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

Abstract

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.

Fig 1. Overview of sediment nourishment study site in Avalon, New Jersey, USA.

(a) Location of the study site. (b) Investigation layout, coir log containment, dredged outfall, and pipeline locations. Aerial images from The National Map Orthoimagery courtesy of the U.S. Geological Survey (2020). (c) An image showing tall- and short-forms of S. alterniflora. Image taken by author.

Fig 2. (a) Performing a cone penetrometer test and (b) cone penetrometer components.

Fig 3. Example of (a) push speed in relation to the (b) sleeve resistance (f_s) values.

Fig 4. Example of Russian Peat Core sample from Transect A at 60 m.

The shallower portion (left) showing the dredged sediment and the deeper (right) showing the vegetated marsh sediment.

Fig 5. Percent of fine grained material (<0.075 mm) with distance from the dredge outfall from field investigations.

Fig 6. Average shear strength and dredged sediment depths along Transect A moving away from the discharge.

Shaded regions represent ±1 SD.

Fig 7. Average shear strength and dredged sediment depths along Transect B moving away from the discharge.

Shaded regions represent ±1 SD.

Fig 8. Average shear strength at the reference sites.

(a) Short- and (b) tall-form S. alterniflora sites. Shaded regions represent ±1 SD.

Fig 9. Analogous trends between belowground biomass (g/cm³) and shear strength (kPa) with depth.

Horizontal bars are ±1 SD.

Fig 10. Shear strength compared to belowground biomass measurements within the dredged sediment and marsh sediment samples.

Arrow illustrates the trajectory of dredged sediment to established marsh sediment.

Fig 11. The influence of dredged sediment depths across varying intervals.

(a) Shear strength, (b) belowground biomass, (c) dry bulk density, and (d) moisture content (%) at ≤5 cm (red), 5–10 cm (blue), 10–30 cm (green), and >30 cm (gray). Shaded regions and horizontal bars represent ±1 SD.

Fig 12. Average tip resistances for the (a) vegetated and (b) ponded zones, and (c) testing locations across construction tracks.

Red and blue circles denote the vegetated and ponded zones, respectively. Shaded regions represent ±1 SD.