Longboat Key Summary & Conclusions

• 3.13 million cubic yards of beach-quality sand over 9.28 miles was placed.
• The shoreline received an average of 66 cubic yards per foot of sand.
• The shoreline was advanced an average of 110 feet.
• The adjacent beaches experienced general erosion between December 1992 and Septemer 1993, although much of this erosion is a result of the "100-year" storm which occurred in mid-March 1993.

• The net change is a loss of 241,350 cubic yards between the north and south limits of beachfill placement which is a loss of 8 percent of 3,129,046 cubic yards, the total amount of sand placed in the nourishment project. The condition of the beaches based on the 6-month post-nourishment surveys shows that the alongshore variability in net sand volume losses was primarily the result of:
  1. the volume losses that occurred during the March 1993 "storm of the century" that resulted in severely eroded beach profiles conditions; and
  2. the unit sand placement volumes.
• Observed beach profile adjustments were greater than expected between Sarasota County monuments R-2 to R-11 as a result of the severely eroded beach profile conditions at the time of nourishment, high wave energy levels following nourishment and unit beachfill sand placement volumes. As a result, the completed nourishment project along these areas experienced severe offshore adjustment and substantial visible beach losses.
• With the exception of the "hot spots" along the center of the Island and a 500 feet length of shoreline at the Islander Club, the Longboat Key beach nourishment project has performed as expected or better than expected.

• On a project-wide basis, volumetric change analyses have shown no significant gains or losses between the 6-month and 1-year surveys, although a longshore redistribution of sand is apparent. The profile adjustments observed between the immediate post-nourishment and 6-month surveys were primarily cross-shore (i.e., formation of the protective bar system). The results of the comparison between 6-month and 1-year surveys indicate the effects of alongshore spreading predominated.
• It is evident that two beach segments experienced significant sand losses and two beach segments experienced significant sand gains. Sand losses occurred at:
  1. the Gulfside Road beach segment, located along approximately 3,500 feet of shoreline between R-48 and R-51, which lost an estimated 30 percent of the initial placement sand quantity and
  2. the Mid-Key beach segment, which lost approximately 15 percent of the initial placement sand quantity.
  In comparison, significant sand gains were observed at:
  1. The north County beach segment which gained approximately 11 percent in volume above the initial placement sand quantity; and
  2. the beach segment located along roughly 16,000 feet of shoreline between R-12.5 and R-29 which gained an estimated 15 percent in volume above the inital placement volume.
• It is clear that the greatest relative changes in sand volumes occurred at the extreme south end of the key where sand moved in a seaward direction.

• By August 1996, of the 3.3 million cubic yards of fill placed within the 1993 project limits, a total of 188,400 cubic yards had eroded from the construction area on Longboat Key.
• In the 36 month time period since the August 1993 construction the beach in the fill area lost 1,372,206 cubic yards of fill on the dry beach and down to the -6 foot contour. At the same time 1,183,822 cubic yards accreted between the -6 foot and -15 foot contours. This indicates that 94% of the fill remains within the active profile of the project area, but the material has experienced a significant offshore adjustment.
• The shoreline since the August 1993 construction has retreated landward an average of 55.9 feet. The average fill width remaining over the length of the fill area is 40.8 feet. Despite these average values, significant reaches of shoreline have receded to the pre-project water line.
• The borrow areas of the New Pass shoal and the Longboat Pass shoal have experienced volumetric changes of +49,100 and +423,800 cubic yards respectively, over the last three years.
• CP&E recommends:
  
  1. The monitoring program should be continued in compliance with the permits, but integrated into the Comprehensive Beach Management Plan and the new interim project requirements.
  2. Hot Spot mitigation is planned for November and December 1996 with a fill project along the island's central reach. Future monitoring will reflect the impacts of this project.

• The Mid-Key project area experienced erosion during the 18-month period following construction. Of the 891,000 cubic yards of sand pumped on the beach, 88%, or 783,000 cubic yards, was found within the project limits.
• The Mid-Key project area shoreline has receded an average of 37.5 feet since 1996/1997 construction. Approximately 14 feet of this recession was due to an offshore adjustment of 10% of the placed sand. This compares well with design allowances of 20% for offshore adjustment. The shoreline was advanced 133 feet seaward by the 1997 Mid-Key Interim Beach Project, currently leaving a net advance of 96.9 feet.
• From August 1997 through August 1998, Longboat Key nourished beaches lost 144,236 cubic yards and receded an average of 8 feet.
• The current width of previously nourished beach in August of 1998 averaged 86.7 feet; 10 feet narrower than they were in January 1993 after the initial nourishment.
• The Longard tube sill has had no significant negative or positive impact on the beach since construction.
• The dry beach adjacent to the New Pass terminal groin extension had receded significantly since the completion of construction. Based on observations, there is no one reason for this recession. Phased remedial action should be considered.
• The section at the north end of the key eroded during the first three years following the 1993 fill project and accreted sand during the subsequent two years. The Comprehensive Plan (1995) called for the construction of three groins in this area. Due to the accretional trend of the last two years, construction of these structures should be deferred.

• Seventy-three percent (73%) of the shoreline remain seaward of the design shoreline established by the 1995 Beach Management Plan.  The remaining shorelines, fall an average of 40.8 feet landward of the design shoreline.
• Eroding beaches were concentrated in two areas:
  1. Beach Section 2, from Beach Walk to Rolling Waves Cottages (R47-R50).
  2. The Mid-Key area between the county line and Sunset Beach (T1-R15).
  Eighty-six percent (86%) of the erosion between August 2001 and April 2003 took place in these areas.
• Between January 1993 and April 2003, 657,300 cubic yards eroded from the island, equal to a rate of 64,200 cy/year.  The loss rate from the eroding profiles was 144,700 cy/year.  Of 4,714,300 cubic yards placed by fill projects since January 1993, 4,056,900 (86%) remain.
• The three Longard tube groins at the Diplomat are mostly buried.  The two groins at the Islander are 85% exposed, but are in good condition.
• The Longard tube sill is now buried.  The sill does not have a noticeable effect on the beach.
• The New Pass terminal groin is completely impounded, allowing sand to deposit along the northern bank of the inlet.
• The 1997 Mid-Key Interim Nourishment Project has performed well, in terms of dry beach width.

• The shoreline of Longboat Key retreated an average of -16.5 feet between April 2003 and October 2004.  The largest retreat of -264.7 feet took place near Seabreeze Avenue (R45).
• During this period approximately -110,300 cubic yards of material was lost within the 1993 renourishment project limits (R46-R29) above the -3' NAVD contour.
• The beaches of Longboat Key lost a net volume of -40,350 cubic yards above the depth of closure between April 2003 and October 2004 surveys.
• Of 4,714,300 cubic yards placed by fill projects since January 1993, 4,016,550 cubic yards (85%) remain.  The adjusted erosion loss between January 1993 and October 2004 was approximately -697,750 cubic yards (-59,000 c.y./year).
• Sixty-one percent (61%) of the shoreline remains seaward of the design shoreline established by the 1995 Beach Management Plan.  The remaining shorelines fall an average of 34.8 feet landward of the design shoreline.
• Sixty-one percent (61%) of the berm (+5' NGVD) remains seaward of the design berm established by the 1995 Beach Management Plan.  For the remaining profiles, the berm is located an average of 52.9 feet landward of the design berm.
• The 2005 renourishment project will require 1,416,200 cubic yards of fill based on the October 2004 survey.  This is an increase of 124,000 cubic yards over that required based on the April 2003 survey.