photographs and short-term changes (2012 through 2015) are analyzed through both shoreline and volumetric change using recent topographic and bathymetric field surveys.
3.1. Near-Term Shoreline Change
The long-term evolution of Big Hickory Island suggests that the island has been highly migratory since the late 1800s [30
]. A major change in the overall barrier island morphology occurred between 1885 and 1927, when the island shortened and widened as the inlet to the north (Little Carlos Pass, approximately 1.8 km north of present-day New Pass [41
] substantially migrated south and Big Hickory Pass (to the south) migrated north. In addition to larger-scale drivers of change, such as the global acceleration of sea level rise [42
], the natural hydrodynamic interactions between the barrier and its bounding inlets and the dynamics of the adjacent barrier islands were the primary localized drivers of morphologic change at that time (i.e
., little to no human-induced change).
Evaluation of decadal trends in the near-term evolution of Big Hickory Island reveals an island continuing to exhibit unstable shoreline conditions and large morphologic variations throughout the 1900s and into the 2000s. In 1944, Big Hickory Island was an elongated barrier with a well-developed channel separating the barrier from the vegetated island to the east (landward of the barrier), similar to the general morphologic conditions of 1927. Between 1944 and 1958, overall landward migration of the barrier island, likely through overwash processes associated with the passage of several hurricanes [41
], closed the channel passage along the bayside of the barrier and connected it to the vegetated landmass to the east (Figure 3
). New Pass, bounding Big Hickory Island to the north, had become a well-developed and dominant inlet [41
]. Shoreline recession at the south tip contributed to an overall shortening of the barrier. However, the most substantial change occurred along the northern end of the barrier island, which not only retreated south but also recurved landward closing the northern extent of the 1944 backbarrier channel. By 1958, development had begun on the barrier island to the south (Little Hickory Island). Any unvegetated areas or sediment shoals to the north of the island in 1944 disappeared by 1958. In addition, significant sediment accumulation along the northern tip of Little Hickory Island appears to have occurred during this time. This period denotes the introduction of human-induced changes to the natural barrier island system.
Between 1958 and 1980, continued morphologic change occurred in conjunction with significant anthropogenic activities within the area. Between 1958 and 1965, a coastal causeway was constructed connecting Estero Island (to the north) to Little Hickory Island, influencing the hydrodynamics of several tidal channels within the area [31
]. Northward sediment transport resulted in the closure of Big Hickory Pass, consequentially connecting Big Hickory Island and Little Hickory Island. Despite anthropogenically reopening the inlet in 1976 [30
], by 1980 the inlet was again infilled by northward longshore sediment transport (Figure 4
). Remnants of the dredged 1976 inlet are apparent from the 1980 aerial image. The closure of this inlet allowed for significant quantities of northward transported sediment to naturally supplement the beaches along Big Hickory Island. The apparent northward longshore sediment transport represents a localized reversal in the regional north to south longshore sediment transport patterns [24
]. Figure 4
shows a comparison of a 1970 aerial photograph to the 1980 shoreline illustrating the widening of the beach as a result of the closure of the southern inlet [25
In November 1995, Big Hickory Pass (south channel-side) was stabilized in an open configuration with two terminal rock groins at the north end of Little Hickory Island [43
] to prevent infilling by northward transported sediment. Due to the predominant northward longshore sediment transport, sediment was depleted from the southern portion of Big Hickory Island, resulting in shoreline recession along the southern extent of the island and northward shoreline advance from sediment accumulation along the northern tip of the island (Figure 4
). It is evident that the groin structures along Bonita Beach (south of Big Hickory Island) had a significant impact on the morphology and sediment supply of Big Hickory Island. It also appears that when Big Hickory Pass is open, Big Hickory Island will erode due to a reduced sediment supply.
Following the stabilization of Little Hickory Pass south of Big Hickory Island, sediment supply to the island was diminished with little mechanism for sediment by-passing. As a result, the barrier island began to migrate landward, with shoreline retreat observed between 1996 and 2005 (Figure 5
). During this time, community facilities were permitted and constructed on Big Hickory Island. The trend of shoreline recession continued through 2012, with rapid shoreline retreat along the northern portion of Big Hickory Island (Figure 5
). The south inlet’s northern ebb tidal delta appears to have equilibrated after groin construction and started contributing sediment to the southern portion of Big Hickory Island, evident from the slight shoreline advance along this section of the island. However, overall the island appears to be in a state of severe sediment depletion as evidenced by the erosive trends exhibited leading up to the 2012 morphologic state.
Near-term evaluation of aerial photographs illustrates the dramatic morphologic changes of Big Hickory Island between 1944 and 2012. Natural processes associated with the hydrodynamic fluctuations of nearby inlets and event-driven changes resulting from the passages of storms dominated the morphodynamics of Big Hickory Island until the late 1950s. The 1960 and 1970s represent the temporal shift from natural processes to human-induced changes dominating the barrier island system. Throughout the last two decades, shoreline-stabilization structures updrift (south) of Big Hickory Island (and removal of sediment for nearby beach nourishment projects [25
]) resulted in a significant deficit of sediment input onto the barrier island, causing marked barrier island retrogradation by 2012.
3.2. Short-Term Shoreline Change
Between 2012 and early 2013, the north end of Big Hickory Island continued to retreat landward, as illustrated by the Mean High Water (MHW) change (Figure 6
, red and orange lines). Beach nourishment and groin construction were implemented in mid- to late-2013 in response to the rapid erosion occurring on Big Hickory Island (Figure 6
, yellow line). Nourishment sediment spreading is evident with advancement of the island shoreline to the north during the two years post-construction (Figure 6
, purple line). By late 2015, a new equilibrium shoreline location is emerging along the groin field. Detailed evaluation of the MHW and volumetric changes across Big Hickory Island between 2012 and 2015 provides information on the short-term morphodynamics on Big Hickory Island in response to the most recent anthropogenic influences (nourishment and groin placement), suggesting a trend toward a new barrier island dynamic equilibrium state [44
] that is more consistent with the 2005 state (Figure 5
Tabulated annual shoreline change from construction completion (November 2013) to November 2015 at the FDEP R-monuments is given in Table 1
. The average shoreline change during the first two years after project construction, from November 2013 to November 2015, was a landward movement of 5.4 (± 12.2) meters. The large standard deviation (σ) implies significant alongshore variability. The greatest shoreline change in the project area occurred at R223, which represents roughly the center of the beach nourishment perturbation. Note that no standard R-monuments exist north of the project area in the volatile region adjacent to New Pass. Shoreline change immediately south of the project area was negative (representing landward change or erosion); whereas, change along southern Big Hickory Island, adjacent to Big Hickory Pass was positive or accretional (Figure 6
Shoreline change within the groin field since construction (November 2013 to November 2015) was on average 15.7 (± 4.2) meters landward. This change is visualized in Figure 6
, illustrating that despite the substantial shoreline retreat, the 2015 shoreline position is seaward of the pre-nourishment shoreline position.
When shoreline change is calculated for all wading depth profiles, including BHI-1 and BHI-2, the total shoreline change averaged only 2.8 (± 8.9) meters landward between November 2013 and November 2015. As suggested by the high
the shoreline location moved 47.9 m seaward at BHI-2 during this time period (Table 2
), representing significant spreading of nourished sediment to the north. As expected [45
], the greatest landward shoreline movement occurred in the G5-6 groin cell, which is located close to the center of the beach erosion mitigation project.
MHW shoreline changes for both the R-monument beach profile surveys (Table 1
) and the groin profile surveys (Table 2
) are summarized in Figure 7
. Note that Figure 7
does not represent shoreline position (i.e
., not a planform or a map). Overall shoreline change after construction followed a typical planform spreading signature [45
] of landward shoreline movement in the center of the nourished area and shoreline advancement to the north and south, with considerably more advancement to the north, the direction of longshore sediment transport. Shoreline change stabilized (i.e
., near zero change) in the vicinity of groins 2, 3, 4, and 5 during the second year after construction; whereas, the pattern of spreading continued along the northern project area with spreading to the north.
Based on the shoreline change performance in the vicinity of R223.5 to G4-5, the groins have stabilized shoreline changes two years after project construction. The data suggest that the groins will serve to stabilize shoreline changes north of G4-5 to R222.5 once the nourished sediment is distributed outside of the project area. Provided periodic renourishment, the island should reach a new dynamic equilibrium shoreline position controlled by the groins that is farther seaward than the pre-nourishment shoreline position. Without periodic renourishment, the groin field may have adverse impacts on the downdrift shoreline located to the north of the project area.
3.3. Short-Term Volumetric Change
Tabulated volumetric changes calculated from construction completion (November 2013) to November 2015 are given in Table 3
. The volumetric analysis is limited to the R-monument surveys because they extend to the depth of closure and capture all volume change across the profile. However, these monument surveys are spaced at roughly 150-m alongshore; therefore, high-resolution changes within the groin field are not analyzed in detail in this section. The volumetric change analysis supports the findings in sections 3.1 and 3.2.
A total of 7132 yd3 of sediment volume change (gain) was measured across the Big Hickory Island study area from R222.5 to R225.5 from November 2013 to November 2015. Based on R-monument calculations, the nourished area was erosional, while the area located to the south (from R224 to R225) was accretional. This corresponds to the planform spreading pattern noted in the previous section.
Relatively low shoreline change statistics south of the project area (Figure 7
) and high volume change data in this area suggest that much of the sediment has accumulated below mean high water. The volumetric change data indicate that the groins have considerably reduced post-nourishment sediment volume losses.
Volumetric changes for the R-monument surveys (Table 1
and Table 3
) are summarized in Figure 8
. Volume loss was measured within the project area and accretion was observed to the south. Photographic and field observations, as well as shoreline change measurements, indicate substantial accretion to the north of the project area. As noted above, this is the expected post-nourishment volume change response. The positive volume change statistics suggest good beach erosion mitigation project performance.