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The formation of the geologic record offshore of small mountainous rivers is event-driven and, more so than many other environments, can result in relatively complete sequences. One such river, the Waipaoa in New Zealand, has been studied from its terrestrial source to its oceanic sink over timescales spanning storms, seasons, and the Holocene. This study focused on the formation of riverine deposits on the Waipaoa Shelf during episodic flood and wave events, contrasting deposition during short-lived events to accumulation patterns created over thirteen months. Sediment fluxes and fate were estimated using the numerical hydrodynamic and sediment transport model ROMS, the Regional Ocean Modeling System, using CSTMS, the Community Sediment Transport Modeling System. During the study period (January 2010–February 2011), the model indicated that initial flood deposition generally occurred near the river mouth and along the coast in water shallower than 40 m, and that deposition during any one event was sensitive to variations in shelf currents and wave energy. Also, the sedimentation due to plume settling and suspended transport during these relatively short flood and wave events were not aligned with longer time-scale accumulation patterns (months or greater) previously reported for the Waipaoa shelf. In the days to months following a flood pulse, waves episodically reworked this initial deposit, resuspending centimeter-scale layers of sediment during energetic periods. Frequent and intense resuspension occurred in shallow areas where bed stresses were high. This encouraged redistribution of material toward deeper areas having lower near-bed wave stresses, including continental shelf depocenters and offshore areas. While fast settling material was preferentially retained near the river mouth, currents dispersed slower settling sediment farther before deposition. Overall, accumulation depended on characteristics of oceanographic transport (wave energy, current velocities), not just source characteristics (flood size, sediment size distribution). [ABSTRACT FROM AUTHOR]

A one-year study examining water-column and near-bed time-series observations from benthic-boundary-layer tripods was undertaken in the Waipaoa Sedimentary System (WSS) on the Poverty Bay continental shelf, offshore of the Raukumara Ranges, New Zealand. Here, the nature of the adjacent post-glacial shelf deposits, fed by the Waipaoa River, and century-scale sediment accumulation is well established, but event-scale sediment dispersal and hydrodynamics had not been measured. Data from two tripods outfitted with acoustic and optical instrumentation, and x-radiograph images of seabed cores collected at 4-month intervals, were analyzed to quantify progressive sedimentological changes, and to document processes responsible for seabed strata formation, destruction, and preservation. At the Northern Tripod (NT) mid-shelf location 15 km southeast of the river mouth, we observed increases in near-bed suspended sediment concentration when shear velocities exceeded 0.03 m s −1 on 33 occasions over an 11.5-month period. Of these events, 9 were depositional, 10 were erosional, and 14 had no bed-elevation change at the 1-cm resolution of our instrumentation. Relatively high (2–5 m) and short-period (<11–12 s) waves, combined with current velocities <0.15 m s −1 , led to deposition on the seabed. Smaller-height (2–3 m) and longer-period (11–13 s) waves with stronger current velocities (>0.15 m s −1 ) typically induced erosion. Despite the short reach of this mountainous riverine dispersal system, we observed a consistent delay between river discharge and apparent deposition on the mid-shelf at the NT location. Following the three largest floods, ~120 h of strong, wave-driven bed shear stress preceded measurable deposition on the mid-shelf, suggesting that event duration and intensity are key for transporting material offshore from Poverty Bay. The largest flux event recorded over the study year was a 130-hour-long storm and associated river flood that resulted in near contemporaneous deposition at the NT site. Suspended-sediment concentration data suggest transport by a wave-supported fluid mud. X-radiographs from the NT location corroborate the instrumental record of 5 cm deposition, but only the two largest depositional events were preserved in the strata. On the southern shelf we observed no evidence of deposition in either the instrumental data or in the x-radiographs of cores. This study suggests that event duration, and spatial and inter-annual variability are critical considerations when evaluating along- and across-shelf sediment transport and seabed stratigraphic evolution associated with discharge from small mountainous rivers. [ABSTRACT FROM AUTHOR]

The Waipaoa River Sedimentary System in New Zealand, a focus site of the MARGINS Source-to-Sink program, contains both a terrestrial and marine component. Poverty Bay serves as the interface between the fluvial and oceanic portions of this dispersal system. This study used a three-dimensional hydrodynamic and sediment-transport numerical model, the Regional Ocean Modeling System (ROMS), coupled to the Simulated WAves Nearshore (SWAN) wave model to investigate sediment-transport dynamics within Poverty Bay and the mechanisms by which sediment travels from the Waipaoa River to the continental shelf. Two sets of model calculations were analyzed; the first represented a winter storm season, January–September, 2006; and the second an approximately 40 year recurrence interval storm that occurred on 21–23 October 2005. Model results indicated that hydrodynamics and sediment-transport pathways within Poverty Bay differed during wet storms that included river runoff and locally generated waves, compared to dry storms driven by oceanic swell. During wet storms the model estimated significant deposition within Poverty Bay, although much of the discharged sediment was exported from the Bay during the discharge pulse. Later resuspension events generated by Southern Ocean swell reworked and modified the initial deposit, providing subsequent pulses of sediment from the Bay to the continental shelf. In this manner, transit through Poverty Bay modified the input fluvial signal, so that the sediment characteristics and timing of export to the continental shelf differed from the Waipaoa River discharge. Sensitivity studies showed that feedback mechanisms between sediment-transport, currents, and waves were important within the model calculations. [ABSTRACT FROM AUTHOR]

Abstract: Nearshore regions act as an interface between the terrestrial environment and deeper waters. As such, they play important roles in the dispersal of fluvial sediment and the transport of sand to and from the shoreline. This study focused on the nearshore of Poverty Bay, New Zealand, and the processes controlling the dispersal of sediment from the main source, the Waipaoa River. Hydrodynamics and sediment-transport in water shallower than 15m were observed from April through mid-September 2006. This deployment afforded observations during 3–4 periods of elevated river discharge and 5 dry storms. Similar wind, river discharge, wave, current, and turbidity patterns were characterized during three of the wet storms. At the beginning of each event, winds blew shoreward, increasing wave heights to 2–3m within Poverty Bay. As the cyclonic storms moved through the system the winds reversed direction and became seaward, reducing the local wave height and orbital velocity while river discharge remained elevated. At these times, high river discharge and relatively small waves enabled fluvially derived suspended sediment to deposit in shallow water. Altimetry measurements indicated that at least 7cm was deposited at a 15m deep site during a single discharge event. Turbidity and seabed observations showed this deposition to be removed, however, as large swell waves from the Southern Ocean triggered resuspension of the material within three weeks of deposition. Consequently, two periods of dispersal were associated with each discharge pulse, one coinciding with fluvial delivery, and a second driven by wave resuspension a few weeks later. These observations of nearfield sediment deposition contradict current hypotheses of very limited sediment deposition in shallow water offshore of small mountainous rivers when floods and high-energy, large wave and fast current, oceanic conditions coincide. Consistently shoreward near-bed currents, observed along the 10m isobath of Poverty Bay, were attributed to a combination of estuarine circulation, Stokes drift, and wind driven upwelling. Velocities measured at the 15m isobath, however, were directed more alongshore and diverged from those at the 10m isobath. The divergence in the currents observed at the 10 and 15m locations seemed to facilitate segregation of coarse and fine sediment, with sand transported near-bed toward the beach, while suspended silts and clays were exported to deeper water. [Copyright &y& Elsevier]

Measurements of waves, currents and sediment suspension from a 35-day deployment of three instrumented tripods on the wave-dominated shoreface off Tairua Beach, New Zealand, are presented. Two tripods were deployed at 22-m depth, one within a coarse-sand rippled scour depression (RSD), and the other nearby on the surrounding, relatively featureless fine-sand plain. A third tripod was deployed at 15-m depth, also on fine sand. To distinguish between suspended fine and coarse sand at each site, a method of simultaneously inverting two-frequency acoustic backscatter data was applied.Background, long-period swell was interrupted by two 6-day periods of high waves. At the 15-m/fine-sand site, only fine sand appeared in suspension. However, at the 22-m/fine-sand site, coarse sand appeared in suspension together with fine sand when the waves were highest. This suggests that coarse sand resuspended from the RSD can be dispersed on the adjacent fine-sand plain, at least along the same isobath. At the RSD, there was no coarse sand in suspension under background swell, but there was a fine-sand “washload”, presumably advected from the surrounding fine-sand plain, even though the coarse bed sediment in the RSD itself was not in motion. Under high waves, both fine and coarse sands were in suspension over the RSD. The fine sand appeared again as a washload, but the coarse sand was confined relatively close to the bed in accordance with a sediment diffusivity that decreased linearly with elevation above the bed.Measurements of fine-sand reference concentration over both fine-sand beds (15 and 22-m depth) plot against wave-induced skin friction in one cluster under the background swell and in another cluster under the high waves, which suggests a change in bedforms, possibly to a hummocky bed, that is related to wave conditions and that alters the suspension dynamics. The coarse-sand reference concentrations over the RSD fall into the “hummocky cluster”, which supports this interpretation.The fine-sand washload is indicative of an unfavourable settling environment over the RSD, which could constitute a positive feedback that causes RSDs to self-organize from initial perturbations. Under high waves, the suspended-sediment load impinging from the surrounding plain will be high, but turbulence will also be more energetic on the RSD, thus more effectively inhibiting deposition. Under low waves, deposition may be less inhibited on the RSD, but the suspended-sediment load arriving from the surrounding fine-sand plain will also be lower. Thus, the fine-sand deposition rate on the RSD will be small, even though conditions are more favourable for settling. Modelling could be expanded to consider the balance between inhibition of settling and tendency for burial by the sediment load that impinges from the area surrounding an RSD, and the way wave climate and water depth might mediate the basic self-organization process. Escape of coarse sand from RSDs under high waves may seed the growth of new RSDs. [Copyright &y& Elsevier]

Foveaux Strait has been commercially fished for oysters for over 100 years, focusing principally upon bryozoan biogenic reefs that once covered large areas of the strait. Bryozoan biogenic reefs consisted of linear swards of bryozoan patch reefs, paralleling the peak tidal current, and were formed by the frame-building bryozoan Cinctipora elegans and associated epifauna. Two side-scan surveys one in the late 1970s and the other in the late 1990s and estimates of distribution of oyster density and distribution of fishing effort, show how fishing has effected the distribution of habitat, oysters and sediments over the last 30 years. Fishers dredged the biogenic reefs for their oysters, damaging the framework structure, removing epifauna and exposing associated sediments, which were then reworked and transported down-current in the strong tidal flow. Large volumes of biogenic sediments released in this process formed thick sheets and dune bedforms. When dredging ceased locally (after removal of biogenic reef habitat and oysters), sediment supply downstream dwindled and ceased, bedforms diminished and the seafloor ultimately reverted to relict pebble gravel. Sediment derived from the dredging of biogenic reefs in northern Foveaux Strait was mostly transported and deposited in deeper water to the east, however, sediment derived from biogenic reefs in southern Foveaux Strait was transported in the opposite direction, giving rise to a group of large dunes in the southwest that in 1999 contained ∼58×106 m3 of coarse biogenic sediment. By 1998 none of the original bryozoan biogenic reefs remained.We interpret sediment distribution in Foveaux Strait in the light of these processes, and compare with earlier data to provide snapshots of human-induced modification of the benthic environment. We also explore evidence for regeneration of simple biogenic reefs in areas no longer fished, and discuss the conditions in which this has occurred. Communities dominated by byssally attached Modiolus appear to provide the early framework and shelter for development of new patch reefs. However the recent discovery of regenerating colonies of the reef-building bryozoan Cinctipora in eastern Foveaux Strait suggests that bryozoan patch reefs may also be capable of re-establishing where conditions are suitable. We favour helical circulation cells in the strongly linear tidal flow of Foveaux Strait as a mechanism for controlling, not only the linear aspect of the reef frameworks, but also for concentrating nutrients, propagules and the settlement of larvae. Our observations of regenerating epifaunal habitat on the fishery-modified seafloor suggest that careful management could reverse much of the deleterious effect of fishing. (4 0 6) [Copyright &y& Elsevier]

To identify the drivers of sediment deposition within a mangrove-lined river delta, we developed a three-dimensional numerical model in an idealized domain. Our model system is based on the Firth of Thames, a modern flat-fronted, bay-head deltaic system, in the Hauraki Gulf, in the North Island of New Zealand. The interactions of the buoyant river plume with mangroves are examined, including sediment deposition and erosion patterns. The morphological study presented here is restricted to cohesive sediments, whose patterns are observed after 4 months of morphological evolution. For the non-vegetated case, represented by a spatially uniform roughness, sediment deposited throughout the central model domain with a formation of a cuspate delta at the river mouth. However, in the vegetated case (with vegetated areas represented by an enhanced and spatially varying roughness coefficient), sediment deposition occurred in the regions of forests and flats, whereas the fringe areas experienced erosion. The sediment deposition patterns along cross-sections were qualitatively consistent with field observations from the Firth of Thames. Analysis of the across-transect and along-transect momentum balances demonstrates that the predominant balance between the bottom shear stress and baroclinic pressure gradient largely controls the sediment deposition in the riverine sections of the domain. However, close to the center of the plume, vertical advection promotes suspension of the sediments in the fringe region during the flood tide. • The presence of mangroves altered sediment deposition patterns. • Sediment deposition occurred in the forests and over the mudflats. • The fringe area between vegetation and bare mudflat experienced erosion. • Sedimentation patterns are explained by across- and along-shore momentum balances. [ABSTRACT FROM AUTHOR]