Seawall Construction in San Marco

A seawall is designed around the energy it absorbs, so the design starts with the water rather than with the wall.

San Marco sits on the east bank of the St. Johns just south of downtown Jacksonville, a historic neighbourhood whose riverfront is among the most visible in the city. Low elevation in parts of San Marco is a well-documented local reality, and it shapes how shoreline structures here have to perform.

Frontage on the St. Johns facing downtown, largely armoured and built up, with a mix of private riverfront and public bulkhead. Elevation relative to the river is modest in places, which makes high-water performance a design consideration rather than an afterthought.

Wind-driven fetch across the river, boat wake from a busy urban reach including commercial traffic, and — the consideration that distinguishes San Marco — elevated water levels during storm and tidal events. The river is tidal this far up, and high water here reaches ground that matters.

Fresh to brackish water. Gentler on timber and on steel hardware than the coastal communities, which widens the material options and extends service life for a given specification. Tidal range is modest this far up the system. Deck elevation is simpler to resolve than on the coast, and the twice-daily drawdown that empties soil through wall joints is less aggressive.

Filled ground over original river margin in much of the developed frontage, with organic layers beneath. The fill behind older walls is frequently uncontrolled, and it is this material that leaves through open joints and produces the settlement seen in streets and hardscape. For a seawall it drives section selection, how much toe protection the wall needs, and where the cap has to sit to limit overtopping.

A new seawall on St. Johns River and Pottsburg Creek has to answer all of these. The ones involving scour and overtopping are the ones most often left out of a cheap design.

• Elevated water levels and overtopping, eroding from the landward side of walls
• Soil loss through joints into a tidal river, drawing fill out twice daily
• Settlement of fill over organics, visible in hardscape and paving
• Toe scour from combined wake and current along an urban reach
• Hydrostatic pressure behind walls during high-water events when drainage cannot keep pace

A seawall is designed around the energy it has to absorb, and that is why we will not quote one by the foot over the phone. Wind-driven fetch across the river, boat wake from a busy urban reach including commercial traffic, and — the consideration that distinguishes San Marco — elevated water levels during storm and tidal events. The river is tidal this far up, and high water here reaches ground that matters.

On San Marco frontage that drives four things: the section selected, how deep it is embedded, whether toe protection is needed and how much, and where the top of the wall sits relative to design water levels.

• Fetch and wake exposure specific to your frontage on the St. Johns River
• Embedment depth, with an allowance for scour rather than for soil load alone
• Toe protection sized for the energy the wall will reflect back into the water
• Cap elevation and freeboard, set to limit overtopping
• Tieback spacing and anchor placement outside the active soil wedge

A vertical wall reflects wave energy downward into the water at its own base, and that reflected energy excavates the material the wall is embedded in. The wall undermines its own foundation, continuously, and none of it is visible from the top.

San Marco seawalls need to be assessed with overtopping explicitly in mind — a wall designed only for the water in front of it is addressing half the problem in this neighbourhood. Cap elevation and the drainage behind the wall deserve as much attention as the structure.

If you are comparing estimates for San Marco and one of them has no toe protection on exposed frontage, that is the question to ask before you compare the totals.

Fresh to brackish water. Gentler on timber and on steel hardware than the coastal communities, which widens the material options and extends service life for a given specification. We build vinyl and concrete seawalls, and the choice turns on exposure, what sits behind the wall, and how long you intend to own the property.

The important qualifier for vinyl is that wave loading is a larger demand than soil retention, so the section has to be selected for it. A profile adequate on sheltered water is not automatically adequate here. The full construction process is here.

A new seawall here is also a soil-retaining structure, so the bulkhead side of the problem does not go away. San Marco bulkheads are retaining fill on low ground against a tidal river, and two failure paths run simultaneously: ordinary soil loss through joints, and scour from behind during high water. Full detail: bulkhead construction in San Marco and bulkhead repair.

A new wall is the natural moment to resolve a dock tie-in properly rather than working around it. The constraints on this frontage:

More at dock construction in San Marco.

• Urban reach with commercial as well as recreational vessel traffic
• Limited private frontage in a built-up neighbourhood
• Elevated water events affecting structure elevation decisions

Putting that together for a new seawall here:

The priority in San Marco is usually the ground behind the wall rather than the wall face. Depressions in lawns and tilting hardscape here are a reliable early signal, and dealing with them early is a fraction of the cost of waiting until the wall itself moves.

Dense historic neighbourhood with constrained access, limited laydown space and hardscape close to the water. Protecting and restoring the work corridor is a significant part of the scope.

A new seawall is reviewed as new construction, and the toe protection it needs is part of the proposal rather than an afterthought added later.

Historic district considerations apply in parts of San Marco, and the urban riverfront brings navigation review given commercial traffic on this reach.

Requirements are revised periodically. Treat this as orientation and confirm current requirements with each reviewing agency before planning work. We handle permitting on the projects we build, and agency review is normally the longest item in the schedule.