Modelling support for port modifications

Over recent years, MetOcean Solutions has modelled the effects of dredging for ports all over New Zealand and Australia. 

Using models we can estimate the percentage of time suspended solid concentration thresholds (here 10 mg/L) are exceeded during dredge spoil disposal operations. Inner and outer dashed circles radius: 500 and 1000 m, respectively. 

Using models we can estimate the percentage of time suspended solid concentration thresholds (here 10 mg/L) are exceeded during dredge spoil disposal operations. Inner and outer dashed circles radius: 500 and 1000 m, respectively. 

“Port modifications and maintenance often involve dredging,” explains Project Director Dr Brett Beamsley. “This can include deepening channels or berth pockets, or maintaining existing depths.” 

“Over the years, we have developed modelling processes to support ports and regulators evaluate the impacts from, and manage, port dredging in the best possible manner.” 

Dredging can have complex effects. It causes sediment release and temporarily increased levels of suspended solids. Dredge spoil disposal changes the shape of the seabed at the disposal site, and deepening channels can alter the waves and tidal flows in nearby areas, potentially affecting structures such as wharves and jetties, as well as processes like shoreline erosion. 

Source of a dredge plume for a Trailing Suction Hopper Dredger. 1. Drag head; 2. Overflow; 3. Prop wash. After Becker et al. (2015).  

Source of a dredge plume for a Trailing Suction Hopper Dredger. 1. Drag head; 2. Overflow; 3. Prop wash. After Becker et al. (2015).
 

The dispersal of sediments occurring during dredge disposal can be modelled.

The dispersal of sediments occurring during dredge disposal can be modelled.

Suspended solids can be harmful to marine life. To minimise adverse effects, managers need to know the extent and duration of suspended sediment plumes, and what levels of suspended solids to expect within the plumes. Because the plumes move with the water, hydrodynamic models can be used to predict their extent and duration.

Managers also need to know the effects of discharging sediment at offshore disposal sites. They need to ensure that dredge spoil is disposed at suitable sites, where potential changes to the seabed shape will not cause adverse effects on flow, tidal flushing and local wave climate, and that disposed sediments will not be carried to nearby areas of environmental or amenity importance.

MetOcean Solutions employs a multi-disciplinary team of scientists with a wide range of expertise in port studies and is therefore well placed to provide scientific advice to ports. Two of the company’s founding directors did PhD research on the wave and sediment dynamics at New Zealand ports.

“We use a variety of techniques to get the modelling spot on,” continues Dr Beamsley. “As a starting point, we collate all information we can find for the site, including any wave or current measurements. Additional field data is often collected to ensure that important flow dynamics are captured.” 

“Once we have enough data, we set up a series of models. First, we develop regional and local-scale wave and hydrodynamic models which capture important processes responsible for sediment entrainment and transport. We nest these into larger-scale wave and hydrodynamic models to ensure atmospheric forcing and river discharges are taken into account where necessary. We calibrate and validate the models against all available measured data.

“Once we’re satisfied our models are performing well, we generate a historical data set for the location, detailing wind, waves and currents for multiple years. This historical data set is used to define the wave and hydrodynamic climate of the site and is used as input into all subsequent studies.

“We simulate dredging plumes using models that track sediment particles as they disperse from the site of dredging. Dredging typically releases sediments near the seabed and just below the water surface. Following an initial near-field phase, dredge plumes move with current flows. Over time, the particulates settle out, with larger grain sizes settling faster and finer sediments being transported further, although flocculation needs to be considered. Through modelling the evolution of the plume over time, our oceanographers simulate the particulates as they settle on the seabed, and trace the levels of suspended solids that remain in the water column. 

Example of plume modelling, showing average near-seabed suspended solid concentrations (SSC) for Lyttelton Harbour, New Zealand.

Example of plume modelling, showing average near-seabed suspended solid concentrations (SSC) for Lyttelton Harbour, New Zealand.

“The dispersal of dredge spoil disposal is investigated using high-resolution morphological models, which account for the surficial sediment grain size within the disposal ground. Using these morphological model we simulate the development of the disposal ground over multiple years to decades, modelling how existing flow patterns will affect the disposal mound and how the mound will affect circulation. As part of this, we determine the likely length of time that discharged sediment resides at the offshore disposal ground and highlight sediment transport pathways, determining where the disposed sediments are likely to ultimately end up.”

After five years the disposed sediments at this site have dispersed. The figure shows the cumulative morphological changes after each year over a 5-year morphological simulation of the disposal of 18 million m3 sediment onto a 12.5 km2 disposal ground. Initial bathymetric contours are shown in black. A positive magnitude (yellow and red colours) indicates sedimentation.

After five years the disposed sediments at this site have dispersed. The figure shows the cumulative morphological changes after each year over a 5-year morphological simulation of the disposal of 18 million m3 sediment onto a 12.5 km2 disposal ground. Initial bathymetric contours are shown in black. A positive magnitude (yellow and red colours) indicates sedimentation.

For a discussion about how MetOcean Solutions can help you predict and manage the effects of your dredging operations, contact us at enquiries@metocean.co.nz.