Rivers discharge around 20 billion tonnes of sediment to the coastal ocean each year (Milliman and Syvitski, 1992). Many supply dominated rivers possess large subareal and subaqueous deltas, whose growth depends on the physical processes controlling sediment deposition or removal. There has yet to be a complete understanding of how short term processes such as settling from buoyant plumes, wave/current resuspension, and transport by currents and gravity flows, interact to produce the depositional products that develop over varying timescales. The Po River in Northern Italy forms a substantial delta in the Adriatic Sea. It has five distributary mouths, and is responsible for a third of all freshwater and sediment entering the Adriatic. Near-field sediment transport is influenced predominantly by Po River floods and two dominant wind regimes. Cold, dry Bora winds blow from the northeast, while warm, wet Siroccos blow from the southeast. Both of these winds produce waves large enough to resuspend sediments off of the Po delta.
A three-dimensional numerical modeling study was conducted using the Regional Ocean Modeling System (ROMS) examining sediment transport near the Po delta. ROMS includes sediment settling, wave/current resuspension and transport, and multiple grain types. Simulations and idealized test cases were run. The simulation spanned September, 2002 – March, 2003, and included a large flood event. The idealized cases examined distinct Bora and Sirocco events. Meteorological forcing was based on realistic wind and wave fields. Fluvial sediment was modeled using both flocculated and unflocculated sediment types. A stretched grid with ~750m resolution over the Po delta was used. Compared to earlier models, this increases resolution at the delta and more accurately resolves near-field sediment dynamics.
This study indicates that high initial settling significantly reduces the influence of buoyant plume transport on sediment fluxes. Most sediment flux, and export from the delta, occurs within a few meters of the seabed, and is driven by the wave action and near-bed currents generated during wind events. Time-averaged currents dictate the direction of net sediment flux. Under idealized conditions Sirocco winds transport sediment predominantly southward, while under Bora winds relatively more sediment is transported east, and northeast, of the delta. Time-variation in the simulation conditions, however, generates significant southward fluxes during more realistic Bora conditions. Most sediment is deposited directly off the river mouths, with the remaining depositional patterns dictated by current flow and bed shear stress patterns during wind events. The footprint and total sediment mass of the 2002 flood deposit is reasonably well represented, taking limitations of the observational sampling methods into account, specifically, by limiting the model’s predicted deposit to that >5mm thick and within water depths >10m. Predicted maximum depositional thicknesses, however, are ~2-5 times those observed. Fluvial sediment deposition predicted for typical meteorological conditions is consistent with accumulation patterns that develop over much longer timescales. The long-term depositional patterns off the Po delta are similar to those produced by Bora winds, implying that the ~100 year accumulation pattern on the Po delta may be predicted through the modeling of idealized Bora events.
| Identifer | oai:union.ndltd.org:wm.edu/oai:scholarworks.wm.edu:etd-2973 |
| Date | 01 January 2006 |
| Creators | Bever, Aaron J. |
| Publisher | W&M ScholarWorks |
| Source Sets | William and Mary |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations, Theses, and Masters Projects |
| Rights | © The Author |
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