Redefining sediment transport models
over sensitive benthic habitats
DREDGING SCIENCE NODE
PROJECT 2/3.3 – RYAN LOWE, MARCO GHISALBERTI , ANDREW
POMEROY, TA J SARKER, MIKE CUT TLER, GRAHAM SYMONDS
Canopies modify
sediment transport
• Most coastal ecosystems have benthic
canopies (e.g., coral reefs, seagrass
meadows, mangroves, etc.)
• Reduced bed shear stresses in canopies
→ reduced sediment transport
Seagrass canopy
• Sediment transport formulations
accounting for canopies are lacking
• Existing sediment transport models
based on open (bare) sediment can
grossly underestimate sediment
deposition within canopies
Coral reef canopy
Pilbara dredging project
Project objectives
(1) Provide direct field and controlled laboratory measurements of
sediment fluxes, deposition and resuspension rates within benthic
canopies.
(2) Develop new empirical relationships of sediment deposition and
erosion rates applicable to a range of habitat types (e.g. coral reefs,
seagrasses, etc.).
(3) Incorporate and validate the new sediment transport routines
applicable to a range of habitat types within a practical
hydrodynamic-sediment transport numerical model (e.g., Delft3D).
Initial field experiments: e.g., Ningaloo Reef
Sediment fluxes in the reef bottom boundary layer
Pump sampling
Field instrument array
Sawhorse /
scaffolding
• Andrew Pomeroy PhD Project
• Detailed field measurements of:
‒ Turbulence (bed stresses), sediment concentrations, and sediment fluxes over reef canopies
‒ Broad (reef-scale) observations sediment concentrations and reef hydrodynamic processes
Suspended sediment properties and
transport thresholds
Bed sediment grain size
 can threshold
• Existing sediment transport models predict bed sediment
(medium-coarse sand) should be mobilised for the  can
• Only much finer sediment (coarse silt) is transported →
consistent with much lower  bed
• Reef roughness substantially reduces sediment transport
Suspended sediment grain size
 bed
threshold
Preliminary laboratory experiments
• Experiments conducted in a large-scale wave-current flume at UWA
• Model sediment – uniform glass spheres (250 mm then 75 mm)
• Model canopy – array of rigid cylinders (variable packing density), emergent & submerged
• Coupling measurement of spatially-averaged erosion rates with measurement of bed
stresses and near-bed turbulence
Sediment motion thresholds and erosion rates
(a) Threshold velocity for incipient motion
(b) Spatially-averaged erosion rate
Summary and future work
• Field experiments of sediment transport processes over coral reefs
have been completed with analysis underway. Future field work over a
seagrass meadow (late 2015)
• Controlled laboratory experiments (unidirectional flow) are revealing
the detailed mechanics of transport. Future work will assess wavedriven transport
• Future work will focus on using these comprehensive datasets to
develop new sediment transport formulations that account for benthic
roughness/canopies
• These will be embedded within process-based hydrodynamic-sediment
transport numerical models
Acknowledgments
Woodside, Chevron, BHP and WAMSI partners for partially funding this
research
Additional funding provided by an Australian Research Council Future
Fellowship and the ARC Centre of Excellence for Coral Reef Studies
Field assistance from Mike Cuttler, Gundula Winter, Jim Falter, Sana
Dandan, Leonardo Ruiz Montoya, Jeff Hansen
Dredging Science Node