Seabed armoring

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Definition of Seabed armoring:
The formation of a top layer of coarse sediment hiding and protecting a sublayer of finer sediment from erosion.
This is the common definition for Seabed armoring, other definitions can be discussed in the article

Seabed armoring processes

Seabed armoring is a ubiquitous process in coastal environments where the sediment bed contains a mixture of fine- and coarse-grained sediment particles (so-called 'graded sediment').

An erosion-resistant top layer of coarse sediment is formed when fine sediment is removed from the seabed surface by the scouring action of energetic waves. This coarse sediment top layer hides the underlying layer of finer sediments, which is therefore protected from scouring.

Wave action can produce seabed armoring also by 'inverse grading', a process called 'kinematic vertical sorting' [1]. This phenomenon has been observed, for example, in the case of wave-induced sheet flow[2]. When wave-induced interparticle collisions lift a coarse sediment particle from its position in the soil matrix, the vacated pore space is readily filled with finer particles, preventing the coarse particle from bouncing back. The coarsest sediments are therefore lifted gradually to the seabed surface, leading to the formation of an armor top layer covering a sublayer of finer sediment. This armor layer inhibits suspension of the fine sediments, whilst the coarser particles are more exposed and set in motion more easily. The term 'inverse grading' points to the intuition that one would expect a layer of fine sediment at the surface rather than beneath it.

If the seabed is loosely packed and poorly drained, wave action can induce soil liquefaction. Il this case coarser particles will sink, rather than being uplifted - see wave-induced soil liquefaction.

Related articles

Coastal and marine sediments
Shoreface profile
Sediment deposition and erosion processes
Sand transport


  1. Legros, F. 2002. Can dispersive pressure cause inverse grading in grain flows? Journal of Sedimentary Research 72: 166– 170
  2. Hassan, W.N. and Ribberink, J.S. 2005. Transport processes of uniform and mixed sands in oscillatory sheet flow. Coastal Engineering 52: 745– 770

The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Job Dronkers (2023): Seabed armoring. Available from [accessed on 22-04-2024]