Seawall and bulkhead materials.
Match the wall to the water.

Start with salinity, not the brochure

USGS measurements show salinity dropping fast up the St. Johns: about 20–21 ppt at the Dames Point Bridge, about 5 ppt at the Acosta Bridge downtown, around 1–2 ppt near the Buckman Bridge and under 0.5 ppt from the Shands Bridge south. The Intracoastal and Amelia River stay close to seawater. Storms can push salt water far upriver for days.

Salinity decides which marine borers attack timber and how fast steel and reinforcing bar corrode. Ask any designer or contractor to say which water their material choice assumes.

Continuous stream discharge, salinity, and associated data collected in the lower St. Johns River and its tributaries, Florida, 2021 (OFR 2022-1111) · NHC Tropical Cyclone Report: Hurricane Irma (AL112017)

Timber bulkheads: common, but borers are real

Many Jacksonville riverfront and creek lots still have timber bulkheads. Florida Sea Grant found wood deteriorates faster in Florida than elsewhere in the continental U.S.: treated lumber expected to last 20–30 years elsewhere may fail in under 10 years in Florida waters because borers are active all year.

Gribble (Limnoria) prefers high-salinity water, while the isopod Sphaeroma favors low-salinity and tidal-fresh water. So brackish river water does not make timber safe. Shipworm damage is especially hard to see: entrance holes are tiny and the animal lives inside the wood, so a timber wall can look sound until it fails.

For new or replacement timber in salt or brackish water, ask the engineer to confirm the AWPA Use Category 5 treatment specified for southern waters, and ask how hidden borer damage in the existing wall will be checked (sounding or coring, not just a visual look).

Wood-Boring Marine and Estuarine Animals in Florida (SGEB-15) · Accredited Agencies for Supervisory and Lot Inspection of Pressure Treated Wood Products (Jan 2026) · UFM 4-150-07 Waterfront Facilities: Operation and Maintenance

Steel sheet pile: watch the splash zone

Carbon steel corrodes in every exposure zone, but federal waterfront guidance says the most severe corrosion is in the splash zone and just below mean low water. Coatings and cathodic protection are the usual defenses. At Mayport and Fernandina those zones are tall bands because the tide range is large; upriver they are narrow, but long-lasting floods move them up the wall.

UFM 4-150-07 Waterfront Facilities: Operation and Maintenance · Mayport (Bar Pilots Dock) 8720218 datums · Fernandina Beach 8720030 datums

Concrete caps, panels and king piles

In concrete, salt and oxygen reach the reinforcing steel fastest just above mean low water, where the surface wets and dries. Expanding rust cracks the concrete along the bars and then spalls it. Rust staining and cracks running parallel to the cap or panel edge are worth photographing and asking about early.

UFM 4-150-07 Waterfront Facilities: Operation and Maintenance

Vinyl and composite sheet pile

Vinyl and fiberglass-composite sheet piles do not rot or rust, which is why they are widely offered for residential replacement. They still depend on the same things every wall does: enough embedment, tiebacks or anchors, a sound cap, filter fabric and drainage. Ask for the product’s published section properties and the engineer’s design for your wall height and soil, and compare warranties by what they actually cover.

We did not find official Florida guidance comparing vinyl and composite products. Treat manufacturer life claims as claims, and compare designs, not brand names.

What is behind and below the wall matters

The only published subsurface description we found for east Duval County is a deep USGS test well: sand, clayey sand, phosphatic sandy clay, coquina and limestone over the Floridan aquifer. That is not a design input for your yard. Any replacement design should rest on a site-specific soil boring and survey.

Hydrogeologic data from a test well in east-central Duval County, Florida (OFR 84-802)

Questions that make quotes comparable

• Which water (salinity and tide range) does the material choice assume?
• What embedment depth, anchor system and cap are included, and who designed them?
• How will drainage and filter fabric keep backfill from washing out during multi-day high water?
• For timber: what treatment standard, and how will hidden borer damage in the old wall be checked?
• For steel: what coating or cathodic protection, and how will the splash zone be inspected later?
• What does the warranty cover, for how long, and what voids it?

Then use the cost guide to compare price ranges by material, and the repair-or-replace guide to decide how much of the wall needs to change.