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We describe and demonstrate algorithms for treating cohesive and mixed sediment that have been added to the Regional Ocean Modeling System (ROMS version 3.6), as implemented in the Coupled Ocean Atmosphere Wave Sediment-Transport Modeling System (COAWST Subversion repository revision 1179). These include: floc dynamics (aggregation and disaggregation in the water column); changes in floc characteristics in the seabed; erosion and deposition of cohesive and mixed (combination of cohesive and non-cohesive) sediment; and biodiffusive mixing of bed sediment. These routines supplement existing non-cohesive sediment modules, thereby increasing our ability to model fine-grained and mixed-sediment environments. Additionally, we describe changes to the sediment bed-layering scheme that improve the fidelity of the modeled stratigraphic record. Finally, we provide examples of these modules implemented in idealized test cases and a realistic application. [ABSTRACT FROM AUTHOR]

Desorption rates for sediment-associated contaminants during resuspension events are investigated for a site on the Palos Verdes shelf, California. This study focuses on p,p′-DDE, the most abundant DDT compound present in an effluent-affected near-surface sediment deposit on the Palos Verdes shelf. Frequency, magnitude, and duration of resuspension events are estimated from bottom boundary layer measurements and a 17-yr record of surface wave conditions. Desorption of p,p′-DDE from sediment in suspension is calculated using a radial diffusion model for porous aggregates. Suspended sediment concentrations and size distributions during resuspension events are found using a shelf sediment-transport model. Calculations are made for a 60-m site where measurements of near-bed flow and sediment properties, including p,p′-DDE concentration and percent organic carbon by size fraction, are available. The results indicate that the time scales of p,p′-DDE desorption from the finest sediment fractions (<20 μm) are typically much shorter than the duration of resuspension events, which average 1.6 days at the study site, whereas desorption time scales are almost always much longer than event duration for the coarser size fractions (>63 μm); the median bed grain size is about 45 μm. Our calculations suggest that 25–50% of the mass of p,p′-DDE in the surface active layer of the bed is lost during a resuspension event; mean active layer depth is 0.2 cm and mean p,p′-DDE concentration in the uppermost 2 cm of the bed at the study site is 11 μg−1. Total annual loss depends on the number of events per year. Wave data indicate an average of 10 resuspension events per year at a depth of 60-m on the Palos Verdes shelf from 1982 to 1999. This leads to an estimated average loss of p,p′-DDE by desorption during wave-driven resuspension of 3–7 μg cm−2 yr−1 for the 60-m site. The range in average loss rates is due to the large range in published values of the partition coefficient for DDE. [Copyright &y& Elsevier]

Particle settling velocity and erodibility are key factors that govern the transport of sediment through coastal environments including estuaries. These are difficult to parameterize in models that represent mud, whose properties can change in response to many factors, including tidally varying suspended sediment concentration (SSC) and shear stress. Using the COAWST (Coupled Ocean-Atmosphere-Wave-Sediment Transport) model framework, we implemented bed consolidation, sediment-induced stratification, and flocculation formulations within an idealized two-dimensional domain that represented the longitudinal dimension of a micro-tidal, muddy, partially mixed estuary. Within the Estuarine Turbidity Maximum (ETM), SSC and median floc diameter varied by a factor of four over the tidal cycle. Downstream of the ETM, the median floc size and SSC were several times smaller and showed less tidal variation (~20% or less). The suspended floc distributions only reached an equilibrium size as a function of SSC and shear in the ETM at peak tidal flow. In general, flocculation increased particle size, which reduced SSC by half in the ETM through increased settling velocity. Consolidation also limited SSC by reduced resuspension, which then limited floc growth through reduced SSC by half outside of the ETM. Sediment-induced stratification had negligible effects in the parameter space examined. Efforts to lessen the computation cost of the flocculation routine by reducing the number of size classes proved difficult; floc size distribution and SSC were sensitive to specification of size classes by factors of 60% and 300%, respectively. [ABSTRACT FROM AUTHOR]

Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures and because of rapid sedimentation have the potential to impact global biogeochemical cycling. The Ayeyarwady Delta in Myanmar ranks as the world's third largest river delta in terms of sediment supply; however, modern increases in regional anthropogenic impacts risk severe alteration to sediment and TerrOC loads within this major system. By investigating modern sediment and terrestrial organic carbon (TerrOC) accumulation within the offshore Ayeyarwady Delta this study estimates baseline sediment and TerrOC budgets for this understudied mega-delta. Using 210Pb geochronology of 27 sediment cores collected from the continental shelf, we estimate that 405 (+52/-47) Mt of sediment, or ~70‐80% of fluvial sediment discharged from the Ayeyarwady and Thanlwin rivers (the main inputs to the delta), accumulates there annually. Sediment not retained on the shelf is likely partitioned between the Ayeyarwady floodplain, shoreline accretion, and minor deep-sea export. Estimates of TerrOC (based on δ13C mixing models) were coupled with modern sediment accumulation rates to determine an annual burial of 1.93 (+1.09/-0.15) Mt C on the shelf, with TerrOC burial fluxes being highest in the foreset beds of the subaqueous delta, coincident with the area of highest sediment accumulation rate. Based on estimates of the Ayeyarwady and Thanlwin rivers' TerrOC delivery, an apparent ~100% of TerrOC input is preserved on the continental shelf. However, an across shelf trend of increasing TerrOC degradation with distance offshore is also observed, indicating that while the shelf has high apparent TerrOC sequestration, carbon remineralization is also occurring prior to deposition within the subaqueous delta. Based on these conflicting outcomes, we suggest that input of TerrOC from additional sources other than the Ayeyarwady and Thanlwin rivers roughly balance the observed carbon remineralization. Main additional sources of TerrOC include the Sittang and several smaller rivers, and the Ayeyarwady delta plain below the river gauging station. As anthropogenic development within the Ayeyarwady and Thanlwin watersheds continues to increase, these sediment and TerrOC budgets provide a baseline from which future changes within the offshore Ayeyarwady Delta can be monitored. • ~80% of the Ayeyarwady-Thanlwin sediment load is trapped on the continental shelf. • Terrestrial organic carbon (wt%) decreases with increased distance offshore, suggesting significant remineralization. • High apparent terrestrial organic carbon sequestration is attributed to unaccounted inputs, e.g. the Ayeyarwady floodplain. • The subaqueous clinoform is the primary depocenter for Ayeyarwady-Thanlwin sediment and particulate organic carbon. [ABSTRACT FROM AUTHOR]

Abstract: The Waiapu River sedimentary system, New Zealand, provides a prototype for investigating the relative importance of wave- versus current-supported gravity flows on continental shelf deposition. A two-dimensional model was used to represent gravity-driven sediment transport and deposition on the Waiapu shelf over an annual cycle of storm events and associated Waiapu River floods. Model inputs of waves and wind-driven currents were derived from WAVEWATCH III hindcasts and constrained by benthic tripod data. The 12-month model run included a low-energy period (September 2003 to May 2004) with weak waves and currents and low river discharge, and a high-energy period (May to August 2004) with stronger waves and wind-driven currents and more frequent river floods. Model results suggested that during the low-energy period, riverine sediment was trapped between the 20- and 80-m isobaths. During the high-energy period, sediment was deposited obliquely across the shelf between the 60- and 120-m isobaths. The predicted deposit locations for the low- and high-energy periods, respectively, were consistent with short- and long-term observed accumulation patterns based on 7Be and 210Pb activity [Kniskern, T.A., Kuehl, S.A., Harris, C.K., Carter, L., 2010. Sediment accumulation patterns and fine-scale strata formation on the Waiapu River shelf, New Zealand. Marine Geology 270, 188–201]. Gravity flows were mainly wave-supported landward of the 60-m isobath, but became increasingly current-supported as wave orbital velocity attenuated in deeper water. Both analytical theory and numerical results indicated that wave-supported gravity currents were sensitive to local water depth and favored deposition parallel to isobaths as depth increased. In contrast, current-supported gravity currents were more sensitive to spatial variations in seabed slope, with seaward decreases in slope and along-shelf embayment of bathymetry favoring transport convergence and deposition. We conclude that the longer term (∼100yr) shelf-oblique mud deposit on the Waiapu shelf mainly reflects current-supported gravity flows responding to local variations in seabed slope and curvature of isobaths. [Copyright &y& Elsevier]

An ephemeral oceanic-flood deposit adjacent to a well-studied small mountainous river (SMR), the Waipaoa River in northeastern New Zealand, was characterized using multiple proxies, including radioisotopes ( 234 Th, 7 Be, and 210 Pb), bulk organic carbon abundance and isotopic signature (%OC, δ 13 C), as well as a biomarker of terrigenous organic matter (lignin). Field sampling was conducted within two weeks after a 1-in-8 year flood that occurred between January 30 and February 6, 2010. Geochemical analyses indicated that initial deposition of fresh riverine material extended alongshore to the north and south from the river mouth. A comparison of prior- and post-flood 7 Be inventories revealed that flood sediments were widely dispersed between 20 and 70 m water depth, accounting for 50–80% of the estimated flood load. Surface (0–2 cm) isotopic carbon values increased with distance from Poverty Bay, positively correlating with total 210 Pb activities, potentially reflecting increasing marine influence with water depth. Abundances of sedimentary organic carbon (OC) were 0.18–0.76% dry weight, and the total nitrogen varied from 0.02 to 0.13%. Stable isotope signatures of carbon (δ 13 C OC ), nitrogen (δ 15 N), and lignin abundances (λ 6 ) throughout the study area ranged from −23.6 to −27.7‰, 1.9 to 5.3‰, and 0.93 to 9.0 mg 100 mg OC −1 , respectively. The spatial distribution pattern of terrigenous organic matter (OM) abundance and interclass ratios (indicative of freshness of organic matter) varied along and across-shelf. Lignin abundances were high and interclass ratios were low in the southern depocenter and inner shelf areas, suggesting that this zone had recently received vascular-plant enriched OM, minimally altered by shelf-bed mixing processes. In contrast, sediments in the northern depocenter and outer shelf also contained elevated amounts of terrigenous sedimentary OM, but this material was generally lower in lignin abundance and had higher interclass ratios (greater degradation). Collectively, these results suggest that the flood-derived sediment and fresh terrigenous OM were mostly constrained between 20 and 70 m water depth, with enhanced deposition overlapping the tectonic-controlled depocenters located to the northeast and southeast of Poverty Bay. [ABSTRACT FROM AUTHOR]