Riprap and Reno Mattress are common solutions for bank protection, and this article aims to solve some doubts people have about the differences between the two.
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The Riprap solution is composed of stones. It is responsible for protecting the bank against erosion, tractive forces, wave forces, etc. The stones sizes can be diversified, and the engineer needs to design the correct size of these stones in order to resist all the possible natural efforts. This solution doesn't have any type of mesh. The stability is guaranteed solely by the stones&#; weights and the stones&#; friction. If the velocity is high the engineer needs to design bigger stones for the protection project. If the velocity is low, the engineer can work with lower diameters. Normally the diameters used for Riprap are bigger than 50cm because, with a smaller diameter, the Riprap understands there isn't a big natural effort acting on the bank, therefore another solution can be chosen. With a large diameter, there will be big gaps, normally used as a geotextile layer on the bank, to receive small stones (for the transition), and then the Riprap.
The Reno Mattress solution is composed of stones and double-twist hexagonal mesh. It is also used to protect the banks against erosion, waves and tractive forces. The average diameter of the stones can be from 90mm to 125mm. With the hexagonal mesh, the solution has an element that works to confine the stones, and it has a total influence that guarantees the solution&#;s stability. There are three thicknesses of mattress: 17cm for low velocities; 23cm for medium velocities; and 30cm for high velocities. With the sizes of stones normally used inside the Reno Mattress it is unnecessary to work with small stones as a transition filter, but the geotextile filter between the Reno mattress and the bank is always indicated.
What are the differences between Riprap and Reno Mattress for longitudinal protections?
Although they have the same velocity, the thicknesses of Reno mattress and Riprap are different. The Riprap&#;s stability is guaranteed by the weight of the stones and the respective interlocking, while the stability of the Reno mattress is guaranteed by the weight of the stones, the interlocking, and also through the Hexagonal mesh confinement. For example, with 4m/s of velocity, we can stabilize the bank with a 23cm Reno mattress, while with the Riprap solution it would be necessary to have a thickness of about 70cm. Look at the picture below and check.
The residual velocity is the flow that passes between the stones and comes into contact with the bank. Both solutions are permeable, therefore there is a residual velocity in both the Reno Mattress and Riprap solutions. The difference is, for the Riprap solution it is necessary to use small stones on the geotextile, before placing the large stones. Why is this necessary?
Because if you just work with the large stones for the Riprap protection, the solution will have a lot of empty spaces, and the flow will be able to enter into these spaces with more facility and generate a higher residual velocity.
The Reno Mattress solution doesn't need an additional amount of stones in the bottom to protect the base, just a deformation platform. This solution is flexible and can follow the bottom erosion. For the Riprap solution, the designers usually use an additional amount of stones in the bottom for the base protection, because if the erosion starts some of the stones are naturally displaced by the erosion. Look at the picture below and check this information:
If the longitudinal protection is submitted to wave action, it is necessary to design the solution taking this natural effort into consideration. In this case, the Riprap doesn't have a theoretical limit of diameter stones, therefore big waves can be resisted by big diameter stones. For the Reno Mattress the stones&#; diameters are limited by the thickness, normally two layers of stones are used, so the diameter can't be bigger than half of the thickness. Reno Mattress can support waves but for big waves (bigger than 1m) it is necessary to be careful and to apply maintenance. Also, with this information, it can be noted that Riprap is more used for coastal protection because in these are bigger waves are often found.
To conclude, both solutions are commonly used in bank protection, and both are composed of stones. But with Reno Mattress, with its hexagonal mesh, in most cases some advantages present themselves, due to the confinement mesh effect. When it comes to resistance to waves, Riprap is more powerful with its use of big stones and for this reason it is more commonly used for coastal protection. Whatever the solution it is necessary to correctly design the material for good performance.
I've done another post talking about Reno Mattress Waves Design:
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MACCAFERRI; Manual de revestimento de Canais, Maccaferri do Brasil, São Paulo,
C. T. BROWN, Gabion Report on some factors affecting the use of Maccaferri gabions and Reno mattresses for coastal revetments, Manly Vale, N. S. W., .
Traditional river bank protections made with loose stones (rip-rap) require a thick layer of material to be effective. This is due to the fact that the stones are not confined and that the stones layer cannot be accurately placed, resulting in higher stone consumption and, consequently, increasing the environmental burden.
Traditional Reno Mattresses are over-designed river bank protections compared to the actual load they are subjected to.
Reno Mattress Plus is the result of the experimental tests at Colorado State University. A deeper understanding of the performance of the mattress enabled us to make more cost-effective designs, giving to our solutions an increased competitiveness in comparison to equivalent and alternative (traditional) systems.
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