For those of us living on Tybee Island, we know that the only thing that does not change is change itself. This is particularly true of the weather and the winds that come along with it. These winds accompanied by currents and tides make up the dynamics that constantly change the shape and location of our
island. Most changes are small but some working over an extended period of time can be significant and even disastrous.

Walking the beach on a regular basis, it is easy to see the changes that take place. The cycle of one tide can turn a rough, shell strewn beach into a smooth clean flat one. A windy day can cause scallops in the sand ten feet in diameter and over a foot deep or water filled channels like perpendicular
fingers. In a week's time, three foot scarps, vertical walls of sand, or gullies hundreds of feet long, can form or disappear.

Other changes happen over months. This summer the level of sand at the north groin increased causing much of the rock structure to all but disappear. Wave action this fall caused the south end of the front beach to become much steeper and scarped. The stronger winter waves have since flattened the beach to its normal configuration. In November, it was
impossible to walk from the south tip to the back river beach without using the top of the seawall, in January sand had filled in, completely covering the first two rock groins and almost covering the previously exposed seawall. In places, seven feet of sand depth, several hundred feet wide, along hundreds of feet of beach moved into place.

Some changes are slow to happen and are only possible to detect due to fixed reference points. One obvious example is at the north east point of the island, where the number one green channel marker is more than one hundred feet into the beach at low tide. Another is the decrease in beach width at the intersection of Highway 80 and Butler Avenue. In order to
monitor these changes the Army Corps of Engineers has the beach measured a couple of times each year. From the results of this, plans can be made to make the necessary repairs by re-nourishment under the provisions of the Federal Shore Protection Program.

Why do these changes occur? Like many things in life, the reasons are complex and cause much disagreement among the experts. Engineers have developed mathematical models that are supposed to forecast the changes to islands and beaches, but the many interrelated dynamic systems renders, in most cases, the predictive quality unsatisfactory. Most of the experts
do, however, agree that the major factors behind beach change are the following:

1. Wind and tidal energy
2. Quality and quantity of sand supply
3. Location and shape.

What is the effect of these seemingly logical and simple factors to Tybee Island?

(1) The dynamics of the islands beaches are caused by the wind driven waves, their size and direction and the action of the tides moving in and out four times a day. In a simple world, sand is going to move in the direction that the water is moving. Thus waves hitting the shore straight on will bring sand as they move onto the beach and remove it as the water retreats. Under normal, somewhat calm conditions these forces are in balance. The beach acts as a shock absorber and as a system of equilibrium the beach will take a shape to maintain itself by dissipating the wave energy. A long near flat beach absorbs energy over a large distance while the opposite, a seawall, will have all its energy react in a small area. There is agreement that a seawall will cause the quickest beach erosion and
eventually destroy the beach.

Waves striking the beach on an angle create sand movement in the direction of the motion of the water. Swimmers call this motion in the water an undertow. In the summer time, Tybee's gentle breezes move from south to north while in the winter, stronger storm winds blow in the opposite direction. With the net movement being stronger by winter winds to the south
there is a general flow of sand in that direction. This is true for the entire east coast. Sand from Hilton Head Island should be nourishing Tybee's beaches, while our sand is sliding off to the south to Little Tybee and beyond. Unfortunately, the Savannah Ship Channel at its present depth captures most of our incoming sand. It is thought that there is a net loss of approximately 200,000 cubic yards of sand from our beach each year due to this angular wave action.

The changes in the tides along both ends of the island effects
these shorelines as sand is carried in and out each day. Tybee has the greatest change of tides in the southeast due to the fact that the Savannah River is at the center of a long arc between Florida and North Carolina. Also, as the moon and sun come into alignment, our tide changes grow from about six
feet to over eight feet. These tide changes make alterations to the shorelines at the back river and north west beach as well as help form the shoals at each end of the island. The wind supplies the energy to develop not only the waves that we feel
on Tybee, but the large waves that form ell out to sea and travel many miles to break on our shore line. The increased energy from such waves can change the stability of our beaches as is noted dur ng periods of storms. These
stronger waves have a tendency to flatten out the beach including sand dunes and the ocean bottom beyond the beach. The wind also moves sand along the beaches. Sometimes being on the beach is a painful experience from the sting of wind. This same force is what builds the dune systems that can be the
main line of protection for structures near the shore during major storms.

(2) As mentioned above, sand moves from north to south. A disruption in this movement as Tybee experiences, due to the channel to our north, creates a net loss of sand and therefore, erosion to the beach. To replace this sand, we must dredge and pump sand from other supply sources. In the 1975 nourishment, sand was obtained from just south of the island between Tybee
and Little Tybee. This sand had recently moved off the beach and was very fine. As such, this sand did not stay in place as long as expected. In 2000, sand was pumped from an off shore source which was more course and has remained in place, decreasing the rate of erosion. The quality of the sand is important to its stability upon the beach and the frequency of
renourishment.

(3) The location of Tybee is a help to its stability. The continental shelf is a line of equal depth offshore. Due in part to the curvature of the coast from mid-Florida to North Carolina, the water offshore in our area is shallow for a greater distance than other locations. Therefore this long gentle slope absorbs storm wave energy that would otherwise be detrimental to our
beaches. Large waves are absorbed over a longer distance becoming much smaller and less damaging when they strike the beach. The dynamic stability of a long sloping beach and near shore also means that sand being removed by storm waves travels a smaller distance from the shore and is easily returned
by the normal gentle waves.

The shape of the island also plays into certain dynamic changes. At the beach where Highway 80 meets Butler there is a relatively large change in angle to the shoreline. This change accentuates the angle of waves striking the beach and is a cause for the sand to be carried in both directions as the direction of the wind varies. The overall effect is to erode this
section of the beach more quickly than the straight beach. The width of the island allows the blowing sand to stay in the beach and dune area instead of completely blowing across the island.

Most experts that study historic charts of Tybee agree that prior to the twentieth century the island was very stable. Changes and erosion were experienced during the 1900's that parallel the dredging of the Savannah Channel to depths that impair the sand transport from the north to Tybee's beaches. Although, residents and other experts unanimously agree that
Tybee's shore erosion is caused, at least in part by the channel, there has not been a federally sponsored study to substantiate that conclusion.

The Beach Task Force has recommended, and Tybee has requested, that the federal government through the Army Corps of Engineers make this study. The study has been authorized to be completed in the next couple of years. Should this study prove what we all think, then the federal government will
be responsible for more of the costs to mitigate the damage caused by the channel. More information on this study and its meaning to Tybee will be in the next article on Tybee Beaches.