Abstract

A methodology that utilizes georeferenced imagery to analyze spatial fluid mud flux measurements pre-, during-, and post-dredging is described. The spatial analysis technique utilizes novel passive sonde sampling techniques and above surface imagery in ArcGIS to geometrically represent fluid mud flux over a specific area of the dredging project waterway. Transect data obtained from the Indian River Lagoon (IRL), south of Sebastian Inlet, Florida is used to mathematically calculate combined fluid mud and flocculation transport in the North, South, East, West, settling (depositional), and upwelling (resuspension) directions across the navigable marine channel at four transects. Satellite imagery is transferred into raster images using ten point image to image georeferencing technique and a 3rd order image transformation providing a root mean square error of < 0.1 m. A kriging technique coupled with contouring was utilized to give a spatial representation of the point sampling data between sampling stations. Digitization of the water area is essential in order to calculate the flux over the surface area of the nearby estuary. The advantages of these techniques are the estimation of fluid mud movement over an estuarine region given sonde data during the dredging of waterways in coastal areas. The techniques are transferrable to other water bodies.

INTRODUCTION

Dredging operations result in the suspension of fine grained sediments decreasing the amount of light that passes through the water column to the seafloor. A large portion of the fine grained materials that are suspended off the bottom remain in a turbid lutocline for extended periods of time. Sediments that are entrained in the lutocline are referred to as fluid mud. The suspension of fluid mud results in the reduction of subsurface irradiance throughout these periods potentially resulting in detrimental effects to the benthic aquatic life that rely on light for vision and productivity (Aumack et al., 2007). In areas where a fluid mud bottom type is dominated, dredging can cause turbulence, suspending and breaking apart the lutocline material. In order for these particles to settle once again they must undergo cohesion between other fine grained particles until the density is large enough to result in a floc to settle out (Winterwerp, 2002). Settling velocities of fluid mud are on the order of 10−1 mm s−1, significantly slower than the tidal current speeds that are encountered. According to Vinzon and Mehta (2003) current in-situ methods used to gain an insight into the mass transport have difficulties measuring concentrations greater than (1 – 2 kg m−3). Due to these difficulties in sampling suspended concentrations, typical sampling is performed instantaneously. This water sampling however, does not include estimates of directional particulate fluxes (g m−2 s−1). McNally et al, 2007 indicates that flux measurements require sampling to be integrated over time. In-situ passive sonde sampling techniques used by Bostater & Rotkiske (2015) measure the mass flux (g) that moves through a sampling area of the sonde during a deployment period. This method is the only reported direct method for flux conserving measurements of moving fluid mud and muck.

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