Revetment

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Rip Rap Drawing

Erosion Control

A revetment provides a cost-effective solution to protect banks and shorelines from erosion caused by wave action or water currents. A revetment is a sloping structure placed on banks and constructed of more extensive rock material, often referred to as a “riprap” or articulated concrete blocks. Revetments work best for low energy wave action, typically 5 ft or less. When designing and installing a revetment and when maintained properly, it can prevent the erosion of shorelines. A revetment provides a more cost-effective and environmentally friendly solution when a seawall is unnecessary or even prohibited.
Engineering and designing a revetment requires determining the cause of the erosion. It is essential to determine if the deterioration is from wind, wave, or current activity, or a combination of several factors.

Riprap Revetments

Most Florida riprap revetments utilize coral or limestone rock because of their ability to withstand saltwater conditions and look. A riprap revetment is different from an armor stone revetment because the riprap is widely graded. Stone armor is usually specified and has a small range of sizes. A typical weight range is 0.75W50 to 1.25W50, where W50 is the median weight of the stone. The nominal diameter, Dn50, is defined as

Equation 1


This equation determines the weight density of the stone. An r allowable diameter range of 0.91Dn50 to 1.08Dn50. The corresponding range for riprap is 0.125W50 to 4.0W50 (0.50D n50 to 1.59Dn50). Riprap revetments are not recommended for large waves above five ft. unless there are other considerations. Typically, the design or a riprap revetment will follow the U.S. Army Corps of Engineers (COE 2001, 1984, 1973). A calculation using the Hudson equation or the Van Der Meer equation to determine the required riprap size.

The Hudson equation is:

Equation 2

in which KD is the empirical Hudson stability coefficient and is the immersed relative density defined as ? = ?r/?w-1, where ?r is the specific weight of the rock and ?w is the specific weight of the water (62.4 pounds per cubic foot (pcf) for freshwater; 64.0 pcf for seawater). The stability coefficient for angular riprap in breaking waves is KD = 2.2 (COE 1984, 1973). The wave height is computed as the wave height at the toe of the structure. The 1973 and 1984 versions of the SPM are inconsistent in the use of the Hudson equation. The 1984 SPM recommends using a wave height of 1.27Hs if this wave height is used in the Hudson equation, the required stone weight doubles.
Precision Marine Construction Inc., (PMC) has been designing and building riprap revetments in Florida for more than 20 years. Call the experts at PMC for your free estimate.

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