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.

Introduction

Coastal wetlands provide a variety of vital ecological services, including fish and wildlife habitat, water filtration, carbon and nutrient sequestration, and flood and storm protections [1-7]. However, due to the sea-level rise, hydrologic and sediment restrictions, rates of wetland fragmentation and loss have accelerated. Thin layer placement (TLP) is a common restoration management strategy used throughout the Gulf, Atlantic, and Pacific coasts of the United States that focuses on improving biotic and abiotic environmental conditions through pumping hydraulically dredged sediments onto the marsh platform [8-17]. While TLP appears to be an effective restoration strategy, there is limited information on the effectiveness of dredged sediments on vegetation establishment and wetland soil strength.

The application of dredged sediments to deteriorating wetlands increase marsh elevation and improve soil aeration in the root zone, thereby increasing redox potentials (Eh), plant productivity and soil accretion allowing marshes to keep pace with relative sea-level rise [9, 16, 18, 19]. However, quantifying belowground soil stability or strength following TLP is still poorly understood and despite being a crucial parameter in the prediction of wetland sustainability (i.e., erosion, ponding, collapse, uprooting) [20, 21]. Only recently has methodology for quantifying wetland strength using a cone penetrometer been standardized [21]. As a result, the following study implements a cone penetrometer test (CPT) to measure belowground soil strength against accepted trajectory performance measures to determine the influence of TLP on biotic and abiotic soil properties.

The CPT is a common method utilized by geotechnical engineers to define stratigraphy and engineering behavior of soil for infrastructure projects (e.g., levees, dams, and bridges) [22]. The standard CPT measures tip resistance, sleeve friction, and pore-water pressure [23], but additional modules can be added to measure soil moisture, resistivity, and temperature [21]. Along with the ability to provide a wider range of data, a major advantage of the CPT over other field methods (e.g., handheld shear vane or torvane) is they provide a continuous resistance profile with depth allowing for a better estimation of subsurface site stratigraphy. In addition, CPTs can be conducted faster than field vane tests which can allow for a more robust spatial data set. The implementation of CPTs in wetlands has not been common due to instrumentation limitations but has become more common over the past decades. In particular, they have been utilized to identify groundwater recharge zones within Massachusetts, USA [24] and to better understand differences in salt marsh stability in coastal Louisiana, USA [20]. Most recently [21], developed a CPT to understand the vertical and spatial variations of geotechnical properties of salt marshes in Louisiana, USA.

In March 2016, the United States of America Corps of Engineers (USACE) Philadelphia District (NAP) began restoration of coastal wetlands in New Jersey, USA via TLP. In total, the restoration deposited 34,405 m3 (45,000 yd3) of dredged sediments from the New Jersey Intracoastal Waterway (NJICW) [19] across five containment sites. The sediment was contained using coir logs that were slashed after stabilization of the dredged sediment (approximately 6 months) to expedite coir log deterioration. Placement target elevations, based on tidal and biological references, ranged from 0.73 to 0.91 m (2.4 to 3.0 ft) NAVD88 [25, 26]. Sediment thicknesses ranged between 5.2 cm to 9.5 cm in the vegetated areas and 32.5 cm to 82.5 cm in the open water features six months after dredging was completed [16].

In this 2019 study, two 100 m transects were conducted within one of the 2016 TLP containment sites moving downstream of the dredge outfall. The first transect traversed a previously ponded section and the second transect traversed a tidal creek. The results are compared to a reference site 500 m northeast of the nourishment site. This study is the first to evaluate the benefits of dredged sediment on a wetland using a CPT and traditional trajectory performance measurements to quantify soil strength gain within a wetland nourishment. These findings can serve as a benchmark for future restoration projects by demonstrating useful establishment monitoring practices for coastal stakeholders.

Background

Site description

The study site is located near Avalon, New Jersey, USA (Fig 1). The sites are within a 17 km2 tidal marsh complex adjacent to Great Sound. Freshwater input to the site is limited, with the nearest rivers located approximately 26 km (Great Egg Harbor River) and 56 km (Mullica River) to the north [27]. The surficial sediments are primarily comprised of Holocene era salt marsh and estuarine deposits (organic silts and clays with sand) to a maximum depth of 18 m [28, 29], which pinches out from east to west [27]. The Holocene sediments are underlain by a Pleistocene sand deposit, which is approximately 37 m thick with some interbedded silts [27, 29].

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.

Wetland degradation of the area was identified in the rapid transition of vegetated marsh platforms to un-vegetated shallow pannes through erosion and vegetation stress over the course of decades [16, 19, 30]. The primary vegetation located across the wetland was a low-form of S. alterniflora that transitions to tall-form S. alterniflora as elevation decreases near ponds and tidal creeks (Fig 1). The vegetated areas are located at an average elevation of 0.61 m NAVD88 (±0.20 m) while shallow open water pannes were found at an average elevation of 0.26 m NAVD88 (±0.13 m) [16]. The site experiences a semidiurnal tidal range of 1.39 m with a MHHW of 0.74 m NAVD88 [30].

Methodology

This field study was comprised of two transects within one of the nourishment sites: (1) a transect that traversed a ponded section, herein referred to as “Transect A”, and (2) a transect that traversed a tidal creek, herein referred to as “Transect B” (Fig 1). Both transects extended 100 m from the dredge outfall with 20 m sample spacing and were selected to show contrasting starting points on the implications of establishment. The field access and sampling collection was approved and overseen by The Wetlands Institute. In addition, the individuals displayed in this manuscript have given written informed consent to publish these case details.

Cone penetrometer

The CPT utilized was specifically developed for use in ultra-soft wetland soils, commonly found in coastal Louisiana, that incorporates both geotechnical parameters (tip resistance, sleeve friction, and pore pressure) and abiotic parameters (soil moisture, electrical resistivity, and temperature) [21]. These ultra-soft soils have high water content and compressibility, may be underconsolidated, and are undergoing self-weight consolidation [31]. For this study, sleeve resistance is considered analogous to shear strength since the goal of this methodology is to provide comparable values as opposed to an engineering design variable. This field equipment consists of a cone piezometer, potentiometer, and a backpack mounted Data Acquisition System (DAS) capable of being manually operated by a three-person crew (Fig 2a). A modified sleeve with 5.2 cm length, perpendicular fins (Fig 2b) was used throughout this study to increase the resolution of soil resistance throughout the vegetated root mass and into the softer organic clay. Four (4) tests were conducted within 1 m2 at each site to better capture substrata variations in root establishment.

Fig 2

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

The CPT soundings are conducted manually, with a target standard penetration rate of 2 cm/s [23]. However, the push speed can vary and occasionally stops if a stiff layer is encountered. A stop in penetration causes a drop to zero push speed (see Fig 3a) and subsequent increase in tip and sleeve resistance values (see Fig 3b at locations with stops) when cone penetration recommences due to soil consolidating around the cone. A Matlab script removes these stops through the application of a Savitzky-Golay filter [32], which applies convolution to smooth the data without distortion of the signal tendency [33]. The push speed in relation to unprocessed and processed sleeve resistance data is shown in Fig 3. The average push speed in Fig 3a is 1.5 cm/s. The unprocessed data (red circles in Fig 3b) show distinct stops at 25 cm and 50 cm. The processed data (black circles in Fig 3b) remove these stops and interpolate between the less dense red circles from 10 cm to 20 cm. The paucity of data in this depth range is due to the significant force required to push through the root mat, causing the CPT to rapidly break through into the underlying soil layer at a rate faster than the sampling frequency of the data acquisition system.

Fig 3

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

Field samples

Along the transects, soil samples were collected using polyvinyl chloride (PVC) cores and a Russian Peat Corer to validate the CPT measurements with traditional performance measures. Soil cores were collected by inserting the PVC pipe (diameter 15 cm) to an approximate depth of 30–35 cm. Samples from these cores were field extruded and cut into 5-cm sections, bagged, shipped on ice, and stored at 4°C until processing. The samples were processed to determine belowground biomass, bulk density, and moisture content with depth. Four (4) Russian Peat Corer samples were taken at each site within the TLP placement area to delineate between the dredged sediment (dark gray and homogenous) and vegetated marsh sediment (brown). An example of the transition from the brown organic to dark grey mineral layer from the Interior at 60 m is shown in Fig 4. The dredged sediment depth was recorded and collected in bags to determine the grain size of the deposited material in accordance with [34].

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.

Results

Sediment redistribution

Post TLP nourishment, researchers from the USACE Engineering Research and Development Center (ERDC) collected dredged sediment samples along Transect A to determine the change in grain size moving down gradient of the dredge outfall. A comparison between the percent of fine-grained material (<0.075 mm) from the 2016 results and the 2019 investigation is shown in Fig 5. The 2016 sampling showed that as distance from the dredge outfall increased, the percentage of fine-grained sediment increased. This is due to coarser material (i.e., sands) falling out of suspension faster than the finer material (i.e., silts and clays). However, the same trend is not evident in 2019, which indicates a more constant percentage of fines across the transect. This redistribution of fines could be attributed to vegetation-driven accretion or movement of the sediment due to hydrodynamic processes and bioturbation [13].

Fig 5

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

Transect A: Soil strength

Along Transect A, short-form S. alterniflora appeared to be the dominant vegetation type, extending out to 40 m from the dredge outfall then transitioned to a mudflat until 80 m where it became sparsely vegetated tall-form S. alterniflora until 100 m. The shear strength (τ) ± 1 standard deviation (SD) with depth and dredged sediment thicknesses for each site through Transect A are shown in Fig 6, and Table 1 summarizes the geotechnical and ecological data. At the dredge outfall and 20 m locations, dredged sediment was recorded down to 5 cm and the sites had approximate shear strengths of 165 kPa. As the amount of placed dredged sediment increases, the corresponding shear strengths decreased with 10 cm of sediment and a strength of 126 kPa at 40 m and ultimately found strengths lower that 90 kPa when the dredged sediment depths were greater than 30 cm. Across Transect A, the underlying soil shear strength averaged 42 kPa with depth.

Fig 6

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

Shaded regions represent ±1 SD.

Table 1

Site description and belowground characteristics along Transect A.

Site	Surface Type	Dredged Sediment (cm)	Belowground Biomass			Underlying Soil
			Peak τ (kPa)	Peak Depth (cm)	Root Depth (cm)	Avg. τ (kPa)
Discharge	Short	5	159	15	29	43
20 m	Short	5	172	14	25	52
40 m	Short	10	126	21	30	55
60 m	Mudflat	37	18	9	-	17
80 m	Tall	29	55	13	-	44
100 m	Tall	36	87	11	22	40

Surface type is Short-form S. alterniflora (Short), Tall-form S. alterniflora (Tall), and Mudflat.

Transect B: Soil strength

Along Transect B, short-form S. alterniflora appeared to be the dominant vegetation type, extending out to 40 m from the dredge outfall then transitioned to tall-form SA at 60 m. Around 80 m, the transect traversed a tidal creek until 100 m where it became a sparsely vegetated tall-form S. alterniflora. The shear strength (τ) ± 1 standard deviation (SD) with depth and dredged sediment thicknesses for each site through Transect B are shown in Fig 7, and Table 2 summarizes the geotechnical and ecological data. At the dredge outfall, dredged sediment was recorded down to 5 cm and the site had a strength of 159 kPa. As with Transect A, the peak shear strengths decreased as the dredged sediment thickness increased with 10 cm of sediment and a strength of 146 kPa at 40 m and then strengths less than 119 kPa for dredge sediment thicker than 31 cm. Across the transect, the underlying soil shear strength averaged 36 kPa with depth.

Fig 7

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

Shaded regions represent ±1 SD.

Table 2

Site description and belowground characteristics along Transect B.

Site	Vegetation Type	Dredged Sediment (cm)	Belowground Biomass			Underlying Soil
			Peak τ (kPa)	Peak Depth (cm)	Root Depth (cm)	Avg. τ (kPa)
Discharge	Short	5	159	15	29	43
20 m	Short	5
40 m	Short	10	146	17	36	36
60 m	Tall	31	119	11	23	49
80 m	Mudflat	37	15	9	18	11
100 m	Tall	40	84	12	24	40

Surface type is Short-form S. alterniflora (Short), Tall-form S. alterniflora (Tall), and Mudflat.

Reference site

Within the Reference Site, CPTs were conducted within a short- and tall-form of S. alterniflora, with short-form being the dominant vegetation. The reference area was assumed as stable (i.e., no discernable deterioration) over the past three decades based on time-lapse aerial images. Due to time constraints, all coring equipment was being utilized within the nourished area so no soil cores were collected. The average shear strength values ±1 SD bands for both forms of vegetation are shown in Fig 8. For the short-form, the peak shear strength of 263 kPa was found at a depth of 11 cm and the root depth was recorded down to 40 cm. The tall-form of S. alterniflora was found at a lower elevation than the short-form, within a shallow confined pond. The peak shear strength of 123 kPa was found at a depth of 11 cm and the vegetation influence was recorded down to 30 cm. At the short- and tall-form sites, the underlying soil exhibited an average shear strength of 46 kPa. The tall-form root strength was weaker than the short-form likely due to the lower elevation resulting in a longer inundation period. The effect of roots on strength was only observed down to 30 cm for the tall-form when compared to the short-form of 40 cm root zone.

Fig 8

Average shear strength at the reference sites.

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

Performance measures

To quantify the effectiveness of the CPT against other performance measures, the belowground biomass, bulk density, and moisture content were measured at each site along the transects and compared to the shear strength profile. Shear strength and belowground biomass exhibited analogous trends with depth, increasing from the marsh surface to a depth of 13 cm where peak belowground biomass is located and then a similar decrease trend with depth to 30 cm, Fig 9.

Fig 9

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

Horizontal bars are ±1 SD.

The larger shear strengths (>75 kPa) correlated to samples of higher belowground biomass (>0.04 g/cm3), Fig 10. The weaker shear strengths were generally found within the dredged sediment which exhibited lower amounts belowground biomass. It is postulated that as the dredged sediment is further established, the presence of belowground biomass will increase the strength to resemble the measurements within the native “marsh sediment”.

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.

A summary of shear strength as it relates to belowground biomass, bulk density, and moisture content plots, divided into varying thicknesses of dredged sediment: ≤5 cm, 5–10 cm, 10–30 cm, and >30 cm are shown in Fig 11. The greatest shear strengths of 160 kPa were found in areas of ≤5 cm dredged sediment thickness with a peak depth of 13 cm, and the lowest shear strength values at peak depth were in areas >30 cm (Fig 11a). The shaded regions in the CPT profiles illustrate the variability, and it signifies that the <5 cm and 5–10 cm thicknesses exhibit less scatter (i.e., uncertainty). The greatest belowground biomass contribution of ~0.8 g/cm3 was found in areas that received ≤5 cm of dredged sediment (Fig 11b). The same trend was observed in the 5 to 10 cm thickness but with lower belowground biomass values. In contrast, the lowest belowground biomass contribution of 0.01 g/cm3 was found in areas of that received greater than 30 cm TLP because of their lower elevation and hence more deteriorated vegetation. Bulk densities were highest in areas that received >30 cm TLP, which is consistent to bulk densities of inorganic sediment [35]. The bulk densities for less than 10 cm TLP exhibit organic-rich soils [36]. At higher TLP thicknesses, bulk densities are approximately 0.6 g/cm3 near the surface for about the first 15 cm, but gradually decrease to bulk densities similar to that of the < 10 cm TLP (Fig 11c). This behavior could be a result of mixing of the dredged sediment with native sediment and provides another means to understand the trajectory of TLP. The moisture content [w (%)] behaves in direct contrast to bulk density because organic material inherently contains more pore water. The moisture content tends to increase with depth to more than 500% for the ≤5 cm and 5–10 cm sites, yet remained at constant with depth in the areas that received >30 cm of sediment (Fig 11d). These higher moisture contents are a key indicator of greater biomass establishment in areas receiving less than 10 cm of sediment whereas greater sediment inputs have lower moisture contents due to lower concentrations of organic material.

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.

Discussion

The purpose of this study was to investigate the usefulness of applying a CPT to investigate the establishment of wetland shear which can provide decision makers with validation information of restorations. A key observation of Fig 11 is that the peak strength and belowground biomass decrease with increasing TLP thickness, which suggests live roots and rhizomes may be providing a sharp increase in strength. Another observation from Fig 11 showed that as the thickness of TLP increases, the scatter of CPT also increased. This signifies the shear strength in the dredged sediment is significantly more variable because of a wide range of particle gradation (sand to clay) and different levels of biomass productivity [23]. In addition, a transition in the belowground biomass profiles occurs at or near 10 cm of TLP, which could signify a tipping point that the geotechnical properties will resemble vegetated wetlands or inorganic sediments from the TLP [14]. This information can help guide future sediment nourishments by providing design constraints on upper limits of placement thicknesses and a methodology to quickly gauge the establishment of vegetation.

During the 2019 field investigation, tracks of ponded marsh were found throughout the area (Fig 1). CPTs were conducted within the ponded tracks (4) and the directly adjacent vegetated zones (6). The tip resistances (q) with depth for both conditions alongside a picture of the test area after a CPT was completed is shown in Fig 12. In the vegetated zones, the q increases to a peak resistance before decreasing to the underlying soil. However, the ponded zones indicate that the peak q occurs at the soil surface and then decreases with depth, exemplifying the site was disturbed through compaction. During the nourishment of the site, this was a common path for marsh buggies carrying the coir logs and dredging pipe and the effects of the continued compaction are still prevalent within the marsh 3.5 years later. It has been previously documented how compaction induced via heavy machinery during the restoration of a wetland results in negative impacts on root development and biomass production, which was attributed to the increase in bulk density inhibiting root penetration [37, 38]. This demonstrates how sensitive wetland environments are to anthropogenic disturbances and careful consideration is required during planning and construction to minimize lasting negative impacts like this.

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.

The results of this study showed that within the nourished site, the shear strength values were higher than the tall-form of S. alterniflora at the reference site but were significantly less than the peak shear strength experienced within the short-form S. alterniflora. Additionally, the peak shear strength was found at a deeper depth than the two reference sites, which could signify that the vegetation roots have yet to fully establish themselves within the dredged sediment. Overall, the average underlying soil shear strength was 39 kPa for the nourished site. If the two mudflat sites were removed, the average increases to 45 kPa, which is consistent with the reference site. This shows that the underlying soil exhibits an intrinsic strength that is consistent throughout the nourished and control areas and not affected by surface wetland functions.

Prior to the TLP nourishment, this site was highly degraded as vegetated portions of wetlands rapidly transitioned to un-vegetated shallow pannes. This downward shift of elevation relative to sea-level induces higher levels of stress on the vegetation health, lowering the wetland resistance to physical stressors [25]. This is evident in the lower shear strength of the tall-form of S. alterniflora compared to the short-form, which resides at a higher elevation in the tidal regime (i.e. lower inundation periods). The rate of sea-level rise in this area is 1 cm/yr [39] while accretion is 0.3 cm/yr [40] leaving an elevation deficit of -0.7 cm/yr. The average elevation gain from this TLP nourishment increased the marsh elevation by 24 cm, indicating that this elevation deficit is offset by 34 years.

Conclusions

This field investigation was performed using co-located cone penetrometer tests and soil cores to measure wetland strength establishment post-TLP nourishment. These measurements were compared to traditional performance measures (e.g., belowground biomass, bulk density, and moisture content) across two transects within a nourished wetland. The primary findings of this study are summarized as follows:

The nourished site exhibited weaker shear strength than the reference site but vegetation establishment does appear to be occurring in the previously ponded areas.

Belowground biomass and CPT shear strength measurements correlated with depth, demonstrating that this methodology can provide accurate quantifications on site development trajectory in a more efficient and less intrusive manner than traditional ecological and geotechnical techniques.

The underlying soil below the roots exhibits an intrinsic strength that is consistent throughout both restored and reference areas and are not affected by surface wetland functions and is likely a function of the long-term geological history of the site.

Sediment sampling directly post-nourishment and 3.5 years later showed a redistribution of grain size gradients, which can be attributed to coastal processes reworking the sediments, accretion, or bioturbation.

Heavily used construction tracks compacted a small area of the marsh platform, inhibiting the establishment of vegetation 3.5 years post-construction.

For this specific case, baseline data would have provided additional information to better indicate wetland establishment trajectories and thus is highly recommended for future field investigations. The results presented herein capture one point in the establishment process of a sediment-nourished wetland, and future monitoring is necessary to fully understand the long-term impacts of TLP on marsh resilience and maintenance. While other meaningful studies utilize extended growing seasons to understand the long-term effects of sediment placement on the biological, chemical, and physical dependencies, this field investigation demonstrated the usefulness of the cone penetrometer when evaluating wetland strength establishment. The utilization of the cone penetrometer in combination with traditional performance measures can provide rapid assessments of wetland status for coastal restoration and management practices.

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16 Oct 2020

PONE-D-20-25909

Establishment of Soil Strength in a Nourished Wetland using Thin Layer Placement of Dredged Sediment

PLOS ONE

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Additional Editor Comments (if provided):

This manuscript on the establishment of soil strength using thin layer placement requires major revisions to address overarching concerns and specific comments identified during the review process. Please carefully consider the comments and recommendations for revision that the reviewers have made, especially as they pertain to the scope and terminology used to describe the project and its implications for wetland restoration. For instance, utilizing standard terminology for nourishment and thin layer placement projects and expanding the literature cited to encompass broader geographic range will help reach the broader audience of Plos One. Such revisions may necessitate a change in title as well. There are also a series of questions and comments about the site description and methods that need to be addressed to improve quality. And finally, it is crucial that the objectives of the project (TLP evaluation relative to reference and use of CPT to measure success) be clarified throughout the paper and substantiated by the data. Reviewer 3 makes some useful suggestions in this regard. Addressing suggestions from these reviews will greatly improve the clarity of the paper and its potential impact in the field of wetland restoration, while also expanding the scope to be more suitable for the international readership of the journal.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Partly

Reviewer #3: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: N/A

Reviewer #3: N/A

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Improvement of English is suggested. For example, the article mentions "At the time of investigation" on several occasions. Rewriting of those sentences is suggested. Also, the article has issues associated with same word once UPPER CASE and once LOWER CASE. Some of the references were not written appropriately. Mixing of present and past tense. The pdf should contain the suggested corrections. Please check those.

Reviewer #2: Overall, I think this is an interesting article that should be accepted for publication in PLOS One pending major revision as detailed in the comments below. The manuscript addresses the impacts on sediment parameters of a thin layer placement. Sea level rise is a challenge faced by wetlands globally, thus evaluating strategies to help marshes keep pace with sea level rise is important. Despite the importance of the topic, this paper needs major revision before being acceptable for publication in PLOS One. The major and minor issues that I found are documented below.

My major comments include

1) Introduction – I think that the overall introduction of this paper needs to be broadened in scope, terminology, and literature review to be more applicable to the global audience of PLOS One.

a. PLOS One is read by a wide range of scientists, not just engineers so I think that some of the background need broader focus, and I have tried to suggest places below (in specific comments) where I feel those additional references or information is needed.

b. A lot of the terminology used (esp. nourishment) is regionally specific.

c. Most of the literature discussed is older and focused on just a couple of ecosystems in the United States (Gulf of Mexico and East Coast). This should be broadened to include more recent studies (e.g. Cahoon et al. 2019 Estuaries and Coasts) and more diverse locations (e.g. Thorne et al. 2019 Ecol Eng from CA, Wigand et al. 2017 Estuaries and Coasts).

2) Methods – Overall, there is a lot of repetitiveness and misplaced information between the Methods and Results. Both sections should be cleaned up and better organized. In addition, the site and experiment description need to clearer as I lost track of what a region versus a site versus a transect versus a TLP area was.

a. The figures and tables are emphasized in the sentence structure instead of the result being discussed with a parenthetic reference to the table or figure that presents the data.

b. A lot of the text in the Results seems to just describe the numbers from the figures which I found confusing. I could see the numbers on the figures and would prefer to have the pattern described in the Results text.

c. Several key methods about vegetation cover and grain size are missing. s

3) Figures – I found several of the figures not as clear as could be due to missing labelling or need of additional details. These are discussed in detail below.

4) Throughout the manuscript, I felt as if the parameters (shear strength) measured were done well but then how broadly these could be applied were overstated. CPT strength measurements may correlate with the measured parameters but it is overstated to claim that they predict restoration success or the entire ecological restoration trajectory. I think being more clear in what is measured at the start and what is inferred would improve the manuscript.

Abstract

L. 59 – This is really not establishment of soil strength but redevelopment of (post-TLP) or changes in.

L. 60 – What are establishment markers? This is too vague for an important point in the abstract.

L. 60 – I think this should read “lower strengths”, not “weaker strengths” which is confusing.

L. 64 – 66 (Starting During the construction process) – This sentences seems unnecessary and like an anecdote, not an abstract sentence.

L. 61 – The root system? Whose roots? Vegetation?

L. 64 – Trajectories is also too vague. You should state vegetation or belowground biomass specifically because there are a lot of restoration trajectory parameters (e.g. invertebrates, algae) that you are not measuring.

Introduction

L. 74 – Habitats should not be plural.

L. 76 hurricanes have accelerated

L. 77 – I think is more true if written “loss, leading to need for comprehensive coastal restoration management plans” so this did not always happen.

L. 81 – 83 – This statement is not always true – it is hypothesized that TLP can be conducted without ecological implications but that varies with location and impacts measured and thickness of sediment applied. A more in-depth literature review is required here. This is also a sentence that needs more modern and more geographically varied references (as discussed above).

L. 83-85 – There are more than just this method to conduct TLP – if you are going to discuss this you should acknowledge other methods or drop this sentence.

L. 91-93 – This repetitive of earlier paragraph.

L. 98 – Spartina-dominated

L. 99 – Where were these experiments? Location and type of marsh would be useful for understanding the context of this reference.

L. 101-102 – You also need to expand this discussion or delete this sentence.

L. 103 – 105 _ I would split this into two sentences after the reference as right now it has a misplaced modifier.

L. 106 – Delete establishment as I think this is confusing.

L. 108 – What do you mean by physical index properties?

L. 109 – You refer to the dredge outfall, but we need more information. Is this permanent or just for construction?

L. 111 – 113 – We also need more information about the reference site to evaluate its choice as a comparison. It is unfortunate that you are missing sediment cores from there but other site parameters will help justify its choice as a reference site.

L. 113 – This is an example of where I feel the implications of this work are overstated. This will not define wetland restoration success but will instead advance our understanding of how CTP relates to specific biomass parameters. You can make the argument (which you should) that AG and BG biomass are related to important ecosystem development metrics.

Background

Figure 1 – This figure needs larger context. I would like to see the inset be expanded to show more of the East Coast, not just a section of NJ and DE. This is hard for a non-local or global reader to understand.

L. 120 – On the bay side of what? And from Figure 1, it appears as if is actually on the Oceanside? Please clarify

L 120 – the wetland – which wetland? The TLP site or the reference site or the whole thing?

L. 127 – When was the area identified as transitioning? The idea of this rapid transition is mentioned several times throughout the paper but not enough detail on the time frame and the observed transition is presented to completely understand.

Figure 1b is useful.

L. 131 onwards in this paragraph – The methods behind these measurements needs to be explained. In fact, these data are repeated in different places. I think the methods for this survey need to be explained in the Methods, and these data need to be presented in the Results.

L. 134 – This is an example of the terminology confusion – what is areas here? Is this the shallow pannes discussed above (L. 132)?

L. 134 – Again my comment about rapid transition. Over what time period?

L. 144 – This seems repetitive of information above or I do not understand the terms for your site versus marsh etc.

Methods

L. 150 – Another landscape term is introduced here “cells”. These should appear on the map.

L. 151 – I would rename “Interior”

L. 153 – 154 – This sentence is confusing. Do both transects begin at the dredge outfall or do they begin at different starting points? Or do you mean that the two transects have different elevations and starting habitats?

L. 157 – Write out the reference name in this case.

L. 159 – I think the more accurate term is abiotic, not ecological.

L. 166 – What four tests? Are these pilot evaluations?

L. 174 onwards should be moved to Results.

L. 188 – Again what are sites? The TLP versus reference marshes? Or the points along the transect?

L. 190 – Here is an example of leading with the Figure instead of stating the results and referring to the figure.

L. 195 – Was the vegetation rinsed to remove mud?

Results

L. 200 – 207 – This should be moved to Methods

L. 209 – The amount of elevation loss over what time period?

L. 201 – Here another landscape term is introduced – region. While these are shown on the map, it is confusing to understand how cell, site, region all fit together in the sampling scheme.

L. 219 – 222 – These sentences have a lot of interpretation and belong in the Discussion.

L. 223 – 227 – These sentences belong in methods.

L. 225 – No grain size methods were discussed at all in the Methods. This needs to be added.

L. 227 – Post-nourishment

L. 227 – I am not sure what is meant by an increase? I think it just needs to be reworded to be clearer.

L. 231 = This belongs in the Discussion, and this is an example of what I find overstated. We don’t know that this is vegetation-driven accretion from your study.

Headings of Establishment here confused me – establishment of what?

L. 234 – 236 – This is already described in the Methods so I think could be deleted.

L. 235 – Vegetation methods need to be described better in Methods (as discussed above) so this should not be “appeared to be” but an actual community analysis or description.

This paragraph has lots of examples of leading with the Figure or Table when the results should be summarized.

L. 240 – Shear strength increased down the core or over time?

This whole section is difficult for me to read – I think the figures speak for themselves and I would prefer to see the pattern described without the exact numbers. With a well-designed figure, I can read the exact numbers myself.

L. 240 – At the time of investigation? Tell us when it was.

Table 1 is not vegetation but AG and BG biomass

L. 268 – Again the opening sentences are repetitive of information from earlier and should be deleted.

L. 296 – More attention needs to be paid to the missing data from Reference site. I understand that things happen but it could affect conclusions.

L. 296 – Dominant, not dominate

Compaction – This section does not contribute much to the manuscript in my mind so either needs to be better linked and connected to the story or deleted.

L. 373 – This is missing something….

Discussion

L. 381 – This is still one of the points on which I am confused. When are we talking about specifically? Why was the site so degraded? Salt pannes can be natural so I am not convinced that the transition to salt pannes alone was bad or a sign of degradation.

While I like linking this back to ecological theory, I am not convinced of the stage model as it relates to your data and conclusions.

Reviewer #3: This manuscript addresses soil strength and biophysical factors of thin layer placement as a restoration technique for degraded marshes. The manuscript describes the use of a cone penetrometer as a proxy for soil strength along two transects, a heavy machinery impacted zone, and a control area. Overall, the manuscript addresses important data gaps in methodology and marsh restoration trajectories following thin layer placement. However, the manuscript requires additional clarification and text in several locations, and mainly in the discussion section. I am recommending major revisions to this manuscript, which upon completion will provide important new data to the wetland literature. Specific comments are provided below.

60 Clarify “traditional wetland establishment markers” to specify belowground biomass, bulk density, and moisture content. Are these parameters only related to wetland establishment, or more broadly applicable to wetland condition?

61 This statement is a little misleading. Based on the site description, large areas were un-vegetated prior to thin layer placement. Post thin layer placement, these areas are beginning to vegetate. Is a comparison between a vegetated control area and an un-vegetated open treatment area valid?

80 The use of “nourishment” is also misleading. Are you nourishing elevation? The objective of the thin layer placement is to increase elevation. A secondary effect is nutrient addition (i.e. “nourishment”) from the dredged material.

85 There are examples of thin layer placement on the West coast, and should be included in this section. Specifically, Seal Beach, CA is an example of a West Coast TLP project.

92 Is there really a significant contribution of organic material from the dredged sediment?

102 Add a transition here; the change in ecological discussion to shear strength is abrupt.

129 The literature uses “short form” and “tall form” of S. alterniflora.

154 Establishment of what?

157 Is ultra-soft wetland soils analogous to unconsolidated soils? If so, include unconsolidated soils as an additional term that people are familiar with. If not, define what “ultra-soft” refers to.

204 Consolidation of the dredged material is expected. However this is not addressed until much father in the text. Recommend this information is moved up in the text to clarify that elevation loss was not un-expected.

231 Vegetation driven accretion? The accumulation of organic material through vegetation growth redistributes fine materials? More likely redistribution of fine material is through hydrodynamic processes and bioturbation; above ground biomass would increase fine material deposition. Clarification of this statement is needed.

265 Table 1. What does Vegetation Depth (cm) describe? The original marsh surface? Clarification is needed.

326 Clarify what “dredge sediment is further established” means. Vegetation establishment within the placement sediment?

361 Correct biomass to organic material.

365 Tracks of what?

308 The discussion section is very thin. One important point to discuss is open water areas that received dredged material in the context of the vegetated control areas. These are not directly comparable, yet offer an opportunity to discuss the trajectory of an open water area that receives dredge material moving towards shear strength of a vegetated area. Additionally, a discussion of Avalon marsh trajectory is missing, and perhaps is not possible given one sampling date? How does this fit into the conceptual model? Going back to the objectives outlined in 1121-115, was this wetland restoration project a “success”? How do you define success with one sampling date with CPT? There seems to be two competing objectives of this paper: 1. how does the TLP site compare to the reference area and 2. use of the CPT as a methodology to measure restoration success and trajectory. Revise the discussion section, and throughout as needed, to address these two objectives.

387 Does this calculation consider additional potential elevation gains from organic matter accumulation and resulting accretion from healthy vegetation growth?

**********

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Submitted filename: PONE-D-20-25909_reviewed.pdf

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8 Mar 2021

A complete response to each comment has been provided in the accompanying word document. Thank you!

Submitted filename: PONE-D-20-25909_Response_to_Reviewers.docx

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27 Apr 2021

Establishment of Soil Strength in a Nourished Wetland using Thin Layer Placement of Dredged Sediment

PONE-D-20-25909R1

Dear Dr. Harris,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Julia A. Cherry

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Thank you for your careful attention to the reviewers' comments.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: I appreciate the careful revisions of this manuscript as well as the detailed response to reviewers. The revisions dramatically improve the manuscript and make it both more understandable as well as more applicable to a variety of readers.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #2: No

29 Apr 2021

PONE-D-20-25909R1

Establishment of Soil Strength in a Nourished Wetland using Thin Layer Placement of Dredged Sediment

Dear Dr. Harris:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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