Andrew W. Stevens

Mississippi Delta mudflow activity and 2005 Gulf hurricanes

Gravity-driven sediment flows can be important mechanisms for transporting sediments and solutes rapidly across continental margins, and therefore may have important impacts on benthic ecosystems and geochemical cycling. Also, infrastructure damage can result from these events, as was the case when mudflow activity during Hurricane Ivan in fall 2004 caused pipeline damage (U.S. Minerals Management Service (MMS) press release on 8 October 2004; http://www.mms.gov/ooc/press/2004/press I 008a.htm).

The Influence of Wave Energy and Sediment Transport on Seagrass Distribution

A coupled hydrodynamic and sediment transport model (Delft3D) was used to simulate the water levels, waves, and currents associated with a seagrass (Zostera marina) landscape along a 4-km stretch of coast in Puget Sound, WA, USA. A hydroacoustic survey of seagrass percent cover and nearshore bathymetry was conducted, and sediment grain size was sampled at 53 locations. Wave energy is a primary factor controlling seagrass distribution at the site, accounting for 73% of the variability in seagrass minimum depth and 86% of the variability in percent cover along the shallow, sandy portions of the coast. A combination of numerical simulations and a conceptual model of the effect of sea-level rise on the cross-shore distribution of seagrass indicates that the area of seagrass habitat may initially increase and that wave dynamics are an important factor to consider in predicting the effect of sea-level rise on seagrass distributions in wave-exposed areas.

Increased sediment load during a large-scale dam removal changes nearshore subtidal communities

The coastal marine ecosystem near the Elwha River was altered by a massive sediment influx—over 10 million tonnes—during the staged three-year removal of two hydropower dams. We used time series of bathymetry, substrate grain size, remotely sensed turbidity, scuba dive surveys, and towed video observations collected before and during dam removal to assess responses of the nearshore subtidal community (3 m to 17 m depth). Biological changes were primarily driven by sediment deposition and elevated suspended sediment concentrations. Macroalgae, predominantly kelp and foliose red algae, were abundant before dam removal with combined cover levels greater than 50%. Where persistent sediment deposits formed, macroalgae decreased greatly or were eliminated. In areas lacking deposition, macroalgae cover decreased inversely to suspended sediment concentration, suggesting impacts from light reduction or scour. Densities of most invertebrate and fish taxa decreased in areas with persistent sediment deposition; however, bivalve densities increased where mud deposited over sand, and flatfish and Pacific sand lance densities increased where sand deposited over gravel. In areas without sediment deposition, most invertebrate and fish taxa were unaffected by increased suspended sediment or the loss of algae cover associated with it; however, densities of tubeworms and flatfish, and primary cover of sessile invertebrates increased suggesting benefits of increased particulate matter or relaxed competition with macroalgae for space. As dam removal neared completion, we saw evidence of macroalgal recovery that likely owed to water column clearing, indicating that long-term recovery from dam removal effects may be starting. Our results are relevant to future dam removal projects in coastal areas and more generally to understanding effects of increased sedimentation on nearshore subtidal benthic communities.

MODELING SEDIMENT TRANSPORT AND DELTA MORPHOLOGY ON THE DAMMED ELWHA RIVER, WASHINGTON STATE, USA

The sediment supply to the delta and adjacent beaches of the Elwha River in Washington State, USA is expected to increase significantly after removal of two dams. This paper describes the initial implementation of a process-based hydrodynamic and sediment transport model to predict sediment transport pathways and delta morphological response to changes in sediment supply in a mixed sediment system. The hydrodynamic model is calibrated and validated against water levels and currents measured in the Strait of Juan de Fuca and on the Elwha delta. Strong instantaneous and residual tidal currents are responsible for the transport and dispersal of fine-grained and sand-sized sediments across the delta. If sediment supply is large enough, some sediment will accumulate on the delta, modifying the delta substrate, which is presently dominated by hardbottom and coarse sediments.

Modeling the Hydrodynamic and Morphologic Response of an Estuary Restoration

Estuary evolution is investigated using the hydrodynamic and sediment transport model, Delft3D, to study the response of a dammed tidal basin to restored tidal processes. The development of decadal (10-year) morphological simulations of the restored estuary required simplifying several data inputs and implementing a time-scale acceleration technique. An innovative river sediment discharge schematization was developed that connected sediment discharge to morphological change in the estuary. Mud erodibility parameters were determined from laboratory analysis of sediment cores from the modern lakebed and statistical refinement with a Bayes network of the probability of occurrence. The changing estuary morphology appears to have a dominant impact on the physical habitat (substrate, inundation frequency, mean salinity, and salinity range). The numerical model provides a tool to compare the functions of the historical estuary and possible future alternatives for a restored estuary. Sensitivity of the morphological model to sediment types and erodibility parameters was also examined. A conceptual model covering morphology and indicators of physical habitat for three phases of estuary evolution during restoration is presented that could be applied to estuarine systems that are severely out of equilibrium.

A regime shift in sediment export from a coastal watershed during a record wet winter, California: Implications for landscape response to hydroclimatic extremes: Sediment flux regime shift

Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10-year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, these flows exported more than half as much sediment as had a 100-year flood 35 years earlier, substantially enlarging the nearshore delta. Annual sediment load in 2017 was an order of magnitude greater than during an average-rainfall year, and 500-fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for several years. Future projections of megadroughts punctuated by major atmospheric-river storm activity suggest that interannual sediment-yield variations will become more extreme than today in the western USA, with potential consequences for coastal management, ecosystems, and water-storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Morphodynamic evolution following sediment release from the world’s largest dam removal

Sediment pulses can cause widespread, complex changes to rivers and coastal regions. Quantifying landscape response to sediment-supply changes is a long-standing problem in geomorphology, but the unanticipated nature of most sediment pulses rarely allows for detailed measurement of associated landscape processes and evolution. The intentional removal of two large dams on the Elwha River (Washington, USA) exposed ~30 Mt of impounded sediment to fluvial erosion, presenting a unique opportunity to quantify source-to-sink river and coastal responses to a massive sediment-source perturbation. Here we evaluate geomorphic evolution during and after the sediment pulse, presenting a 5-year sediment budget and morphodynamic analysis of the Elwha River and its delta. Approximately 65% of the sediment was eroded, of which only ~10% was deposited in the fluvial system. This restored fluvial supply of sand, gravel, and wood substantially changed the channel morphology. The remaining ~90% of the released sediment was transported to the coast, causing ~60 ha of delta growth. Although metrics of geomorphic change did not follow simple time-coherent paths, many signals peaked 1–2 years after the start of dam removal, indicating combined impulse and step-change disturbance responses.

Bathymetry and topography data from the Elwha River delta, Washington, September 2010

Two dams on the Elwha River, Washington State, USA trapped over 20 million cubic meters of sediment, reducing downstream sediment fluxes and contributing to erosion of the rivers coastal delta. The removal of the Elwha and Glines Canyon dams between 2011 and 2014 induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the response of a delta system to changes in sediment supply. The U.S. Geological Survey (USGS) developed an integrated research program aimed at understanding the ecosystem responses following dam removal. The research program included repeated surveys of beach topography, nearshore bathymetry, and surface sediment grain size to quantify changes in delta morphology and texture following the dam removals. For more information on the USGS role in the Elwha River Restoration Project, please visit http://walrus.wr.usgs.gov/elwha/. This USGS data release presents data collected during surveys of nearshore bathymetry and beach topography of the Elwha River delta, Washington. Survey operations were conducted between September 5 and September 7, 2010 (USGS Field Activity Number W-03-10-PS) by a team of scientists from the U.S. Geological Survey Pacific Coastal and Marine Science Center (PCMSC), Washington State Department of Ecology (WA DOE), and Washington Sea Grant. Nearshore bathymetry data were collected using a personal watercraft (PWC) and a small boat, each equipped with single-beam echosounders and survey-grade global navigation satellite system (GNSS) receivers. Topography data were collected on foot with GNSS receivers mounted on backpacks. Positions of the survey platforms were referenced to a GNSS base station placed on a nearby benchmark with known horizontal and vertical coordinates. A total of 118 km of nearshore bathymetric survey lines and 49 km of topographic survey lines were collected during the 3 days of survey operations. Environmental conditions were favorable, resulting in good coverage of the beach and nearshore region. A continuous DEM surface of the primary survey area was produced from all available bathymetry and topography data using linear interpolation and a grid-spacing of 5 m. An additional DEM with 1-m resolution grid-spacing was produced using linear interpolation for areas adjacent to the river mouth. Digital files containing the nearshore bathymetry data, beach topography data and derived DEMs from this survey are available for download from the child item pages.

Bathymetry and topography data from the Elwha River delta, Washington, August 2011

Two dams on the Elwha River, Washington State, USA trapped over 20 million cubic meters of sediment, reducing downstream sediment fluxes and contributing to erosion of the rivers coastal delta. The removal of the Elwha and Glines Canyon dams between 2011 and 2014 induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the response of a delta system to changes in sediment supply. The U.S. Geological Survey (USGS) developed an integrated research program aimed at understanding the ecosystem responses following dam removal. The research program included repeated surveys of beach topography, nearshore bathymetry, and surface sediment grain size to quantify changes in delta morphology and texture following the dam removals. For more information on the USGS role in the Elwha River Restoration Project, please visit http://walrus.wr.usgs.gov/elwha/. This USGS data release presents data collected during surveys of nearshore bathymetry and beach topography from the Elwha River delta, Washington. Survey operations were conducted between May 18 and May 20, 2012 (USGS Field Activity Number W-03-12-PS). The survey team included scientists from the U.S. Geological Survey Pacific Coastal and Marine Science Center (PCMSC), Washington State Department of Ecology (WA DOE), Oregon State University (OSU), San Jose State University (SJSU), and Washington Sea Grant. Nearshore bathymetry data were collected using two personal watercraft (PWCs), each equipped with single-beam echosounders and survey-grade global navigation satellite system (GNSS) receivers. Topography data were collected on foot with GNSS receivers mounted on backpacks. Positions of the survey platforms were referenced to a GNSS base station placed on a nearby benchmark with known horizontal and vertical coordinates. Depths from the echosounders were computed using sound velocity profiles measured with a conductivity-temperature-depth (CTD) sensor during the survey. A total of 130 km of nearshore bathymetric survey lines and 120 km of topographic survey lines were collected during the 3 days of survey operations. Environmental conditions were favorable, resulting in good coverage of the beach and nearshore region. A continuous DEM surface of the primary survey area was produced from all available bathymetry and topography data using linear interpolation and a grid-spacing of 5 m. An additional DEM with 1-m resolution grid-spacing was produced using linear interpolation for areas adjacent to the river mouth. Digital files containing the nearshore bathymetry data, beach topography data, and derived DEMs from this survey are available for download from the child item pages.

Coastal change from a massive sediment input: Dam removal, Elwha River, Washington, USA

The removal of two large dams on the Elwha River, Washington, provides an ideal opportunity to study coastal morphodynamics during increased sediment supply. The dam removal project exposed ~21 million cubic meters (~30 million tonnes) of sediment in the former reservoirs, and this sediment was allowed to erode by natural river processes. Elevated rates of sand and gravel sediment transport in the river occurred during dam removal. Most of the sediment was transported to the coast, and this renewed sediment supply resulted in hundreds of meters of seaward expansion of the river delta since 2011. Our most recent survey in January 2015 revealed that a cumulative ~3.5 million m of sediment deposition occurred at the delta since the beginning of the dam removal project, and that aggradation had exceeded 8 m near the river mouth. Some of the newly deposited sediment has been shaped by waves and currents into a series of subaerial berms that appear to move shoreward with time.