Morphological evolution of a mud-capped dredge pit on the Louisiana shelf: Nonlinear infilling and continuing consolidation

Geomorphology 354 (2020) 107030

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Morphological evolution of a mud-capped dredge pit on the Louisiana shelf: Nonlinear infilling and continuing consolidation Patrick Robichaux a,b,⁎, Kehui Xu a,b,⁎⁎, Samuel J. Bentley b,c, Michael D. Miner d, Z. George Xue a,b,e a

Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA d The Water Institute of the Gulf, New Orleans, LA 70122, USA e Center for Computation and Technology, Louisiana State University, Baton Rouge, LA 70803, USA b c

a r t i c l e

i n f o

Article history: Received 16 February 2019 Received in revised form 23 December 2019 Accepted 29 December 2019 Available online 02 January 2020 Keywords: Sediment transport Dredge pit Louisiana shelf Coastal restoration

a b s t r a c t Buried sandy paleo-river channels on the continental shelf can be accessed through dredging for the restoration of beaches and barrier islands which are under the threat of land loss and rising sea levels. However, our knowledge on post-dredging evolution of paleo-river channels is still very limited. In this study we focus on infilling rate, sediment acoustic characteristics, slope change and surficial morphology, all of which have implications for the management of seafloor oil and gas infrastructure. Bathymetric data collected during nine bathymetric surveys over 13 years were used to study the long-term changes of the Peveto Channel dredge pit, which is ~5 km offshore of Holly Beach in western Louisiana, USA. This pit was constructed in 2003 and was completely infilled in or before 2016. Initial pit infilling rates began at 214 cm/year from 2003 to 2004 and then rapidly decreased to an average rate of only 33 cm/year from 2006 to 2016. This corresponds volumetrically to 418,753 and 80,816 m3/year for the same intervals. Pit wall margins displayed small lateral migration, moving b30 m outwards over 13 years, posing minimal threat to adjacent infrastructure. After complete infilling, numerous small mounds were found on 2016 bathymetry, morphologically resembling ‘mud volcanoes’ that have been reported on deltas, continental shelves and slopes around the world. Newly deposited sediment on seafloor surface had a reflectivity lower than the ambient seafloor environment. Massive amounts of sediments were transported to the pit despite no proximity to direct riverine inputs, making the pit act as an efficient sediment trap. Inferred depthintegrated lateral sediment transport rates were 9.91–12.73 g/m/s. Despite the fact that Peveto Channel pit was filled up, consolidation continues and the filled sediment surface hasn't reach a condition in equilibrium with ambient seafloor yet. Our results highlight the continuing dewatering and long consolidation processes at the pit site even after a complete infilling. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Buried paleo-river channels present a viable option as sources of sandy material for beaches and barrier islands in need of restoration. These paleochannels usually contain point bars and channel sands covered by muddy overburden that can be up to several meters thick and must be removed to access the sand underneath. Many paleochannels on continental shelves were previously formed during the Last Glacial Maximum (LGM), which occurred following the Sangamon Highstand ⁎ Correspondence to: P. Robichaux, Department of Oceanography and Coastal Sciences, Louisiana State University, 2165 Energy, Coast and Environment Building, Baton Rouge, LA 70803, USA. ⁎⁎ Correspondence to: K. Xu, Department of Oceanography and Coastal Sciences, Louisiana State University, 2165 Energy, Coast and Environment Building, Baton Rouge, LA 70803, USA. E-mail addresses: [email protected] (P. Robichaux), [email protected] (K. Xu).

https://doi.org/10.1016/j.geomorph.2019.107030 0169-555X/© 2020 Elsevier B.V. All rights reserved.

of the late Wisconsinan glacial stage, approximately 18,000 years B.P. Sea level during this time was 110 m to 120 m below the modern sea level (Chappell and Shackleton, 1986; Suter, 1987; Fairbanks, 1989; Stright, 1990). Deltaic systems adjusted accordingly during this time and extended seaward onto the current-day continental shelf, incising fluvial channels as they went. This resulted in the deposition of large sand bodies mobilized by fluvial systems along the flat continental shelves. The exposed shelf surface left behind the late Wisconsinan unconformity, a weathered and oxidized surface that is still present in the subsurface. Paleochannels carved into the Wisconsinan unconformity by rivers can contain fills comprising some of the coarsest material present on the coastal shelf (Kuecher, 1994). During late Pleistocene and into early Holocene, the Calcasieu River met with the Sabine River valley complex (in present day Louisiana, USA), which incised a valley and that ultimately merged with the larger Trinity River valley complex 40 km offshore from the current Bolivar

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Peninsula, Texas (Simms et al., 2007). Extensive surveys in the ancient Trinity River valley on the continental shelf reveal 8 m thick of quartz rich sandy material overlain by 20 m of fine marine sediment (Thomas and Anderson, 1994). As sea levels began to rise ~18,000– 12,000 years B.P., braided fluvial systems transitioned to meandering systems. Estuarine and marsh environments matured in the former valleys, leaving behind a record of organic-rich, fine-grained deposits overlying coarser material. As the Holocene transgression waned, sediment sourced from rivers (mostly the Mississippi and Atchafalaya Rivers to the east) was transported laterally along the shelf and deposited to develop the Louisiana and Texas Chenier Plains (Gould and McFarlan, 1959; Coleman et al., 1983; Penland and Suter, 1989). The Louisiana Chenier Plain environment of today is characterized by straight expanses of shoreline backed by wetlands that are subdivided into alternating shore-parallel sandy/shelly coarse ridges, and estuarine and marsh environments dominated by fine sediment (Penland and Suter, 1989). The study of Quaternary deposition patterns is what initially revealed the paleochannels which were recognized as potentially useful sand-bodies (Suter, 1987). While some focus has been directed towards the eastern Chenier Plain of Louisiana, the western edge of the present-day Chenier has not been well studied. The eastern half of the Chenier Plain only receives an estimated 2–7% of the finegrained load of the Atchafalaya River system (Draut et al., 2005). Until the past half century, the Chenier Plain was receding due to the lack of sediment supply. However, as the flow from the Mississippi River shifted towards the Atchafalaya and Wax-Lake Outlet, local progradation and the formation of a subaqueous delta outside of the mouth of Vermilion Bay has been documented in recent years (Allison & Neill, 2002). Despite receiving little direct sediment deposition from riverine sources, the subaqueous Chenier Plain further west of the Atchafalaya still receives sedimentary input from resuspended deposits. The main mechanism driving cross-shore oriented sediment transport across the inner-continental shelf along the Chenier Plain is fluid mud flows (Jaramillo et al., 2009; Sheremet et al., 2011; Traykovski et al., 2015; Denommee et al., 2016). These flows are triggered mainly by the passage of weather events, most frequently cold-fronts, and tropical cyclones with less frequency (Rotondo and Bentley, 2003; Neill and Allison, 2005; Xu et al., 2011; Kineke et al., 2006). These events produce increased wave action, which resuspends the surficial sediment deposits, causing them to liquefy and become unconsolidated (Sheremet et al., 2011). This creates fluid mud bodies which are then susceptible to reworking and transport from further wave action, currents, and gravity, even on low gradient shelf environments (Denommee et al., 2016). Despite the areas near the Mississippi River and Atchafalaya shelf experiencing progradation, there are exceptions when it comes to the maintenance and building of new land. Coastal landscapes today must contend with sea level rise and rapid subsidence. These problems are compounded by manmade alterations to the natural system (dams, levees, and other hard structures) which deprive areas of sediment. Louisiana has been impacted severely by land loss in recent decades, with N50% decrease in sediment supply due to damming of the Mississippi River drainage basin, high rates of subsidence and others (Blum and Roberts, 2009). Restoration efforts in Louisiana are estimated to cost more than $50 billion over 50 years, so any opportunity to reduce costs should be taken into consideration (Davis et al., 2015; Xu et al., 2016b; Xu et al., 2019). The western Louisiana Chenier Plain, as a region dominated today mainly by muddy sedimentary sources and low riverine input, stands to benefit greatly from using local buried paleochannels to source restoration quality sand for local beaches (Dean and Yoo, 1992). Restoration quality here is defined as being of equal or slightly coarser size than the predominant grain size of the area being restored. The muddy overburden (or “mud cap”) that overlies paleochannels can be stripped away during dredging to access the sand beneath. The excavation sites produced during dredging paleo-river channels with a muddy overburden

have been referred to as mud-capped dredge pits (MCDPs) (O'Connor, 2017; Obelcz et al., 2018; Wang et al., 2018). MCDPs are distinguished in this way from sand dominated dredge pits (SDDPs), in which the sand deposits are exposed on the seafloor (these are most often shoals or ebb-tidal deltas) (Van Rijn et al., 2005). One advantage of MCDPs is that they are often closer to areas in need of restoration, eliminating the need to transport large amounts of sand to areas that may not have native sand resources. This shorter distance reduces the costs of transportation via barges or pipelines (Nairn et al., 2005). One of the earliest MCDPs used for restoration is Peveto Channel (PC), located just south of Lake Calcasieu, Louisiana, USA (Ramsey and Penland, 1992; Fig. 1). The Peveto Channel sand resource was used to restore nearby Holly Beach, Louisiana in 2003 for a beach restoration project. As part of a project from the former Minerals Management Service (MMS) to monitor the behavior of PC dredge pit after dredging, a total of eight surveys were conducted at the PC dredge pit from 2003 to 2007. These included bathymetric surveys and an assessment of possible effects on benthic life. Because of ongoing interest in the behavior of these dredged areas, PC dredge pit is the focus of our study with integration of these historical surveys to create a more robust and comprehensive dataset. 2. Objectives and hypotheses MCDPs present a novel and cost-effective approach for restoration projects in Louisiana, but there is still limited knowledge of how they evolve after dredging and the environmental risk that they potentially pose. Many past studies have been conducted on SDDPs (Bokuniewicz et al., 1986; Cialone and Stauble, 1998; Van Dolah et al., 1998; Byrnes et al., 2004; Ribberink et al., 2005; Van Rijn et al., 2005; Kennedy et al., 2009; Liu et al., 2019). The existing literature on MCDPs (Obelcz et al., 2018; O'Connor, 2017; Wang et al., 2018) is recent and those studies have only focused on sites that were still undergoing infilling. It is documented that SDDPs in some settings are prone to significant wall collapse after dredging, whereby slope failures occur and the pit expands outward laterally (Nairn et al., 2005; Van Rijn et al., 2005). In a region like the northern Gulf of Mexico, with well-established oil and gas infrastructures, pit wall lateral migration could expose buried pipelines. Additionally, it is unknown how long it takes for MCDPs to fill in with sediment after dredging, or how long it will take for the pit to reequilibrate to the ambient seafloor after complete infilling. PC is valuable in this regard as it is the only known completely-filled MCDP in northern Gulf of Mexico to date, making it a prime example for studying the entire lifecycle of a MCDP from excavation to infilling, and then to post-infilling. Additionally, a total of eight bathymetric surveys were conducted on PC from 2003 (before dredging) to 2007 (see details in Nairn et al., 2005), providing a rich dataset for detailed analysis. We enhance this dataset with our own geophysical survey conducted in 2016 (after infilling) as an endmember. Our study seeks to answer the following questions that will enhance our understanding of MCDP evolution and behavior after dredging, using PC as an example: 1) Do the walls of PC dredge pit migrate over time, and if so, to what extent? 2) How long does it take for the pit to fill with sediment and what are the acoustic characteristics of sediment infilling? 3) Does the pit reach an equilibrium condition after complete infilling, whereby geological and morphological characteristics of the dredge site once again resemble that of the ambient undisturbed seafloor? Several hypotheses can be formed in our study. First, the pit walls will experience migration rates like those observed in SDDPs at other locations like the North Sea (Ribberink, 2005). Secondly, we hypothesize that PC dredge pit will infill at rates according to predictive models in Nairn et al. (2005) and Lu and Nairn (2011), hereafter defined as the “Nairn model” in this paper. Lastly, the pit site will return to its equilibrium state, a flat surface after complete infilling will be formed and the acoustic reflectivity of infilled sediment should be similar to that of

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Fig. 1. A base map of the study area. Isobath contours are at 1 m interval. The study site sits within the northern Gulf of Mexico off the Louisiana coast and is about 30 km east of TexasLouisiana border (red dashed line).

ambient undisturbed seafloor. The answers to these questions will help determine whether MCDPs pose a potential risk to infrastructure, whether they will continue to be a viable resource in the future and whether new pipelines can be constructed over a filled-up pit. 3. Methods The PC dredge pit was surveyed in 2016 using a full suite of highresolution geophysical instruments, including interferometric sonar for swath bathymetry, sidescan sonar, and chirp subbottom profiler. An Edgetech 4600 interferometric swath bathymetry and sidescan sonar system was used to collect data with a swath width about 3–5 times the water depth. The 4600 system produces real-time highresolution three-dimensional maps of the seafloor while providing coregistered simultaneous sidescan and bathymetric data. An Edgetech 0512i subbottom profiler was used for collection of subbottom profiles at a frequency of 0.5–4.5 kHz. The R/V Coastal Profiler from Coastal Studies Institute of Louisiana State University was used for all fieldwork. The bathymetry and sidescan acquisition device were pole-mounted and fixed from a bowsprit ahead of the vessel. Sub-bottom profiler was towed off the port side of the vessel about 0.5 m below the sea surface. All geophysical data from the 2016 survey were combined with previous surveys performed in 2003, 2004, 2006, and 2007 by Weeks Marine and ERIS, provided by Baird and Associates. These surveys utilized various single beam echosounders and varying line spacings, ranging from 15 m to 30 m for the surveys that provided full coverage. The total number of all surveys was 9, with each survey varying in acquisition method and coverage (Nairn et al., 2005, 2007). Sonar data were processed using Caris HIPS/SIPS and then exported to ArcMap to create Digital Elevation Models (DEMs), which were then used to analyze slope, Difference of Depth (DoD), volume changes, and surface morphology.

Subbottom profiler data were processed using Sioseis, Kingdom Suite, and Fledermaus. Detailed geophysical methods can be found in Obelcz et al. (2018) and Liu et al. (2019). In order to acquire an estimate of uncertainty for the DoDs, the “fixed reference uncertainty” was calculated, following the methods detailed in Schimel et al. (2015). Cores were not taken during any of the previous surveys conducted, so no ground truthing data were available. In the time between the dredging of the pit in 2003 and 2016, an oil pipeline was constructed, which traversed the middle of the pit. As pipelines tend to move during storms and hurricanes, the original location of the pipeline may have shifted since its construction and, for this reason, cores were not taken during the 2016 surveys. During surveys conducted in 2007, surficial sediment samples were taken by divers; these are the only sediment samples collected during the historical surveys. The volumetric changes of the pit were calculated and compared with a model constructed by Lu and Nairn (2011) based on their initial study on the PC dredge pit; the equation is as follows: "
3 # C0 ωs T h0 ΔZp ¼ k1 1− ρdry hp

Where ΔZp is the total deposition thickness per tide, C0 is background concentration of suspended sediment outside of the pit, k1 is an empirical coefficient, ωs is settling velocity of mud, T is tidal period, ρdry is dry bulk density, h0 is water depth above the natural seabed outside the dredged area, and hp is the water depth inside the dredged area. All the values can be adjusted for any site, and the ones used for PC dredge pit are listed in Nairn et al. (2007). The purpose of comparing our measured values with the model is to evaluate whether the Nairn

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model, designed around SDDPs, is also suitable for MCDPs and whether it can effectively predict the infilling rates of an actual dredge site. 4. Results 4.1. Bathymetry Survey data from 2016 reveals that PC has undergone complete infilling, and the topographic lows (compared to the surrounding seafloor) caused by the dredging were no longer present (Fig. 2). Ambient seafloor depth around the pit was approximately 8 m. Immediately after dredging in 2003, the maximum pit depth reached 19 m. In 2004, the pit depth decreased to ~13–14 m, while in 2006 it was reduced to ~11 m. Despite complete infilling, bathymetric map indicates that there was slight positive relief in 2016, implying that total “fill” volume exceeds the accommodation volume in the pit. This positive relief was evident in a large shoaling area in the northern half of the pit (Fig. 2D). In addition to the prominent raised seafloor feature over the pit, the central part of the pit was covered by numerous mounds that were ~1 m in height, and their occurrence was frequent (Fig. 3). These small

morphological features resembled mud volcanoes observed in subaerial environments such as near the modern Balize delta of the Mississippi River. The NW corner of the pit contained a dredge spoil, characterized by a small positive relief, which was the highest feature in all the DEMs. Depth profiles are shown in Fig. 4. These longitudinal and latitudinal profiles include all surveys taken from 2003 to 2016 and illustrate the full-time lapse of pit infilling. Partial coverage surveys were included because they provide transects over the middle of the pit. The mound features are present in the 2016 transects, as evidenced by their positive relief compared to the surrounding seafloor (Fig. 4). In 2003 the pit floor was uneven and lumpy immediately after dredging, and similar features can be found in the 2004 surveys, albeit at a shallower depth. This indicates that over time these features were reworked by oceanographic processes to try to equilibrate with the infilling floor. 4.2. Gradient Analysis of the elevation gradient derived from bathymetric data shows that the slope of the pit walls decreased rapidly over time (Fig. 5). After initial dredging, the maximum slope around the pit wall

Fig. 2. Bathymetric maps of Peveto Channel dredge pit from 2003, 2004, 2006, and 2016 (A, B, C, and D, respectively). Warmer colors are shallower depths and cooler colors indicate deeper areas. As time progresses, the initially rough surface of the pit smooths out.

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Fig. 3. Zoom-in views of bathymetric map of Peveto Channel dredge pit of 2016. The figures use the same dataset as Fig. 2D but a different color bar. The blue line delineates the extent of numerous small mounds which are morphologically like “mud volcanoes”.

was 40° in some locations. In 2004, the maximum slope around the pit was ~20°, and in 2006 the maximum slope was only ~10°. Although the pit was completely filled in 2016, there were still sloping features evident in what was once the pit area, which are caused by the mounds. As the magnitude of slope is not directly tied to the magnitude of elevation, these features are present in gradient maps even though the small mounds have a relief of only 1 m.

4.3. Depth change Difference of Depth (DoD) maps were created by subtracting one surface from another to create a time series of depth change between two surveys (Fig. 6). Light colors in Fig. 6 indicate a statistical “no change” area where the depth change is within two standard deviations of the means. Blue values indicate shoaling or deposition of sediment, while red values indicate deepening or a removal of sediment. Fig. 6A illustrates the initial dredging, wherein the depth of the excavation area was increased by ~11 m at its deepest point. Between 2003 and 2004 the pit began to shoal with higher rates of deposition in deeper portions of the pit (Fig. 6B), which can be correlated to original dredging depth shown in Fig. 2. Fig. 6 confirms that deeper areas initially accumulate sediment as there is more accommodation space to be filled, and this accumulation decreases as the pit is filled in. For each DoD afterwards, the pit filled in approximately the same amount despite longer survey intervals, indicating that as time went by, the rate at which sediment filled the pit decreased (Table 1). Using statistical values derived from the DoD maps, average vertical accretion rates were as follows: 2003–2004 with deposition of 2.14 m/yr; 2004–

2006 with deposition of 2.02 m/yr; 2006–2016 with deposition of 0.33 m/yr. 4.4. Sidescan sonar Sidescan sonar data revealed that in 2016 the varying backscatter intensity within the survey area correlated to different morphological features in and around the pit (Fig. 7). Areas where material had been cast during dredging operations (known as “dredge spoils”) had a relatively high backscatter intensity, possibly due to the coarser grain size of the dredged material relative to adjacent seafloor. Diving operations were conducted in 2007 that included surficial sediment samples which were analyzed for grain size (Nairn et al., 2007). Divers observed that the sediment samples taken at the dredge spoil were over 93% sandy material, which should be the cause of higher backscatter intensity. Within the area of the dredge pit, the backscatter intensity was much lower. This is surprising considering that infilling had completed yet the borrow area still had a unique signature different from adjacent seafloor (Fig. 7). It is likely that newly deposited sediment has yet to reach equilibrium with respect to the ambient seafloor surrounding the pit in term of acoustic characteristics. Sedimentary environments in PC dredge pit in 2016 can then be classified based upon the reflectance patterns observed: 1. dredge spoil material, characterized by uniform high intensity and bright colors (yellow) on sidescan imagery, 2. ambient seafloor, characterized by an intermediate range of intensities and moderate variation due to uneven surfaces or trawl scars, and 3. interior of the dredge pit, which is characterized by low reflectivity and dark colors (brown) and a general conformance to the borrow area.

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Fig. 4. Depth profiles at Peveto Channel dredge pit. Transects A-A' and B-B′ are shown in Fig. 2A. Profiles include 2003 before dredging, after preliminary stripping, and all subsequent surveys after dredging. Positive relief above the original pit surface (shoal) can be seen on the 2016 profiles.

4.5. Chirp seismic profiles Subbottom chirp seismic profiles collected in the survey area reveal multiple layers in stratigraphy (Fig. 8). The most prominent features were the remnants of buried paleovalleys in the surrounding area outside of the dredge pit. Within the pit area, there was a wide acoustically-opaque unit filling the pit. This acoustically opaque seismic facies characterized the entirety of the pit up to depths where multiples begin to occur, indicating that the acoustic properties of the pit-fill material disrupt the return signal.

5. Discussion 5.1. Pit wall migration One of the main focuses of this study is to quantify how much MCDP walls migrate outward. As evidenced by multiple datasets (sidescan, DEMs, gradient, and DOD), the pit walls appear to remain in place laterally as the pit fills in (Figs. 2, 5, 9). Presently, policies for dredging offshore require a horizontal setback buffer distance of ~300 m (1000 ft) from oil and gas pipelines to prevent exposure or spanning. In general, a minimum sediment cover of 1 m thick is needed for all oil and gas pipelines in northern Gulf of Mexico for water depths b60 m (Nairn et al., 2005). This is because when exposed on the sea floor, these

pipelines are not in a safe condition because of tide and wave activity and frequent trawling activities. The 300-m setback buffer distance, however, is a figure that is based on highly mobile sandy dredge pits, which experience rapid wall slumping due to the non-cohesive nature of coarse material when seated at a high angle of repose. MCDPs differ from SDDPs by having a muddy cap overlying the sand body beneath. We hypothesize that the pit walls of PC dredge pit have become gentler, but faced little migration due to slumping because of the cohesive overburden of the relatively fine grained material that forms the mud cap. It is likely that the mud cap acts as a stabilizing structure to help the pit retain its original shape. The steep part of the wall, when viewed over time, moved no N30 m from 2003 to 2006 (Fig. 9). These results demonstrate that MCDPs remain relatively stable as they fill completely and are not prone to wall failures due to the stabilizing mud cap. This considers the normal oceanographic conditions and weather events such as cold fronts and hurricanes. The northeastern corner of Hurricane Rita, a category 3 hurricane at the time, passed directly over our study site in 2005. As mentioned earlier, hurricanes frequently rework sediment on continental shelves, but Nairn et al. (2005) hypothesized that hurricanes have only a small effect on dredge pits due to their short duration as well as the possible winnowing of fine-grained material due to bed armoring. This effect made the pit walls resistant to large scale slumping even from bottom shear stresses induced by storms or hurricanes.

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Fig. 5. Map of slope evolution at Peveto Channel dredge pit. A, B, C, and D correspond to years 2003, 2004, 2006, and 2016, respectively. Green colors are flat surfaces, while orange and yellow colors indicate steep surfaces.

5.2. Pit infilling Bathymetric data from 2016 show that PC has been completely filled in. The 2016 survey data serve as an endmember in estimating infilling time, as it is the most recent observation made on the study area and the first to document complete infilling. It should be noted that it is possible that PC pit was filled in completely multiple years before 2016. No surveys were performed between 2007 and 2016 and thus there are no data on pit infilling during this time. Infilling rate, however, can be inferred using a numerical model. Models developed specifically to estimate PC infilling (Nairn et al., 2005; Lu and Nairn, 2011) predicted that some of the infilling sediment would be sourced from the pit walls as they collapsed due to instability, but our slope data in Fig. 9 indicate that there is limited lateral pit wall movement. We have estimated pit infilling based on volume calculations taken from DoD maps to compare to the results derived from the Nairn model. The infilling models developed by Nairn et al. (2005) and Lu and Nairn (2011) (Eq. 1) over-predicted infilling at PC when compared to observed volumetric data (Fig. 10). Infilling predictions from Nairn's

models estimate infilling at a rate of 10–15% faster than observed infilling in later stages based on volume calculations. One reason for this over-prediction may be due to the lack of a consolidation coefficient in the model equation. As sediment is deposited, it goes through different processes including hindered settling and self-weight consolidation. Over time, a sediment packet will decrease in porosity and thickness, while increasing in density (Lo et al., 2014; Sha et al., 2018). Even while the sediment is being deposited, older packets are simultaneously descending as consolidation processes take place. The Nairn model does not explicitly take this decrease in height associated with cohesive sediment consolidation and dewatering behavior into account, despite continued infilling. Another possible reason for the discrepancy between model and observation is that the model is 1-dimensional, and only measures distance from the pit floor to ambient seabed depth as a marker for pit infilling, making the estimate elevation-based (thus assuming the pit has one depth value throughout), while volumetric analysis is based on three dimensions (surface, area, and time), making it spatially variable and thus a more accurate representation of change. The next

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Fig. 6. Difference of depth maps which display depth changes. Red values indicate deepening, while blue values indicate shoaling. Light colors indicate little change and are within the uncertainty values.

possible reason is the fact that the Nairn model uses static parameterizations such as sediment concentration and dry bulk density, while in reality these values are prone to change, hourly, daily and seasonally, and may be affected by weather events such as cold fronts or hurricanes, as well as variable periods of high discharge from the Atchafalaya River (Allison & Neill, 2002; Rotondo and Bentley, 2003). Although Nairn et al. (2005) predicted that hurricanes would not detract from infilling

Table 1 Volumetric change and infilling at Peveto Channel dredge pit. Raster title

Volume change (m3)

Infilling rate (m3/yr)

Uncertainty of infilling rate (m3)

Percent infill (%)

Pre-Postdredge Postdredge-2004 2004–2006 2006–2016

−2,059,481 766,318 565,980 808,161

N/A 418,753 377,320 80,816

42,496 107,357 214,201 121,161

0 37 65 104

Note: 100% was designated as the amount of volume removed during initial dredging. Uncertainty was calculated using the fixed reference point method from Schimel et al. (2015). Standard deviation of each cut-fill volume was calculated and then multiplied by the area to calculate uncertainty.

processes, Hurricane Rita may have accelerated the deposition by causing a higher rate of initial sediment deposition in the first three years of the pit's existence by mobilizing sediment from the nearby continental shelf to be deposited in the pit, as evidenced by the higher shear stress and greater seabed erosion/deposition that is predicted to have occurred (Rotondo and Bentley, 2003; Xu et al., 2016a). Lastly, the Nairn model assumes that some material will be derived from collapses of the pit walls, and it may overestimate the amount of material that is drawn from this source since the walls remain relatively stable. Improvements can be made on the Nairn model through collecting site specific data for extended periods of time (several months). The infilling of any dredge pit will be site specific and based on a number of factors such as proximity to sediment source, pit size, and oceanographic conditions. The case of PC dredge pit can be used to strengthen our overall understanding of infilling behavior and synthesized with other studies on dredge pits (Bokuniewicz et al., 1986; Cialone and Stauble, 1998; Byrnes et al., 2004; Ribberink et al., 2005) to create a more complete understanding of dredge pit characteristics. Models constructed for specific areas can vary widely in the prediction of infilling time. Obelcz et al. (2018) demonstrated that there is little correlation

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Fig. 7. Sidescan sonar mosaic derived from 2016 survey data. Brighter colors indicate high backscatter, while darker colors indicate lower backscatter. A total of three sedimentary environments are present: dredge spoil (bright areas), ambient seafloor (intermediate areas), and the dredged area (dark area within pit). Possible trawling marks can be seen on the seafloor.

between dredged volume and infilling time among various dredge pits. Another important factor is the proximity of dredge pits to sediment sources. In incorporating our knowledge of PC dredge pit, future infilling predictors such as models can be improved and updated for future sites where dredging projects may occur. The lack of cores or ground samples from this study makes it impossible to conclude for texture and geotechnical characteristics of PC dredge pit, such as grain size of infilled material and sediment sourcing. The only existing physical samples from the study site were taken during surveys in 2006 and 2007 and were reported in Nairn et al. (2007). The purpose of these surveys was to determine ecological effects of dredging on marine fauna and as such was not focused specifically on grain size. The results state that sediment samples taken within the pit were over 94% silt and clay, and finer than the surrounding seafloor. To be certain, core samples would need to be taken on site to determine the properties of the newly deposited sediment and to measure whether this differs from the surrounding seafloor. Using volume calculations derived from the cut-and-fill analysis, it is possible to calculate average vertical accretion rates (Table 1). The values are: 2.14 m/yr from 2003 to 2004, 2.02 m/yr from 2004 to 2006, and 0.33 m/yr from 2006 to 2016. Be7 based sedimentation rates near the Mississippi Delta range from 11 to 48 cm/yr (Corbett et al., 2004). Pb210 based sedimentation (which is used for estimating decadal-scale accumulation) for the same region is 1.3–7.9 cm/yr. Compared to the Mississippi Delta area, the average sediment accumulation rate at PC dredge pit from 2003 to 2004 was 4.5–19.5 times greater than Be7 short term accumulation rates. It can be seen from these comparisons that the PC dredge pit is indeed a highly efficient sediment trap. As previous studies have shown (Kemp, 1986; Rotondo and Bentley, 2003; Jaramillo et al., 2009; Sheremet et al., 2011; Denommee et al., 2016; Denommee et al., 2018), the western inner Louisiana shelf has

Fig. 9. Map showing pit margins from 2003 to 2006. The colored outline encompasses the entirety of the steep wall slope. Red, blue and green zones are slopes N16, 7 and 3° in 2003, 2004 and 2006, respectively.

frequent reworking of surficial sediment deposits, as they are liquefied and transported as fluid mud flows by currents, waves, cold fronts, and hurricanes. Kineke et al. (2006) observed the role of cold fronts in mobilizing fine grained material along the inner continental shelf south of Atchafalaya Bay. They reported transport rates of 20– 50 g/m/s, constrained mostly near the seabed. Although PC dredge pit is ~100 km from the mouth of Atchafalaya Bay, it may possibly receive sediment from mud transported in suspension westward along the shelf as its main source of sediment.

5.3. Pit equilibrium Diving surveys conducted in 2006 and 2007 describe the pit floor as “soft” compared to the surrounding undisturbed seafloor. Whether or not the material in the pit today is finer than the surrounding area is unknown, as perhaps the grain size of the former dredge site has equilibrated with the surrounding seafloor. Regardless, the divers' description of a “soft pit floor” gives some credibility to the idea that the pit floor is not yet consolidated. The sidescan data corroborates

Fig. 8. Chirp seismic profile of transect S-S′, located in the northern part of dredge pit. The acoustically opaque sediment package extends until depths where multiples begin to occur. Remnants of paleochannels are scattered across the survey area, evidence of the vast river network that once cut across this area.

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Fig. 10. The Peveto Channel dredge pit infilling over time based on volumetric calculations and compares it to Nairn's model. Nairn's 1-dimensional model overestimates the infilling rate when compared to observational data. In 2016 Peveto Channel has gained volume compared to the predredge surface.

there is a low backscatter reflectance from pit surface that absorbs much of the sound signal (Fig. 7). The pit surface is characterized by rough, uneven mounds in the 2016 data. These form small topographic highs and lows across the surface of pit, resembling some shelf mud volcanos (Milkov, 2000; Yin et al., 2003) and some subaerial mud volcanoes in the Mississippi River delta (Morgan et al., 1968). Mud volcano-like structures can be found throughout the world in a large variety of sizes, ranging from metric (like this study site) all the way to several kilometers. They are generally associated with the expulsion of mud, fluid, and or gas from unconsolidated sedimentary packets. Table 2 illustrates a number of mud volcanoes from different environments and compares them to the ones found at PC dredge pit. Mud volcano-like structures are linked to a wide number of causes, including tectonic activity, gaseous discharge, and/or large unconsolidated sedimentary packets expelling fluid and mud. The reason for rough pit surface can be possibly contributed to: (a) gaseous discharge, (b) consolidation, and (c) dewatering

processes. Gas created by the decomposition of trapped organic matter could produce bubbles which disrupt the sediment as they escape upward. It is unlikely that such rough morphology was caused by thermogenic methane as the original depth of construction of the pit was far too shallow to reach any locations suitable for methane storage (Kvenvolden, 1993). However, gas “wipe-out” can be clearly seen on the subbottom seismic profile in Fig. 8, indicating the presence of some types of gases. Studies have shown that fluid mud flows from volcanoes coincide with high amounts of biogenically sourced methane in surficial sediments and it has been shown that methane is readily produced in shallow surficial sediments due to methanogens (Nyman, 1999). Other possibilities are dewatering and consolidation of newly deposited sediment. As sediment consolidates under the control of gravity and waves, water and air are forced out, leaving behind the mounds. Sidescan sonar results and the presence of mounds indicate that the PC dredge pit is still in a state of continuing consolidation and dewatering. To determine the consolidation of newly deposited sediment, geotechnical experiments would need to be performed, such as cone penetrometer and shear strength testing. The possibility that deposited material within the pit is still consolidating and trying to reach equilibrium with the seafloor would signify that even after complete infilling, more time is still required before the pit's geotechnical properties approach those of the pre-existing seafloor. As a result, more time is needed before any new pipelines and platforms can be constructed in and surround this pit area. A noticeable feature of the surface from 2016 is the fact that positive relief features are visible over areas of the pit that were not dredged as deep as other portions of the pit (Fig. 11). This may be due to differential amount of consolidation over shallower areas of the pit, since deposited sediment thickness is most likely different over these areas as well. Fig. 11 shows how the newly deposited surface elevation matches the small “peninsulas and islands” that were not dredged. This shows that the initial depths of the surface after dredging in 2003 have a “footprint effect” on later sediment deposition in 2016, altering the properties of material that is deposited after dredging. Thus, the overall shape, or “footprint”, of the original dredge pit can be observed in the current surface even 13 years later. 5.4. Future work One of the major limitations of studying PC dredge pit was the lack of sediment cores, as mentioned earlier. Cores would allow quantification

Table 2 Comparison of multiple papers dealing with morphological features similar to the mud mounds observed at Peveto Channel dredge pit. This chart illustrates the diverse environments of such phenomena. Paper

Physical setting

Nomenclature

Mazzini (2009)

Fault zone along Inactive depocenter

Gryphons, Mud volcanoes

Water depth (m)

Height (m)

Width (m)

Subaerial

2–3

2–5

Prograding River Delta

Mud lumps

Subaerial

2–4

70m -several acres

Tectonic activity via faulting, subsidence, and degassing Pressure loading via rapid uneven deposition and river discharge events

Martian surface

Mud diapirs, Cones

Subaerial

10–20

400–800

Pressure loading caused by rapid sedimentation

Martian surface

Mud volcanoes, Mounds

Subaerial

100–500 2200–4000

Long-term load induced subsidence

pockmarks, mounds

~8

0.5–1

1–2

Fluid expulsion/wave loading/degassing

10

2–3

2–3

Degassing due to tectonic activity

365–760

65–345

680–4100

Mud volcanoes

250–850

N/A

1000–2000

Mud volcanoes

2100

2500

20–150

Tectonic loading via subduction, rapid sedimentation, and degassing Fluid expulsion/degassing, pressure loading via sedimentation Degassing due to tectonic activity

Mud volcanoes

400–4000

300

800–3500

Degassing caused by tectonic action

2

Morgan (1951) Skinner Jr and Mazzini (2009) Skinner Jr and Mazzini (2009) This Study Sturz et al. (1992) Chen et al. (2014) Graue (2000) Ivanov et al. (1996) Medialdea et al. (2009)

Driving mechanism

Inner continental shelf, passive margin Strike slip fault zone Accretionary wedge on active subduction zone Upper continental slope near prograding delta Extensional fault zone Active continental margin

Mud cones, mud volcanoes Mud volcanoes, mud diapirs

P. Robichaux et al. / Geomorphology 354 (2020) 107030

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Fig. 11. Overlay of bathymetry from 2003 and 2016 illustrating the “footprint effect”. Different color bars were used to allow the visualization of each layer on top of each other. The dark red/brown outline from 2003 illustrates the main dredged area of the pit, while the blue outline from 2016 illustrates the same approximate shape on the 2016 surface. C is an overlay of the 2003 surface and the 2016 surface, in which the original dredged area correlates to the surface features from 2016.

of physical parameters such as the age of new sediment packets, grain size, accumulation rate and possible sourcing of sediment. This would also allow us to compare physical properties of the pit to nearby undisturbed areas. Geotechnical testing would also be needed to understand long term consolidation within dredge pits. Cone penetrometer testing could provide values for sediment consolidation and a times series of these values would provide information on the processes by which dredged areas equilibrate. Lastly, long term monitoring at dredge sites will allow us to customize the parameters needed to better predict their infilling. Collecting data such as the characteristics of sediment, oceanographic conditions, water quality, benthic ecology, and fish population in the area can be used to better understand how geophysical and biological processes co-evolve with morphological change of dredge pits.

setback rules requiring 300 m between dredge pits and infrastructure are thus overly conservative at this location characterized by low wave and tidal energy with high suspended sediment concentrations. The dredge pit acts as an extremely efficient sediment trap and illustrates that large amounts of sediment that are available for deposition (given accommodation space) exist in an environment where sediment would otherwise bypass the region.

Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

6. Conclusions Acknowledgments The infilling of sediment in Peveto Channel dredge pit illustrates the evolution of an artificially created seabed feature. Although our study is focused on one site, the findings from this study shed light on studies of other coastal dredge pits around the world (Bokuniewicz et al., 1986; Byrnes et al., 2004; Cialone and Stauble, 1998; Ribberink et al., 2005) and can increase our understanding of them in the future. This is particularly useful for muddy or mixed mud-sand environments on passive continental margins in which rates of relative sea level rise are high. Moreover, our total nine bathymetric surveys in a 13-year period provides a unique opportunity of long-term monitoring of a dredge pit. To the best of our knowledge, our study is the first one to present mound morphology after complete infilling. Our work also provides a basis for future in-depth comparison between MCDPs and SDDPs. Our results show that the pit filled in b13 years. Sediment transport processes near this pit might be weather-triggered resuspension and fluid mud flows along the inner Louisiana shelf. The newly deposited sediment is continuing to undergo settling and consolidation processes, even after 100% infilling. The walls of the dredge pit are relatively stable, exhibiting b30 m of lateral outward movement during the period between initial excavation and complete infilling. Because of this, the pit walls pose little threat for slope failure. This means that nearby pipelines, submerged cultural resources and sensitive seafloor habitat are not at direct risk from sediment movement around dredge pits. Current

This study was funded by the U.S. Department of the Interior, Bureau of Ocean Energy Management, Coastal Marine Institute, Washington D.C. under Cooperative Agreement Number M14AC00023. In 2016 geophysical data were collected using the LSU Coastal Studies Institute Field Support Group's R/V Coastal Profiler. Previous datasets were provided by Dr. Rob Nairn and Dr. Qimiao Lu of Baird & Associates. Special thanks to Dr. Jeffrey Obelcz, Haoran Liu and others from LSU Sediment Dynamics Lab for their help with data collection and processing. Two anonymous reviewers and Dr. Zhongyuan Chen (Editor-in-Chief) are appreciated for their constructive suggestions and comments. References Allison, M.A., Neill, C.F., 2002. Accumulation rates and stratigraphic character of the modern Atchafalaya River prodelta, Louisiana. Transactions of the Gulf Coast Association of Geological Societies 52, 1031–1040. Blum, M.D., Roberts, H.H., 2009. Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise. Nat. Geosci. 2 (7), 488–491. Bokuniewicz, H., Cerrato, R., Hirschberg, D., 1986. Studies in the Lower Bay of New York Harbor Associated with the Burial of Dredged Sediment in Subaqueous Borrow Pits: Marine Sciences Research Center. State University of New York Stony Brook. Byrnes, M.R., Hammer, R.M., Thibaut, T.D., Snyder, D.B., 2004. Effects of sand mining on physical processes and biological communities offshore New Jersey, U.S.A. J. Coast. Res. 20 (1), 25–43. Chappell, J., Shackleton, N., 1986. Oxygen isotopes and sea level. Nature 324 (6093), 137.

12

P. Robichaux et al. / Geomorphology 354 (2020) 107030

Chen, S.C., Hsu, S.K., Wang, Y., Chung, S.H., Chen, P.C., Tsai, C.H., Liu, C.S., Lin, H.S., Lee, Y.W., 2014. Distribution and characters of the mud diapirs and mud volcanoes off southwest Taiwan. J. Asian Earth Sci. 92, 201–214. Cialone, M.A., Stauble, D.K., 1998. Historical findings on ebb shoal mining. J. Coast. Res. 14 (2), 537–563. Coleman, J.M., Prior, D.B., Lindsay, J.F., 1983. Deltaic Influences on Shelf Edge Instability Processes. Corbett, D.R., McKee, B., Duncan, D., 2004. An evaluation of mobile mud dynamics in the Mississippi River deltaic region. Mar. Geol. 209 (1–4), 91–112. Davis, M., Vorhoff, H., Boyer, D., 2015. Financing the Future: Turning Coastal Restoration and Protection Plans into Realities–How Much is Currently Funded. Second in an Occasional Series An Issue Paper of the Tulane Institute on Water Resources Law and Policy. Tulane Institute on Water Resources Law & Policy, New Orleans, LA. Dean, R.G., Yoo, C.H., 1992. Beach-nourishment performance predictions. J. Waterw. Port Coast. Ocean Eng. 118 (6), 567–586. Denommee, K.C., Bentley, S.J., Harazim, D., Macquaker, J.H., 2016. Hydrodynamic controls on muddy sedimentary-fabric development on the Southwest Louisiana subaqueous delta. Mar. Geol. 382, 162–175. Denommee, K.C., Bentley, S.J., Harazim, D., 2018. Mechanisms of muddy clinothem progradation on the Southwest Louisiana Chenier Plain inner shelf. Geo-Mar. Lett. 38 (3), 273–285. Draut, A.E., Kineke, G.C., Velasco, D.W., Allison, M.A., Prime, R.J., 2005. Influence of the Atchafalaya River on recent evolution of the chenier-plain inner continental shelf, northern Gulf of Mexico. Cont. Shelf Res. 25 (1), 91–112. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342 (6250), 637. Gould, H.R., McFarlan, E., 1959. Geological history of the chenier plain, southwestern Louisiana. Transactions of the Gulf Coast Association of Geological Societies 9, 261–272. Graue, K., 2000. Mud volcanoes in deepwater Nigeria. Mar. Pet. Geol. 17 (8), 959–974. Ivanov, M.K., Limonov, A.F., Van Weering, T.C., 1996. Comparative characteristics of the Black Sea and Mediterranean Ridge mud volcanoes. Mar. Geol. 132 (1–4), 253–271. Jaramillo, S., Sheremet, A., Allison, M.A., Reed, A.H., Holland, K.T., 2009. Wave-mud interactions over the muddy Atchafalaya subaqueous clinoform, Louisiana, United States: wave-supported sediment transport. Journal of Geophysical Research: Oceans 114 (C4). Kemp, G.P., 1986. Mud Deposition at the Shoreface: Wave and Sediment Dynamics on the Chenier Plain of Louisiana. Louisiana State University, Baton Rouge, LA, p. 163 Ph.D. dissertation. Kennedy, A.B., Slatton, K.C., Starek, M., Kampa, K., Cho, H.C., 2009. Hurricane response of nearshore borrow pits from airborne bathymetric lidar. J. Waterw. Port Coast. Ocean Eng. 136 (1), 46–58. Kineke, G.C., Higgins, E.E., Hart, K., Velasco, D., 2006. Fine-sediment transport associated with cold-front passages on the shallow shelf, Gulf of Mexico. Cont. Shelf Res. 26 (17), 2073–2091. Kuecher, G.J., 1994. Geologic Framework and Consolidation Settlement Potential of the Lafourche Delta, Topstratum Valley Fill Sequence: Implications for Wetland Loss in Terrebonne and Lafourche Parishes, Louisiana. [Ph.D. Thesis]. Louisiana State University, Baton Rouge, p. 346. Kvenvolden, K.A., 1993. Gas hydrates—geological perspective and global change. Rev. Geophys. 31 (2), 173–187. Liu, H., Xu, K.H., Li, B., Han, Y., Li, G., 2019. Sediment identification using machine learning classifiers in a mixed-texture dredge pit of Louisiana shelf for coastal restoration. Water 11, 1257. https://www.mdpi.com/2073-4441/11/6/1257. Lo, E.L., Bentley, S.J., Xu, K., 2014. Experimental study of cohesive sediment consolidation and resuspension identifies approaches for coastal restoration: Lake Lery, Louisiana. Geo-Mar. Lett. 34 (6), 499–509. Lu, Q., Nairn, R.B., 2011. Prediction on morphological response of dredged sand-borrow pits. Coastal Engineering Proceedings 1 (32), 74. Mazzini, A., 2009. Mud volcanism: processes and implications. Mar. Pet. Geol. 9 (26), 1677–1680. Medialdea, T., Somoza, L., Pinheiro, L.M., Fernández-Puga, M.C., Vázquez, J.T., León, R., Ivanov, M.K., Magalhaes, V., Díaz-del-Río, V., Vegas, R., 2009. Tectonics and mud volcano development in the Gulf of Cádiz. Mar. Geol. 261 (1–4), 48–63. Milkov, A.V., 2000. Worldwide distribution of submarine mud volcanoes and associated gas hydrates. Mar. Geol. 167 (1–2), 29–42. Morgan, J.P., 1951. Mudlumps at the mouths of the Mississippi River. Coastal Engineering Proceedings 1 (2), 11. Morgan, J.P., Coleman, J.M., Gagliano, S.M., 1968. Mudlumps: diapiric structures in Mississippi Delta sediments. In: Braunstein, J., O’Brien, G.D. (Eds.), Diapirism and Diapirs. 8. American Association of Petroleum Geologists, Memoir, pp. 145–161. Nairn, R. B., Lu, Q., & Lagendyk, S. (2005). A study to address the issue of seafloor stability and the impact on oil and gas infrastructure in the Gulf of Mexico: U.S. Department of Interior. Nairn, R. B., Lu, Q., Langendyk, S. K., Montagna, P. A., & Powers, S. P. (2007). Examination of the Physical and Biological Implications of Using Buried Channel Deposits and other Non-Topographic Offshore Features as Beach Nourishment Material. US Dept. of the Interior, Minerals Management Service. OCS Study MMS, 48.

Neill, C.F., Allison, M.A., 2005. Subaqueous deltaic formation on the Atchafalaya Shelf, Louisiana. Mar. Geol. 214 (4), 411–430. Nyman, J.A., 1999. Effect of crude oil and chemical additives on metabolic activity of mixed microbial populations in fresh marsh soils. Microb. Ecol. 37 (2), 152–162. Obelcz, J., Xu, K.H., Bentley, S.J., O’Connor, M., Miner, M., 2018. Mud-capped dredge pits: an experiment of opportunity for characterizing cohesive sediment transport and slope stability in the northern Gulf of Mexico. Estuar. Coast. Shelf Sci. 208, 161–169. O’Connor, M.C., 2017. Mud-Capped Dredge Pit Evolution in Two Northern Gulf of Mexico Sites: What they Add to our Understanding of Continental Shelf Sedimentation. Master’s Thesis. Louisiana State University. Penland, S., Suter, J.R., 1989. The geomorphology of the Mississippi River Chenier Plain. Mar. Geol. 90, 231–258. Ramsey, K.E., Penland, S., 1992. Stratigraphic Assessment of Sand Resources Offshore Holly and Peveto Beaches, Louisiana. U.S. Department of Interior, Minerals Management Service, Gulf of Mexico Region, New Orleans, LA (42 pp). Ribberink, J.S., Roos, P.C., Hulscher, S.J., 2005. Morphodynamics of trenches and pits under the influence of currents and waves—simple engineering formulas. Coastal Dynamics 2005: State of the Practice, pp. 1–13. Rotondo, K.A., Bentley, S.J., 2003. Deposition and resuspension of fluid mud on the western Louisiana inner shelf. 53. Gulf Coast Association of Geological Societies Transactions 722–731. Schimel, A.C., Ierodiaconou, D., Hulands, L., Kennedy, D.M., 2015. Accounting for uncertainty in volumes of seabed change measured with repeat multibeam sonar surveys. Cont. Shelf Res. 111, 52–68. Sha, X., Xu, K.H., Bentley, S.J., Robichaux, P.A., 2018. Characterization and modeling of sediment settling, consolidation, and suspension to optimize coastal Louisiana restoration. Estuar. Coast. Shelf Sci. 203, 137–147. Sheremet, A., Jaramillo, S., Su, S.F., Allison, M.A., Holland, K.T., 2011. Wave-mud interaction over the muddy Atchafalaya subaqueous clinoform, Louisiana, United States: Wave processes. Journal of Geophysical Research: Oceans 116 (C6). Simms, A., Anderson, J.B., Taha, P.Z., Rodriguez, A.B., 2007. Over-filled versus under-filled incised valleys: examples from the Quaternary Gulf of Mexico. Society of Sedimentary Research, Special Publication 88, 117–139. Skinner Jr., J.A., Mazzini, A., 2009. Martian mud volcanism: terrestrial analogs and implications for formational scenarios. Mar. Pet. Geol. 26 (9), 1866–1878. Stright, M.J., 1990. Archaeological sites on the North American continental shelf. Geological Society of America, Centennial Special 4, 439–465. Sturz, A.A., Kamps, R.L., Earley, P.J., 1992. Temporal changes in mud volcanos, Salton Sea geothermal area. Water-rock interaction 1363–1366. Suter, J.R., 1987. Fluvial systems. In: Berryhill, H.L. (Ed.), Late Quaternary Facies and Structure, Northern Gulf of Mexico — Interpretations from Seismic Data, AAPG Studies in Geology. 23, pp. 81–129. Thomas, M.A., Anderson, J.B., 1994. Sea-level controls on the facies architecture of the Trinity/Sabine Incised-Valley System, Texas Continental Shelf. In: Dalrymple, R., Boyd, R., Zaitlin, B.A. (Eds.), Incised Valley Systems: Origin and Sedimentary Sequences. SEPM Special Publication 51, Tulsa, Oklahoma, pp. 63–82. Traykovski, P., Trowbridge, J., Kineke, G., 2015. Mechanisms of surface wave energy dissipation over a high-concentration sediment suspension. Journal of Geophysical Research: Oceans 120 (3), 1638–1681. Van Dolah, R.F., Digre, B.J., Gayes, P.T., Donovan-Ealy, P., Dowd, M.W., 1998. An evaluation of physical recovery rates in sand borrow sites used for beach nourishment projects in South Carolina. Final Report. Prepared for The South Carolina Task Force on Offshore Resources and the Minerals Management Service. Office of International activities and Marine Minerals 76pp. Van Rijn, L.C., Soulsby, R.L., Hoekstra, P., Davies, A.G., 2005. SANDPIT, Sand Transport and Morphology of Offshore Sand Mining Pits. Process knowledge and guidelines for coastal management. End Document, May 2005. Part 1. EC Framework V Project No. EVK3-2001-00056. Aqua Publications, The Netherlands. Wang, J., Xu, K., Li, C., Obelcz, J., 2018. Forces driving the morphological evolution of a mud-capped dredge pit, Northern Gulf of Mexico. Water 10 (8), 1001. Xu, K., Harris, C.K., Hetland, R.D., Kaihatu, J.M., 2011. Dispersal of Mississippi and Atchafalaya sediment on the Texas–Louisiana shelf: model estimates for the year 1993. Cont. Shelf Res. 31 (15), 1558–1575. Xu, K., Mickey, R.C., Chen, Q., Harris, C.K., Hetland, R.D., Hu, K., Wang, J., 2016a. Shelf sediment transport during hurricanes Katrina and Rita. Comput. Geosci. 90, 24–39. Xu, K.H., Bentley, S.J., Robichaux, P., Sha, X., Yang, H., 2016b. Implications of texture and erodibility for sediment retention in receiving basins of coastal Louisiana diversions. Water 8, 26. https://doi.org/10.3390/w8010026. Xu, K.H., Bentley, S.J., Day, J.W., Freeman, A.M., 2019. A review of sediment diversion in the Mississippi River Deltaic Plain. Estuar. Coast. Shelf Sci. 225, 106241. https://doi. org/10.1016/j.ecss.2019.05.023. Yin, P., Berné, S., Vagner, P., Loubrieu, B., Liu, Z., 2003. Mud volcanoes at the shelf margin of the East China Sea. Mar. Geol. 194 (3–4), 135–149.