Waterfront Erosion Explained: Waves, Wakes, Tides and Walls

Erosion is a natural coastal process. Shorelines have always moved, gaining sediment in some seasons and losing it in others. It becomes a problem when the land that is moving holds a house, a yard or a wall.

For a waterfront owner, the useful questions are practical. Where does the energy hitting your shoreline come from? Where does the eroded material go? And how does a wall, if you have one, change the answer? This guide covers the main forces, how hardened shorelines interact with them, and where living shorelines fit.

The forces that move a shoreline

Wind waves

Most shorelines are shaped by wind-driven waves. The U.S. Army Corps of Engineers explains that wave height depends on wind speed, how long the wind blows, water depth and fetch, which is the open-water distance the wind travels over. At a given site, the longest fetch is usually the most important. A lot facing a wide bay in the prevailing wind direction sees far more wave energy than one tucked inside a narrow canal.

Boat wakes

The same Corps guide notes that pleasure craft and large vessels can generate significant wave activity as wakes. On sheltered waterways with little fetch, wakes may be the largest waves a shoreline regularly sees.

A Chesapeake Bay Program scientific review, summarized by Virginia Sea Grant in 2017, found evidence associating boat activity with erosion and turbidity. Its authors pointed to shallower, narrower waterways where wakes have less room to dissipate before reaching shore. They were also candid that the data to quantify how much erosion wakes cause did not yet exist, and they identified no-wake zones as one of the more feasible responses.

At the wall itself, Key Colony Beach, Florida, attributes the loss of the supporting berm at the toe of its seawalls to wave action, natural or from speeding boats, and asks boaters to keep to idle speed in canals.

Tides and currents

Tides raise and lower the water level once or twice a day and drive currents in bays and inlets as water floods in and ebbs out. The Corps guide notes that tide ranges vary through the lunar month, with larger spring tides and smaller neap tides. Changing water levels move the zone where waves attack a shoreline up and down. Strong tidal currents can also carry away sediment at the base of a wall. Key Colony Beach's seawall manual describes riprap at the toe as particularly useful where tidal velocity is high, because it keeps the supporting berm from washing away.

Storm surge

The National Hurricane Center defines storm surge as an abnormal rise in water level generated by a storm, over and above the predicted tide. The combined level of surge plus tide is the storm tide. Surge lets waves reach higher on a shoreline than they normally can. The Center notes that battering waves can demolish structures not designed for them, and that erosion can undermine foundations even when a building survives the wind.

Runoff and groundwater

Not all erosion comes from the water side. Rainfall, irrigation and roof runoff that soak into the ground behind a shoreline must drain somewhere. On bluffs, the Corps guide treats drainage controls as essential where groundwater affects slope stability. Behind a wall, poorly managed drainage adds pressure and can carry soil out through openings. See why is my yard sinking behind my seawall.

What happens when waves meet a wall

A vertical wall changes how wave energy behaves at the shoreline. Three processes matter most.

Reflection and toe scour

The Corps guide explains that a smooth vertical wall reflects waves almost perfectly. When reflected and incoming waves line up, they can form standing waves up to twice the height of the incoming waves, which may cause considerable scouring of the bottom. NOAA's living shorelines guidance makes the same point directly: the vertical face of a bulkhead reflects wave energy, resulting in erosion along the toe of the structure.

This is why toe protection, such as stone placed at the base, or deep embedment is central to wall design. Loss of the toe is also one of the most common routes to failure, covered in signs of bulkhead failure.

Effects beyond the property line

The Corps guide states that bulkheads protect only the land immediately behind them and offer no protection to adjacent areas or to a fronting beach. If eroding land once supplied sediment to beaches down the shore, armoring it can starve those beaches.

A NOAA-funded study on the Oregon coast found that armored properties can change sediment flows and redirect wave action to erode adjacent, unarmored shoreline. In that market, at-risk unarmored neighbors sold for about 8 percent less. That is one regional study, but it illustrates that a wall's effects do not stop at the property line.

Flanking and overtopping

Erosion can continue around the ends of a wall, a process called flanking. The Corps guide stresses tying the ends of structures into high ground and notes that a community approach to shoreline protection usually reduces the number of vulnerable ends. Waves can also overtop a wall that is too low for conditions. White spray is generally harmless. Solid "green water" over the top can scour soil from behind it.

Sea-level rise: the long-term backdrop

All of these forces act on a water level that is rising. NOAA's 2022 interagency report projects that sea level along U.S. coastlines will rise an additional 10 to 12 inches by 2050, with local amounts varying mainly because land in some areas is sinking or rising. NOAA expects moderate coastal flooding to occur more than 10 times as often as it does today by 2050, and notes that rising seas increase flooding even without storms.

For a wall owner, higher water means waves break higher on the face, overtopping becomes more frequent, and a wall built to a given height offers less margin than it did when it was designed. That matters when you plan the elevation of a replacement wall or cap.

How hardened and living shorelines compare

Shoreline protection runs along a continuum, from natural vegetation through hybrid designs to fully hard structures. The table summarizes how each broad approach handles wave energy, based on NOAA and VIMS guidance.

What NOAA and VIMS say

NOAA's 2015 guidance encourages living shorelines on sheltered coasts, meaning shorelines not exposed to open-ocean waves. It states that bulkheads have adverse effects on adjacent habitats, including scour of shallow intertidal bottom and loss of fringing marsh. It also acknowledges that hardened structures may be appropriate for shoreline stabilization at specific sites.

The guidance cites a North Carolina study in which marshes, with and without sills, outperformed bulkheads during Hurricane Irene in 2011, while 75 percent of the regional bulkheads surveyed were damaged. It is equally clear that no technique, hardened or living, is guaranteed to prevent losses in the most severe storms.

The VIMS Shoreline Studies Program prefers living shoreline methods where they fit. It also describes rock revetments as an effective last line of defense in high-energy settings, because their sloped, rough faces reduce wave reflection and bottom scour.

Honest limits on both sides

Living shorelines are not a universal substitute. Many developed canals have no room for a marsh fringe, deep water at the shoreline may be needed for boats, and open-coast wave energy may exceed what vegetation can handle. The Corps guide notes that bulkheads remain necessary where water depth for boating is needed at the shore.

Equally, a wall is not a permanent fix. NOAA observes that bulkheads perform best when first installed and must eventually be replaced in full. Our guide to riprap and living shorelines compares the options in more detail.

Reading erosion on your own shoreline

Before talking to a professional, it helps to observe a few things over several weeks and after at least one storm or busy boating weekend.

1. Note your exposure. Which direction faces the longest stretch of open water? Is there regular boat traffic close to shore?
2. Check the toe at low tide. Look for exposed wall face that used to be buried, missing stone, or a scour trench along the base.
3. Look at both ends. Is soil cutting in behind the wall where it meets a neighbor or a natural bank?
4. Watch after storms and high tides. Wet ground, debris or fresh soil loss behind the wall suggests overtopping.
5. Compare with neighbors. Erosion patterns along a reach of shoreline often say more than one lot alone.

These observations help a marine contractor, coastal engineer or your state's Sea Grant or coastal management program give site-specific advice. For broader options, see shoreline erosion control.