Dr. Fred Speaks

Is any body going to listen?

Adjustment of Beach profile of runnel and bar after 2018 granite revetment (9, 300 feet) near N. end of Jekyll island, GA. Data lacks.

James:
Two very recent pictures on the ground of the granite revetment on the N. end of Jekyll near Dexter Lane were taken by a friend. Both show the new cross section adjustment of the bar and runnel system as the result of the 2018 rock revetment. Apparently it is a single and not a multiple of two or three runnels and bars out beyond low tide. The Holland aerial picture was taken a day or two earlier and depict the same runnel and bar..

A healthy beach profile or cross section generally shows a back dune often with a 20′ tree to indicate stability (one basis of GA 1969 Shore Law, SPA), a fore dune(s), dry sand beach, intertidal beach and then a series of runnels and bars going out below Mean Low Water (MLW). Apparently in the pictures above we have a single runnel and bar seaward with access to a low to mid-tide beach now adjusting its bar/runnel profile following the 2018 rocks.

The James Holland aerial photo shows the same single runnel and bar system as the ground photos. The dark is an old salt marsh. The Island is rolling over as the ocean level rises exposing a ~ several thousand year old salt marsh.

The need for sand is acute in front of the rocks. If the shoreline were nourished with sand it would be very helpful to beach goers and tourism. Some sand would be sacrificial in the form of three rows of dunes that would in some cases be rebuilt by NE winds using zig-zag snow fences. In places with sand renourishment, studies would reveal where holding structures would be beneficial.

Jekyll is down drift of a 50 foot deep, 600 foot wide GA Ports Authority Ship Channel aka SAND SINK. Historically the sand from SSI would by-pass to Jekyll via the outer bar of 12 feet which has been dredged to > 50′ deep by near continuous harbor widening and deepening. The hopper dredge — the ship often seen on the horizon– cuts new material (nascent strata of the Satilla and Hawthorne Formations) from the bottom and sides of the ship channel and then dumps the sand seven (7) miles out to sea. This valuable sand is lost from the sand sharing system, dunes, dry sand and high tide beaches and runnel/ bar systems and intertidal swash bars. [Please note: The Corps, GA Ports, etc., for years has said the dredged sand is polluted — Not from the new strata when deepening and widening.]

Beaches and the economic tourism it supports is diminished and will be largely gone in a very few years hence. The obvious need for a vibrant port is wagging the tourism dog. Large economic benefits of beach sand go unattended. Valuable dredged sand fill is dumped far at sea (in an ‘EPA approved site’) and unavailable for many useful purposes such as overpasses, elevate island causeways and beach renourishment.

Glynn County is sinking due to groundwater withdrawal from the limestone strata of the Ocala or Floridan Aquifer. When the pore water is removed from the limestone geological formations some 600 feet below, without water in the voids, the strata collapses to cause subsidence. For every 100 foot drop in the piezometric surface in the cone of depression the land slowly subsides 1 foot or ~1% according to the late Dr. Phil LaMoreaux https://www.deepdyve.com/lp/springer-journals/obituary-for-dr-philip-e-lamoreaux-V5tD703dOJ

The Glynn sinking of the geological strata is twice sea level rise (SLR) — approaching a total for both of > 4 to 5 mm/year, more than the thickness of two pennies/year. In the life of a 90 year old person it amounts to nearly a foot of rise. For many the encroachment of the sea: ” It never happened.”

The NOS Co-op tide gauge at SSI pier has been discontinued. There is little way to know precisely the changes in sea level datums and elevations of Max storm tide events of Matthew (2016) and Irma (2017).
I looked and do not find the average top elevations of the present revetments on SSI and Jekyll in relation to Mean Sea Level (MSL) or other tidal datums. Original elevation of SSI granite was 12 feet above Mean Low Water.

When the one man granite stones were installed on Jekyll (2018) James Holland’s photos showed there were also shards and cuttings from black gravestones particularly in front of the new condos. These objects (missiles) are apparently not visible now. The small granite stones that the kids picked up this week are interesting also. These small crumbles of granite stones that kids discovered may soon be a hazard to upland property during storm waves.

N & S pictures on the ground of the revetment show mainly granite the size of one man stones—with some slightly larger granite boulders. The one man stones are comparatively light in breaking storm waves. The one man stones maybe transported against upland property. The Johnson Rocks installed on SSI after Hurricane Dora in 1964 are much larger by a factor of 2 or 3. They too are being buried in the sand and are at lower elevations now from their installation. New surveys of the top elevations of the rock revetments need to be carried out. They have performed very well to protect the S. end of SSI and the pier village. The main accreting beaches of Glynn are the southern recurved spit on Jekyll and between Gould’s Inlet and the King & Prince Hotel. The presence of that sandy beach on SSI is due to the severe erosion on the S. end of Sea Island spit. It is reckoned the life span of that East Beach sand is less than a decade or a very few years in the event of a storm event. Breaking and crashing storm waves will mobilize the sand to wash it into the SINK = the ship channel.

The proposed HB 445 in 2019 amendments for a 25 foot setback to the SPA is a hazardous step and retrograde for tourism, beaches, public safety and private property.

The State of Georgia through its Port Authority, the DNR, JIA and the County of Glynn need to bring data to address these continuing policy shortcomings and happenings to the beaches.
Best:
Fred Marland
(Frederick C. Marland, PhD, retired marine scientist)
PO Box 636, Marshwrack