Jacksonville water levels.
Know the water your wall faces.

Three kinds of water, three kinds of wall

Northeast Florida walls sit on three very different water bodies. Before comparing quotes, materials or a neighbor’s repair, work out which one your property faces.

Figures are NOAA great diurnal ranges (mean lower low water to mean higher high water) at the nearest long-term gauges. Your creek or canal can differ; ask your surveyor to tie the wall to a published datum.

Mayport (Bar Pilots Dock) 8720218 datums · Fernandina Beach 8720030 datums · Dames Point 8720219 datums · Southbank Riverwalk 8720226 datums · I-295 Buckman Bridge 8720357 datums · St. Augustine Beach and ICWW station datums (8720587, 8720494, 8720398, 8720305, 8720267) · Continuous stream discharge, salinity, and associated data collected in the lower St. Johns River and its tributaries, Florida, 2021 (OFR 2022-1111)

Why the St. Johns River floods differently

The St. Johns is a slow, 310-mile river that flows north and drops less than 30 feet over its whole length. Atlantic tides reverse its flow twice a day, and sustained northeast winds can hold water in the river for days. NOAA’s river model notes that freshwater outflow is only about one-seventh of the average tidal flow.

For a riverfront wall, that means the dangerous water is often not the daily tide. It is several tide cycles of elevated water during a nor’easter or tropical storm, which saturates the soil behind the wall and can wash fill out through open joints and weep holes when the water drops.

In October 2025 the National Weather Service issued a coastal flood warning for the whole St. Johns River basin, explaining that persistent onshore winds were trapping tides in the river.

St. Johns River · About St. Johns River, Florida (GNOME Location File) · NWS Jacksonville Area Forecast Discussion, Oct 3, 2025

Normal high water sits close to NAVD88 upriver

Seawall drawings usually state cap elevations in NAVD88. Normal high water (mean higher high water, MHHW) is about 1.96 ft above NAVD88 at Mayport, about 1.42 ft at Dames Point, about 0.65 ft downtown at the Southbank Riverwalk gauge and only about 0.39 ft at the Buckman Bridge.

So the same cap number gives very different freeboard in different places. A cap at elevation 4 ft NAVD88 sits more than 3.5 ft above normal high water in Mandarin, but only about 2 ft above it at Mayport. Never compare a measurement taken from today’s waterline with an elevation on a survey, and never mix NAVD88 with older NGVD29 figures.

Mayport (Bar Pilots Dock) 8720218 datums · Dames Point 8720219 datums · Southbank Riverwalk 8720226 datums · I-295 Buckman Bridge 8720357 datums · Frequently Asked Questions: Datums

Flood stages at the river gauges

The National Weather Service measures river flooding in feet above MHHW. At the Main Street Bridge gauge downtown, action stage is 1.0 ft, minor flooding starts at 1.5 ft, moderate at 2.0 ft and major at 3.0 ft. At minor stage, NWS says some boat ramps and docks along the St. Johns and other waterways begin to flood.

At the St. Augustine marina gauge, NWS notes water overtops the seawall along Avenida Menendez at the 3.7 ft major stage. Use the gauge nearest you: Mayport or Fernandina for coastal property, Dames Point or Main Street for the urban river, and Buckman Bridge for Mandarin, Orange Park and the St. Johns County riverfront.

St. Johns River at Main Street Bridge (MSBF1) · Main Street Bridge flood impacts · St. Johns River at Mayport (MYPF1) · NWPS St. Johns River and coastal gauge flood categories · Tide Station at St Augustine Municipal Marina (SMFF1) · CO-OPS water level station list

The storms that set local records

• Irma, September 2017: the modern record at Main Street Bridge, 4.89 ft above MHHW; 5.2 ft at Buckman Bridge. The National Hurricane Center described flood water up to 5 ft deep in some Jacksonville streets, and downtown salinity jumped from under 5 ppt to about 30 ppt as easterly winds pushed seawater upriver.
• Matthew, October 2016: a then-record 3.28 ft above MHHW at Mayport and a 4.17 ft storm tide at Fernandina Beach, with 5–7 ft of inundation along the St. Johns and Duval coasts.
• Ian and Nicole, fall 2022: back-to-back storms six weeks apart. Nicole set the highest water level on record at the Mayport gauge, about 3.41 ft above MHHW, and NHC notes Ian likely left the coast more vulnerable first.
• Milton, October 2024: 1–3 ft of surge from Port Canaveral to the Georgia line; 3.01 ft above MHHW at Buckman Bridge.
• October 2025 nor’easter: NWS expected moderate tidal flooding near 3 ft MHHW on the beaches and 2.0–2.5 ft on the river, the highest since Nicole.

A wall weakened in one storm can fail in the next. Record its condition after each event, even when nothing looks wrong.

Main Street Bridge historic crests · NWPS historic crests (BKBF1, DMSF1, MYPF1, SRPF1, GCEF1, FRDF1) · NHC Tropical Cyclone Report: Hurricane Irma (AL112017) · NHC Tropical Cyclone Report: Hurricane Irma (AL112017) · NHC Tropical Cyclone Report: Hurricane Matthew (AL142016) · NHC Tropical Cyclone Report: Hurricane Nicole (AL172022) · Mayport 8720218 datums (max observed) · NHC Tropical Cyclone Report: Hurricane Milton (AL142024) · NWS Jacksonville Area Forecast Discussion, Oct 9, 2025 (8:26 PM)

High water is becoming more frequent

NOAA measures relative sea level rising about 2.9 mm a year at Mayport (1928–2025) and about 2.3 mm a year at Fernandina Beach (1897–2025). Its outlook for May 2026 to April 2027 projects 4–6 minor high-tide-flooding days at Mayport and 6–9 at Fernandina Beach, up from 2–3 and 0–5 in its 2019 outlook.

Average monthly sea level here peaks in October, then September and November. Fall is when king tides, nor’easters and late-season storms overlap, so plan inspections before September and repairs before the next fall season where you can.

Relative sea level trends (Mayport 8720218) · Relative sea level trends (Fernandina Beach 8720030) · High tide flooding annual outlook (Mayport, Fernandina Beach) · High tide flooding outlook history · Seasonal sea level cycle (Mayport, Fernandina Beach)

A practical inspection calendar

• Walk the wall and photograph it from the same safe positions every summer, before the fall high-water season.
• Look again after any event that reaches NWS minor flood stage at your nearest gauge, after every named storm and after multi-day nor’easters.
• Check for depressions or sinkholes behind the cap, open joints, new lean, rust staining, cracked caps and fill on the bottom in front of the wall.
• As a federal reference point, the Defense Department’s waterfront maintenance manual recommends annual above-water inspection and more frequent timber checks where marine borers are active. Your engineer sets the schedule for your wall.

Use the warning-sign check to decide what needs attention first, and the storm records guide after an event.

UFM 4-150-07 Waterfront Facilities: Operation and Maintenance · St. Johns River at Main Street Bridge (MSBF1)

Know your flood zone and your discount

Jacksonville takes part in FEMA’s Community Rating System at class 6, which gives eligible flood-insurance buyers a 20% discount inside the special flood hazard area and 10% outside it. Check your zone on FEMA’s Flood Map Service Center, and ask the city’s floodplain coordinator about determinations or copies of available elevation certificates. Remember that flood policies generally exclude the seawall itself; see the seawall insurance guide.

Floodplain Management · CRS Eligible Communities, April 1, 2024 · Flood Protection Information