The highest water predicted for downtown St. Augustine this week arrives at 8:58 on Saturday morning, at 5.605 feet above mean lower low water on the city dock prediction table. That is the highest of the whole run. It started Wednesday, and the morning high stays above five feet every day through Wednesday of next week, easing a little each day after Saturday.

In NAVD88, which is the datum an elevation certificate is written in, that 5.605 feet works out to about 2.7 feet. The conversion is one subtraction. The same station publishes mean lower low water at 1.77 feet and NAVD88 at 4.70 feet on its own station datum, a difference of 2.93 feet, and the predicted height comes down by that much. At a doorstep that is the water surface. It is not the depth in the street, and whether it reaches a floor depends on how high that floor was built and on what the ground does between the street and the door.

The station also publishes a Great Diurnal Range of 4.98 feet, which is the average height of the higher of the two daily high tides. Saturday’s prediction sits roughly six tenths of a foot above that average, which is the difference between the 5.605 feet in the table and the 4.98 feet in the datums record. Those datums are on the 1983 to 2001 epoch, and the record for the city dock notes they were worked out from a two year analysis in 1977 and 1978 against the control station at Mayport.

The tide and the rain are two different things

A prediction at this station is harmonic. It is the moon and the sun and the shape of the bottom, worked out in advance, and it carries no wind, no rain and no surge of any kind. It is a floor, not a forecast. None of the seven tide stations in this county records water continuously, so no federal gauge publishes what the water actually reached downtown, and every height here that is not an outlet’s observation is a predicted one.

Friday is the test. It measures how much the weather has been adding on top of what the table already said was coming. News4JAX reported that the morning high reached about five and a half feet just before 8:15. The prediction table carries 5.51 feet at 8:13 a.m. for that station that day. Within the precision either of those numbers holds, Friday’s water downtown was the tide arriving on schedule.

The rain is the part that moved overnight. The National Weather Service in Jacksonville issued a flood advisory at 12:12 a.m. Saturday covering northern St. Johns County and four areas around it, and wrote that between 1 and 4 inches of rain had fallen with up to 2 inches more expected. That advisory ran until 2:30 a.m. It describes urban and small stream flooding from rainfall, which is a different mechanism from the tide and arrives on a different clock, so a street can be dry at high tide and under water two hours later while the tide is falling.

A coastal flood advisory issued Friday afternoon runs until 4 p.m. Sunday. It covers the St. Johns River basin, including western St. Johns County, and forecasts minor coastal flooding: “Flooding of lots, parks, and roads with only isolated road closures expected.”

A rip current statement for the northeast and southeast St. Johns coast runs through late Saturday night, for “Dangerous rip currents and rough surf.”

What the city has said, and where it said it

City and county crews are monitoring roads, clearing drains and putting out signs, News4JAX reported Friday evening. City officials told the station that several cars have stalled on flooded roads over the past week. There was standing water along Oglethorpe Boulevard in Davis Shores during a drive through the city, and the historic downtown was wet without significant flooding.

The city’s warning to residents went out on Facebook. Find another route around a flooded road. If you have to drive through standing water, slow down, because a vehicle pushes a wave onto the properties beside it.

The city’s News Flash page, which is where it posts official notices, carried nothing about the tides when this paper read it early Saturday. The three items on it are an events policy, the city manager’s retirement and one item below them.

“Our storm drains are like a funnel, and so it can handle as much water that you pour into it, and it’s basically the rain. So if you pour really fast, the funnel will overflow,” Jessica Beach, the city’s chief resilience officer, told News4JAX.

The wall being built for this

The state is replacing the seawall that holds State Road A1A off the Matanzas between the Bridge of Lions and Charlotte Street. The Department of Transportation’s project page gives the reason in its own words. The work is for “enhancing pedestrian safety and accessibility, strengthening protection of State Road A1A from king tides, and preserving the aesthetics of the historic area.”

The same page prints the cost at $19.9 million and the project number as 428271-2, and it lists the work: replacing the wall, rebuilding the promenade, putting a solid three and a half foot coquina concrete barrier where the post and chain is now, taking down and rebuilding the timber pergolas, and removing an event platform. It is staged from south to north and run mainly by barge. Vecellio and Grogan, Inc. holds the contract. Construction started on June 15 of this year and expected completion is fall 2027.

So the wall the state says is being built against king tides is a year out, and the tides it is named for are running this week. A second one, at Maria Sanchez Lake, is still in design. A pump is planned for the south end, News4JAX reported.

The next high tides at the city dock are predicted for 9:12 Saturday night at 5.2 feet, 9:40 Sunday morning at 5.603 feet, and 10:21 Monday morning at 5.519 feet.