Abstract

This paper describes a simple mathematical model of hopper dumped dredge spoil dispersion that is based upon a modified version of Stokes’ law to estimate vertical particle velocities, and empirical metocean data to estimate horizontal particle velocities. The horizontal and vertical components are combined to simulate particle paths for computational packets of different grain size fractions under metocean conditions representative of those during the discharge period. Stokes’ law is modified using published relationships to account for large and non-spherical particles. Horizontal particle transport distance is controlled by water depth, particle size, current speed, and current direction variability based upon empirical metocean current speed and direction time series. The resultant dredge spoil depositional pattern is an irregular bull's eye in which particle size and deposit thickness decrease radially from the deposit center. Transport distances for each grain size fraction generally follow a lognormal-like distribution, with distances being inversely proportional to grain size. If there is a predominant current direction during disposal, the resulting deposit will be elongated in that direction. Application of the method, variations of which have been used to support dredge spoil discharge permit applications on several unnamed projects, is illustrated for a hypothetical site.

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