VTechWorks Repository :: Browsing by Author "Stark, Nina"

Browsing by Author "Stark, Nina"

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Advancement of Using Portable Free Fall Penetrometers for Geotechnical Site Characterization of Energetic Sandy Nearshore Areas
Albatal, Ali Hefdhallah Ali (Virginia Tech, 2018-04-24)
Portable Free Fall Penetrometers (PFFPs) are lightweight tools used for rapid and economic characterization of surficial subaqueous sediments. PFFPs vary in weight, shape and size with options for using add-on units. The different configurations enable deployments in various environments and water depths, including the nearshore zone where conventional methods are challenged by energetic hydrodynamics and limited navigable depth. Moreover, PFFPs offer an opportunity to reduce the high site investigation costs associated with conventional offshore geotechnical site investigation methods. These costs are often a major obstacle for small projects serving remote communities or testing novel renewable energy harvesting machines. However, PFFPs still face issues regarding data analysis and interpretation, particularly in energetic sandy nearshore areas. This includes a lack of data and accepted analysis methods for such environments. Therefore, the goal of this research was to advance data interpretation and sediments characterization methods using PFFPs with emphasis on deployments in energetic nearshore environments. PFFP tests were conducted in the nearshore areas of: Yakutat Bay, AK; Cannon Beach, AK; and the U.S. Army Corps of Engineers' Field Research Facility's beach, Duck, NC. From the measurements, the research goal was addressed by: (1) introducing a methodology to create a regional sediment classification scheme utilizing the PFFP deceleration and pore pressure measurements, sediment traces on the probe upon retrieval, and previous literature; (2) investigating the effect of wave forcing on the sediments' behavior through correlating variations in sediment strength to wave climate, sandbar migration, and depth of closure, as well as identifying areas of significant sediment mobilization processes; and (3) estimating the relative density and friction angle of sand in energetic nearshore areas from PFFP measurements. For the latter, the field data was supported by vacuum triaxial tests and PFFP deployments under controlled laboratory conditions on sand samples prepared at different relative densities. The research outcomes address gaps in knowledge with regard to the limited studies available that investigate the sand geotechnical properties in energetic nearshore areas. More specifically, the research contributes to the understanding of surficial sediment geotechnical properties in energetic nearshore areas and the enhancement of sediment characterization and interpretation methods.
Advancements for the Numerical Simulation of Free Fall Penetrometers and the Analysis of Wind Erosion of Sands
Zambrano Cruzatty, Luis Eduardo (Virginia Tech, 2021-08-27)
The coastal population is growing, putting extra stress on coastal sediments and protection features, such as beach dunes. Moreover, global warming will increase the frequency of storms, and coastal dunes and other defense infrastructure will be subjected to increased erosion and scouring, endangering the people they are meant to protect. Understanding soil dynamics and fluid interaction is crucial to predict the effects of sand erosion. In particular, the study of wind erosion of sands in coastal dunes is essential due to the protective role these earthen structures have during storm events. One of the challenges about predicting wind erosion in coastal dunes is its extended spatial scale and the associated economic and logistics costs of sampling and characterizing the sediments. Because of this, in-situ testing for sediment characterization is essential. In particular, the usage of free-fall penetrometers (FFP) is appealing due to their portability and robustness. The sediment properties obtained with this type of testing can later be used to assess wind erosion susceptibility by determining, for example, the wind velocity to initiate the erosion process. FFP testing involves dropping an instrumented probe that impacts the soil and measures the kinematics or kinetics during the penetration process. For example, deceleration measurements are used to compute an equivalent quasi-static failure, which is not in line with the dynamic process characteristic of FFP testing. This preassumed failure mechanism is used to back-calculate the sand's geomechanical properties. However, soil behavior is highly complex under rapid loading, and incorporating this behavior into FFP sediment characterization models is challenging. Advanced numerical modeling can improve the understanding of the physics behind FFP testing. This thesis presents various advancements in numerical modeling and erosion models to bridge FFP in-situ testing with predicting the initiation of wind erosion of sands. First, improvements oriented to the Material Point Method (MPM) for modeling in-situ FFP testing are proposed. The numerical results show that the simulation of FFP deployment in sands is affected by strain localization and highlight the importance of considering constitutive models sensitive to different loading rates. Because of the importance of rate effects in soil behavior, the second aspect of this thesis proposes a novel consistency framework. Two constitutive models are adapted to study strain-rate sensitive non-cohesive materials: i) a strain-softening Mohr-Coulomb, and ii) a NorSand model. In addition to increased strength, the proposed framework captures increased dilatation, an early peak deviatoric stress, and relaxation. Finally, a novel sand erosion model is derived using a continuum approximation and limit equilibrium analysis. The erosion law considers geotechnical parameters, the effects of slope, and moisture suction, in a combined manner. The proposed model is theoretically consistent with existing expressions in the literature. It covers a wide range of environmental and geometrical conditions and helps to reconcile the results from FFP testing with the prediction of the initiation of wind erosion. The model was validated in a wind tunnel and is demonstrated to be a viable alternative for predicting sand erosion initiation. This thesis opens up new research prospects, such as improving the soil characterization models or the direct prediction of sand erosion using rapid, reliable, and efficient in-situ testing methods.
Coastal topography and hydrogeology control critical groundwater gradients and potential beach surface instability during storm surges
Paldor, Anner; Stark, Nina; Florence, Matthew; Raubenheimer, Britt; Elgar, Steve; Housego, Rachel; Frederiks, Ryan S.; Michael, Holly A. A. (Copernicus, 2022-12)
Ocean surges pose a global threat for coastal stability. These hazardous events alter flow conditions and pore pressures in flooded beach areas during both inundation and subsequent retreat stages, which can mobilize beach material, potentially enhancing erosion significantly. In this study, the evolution of surge-induced pore-pressure gradients is studied through numerical hydrologic simulations of storm surges. The spatiotemporal variability of critically high gradients is analyzed in three dimensions. The analysis is based on a threshold value obtained for quicksand formation of beach materials under groundwater seepage. Simulations of surge events show that, during the run-up stage, head gradients can rise to the calculated critical level landward of the advancing inundation line. During the receding stage, critical gradients were simulated seaward of the retreating inundation line. These gradients reach maximum magnitudes just as sea level returns to pre-surge levels and are most accentuated beneath the still-water shoreline, where the model surface changes slope. The gradients vary along the shore owing to variable beach morphology, with the largest gradients seaward of intermediate-scale (1-3 m elevation) topographic elements (dunes) in the flood zone. These findings suggest that the common practices in monitoring and mitigating surge-induced failures and erosion, which typically focus on the flattest areas of beaches, might need to be revised to include other topographic features.
Combined portable free fall penetrometer and chirp sonar measurements of three Texas river sections post hurricane Harvey
Jaber, Reem; Stark, Nina; Jafari, Navid; Ravichandran, Nadarajah (2021-12-05)
The US Gulf of Mexico coastal region has repeatedly been subjected to major flood events. Local geotechnical site characteristics and geomorphology can change due to sediment transport processes during such events. However, field measurements during extreme conditions are challenging. This paper discusses initial attempts at a combined geotechnical and geophysical site investigation of the uppermost layers of riverbeds following severe flooding events at three different rivers in Texas: the Guadalupe, Brazos, and Colorado Rivers in terms of sediment strength derived from a portable free fall penetrometer, backscatter intensity recorded by a chirp sonar, and soil sample characterization. Results show low strength sediments (<40 kPa) along the investigated sections of the Guadalupe and Brazos riverbanks. Although sediments in the center of the Brazos River were characterized with higher strength (>50 kPa) and larger grain sizes (d(50) similar to 0.3 mm), sediment strength of the Guadalupe and Colorado Rivers displayed more variations around bridge piers. The spatial variations likely resulted from sediment remobilization processes and local scour under severe hydrodynamic conditions. Both, geotechnical and geophysical results, reflected the observed variations in the riverbed sediments; nonetheless, a quantitative correlation among the rivers was impeded by challenges primarily related to limitations of spatial accuracy and the significant riverbed heterogeneity, as well as shallow water limitations of the chirp sonar.
Computational Analysis of Internal Coral Hydrodynamics
Hossain, Md monir (Virginia Tech, 2020-07-30)
Knowledge of the detailed flow dynamics at the interior of branching corals is critical for a full understanding of nutrient uptake, mass transport, wave dissipation, and other essential processes. These physiological processes depend on the local velocity field, local concentration gradients of nutrients and waste, and the turbulent stresses developed on and above the coral surface. Though the large-scale hydrodynamics over coral reefs are well studied, the interior hydrodynamics, between the branches, remains uncharacterized due to limited optical and acoustic access to the interior. In the current thesis, a three-dimensional immersed boundary method in the large eddy simulation framework was used to compute the flow inside several branching coral colony geometries in order to study the effects of branch density and surface structure on the flow fields in the coral interiors. Two different Pocillopora colony species were studied at different Reynolds numbers. A ray-tracing algorithm was used for capturing the arbitrary branches of these complex geometries to obtain the three-dimensional flow fields within these colonies for the first time. The analysis showed the formation of vortices at the colony interior that stir the water column and thus passively enhance mass transport, compensating for the reduced mean velocity magnitude compared to the free stream value, within the densely branched Pocillopora meandrina colony. Further analysis showed that the mean streamwise velocity profile changes shape along the streamwise direction inside P. meandrina, whereas the mean velocity profile did not change shape from the front to the back for the loosely branched Pocillopora colony, Pocillopora eydouxi. Moreover, turbulent flow field quantities were computed for both these structures, and for two almost identical Montipora capitata colony geometries, one with, and one without roughness elements called verrucae. The analyses demonstrated significant differences in the mean velocity profiles, Reynolds stress, and other flow quantities with changes in colony branch density and surface structure.
Deepwater soil investigation using a free fall penetrometer
Stark, Nina; Parasie, Nico; Peuchen, Joek (Canadian Science Publishing, 2022-12)
Free fall penetrometers (FFP) can improve the efficiency of deepwater geotechnical site investigations. This study investigates the derivation of geotechnical properties from piezocone FFP measurements in the upper 4 m of the seabed at a clayey site with water depths of similar to 1500 m. Data analysis methods as commonly applied in cone penetration test (CPT) analysis were tested with strain rate normalization methods previously suggested for FFP analysis. The logarithmic, inverse hyperbolic sine, and power law approaches were applied for strain rate normalization. Strain rate effects appeared small for FFP cone resistance (with mu = 0.1 for the logarithmic and ,8 = 0.06 for the power law approach), but were apparent. Undrained shear strength derived from FFP data matched overall the CPT results, using an N factor Nkt = 12. Undrained shear strength derived from excess pore pressure measured at the u1 position also yielded favorable results using NAu_FFP = 6. Two soil behavior type classifications showed acceptable agreement between strain rate normalized FFP and CPT results.
Development and Uncertainty Quantification of Hurricane Surge Response Functions and Sea-Level Rise Adjustments for Coastal Bays
Taylor, Nicholas Ramsey (Virginia Tech, 2014-06-16)
Reliable and robust methods of extreme value based hurricane surge prediction, such as the Joint Probability Method (JPM), are critical in the coastal engineering profession. The JPM has become the preferred surge hazard assessment method in the United States; however, it has a high computational cost: one location can require hundreds of simulated storms, and more than ten thousand computational hours to complete. Optimal sampling methods that use physics based surge response functions (SRFs), can reduce the required number of simulations. This study extends the development of SRFs to bay interior locations at Panama City, Florida. Mean SRF root-mean-square (RMS) errors for open coast and bay interior locations were 0.34 m and 0.37 m, respectively; comparable to expected ADCIRC model errors (~0.3 m—0.5 m). Average uncertainty increases from open coast and bay SRFs were 10% and 12%, respectively. Long-term climate trends, such as rising sea levels, introduce nonstationarity into the simulated and historical surge datasets. A common approach to estimating total flood elevations is to take the sum of projected sea-level rise (SLR) and present day surge (static approach); however, this does not account for dynamic SLR effects on surge generation. This study demonstrates that SLR has a significant dynamic effect on surge in the Panama City area, and that total flood elevations, with respect to changes in SLR, are poorly characterized as static increases. A simple adjustment relating total flood elevation to present day conditions is proposed. Uncertainty contributions from these SLR adjustments are shown to be reasonable for surge hazard assessments.
Development of a Sediment Sampling Free Fall Penetrometer Add-on Unit for Geotechnical Characterization of Seabed Surface Layers
Bilici, Cagdas (Virginia Tech, 2018-06-27)
In-situ geotechnical testing of surficial sediment layers in areas of active sediment dynamics can provide essential information about physical and geotechnical variations of sediment properties with regards to active sediment remobilization processes. For example, portable free fall penetrometers (PFFPs) can assist with the detection of mobile sediment layers. They are easy to deploy, and can provide a large spatial coverage in a time- and cost-effective manner. However, they often struggle to provide more detailed information about the properties of mobile sediment layers due to a lack of calibration and validation in existing data sets. Currently, existing sediment samplers often disturb, or ignore the uppermost sediment layers. Simultaneous sediment sampling and geotechnical profiling is needed to fill this gap, and to drive data interpretation forward. A field investigation of surficial sediments was conducted in the wetland waterways of coastal Louisiana in 2014. In-situ tests were conducted using PFFP, and disturbed sediment samples were collected in selected locations. The results allowed us to map changes in sediment strength and stratification, and correlate the geotechnical results to local site characteristics. However, the need for high quality sediment samples for calibration and validation was emphasized by the results. Three different sediment sampler add-on units targeting mobile layers were designed and manufactured based on lessons-learned from the literature. The designs were tested in the laboratory and in the field (Yakutat, Alaska and York River, Virginia) in 2017. The samples were analyzed to understand the influence of different sampler characteristics on collected sample quality, and, to define mobile layer sampler characteristics that enable simultaneous geotechnical testing and the collection of high quality samples. Following field survey campaigns in the York River, Virginia in 2016 allowed to assess surficial sediment layer characteristics and behavior based on a coupled analysis of geotechnical data from in-situ PFFP tests and the sedimentological data collected using box cores and the novel sediment sampler. In summary, novel strategies and instrumentation to carry out simultaneous sediment sampling and geotechnical profiling of seabed surface layers were tested, and new pathways for geotechnical data analysis for the investigation of mobile seabed layers were presented.
The Development of an In-situ Mud Floc Microscope Imaging Device and In-situ Floc Observations from the Lowermost Mississippi River
Osborn, Ryan Todd (Virginia Tech, 2021-05-20)
Mud makes up a large fraction of sediment transported within rivers to the coasts. Predicting where mud will settle is complicated by the cohesive nature of silts and clays, which can combine to form larger aggregates known as flocs. The size and density, and consequently, the settling velocity, of flocs is highly dynamic and depends on factors such as turbulence levels within the flow and biogeochemical components of the water and sediment. To better predict where mud will deposit, more observations of flocs while in their natural environment is required to better understand the controls on when, where, and to what degree mud is flocculated. However, the need for more field observations is complicated by the dynamic and fragile nature of flocs. This necessitates the need for developing in-situ observation methods to ensure that measured floc sizes are representative of their in-situ size, and not a result of sampling methods. In this thesis, a new instrument for in-situ observation of flocs is presented. In addition, two methods using the data collected from the instrument allow the user to: (1) identify sand within the particle data using a machine learning algorithm, and (2) estimate the mass suspended sediment concentration of the mud and sand fractions of suspended sediment independently. Results from using the instrument in the lowermost Mississippi River reveal differences in floc sizes over the water column, and by season. In addition, a unique observation of flocs in the presence of a salt wedge is presented. Overall, the instrument provided the first known observations of flocs within the Mississippi River, and provides a start to better understanding controls on floc sizes within the fluvial environment.
Effects of Shell Hash on Friction Angles of Surficial Seafloor Sediments near Oysters
Consolvo, Samuel T.; Stark, Nina; Castellanos, Bernardo; Castro-Bolinaga, Celso F.; Hall, Steven; Massey, Grace (ASCE, 2022-09-01)
Oysters are hypothesized to affect the shear strength of nearby surficial seafloor sediment as fragments of oyster shells (shell hash) are typically more angular relative to sand particles alone, among other differences. Resistance to shearing is well characterized by the friction angle, which is estimated in this study from vacuum triaxial laboratory and portable free-fall penetrometer field tests. Friction angles of sediment with shell hash were higher relative to those of sediment without shell hash (via hydrochloric acid treatment) on average by about 19% (36.0 degrees-30.2 degrees, respectively). Triaxial confining pressures ranged between 2.1 and 49.0 kPa to simulate subtidal and intertidal aquatic conditions. Regularity (average of particle roundness and sphericity) values of sediment samples with shell hash were found to be less than those of samples without by about 6% (0.66 and 0.70, respectively), which indicates the particle shapes of the former are, overall, more angular and less spherical. Further study and methodology improvements are needed to decrease the approximate 9 degrees friction angle discrepancy estimated from field- and laboratory-based tests. Knowing oysters have the potential to increase sediment shearing resistance helps establish a pathway of how shellfish colonies may contribute to mitigating surficial erosion around coastal infrastructure.
Experiments on the Transformation of Mud Flocs in Turbulent Suspensions
Tran, Duc Anh (Virginia Tech, 2018-06-21)
This dissertation aims to better understand how floc aggregate characteristics and behaviors are modified under different local conditions and how such alterations impact the floc settling velocity, which is one of the most crucial parameters influencing sediment transport modeling. A series of laboratory experiments were conducted to examine the impact of suspended sediment concentration, mixes of clay and silt, and resuspension process to equilibrium floc size and floc settling velocity. In order to observe floc size evolution, a new floc imaging acquisition was first developed. This new method allows flocs in suspended sediment concentration up to C = 400 mg/L can be imaged non intrusively. This new method was applied in all three individual studies, which are composed of this dissertation. The first chapter investigates the behaviors of flocs under constant and decay suspended sediment concentrations within a steady turbulent suspension. In the constant-concentration set of experiments, floc size time series were measured for 12 h for each of the concentration C = 15, 25, 50, 100, 200, 300, and 400 mg/L. In the decay-concentration experiments, clear water was introduced to the mixing tank, simultaneously the suspension was drained out of the mixing tank at the same rate to make the suspended sediment concentration reduce while the turbulent shear was remained unchanged. The data shows that the equilibrium floc size is a weak, positive function of concentration. For example, in order to increase 20% of floc size (approximate 22 um) the concentration needs to be increased by 700% (going from 50 to 400 mg/L). The data also illustrates that during the decrease of concentration from C = 400 to 50 mg/L, the floc size responses to the changes of concentration in the order of 10 min or less. The second chapter examines how silt particles and clay aggregates interact in a turbulent suspension. Floc sizes and settling velocity of three different suspensions, i.e., pure clay, pure silt, and a mixture of clay and silt, were monitored. The floc size data show that the presence of silt particles does not have significant impacts on clay aggregate sizes. Silt particles, however, get bound up within floc aggregates, which in turn increase the settling velocity of the floc by at least 50%. The third chapter examines whether any changes in floc properties during the deposition and resuspension processes. The floc sizes and shapes in a set of experiments with different consolidation times, concentrations, and shear patterns were measured. The conditions at which the flocs deposited or resuspended were maintained the same. The data reveal that floc size and shape of freshly deposited and after resuspended are unchanged. The erosion rate and concentration is a function of consolidation time and the applied shear stress during the deposition phase. Hence, there is a small reduction in resuspended concentration resulting in a slight decrease in resuspension floc size since floc size is also a function of concentration.
A Framework for Assessing Lower-Bound Bearing Capacity of Sandy Coastal Sediments from Remotely Sensed Imagery
Paprocki, Julie Anna (Virginia Tech, 2022-04-28)
With advances in modern technology, satellite-based data is rapidly becoming a viable option for geotechnical site characterization. Commercial satellite data offers high resolution (~25-200 cm), increased spatial coverage on the order of kilometers, short revisit times leading to high temporal coverage, and allows for data to be analyzed rapidly and remotely without the need for physical site access. These advantages are particularly attractive for characterizing coastal sites, where both the strength properties and moisture content can change rapidly in response to tidal stages, wave runup, and storm events. To date, there have been limited investigations into the use of satellite-based data for characterizing geotechnical properties of sandy beach sediments. Furthermore, the use of these moisture contents to estimate the soil strength of beaches has been limited. The goal of this research was to develop pathways to estimate the moisture content of sandy beach sites utilizing satellite-based data. For this study, both optical and synthetic aperture radar (SAR) images were collected at two sites: the Atlantic beach near the US Army Corps of Engineers Field Research Facility in Duck, North Carolina and three distinct sites located near Yakutat, Alaska (Cannon Beach, Ocean Cape, and Point Carrew). During satellite overflight, ground measurements of moisture content, grain size, unit weight, porosity, and bearing capacity were collected. Using the field measurements, this research (1) developed a framework to estimate the moisture content of sandy beach sediments from satellite-based optical images; (2) investigated the necessary collection parameters to estimate the moisture content from SAR images; and (3) developed a framework to estimate the bearing capacity of sandy beaches using moisture contents derived from satellite-based images. The results of this study demonstrated that optical images can produce reasonable estimates of the moisture content when compared to field measurements and are strongly influenced by local morphology. Additionally, SAR images with incidence angles of 30°-50° produced the best results when compared to field measurements. Finally, using the spatial estimates moisture content produced from satellite data and standard sediment, maps of bearing capacity can be developed to predict beach trafficability.
Geotechnical Investigation and Characterization of Bivalve-Sediment Interactions
Consolvo, Samuel Thomas (Virginia Tech, 2020-06-24)
Scour around important foundation elements for bridges and other coastal infrastructure is the leading cause of failure and instability of those structures. Traditional scour mitigation methods, such as the placement of riprap, the use of collars or slots, embedding foundations deeper, or a combination thereof can be costly, require long-term maintenance, and can potentially have detrimental environmental effects downstream. These difficulties with traditional methods are potentially alleviated with the implementation of self-sustaining bivalve (e.g., mussel, oyster, scallop) farms that could act as mats of interconnected living barriers, protecting the seabed from scour. The mats would help to attract larval settlement by making the substrate a more suitable habitat, contributing to the sustainability of the bivalve farms. Colonies of bivalves are already being used as living shorelines for retreatment mitigation, embankment stabilization, and supporting habitat for other marine life. These applications are accomplished, in part, by bivalves' strong attachment capabilities from the bioadhesives they secrete that act as a strong underwater glue, adhering their shells to granular substrate. Some species of mussels have been shown to withstand water flow velocities greater than 6 m/s without detaching. For reference, riprap with a median grain size of about 655 mm has been shown to require a flow velocity of at least 1.7 m/s for incipient motion of the boulder-sized riprap. In addition to the contiguous living bivalve mat offering scour protection, the whole or fragmented shells (i.e., shell hash) that are left behind from dead bivalves are hypothesized to reduce erosion potential. Shell hash-laden sediments should be able to better withstand shearing, thereby increasing the critical shear stress required to erode material, compared to sediment without shell hash. Habitat suitability for bivalve colonies is also an important consideration to evaluate what surface enhancements may be needed for a site to be selected for implementation of bivalve scour mats. Bed surfaces that consist of unconsolidated fine-grained sediment are unlikely to be able to support bivalve species as the organisms could sink into the sediment, not allowing solid anchoring points. In contrast, harder substrates typically found in granular sediments offer much more suitable habitats. Along with testing the influence of shell hash and bioadhesive on sediment behavior, this thesis aims to establish a methodology to evaluate whether a section of seafloor can support bivalves or enhancement materials (e.g., shell, shale, or slag fragments) without them sinking, thereby depriving them of oxygen. Together, the examining of geotechnical aspects of bivalve habitat enhancement through seabed soil alteration and the influence of shell hash and bioadhesives on sediment shear behavior are part of a novel multidisciplinary approach toward this proposed bioengineered scour solution. Consequently, the research objectives explored in this thesis are as follows: (1) characterize morphology of existing bivalve colonies through acoustic and direct field measurements; (2) evaluate the spatial variation of the sediment shear strength in terms of proximity to bivalve colonies; (3) expand the domain of confining pressures and shell hash weight fractions used in sediment strength testing; (4) quantify the changes in shear strength and erodibility from laboratory tests on sampled material with and without the presence of bioadhesives, as well as shell fragments mixed in with the sediment; and, (5) develop a methodology ranking system for the suitability of a surficial sediments to support seeding material to improve benthic life habitat substrates. Three exploratory field surveys were conducted where colonies of oysters and other benthic life were present: in the Piankatank River in Virginia, in the Northwest Arm of the Sydney Harbour in Nova Scotia, Canada, and at the Rachel Carson Reserve in North Carolina. Field sampling techniques included Ponar grab samples, hand-dug samples, X-ray rectangular prism cores, and cylindrical push cores, which were all pivotal to understanding sediment composition, size and shape of particle distributions, as well as in-situ depth profiles of shells. Remote sensing and intrusive instrumentation included a rotary scanning sonar, acoustic Doppler current profilers, CTD (Conductivity, Temperature, Depth) probes, underwater cameras, a portable free-fall penetrometer, and in-situ jet erosion testing which helped to characterize the morphology of the bivalve colonies and the spatial variability of sediment strength. Subsequent laboratory experiments included grain size distribution analyses, vacuum triaxial tests to measure changes in shear strength with and without shell hash, and miniature vane and pocket erodometer tests on bioadhesive-treated sediments. The results showed: (1) a significant increase in the standard deviation of the backscatter intensity where the oyster reef was located; (2) the in-situ sediment shear strength increased slightly closer to the oyster reef at the Piankatank River site; (3) samples with a higher oyster density exhibited less uniform particle size distributions; (4) the presence of less than approximately 4% (by weight) of shell fragments increased the secant friction angle by approximately 6° relative to samples with no shell fragments; and, (5) the harbor bed of the Northwest Arm of the Sydney Harbour is a suitable stiffness for enhancement with shell hash over about 23% of its area. Preliminary testing showed a subtle increase in the torsional shear resistance and a decrease in erodibility for bioadhesive-treated samples; however, further testing is needed for confidence to be achieved in the results due to bioadhesive supply issues.
Geotechnical Measurements for the Investigation and Assessment of Arctic Coastal Erosion — A Review and Outlook
Stark, Nina; Green, Brendan; Brilli, Nick; Eidam, Emily; Franke, Kevin W.; Markert, Kaleb (MDPI, 2022-07-01)
Geotechnical data are increasingly utilized to aid investigations of coastal erosion and the development of coastal morphological models; however, measurement techniques are still challenged by environmental conditions and accessibility in coastal areas, and particularly, by nearshore conditions. These challenges are exacerbated for Arctic coastal environments. This article reviews existing and emerging data collection methods in the context of geotechnical investigations of Arctic coastal erosion and nearshore change. Specifically, the use of cone penetration testing (CPT), which can provide key data for the mapping of soil and ice layers as well as for the assessment of slope and block failures, and the use of free-fall penetrometers (FFPs) for rapid mapping of seabed surface conditions, are discussed. Because of limitations in the spatial coverage and number of available in situ point measurements by penetrometers, data fusion with geophysical and remotely sensed data is considered. Offshore and nearshore, the combination of acoustic surveying with geotechnical testing can optimize large-scale seabed characterization, while onshore most recent developments in satellite-based and unmanned-aerial-vehicle-based data collection offer new opportunities to enhance spatial coverage and collect information on bathymetry and topography, amongst others. Emphasis is given to easily deployable and rugged techniques and strategies that can offer near-term opportunities to fill current gaps in data availability. This review suggests that data fusion of geotechnical in situ testing, using CPT to provide soil information at deeper depths and even in the presence of ice and using FFPs to offer rapid and large-coverage geotechnical testing of surface sediments (i.e., in the upper tens of centimeters to meters of sediment depth), combined with acoustic seabed surveying and emerging remote sensing tools, has the potential to provide essential data to improve the prediction of Arctic coastal erosion, particularly where climate-driven changes in soil conditions may bias the use of historic observations of erosion for future prediction.
Global Change Impacts Education Tool
Burke-Flask, Abby; Stark, Nina; Rodriguez-Marek, Adrian (Center for Geotechnical Practice and Research, 2020)
This presentation provides a broad overview of global change processes and impacts in coastal, permafrost, and inland regions. The presentation was developed to supplement a report for the Center for Geotechnical Practice and Research at Virginia Tech.
High-Fidelity Numerical Simulation of Shallow Water Waves
Zainali, Amir (Virginia Tech, 2016-12-09)
Tsunamis impose significant threat to human life and coastal infrastructure. The goal of my dissertation is to develop a robust, accurate, and computationally efficient numerical model for quantitative hazard assessment of tsunamis. The length scale of the physical domain of interest ranges from hundreds of kilometers, in the case of landslide-generated tsunamis, to thousands of kilometers, in the case of far-field tsunamis, while the water depth varies from couple of kilometers, in deep ocean, to few centimeters, in the vicinity of shoreline. The large multi-scale computational domain leads to challenging and expensive numerical simulations. I present and compare the numerical results for different important problems --- such as tsunami hazard mitigation due to presence of coastal vegetation, boulder dislodgement and displacement by long waves, and tsunamis generated by an asteroid impact --- in risk assessment of tsunamis. I employ depth-integrated shallow water equations and Serre-Green-Naghdi equations for solving the problems and compare them to available three-dimensional results obtained by mesh-free smoothed particle hydrodynamics and volume of fluid methods. My results suggest that depth-integrated equations, given the current hardware computational capacities and the large scales of the problems in hand, can produce results as accurate as three-dimensional schemes while being computationally more efficient by at least an order of a magnitude.
Impact of Patchy Vegetation on Wave and Runup Dynamics
Yang, Yongqian (Virginia Tech, 2016-08-18)
Coastal regions are vulnerable to various natural processes, ranging from normal waves to extreme events. Given the flourishing development and large population along coastlines, various measures have been taken to mitigate the water-induced damage. Nature-based coastal protection, especially vegetation, has attracted unprecedented studies over the past two decades. To enhance understanding of this subject, this dissertation evaluates the impact of patchy vegetation on wave and runup dynamics along coastlines. Selecting from a prototype in Dalehite Cove, Galveston Bay, TX, results from a Boussinesq model (COULWAVE) showed patchy vegetation reduced up to 75% mean shoreward current in the mound-channel wetland systems. These vegetation patches also reduced the primary circulation around mounds, with a power-form relation between circulation size and various parameters (i.e., bathymetry, incident wave and vegetated roughness). Substituting spectral waves for regular waves in the similar wetlands, more energy was transferred into the higher frequencies. The impact of patchy vegetation on wave energy was frequency- and space-dependent, with increased energy observed in specific harmonics and locations. Comparison with unvegetated horizontal bathymetry demonstrated that mound-channel bathymetry was the dominant factor in transferring and dissipating wave energy, while vegetation patches added a fair contribution. As for extreme events, such as tsunamis, laboratory experiments and numerical simulations were conducted to assess the effectiveness of patchy vegetation with various roughness levels, spacings and sizes. Overall, vegetation patches reduced the most destructive loads onshore by up to 80%. Within-patch roughness variation only caused uncertainty on the hydrodynamics around the seaward patches, while the mitigation of extreme loads was not undermined. A logarithmic relation was observed between the protected area from extreme loads and the vegetated coverage. These findings will fill the knowledge gap of hydrodynamics in the presence patchy vegetation, and improve the engineering practice of coastal protection using nature-based infrastructure.
Improvements to the Modeling of Average Floc Size in Turbulent Suspensions of Mud
Kuprenas, Rachel Leah (Virginia Tech, 2018-06-25)
The accuracy of sediment transport models depends on identifying an appropriate sediment settling velocity. Determining this value for mud suspensions can be difficult because cohesive mud particles can aggregate, forming flocs whose sizes are a function of hydrodynamic and physiochemical conditions of the suspension. Here we present a new model refining the predicted floc size based on hydrodynamic conditions and inherited floc sizes, as well as on the salinity of the fluid environment. The improvements come from modifications made to the Winterwerp (1998) (W98) model. These improvements include: limiting floc size to the Kolmogorov microscale and including an initial salinity dependence. Limiting floc size in this way brings the model predictions more in line with flocculation theory and experimental observations. The salinity dependence was introduced based on a preliminary set of experiments that were conducted to examine floc growth rate and equilibrium size under different salinity conditions. In these experiments, increasing salinity from 2.5 to 10 PSU did not affect equilibrium floc size. However, the increases in salinity did result in longer times to equilibrium and an apparent increase in floc density or fractal dimension. The modified W98 model allows calibrated aggregation and breakup coefficients obtained under one set of concentration values (for both sediment and salinity) to be used to predict floc size under other concentration conditions. Comparing the modified W98 model with laboratory data shows more accurate predictive values, indicating that the modified W98 equation is a promising tool for incorporation into larger sediment transport models.
In-Situ Geotechnical Characterization of Soft Estuarine Surficial Sediments Using a Portable Free Fall Penetrometer
Kiptoo, Dennis Kipngetich (Virginia Tech, 2020-07-02)
Knowledge of geotechnical soil properties in the upper meter of the seabed is important for challenges such as scour around submerged structures, management of unexploded ordnances, and generally issues associated with active sediment transport and deposition. Portable free fall penetrometers have been previously used to provide initial information on sediment type, strength, and stratification, but challenges with the calibration of empirical parameters such as the cone factor and strain rate factor hampered the derivation of geotechnical design parameters such as undrained shear strength. This challenge applies particularly in areas of more rare seabed soil conditions such as very soft estuarine sediments. This study aims to advance the analysis procedure of portable free fall penetrometers (PFFP) in soft subaquatic fine-grained soils with natural water contents greater than the liquid limit by estimating the undrained shear strength (su). The logarithmic and power law methods for strain rate correction were investigated at sites in the York River Estuary and yielded a match to vane shear results at a logarithmic multiplier of k=0.1-0.3 and a power law rate exponent of β=0.01-0.03, indicating minimal strain rate effects. Resulting representative cone factors based on sediment strength and profile groupings ranged from 7 to 12 for logarithmic, power law, and no strain correction, and were tested at sites in the Potomac River with similar sediment properties. The PFFP su compared well with mini-vane shear measurements with differences of less than ± 0.5 kPa. Additionally, the PFFP su showed inappreciable differences in strength with or without strain rate application. Therefore, these high water content soils that exhibit little strain rate effects within a soil behavior context, can be better understood through rheological studies. Rheological studies were conducted, and the storage and loss modulus were observed to remain constant when the soil is tested over a range of frequencies. This indicates that the sediment strength is not affected by the rate of soil testing. The outcome of this study is the advanced the use of the PFFP by quantifying the strain rate effects and defining the applicable cone factors for use in estimating the undrained shear strength of soft estuarine marine soils. Furthermore, the understanding of soil behavior of these soils has been explored from rheological context.
Influence of Geotechnical Properties on Sediment Dynamics, Erodibility, and Geomorphodynamics in Coastal Environments Based on Field Measurements
Brilli, Nicola Carmine (Virginia Tech, 2023-06-06)
Geotechnical sediment properties such as moisture content, relative density, bearing capacity, and undrained shear strength have been discussed in the context of coastal sediment dynamics. However, these properties have rarely been assessed in their respective relevance or quantitatively related to sediment transport and erodibility. Also, to date there is no framework available for collecting direct measurements of these properties for estimating initiation of motion and erosion rates. Here, it is postulated that improving the ability to measure geotechnical sediment properties in energetic foreshore environments can improve our ability to predict coastal response to climate change. Through a series of field measurements, the research presented here (1) provides a framework for conducting geotechnical measurements of beaches, (2) advances portable free fall penetrometer (PFFP) data analysis in intertidal environments through the introduction of an impact velocity dependent strain-rate correction factor, (3) relates textural and sediment strength properties derived from PFFP measurements to an erosion rate parameter and hydrodynamically driven bed-level change, and (4) uses PFFP measurements to develop a sediment classification scheme in terms of soil behavior and erosion behavior for a mixed sediment type Arctic environment. Relationships between sediment properties other than grain size, most significantly void ratio, and erodibility parameters highlight the relevance of these measurements in geomorphodynamically active sandy beach environments. For the cohesive sediments in the Arctic, undrained shear strength was also related to an erosion rate parameter, allowing for a categorical framework for erodibility classification to be developed. The cohesive framework was combined with the relationships developed for sandy sediments and used to highlight areas of active sediment transport in the context of local morphodynamic and ice gouging processes. Finally, a simple case study showed how implementing in-situ erodibility parameters was important for long-term morphological modelling. The results represent a step forward in our ability to predict and mitigate climate change related issues from coastal erosion.

Browsing by Author "Stark, Nina"

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