Geology topics

Jessica R. Lacy

Publications and source records attributed to Jessica R. Lacy.

At least 37 records · Page 2Linked to original sources

USGS2020

Carbon storage and sediment trapping by Egeria densa Planch., a globally invasive, freshwater macrophyte

Invasive plants have long been recognized for altering ecosystem properties, but their long-term impacts on ecosystem processes remain largely unknown. In this study, we determined the impact of Egeria densa Planch, a globally invasive freshwater macrophyte, on sedimentation processes in a large tidal freshwater region. We measured carbon accumulation (CARs) and inorganic sedimentation rates in submerged aquatic vegetation SAV dominated by E. densa and compared these rates to those of adjacent tidal freshwater marshes. Study sites were chosen along a range of hydrodynamic conditions in the Sacramento-San Joaquin Delta of California, USA, where E. densa has been widespread since 1990. Cores were analyzed for bulk density, % inorganic matter, % organic carbon, 210 Pb, and 137 Cs. Our results show that E. densa patches constitute sinks for both “blue carbon” and inorganic sediment. Compared to marshes, E. densa patches have greater inorganic sedimentation rates ( E. densa : 1103–5989 g m −2 yr −1 , marsh: 393–1001 g m −2 yr −1 , p < 0.01) and vertical accretion rates ( E. densa : 0.4–1.3 cm yr −1 , marsh: 0.3–0.5 cm yr −1 , p < 0.05), but similar CARs ( E. densa : 59–242 g C m −2 yr −1 , marsh: 109–169 g C m −2 yr −1 , p > 0.05). Sediment stored by E. densa likely reduces the resilience of adjacent marshes by depleting the sediment available for marsh-building. Because of its harmful traits, E. densa is not a suitable candidate for mitigating carbon pollution; however, currently invaded habitats may already contain a meaningful component of regional carbon budgets. Our results strongly suggest that E. densa patches are sinks for carbon and inorganic sediment throughout its global range, raising questions about how invasive SAV is altering biogeochemical cycling and sediment dynamics across freshwater ecosystems.

California↗

USGS2019

Seasonal, spring-neap, and tidal variation in cohesive sediment transport parameters in estuarine shallows

Numerical models for predicting sediment concentrations and transport rely on parameters such as settling velocity and bed erodibility that describe sediment characteristics, yet these parameters are rarely probed directly. We investigated temporal and spatial variation in sediment parameters in the shallows of San Pablo Bay, CA. Flow, turbulence, and suspended sediment data were measured at sites located at 1 and 2 m below mean lower low water (MLLW) from November 2013 through April 2015, supplemented by monthlong periods in 2011, 2012, and 2016. Maximum current velocities were 0.40–0.47 m s -1 at these depths; the strongest currents decreased to 0.27–0.34 m s -1 during neap periods. Winters 2013–2014 and 2014–2015 experienced strong drought conditions, limiting the potential for seasonal impact on sediment conditions during this experiment. Despite this, the more storm‐influenced site showed clear changes during the winter: the roughness parameter decreased from 10 −4 to 10 −5 m, from hydrodynamically rough to smooth conditions, and bed erodibility increased by an order of magnitude. Median settling velocity was 2.05·10 −4 m s -1 ; it varied twofold within a tidal cycle, decreasing as current velocity grew during flood and ebb. This tidal control on floc size affected settling velocity on the spring‐neap timescale, possibly driving a spring‐neap oscillation in erodibility. Our findings highlight variation in sediment dynamics that is commonly ignored in numerical models and the need for field observations to ground truth ongoing modeling efforts.

California↗

USGS2019

Measuring settling velocity in a strongly tidal estuary

Predicting sediment transport in estuarine systems requires understanding sediment settling velocity, its range of fluctuations, and the shortcomings of the tools to measure it. Previous studies have used Laser In-Situ Scattering and Transmissometry (LISST) instruments to measure particle size and Acoustic Doppler Velocimeters (ADV) to return estimates of settling velocity. We deployed both instruments over a 4-week period in summer 2016 in San Pablo Bay, CA, and observed conflicting results from the two instruments. The LISST indicated the settling velocity decreased with increasing bed shear stress, while the ADV showed that settling velocity increased. Comparing the experimental conditions from this study with previous research reveals that the ADV method requires steady state conditions that were not achieved. Although the ADV method uses easily measured quantities to return the settling velocity, it relies on assumptions that are not always valid in estuarine settings.

California↗

USGS2018

Three-dimensional modeling of ﬁne sediment transport by waves and currents in a shallow estuary

A suspended sediment transport model is implemented in the unstructured‐grid SUNTANS model and applied to study fine‐grained sediment transport in South San Francisco Bay. The model enables calculation of suspension of bottom sediment based on combined forcing of tidal currents and wind waves. We show that accurate results can be obtained by employing two‐size classes which are representative of microflocs and macroflocs in the Bay. A key finding of the paper is that the critical calibration parameter is the ratio of the erosion of the microflocs to macroflocs from the bed. Different values of this erosion ratio are needed on the shallow shoals and deeper channels because of the different nature of the sediment dynamics in these regions. Application of a spatially variable erosion ratio and critical shear stress for erosion is shown to accurately reproduce observed suspended sediment concentration at four‐field sites located along a cross‐channel transect. The results reveal a stark contrast between the behavior of the suspended sediment concentration on the shoals and in the deep channel. Waves are shown to resuspend sediments on the shoals, although tidal and wind‐generated currents are needed to mix the thin wave‐driven suspensions into the water column. The contribution to the suspended sediment concentration in the channel by transport from the shoals is similar in magnitude to that due to local resuspension. However, the local contribution is in phase with strong bottom currents which resuspend the sediments, while the contribution from the shoals peaks during low‐water slack tide.

San Francisco Bay↗

USGS2018

The influence of neap-spring tidal variation and wave energy on sediment flux in salt marsh tidal creeks

Sediment flux in marsh tidal creeks is commonly used to gage sediment supply to marshes. We conducted a field investigation of temporal variability in sediment flux in tidal creeks in the accreting tidal marsh at China Camp State Park adjacent to northern San Francisco Bay. Suspended-sediment concentration (SSC), velocity, and depth were measured near the mouths of two tidal creeks during three six-to-ten-week deployments: two in winter and one in summer. Currents, wave properties and SSC were measured in the adjacent shallows. All deployments spanned the largest spring tides of the season. Results show that tidally-averaged suspended-sediment flux (SSF) in the tidal creeks decreased with increasing tidal energy, and SSF was negative (bayward) for tidal cycles with maximum water surface elevation above the marsh plain. Export during the largest spring tides dominated the cumulative SSF measured during the deployments. During ebb tides following the highest tides, velocities exceeded 1 m/s in the narrow tidal creeks, resulting in negative tidally-averaged water flux, and mobilizing sediment from the creek banks or bed. Storm surge also produced negative SSF. Tidally-averaged SSF was positive in wavey conditions with moderate tides. Spring-tide sediment export was about 50% less at a station 130 m further up the tidal creek than at the creek mouth. The negative tidally-averaged water flux near the creek mouth during spring tides indicates that in the lower marsh, some of the water flooding directly across the bay--marsh interface drains through the tidal creeks, and suggests that this interface may be a pathway for sediment supply to the lower marsh as well.

California↗

USGS2018

Wave attenuation across a tidal marsh in San Francisco Bay

Wave attenuation is a central process in the mechanics of a healthy salt marsh. Understanding how wave attenuation varies with vegetation and hydrodynamic conditions informs models of other marsh processes that are a function of wave energy (e.g. sediment transport) and allows for the incorporation of marshes into coastal protection plans. Here, we examine the evolution of wave height across a tidal salt marsh in San Francisco Bay. Instruments were deployed along a cross-shore transect, starting on the mudflat and crossing through zones dominated by Spartina foliosa and Salicornia pacifica . This dataset is the first to quantify wave attenuation for these vegetation species, which are abundant in the intertidal zone of California estuaries. Measurements were collected in the summer and winter to assess seasonal variation in wave attenuation. Calculated drag coefficients of S. foliosa and S. pacifica were similar, indicating equal amounts of vegetation would lead to similar energy dissipation; however, S. pacifica has much greater biomass close to the bed (<20 cm) and retains biomass throughout the year, and therefore, it causes more total attenuation. S. foliosa dies back in the winter, and waves often grow across this section of the marsh. For both vegetation types, attenuation was greatest for low water depths, when the vegetation was emergent. For both seasons, attenuation rates across S. pacifica were the highest and were greater than published attenuation rates across similar ( Spartina alterniflora ) salt marshes for the comparable depths. These results can inform designs for marsh restorations and management plans in San Francisco Bay and other estuaries containing these species.

California↗

USGS2018

Bed shear stress estimation under wave conditions using near-bottom measurements: Comparison of methods

Understanding the influence of waves on bed shear stress is critical for predicting morphodynamical behaviours in coastal areas. Near-bed flow was measured on the middle and lower intertidal mudflats along the Jiangsu coast, China, using a three-dimensional acoustic velocimeter that collected a 3.5-cm vertical profile at 1mm resolution and sample rate of 25 Hz. On the lower and middle tidal flats, velocities from ~2.5-6 cmab (cm above bed) and ~0-3 cmab were measured, respectively. Current-induced bed shear stresses were calculated from turbulent kinetic energy (TKE) at the 11th measurement layer (i.e., 5.1 cm below the probe) using wave-turbulence decomposition and from a logarithmic fit to the horizontal mean velocity profile (LP). A wave boundary layer extended from the bed up to 3 cmab when the significant wave height was 0.23 m; when it was present the near-bed mean velocity profile was non-logarithmic. Waves suppress the development of a vertical velocity gradient and lead to an overestimation of bed shear stress when calculated using the log profile assumption. The TKE method is more accurate than the LP method when waves are present and measurements are at least partially within the wave boundary layer. Accurate calculation of current-induced bed shear stress depends on probe height and wave conditions.

Jiangsu coast↗

USGS2016

Bathymetric survey and digital elevation model of Little Holland Tract, Sacramento-San Joaquin Delta, California

The U.S. Geological Survey conducted a bathymetric survey in Little Holland Tract, a flooded agricultural tract, in the northern Sacramento-San Joaquin Delta (the “Delta”) during the summer of 2015. The new bathymetric data were combined with existing data to generate a digital elevation model (DEM) at 1-meter resolution. Little Holland Tract (LHT) was historically diked off for agricultural uses and has been tidally inundated since an accidental levee breach in 1983. Shallow tidal regions such as LHT have the potential to improve habitat quality in the Delta. The DEM of LHT was developed to support ongoing studies of habitat quality in the area and to provide a baseline for evaluating future geomorphic change. The new data comprise 138,407 linear meters of real-time-kinematic (RTK) Global Positioning System (GPS) elevation data, including both bathymetric data collected from personal watercraft and topographic elevations collected on foot at low tide. A benchmark (LHT15_b1) was established for geodetic control of the survey. Data quality was evaluated both by comparing results among surveying platforms, which showed systematic offsets of 1.6 centimeters (cm) or less, and by error propagation, which yielded a mean vertical uncertainty of 6.7 cm. Based on the DEM and time-series measurements of water depth, the mean tidal prism of LHT was determined to be 2,826,000 cubic meters. The bathymetric data and DEM are available at http://dx.doi.org/10.5066/F7RX9954 .

California↗

USGS2016

Wave attenuation in the shallows of San Francisco Bay

Waves propagating over broad, gently-sloped shallows decrease in height due to frictional dissipation at the bed. We quantified wave-height evolution across 7 km of mudflat in San Pablo Bay (northern San Francisco Bay), an environment where tidal mixing prevents the formation of fluid mud. Wave height was measured along a cross shore transect (elevation range−2mto+0.45mMLLW) in winter 2011 and summer 2012. Wave height decreased more than 50% across the transect. The exponential decay coefficient λ was inversely related to depth squared (λ=6×10 −4 h −2 ). The physical roughness length scale k b , estimated from near-bed turbulence measurements, was 3.5×10 −3 m in winter and 1.1×10 −2 m in summer. Estimated wave friction factor f w determined from wave-height data suggests that bottom friction dominates dissipation at high Re w but not at low Re w . Predictions of near-shore wave height based on offshore wave height and a rough formulation for f w were quite accurate, with errors about half as great as those based on the smooth formulation for f w . Researchers often assume that the wave boundary layer is smooth for settings with fine-grained sediments. At this site, use of a smooth f w results in an underestimate of wave shear stress by a factor of 2 for typical waves and as much as 5 for more energetic waves. It also inadequately captures the effectiveness of the mudflats in protecting the shoreline through wave attenuation.

California↗

USGS2015

Mechanisms of sediment flux between shallows and marshes

We conducted a field study to investigate temporal variation and forcing mechanisms of sediment flux between a salt marsh and adjacent shallows in northern San Francisco Bay. Suspended-sediment concentration (SSC), tidal currents, and wave properties were measured over the marsh, in marsh creeks, and in bay shallows. Cumulative sediment flux in the marsh creeks was bayward during the study, and was dominated by large bayward flux during the largest tides of the year. This result was unexpected because extreme high tides with long inundation periods are commonly assumed to supply sediment to marshes, and long-term accretion estimates show that the marsh in the study site is depositional. A water mass-balance shows that some landward transport bypassed the creeks, most likely across the marsh-bay interface. An estimate of transport by this pathway based on observed SSC and inferred volume indicates that it was likely much less than the observed export.

California↗

USGS2015

Model-based interpretation of sediment concentration and vertical flux measurements in a shallow estuarine environment

A one-dimensional numerical model describing tidally varying vertical mixing and settling was used to interpret sediment concentrations and vertical fluxes observed in the shoals of South San Francisco Bay by two acoustic Doppler velocimeters (ADVs) at elevations of 0.36 m and 0.72 m above bed. Measured sediment concentrations changed by up to 100 g m −3 over the semidiurnal tidal cycle. These dynamics were dominated by local resuspension and settling. Multiple particle class models suggested the existence of a class with fast settling velocities ( w s of 9.0 × 10 −4 m s −1 in spring and 5.8 × 10 −4 m s −1 in fall) and a slowly settling particle fraction ( w s of <1 × 10 −7 m s −1 in spring and 1.4 × 10 −5 m s −1 in fall). Modeled concentrations of slowly settling particles at 0.36 m were as high as 20 g m −3 during fall and varied with the spring-neap cycle while fine sediment concentrations in spring were constant around 5 g m −3 . Analysis of in situ water column floc size distributions suggested that floc properties in the lower part of the water column were most likely governed by particle-size distribution on the bed and not by coagulation, validating our multiple particle size approach. A comparison of different sediment bed models with respect to model performance, sensitivity, and identifiability suggested that the use of a sediment erosion model linear in bottom shear stress τ b ( E = M ( τ b − τ c )) was the most appropriate choice to describe the field observations when the critical shear stress τ c and the proportionality factor M were kept constant.

California↗

USGS2014

Autonomous bed-sediment imaging-systems for revealing temporal variability of grain size

We describe a remotely operated video microscope system, designed to provide high-resolution images of seabed sediments. Two versions were developed, which differ in how they raise the camera from the seabed. The first used hydraulics and the second used the energy associated with wave orbital motion. Images were analyzed using automated frequency-domain methods, which following a rigorous partially supervised quality control procedure, yielded estimates to within 20% of the true size as determined by on-screen manual measurements of grains. Long-term grain-size variability at a sandy inner shelf site offshore of Santa Cruz, California, USA, was investigated using the hydraulic system. Eighteen months of high frequency (min to h), high-resolution (μm) images were collected, and grain size distributions compiled. The data constitutes the longest known high-frequency record of seabed-grain size at this sample frequency, at any location. Short-term grain-size variability of sand in an energetic surf zone at Praa Sands, Cornwall, UK was investigated using the ‘wave-powered’ system. The data are the first high-frequency record of grain size at a single location of a highly mobile and evolving bed in a natural surf zone. Using this technology, it is now possible to measure bed-sediment-grain size at a time-scale comparable with flow conditions. Results suggest models of sediment transport at sandy, wave-dominated, nearshore locations should allow for substantial changes in grain-size distribution over time-scales as short as a few hours.

California↗

USGS2014

Interactions between waves, sediment, and turbulence on a shallow estuarine mudflat

Measurements were collected on a shallow estuarine mudflat in northern San Francisco Bay to examine the physical processes controlling waves, turbulence, sediment resuspension, and their interactions. Tides alone forced weak to moderate currents of 10–30 cm s -1 in depths of 0–3 m, and maintained a background suspension of 30–50 mg L21 of fine sediment. In the presence of wind waves, bottom orbital velocities spanned 20–30 cm s -1 , suspended-sediment concentrations (SSC) at 15 and 30 cm above the bed (cmab) increased by 1–2 orders of magnitude, and vertical gradients in SSC were strong enough to produce turbulence-limiting stratification, with gradient Richardson numbers exceeding 0.25. Simultaneously, turbulent stresses (decomposed from wave motions) increased by an order of magnitude. The apparent contradiction of energetic turbulence in the presence of strong stratification was reconciled by considering the turbulent kinetic energy (TKE) budget: in general, dissipation and buoyancy flux were balanced by local shear production, and each of these terms increased during wave events. The classic wave-current boundary layer model represented the observations qualitatively, but not quantitatively since the velocity profile could not be approximated as logarithmic. Rather, the mean shear was elevated by the Stokes drift return flow and wind-generated surface stress, which diffused sediment upward and limited stratification. Our findings highlight a pathway for waves to supply energy to both the production and destruction of turbulence, and demonstrate that in such shallow depths, TKE and SSC can be elevated over more of the water column than predicted by traditional models.

California↗

USGS2014

Lateral baroclinic forcing enhances sediment transport from shallows to channel in an estuary

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologic and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperature- and SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.

California↗

USGS2012

Currents, drag, and sediment transport induced by a tsunami

We report observations of water surface elevation, currents, and suspended sediment concentration (SSC) from a 10-m deep site on the inner shelf in northern Monterey Bay during the arrival of the 2010 Chile tsunami. Velocity profiles were measured from 3.5 m above the bed (mab) to the surface at 2 min intervals, and from 0.1 to 0.7 mab at 1 Hz. SSC was determined from the acoustic backscatter of the near-bed profiler. The initial tsunami waves were directed cross shore and had a period of approximately 16 min. Maximum wave height was 1.1 m, and maximum current speed was 0.36 m/s. During the strongest onrush, near-bed velocities were clearly influenced by friction and a logarithmic boundary layer developed, extending more than 0.3 mab. We estimated friction velocity and bed shear stress from the logarithmic profiles. The logarithmic structure indicates that the flow can be characterized as quasi-steady at these times. At other phases of the tsunami waves, the magnitude of the acceleration term was significant in the near-bed momentum equation, indicating unsteady flow. The maximum tsunami-induced bed shear stress (0.4 N/m 2 ) exceeded the critical shear stress for the medium-grained sand on the seafloor. Cross-shore sediment flux was enhanced by the tsunami. Oscillations of water surface elevation and currents continued for several days. The oscillations were dominated by resonant frequencies, the most energetic of which was the fundamental longitudinal frequency of Monterey Bay. The maximum current speed (hourly-timescale) in 18 months of observations occurred four hours after the tsunami arrived.

California↗

USGS2011

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.

Washington↗

USGS2011

The influence of current speed and vegetation density on flow structure in two macrotidal eelgrass canopies

The influence of eelgrass ( Zostera marina ) on near-bed currents, turbulence, and drag was investigated at three sites in two eelgrass canopies of differing density and at one unvegetated site in the San Juan archipelago of Puget Sound, Washington, USA. Eelgrass blade length exceeded 1 m. Velocity profiles up to 1.5 m above the sea floor were collected over a spring-neap tidal cycle with a downward-looking pulse-coherent acoustic Doppler profiler above the canopies and two acoustic Doppler velocimeters within the canopies. The eelgrass attenuated currents by a minimum of 40%, and by more than 70% at the most densely vegetated site. Attenuation decreased with increasing current speed. The data were compared to the shear-layer model of vegetated flows and the displaced logarithmic model. Velocity profiles outside the meadows were logarithmic. Within the canopies, most profiles were consistent with the shear-layer model, with a logarithmic layer above the canopy. However, at the less-dense sites, when currents were strong, shear at the sea floor and above the canopy was significant relative to shear at the top of the canopy, and the velocity profiles more closely resembled those in a rough-wall boundary layer. Turbulence was strong at the canopy top and decreased with height. Friction velocity at the canopy top was 1.5–2 times greater than at the unvegetated, sandy site. The coefficient of drag C D on the overlying flow derived from the logarithmic velocity profile above the canopy, was 3–8 times greater than at the unvegetated site (0.01–0.023 vs. 2.9 × 10 −3 ).

Washington↗

USGS2011

The dynamics of fine-grain sediment dredged from Santa Cruz Harbor

In the fall and early winter of 2009, a demonstration project was done at Santa Cruz Harbor, California, to determine if 450 m 3 /day of predominantly (71 percent) mud-sized sediment could be dredged from the inner portion of the harbor and discharged to the coastal ocean without significant impacts to the beach and inner shelf. During the project, more than 7600 m 3 of sediment (~5400 m 3 of fine-grain material) was dredged during 17 days and discharged approximately 60 m offshore of the harbor at a depth of 2 m on the inner shelf. The U.S. Geological Survey’s Pacific Coastal and Marine Science Center was funded by the U.S. Army Corps of Engineers and the Santa Cruz Port District to do an integrated mapping and process study to investigate the fate of the mud-sized sediment dredged from the inner portion of Santa Cruz Harbor and to determine if any of the fine-grain material settled out on the shoreline and/or inner shelf during the fall and early winter of 2009. This was done by collecting high resolution oceanographic and sediment geochemical measurements along the shoreline and on the continental shelf of northern Monterey Bay to monitor the fine-grain sediment dredged from Santa Cruz Harbor and discharged onto the inner shelf. These in place measurements, in conjunction with beach, water column, and seabed surveys, were used as boundary and calibration information for a three-dimensional numerical circulation and sediment dynamics model to better understand the fate of the fine-grain sediment dredged from Santa Cruz Harbor and the potential consequences of disposing this type of material on the beach and on the northern Monterey Bay continental shelf.

California↗