Shifting sediment dynamics in the Coos Bay Estuary in response to 150 years of modification

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(1), (2021): e2020JC016771, https://doi.org/10.1029/2020JC016771.Estuaries worldwide have experienced modifications including channel deepening and intertidal reclamation over several centuries, resulting in altered fine sediment routing. Estuaries respond differently based on preexisting geometries, freshwater and sediment supplies, and extents and types of modification. The Coos Bay Estuary in Oregon is a relatively small estuary with complex geometry that has been extensively modified since 1865. A sediment transport model calibrated to modern conditions is used to assess the corresponding changes in sediment dynamics. Over ∼150 years, channel deepening (from ∼6.7 to 11 m), a 12% increase in area, and a 21% increase in volume have led to greater tidal amplitudes, salinity intrusion, and estuarine exchange flow. These changes have reduced current magnitudes, reduced bed stresses, and increased stratification, especially during rainy periods. Historically, fluvially derived sediment was dispersed across broad, deltaic‐style flats and through small tidal channels. Now, river water and sediments are diverted into a dredged navigation channel where an estuarine turbidity maximum (ETM) forms, with modeled concentrations >50 mg/L and measured concentrations >100 mg/L during discharge events. This “new” ETM supplies sediment to proximal embayments in the middle estuary and the shallow flats. Overall, sediment trapping during winter (and high river discharges) has increased more than two‐fold, owing to increased accommodation space, altered pathways of supply, and altered bed stresses and tidal asymmetries. In contrast to funnel‐shaped estuaries with simpler geometries and river‐channel transitions, these results highlight the importance of channel routing together with dredging in enhancing sediment retention and shifting pathways of sediment delivery.The Science Collaborative is funded by the National Oceanic and Atmospheric Administration and managed by the University of Michigan Water Center (NAI4NOS4190145).2021-06-1

Light availability controls in the benthic nearshore ecosystem of the Elwha River

The Elwha River Restoration Project was the largest US dam removal project to date, both in dam height and sediment released. During dam removal in 2011–2014, ~18 Mt of sediment washed downriver, and macroalgae virtually disappeared from the adjacent nearshore ecosystem. The link between current benthic light availability and sediment delivery and transport has been investigated in order to understand conditions during dam removal. Seven instrument platforms were deployed on the 10-m isobath along a 16 km transect centered on the river mouth for seven fortnightly periods in 2016 and 2017 to monitor near-bed photosynthetically available radiation (PAR), suspended sediment, wave climate, current velocity, temperature, and salinity. Water-column profiles, bed sediment, and water samples were collected during deployments. Seasonally variable chlorophyll-a and colored dissolved organic matter did not contribute substantially to light attenuation compared to suspended sediment. Along the 10-m isobath within 1.5 km of the river mouth, the greatest light attenuation occurred when wave events coincided with or followed periods of high river discharge. However, discharge events lasting attenuation; energetic tidal currents promote rapid sediment export out of the nearshore environment. In the buoyant plume, maximum light attenuation occurred within 1 m of the surface, reducing light through the rest of the water column. Benthic PAR varied more during spring tides when plume location was more variable. Alongshore 1.5 to 8 km from the river mouth, light availability was not directly coupled to river discharge. Light attenuation occurred throughout the water column, influenced by resuspension due to strong currents and wave events. This subsurface attenuation would not be captured by remote sensing. Predicting benthic light availability over event, tidal, and seasonal timescales will improve management strategies designed to limit ecosystem damage during other dam removals or sediment delivery events

Experiential education and outreach based on nearshore monitoring of the Elwha River restoration project

Nearshore monitoring of benthic habitats and the coastal environment following the Elwha River Restoration project has engaged students and citizens with coastal science and management issues. In the post-dam-removal period, the lessons learned will continue to be disseminated via a UW undergraduate course and an interactive digital map, both designed to engage students and communities in restoration science. The research-focused course developed at the UW Friday Harbor Labs has allowed us to engage diverse undergraduate students (and graduate teaching assistants) in the research process. The course integrates interdisciplinary lectures and workshops on data analysis and laboratory methods, with the research process; from proposal to oceanographic data collection to analysis to publication. The course provides opportunities for student creativity and leadership. Outcome tracking indicates that these undergraduate (and post-bac) students are generally attending graduate school at a high rate, and launching careers in education, coastal management, and other STEM fields. To engage a broader segment of the community and to support decision-making about large-scale coastal restoration projects, we have developed an interactive digital map that will be available on-line, and will also be piloted as a physical interpretive display at the Feiro Marine Life Center in Port Angeles, WA. The interactive digital map is designed to effectively tell the story of the Elwha restoration in the coastal environment through the compilation and display of multiple data sets, some of which have never before been publicly available. Ultimately, the result of long-term monitoring of the Elwha nearshore system will provide a better understanding of the effects of restoration activities, such as dam removal on benthic habitats, and this knowledge will be passed to future managers and citizens through educational and outreach activities that captivate and inspire a broad audience

Attenuation of ocean surface waves in pancake and frazil sea ice along the coast of the Chukchi Sea

Alaskan Arctic coastlines are protected seasonally from ocean waves by the presence of coastal and shorefast sea ice. This study presents field observations collected during the autumn 2019 freeze up near Icy Cape, a coastal headland in the Chukchi Sea of the Western Arctic. The evolution of the coupled air-ice-ocean-wave system during a four-day wave event was monitored using drifting wave buoys, a cross-shore mooring array, and ship-based measurements. The incident wave field with peak period of 2.5 s was attenuated by coastal pancake and frazil sea ice, reducing significant wave height by 40\% over less than 5 km of cross-shelf distance spanning water depths from 13 to 30 m. Spectral attenuation coefficients are evaluated with respect to wave and ice conditions and the proximity to the ice edge. Attenuation rates are found to be three times higher within 500 m of the ice edge, relative to values farther in the ice cover. Attenuation coefficients are in the range of $\langle 2.3,2.7\rangle$ \si{\meter^{-1}}, and follow a power-law dependence on frequency

Processes and records of coastal sediment dispersal in contrasting deltaic systems

Thesis (Ph.D.)--University of Washington, 2017-07Rivers are responsible for most sediment delivery to the ocean, and at the river-ocean transition, a complex set of processes and pathways shape the ultimate fate of particulates. Conceptual and numerical models of fluvial-marine dispersal have become increasingly sophisticated through several decades of observational and modeling studies. However, many key processes remain poorly constrained, such as particle clearance from river plumes and episodic sediment-gravity flows, and connections between these types of discrete processes to subaqueous-delta evolution. This study investigates sediment-dispersal processes and their products in two deltaic systems: the small mountainous Elwha River delta (Washington State), and the large Mekong River delta (Vietnam). Sediment clearance from buoyant river plumes is complicated by flocculation dynamics, which remain difficult to quantify. One option for characterizing sediment-clearance rates is estimation of an “effective settling velocity” (we), i.e., a single settling velocity which describes the clearance rate of all flocculated and unflocculated sediment in a plume. This study tests the validity and key assumptions of a published 1D clearance model for we using measurements from the small Elwha River plume during and after a dam removal project. The difference between effective settling velocity (we) and total effective settling velocity (we’) is also explored, based on previous studies suggesting that we’ is the sum of we (representing gravitational settling) and an additional term representing enhanced removal at the base of a plume. Four variations of the 1D clearance model are applied, using both point and depth-integrated measurements of suspended sediment concentrations, as well as estimated and modeled plume-water residence times. For depth-integrated measurements and modeled residence times, we’ are ~0.13–3.5 mm/s, and are greater than we values obtained from point measurements (~0.041–0.33 mm/s). The depth-integrated results are interpreted to be a reasonable approximation of we’ based on measured grain-size distributions and the scale of the plume. The differences between we’ and we are attributed to concentration gradients and turbulence-induced removal of sediment at the base of the plume within a few kilometers of the river. During extreme sediment-loading events, near-bed sediment-gravity flows may dominate dispersal pathways, rather than buoyant surface plumes. These events occur episodically, and thus their mechanics are not well-constrained due to sparse in situ measurements. Instruments deployed near the Elwha River mouth for ~3.5 y during dam removal recorded a gravity flow associated with a river flood. The event lasted ~10 h and resulted in rapid deposition of >15 cm of sand within a few hundred meters of the mouth, and was interpreted to be a hyperpycnal plume or hyperpycnal flow. After a few hours, the flow deteriorated through rapid deposition of sand and vertical mixing of muddy water. The extinction dynamics were characteristic of a hydraulic jump and lofting plume at the base of the steep delta front, resulting in a neutrally buoyant plume filling most of the water column. The muddy-sand deposit was entirely eroded within three weeks, leaving little record of the event. The sandy composition, short runout distance, and rapid extinction of the gravity flow highlight the challenges to forming and maintaining hyperpycnal flows. Post-event erosion of the deposit suggests that in similarly energetic environments, such events may be under-represented in the stratigraphic record. In contrasting large-river systems, sediment dispersal processes tend to be modulated by seasonal shifts in river discharge and ocean energy, rather than by storms and other extreme events. These seasonal processes coupled with abundant sediment supply generate subaqueous deltas near many of the world's largest rivers. The geometry of these subaqueous deposits is generally controlled by wave and current energy, which produce a topset-foreset transition (or “rollover point” between zones of moderate and rapid accumulation) at 25–40-m water depth. Instrument measurements, cores, and a simplistic wave model are used to evaluate morphodynamics of the subaqueous Mekong Delta, which has an unusually shallow rollover at 4–6 m depth. Results suggest that the foreset experiences rapid accumulation and exhibits internal structures characteristic of many subaqueous deltas. However, based on the wave model, the foreset is not energy-limited and does not exhibit a classic subaqueous-delta-front stress refuge. During the high-discharge season, sediment is delivered to the topset and foreset, but further seaward dispersal is limited by landward return flow under the plume and regional circulation patterns. During the windy monsoon season, landward currents driven by regional circulation (at greater depths) and by winds (at shallow depths) serve to retain sediment near shore. Thus, persistent landward sediment fluxes in both seasons likely help shape the subaqueous delta, allowing a shallow topset to exist, despite sufficient transport energy to erode muds and fine sands during much of the year. These results highlight the importance of considering both transport energy and transport direction (leading to sediment convergence) when interpreting the evolution of large subaqueous deltas

Elwha Delta coastal marine boundary-layer time-series observations

Please see the metadata file for a complete description of variables contained in the .mat data file.This dataset consists of a Matlab data file, 'ElwhaGF20132014.mat', and a text metadata file, 'MetadataElwhaGF20132014.txt.' These data were collected by PIs Andrea Ogston and Chuck Nittrouer, graduate student Emily Eidam, and the UW Sediment Dynamics lab from November 2013 to April 2014 on the Elwha Delta. The data are time-series measurements from acoustic, optical, and peripheral sensors deployed on two bottom-boundary-layer tripod frames deployed at ~14 m and ~23 m water depth within 1 km of the Elwha River mouth. Measurements were collected as part of an ongoing effort to document sediment gravity flows generated during the removal of two hydroelectric dams from the river (a large-scale inter-agency restoration project conducted between 2011 and 2014). Data from prior instrument deployments were reported in Eidam et al., 2016 (see citation below); data from this deployment are reported in a new paper currently in prep (as of Dec 2017). These data were collected under NSF grant 0960788. The Matlab file is divided into structures by instrument. The supporting metadata file gives variable descriptions and units.NSF grant 096078

Mooring results from Coastal Ocean Dynamics in the Arctic (CODA)

National Science Foundation, Office of Naval Researc

Increasing Wave Energy Moves Arctic Continental Shelves Toward a New Future

Arctic continental shelves, including the Alaskan Beaufort Shelf (ABS), are experiencing declines in sea ice coverage leading to increasingly energetic sea states and coastal erosion. In this study we investigated the morphologic response of the ABS to increasing wave energy, and how shelf profile adjustments modify wave energy propagating toward the coast. We developed a 2D cross-shelf morphodynamic model using Delft3D and tested shelf response to a present-day wave climate and a future Arctic wave climate projected under the RCP8.5 climate-change scenario. Simulations lasting 1000 years were conducted for relatively steep (Flaxman Island, AK, slope 0.0008) and flat (Harrison Bay, AK, slope 0.0003) cross-shelf profiles. We found that morphologic evolution and regulation of future waves depends primarily on existing shelf morphology. On the steeper profile, RCP 8.5 waves drove sediment erosion at 0–15 m water depth and redeposition at 15–30 m water depth. Over 1000 years, this redistribution of sediment from the inner to middle shelf resulted in a 7.6% reduction in wave heights at the 2 m isobath. This morphologic adjustment represented a regulatory feedback in which shallowing of the middle shelf led to attenuation of waves reaching the inner shelf. In contrast, effective wave attenuation across the flatter and wider Harrison Bay section limited cross-shelf transport and morphologic change under both wave climates. Together our results suggest that coastal changes in response to the growing Arctic wave climate may be dependent on shelf morphology, and even mitigated in some regions by morphologic adjustment

Increasing Wave Energy Moves Arctic Continental Shelves Toward a New Future

Arctic continental shelves, including the Alaskan Beaufort Shelf (ABS), are experiencing declines in sea ice coverage leading to increasingly energetic sea states and coastal erosion. In this study we investigated the morphologic response of the ABS to increasing wave energy, and how shelf profile adjustments modify wave energy propagating toward the coast. We developed a 2D cross-shelf morphodynamic model using Delft3D and tested shelf response to a present-day wave climate and a future Arctic wave climate projected under the RCP8.5 climate-change scenario. Simulations lasting 1000 years were conducted for relatively steep (Flaxman Island, AK, slope 0.0008) and flat (Harrison Bay, AK, slope 0.0003) cross-shelf profiles. We found that morphologic evolution and regulation of future waves depends primarily on existing shelf morphology. On the steeper profile, RCP 8.5 waves drove sediment erosion at 0–15 m water depth and redeposition at 15–30 m water depth. Over 1000 years, this redistribution of sediment from the inner to middle shelf resulted in a 7.6% reduction in wave heights at the 2 m isobath. This morphologic adjustment represented a regulatory feedback in which shallowing of the middle shelf led to attenuation of waves reaching the inner shelf. In contrast, effective wave attenuation across the flatter and wider Harrison Bay section limited cross-shelf transport and morphologic change under both wave climates. Together our results suggest that coastal changes in response to the growing Arctic wave climate may be dependent on shelf morphology, and even mitigated in some regions by morphologic adjustment