OCE 3016 Class Handout Coastal Structures In the design of jetties, breakwaters, etc. the size and density of armor stone must be determined. One method of estimating the “stability” of the armor stone is to use the so called Modified Hudson Stability Number, Ns*, *Know this formula for Quiz* ()()1/32sp*ss50wHLN=D1cotρ−αρ Where: Hs= significant wave height Lp= wave length corresponding to Tp Tp= wave period at peak of wave energy density spectrum D50= median stone diameter = 1/350smρHas dimensions of 1/33massmassLL=  m50= median mass of stone ρs= mass density of rock (stone) ρw= mass density of water cot (α)= slop of structure And the Dimensionless Damage, DD, ()d250ADDD= Where: Ad= cross sectional area of damageAn Example: Before storm shape After storm shape Ad= Area of damage Recall: Wave frequency ω ωp Wave Energy Density pp2T π=ω And Lp = wave length of wave with period Tp The relationship between DD and Modified Hudson Stability Number (sometimes called the Spectral Stability Number) is determined experimentally. Thus for Ns* < 6 the Ns* Dimensionless Damage 0 6 8structure will be