The proposed research will examine sediment transport and deposition under varying fluvial discharge and oceanic conditions on the Waipaoa River shelf, New Zealand, and combined with work done at other river-shelf sites along the US West coast margin, create a framework to explain the differences and similarities amongst these river systems and assess transport and depositional patterns on as yet unexamined river-shelf systems.Objectives are to investigate the conditions under which preservable flood and storm beds form on the river shelf systems and to create a conceptual framework to predict preservation potential of flood and storm beds on margins.The study will entail rapid response sampling, experimental numerical modeling of hydrodynamics and sediment dynamics, and a global-scoped statistical analysis of the fluvial and oceanic conditions during sediment transport on shelf systems. Broader Impact include the support of a new, young female post-doctoral researcher and involvement of undergraduate and graduate students. The results could be useful to wide range of workers concerned with the coastal research and policy issues.
To better understand how small rivers (< 15,000 km2) with steep basins deliver sediment to the ocean during floods and how and where that sediment is deposited in the ocean, a post-doctoral researcher, Tara Kniskern, investigated rivers in New Zealand and along the US West Coast. The objectives were to investigate the conditions under which preservable flood and storm beds form on the continental shelf systems and assess their preservation potential. The proposed work built on the strength of previous research in both locations, utilized data bases from the US Geological Survey and National Oceanic and Atmospheric Administration, and strengthened collaborative relationships between several domestic academic institutions and New Zealand’s National Institute for Water and Atmospheric Research. Additionally, opportunities were presented during the course of this study to advance discovery, understanding, and training for the post-doctoral researcher this project funded, as well as 3 graduate students, an undergraduate student, and York High School, VA students enrolled in Earth Science. Thus far, results from this study were presented at international meetings and in a peer-reviewed journal. The study entailed rapid-response sampling after a flood, experimental numerical modeling of ocean circulation and sediment dynamics, and statistical analyses of the riverine and oceanic conditions during floods. In the first part of the study, sediment samples were collected from the Waipaoa River shelf in New Zealand after a flood in January 2010 (Fig. 1). Sediment cores were x-radiographed and then sub-sampled for radioisotopic and geochemical analyses. A flood deposit was identified on the Waipaoa River shelf confined to water depths less than 70 m (Fig. 2). Radioisotopic and geochemical results indicated that sediments were from a terrestrial source, but also reflected a combination of sources from within the river basin (Fig. 3). The source and quality of the organic material changed over the duration of the flood, and was most likely the dominant driver of spatial differences. Finally, a comparison of the results with samples collected by colleagues after the January, 2010 flood indicated that high waves that occurred more than a month later resuspended and redistributed the flood sediments on the shelf. The second part of the study focused on analyzing the similarities and differences among small rivers with mountainous basins. To accomplish this, the post-doctoral researcher worked with colleagues to analyze decades of river discharge, wave, and wind data from New Zealand and the US West Coast. Results indicated that several of these rivers flood into the open ocean within hours of each other in response to the same storm system (Fig.4). This occurs because the size of the storms are much larger than the rivers. Numerical modeling experiments were then used to show that when multiple rivers discharge simultaneously, the freshwater plumes commingle, strengthening distribution of flood sediment both along- and across-shelf (Table 1). As a result, more sediment was deposited on the shelf than estimated when modeling a single river. Additional experiments indicated that seasonal patterns in the hourly timing of storm waves and floods influence sediment distribution and deposition. Large storm waves, which typically occur in winter months, result in thicker, but localized deposits. Understanding how flood sediments are distributed and deposited on continental shelves is important to resolving the global carbon cycle, determining coastal abatement strategies, explaining ancient sedimentary bedding structures, and is of interest to the oil industry. Therefore, hourly time-series data from rivers and near-shore coastal environments are extremely useful. Over the last several decades, federal funding of the USGS river monitoring system has declined, resulting in fewer rivers monitored. Furthermore, this study indicated a distinct spatial scale of interest commensurate with regional flooding for engineers considering coastal abatement and beach replenishment. Continental shelf environments are increasingly over-fished, polluted, and subject to seasonal hypoxia, influencing fisheries health. Since nutrients and pollutants are adsorbed to sediments, and thick flood deposits also contribute to organism stress, understanding transport in these important economic zones can influence policy makers. The results from this project directly contribute to understanding the distribution of sediment and associated nutrients and pollutants.
