Jack A. Cadigan

The evolution of coastal wetlands is a complex process which is difficult to forecast, made more complicated by the addition of changing climatic conditions. Here, long term ecological and geomorphological data are coupled to geotechnical measurements at a coastal wetland in North Inlet estuary, South Carolina. The coupled methodology is presented and discussed in context of understanding coastal wetland system evolution in a changing climate. Specifically, the root shear strength of Spartina alterniflora across a range of elevations was investigated using a cone penetrometer test. Elevation, shear strength, and biomass are shown to be critically interconnected. Root strength was shown to decrease with increased inundation time and decreased elevation (i.e., mudflats). Conversely, the data set illustrates the importance of maintaining key elevation ranges in relation to sea‐level to optimize wetland resilience.

Mechanisms that control marsh edge erosion include wind‐generated waves, vegetation productivity, land use and land change, and geotechnical properties of sediments. However, existing models for predicting marsh edge evolution focus primarily on edge retreat rates as a function of wave energy while accounting for other controlling factors as empirical constants. This simplification arises from a lack of high‐frequency monitoring of marsh evolutions. In particular, marsh erosion is timescale dependent, and conducting field observations on short temporal and spatial scales could elucidate the progression of erosion, which may improve marsh erosion predictive models. This study developed and validated a near‐continuous camera and erosion pin monitoring system to document marsh edge erosion at a high frequency (i.e., daily) in Terrebonne Bay, Louisiana. This was supplemented with daily wave power to explore the relationships between daily erosion and wave power. Long‐term average erosion rates derived from satellite and aerial imagery from 1989 through 2019 compare similarly to rates derived from longer‐term site visits (i.e., monthly) at approximately 2.2 m/yr. High‐magnitude erosion events (>20 cm/day) are driven by a buildup in wave energy over a 7‐day time period coupled with a strong 1‐day wave event, indicating a gradual reduction in marsh edge resistance with continued wave attack. Long‐term erosion monitoring methods, including monthly field visits, provide results that align well with previously reported relationships between wave power and erosion. High‐frequency measurements, however, illustrate that the previously published trends smooth over the large‐magnitude short‐term erosion events, potentially obscuring the physical processes of marsh edge erosion. For example, satellite and aerial imagery provide a long period of record, but they may underestimate the average annual erosion rate in the region, the effect of which may become exasperated over the varying temporal scales considered in coastal planning efforts across the USA and worldwide.

The Rockefeller Wildlife Refuge, located along the Chenier Plain in Southwest Louisiana, was the location of the sequential landfall of two major hurricanes in the 2020 hurricane season. To protect the rapidly retreating coastline along the Refuge, a system of breakwaters was constructed, which was partially completed by the 2020 hurricane season. Multi-institutional, multi-disciplinary rapid response deployments of wave gauges, piezometers, geotechnical measurements, vegetation sampling, and drone surveys were conducted before and after Hurricanes Laura and Delta along two transects in the Refuge; one protected by a breakwater system and one which was the natural, unprotected shoreline. Geomorphological changes were similar on both transects after Hurricane Laura, while after Delta there was higher inland sediment deposition on the natural shoreline. Floodwaters drained from the transect with breakwater protection more slowly than the natural shoreline, though topography profiles are similar, indicating a potential dampening or complex hydrodynamic interactions between the sediment—wetland—breakwater system. In addition, observations of a fluidized mud deposit in Rollover Bayou in the Refuge are presented and discussed in context of the maintenance of wetland elevation and stability in the sediment starved Chenier Plain.

This study compares the differences in calculated relief well performance at the Profit Island vicinity levee (PIVL) near Baton Rouge, Louisiana, between the three-dimensional (3D) USGS MODFLOW-USG groundwater model and the blanket theory. The 3D hydrogeological model was built using indicator kriging with 192 boring logs and calibrated against measured well discharge rates and piezometric heads during the March to May 1997 flood event. The blanket theory indicates that the total heads at 19 relief wells were lower than the ground surface. However, relief well flow rates observed during the 1997 Mississippi River flood imply that the aquifer pressure was sufficient to overcome both the total hydraulic head equivalent to the ground surface and any frictional head losses. The discrepancy is mainly due to the complex geological setting, effects resulting from relief well installation, and the influence of adjacent relief wells that cause observations to diverge from blanket theory assumptions. The 3D MODFLOW model found that all 84 relief wells maintain a factor of safety (FS) above 1.5 during the peak flood stage in 1997 even though several relief wells produced negligible discharge rates. As a result, this case study indicates that 3D seepage modeling is an accountable and precise tool.

Coastal wetlands are experiencing accelerated rates of fragmentation and degradation due to sea-level rise, sediment deficits, subsidence, and salt-water intrusion. This reduces their ability to provide ecosystem benefits, such as wave attenuation, habitat for migratory birds, and a sink for carbon and nitrogen cycles. A deteriorated back barrier wetland in New Jersey, USA was nourished through thin layer placement (TLP) of dredged sediment in 2016. A field investigation was conducted in 2019 using a cone penetrometer (CPT) to quantify the establishment of soil strength post sediment nourishment compared to adjacent reference sites in conjunction with traditional wetland performance measures. Results show that the nourished area exhibited weaker strengths than the reference sites, suggesting the root system of the vegetation is still establishing. The belowground biomass measurements correlated to the CPT strength measurements, demonstrating that shear strength measured from the cone penetrometer could serve as a surrogate to monitor wetland vegetation trajectories. In addition, heavily trafficked areas underwent compaction from heavy equipment loads, inhibiting the development of vegetation and highlighting how sensitive wetlands are to anthropogenic disturbances. As the need for more expansive wetland restoration projects grow, the CPT can provide rapid high-resolution measurements across large areas supplying government and management agencies with vital establishment trajectories.

To measure the fully softened strength, the normally consolidated and reconstituted specimen must be drained, i.e., no development of shear-induced pore-water pressures, during shearing. This article investigates (1) typical shear displacement rates in torsional ring shear tests that yield drained shearing conditions for a range of fine-grained soils and (2) the problems that can develop if the porous discs in a torsional ring shear device are not clean and saturated prior to placement of the reconstituted soil paste in the specimen container. The data presented herein show that using a clogged or unsaturated porous disc will require a drained shear displacement rate that is one to two orders of magnitude slower than required, e.g., 0.018 mm/min, if the porous discs are clean, maintained, and saturated as recommended by the original equipment manufacturer and the ASTM D7608, Standard Test Method for Torsional Ring Shear Test to Measure Drained Fully Softened Shear Strength and Stress Dependent Strength Envelope of Fine-Grained Soils.

Mechanisms that control marsh edge erosion include wind-generated waves, vegetation, mudflats, anthropogenic factors, and geotechnical properties of sediments. However, existing models for predicting marsh edge evolution focus primarily on edge retreat rates as a function of wave energy while accounting for other controlling factors as empirical constants. This simplification rises from a lack of high frequency monitoring of marsh evolutions. In particular, marsh erosion is time-scale dependent and conducting field observations on short time and spatial scales could elucidate the progression of erosion, which may improve marsh erosion predictive models. This study developed and validated a near continuous camera monitoring system to document marsh edge erosion at a high frequency in Terrebonne Bay, Louisiana. Erosion pins were monitored with the cameras and daily erosion rates were estimated. This was supplemented with daily wave power to explore the relationships between daily erosion and wave power. The largest magnitude erosion events are driven by a buildup in wave energy over a seven-day time period coupled with a strong one-day wave event, indicating a gradual reduction in marsh edge resistance with continued wave attack. Long-term erosion monitoring methods, including monthly field visits, smooths over the large magnitude short-term erosion events. For example, satellite and aerial imagery provide a long period of record, but they seem to underestimate the average annual erosion rate in the region, the effect of which may become exasperated over the varying temporal scales considered in the planning efforts of projects meant to protect the Louisiana coastline.

The Louisiana coastline experiences drastic levels of deterioration due to marsh edge erosion, pipeline and navigation channel construction, saltwater intrusion, and restricted sediment supply. The role of geotechnical properties as a driver of wetland behavior is underscored by the many studies on wetland loss, e.g., marsh edge erosion, shallow subsidence, wetland collapse, and uprooting. A modified piezocone penetrometer was implemented alongside soil cores to evaluate shear strength and hydraulic properties of the vegetated mats at two marshes in Terrebonne Bay, Louisiana. Based on the field measurements, the test method was found to be repeatable and consistent with the ASTM D5778 standard guideline. This study provides a geotechnical assessment of the two locations to better understand the implications of the root-soil matrix on wetland stability. The results show these marshes are supported primarily by an offshore sediment supply and a progression inland from the shore is accompanied by a reduction in soil strength.