K-type coefficients in the littoral drift system

Abstract

The quantity q (volume of sand transported past a given point, per unit time; L 3T −1, can be related to P L (the littoral component of wave power, per unit length of coast; MLT −3) by using the expression k γρa (where gamma is the acceleration of gravity, rho is the mass density of sediment, and a is a packing coefficient). In this third expression, three of the values can be taken as constants which provide that the resulting equation will be dimensionally homogeneous: q = kP L/γϱs a The coefficient k is, therefore, dimensionless. Its value, as reported in the literature, ranges from numbers close to (but smaller than) 1.0, down to numbers such as 10 −4 and 10 −5. If one wishes to use P L in order to predict q, and hence dq dx (erosion or deposition, according to the algebraic sign on the derivative), there must be a better assessment of k than a range of four, five, or six orders of magnitude. The coefficient k was originally defined in an instantaneous sense, so it can have values greater than 0.1. For long term calculations, a similar coefficient can be defined as: In K = C t 1P 1 t 2P 2 − 1.0 where C can be taken as −11, and the tP-ratio expresses the extent to which wave power is cancelled in a bidirectional drift system, thus producing values of K in the general range down to 10 −5 or perhaps smaller.