Rock Armour

The main difference between rock armour and seawalls is that rock armour is permeable. It is this permeability that allows it to dissipate the energy of storm waves and prevent erosion. A similar effect may be created through the use of pre cast concrete units that are designed to be permeable

The rocks are placed against a cliff, bank or existing seawall in order to reduce the energy of incident waves. Dissipation of the wave energy should reduce wave reflection from the structure, thus reducing scour from the toe of the defence.

Rocks are often brought in to an area by boat due to the weight and amount of room required to transport them. They are then positioned using heavy lifting gear. In the video bellow rocks are being positioned at Amroth, South Wales.

The placing of rock armour stones at the foot of a cliff relies on several factors to protect the area behind. These are:

the slope of the front face of the structure,

the voids within the structure itself, and

the weight of the individual blocks.

If you imagine a boxing match when one of the contestants is hit full in the face the effect of the blow is much more than a "glancing " blow which slides off the opponents chin. This is the same with a rock structure where the front slope shape means that the force of the wave attack is deflected and its impact is reduced.

When you add to this the fact that a rock armour structure is full of voids between each individual piece of rock we can see that any wave energy is dissipated by the fact that the incoming wave passes into the voids and as it can expand into the void some of the pressure is released and the energy is lost.

This loss of wave energy has two effects both in reducing the impact of the wave but also in preventing reflection of the wave when it hits the structure and it is this reflection which generally cause undercutting of more "solid" forms of structure such as the old Victorian vertical sea walls we see in many seaside resorts.

For the above effects to happen it is vital that each of the individual blocks are of sufficient "size" that they are not picked up by the waves and tossed about into the cliff causing even more damage. The term "size" is a mixture of block weight and specific gravity of the rock piece and these two factors combined with the slope angle of the front face dictates the design of the structure required to withstand a particular wave height.

In addition to standard rock shapes, manufactured armour such as tetrapods or X blocs are also used in some highly volatile areas. The use of these type of armour reduces any possible movement of the structures as they are made to interlock. Examples of these structures are shown below.

The following photograph shows the armoured protection of the cliff toe at Downderry, Cornwall, with a rip-rap design. The porosity of the structure allows wave energy to dissipate with minimal reflection