Tides and dock height on Hilton Head Island

On a Hilton Head creek, the water at the end of your dock can rise and fall by more than the height of a person twice a day. That swing decides how high the walkway sits, how steep the ramp to a floating dock gets, and whether a boat on the lift can reach water at low tide. NOAA publishes the numbers you need for free.

The numbers for Hilton Head

NOAA's tide stations report "datums," long-term averages of the water level computed over a 19-year period. The current datums for these stations use the 1983 to 2001 period.

The terms come from NOAA's definitions. Mean high water (MHW) averages all the high tides. Mean higher high water (MHHW) averages the higher of each day's two highs, and mean lower low water (MLLW) averages the lower of the two lows. Nautical charts measure depth from MLLW.

The two island stations differ by more than a foot. Use the station closest to your lot, and treat it as a guide rather than a measurement of your creek.

Walkway height

South Carolina's dock regulation, R.30-12, says a dock walkway should be elevated at least 3 feet above mean high water. At the Broad Creek station, mean high water sits about 7.7 feet above MLLW, so a walkway that meets the guideline would sit about 10.7 feet above MLLW. At Port Royal Plantation, mean high water sits about 6.3 feet above MLLW, which puts the walkway at about 9.3 feet.

Those figures come from subtracting NOAA's MLLW value from its MHW value at each station. Your surveyor will tie your dock to a datum at your own site. Ask what reference the permit drawings use and where mean high water falls on your shoreline.

Averages leave room for higher water. Spring tides, wind, and storms push water above MHHW. During Hurricane Matthew in October 2016, the Fort Pulaski gauge peaked at 5.05 feet above MHHW, according to the National Hurricane Center's report on the storm.

Storm surge goes far past the datums. The same National Hurricane Center report says a National Weather Service survey found Matthew's surge reached the Harbour Town Golf Course on Hilton Head, more than 500 feet inland from the small harbor. No walkway height keeps a dock above that kind of water, so storm planning is about records and repairs. The storm rebuild guide covers what the state allows after damage.

Low-water access

The same datums tell you what to expect at the other end. MLLW is an average of the daily lower lows, so many of those lows fall below it. If you plan to keep a boat at the dock, measure the depth where the pierhead or lift would go at a low tide, and compare it to your boat's draft with the motor down.

R.30-12 requires a dock to extend to the first navigable creek within your extended property lines, so the walkway length follows from where that creek runs.

Floating docks and ramps

A floating dock rises and falls with the tide, so boarding height stays about the same. The ramp from the fixed pierhead to the float takes up the change. At high tide the ramp sits at its flattest. At low tide it tilts at its steepest, and with a range of 7 or 8 feet, a short ramp gets steep.

A longer ramp makes the low-tide slope gentler. Ask your designer to show the ramp angle at MLLW and at a low spring tide, and to tell you whether the float rests on the bottom at extreme low water. Under R.30-12, the float counts toward the dock's area limit, and the ramp does not.

Questions for your designer

• Which NOAA station or site survey did you use for mean high water and MLLW?
• How high above mean high water does the walkway sit, and how high above the marsh?
• How deep is the water at the pierhead or lift at MLLW, and at a low spring tide?
• How steep is the ramp at MLLW, and does the float rest on the bottom at low water?

What's under the water

Tidal marsh bottoms in Beaufort County are soft. The largest soil map unit in the county by area is the Bohicket association. USDA describes Bohicket soils as very poorly drained and very slowly permeable, formed in marine sediments in tidal marshes and flooded twice daily by sea water, and notes they are too soft for cattle grazing. The Capers soils, the county's next-largest soil unit after open water and Bohicket, are also very deep, very poorly drained tidal marsh soils.

Soft mud affects how piles are placed and how deep they go. Pile depth is an engineering call for each site, and no single number fits every lot. Ask how the designer or contractor decided the pile depth for yours.

For the size limits that go with these rules, read South Carolina's dock size rules. To describe your project, send a request.