Erosion forces from running water

“Application of the Marecom Mattress is a cost-effective solution with long term protection and reduced maintenance costs”

say Rune Nilsen and Bjørnar Sand, senior scientists at Norut Northern Research Institute in Narvik. They are both experts on construction technology and have been working with the steel reinforced concrete mattress for several years – doing strength and load calculations and testing. Their work has provided valuable input data to the engineering of the mattresses.

The erosion forces in play can be divided into four types:

Wave erosion occurs in two phases: Incoming upstream flow and outgoing downstream flow. The last is normally the worst when it comes to erosion. Incoming waves can bring powerful blows to the riprap slope, plastering and constructions, pressure, buoyance and drag through an upstream flow. The return wave makes a strong drag from the downstream flow, pressure fall along the water front, washing out of loose gravel and sand – which in turn destabilizes the bigger stones in the riprap. Even huge stones used to plaster a riprap (i.e. a drywall) can be destabilized by these forces.

The buoyancy is 1 metric ton per m³, and the tides will change these forces in a cyclic pattern. When all these forces mentioned are working together, the erosion effect is strong, and in time the damage can become catastrophic, leading to slides, undermining and collapse in heavy constructions.

The water resistant concrete mattress from Marecom will shield the riprap permanently under a thick layer of solid steel reinforced concrete. The mattress can absorb extreme wave forces because it is very robust and heavy. As an example a Marecom mattress of 10 x 20 meters has a weight of more than 100 metric tons. It is also very durable, with an expected life time of more than 100 years.

Load forces

Typical loads on the mattress in its operative phase (i.e. submerged in the sea and fully concreted) are the various forces from waves and propeller wash.

The Marecom Mattress can be used in many situations, some of which are listed below:

• Securing jetties of various sizes and filled in areas that are exposed to natural forces such as waves, currents, etc.
• Protection of foundations and pilings for docks and piers and to prevent weakening and undermining caused by, for example, propeller turbulence
• Environmentally securing and covering deposited material
• Covering sunken ships, discarded offshore structures, etc.
• As overburden on oil and gas pipelines
• Flood protection in watersheds