Modelling the effects of dredge spoil disposal

As port weather experts, MetOcean Solutions regularly provides modelling support for port dredging operations. 

“The disposal of dredge spoil requires careful consideration,” explains Oceanographer Simon Weppe. “Over time, disposed sediments will be redistributed by currents and waves. When selecting offshore disposal sites, ports have to consider the hydrodynamic processes of the area and evaluate the longer-term changes to the shape of the seabed (morphological changes). Understanding the general morphological behaviour of the region is essential if we are to reduce the potential for sediment recirculation in the channel and port.

“Through our work with numerous ports, we’ve developed a process for modelling the effects of dredge spoil disposal. The changes to the shape of a disposal mound are governed by long-term hydrodynamic patterns slowly transporting sediments, and by extreme events such as storms, which can drastically change the shape of the seabed over short timescales. We use a coupled wave, current, and morphological model system, Delft3D, to understand the processes governing sediment transport in the area, and to identify key transport pathways. This gives us information to help predict the expected long-term morphological changes, as well as those arising from extreme events.”
 

Morphological changes predicted at the end of an accelerated 1-year simulation. A positive magnitude indicates sedimentation. The black polygons show the proposed disposal grounds and channel.

Morphological changes predicted at the end of an accelerated 1-year simulation. A positive magnitude indicates sedimentation. The black polygons show the proposed disposal grounds and channel.

The Delft3D modelling system is placed within broader regional wave and 3D hydrodynamic models of the area. 

For waves, a 10-year historical data set (hindcast) is prepared using the Simulating WAves Nearshore (SWAN) model, carefully downscaled from a New Zealand-wide model to the high-resolution location domain. 

For currents, the Regional Ocean Modelling System (ROMS) is used to create a 10-year hydrodynamic hindcast using a three-step nesting approach, transferring the energy from a New Zealand-scale domain to a high-resolution local domain. 

The Delft3D model is used to simulate coupled wave, current and sediment transport. The modelling system uses boundary conditions from the regional wave (SWAN) and hydrodynamic (ROMS) models.

“Sediment transport depends on the particle size,” continues Simon. “To accurately predict the dispersal of sediments from the disposal site, we need reliable sediment grain size information. The settlement of larger particles, such as sandy sediments, is governed by gravitational forces, whereas for very fine sediments like silts and clays, inter-particle forces caused by ionic charges become significant. 

“When applying process-based models to predict morphological evolution, the main challenge is that the morphology of coastal systems develops over time scales several orders of magnitude larger than the time scale of the hydrodynamic fluctuations responsible for sediment transport, i.e. hours to days versus years to decades, and more. This means that a model system that can predict the time series of instantaneous hydrodynamics and sediment transport will require an unfeasibly long period to compute a multi-year real-time simulation. 

“To overcome this, we use the Morphological Acceleration Factor (MORFAC) method. MORFAC combines the reduction of the input forcing with the use of morphological factors. This means that we reduce the ambient wave and hydrodynamic forcings to a set of representative conditions that reproduce the same morphological evolution as the real-time forcing would. We do this by combining representative wave events with representative tides and current patterns for the site. 

Morphological Acceleration Factors (MORFACs) are used to represent the long-term effects of tides and waves on the seabed morphology within morphological models. Adapted from Ranasinghe et al. (2011).

Morphological Acceleration Factors (MORFACs) are used to represent the long-term effects of tides and waves on the seabed morphology within morphological models. Adapted from Ranasinghe et al. (2011).

“To account for long-term sediment movement as well as that resulting from storms, we generate two types of results: annual morphological simulations and historical storm simulations. The annual simulations provide information on the net sediment dispersion around the disposal mound each year. The real historical storm simulations model the detailed wave, circulation and sediment transport patterns that develop during energetic events, which normally cause the most significant morphological changes.

“Our modelling provides a best estimate of sediment dispersal from the disposal site over time, taking into consideration both day-to-day currents and waves and storm conditions which move large amounts of sediment in one go. The actual sediment transport may differ slightly depending on the timing of storms, but the overall pattern of sediment movement will remain valid. 

“Regulators use the information to determine the potential effects of the dredge disposal on local hydrodynamic processes, and to ensure there is minimal impact on sites of particular ecological or amenity value. Knowing where sediments will end up helps the mitigation, monitoring and management of dredging operations.”

For more information on dredge disposal modelling, contact us at enquiries@metocean.co.nz.
 

Mean wave, current velocity and sediment transport fields for a disposal site for Lyttelton Port off the Canterbury coast in New Zealand. The black polygons show the proposed disposal grounds and channel.

Mean wave, current velocity and sediment transport fields for a disposal site for Lyttelton Port off the Canterbury coast in New Zealand. The black polygons show the proposed disposal grounds and channel.