Seawall Replacement: Exposure, Elevation and Tie-Ins

A replacement seawall will likely face higher water, and possibly stronger storms, than the wall it replaces did when it was new. That makes replacement more than a like-for-like swap. It is the point at which wall type, height, toe protection and end details can be reconsidered with current information.

This article focuses on the design questions specific to seawalls. For the general construction sequence, see bulkhead replacement. For a step-by-step project plan, see planning a seawall replacement.

Start with wave exposure

The amount of wave energy a site receives shapes nearly every other decision. FEMA's shoreline guidance describes seawalls as built to resist waves and suitable for ocean shorelines, while bulkheads provide protection against low to moderate wave action and are not suitable for ocean shorelines. FEMA also notes that vertical seawalls often deflect wave energy rather than dissipate it, which can make the shoreline in front more subject to erosion, and that seawalls can be built with different face shapes to deflect waves.

The Virginia Institute of Marine Science's Chesapeake Bay shoreline report adds two practical points: walls on open, high-energy shorelines should be fairly massive, and the potential for scour and increased bottom depth over time is greatest with vertical structures. A replacement on an exposed site may warrant a different wall type, more toe protection or a sloped alternative. See waterfront erosion explained.

Setting the design elevation

Know which datum is being used

Elevations on drawings are measured from a reference called a datum. NOAA distinguishes tidal datums such as Mean High Water, the average of all high water heights over the National Tidal Datum Epoch, from fixed geodetic datums such as NAVD88, which NOAA cautions should not be treated as mean sea level. Local codes often state minimum elevations in NAVD88. Make sure the drawings, survey and local requirement all use the same reference.

Factor in sea level rise

NOAA reports that sea level along U.S. coastlines is projected to rise 10 to 12 inches on average by 2050, with amounts varying by region, and that moderate flooding is expected to occur more than 10 times as often as it does today. NOAA's Sea Level Rise Viewer lets owners visualize potential inundation and high tide flooding, though NOAA notes it is a planning tool, not for permitting. FEMA's guidance recommends considering future conditions such as sea level rise and post-storm beach profiles when designing a seawall.

Check local minimums

Some local governments now require minimum seawall heights. The City of Fort Lauderdale updated its seawall ordinance on March 23, 2023 to adopt regionally consistent standards, and states that it requires new seawalls to reach a minimum top elevation of 5 feet. Requirements like this may apply to replacements and substantial repairs, so confirm with your local building or engineering department.

Height has structural consequences

A taller wall retains more soil and resists more water pressure. That can call for deeper embedment, larger sections or additional anchors. North Carolina's state homeowner handbook notes simply that the taller a bulkhead needs to be, the more expensive it will be. If a future cap extension is planned, the wall should be designed for that added height from the start.

Toe and scour protection

FEMA lists undermining of the seawall foundation by storm waves and tides as a pathway to failure. The design response is some combination of adequate embedment below the expected scour depth and toe protection such as armor stone. Maryland's guidelines describe armor stone placed on filter cloth and a bedding layer at the toe, and note that in some places the stone can cost nearly as much as a revetment, in which case a revetment may be the more prudent choice.

Return walls and flanking

Waves and currents that cannot cut through a wall can work around its ends. FEMA describes this active erosion as localized scour on the water side of the structure and on unprotected property beyond its ends. Maryland's guidelines recommend designing structures to protect their sides and back for the erosion rate and design life, and caution that where only a segment of shoreline is protected, the adjacent shoreline could erode rapidly and endanger the structure.

Practical end details include:

• Returns turned landward and carried far enough into the bank to stay ahead of erosion on the unprotected side.
• Tie-ins to an adjacent wall or revetment, with a sealed joint that does not leak soil.
• Transition protection such as stone where a vertical wall meets a softer shoreline.

VIMS notes that severe flanking can cause structural failure of a wall and that treating longer reaches of shoreline together is a more cost-effective means of controlling erosion.

Tying in with neighbors

Property lines rarely match natural shoreline breaks, so a replacement wall often meets a neighbor's older wall. Common issues at that joint include different top elevations, misaligned faces, mismatched materials and a gap that leaks soil. A low neighboring wall can also become a path for tidal water around a raised wall. Fort Lauderdale says it will cite owners whose seawalls contribute to roadway tidal flooding, which shows that one low wall can affect others.

Options worth discussing with your engineer and neighbors:

1. A designed transition at the property line, including a return if the neighbor's wall is lower or failing.
2. Matching or stepping elevations so water cannot bypass the new wall.
3. Coordinating timing with neighbors who are also planning work, which can simplify access and transitions.
4. Recording the agreed joint detail on the drawings so it can be permitted and built as designed.

Regulatory context on open coasts

Open-coast seawalls often fall under separate state beach and dune programs. In Florida, for example, the statute that exempts certain seawall restorations from state permits states that the exemption does not affect permitting under Chapter 161, the beach and shore preservation chapter. FEMA notes that some jurisdictions prohibit or limit new seawalls. Check early with the permit navigator and the bulkhead permit guide.