Establishment of soil strength in a nourished wetland using thin layer placement of dredged sediment.

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.

Wetland Shear Strength with Emphasis on the Impact of Nutrients, Sediments, and Sea Level Rise.

This paper presents a comprehensive review of shear strength measurements in wetland soils, which can be used to make inferences of the influence of nutrients and sediments on wetland health. Ecosystem restoration is increasing across the Gulf of Mexico and in other coastal systems, with management questions related to soil strength among the most critical to address for the sustainability of restoration programs. An overview of geotechnical engineering principles is provided as a starting point to understand basic soil mechanics concepts of stress, effective stress, pore-water pressure, unit weight, and shear strength. The review of wetland shear strength measurements focuses on the hand-held vane shear, torvane, cone penetrometer, and wetland soil strength tester. This synthesis shows that vane shear measurements can identify the shear strength trend in horizontal and vertical spaces and may be an indicator of wetland soil strength. However, the significant uncertainty of the vane shear measurements may preclude making conclusions about shear strength values without further testing and calibration of the devices. The torvane results show considerable scatter such that it is not recommended for quantitative shear strength measurements. The cone penetrometer represents a technique that is independent of operators and provides a high density of measurements with depth. It signifies the state-of-practice of wetland shear strength testing and is a reasonable tool to measure spatial and temporal variations in soil strength and other geotechnical properties (e.g., pore-water pressure, soil moisture, resistivity, and temperature) in wetlands. The wetland soil strength tester provides insight into the wetland soil resistance in the first 15 cm, which is the zone where most belowground biomass is present. Recommended future research includes evaluating the uncertainty in all in-situ soil strength testing methods, developing relationships between different field instruments, and establishing consistent statistical methods and field-testing procedures to make inferences and assessments.