No significant changes in wave energy affecting Siesta Key found in aftermath of Lido Beach Renourishment Project, Army Corps of Engineers says

First monitoring report provided to FDEP in January

This is an aerial view of Lido Key Beach taken in early December 2020, as the renourishment portion of the project was nearing its end. Image courtesy of Michael Holderness

In its first monitoring report since its contractor completed the Lido Key Hurricane and Storm Damage Reduction Project last year, the U.S. Army Corps of Engineers (USACE) says that its analysis of wave action showed that the effects on Siesta Key were “statistically insignificant” between its pre-construction post-construction surveys.

Leaders of one Siesta Key organization that fought to prevent the dredging of Big Sarasota Pass to renourish Lido Key Beach had referenced documentation that the USACE submitted to the Florida Department of Environmental Protection (FDEP) in March 2015, predicting that the wave energy would increase greatly, causing problems for homeowners on the northern part of Siesta, facing Big Pass.

During the July 2017 meeting of the Siesta Key Association (SKA), Robert Luckner, husband of SKA President Catherine Luckner, predicted the wave energy would increase in a range from 60% to 100%

The USACE’s monitoring report, submitted to FDEP in January, explains, “On average, 50% to 60% of wave height” arriving from the southwest and north-northwest was transmitted to Sarasota Point during both the pre- and post-construction measurements. Those were “the most energetic quadrants,” the report notes.

“An observer at Sarasota Point would have experienced less than a 10% increase in wave energy” in the aftermath of the completion of the Lido project, the report says.

Sarasota Point is “located at the western tip of Siesta Key between monuments R-45 and R-50,” the report adds. The monuments are designations used in referencing specific sites on shorelines; they are not physical markers.

This graphic shows Sarasota Point on the western tip of north Siesta Key. Image from Google Maps

“Engineers collected directional wave data” through use of Acoustic Doppler Current Profilers (ADCPs) “to ensure that wave energy off Sarasota Point had not increased appreciably post-construction,” the report points out.

The National Oceanic and Atmospheric Administration (NOAA), explains, “An acoustic Doppler current profiler, or ADCP, is a device that uses sound waves to measure the speed and direction of currents throughout the water column. Understanding how water in the ocean moves provides important information about biological, chemical, and physical properties of the ocean.”

NOAA adds, “The ADCP uses the Doppler effect by transmitting ‘pings’ of sound using a sequence of consistent rapid pulses that ricochet off particles suspended in moving water and reflect back to the instrument. Particles moving toward the instrument return waves with a higher frequency (or pitch), while particles moving away produce a lower-frequency return. Since the particles move at the same speed as the water that carries them, the difference in frequency between the sound waves the profiler sends out and the sound waves it receives can be used to calculate how fast the particle and the water around it are moving. The system tracks when each of the pings is returned; as pings that travel further (lower) will return later, this translates to current data across a variety of depths.”

The wave energy data were collected in June and July 2019 and again in May and June 2021, the USACE report says.

This graphic shows the locations of two pieces of equipment used to monitor wave energy for the report. Image courtesy U.S. Army Corps of Engineers

Among details of the wave energy analysis, the report also notes that the data measured at one location — at the entrance to Big Pass, directly north of the western tip of Siesta Key — “exhibited an unusual feature with repeated events where the wave period jumped to approximately 10 seconds for several hours. In reviewing the data,” the report adds, analysts found that those events “correlated with strong ebb currents that flowed in the opposite direction from which the waves were traveling. This suggested that as the waves encountered an ebb current, the speed of the waves over the bottom slowed down,” which resulted in an increase in the measured peak period.

Cottrell Contracting Corp.’s dredge Rockbridge removes sand from Borrow Area C in Big Pass on Aug. 8, 2020 as evidenced by the diminishment of the sandbar off Siesta Key. Image courtesy of Michael Holderness

Among general facts in the report, the USACE notes that 683,084 cubic yards “of beach-compatible sediment was hydraulically dredged from two borrow areas” within Big Pass for placement “onto the eroded shoreline [on] Lido Key.” The project area encompassed about 1.4 miles of coastline.

The sand placement began on June 1, 2020 and was completed on April 30, 2021, the report points out. Additionally, two groins were constructed on South Lido in an effort to try to keep the sand in place between subsequent renourishment initiatives, which the USACE has estimated will be necessary approximately every five years.

The first groin on Lido was constructed between Feb. 16, 2021 and March 24, 2021, the report adds. The second was put in place between March 12 and April 29, 2021. “Following construction,” the report continues, “the seaward limit of both groins remained well landward of the [Mean High Water Line] and [both structures] were completely buried … following backfill operations.”

The construction documents the USACE provided to FDEP as it sought to win the necessary permit for the Lido project had explained that the goal was to keep the groins buried.

This is an overhead view of the two groins under construction on Lido Key Beach in late April 2021. Image courtesy Jacksonville District of the U.S. Army Corps of Engineers

‘General stability’ along Siesta’s shoreline documented

Further, the report says that “trends over the year between the pre- and post-construction surveys indicated general stability [along Siesta Key’s shoreline].”

Siesta Key Beach “experienced an average shoreline advance of 15.0 feet and an average volume gain of 127,000 [cubic yards],” which translated into an increase of 6.6 cubic yards per foot, the report notes.

Siesta opponents of the project also had voiced worries that the groins would create significant disruption of the north-to-south downdrift of sand, which is the common direction for sediment to move off the west coast of Florida. They have feared that Siesta’s beaches eventually will begin to erode.

As for details about the surveys of the Siesta Key shoreline: The report focuses on the area between monuments R-45 and R-63. (Those monuments appear to denote the shoreline between Sarasota Point and Point of Rocks.) The mean high water (MHW) position on Siesta remained stable between the pre- and post-construction surveys, the report says.

This graphic shows the survey monuments on Lido Key and Siesta Key. Image courtesy U.S. Army Corps of Engineers

“This was most likely due to natural and man-made hard structures at the northern and southern extents of the Siesta Key segment,” the report continues. “These hard features on either side of the Siesta Key segment acted like headlands fronting a pocket beach, as they had likely kept the sediment in place. The majority of the Siesta Key segment remained stable, although R-49 exhibited the largest shoreline retreat within the segment.”

The report then explains, “The Study of Big Sarasota Pass Sediment Mining Alternatives (USACE, 2014) documented the ebb shoal attachment bar, generally located just offshore between R-47 and T-51, as highly variable.” That factor, the report adds, “likely drove the shoreline retreat at R-49. Aerial imagery from January 2019 and February 2020 suggests that the attachment point of the ebb shoal had migrated from R-49 to R-47. This supported the shoreline advance seen around R-47 and the shoreline retreat documented at R-49. An envelope of variability could be expected as the ebb shoal migrated northwest and southeast offshore of the northern section of Siesta Key.”

On another point, the report notes that surveys of the borrow areas in Big Pass revealed that 154,458 cubic yards of sediment “were eroded” between the pre- and post-construction survey, even though the final volume of sand placed on Lido Beach was 683,084 cubic yards.

“The large difference” between the amount of sand added to the Lido shoreline and the change in volume in the borrow areas, the report says, “was likely a result of rapid infilling of the borrow areas. A combination of waves and tidal currents, which suspend and transport sediments within the [Big Pass] ebb shoal system, likely caused this rapid infilling.”

This Nov. 27, 2020 aerial view of Big Sarasota Pass shows a portion of the waterway where sand was removed for placement on Lido Beach. Image courtesy of Michael Holderness

The report explains that the authorized borrow areas for the project were Areas C, D1 and D2. “Borrow Area C is an approximately 450- to 500-foot-wide channel through the Big Pass Ebb Shoal, connecting the Gulf of Mexico with Sarasota Bay,” the report continues. “USACE identified Borrow Area C as ideal in providing a replenishable source of beach quality sand while also allowing for additional hydraulic connectivity through the Pass to reduce erosional pressure along the northern shoreline of Siesta Key.”

These are major construction milestones, and other details, documented during the Lido Key Hurricane and Storm Damage Reduction Project. Image courtesy U.S. Army Corps of Engineers

Borrow Areas D1 and D2, it adds, encompass an area that is approximately 1,000 feet wide and 4,000 feet long, “along the offshore and northern lobe of the Big Sarasota Pass ebb shoal.”

The report also notes, “FDEP authorized a target dredge depth of -12 feet NAVD88 and a maximum excavation depth of -13.5 feet NAVD88 for all three borrow areas.”

NAVD88 refers to a North American standard for determining elevation.

Cottrell Contracting Corp. of Chesapeake, Va., was the USACE contractor for the Lido initiative. It handled all of the dredging and placement of sand on the beach, the report notes.

The company’s subcontractor, Earth Tech, built the two groins on South Lido, the report adds.

Immediately after the second groin was finished, the report continues, Cottrell and Earth Tech demobilized all of the equipment that had been on the beach, and they “restored the dune vegetation in the vicinity of the south construction access,” which was Ted Sperling Park. That facility is Sarasota County property.

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