Hudson's equation
Hudson's equation, also known as Hudson's formula, is an equation used by coastal engineers to calculate the minimum size of riprap (rock armour blocks) required to provide satisfactory stability characteristics for rubble structures such as breakwaters under attack from storm wave conditions.
The equation was developed by the United States Army Corps of Engineers, Waterways Experiment Station (WES), following extensive investigations by Hudson (1953, 1959, 1961a, 1961b) (see Shore Protection Manual and Rock Manual referenced below).
Initial equation
The equation itself is:
where:
 W is the design weight of the riprap armour (Newton)
 is the specific weight of the armour blocks (N/m^{3})
 H is the design wave height at the toe of the structure (m)
 K_{D} is a dimensionless stability coefficient, deduced from laboratory experiments for different kinds of armour blocks and for very small damage (a few blocks removed from the armour layer) ():

 K_{D} = around 3 for natural quarry rock
 K_{D} = around 10 for artificial interlocking concrete blocks
 Δ is the dimensionless relative buoyant density of rock, i.e. (ρ_{r} / ρ_{w}  1) = around 1.58 for granite in sea water
 ρ_{r} and ρ_{w} are the densities of rock and (sea)water ()
 θ is the angle of revetment with the horizontal
Updated equation
This equation was rewritten as follows in the nineties:
where:
 H_{s} is the design significant wave height at the toe of the structure (m)
 Δ is the dimensionless relative buoyant density of rock, i.e. (ρ_{r} / ρ_{w}  1) = around 1.58 for granite in sea water
 ρ_{r} and ρ_{w} are the densities of rock and (sea)water ()
 D_{n50} is the nominal median diameter of armour blocks = (W_{50}/ρ_{r})^{1/3} (m)
 K_{D} is a dimensionless stability coefficient, deduced from laboratory experiments for different kinds of armour blocks and for very small damage (a few blocks removed from the armour layer) ():

 K_{D} = around 3 for natural quarry rock
 K_{D} = around 10 for artificial interlocking concrete blocks
 θ is the angle of revetment with the horizontal
The armour blocks may be considered stable if the stability number N_{s} = H_{s} / Δ D_{n50} < 1.5 to 2, with damage rapidly increasing for N_{s} > 3.
Obviously, these equations may be used for preliminary design, but scale model testing (2D in wave flume, and 3D in wave basin) is absolutely needed before construction is undertaken.
See also
References
 US Army Corps of Engineers (1984). "Shore Protection Manual." Vol. II.
 CiriaCUR (2007)  Rock Manual  The use of rock in hydraulic engineering.