Martin 4-Mile Beach Summary & Conclusions

• With the exception of the "hot spots" along the southerly mile of the Project shoreline and a 1,000-foot length of shoreline at the North Martin County Line, the Martin County Beach Nourishment Project has less remaining "in-place" sand than projected in the original design documents (refer to the Project Permit drawings).  The single most important factor contributing to the higher than predicted sand losses along the island is the modification to the pre-construction profile conditions that resulted from the March 1996 storm.

• This storm (the severe March 1996 northeaster) excavated a massive trench at elevations between -8 and -20 feet NGVD.  Sand eroded from the trench was typically transported offshore the -20 foot contour.   Given the massive erosion that resulted, the post-fill survey severly underestimates the 1996 sand placement volume.
• Comparing the 4-year and construction template beach volumes, the subaerial beach has lost 263,000 cy, the intertidal beach has lost 181,000 cy, and the subaqueous beach has lost 471,000 cy.  The total beach has lost 916,000 cy of sand, which is about 75% of the total quantity of sand placed on the beach during the 1996 nourishment.  Using this historical background erosion rate and not accounting for additional erosion due to dispersion processes (which is not documented in USACE, 1993) the beach nourishment project would be expected to erode about 200,000 cy in the placement area.   In contrast, the project has eroded about 916,000 cy, which is about 4.6 times the historical background erosion rate.  With these sand losses, the beach requires renourishment well before the original 11-year estimate.
• The high sand loss rate can be attributed to (a) the impacts of the March 1996 northeaster and the series of hurricanes in 1999, and (b) the normal dispersion of the beach fill.  Other factors possibly influencing beach fill evolution might be sand quality (the borrow sand was coarser and had higher carbonate content than native sand) and beach compaction (the beach fill was tilled between R-1 and R-14 and left untilled between R-15 and R-25).  Coarse sand is typically expected to perform better than fine sand; however, the size, erosional and depositional characteristics of carbonate sands are possibly distinct from those of silica sand.  The effects of these characteristics on beach fill performance are presently unknown; future beach fill designs should attempt to better quantify their impacts on performance.

• The 2005 monitoring period includes the passage of Hurricanes Frances and Jeanne in September 2004 and the spring 2005 renourishment.
• The 2005 post-project average MHW shoreline positions in the project area lie 61 feet seaward of the 2001 pre-project locations.
• The 2005 project area has received approximately 1.6 million cy since 2001 (from the 2001, 2002, and 2005 projects) of which approximately 400,000 cy (25%) remains in the 2005 project area. From 2001 to 2005 the cross-shore segment from the dune to MHW accreted material while the segment from MHW to -20 ft-NGVD eroded.