Dredge Pit Research Articles

Deep Learning-Based Geomorphic Feature Identification in Dredge Pit Marine Environment

Deep Learning-Based Geomorphic Feature Identification in Dredge Pit Marine Environment

Short-Term Coastal Impact Of Lakeshore For Natural Reserve Protection

The site of Grangettes, which is registered as a natural reserve of national and international importance for waterowl and migratory birds (i.e. OROEM site), is considered the last part of the Swiss lakeshore of Lake Geneva that remains natural. This area hosts a wide variety of coastal ecosystems, containing significant natural values located both in aquatic and in terrestrial areas of the reserve. Because of repetitive localized dredging activities of the lake bottom since the last century, a severe withdraw of the shoreline and its associated natural values has been observed between 1964 and 2001 close to the « Gros Brasset » dredge pit.

Sedimentation processes and morphological changes in a dredge pit and surrounding environment on Ship Shoal in the northern Gulf of Mexico

The use of dredged material from continental shelves and estuaries for degraded barrier islands has become a common practice worldwide. South Pelto block on eastern Ship Shoal (SS) off the coast of Louisiana has been dredged for high-quality sand resources, including Caminada Dredge Pit (CDP) and multiple pits in East Timbalier borrow areas (ET) adjacent to CDP. To understand how these dredge pits change and sediments infill over time, multiple surveys were conducted, including bathymetry, sidescan sonar, subbottom CHIRP, and sediment samples were collected for grain size analysis.Although there is no nearby muddy sediment, sidescan and grain size data both show low-reflectivity areas inside CDP became larger over time, indicating continual supply from suspended mud. Subbottom data show decreasing pit wall slopes over time and degassing over patchy low reflectivity areas on the pit bottom. Arc-chord rugosity analysis was performed with emphasis applied to ET to examine the difference between cutter and hopper dredge techniques. It was revealed that dredging from the hopper created a higher rugosity value than the cutter, leading to a shorter recovery time for the benthic environment. Bathymetric results reveal approximately 20 m to 60 m of horizontal outward pit wall migrations over 3–5 years. Hurricanes Marco, Zeta, and Ida as well as Tropical Storm Claudette passed the study site (within 50 Km) during the study period which increased wind-wave energies sufficiently to develop new sand ripples discovered on sidescan sonar data near ET and CDP. By integrating various datasets and investigating the effects of tropical cyclones on a dredged area in SS, this study contributes valuable insights that fill existing knowledge gaps and enhance decision-making for future management strategies.

Spatial and temporal variations of seabed sediment characteristics in the inner Louisiana shelf

Although many studies have been performed in the northern Gulf of Mexico, there have been a paucity of high-resolution sediment data collected from seabed. New multicores were collected repeatedly at 4 fixed stations in the inner Louisiana shelf on March 12, 2021, July 7, 2021, November 20, 2021, and March 5, 2022, respectively, to study detailed spatial and temporal variations of surficial sediment. Being a very high sediment accumulation zone, Raccoon Island dredge pit and its nearby shelf were selected as our study area and additional six vibracores were collected inside and outside the pit for comparison. This study area is about 140 km west of the modern Mississippi Delta and 70 km southeast of the Atchafalaya Delta. A total of 624 samples from both vibracores and multicores were analyzed for this study. Spatially, our results show that sediment near Ship Shoal was the coarsest and sediment deposited in the pit was the finest. The organic matter was highest inside the pit and lowest near Ship Shoal. The carbonate content was the lowest near Ship Shoal. Temporally, surficial sediments were generally coarser in July and November of 2021 and finer in March of 2021 and March of 2022. Although the Mississippi and Atchafalaya Rivers supply a large amount of sediment during spring season, it takes time for the river-derived sediment to be delivered into the study area. Hurricanes Delta, Zeta and Ida passed this area and greatly impacted sediment transport in the years 2021 and 2022. The comparatively coarser sediment in July and November of 2021 were likely related to the resuspension of some coarse sediment by these hurricane events. Besides the river supply, Ship Shoal and barrier islands are nearby localized sandy sources and inner Louisiana shelf is a widespread mud source. This spatial heterogeneity of surficial sediment interplays with the timing of Mississippi and Atchafalaya River flood season and hurricanes and together dictates the sediment texture. This research on a filled-up dredge pit can benefit future evaluation of the safety of existing oil infrastructure and the feasibility of building new pipelines and platforms after dredging activities. Our results also shed light on the study of river- and hurricane-impacted sediment transport in muddy and mixed-texture passive continental margins around the world.

Sediment dynamics over a dredge pit during summer fair weather conditions: A numerical study for Sandy Point, west flank of the Mississippi River

Sediment dynamics over a dredge pit during summer fair weather conditions: A numerical study for Sandy Point, west flank of the Mississippi River

Contrasting modes of sediment infilling and geomorphic change within a sand dominated dredge pit on the inner Louisiana shelf

While many sediment transport studies have been performed on mud capped dredge pits (MCDP), there is a paucity of sediment infilling data for sand dominated dredge pits (SDDP) in the Northern Gulf of Mexico (NGoM). In this study, bathymetry, sidescan, and subbottom data were collected in 2018, 2019, and 2020 and sediment cores were collected between the 2019 and 2020 surveys at Block 88: a dredge pit sitting on the sand-dominated Ship Shoal of Louisiana's inner shelf. The crest of Ship Shoal was 5 m below mean sea level before dredging and was excavated to a depth of 10 m below mean sea level after the dredging of 8.4 million m3 of sediment. Two years after dredging, the northern end of the eastern pit wall accumulated ∼0.09 million tons (approximately 500 m long × 50 m wide × 2.5 m deep) of sediment, driven by westward longshore sediment transport and the deposition of bed load on the shoal crest. Sediment accumulation on the pit bottom was mainly driven by suspended load at a rate of ∼0.1 m yr−1 from 2018 to 2019 and ∼0.2 m yr−1 from 2019 to 2020. Thus, the sediment infilling in Block 88 was from both bed- and suspended-loads from contrasting sources. These pit-bottom sediment accumulation rates are approximately one order of magnitude lower than the predicted rate from a previous empirical modeling study conducted on Ship Shoal and lower than the rates (0.5–2 m yr−1) of three other MCDPs on the inner Louisiana shelf. The infilling sediment was patchy and predominantly silt and clay, with lower sidescan reflectivity. The slopes of Block 88's pit walls decreased from ∼20° in 2018 to ∼8° in 2020. These values were smaller than the values of pit wall slopes in MCDPs at a similar stage (i.e., 2–3 years after dredging). Our results indicate that the highest sediment accumulation rates occur immediately adjacent to the eastern pit wall of Block 88, which is down-current of the longshore transport direction. This study helps sand resource managers and policymakers to better plan and design future sediment dredging projects on sandy shoals. This study concludes that the longest axis of a dredge pit should be aligned perpendicular to the longshore transport direction to trap the most sediments for possible reuse. Additionally, both bed load and suspended load should be considered in future modeling to better predict sediment infilling.

Geomorphologic change and patchy mud infilling in a sandy dredge pit in eastern Ship Shoal, Louisiana shelf, USA

To restore degraded barrier shorelines of the Mississippi River Deltaic Plain of Louisiana, sand is dredged from borrow areas in estuaries and continental shelves. The dredge pit in South Pelto Blocks 12 & 13 on eastern Ship Shoal used for Caminada-Moreau Headland restoration project is one of several sand dredge pits for barrier island restoration in Louisiana. Ship Shoal Sand Complex is in a sand-dominated, energetic, continental shelf environment. During energetic events, transient mud can blanket Ship Shoal, and then bypasses the shoal and deposits in deeper water. In this study, we collected and interpreted a suite of geophysical data from South Pelto Dredge Pit which is located in water depths of ∼9 m on eastern Ship Shoal. Repeat bathymetric survey data show that South Pelto Dredge Pit is presently infilling at an average rate of approximately 27,480 m3/year, which is slower than the predicted rate presented by an earlier numerical model. Side-scan mosaic maps indicate low-backscatter (muddy) sediments were transported into this pit and deposited in many patches within two years after dredging. The seabed inside this pit became smoother over time and the slope of the pit walls became gentler. Infilling of the South Pelto Dredge Pit seem to be controlled by sediment availability, which is volumetrically dominated by (a) far-field fluvial suspended sediment, (b) the adjacent seabed sediment resuspended by tides, waves, and currents, but presumably less impacted by (c) hurricanes or tropical storms during our study period of 2017–2018. The South Pelto Dredge Pit thus displays a unique mixing and redistribution of cohesive mud and non-cohesive sand. Initial spatial control on sediment infilling appears to be dictated by micro-bathymetry on the pit bottom created by the dredging activities. Bathymetric, side scan and grain-size data all indicate that mud was prone to deposit in the troughs.

Hydrodynamics, sediment transport, and water quality of two contrasting dredge pits on the Louisiana shelf

Sediment is needed for coastal restoration in Louisiana and is often excavated offshore from mud-capped and sandy dredge pits. To better understand the post-dredging effects on hydrodynamics, sediment transport, and water quality, two tripods were deployed at sandy Caminada pit and mud-capped Sandy Point pit in summer 2018 and 2019, respectively, and profiling data were collected during a total of 34 casts in 2018 and 2019. The results reveal similarities and differences between pit and adjacent waters and are used to compare two pits. Tropical storm events were observed during each deployment which provided insight concerning the effects these storms have on dredge pits. Water masses inside Caminada pit were relatively stable and sluggish which resulted from the pit's deep depth and location. Sandy Point dredge pit was highly impacted by the Mississippi River plume due to its proximity, which effectively stratified the water column in summer 2019, affecting ventilation near the seabed. Strong winds, taller waves, higher shear stresses, elevated turbidity and increased dissolved oxygen were generally found during the passages of tropical storms over both pits. Comparing with outside tripod station, the inside station of Caminada pit experienced a longer duration of low dissolved oxygen in bottom waters. Water mass movements and oxygen consumption from resuspension of pit bottom are likely key mechanisms driving oxygen dynamics. Our results provide observational data for future studies on post-dredging morphologic change, marine communities, and oxygen dynamics of dredge pits on the Louisiana shelf and other coastal dredge areas worldwide.

Tidal and Storm Impacts on Hydrodynamics and Sediment Dynamics in an Energetic Ebb Tidal Delta

Bottom-mounted instrumentation was deployed at two sites on a large sandy shoal of an ebb tidal delta offshore of the Port Royal Sound of South Carolina of USA to collect hydrodynamics and sediment dynamics data. One site (“borrow site”) was 2 km offshore in a dredge pit for nearby beach nourishment and the other site (“reference site”) was 10 km offshore. In situ time-series data were collected during two periods after the dredging: 15 March–12 June (spring) and 18 August–18 November (fall) of 2012. Data at the reference site indicated active migrating bedforms from centimeters to decimeters tall, and sediment concentrations were highly associated with semidiurnal and fortnightly tidal cycles. In the fall deployment, waves at the reference site were higher than those at the shallow borrow site. Both Tropical Storm Beryl and Hurricane Sandy formed high waves and strong currents but did not generate the greatest sediment fluxes. The two sites were at different depths and distances offshore, and waves contributed more to sediment mobility at the reference site whereas tidal forcing was the key controlling factor at the borrow site. This study provides valuable datasets for the selection of sites, prediction of pit infilling, and the modeling of storm impact in future beach nourishment and coastal restoration projects.

Sediment infilling and geomorphological change of a mud-capped Raccoon Island dredge pit near Ship Shoal of Louisiana shelf

To restore degraded barrier shorelines of the Mississippi River Delta of coastal Louisiana, sand is dredged from high-quality borrow areas on the adjacent continental shelf. This dredging process generally creates a topographic low that facilitates the rapid capture of sediments. Our understanding of sedimentary processes and influences on seabed morphologic evolution is still limited. In this study, Raccoon Island dredge pit in a paleo-river channel on the Louisiana shelf was studied via multiple bathymetric surveys. Physical parameters include pit geometry and topographic change from post-dredging, and infilling sediment sources were analyzed. Raccoon Island pit was 100% filled up less than six years after dredging. The average infilling rate in the Raccoon Island pit was 1.10 m/year from 2013 to 2018. During the surveying period, the pit wall slope decreased by 6.8° within two years. The sediment infilling process in the Raccoon Island pit was likely under the combined influence of topography, wind-driven currents, storm waves, and the dynamic Atchafalaya River dispersal system. The Atchafalaya River reached high discharge in 2016, which likely contributed to a surprisingly high sediment infilling rate at Raccoon Island pit between 2015 and 2018. Raccoon Island pit is in a mixed texture environment in which the sandy pit was fully filled with mud. Mud is useful for marsh restoration but is not considered a quality resource for barrier island or beach restoration. Thus, the Raccoon Island pit is not regarded as a renewable resource for future barrier island restoration.

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

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 <30 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 depth-integrated 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.

A numerical study of sediment dynamics over Sandy Point dredge pit, west flank of the Mississippi River, during a cold front event

Sediment transport over Sandy Point dredge pit in the northern Gulf of Mexico during a cold front event in November 2014 was examined using a finely resolved numerical model. The Delft3D model was used to perform numerical experiments that simulate the effect of wind-generated waves, wind-driven currents, river discharge, and tides on sediment dynamics in Sandy Point dredge pit. The hydrodynamics and sediment models were validated and calibrated using field data of current, wave, water level, and suspended sediment concentration. Two potential sources of sediment were examined: fluvial sediment from the Mississippi River and resuspended sediments from the seabed. Results showed that during a cold front, shear stress from wave motions played a significant role in the resuspension of sediments in Sandy Point dredge pit. The maximum cold front-related wave impact on sediment resuspension could increase near-bed sediment concentration in Sandy Point dredge pit by 20–50 times. In addition, the results suggest that the primary source of sediment for Sandy Point dredge pit during a cold front was resuspension from the ambient seabed due to increased bottom shear stress by wind-induced waves and strong southward wind-driven currents. Currents dispersed sediments from the Mississippi River passes and inhibited riverine sediment supply from Sandy Point dredge pit. Results also showed that cold fronts contribute 16%–24% of the annual sedimentation in Sandy Point dredge pit.

Sediment Identification Using Machine Learning Classifiers in a Mixed-Texture Dredge Pit of Louisiana Shelf for Coastal Restoration

Machine learning classifiers have been rarely used for the identification of seafloor sediment types in the rapidly changing dredge pits for coastal restoration. Our study uses multiple machine learning classifiers to identify the sediment types of the Caminada dredge pit in the eastern part of the submarine sandy Ship Shoal of the Louisiana inner shelf of the United States (USA), and compares the performance of multiple supervised classification methods. High-resolution bathymetry and backscatter data, as well as 58 sediment grab samples were collected in the Caminada pit in August 2018, about two years after dredging. Two primary features (bathymetry and backscatter) and four secondary features were selected in the machine learning models. Three supervised classifications were tested in the study area: Decision Trees, Random Forest, and Regularized Logistic Regression. The models were trained using three different combinations of features: (1) all six features, (2) only bathymetry and backscatter features, and (3) a subset of selected features. The best performing model was the Random Forest method, but its performance was relatively poor when dealing with a few mixed (sand and mud) surficial sediment samples. The model provides a new and efficient method to predict the change of sediment distribution inside the Caminada pit over time, and is more reliable when predicting mixed bed with rough pit bottoms. Our results can be used to better understand the impacts on biological communities by (1) direct defaunation after initial sand excavation, (2) later mud accumulation in topographic lows, and (3) other geological and physical processes. In the future, the deposition and redistribution of mud inside the Caminada pit will continue, likely impacting benthos and water quality. Backscatter, roughness derived from bathymetry, rugosity derived from backscatter, and bathymetry (in the importance order from high to low) were identified as the most effective predictors of sediment texture for mineral resources management.

Forces Driving the Morphological Evolution of a Mud-Capped Dredge Pit, Northern Gulf of Mexico

Sandy sediments preserved as paleo-channel fill on the inner shelf, some of which are overlain by modern muds, have been mined for barrier island restoration along the northern Gulf of Mexico. These mined areas have been termed “mud-capped” dredge pits. The processes governing the morphological evolution of the pits are poorly constrained due to limited observational data. Physical oceanographic (e.g., currents and waves) and sedimentary data were collected at Sandy Point dredge pit offshore Plaquemines Parish, Louisiana in summer 2015. Currents outside the pit flowed southward and/or southeastward at speeds of 8–20 cm/s, while currents inside the pit had speeds less than 2 cm/s with no clear dominant direction. Wave heights detected inside the pit were less than 0.4 m. A high turbidity layer with suspended sediment concentration around 4 g/L was observed above the pit floor, and its thickness was ~0.5 m. With observational data as input, three 2–D numerical models were employed to predict pit morphological responses, including pit infilling, margin erosion and slope change. The model results suggest that resuspension events were rare on the seafloor adjacent to the pit under summer fair weather conditions. Modeled pit margin erosion was very limited. With little resuspension of seafloor sediment locally, weak margin erosion and stable pit walls, the dominant process governing pit evolution was infilling sourced by the deposition of suspended sediments from the Mississippi River plume.

A morphological modeling study to compare different methods of wave climate schematization and evaluate strategies to reduce erosion losses from a beach nourishment project

Beach nourishment on open ocean beaches not bounded by headlands or other structures suffers from high rates of lateral losses of fill volume as the nourished shoreline equilibrates with its surroundings. Estimates of lateral losses are essential for beach nourishment design, these predictions have been made in the last decade utilizing empirical formulations, one line models or lately, process-based coastal morphology models. Coastal morphology models are, however, complex and computationally intensive and in order to maintain a balance between model complexities, computational effort and processing capacity, schematization of model input (input reduction) is necessary. This paper is divided into two main sections. In the first section techniques of wave input reduction for morphological models are evaluated with focus on open ocean wave-dominated coasts. Subsequently, the optimized morphological model is applied to evaluate coastal engineering interventions aimed at reducing volume losses from the Delray Beach nourishment project. Wave input reduction is defined here as the process of reducing the full wave climate of a given coastal region to a set of representative wave-wind conditions, ‘running’ a model with these representative wave conditions in sequence for a smaller time period (i.e. a few tide cycles) and multiplying its effect on the morphology by a Morfac value, that is related to the frequency of occurrence of that wave condition in nature, or its weight in the overall wave climate. Five different techniques of wave input reduction were tested. Of all the methods of wave climate schematization tested the methods defined as ‘Energy Flux Method’ and ‘Opti Method’ showed best results in terms of representing accurately the sediment transport patterns of the study area. The tests conducted indicate that a number around 12 representative wave cases was enough to represent an annual wave climate compared to a very detailed wave climate used as benchmark. The optimized model was used to evaluate alternative engineering solutions to reduce volumetric losses from the beach nourishment project. Engineering solutions evaluated included a construction of a breakwater field, backfilling all dredge pits located offshore of the project site, construction of a groin field at the downdrift end of the project, and backfilling the deepest dredge pit. These engineering interventions caused a reduction in beach volume losses within the project limits with varying levels of effect on the downdrift beaches. Reduction of the volume loss from the project site is technically feasible and may be economically feasible pending further economic feasibility evaluation.

Optimization of Nearshore Dredge Pit Design to Reduce Impacts on Adjacent Beaches

ABSTRACT Benedet, L.; Finkl, C.W., and Dobrochinski, J.P.H., 2013. Optimization of nearshore dredge pit design to reduce impacts on adjacent beaches. The magnitude of the effects of nearshore dredge pits on adjacent beaches depends on a range of parameters, including seabed geomorphology, local wave climate, sediment supply, and pit design characteristics (e.g. distance offshore, depth of cut, cross-shore and alongshore extents, shape of pit). Delft3D, a morphological model developed by Deltares, was used to investigate relationships between dredge pit design parameters and impacts on adjacent beaches. The purpose of this study was to identify design parameters that affected the magnitude of dredge pit effects on adjacent beaches. An ancillary purpose was to develop a scientific basis for dredge pit design recommendations for beach restoration and other sediment needs of coastal infrastructure projects. Dredge pit design sensitivity tests were conducted using Delft3D. A schematic model was constructed usi...

SAND BORROW AREA DESIGN REFINEMENT TO REDUCE MORPHOLOGICAL IMPACTS: A CASE STUDY OF PANAMA CITY BEACH, FLORIDA, USA.

The coastline of Panama City Beach, Florida (FL) has been stricken by several hurricanes during the last decades, especially after 1995. In 1998, beach nourishment projects started being implemented to address the impacts of the hurricanes on the coast. Sources of sand for that purpose are commonly from borrow areas located just offshore of the nourishment site. Impacts of these nearshore dredge pits on adjacent coasts will depend on incident wave conditions, nourishment sediment characteristics and some features of the borrow pit (distance from the shore, depth of cut, cross-shore extent, alongshore extent and orientation - Bender &amp; Dean, 2003; Benedet &amp; List, 2008). The practical goal of the current study was to mitigate for the negative potential effects by discovering the less impactful design of dredge pit geometries on the Borrow Area S1 in Bay County-FL. Five different cut widths and excavation depths within the permitted limits were herein evaluated. Evaluation of morphological impacts on adjacent beaches was carried with the processed-based morphodynamic model Delft3D, calibrated and simulated for a period of 13 years. Results were evaluated in terms of beach volume changes compared against a baseline simulation (no action).Switching from Alternative 1 (6,260,000 m³) to Alternative 2 (5,380,000 m³) does not result in a substantial reduction of the borrow area's projected impact. The cut depth is still deep, and the surface area is unchanged. Alternative 3 (3,555,000 m³) is able to provide more substantial reductions in the borrow area's impact. By reducing the acreage of the borrow area and switching to a uniform cut depth, the projected impact of the borrow area decreases 39% for 1.56 km along the downdrift beach. Under Alternatives 4 (3,060,000 m³) and 5 (2,755,000 m³), the impacts of the borrow area are projected to be less than 3.75 m³/m/yr. While both alternatives are viable, Alternative 5 minimizes potential impacts, and has a uniform cut depth and a volume that still satisfies the project's requirements. Given these considerations, Alternative 5 is the preferred alternative. Additionally, all the alternatives increase the net-accretion along 6.5 km of Shell Island between 0.25 to 1 m³/lm/yr., a valuable side effect in a region with high net erosion. By conducting various simulations an optimal borrow area design has been identified that reduces its effects on the adjacent beaches.

Modeling Regional-Scale Sediment Transport and Medium-Term Morphology Change at a Dual-Inlet System Examined with the Coastal Modeling System (CMS): A Case Study at Johns Pass and Blind Pass, West-Central Florida

The Coastal Modeling System (CMS), developed by the US Army Engineer Research and Development Center's (ERDC) Coastal Inlets Research Program (CIRP), is applied to model morphology change at a dual-inlet system, the Johns Pass and Blind Pass system in West-Central Florida. The CMS combines computation of current, wave, and sediment transport, leading to the prediction of morphology change at tidal inlets and the surrounding beaches. Medium-term CMS runs, with simulated times of 1.2 to 1.6 years, were completed and compared with extensive field data. Stronger tidal flow through the dominating Johns Pass and weaker flow through the secondary Blind Pass were calculated, indicating that the model reproduced an essential aspect of this interactive two-inlet system. The complicated wave refraction and breaking over the ebb tidal deltas and along the adjacent shorelines were accurately modeled, leading to a realistic representation of the wave-current interaction. Wave-breaking induced elevated sediment suspension and transport were described by the model. The predicted morphology change agreed well with field data. The CMS captured several key spatial trends of morphology change, e.g., erosion along the downdrift beach and accretion at the attachment point. The computed 32,000 m3/yr sedimentation volume in the dredge pit at the updrift side of Blind Pass matched the measured value of 35,000 m3/yr with a similar spatial distribution pattern, suggesting that the calculated net longshore sediment transport rates are accurate. The computed sedimentation rate of 60,000 m3/yr at a designed dredge pit on Johns Pass ebb-delta agrees with the generally accepted gross longshore transport rate. Rapid and large morphology change in response to high wave-energy events is predicted and is consistent with field observations.

The Effects of a Dredge Excavation Pit on Benthic Macrofauna in Offshore Louisiana

Over two years after the original creation of a sand excavation pit 8 km off the Louisiana coast, benthic macrofauna communities and sedimentary characteristics are still effected. Macrofaunal communities inside the pit had lower abundance, biomass, and diversity than communities outside the pit. This difference, however, was only significant with some of the stations outside the pit. Results from multi-dimensional scaling and cluster analysis showed that macrofaunal communities were less than 32% similar inside the pit to communities outside the pit. The polychaete Mediomastus ambiseta was the most abundant species outside the excavation pit, but the species was only counted once inside the pit. The most dominant species, which made up over 90% of organisms inside the pit, was the pioneer polychaete Paraprionospio pinnata. Only three species were found at each station inside the pit as opposed to 9-27 species at stations outside the pit. All species inside the pit were also found outside the pit; thus, change was due to a loss of species rather than replacement by different species. Sediment inside the pit contained more silt and clay; however, no difference in water quality was detected compared with outside the pit. Hurricanes Katrina and Rita passed near the dredge pit in 2005 and could have effected sediment transport in the region. Because the macrofaunal community inside the pit has not recovered within 38 months, it is likely that it will require more time before it resembles the surrounding conditions.

Nearshore Impacts of Dredging for Beach Nourishment

A methodology is presented for predicting the impacts of nearshore dredging on shoreline change, using numerical models for waves, currents, and shoreline change. The dredge pit, outside of the surf zone, was assumed to be steady state, but alters waves reaching the surf zone. The methodology accounts for the transformation of directional wave energy spectra due to spatial gradients in both mean currents and bathymetry, wind inputs, and energy dissipation via several mechanisms. The wave transformation model SWAN (Simulating Waves Nearshore) was used to simulate waves at Folly Island, S.C., which served as the test case. Wave model results were validated using short-term field measurements at three locations. Longer-term (1 year) simulations were driven using hindcast wave and wind data. Velocity fields and tidal stages were simulated using the hydrodynamic model Environmental Fluid Dynamics Code in two-dimensional mode, after calibrating with field measurements. Wave model results were used to estimate longshore sediment transport and shoreline change due to longshore gradients of the longshore sediment transport rate. Empirical sediment transport coefficients were chosen to match model results to measured beach volume changes derived from beach profile data. The calibrated model was then used to predict impacts of dredging on long-term shoreline change. Results reproduced observed trends of erosion and accretion along approximately 90% of the shoreline.