Evaluation Techniques for the Beneficial Use of Dredged Sediment Placed in the Nearshore

McFall, Brian C.; Brutsché, Katherine E.; Priestas, Anthony M.; Krafft, Douglas R.

doi:10.1061/(ASCE)WW.1943-5460.0000648

Open access

Technical Papers

May 31, 2021

Evaluation Techniques for the Beneficial Use of Dredged Sediment Placed in the Nearshore

Authors: Brian C. McFall, Ph.D., M.ASCE https://orcid.org/0000-0001-9575-0012 [email protected], Katherine E. Brutsché, Ph.D. [email protected], Anthony M. Priestas, Ph.D. https://orcid.org/0000-0002-5328-927X [email protected], and Douglas R. Krafft [email protected]Author Affiliations

Publication: Journal of Waterway, Port, Coastal, and Ocean Engineering

Volume 147, Issue 5

https://doi.org/10.1061/(ASCE)WW.1943-5460.0000648

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Abstract

Maintaining navigable waterways through dredging is vital to society and the economy, especially as global transport and cargo loads increase. Decision makers require rapid techniques to evaluate potential placement alternatives for the dredged sediment. When appropriate, dredged sediment can be placed in the nearshore as a berm or a mound, often with the expectations of dissipating wave energy further offshore and inducing shoreward sediment transport. However, the exact placement area requires the water depth, hydrodynamic climate, and sediment size to be considered. Previous work used the well-known depth of closure concept to assess berm stability. A web-based Sediment Mobility Tool (SMT) was recently developed to assess the probability of sediment transport and direction with the knowledge of a few simple parameters. In this study, 20 historical projects that placed dredged sediment in the nearshore are reviewed to evaluate the depth of closure equations predictive capability on berm stability. The frequency of sediment mobilization and cross-shore transport direction are estimated for nine historical projects. The evaluated projects represent a diverse range of construction methods, placement geometries, sediment characteristics, and wave climates. The techniques used to estimate sediment mobilization frequency and transport direction are universal. These techniques are an essential step for engineers and planners to evaluate the potential volumes of dredged material that nearshore placement operations might yield to nourish a beach profile.

Introduction

As waterways worldwide experience increased cargo loads, the importance of dredging channels to maintain navigable waterways is as imperative as ever, and there is a need for efficient techniques to rapidly evaluate sites to place the dredged sediment. The beneficial use of dredged material through placement in the nearshore region allows sediment to remain within the active littoral system, and therefore, the natural wave action may transport fine material offshore and coarse material will be transported toward the shoreline. Projects that place dredged sediment in the nearshore have many names: nearshore nourishment, nearshore placement, profile nourishment, or shoreface nourishment. When sediment is intentionally placed as a shore parallel submerged feature, which creates an artificial sand bar, it is called a nearshore berm. If the berm footprint is approximately equidimensional, it is commonly referred to as a mound (Hands et al. 1992).

Construction of an offshore bar or mound can fulfill different objectives depending on placement depth. In deeper waters, berms can be constructed as stable features that are intended to dissipate high energy storm events. Alternatively, berms can be constructed at shallower depths with the goals of actively nourishing the beach profile, temporarily dissipating wave energy, and retaining the sediment in the littoral system. McLellan and Kraus (1991) developed an engineering strategy to design and construct a successful nearshore berm project. This included sediment compatibility and quantity, dredging equipment and operations, placement timing, local wave conditions, and economic considerations.

Although beach profile nourishment through the nearshore placement of dredged sediment is common practice, the critical questions of how often the sediment will be mobilized and where the sediment will go remain poorly understood (Huisman et al. 2019). Projects have been monitored (Brutsché et al. 2019a) and physical models have been conducted (Bryant and McFall 2016; Smith et al. 2015, 2017) to gain insight into the processes that drive the morphological evolution and shoreline response to these projects. Historically, the empirical depth of closure (DoC) equations were used to determine whether sediment that was placed in the nearshore would be actively mobilized. The Sediment Mobility Tool (SMT) is a web application that was developed by the U.S. Army Corps of Engineers (USACE) to answer essential mobilization and transport questions (McFall and Brutsché 2018). The goal of this study is to document techniques applied to case studies for the rapid evaluation of nearshore nourishment sites using dredged material.

Methods

The SMT web application calculates the DoC, sediment mobilization frequency, and cross-shore transport direction of the sediment placed in the nearshore (McFall and Brutsché 2018). For the calculations, the user defines the shoreline by drawing it with a graphical user interface, then input the sediment grain size, placement depth, longshore current 1 m above the bed, water temperature, and salinity. The SMT web application uses offshore wave hindcast data from the closest Wave Information Study [WIS (Hubertz 1992)] station to a proposed nearshore placement site and converts the wave characteristics into a shore-normal polar coordinate system following the convention of McFall et al. (2016). Waves are transformed to the nearshore using the conservation of energy flux and Snell's Law. The SMT web application applies hourly wave characteristics for 10 years (from January 1990 to January 2000). For application to historical projects in this study, the wave hindcasts from the actual monitoring dates were used for validation of the model. The local wave characteristics are then used to evaluate the potential sediment mobility of the proposed nearshore placement site. The equations used to calculate the DoC, sediment mobility frequency, and cross-shore transport direction are detailed in the following sections.

DoC

The DoC is the depth along a beach profile where sediment is not significantly moved by the nearshore processes or the depth beyond which the cross-shore beach profiles compiled over time converge for a given site (Dean and Dalrymple 2002). Waves might move sediment seaward of this depth, but the net transport does not significantly change the bed elevation. Hallermeier (1978, 1981a) defined an Inner and an Outer DoC. The Inner DoC delineates the littoral zone's seaward extent as identified by significant bed activity due to waves and nearshore circulation. The Outer DoC is the seaward limit of the shoaling zone where surface waves cause little sediment transport. Hallermeier (1978) defined the Inner DoC (h_l) as

h_{l} = 2.28 H_{e} - 68.5 (\frac{H_{e}^{2}}{g T_{e}^{2}})

(1)

where H_e = effective wave height or wave conditions that are exceeded only 12 h/year (0.137% of the time); T_e = associated wave period; and g = gravitational constant. The effective wave height (H_e) can be determined from the annual mean significant wave height (

\bar{H_{s}}

) and the standard deviation of the significant wave height (σ_s) as

H_{e} = \bar{H_{s}} + 5.6 σ_{s}

(2)

Using this relationship, Hallermeier (1981a) derived a simplified approximation for the Inner DoC as

h_{l} = 2 \bar{H_{s}} + 11 σ_{s}

(3)

The Outer DoC (h_i) was defined by Hallermeier (1981a) as

h_{i} = (\bar{H_{s}} - 0.3 σ_{s}) \bar{T_{s}} {(\frac{g}{5000 d_{50}})}^{0.5}

(4)

where

\bar{T_{s}}

= wave period associated with

\bar{H_{s}}

; and d₅₀ = median grain size.

Birkemeier (1985) used high fidelity bathymetric profiles from June 1981 to December 1982 at the USACE Field Research Facility in Duck, North Carolina, to evaluate the Hallermeier (1978) Inner DoC equation and found a better approximation for the data as

h_{l} = 1.75 H_{e} - 57.9 (\frac{H_{e}^{2}}{g T_{e}^{2}})

(5)

and a simplified approximation as

h_{l} = 1.57 H_{e}

(6)

Hands and Allison (1991) plotted the relationship of Hallermeier (1978, 1981a) Inner and Outer DoC's [Eqs. (1) and (4)] to historical nearshore berms to compare whether 11 historical placements were active or stable. The stable nearshore berms maintained most of the placed volume for several years and the active nearshore berms dispersed within a few months. All nearshore berms that were placed shallower than the Inner DoC were active, and the one nearshore berm that was constructed deeper than the Outer DoC was stable. In addition, nearshore berms that were constructed 50% shallower than the Outer DoC were active, but to different degrees. This stability figure is extended in the results section to include nine additional sites.

Sediment Mobility

It is important to know whether sediment placed in the nearshore will be actively mobilized. The SMT uses two methods to investigate sediment mobility (McFall et al. 2016). The first method analyzes the bed shear stress that is calculated using linear wave theory. The second method analyzes the near-bed velocity that is calculated with nonlinear stream function wave theory. Applying both techniques to critical mobilization thresholds provides a range for the sediment mobilization frequency.

The critical shear stress for the first method is determined following a procedure given by Soulsby (1997) and Soulsby and Whitehouse (1997) as

D_{*} = d_{50} {(\frac{g (\frac{ρ_{s}}{ρ} - 1)}{ν^{2}})}^{1 / 3}

(7)

θ_{c r} = \frac{0.30}{1 + 1.2 D_{*}} + 0.055 [1 - \exp (- 0.020 D_{*})]

(8)

and

τ_{c r} = θ_{c r} g (ρ_{s} - ρ) d_{50}

(9)

where

D_{*}

= dimensionless grain size; ρ_s = sediment density; ρ = water density; ν = kinematic viscosity of water; θ_cr = Shields parameter; and τ_cr = critical shear stress.

The bottom shear stress is calculated using a method described by Soulsby (1997) for combined currents and waves. The current induced shear stress (τ_c) is given as

τ_{c} = ρ {(\frac{\bar{U} κ}{\ln (z / z_{0})})}^{2}

(10)

where

\bar{U}

= assumed mean current velocity; κ = von Karman's constant (κ = 0.4); z₀ = bed roughness length (z_o = d₅₀/12 for flat sand); and z = height of the current velocity assumed by the user (z = 1 m). The wave-induced shear stress (τ_w) is given as

τ_{w} = \frac{1}{2} ρ f_{w} U_{w}^{2}

(11)

where f_w = wave friction factor; and U_w = bottom wave orbital velocity as determined by Soulsby (1997), which integrates the linear wave theory orbital velocity across the JONSWAP wave spectra. Linear wave theory is appropriate when the wave steepness (height/wavelength) is small, and the magnitude of the orbital velocity is the same beneath the crest and the trough.

The maximum shear stress (τ_max) from the waves and currents shear stresses is calculated as

τ_{m} = τ_{c} [1 + 1.2 {(\frac{τ_{w}}{τ_{c} + τ_{w}})}^{3.2}]

(12)

and

τ_{max} = [{(τ_{m} + τ_{w} \cos ϕ)}^{2} + {(τ_{w} \sin ϕ)}^{2}]^{1 / 2}

(13)

where τ_m = mean bed shear stress; and ϕ = angle between the wave and current directions.

Fig. 1 shows the histogram of the maximum bed shear stress calculated by the SMT. The vertical dashed lines show the critical bed shear stress for several grain sizes. The legend shows the various grain sizes, critical shear stress, frequency of mobility (f_M), and mean mobility score (M) which is calculated as

M = \bar{(\frac{τ_{max} - τ_{c r}}{τ_{c r}})}

(14)

Fig. 1. Histograms of (a) bed shear stress; and (b) near-bed velocity calculated by SMT for a historical nearshore placement site at Newport, California, in a depth of 5.5 m. The vertical dashed lines indicate the critical thresholds to initiate sediment mobility for the various grain sizes.

The second method to investigate the sediment mobility uses the near-bottom critical velocity, and the near-bottom velocity is calculated using nonlinear stream function wave theory. The critical near-bottom velocity (u_cr) is calculated by a procedure given in Ahrens and Hands (1998), which is based on research by Hallermeier (1980) and Komar and Miller (1974) as

u_{c r} = \sqrt{8 g γ d_{50}} for d_{50} \leq 2.0 mm

(15)

and

u_{c r} = [0.46 γ g T^{1 / 4} {(π d_{50})}^{3 / 4}]^{4 / 7} for d_{50} > 2.0 mm

(16)

where γ is defined as γ = (ρ_s − ρ)/ρ; ρ_s = sediment density; and ρ = water density. Ahrens and Hands (1998) used Dean’s (1974) stream function wave theory table to derive the following equations for the near-bottom wave-induced velocity based on stream function wave theory for the wave crest (u_{max crest}) and trough (u_{max trough}) as

u_{\max crest} = (\frac{H}{T}) {(\frac{h}{L_{0}})}^{- 0.579} \exp [0.289 - 0.491 (\frac{H}{h}) - 2.97 (\frac{h}{L_{0}})]

(17)

and

u_{\max trough} = - (\frac{H}{T}) \exp [1.966 - 6.70 (\frac{h}{L_{0}}) - 1.73 (\frac{H}{h}) + 5.58 (\frac{H}{L_{0}})]

(18)

where h = water depth; H = wave height in the placement site; and L₀ = offshore wavelength given by L₀ = (g T²)/2π. The maximum near-bottom velocity was taken as u_max = max(|u_{max crest}|, |u_{max trough}|).

Similar to the first method, the SMT generates a histogram with the maximum near-bed velocity and the critical thresholds for several grain sizes as shown in Fig. 1. The frequency of mobility (f_Mu) for each grain size is shown in the legend and their respective mean mobility scores (M_u) are calculated as

M_{u} = \bar{(\frac{u_{max} - u_{c r}}{u_{c r}})}

(19)

The epistemic uncertainty for these techniques was quantified using a case study at Vilano Beach, Florida (McFall et al. 2020). Two nearshore berms were constructed with dredged sediment at approximately 3 m depth during the summer of 2015 (Brutsché et al. 2017; McFall et al. 2017). The median grain size of the placed sediment was 0.33 mm. The nearshore berms were modeled with the SMT and Coastal Modeling System. The confidence intervals that were calculated for the frequency of mobility encompassed the epistemic uncertainties associated with each step of the SMT, which included the offshore hindcast wave conditions, wave transformation, critical thresholds for sediment motion, maximum bed shear stress, and maximum near-bottom velocity. The random sampling scheme for wave height (H), period (T), and wave direction parameters consisted of first sampling the H value from a normal distribution with μ = H and σ = 0.30μ. Normal distribution fits were then applied to the subsamples of the parameters T and wave direction in the WIS data set that corresponded to the value of the sampled H. Wave T and direction values were sampled from these distributions and correlated to the sampled H. A collection of 10,000 u_max and τ_max values were generated using the Monte Carlo sampling procedure for each of the 87,590 nearshore H values that were transformed from the WIS station, making it possible to develop histograms that accounted for the uncertainty contributed by the wave forcing and the equations.

The median grain size's frequency of mobility was calculated using linear wave theory at Vilano Beach, Florida, as 94.8%, with 95% confidence limits of 85.4% and 99.4%, respectively. Using stream function wave theory, the median grain size was estimated to be mobilized by 98.5% of the waves with 95% confidence limits of 97.2% and 99.2%. The frequency of sediment mobility for the linear wave theory method (bed shear stress) tended to have an increased confidence interval compared with the stream function method (near-bed velocity). This increase was likely due to the high frequencies of mobility and the physical restriction that the frequency of mobility cannot exceed 100%. The uncertainty from the wave transformation from offshore to nearshore is case dependent according to the bathymetry. Therefore, the uncertainty and confidence intervals noted for this case study provides a qualitative understanding of the uncertainty at other sites.

Cross-Shore Sediment Transport

Numerous researchers have hypothesized that nearshore berm behavior should be similar to natural sand bars, and various attempts have been made to predict their movement, as presented in the extensive literature review provided in Larson and Kraus (1989). Continuing this work, Larson and Kraus (1992) used the time series of cross-shore profiles that were collected at Duck, North Carolina, from 1981 to 1989, to correlate offshore bar migration patterns to various dimensionless wave and sediment parameters. From these, the dimensionless Dean number (D) reasonably predicted the offshore bar migration and is given as

D = \frac{H_{0}}{ω T}

(20)

where H₀ = offshore wave height; ω = sediment fall speed; and T = wave period. D values >7.2 induced erosive, offshore bar migration, and values <7.2 resulted in accretionary, onshore bar migration. The sediment fall speed depends on the grain size diameter and was calculated with the equations derived by Hallermeier (1981b). With this method, finer sediments tend to be transported offshore, and coarser sediments tend to migrate onshore.

Historical Sites

The monitoring of dredged sediment placed in the nearshore is essential to improve the understanding of project performance (Brutsché et al. 2019a). The transport of the placed sediment and the morphological response to the placed sediment are dependent on the site-specific hydrodynamic conditions, sediment characteristics, and morphologic conditions. Data collected from historical monitoring campaigns are used to validate and calibrate models. A summary of historical nearshore berms that were constructed with dredged sediment is listed in Table 1, and a brief description of the sites evaluated in this study is detailed in the following section in chronological order. All of the evaluated sites are from the United States.

Table 1. Summary of historical monitored nearshore placement projects

Placement site	Year	Geometry	Volume × 10³ (m³)	Depth (m)	Berm height (m)	d₅₀ (mm)	Observed activity	Reference
Santa Barbara, California	1935	Berm	154	6.1	1	—	Stable	Hall and Herron (1950)
Atlantic City, New Jersey	1935–1948	Berm	2,700	4.5–7.6	—	—	Stable	Hall and Herron (1950)
Long Branch, New Jersey	1948	Berm	460	11.6	2.1	0.34	Stable	Hall and Herron (1950)
Durban, South Africa	1970	Berm	2,500	7–16	0–8.3	0.13–0.495	Active and stable	Zwamborn et al. (1970)
Copacabana Beach, Brazil	1970	Berm	2,000	4–6	—	0.4–0.5	Active	Vera-Cruz (1972)
New Haven, Connecticut	1974	Mound	1,170	18.3	9.1	Sand/silt	Stable	Bokuniewicz et al. (1977)
Lake Erie, Ohio	1975	Berm	18	17	0.36	Silt	Stable	Danek et al. (1978)
New River Inlet, North Carolina	1976	Multiple mounds	27	2–4	1.8	0.49	Active	Schwartz and Musialowski (1977)
Limfjord Barriers, Denmark	1976	Berm	30	4–5	2.1	0.25–0.3	Active	Mikkelsen (1977)
Tauranga Bay, New Zealand	1977–1978	Mound	2,000	11–17	9	—	Stable	Healy et al. (1991)
Dam Neck, Virginia	1982	Mound	650	11	3.3	0.08	Stable	Hands and DeLoach (1984)
Fire Island, New York	1987	Berm	323	4.9	1.5–3	—	Active	McLellan et al. (1988)
Sand Island, Alabama	1987	Berm	352	5.8	2	0.22	Active	Hands and Bradley (1990)
Coos Bay, Oregon	1988	Mound	4,000	20–26	4.6–7.6	0.25–0.3	Stable	Hartman et al. (1991)
Mobile, Alabama (Outer Mound)	1988	Irregular mound	14,300	14	6.6	0.0015–0.25	Stable	Hands et al. (1992)
Silver Strand, California	1988	Berm	113	4.8–8.5	2.1	0.18	Active	Andrassy (1991); Juhnke et al. (1990)
Humboldt, California	1988–1989	Two berms	—	15.8, 21.3	—	0.23	Active	Hands and Allison (1991)
Kira Beach, Australia	1988	Berm	1,500	7–10	2	—	Active	Smith and Jackson (1990)
South Padre Island, Texas	1989	Berm	125	7.9	—	—	Active	Aidala et al. (1992)
Mt. Maunganui, New Zealand	1990	Berm	80	4–7	2	—	Active	Foster et al. (1994)
Perdido Key, Florida	1991	Berm	3,000	5–6.5	1.5	0.3	Stable	Otay (1994)
Port Canaveral, Florida	1992	Berm	121	5.5–7.0	1.6	0.4	Active	Bodge (1994)
Newport Beach, California	1992	Multiple mounds	980	1.5–9.1	4.4	0.27	Active	Mesa (1996)
Terschelling, the Netherlands	1993	Berm	2,100	5–7	2	0.2	Active	Kroon et al. (1994)
Chetco Inlet, Oregon	1996–2013	Irregular mound	277	4.9–7.9	—	8.4	Active	Gailani et al. (2019)
Noordwijk, the Netherlands	1998	Berm	1,700	5–8	1	0.4	Active	Ojeda et al. (2008)
Egmond aan Zee, the Netherlands	1999	Berm	900	7.5	1	0.228	Active	van Duin et al. (2004) and Gijsman et al. (2019)
Brunswick, Georgia	2003	Mound	—	6	4–5	0.35	Active	Smith et al. (2007)
Ocean Beach, California	2005–2007	Irregular mound	690	9–14	—	0.18	Active	Barnard et al. (2009)
Fort Myers, Florida	2009	Berm	175	1.2–2.4	1	0.16–0.18	Active	Brutsché et al. (2014)
Vilano Beach, Florida	2015	Berm and mound	115	3	—	0.33	Active	Brutsché et al. (2019b)
Ogden Dunes, Indiana	2016	Multiple mounds	107	5.5	0–1.5	0.15	Active	Young et al. (2020)

Santa Barbara, California (1935)

In 1935, a 154,000 m³ nearshore berm was constructed in a depth of 6.1 m with a hopper dredge updrift of eroding beaches in Santa Barbara, California (Hall and Herron 1950). The placed material had a height or ridge elevation of approximately 1 m. The volume of sediment placed was reported as “exceptionally stable”, although the trough landward of the mound filled 0.7–1 m with sediment.

Atlantic City, New Jersey (1935–1942)

Dredged sediment from a nearby channel was placed in the nearshore of Atlantic City, New Jersey, in 4.5–7.6 m of water. The sediment was placed as shallow as the hopper dredge could operate. A total of 2,700,000 m³ from four dredging events was placed in the nearshore from 1935 to 1942. No substantial amount of sediment from the placement was observed on the beach (Hall and Herron 1950).

Long Branch, New Jersey (1948)

In the spring and summer of 1948, a hopper dredge placed 460,000 m³ of sediment in 11.6 m of water that resulted in a nearshore berm 2.1 m high, 230 m wide, and 1,130 m long north of the pier in Long Branch, New Jersey (Hall and Herron 1950). The mean grain size on the beach ranged from 0.35 mm in the summer to 0.91 mm in the winter with a mean diameter of 0.6 mm. The nearshore nourishment had a median grain size of 0.34 mm and was monitored with a wave gauge and regular bathymetric surveys for 18 months. The placement was observed to be stable.

New River Inlet, North Carolina (1976)

A split-hull barge placed 26,750 m³ in the nearshore of New River Inlet, North Carolina, during the summer of 1976 between the 2 and 4 m depth contours (Schwartz and Musialowski 1977). The dredged sediment was placed in small discrete mounds, which had relief dimensions of up to 1.8 m, within a 215 m coastal reach. The placed sediment created an initial wave shoaling area with a minimum depth of 0.6 m. The placed sediment was coarser (d₅₀ = 0.49 mm) than the native material (d₅₀ = 0.14 mm). The placement was studied for 13 weeks to assess the shoreline response and the placed material's net transport direction. The nearshore berm migrated onshore and displaced the natural inner bar further shoreward, and the trough downdrift of the placement site filled with the placed sediment, which demonstrated the longshore transport. The placed sediment's dominant transport direction was shoreward (toward the coarsest native sediment) and then alongshore.

Dam Neck, Virginia (1982)

A total of 650,000 m³ of sediment was dredged from nearby ship channels and placed in 11 m of water in the Dam Neck disposal area 4–5 km offshore of Virginia Beach, Virginia (Hands and DeLoach 1984). The single mound had a base of 40 ha and a height of 3.3 m. The dredged sediment had a median grain size of 0.08 mm (Ahrens and Hands 1998). The mound was considered stable, because most of the volume remained in the placement footprint for years and no significant gains or losses were observed around the perimeter of the mound; however, sediment from the crest of the mound appeared to move landward during storms (DeLoach 1985).

Fire Island, New York (1987)

During the summer of 1987, a hopper dredge placed 323,000 m³ of sediment dredged from nearby Fire Island and Jones Inlets in the nearshore of Gilgo Beach, New York, to mitigate erosion. A shore parallel nearshore berm was constructed along the 4.9 m contour with a height ranging from 1.5 to 3 m. The nourishment was monitored with bathymetric transect surveys at 30 m spacing, sediment samples, and aerial photography (McLellan et al. 1988). The placed sediment was quickly mobilized and incorporated into the littoral system (Hands and Allison 1991).

Sand Island, Alabama (1987)

The Sand Island nearshore berm was constructed in 1987 with 352,000 m³ of maintenance dredged material from the Mobile Bay, Alabama, entrance channel in a depth of 5.8 m and was 1,800 m long with a 2 m height. The berm was constructed of fine sand with a median grain size of 0.22 mm to maintain the sediment in the system and test the limits of nearshore nourishment in a low wave energy environment (Hands and Bradley 1990). At the same depth of the nearshore berm, and with the same dredged sediment, a mound with a diameter of 200 m was constructed southeast of the berm. The mound steadily migrated onshore during the first 2 years following construction. The longer berm did not migrate as a unit, but both features migrated landward (Hands and Allison 1991).

Mobile Outer Mound, Alabama (1988)

The Mobile Outer Mound was constructed with 14,300,000 m³ of dredged sand, silt, and clay material in a depth of 14 m (Hands et al. 1992). The mound was 6.6 m high. The sediment was dredged from the navigation channel in Mobile Bay, Alabama, and the bar channel at the bay entrance. The sediment from Mobile Bay, Alabama, was mostly clay-rich mud of high plasticity, and the sediment dredged from the entrance channel was predominately sand (McLellan and Imsand 1989). The median grain size from 101 surface samples on the mound was from 0.0015 to 0.25 mm (Hands et al. 1992). The mound was monitored to comply with management regulations, which provided insight into fine-grained material stability and improved the understanding of the effects of a large mound on fishery resources and the wave climate. McLellan and Imsand (1989) documented the monitoring efforts from March 1987 to January 1988. The mound was stable (Hands and Allison 1991), and was expected to be stable for many years (McLellan et al. 1990).

Silver Strand, California (1988)

A nearshore berm was constructed approximately 10 km south of the entrance to San Diego Bay, California, in Silver Strand State Park's nearshore with 113,000 m³ in December 1988 in depths from 4.6 to 8.5 m with a height of approximately 2.1 m (Andrassy 1991). The sediment was dredged from the entrance channel to San Diego Bay, California, and consisted of medium sand (d₅₀ = 0.18 mm) with 12% fine material (Juhnke et al. 1990). The evolution of the nearshore berm was monitored with surveyed profile transects. The nearshore berm moved on average 34 m shoreward the first 70 days post placement and the shoreline leeward of the berm widened an average of 40 m during the same timeframe. The shoreline gains indicated that the berm created a reduced wave climate on the leeward side, which captured the longshore transported sediment. The berm was considered active and 63% of placed volume remained in the project's local beach profile.

Humboldt, California (1988–1989)

Sediment dredged from the entrance channel to Humboldt Bay, California, in 1988 and 1989 was placed in a nearshore disposal site in depths from 15.2 to 18.3 m. Before this placement, another disposal facility (SF3) was used with water depths from 18.3 to 27.4 m. Data are limited for these nearshore placement projects and both placement sites were discontinued in 1990 due to concerns of possible fishery impacts (Conner et al. 2006). Hands and Allison (1991) noted two nearshore berms at Humboldt, California, one in a depth of 15.8 m and the second, given the acronym SF3 in their table, in 21.3 m. Both nearshore berms were noted as active, but the original designs were too conservative. The median grain size was 0.23 mm (Ahrens and Hands 1998). Additional details about these projects could not be located.

South Padre Island, Texas (1989)

A 1.2 m high nearshore berm was constructed in a depth of 7.9 m in the nearshore of South Padre Island, Texas, with approximately 125,000 m³ of sediment that was dredged from Brazos Santiago Pass, Texas. The dredged sediment was primarily sandy material and construction was completed in January 1989. The nearshore berm monitoring consisted of bathymetric surveys, sediment samples, wind measurements, and seabed drifters. The berm crest moved approximately 60 m onshore from January to March 1989 and had little movement during the calmer summer months. The May 1990 survey indicated only 26,800 m³ of material remained within the berm footprint (Aidala et al. 1992).

Perdido Key, Florida (1991)

In 1991, a nearshore berm was constructed with 3.0 × 10⁶ m³ of dredged sediment at Perdido Key, Florida, in depths from 5 to 6.5 m (Otay 1995). The median grain size of the dredged material was 0.3 mm. The sediment was placed in a nearshore berm shape 1.5 m high, which left 4.5 m of water above the berm (Work and Otay 1996). The nearshore berm remained stable with only lateral spreading of sediment and the berm height did not change more than 9 cm during the 5 years of monitoring. The nearshore berm provided shoreline protection, because the shoreline on the berm's lee side receded 35 m compared with 60 m outside of the sheltered area (Otay 1994).

Port Canaveral, Florida (1992)

In July 1992, 121,000 m³ of sediment was dredged from the entrance channel of Port Canaveral, Florida, and placed as a nearshore berm at Cocoa Beach, Florida. The berm was placed between 5.5 and 7 m depths with a height of approximately 1.6 m (Bodge 1994). The dredged sediment had a median grain size of 0.4 mm and 3% fine material (defined as passing the #230 US Standard sieve). The nearshore berm migrated landward, and after 1 year only 42% of placed volume could be detected in the survey area. Although Bodge (1994) did not detect longshore transport, the volume removed from the survey area may have been from longshore transport.

Newport Beach, California (1992)

In 1992, approximately 980,000 m³ of dredged sediment was placed in multiple mounds in 1.5–9.1 m water depths in the nearshore of Newport Beach, California. The mean placement depth was 5.5 m and the dredged sediment consisted of 83% sand and 17% fine material, with a median grain size of 0.27 mm. The berm was considered moderately active, because the berm crest eroded, and the material was transported shoreward. However, the berm's centroid remained fairly stable (Mesa 1996).

Chetco Inlet, Oregon (1996–2013)

From 1996 to 2013, 277,000 m³ of dredged sediment was placed south of Chetco Inlet, Oregon, in depths from 4.9 to 7.9 m. The placed sediment was actively transported in the littoral system and the sediment was placed in the same location annually (Gailani et al. 2019). The dredged material was described as silty gravel. The channel's sediment outside the breakwater consisted of 79% gravel and the channel sediment inside the breakwater consisted of 69% fine material. The weighted average of the channel samples' median grain size was 8.4 mm.

Brunswick, Georgia (2003)

Since 2000, sandy dredged sediment from the Brunswick Federal Entrance Channel on the Atlantic coast of Georgia has been placed in designated nearshore sites in depths from 6 to 20 m (Smith et al. 2007). Mound C was constructed in 6 m of water with a mound height of 3–4 m. During the 2003 monitoring campaign, the mound crest was 600 × 600 m. The mound's sediment was active, and the mound deflated during the 5-month monitoring campaign (Johnson and Work 2005).

Ocean Beach, San Francisco, California (2005–2007)

Approximately 690,000 m³ of dredged sediment was placed in Ocean Beach's nearshore in San Francisco, California, at depths between 9 and 14 m. The sediment was placed between 2005 and 2007. The sediment was moderately active with large waves during the winter 2006–2007, which transported the mound onshore (Barnard et al. 2009).

Fort Myers, Florida (2009)

A nearshore berm was constructed of dredged sediment in depths of 1.2–2.4 m in 2009 at Fort Myers Beach, Florida. The 175,000 m³ of dredged sediment placed in the nearshore consisted of sand and fine material. The northern half of the berm had a median grain size of 0.18 mm and the southern half had a median grain size of 0.16 mm. The berm was 1,600 m long, 120 m wide, and 1 m tall. The berm was segmented with multiple gaps smaller than 15 m along the berm crest (Wang et al. 2013) due to the construction technique. This berm was active and the transport was accelerated by tropical storms (Brutsché et al. 2014). It migrated landward and there was a gain of sediment on the subaerial beach leeward and on the adjacent beach southwest of the berm. Sediment samples taken during the post placement monitoring indicated the fine sediment (silt) in the placed material was transported offshore and did not transport shoreward onto the beach.

Vilano Beach, Florida (2015)

Two nearshore berms were built with 115,000 m³ of dredged sediment in approximately 3 m of water at Vilano Beach, Florida, during the summer of 2015. Construction was completed with a split-hull hopper dredge and waves were observed breaking over the placed material during construction (Brutsché et al. 2019b). The two berms were built with equivalent volumes but had different shapes. One was constructed as an elongated bar and the other had a peaked mound. Each berm was built approximately 300 m in the alongshore direction with a 300 m space between them. Both berms were very active and dispersed within 4 months of construction (Brutsché et al. 2017). Shoreline salients were observed on the lee side of both berms, but with different shapes. The salient behind the bar-shaped berm was more extensive and more gradual than the narrower and more peaked salient behind the mound-shaped berm. Based on the surveys, it was unclear whether the volume gained on the shoreline was from the berms or due to waves breaking further offshore, which caused the natural longshore transport to be retained.

Ogden Dunes, Indiana (2016)

A total of 107,000 m³ of dredged sediment was placed at a depth of 5.5 m in the Lake Michigan nearshore at Ogden Dunes, Indiana, during the summer of 2016. The placed sediment had a median grain size of 0.15 mm and was placed with bottom-dumping scows. Concerns about damaging the scow's bottom hanging gates resulted in the construction of an irregular mound with multiple discrete peaks within the nearshore placement area. The placed sediment was active. The mound's centroid moved onshore and a similar distance alongshore during monitoring (Young et al. 2020). An extended monitoring campaign was not conducted because the lake freezes during winter.

Application to Historical Sites

To calculate the Hallermeier (1981a) Inner and Outer DoC's for 11 historical sites before 1990, Hands and Allison (1991) used 20 years of WIS wave hindcasts that were transformed to the placement depth of the nearshore berm. These DoC values were preserved in this study to maintain the relationship to the placement depth described in their paper, and because median grain size for some of the sites is unavailable, which prevented the Outer DoC's recalculation. The placement depths for the historical sites in Hands and Allison (1991) analysis was verified using additional references and all but one was maintained for this study. The Dam Neck, Virginia, project placement depth was changed to 11 m from 7.6 m to be consistent with the original reference. The DoC for nine additional project sites from 1991 to 2016 was calculated using 30 years of WIS wave hindcasts from 1980 to 2010 transformed to the placement depth. The additional sites were added to the stability figure to expand the range of project sites tested. A map that shows all the historical project sites that were investigated in this study is shown in Fig. 2.

Fig. 2. Map of historical nearshore placement project sites examined. Circles indicate sites with DoC stability analysis, and stars indicate the full SMT analysis that includes DoC, sediment mobility, and cross-shore transport analysis.

The SMT sediment mobility techniques were applied to 9 out of the 20 historical sites in the United States from 1982 to 2009 (Priestas et al. 2019). These sites were chosen based on their proximity to a WIS station, temporal alignment with the available WIS hindcasts (1980–2014), documentation of the required SMT input parameters of placement depth and median grain size, and documented morphological evolution. The sites span various conditions that include grain size, geography, wave climate, and placement depth. Projects were included from the US Atlantic, Gulf of Mexico, and Pacific shorelines. Three out of the nine projects were reported stable, and the remaining sites reported onshore migration.

The required input to apply the SMT mobility and cross-shore transport equations are given in Table 1. When a range of values was provided, such as a range of depths, the average of the range was applied. WIS wave characteristics were transformed to the nearshore for the exact dates of monitoring noted in each project's reference. In many cases, the exact dates of monitoring were explicitly noted in the project reference. However, in some cases, the monitoring timeline was more generalized (e.g., May 1990 to May 1992). For these cases, the first day of the month for the range of the monitoring period was used (e.g., May 1, 1990 to May 1, 1992). The qualitative morphological evolution of the placed sediment from the references was compared with the SMT results.

Results

The Hallermeier (1981a) Outer and Inner DoC for the 20 historical projects are given in Table 2. Because the Inner and Outer DoC equations were based on different criteria, the Outer DoC could be calculated as shallower than the Inner DoC. This was true for 6 out of the 20 sites. Fig. 3 shows the relationship of the placement depth to the DoC equations that were plotted on the stability figure. With the additional sites plotted, the range of projects extended to all four quadrants of the figure. Similar trends emerged in Fig. 3 to those observed in Hands and Allison (1991). All nearshore berms that were placed deeper than the Outer DoC were stable. Ogden Dunes, Indiana, was a site where the Inner DoC was deeper than the Outer DoC, and the sediment was placed at the Outer DoC. This placement was mildly active. Perdido Key, Florida, was another unique site. It was a stable project that was placed slightly deeper than the Outer DoC and shallower than the Inner DoC. This berm maintained the trend that berms placed deeper than the Outer DoC were stable. Berms placed shallower than the Inner and Outer DoC tended to be very active. The buffer zone in the upper left quadrant shown in Fig. 3 contains stable berms placed 0%–50% shallower than the Outer DoC and deeper than the Inner DoC.

Fig. 3. Relationship between DoC equations and stability of historical nearshore berms.
(Adapted from Hands and Allison 1991.)

Table 2. Calculated DoC values for historical nearshore placement projects

Placement site	Symbol	Year	Mean placement depth (m)	Outer DoC (m)	Inner DoC (m)	Observed activity
Santa Barbara, California	SB	1935	6.1	10.2	3.0	Stable
Atlantic City, New Jersey	AC	1935–1948	5.8	8.1	5.9	Stable
Long Branch, New Jersey	LB	1948	11.6	4.9	5.5	Stable
New River Inlet, North Carolina	NR	1976	2.1	4.3	5.8	Active
Dam Neck, Virginia	DN	1982	11	10.7	6.8	Stable
Fire Island, New York	FI	1987	4.9	7.4	5.9	Active
Sand Island, Alabama	SI	1987	5.8	17.8	6.7	Active
Silver Strand, California	AG	1988	4.9	35.3	8.1	Active
Humboldt, California	HB1	1988–1989	15.8	57.3	10.3	Active
Humboldt, California	HB2	1988–1989	21.3	57.3	10.3	Active
South Padre Island, Texas	SP	1989	7.9	41.4	7.9	Active
Perdido Key, Florida	PK	1991	6	5.8	9.1	Stable
Port Canaveral, Florida	PC	1992	6.3	24.1	9.9	Active
Newport Beach, California	NB	1992	5.5	23.2	5.8	Active
Brunswick, Georgia	BG	2003	6	22.9	8.4	Active
Ocean Beach, California	OB	2005–2007	11.5	67.8	12.9	Active
Fort Myers, Florida	FM	2009	1.8	2.3	2.8	Active
Chetco Inlet, Oregon	CI	1996–2013	6.4	9.3	12.0	Active
Vilano Beach, Florida	VB	2015	3	24.4	9.4	Active
Ogden Dunes, Indiana	OG	2016	5.5	5.5	7.2	Active

Source: DoC values for projects prior to 1990 are data from Hands and Allison (1991).

The results of the SMT sediment mobility and cross-shore transport techniques that were applied to nine historical projects are given in Table 3. The near-bed velocity technique tended to have an increased frequency of sediment mobility and mobility score compared with the bed shear stress technique. This increased frequency was because the near-bed velocity technique used the nonlinear stream function wave theory, which was more appropriate for more asymmetric waves in water depths from 0.006gT² to 0.016gT² (Soulsby 1997). The bed shear stress technique uses linear wave theory, which is generally more conservative and is appropriate when wave steepness is small. Applying both techniques at a site provided a range of estimated sediment mobility.

Table 3. Predicted sediment mobilization and cross-shore transport direction from the SMT

Placement site	Year	d₅₀ (mm)	SMT results					Observed cross-shore migration
Placement site	Year	d₅₀ (mm)	f_M (%)	M	f_Mu (%)	M_u	Predicted migration	Observed cross-shore migration
Dam Neck, Virginia	1982	0.08	84	1.6	99	4.0	88% offshore	Stable
Mobile, Alabama (Outer Mound)	1988	0.25	8	−0.7	17	−0.4	100% onshore	Stable
Silver Strand, California	1988	0.18	99	2.8	100	5.0	98% onshore	Onshore
Perdido Key, Florida	1991	0.3	36	0.4	46	0.4	100% onshore	Stable
Port Canaveral, Florida	1992	0.3	97	4.4	99	3.7	100% onshore	Onshore
Newport Beach, California	1992	0.27	60	0.7	98	1.6	100% onshore	Onshore
Brunswick, Georgia	2003	0.35	97	2.8	99	2.6	100% onshore	Onshore
Ocean Beach, California	2005	0.18	75	0.7	99	2.4	98% onshore	Onshore
Fort Myers, Florida	2009	0.17	30	−0.2	38	0.2	99% onshore	Onshore

The sediment mobility and cross-shore transport direction were correctly estimated for eight out of nine sites. The Dam Neck, Virginia, project was predicted to have a high frequency of sediment mobility and large mean mobility scores, but the site was stable. It is uncertain why the predictions differed from the observations, but there are several possible explanations. The Dam Neck, Virginia, project had the finest sediment of the projects tested and was placed relatively deep (11 m) as a mound. Although the sediment was noted to consist of fine sand and silt, the presence of any cohesive sediment would encourage stability. The mound was considered stable with no significant sediment gain around the mound's edges, but sediment on the mound crest appeared to move landward during storms. The stability figure that used the DoC equations in Fig. 3 shows the site to be in the stable quadrant. These conflicting results would have encouraged a more comprehensive investigation of the project site if these rapid evaluation techniques had been applied during the project planning phase.

Five out of the six active sites were estimated to have the median grain size mobilized by most waves using both mobility techniques. These projects' high mobility scores indicated the thresholds to initiate sediment mobilization were significantly exceeded. The high frequency of mobilization and mobility score indicated an energetic environment with a high probability of the sediment actively contributing to the littoral system.

Mean mobility scores of less than one were observed at Mobile, Alabama, and Perdido Key and Fort Myers, Florida. Mean mobility scores of less than one generally indicate a stable placement, as observed at Mobile, Alabama, and Perdido Key, Florida. The negative mobility scores at Mobile, Alabama, highlighted the stability of the site. In addition, the presence of cohesive sediment, which is beyond the range of capabilities for these evaluation techniques, encouraged placement stability. Fort Myers, Florida, had a low frequency of mobility and low mobility scores, but the placed sediment actively migrated. This was probably due to the mild wave climate at the site, and the transport was noted to be accelerated by tropical storms during the monitoring campaign (Brutsché et al. 2014).

Summary and Conclusions

As navigation channels are regularly dredged to maintain navigable waterways, decision makers require rapid techniques to assess the potential placement options for the dredged sediment. This need is compounded when shipping channels and ports are deepened to accommodate larger cargo vessels. Several techniques were used to rapidly evaluate nearshore sites for dredged sediment placement. These first-order evaluation techniques are useful as a scoping level tool to quickly compare several potential nearshore placement sites or for small projects that do not warrant an extensive numerical modeling investigation.

The stability diagram, which was originally developed by Hands and Allison (1991) to estimate whether a nearshore placement project would be active or stable, was extended to include 20 historical projects from the US Atlantic, Gulf of Mexico, Pacific, and Great Lakes coasts. The project placement depths ranged from relatively shallow (e.g., 3 m at Vilano Beach, Florida) to relatively deep (e.g., Long Branch, New Jersey, was more than 100% deeper than the Hallermeier Outer DoC). All projects that were deeper than the Outer DoC limit were stable, and projects that were shallower than the Inner DoC were active. Projects that were deeper than the Inner DoC and less than 50% shallower than the Outer DoC were in a buffer zone with stable historical nearshore berms.

The sediment mobility and cross-shore transport direction techniques were tested on nine historical nearshore placement projects, which consisted of three stable and six active placements. The sediment mobility was calculated using two methods: linear wave theory and nonlinear stream function wave theory. Using two methods provided a range of predicted mobility. Most of the active sites were estimated to have the median grain size mobilized by more than half of the waves and the large mean mobility scores indicated that the critical threshold to initiate sediment mobilization was significantly exceeded. Mobility scores less than one generally indicated stable placements, as observed at Mobile, Alabama, and Perdido Key, Florida.

The historical projects at Dam Neck, Virginia, and Perdido Key, Florida, were stable and intended to be active. Both projects would have benefited from these scoping level evaluation techniques during the project planning phase. Dam Neck, Virginia, was predicted to have the median grain size mobilized frequently, but the placement depth was deeper than the Outer DoC. Perdido Key, Florida, was placed slightly deeper than the Outer DoC and had a mean mobility score of less than one. The rapid results for these sites would have warranted the reevaluation of the project design considerations and placement depth, and underscore the value of rapid and straightforward evaluation techniques for projects that place dredged sediment in the nearshore.

Data Availability Statement

The WIS hindcast records are available online at wis.usace.army.mil, and the SMT web application is available online at https://navigation.usace.army.mil/SEM/SedimentMobility. No additional datasets were used beyond values reported in the manuscript and the cited literature.

Acknowledgments

This project was funded by the USACE through the Coastal Inlets Research Program and the Regional Sediment Management Program.

References

Ahrens, J. P., and E. B. Hands. 1998. “Velocity parameters for predicting cross-shore sediment movement.” J. Waterw. Port Coastal Ocean Eng. 124 (1): 16–20. https://doi.org/10.1061/(ASCE)0733-950X(1998)124:1(16).

Abstract

Introduction

Methods

DoC

Sediment Mobility

Cross-Shore Sediment Transport

Historical Sites

Santa Barbara, California (1935)

Atlantic City, New Jersey (1935–1942)

Long Branch, New Jersey (1948)

New River Inlet, North Carolina (1976)

Dam Neck, Virginia (1982)

Fire Island, New York (1987)

Sand Island, Alabama (1987)

Mobile Outer Mound, Alabama (1988)

Silver Strand, California (1988)

Humboldt, California (1988–1989)

South Padre Island, Texas (1989)

Perdido Key, Florida (1991)

Port Canaveral, Florida (1992)

Newport Beach, California (1992)

Chetco Inlet, Oregon (1996–2013)

Brunswick, Georgia (2003)

Ocean Beach, San Francisco, California (2005–2007)

Fort Myers, Florida (2009)

Vilano Beach, Florida (2015)

Ogden Dunes, Indiana (2016)

Application to Historical Sites

Results

Summary and Conclusions

Data Availability Statement

Acknowledgments

References

Information

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Authors

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