Modeling suspended sediment transport under combined wave current actions in Indian River Lagoon (Florida)

Aquatic systems that have received nutrient loads for long period of time accumulate nutrients in bottom sediments. These sediments become a potential source of nutrients to the overlying water column. Sediment bound nutrients can then be released through mineralization of organic matter, followed by resuspension and diffusion of nutrients into the water column. Sediment transport processes are one of the controlling factors in these sediment-nutrient cycles. Accurate understanding of the sediment transport processes is, therefore, very important for water quality studies in the aquatic systems.; The Indian River Lagoon (IRL) is a unique estuary of national significance providing home to more than 4300 kinds of plants and animals. However, in recent years, increased population along the shoreline of the IRL and its surroundings has strained its natural resources. The stress caused by the population growth and associated activities have increased total suspended solids (TSS) and nutrient loadings into the lagoon, which have led to poor water quality. In this study, sediment samples have been collected and analyzed in laboratory to obtain sediment parameters such as size distributions, settling velocities, critical shear stresses and erosion rate constants. Four episodic events were conducted during 1997 to 2000 to study sediment resuspension in the IRL, during which waves, currents, water levels, TSS and water quality data were collected. Twelve synoptic events were conducted between April, 1997 and May 1998, during which TSS, salinity and other water quality data were collected at 45 stations. Results of the episodic experiments showed that significant sediment resuspension occurred during these events. To quantitatively study suspended sediment transport in the IRL, a two-group suspended sediment transport model has been developed, in which suspended sediments were categorized into two size groups: a fine group with sediment size less than 62 microns and a coarse group with sediment size larger than 62 microns. Coupled with the CH3D hydrodynamic model, originally developed by Sheng (1986,1989) and the SMB wave model, the sediment transport model is successfully calibrated with field data. Applications of the model to the IRL successfully simulated the episodic and synoptic events. Results showed that: (1) Resuspension and deposition processes are realistically formulated in the model with sufficient accuracy. (2) The internal loadings (sediments resuspended from bottom) were dominant during the episodic events while for longer term (1 year or longer) the external loadings (sediments from river run-offs and other external sources) were more important. (3) Waves played a very important role in sediment resuspension in the IRL.