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Sediment Dispersal Offshore Of Small Mountainous Rivers

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Courtney K. HarrisIn collaboration with:Aaron Bever, Carl Friedrichs, Tara Kniskern, Julia Moriarty, Yanxia Ma, Jesse McNinch, and Don [email protected]/23/2011 1Waipaoa River; 15MT/Yr ± 25%Waiapu River; 35MT/Yr ± ?Sediment Dispersal Offshore Of Small Mountainous Rivers: Insights from Numerical ModelsFrom source to sink: triggers on the inner shelf?• High sediment yields.• River mouth often exposed to oceanographic forces.• Flood plume might carry sediment directly to mid-shelf depocenters(non-stop delivery).• Sediment delivery during significant events may overwhelm inner shelf: cross-shelf flux seems limited.• Sediment trapped, deposited, or somehow remains on inner shelf. • Eventually something triggers cross-shelf flux (gravity flow induced by waves or currents).1/23/2011 2Figure courtesy of JP Walsh, ECUSediment delivery by small mountainous rivers• Approach: • Three-dimensional.• Include water column and sediment bed. • Coupled sediment transport and physical oceanographic models.• Account for:• Waves and currents. • Suspended transport.• Bed armoring.• Fluvial input.• Gravity flows (*). 1/23/2011 3Grid for ROMS – SWAN Waipaoa shelf model under development by J. Moriarty (VIMS)WaterSeabedSediment Transport Model1/23/2011 4OBJECTIVES Contrast delivery mechanisms for the nearshore and inner shelf of three small mountainous river systems (the Eel, Waiapu, and Waipaoa Rivers).  Illustrate capabilities of event- to seasonal-timescale three-dimensional numerical models for evaluating sediment dispersal within the coastal zone of continental margins.Eel River shelf• Muddy deposits on mid-shelf.• Flood plume delivers sediment to inner shelf.• Wave supported gravity flows deliver material to mid-shelf. • Plan view of model1/23/2011 5Harris et al. 2004, 2005Cross-shelf flux offshore of Eel River• Sediment delivery to mid-shelf determined by wave energy.• Thickness of mid-shelf flood deposits function of wave energy. 1/23/2011 6Harris et al. 2004, 2005.•210Pb from Kniskern et al. 2010.• Inner shelf storage (Wadman and McNinch, 2008).• Accumulation rates peak ~80 m isobath.• Cross-shelf transport mechanisms?1/23/2011 7Waiapu River mid-shelf depositFigure from Kniskern et al. 2010Wave supported gravity flows carry sediment to mid-shelf• Wave – supported gravity flows: Model estimated flux to 30 – 40 m isobath(Kniskern, 2008). 1/23/2011 8Figure from T. KniskernWaiapu River Shelf178.4° 178.6° 178.8° 179°-38.2°-38°-37.8°-37.6°North Island, NZWaiapu RiverWaiapu RiverTripods2040300 10 kmN• Tripods deployed at 40 and 60 m water depth.• Current supported gravity flows carry sediment across-shelf (Ma et al. 2008). • Post-flood turbidity triggered by cross-shelf currents.1/23/2011 9Figure from Y. Ma01100220005010002402400.30.606/01 06/10 06/19 06/28 07/07 07/16-0.200.30.6Date in 20041/23/2011 10a) Water Discharge (m3/s, ) and TSS (g/l, )b) Near-bed Sediment Concentration (g/l)c) RMS Wave Height (m, ) and Current Speed (m/s, )d) Across-shelf Velocity (m/s)Seaward60-m YSI, 91 cmab60-m ADV, 42.5 cmab60-m ADV, 42.5 cmabProvided by NIWAFrom Ma, et al. in prep.Cross-shelf flux: strong waves and seaward currentsFloodResuspension178.4 178.5 178.6 178.7-38-37.95-37.9-37.85-37.8-37.75-37.7-37.65 Waiapu20406080100120140Deposit thickness (cm)012341/23/2011 11From Ma et al. 2010.Current supported gravity flows move sediment offshore to 80 m isobath.Waipaoa River shelf• Sediment delivered to Poverty Bay.• Bed stresses relatively low in Poverty Bay.1/23/2011 12Miller and Kuehl, 2010J. Moriarty’s PosterSediment export during flood pulse, and wave resuspension• ROMS model estimates of suspended sediment flux.• Top panel: Non-stop to the shelf during flood pulse.• Bottom panel: Later export to mid-shelf, triggered by energetic swell waves. about one week after floods. From Bever dissertation (2010), in prep. as Bever and Harris.1/23/2011 131/23/2011 14Conclusions1/23/2011 15Three systems considered: each had different triggers for sediment delivery to shelf.Coherence does not guarantee delivery, dispersal, and deposition to be in phase.Shallow coastal areas act to modulate sedimentary signals on continental margins.Currents, as well as waves and fluvial load, can create gravity flows on continental shelves, thereby triggering cross-shelf flux.References1/23/2011 16Bever, A.J. 2010. Integrating Space and Time-Scales of Sediment-Transport for Poverty Bay, New Zealand. Ph.D. Dissertation, School of Marine Science, College of William and Mary, Gloucester Point, VA, USA. 263 PP.Harris, C.K., P. Traykovski, and W.R. Geyer. 2004. Including a near-bed turbid layer in a three dimensional sediment transport model with application to the Eel River shelf, northern California. Estuarine and Coastal Modeling; Proceedings of the Eighth International Conference. M.L. Spaulding, et al. (editors), American Society of Civil Engineers. 784—803. Harris, C.K., P. Traykovski, and W.R. Geyer. 2005. Flood dispersal and deposition by near-bed gravitational sediment flows and oceanographic transport: A numerical modeling study of the Eel River shelf, northern California. Journal of Geophysical Research. 110(C09025): d0i: 10.1029 / 2004JC002727.Kniskern, T.A., 2007. Shelf sediment dispersal mechanisms and deposition on the Waiapu River Shelf, New Zealand. Ph.D .Dissertation, School of Marine Science, College of William and Mary, Gloucester Point, VA, USA. 283 PP.Kniskern, T.A., S.A. Kuehl, C.K. Harris, and L. Carter. 2009. Sediment accumulation patterns and fine-scale strata formation on the Waiapu River shelf, New Zealand. Marine Geology, 270 (1-4): 188-201.Ma, Y., Wright, L.D., Friedrichs, C.T., 2008. Observations of sediment transport on the continental shelf off the mouth of the Waiapu River, New Zealand: evidence for current-supported gravity flows. Continental Shelf Research, 28, 516-532.Ma, Y. 2009. Continental shelf sediment transport and depositional processes on an energetic, active margin: the Waiapu River Shelf, New Zealand. Ph.D .Dissertation, School of Marine Science, College of William and Mary, Gloucester Point, VA, USA. 206 PP.Ma, Y., C.T. Friedrichs, C.K. Harris, and L.D. Wright. 2010. Deposition on the Waiapu, New Zealand, continental shelf by seasonal wave- and current-supported sediment gravity flows interacting with spatially varying


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