On a river, current is the load. On a coast, a lake, an estuary, or a berth, waves act as well, and the two together are not simply the larger of the two.
Why the combination is worse
Orbital velocities near the bed. A passing wave drives a near-bed orbital motion. In shallow water that motion reaches the bed and mobilises material.
Superposition. Where wave orbital motion runs in the same direction as the current, the instantaneous velocity is higher than either alone. Sediment transport responds strongly to velocity, so the combined condition can move material that neither component would.
Mobilise and transport. Waves stir material into suspension and current carries it away. The two mechanisms complement each other, which is why a site with modest current and modest waves can erode faster than either figure suggests.
Cyclic loading and drawback. Wave action is oscillatory. The repeated loading and the return flow work material loose in a way steady current does not, and it is why a shoreline notch forms at the waterline. See boat wake erosion and lakefront erosion.
Where it applies
- Open coast, the obvious case
- Estuaries, where tidal current and wave action coincide
- Large lakes and reservoirs, where fetch generates real wave energy. See lakefront erosion
- Berths and harbours, where vessel-generated waves and propeller wash add to the natural regime. See scour at marine piles and dolphins
- Offshore, where wave and current combine over a seabed. See subsea free spans
What the design needs
Wave conditions: significant wave height, period, and direction, for the design condition. Whether they are locally generated by fetch or arriving as swell.
Water depth, because whether wave motion reaches the bed depends on depth relative to wavelength.
Current, its magnitude and direction relative to the waves.
Whether they coincide in the design condition, which is not automatic.
Fetch and wind, on a lake or enclosed water where waves are locally generated.
The bank covers how combined wave and current forces are addressed on coastal or offshore sites, and what site data supports that.
The honest position on our evidence
This is the part that matters and it is stated plainly.
FES holds no citable physical model test results for wave stability.
That is set out on when not to use rock bags, which is built from the questions our own data does not answer.
The consequence for a design:
Cast concrete armor units have a framework we do not. Tetrapods, dolosse, accropodes and the rest come with stability coefficients derived from decades of physical model testing, which lets a coastal engineer size an armor layer against a design wave by an accepted method and demonstrate it. See rock bags vs tetrapods and dolosse.
On a design-wave-governed structure, that is a real gap. Where a specification requires armor stability demonstrated against a design wave by an established method, that is a legitimate requirement and one we cannot meet from published material. A specifier is entitled to write it and we would rather they knew at specification stage. See writing a spec that stays open.
Where the wave-affected duties still work
Being precise rather than either overclaiming or writing the product out:
- Toe protection and scour control at a structure, where the duty is preventing bed material loss rather than resisting design-wave attack as a primary armor layer
- Sheltered and moderate-energy sites — estuaries, harbours, lakes with limited fetch
- Behind or beneath a primary armor layer
- Repair and infill within an existing armor system
- Berth and harbour interiors, where the loading is vessel-related rather than ocean swell
That pattern — secondary and scour duties rather than primary wave armor on an exposed coast — is an accurate description of where bagged armor fits on a wave-loaded site, and it is what coastal bluff and dune toe protection says as well.
What to do if your site is wave-governed
- Establish whether waves or current govern. If the answer is a design wave on an exposed coast, that shapes the whole conversation
- Get the wave data rather than inferring it from current
- Ask what demonstration the specification requires
- Raise the gap early. If an accepted-method stability demonstration is required, we will tell you we cannot provide it
- Consider the hybrid. Primary armor by a system with the framework, toe and scour protection by one that suits the placement conditions. See combining rock bags with other systems