Modeling Of Sediment Dispersion During Deep Ocean Mining Operations
Authors: Ichiye Takashi , Carnes Michael 
Published: October 2008

Mathematical models are constructed to predict the transport and dispersion of waste sediments from operations of mining manganese nodules in the tropical western Pacific for the DOMES (Deep Ocean Mining Environmental Study) project. Solutions for stationary Fickian diffusion from a vertical line source indicates that concentration Of SPM (suspended particulate matter) with settling velocity of 10-5 m/s may exceed natural values in the mixed layer within 10 to 100 kin of the source for a discharge rate of 1,000 tons/day. Particularly, if there is upwelling comparable to the settling velocity in the thermocline, the SPM tends to accumulate above it. The Lagrangian diffusion of. SPM from a point source indicates that the distribution becomes stationary within half a day within 5 km of the source, whereas about 4 months is necessary to reach a stationary state in the range of 500 kin from the source. The models are also expressed in non dimensional form to be used with arbitrary environmental parameters.

INTRODUCTION

Although the Challenger discovered manganese nodules on the deep-sea floor a century ago, only recently have developments in deep ocean engineering stimulated the interests of industry in retrieving these nodules as mineral resources. Particularly, assessment of distributions of such nodules confirmed that nodules with nickel content equal to or greater than 1 percent are found on a narrow belt of the northwestern part of the tropical Pacific Ocean. 16 Therefore, several mining firms and international mining consortiums are planning manganese-nodule mining in the area on a commercial basis in the early 1980' s with a goal of 1 to 3 million metric tons. (mt) in dry weight of nodules annually for each firm or consortium.

The National Oceanic and Atmospheric Administration initiated a multidisciplinary research project DOMES (Deep Ocean Mining Environmental Studies for assessing the environmental impact of commercial mining. The project designated a. rectangular area surrounded by 5° and 20° N latitudes and by 110° and 180° W longitudes as the DOMES area since most mining operations will be carried out in this area. A mining ship may recover 5,000 to 10,000 mt of nodules per day and operate about JOO days/year. It is estimated that about .1,000 mt of sediments will be discharged by each ship as waste if 5,000 mt/day of dry nodules is to be produced. 19 Although there is natural suspended particulate matter (SPM) consisting of organic materials and mineral sediments in the upper layer' of the ocean, the discharged sediments may cause serious environmental disturbances in the upper layer of the ocean by interacting with planktonic organisms and by changing radiation absorption of the water if the concentration is excessive. Therefore it is important to know how the waste sediments are transported and dispersed in order to assess their impact on environment.

Mathematical models are developed to determine the concentration of SPM in the ocean as the waste sediments are discharged in the upper layer of the ocean.