Delta Morphology Research Articles

Effects of upstream and downstream boundary conditions on lacustrine shallow-water delta morphologies: A numerical modeling approach

Lacustrine shallow-water deltas exhibit highly variable morphologies, owing to their high sensitivity to changes in the upstream and downstream boundary conditions, where a small change significantly alters their morphology. In this study, we present the results from a numerical model (DELFT3D) used to explore the mechanism of distributary network amalgamation and delta evolution under various boundary conditions. We established a reference case to model the delta building-out process considering the basin water depth difference (shallow and deep). Geological and hydrodynamic models were established by varying the water level under a steady river discharge and sediment supply. Despite similar initial conditions, each modeled delta exhibits a unique morphological evolution, progressing through distinct stages: (i) formation of unstable, narrow channels with quasi-radial shapes; (ii) channel development, shaping the delta based on controlling factors; (iii) reduction of deltaic activity and increased channel avulsion due to delta submergence, and; (iv) new channels emerging, paving the way for reshaping and migration towards new paths, resulting in fewer bifurcations and straighter channels. Quantitative relationships characterizing delta morphology and distributary channels were established, and the following results were noted: (i) a positive correlation between active and abandoned channels, distributary channel network opening angle, and delta area with discharge and sediment grain size, and negative correlation with basin depth and falling water level; (ii) a positive correlation of lobe elongation with basin depth and discharge, and a negative correlation with sediment grain size; and (iii) roughness and length-to-width ratio emerging as key predictors of delta shape. By applying the model predictions to real-world scenarios, including modern deltas (Sile and Chiemsee) and ancient systems (Bohai Bay Basin), this study can provide valuable insights into the dynamics of lacustrine deltaic systems and their responses to changing boundary conditions. This highlights the importance of understanding deltaic systems for both theoretical research and practical applications.

Sustained increase in suspended sediments near global river deltas over the past two decades

River sediments play a critical role in sustaining deltaic wetlands. Therefore, concerns are raised about wetlands’ fate due to the decline of river sediment supply to many deltas. However, the dynamics and drivers of suspended sediment near deltaic coasts are not comprehensively assessed, and its response to river sediment supply changes remains unclear. Here we examine patterns of coastal suspended sediment concentration (SSC) and river sediment plume area (RPA) for 349 deltas worldwide using satellite images from 2000 to 2020. We find a global increase in SSC and RPA, averaging +0.46% and +0.48% yr−1, respectively, with over 59.0% of deltas exhibiting an increase in both SSC and RPA. SSC and RPA increases are prevalent across all continents, except for Asia. The relationship between river sediment supply and coastal SSCs varies between deltas, with as much as 45.2% of the deltas showing opposing trends between river sediments and coastal SSCs. This is likely because of the impacts of tides, waves, salinity, and delta morphology. Our observed increase in SSCs near river delta paints a rare promising picture for wetland resilience against sea-level rise, yet whether this increase will persist remains uncertain.

Flexural isostatic response of continental-scale deltas to climatically driven sea level changes

Abstract. The interplay between climate-forced sea level change, erosional and depositional processes, and flexural isostasy in deep time on passive margin deltas remains poorly understood. We performed a series of conceptual simulations to investigate flexural isostatic responses to high-frequency fluctuations in water and sediment load associated with climatically driven sea level changes. We model a large drainage basin that discharges to a continental margin and produces a large deltaic depocenter, then prescribe synthetic and climatic-driven sea level curves of different frequencies to assess flexural response. Results show that flexural isostatic responses are bidirectional over 100–1000 kyr timescales and are in sync with the magnitude, frequency, and direction of sea level fluctuations and that isostatic adjustments play an important role in driving along-strike and cross-shelf river mouth migration and sediment accumulation. Our findings demonstrate that climate-forced sea level changes produce a feedback mechanism that results in self-sustaining creation of accommodation into which sediment is deposited and plays a major role in delta morphology and stratigraphic architecture.

Sediment Grain Size Affects Vegetation Patterns in River-Dominated Deltas

River deltas contain some of the most densely populated areas in the world and are characterized by a rich biodiversity and highly productive ecosystems. Although previous studies have revealed that sediment grain size plays a key role in determining delta morphology, its subsequent effects on deltaic vegetation distribution patterns remain elusive. We used a theoretical templet to facilitate our study by conducting numerical experiments to simulate deltaic morphological evolution in response to various sediment grain sizes and explored the vegetation distribution determined by the elevation profile of a typical river-dominated delta. Our results showed that a fine-grained, cohesive delta would develop vegetation patterns transitioning from high- to low-elevation adapted vegetation species with increasing distance, whereas the same vegetation transformation trend could occur with decreasing distance to the center of a large-grained, noncohesive delta. The modeled vegetation distribution pattern of fine-grained, cohesive and large-grained, noncohesive deltas could be well demonstrated in the natural deltas. In addition, vegetation distribution patterns are related to the grain size-driven morphological processes of deltas, and the leading role of cohesive and noncohesive sediments on constructed delta is critical in determining the resulting vegetation distribution pattern. The dominance of each vegetation type would gradually become stable as delta matures. The interspecies competitions could influence distribution patterns of each vegetation type by enhancing the fragmentation of mid-elevation adapted vegetation. The effects of sediment grain size on the deltaic vegetation patterns associated with a delta’s morphological processes have potential implications for the conservation and restoration of deltaic habitats.

Marsh-induced backwater: the influence of non-fluvial sedimentation on a delta's channel morphology and kinematics

Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta.

Quantifying Formative Processes in River‐ and Tide‐Dominated Deltas for Accurate Prediction of Future Change

AbstractAlthough most deltas are expected to lose land due to climate change and urbanization, tide‐dominated deltas have been suggested to gain land. The processes of land change in such deltas are, however, not well understood, and tide‐dominated deltas with all their known morphological attributes have not been simulated before. Our Delft3D simulations successfully reproduce tide‐dominated delta morphology, and show that tide‐dominated deltas with seaward widening and stable channels, elongate tidal bars and a funnel shape form when bidirectional tidal flow occurs along the whole delta. In contrast, tide‐influenced but river‐dominated deltas protrude from shoreline, and are efficient at gaining land by mouth bar deposition and bifurcations. These differences are a function of tidal‐fluvial interactions expressed by month‐averaged seaward and lateral velocities. Identification of delta types is thus critical for predicting future land change. We propose morphological metrics to differentiate river‐ and tide‐dominated deltas and test metrics on 40 modern deltas.

Modeling the Morphological Responses of the Yellow River Delta to the Water‐Sediment Regulation Scheme: The Role of Impulsive River Floods and Density‐Driven Flows

AbstractMorphological evolution of river deltas depends to a large extent on river discharges, which are usually highly unsteady due to natural hydrological cycles and anthropogenic regulations. However, it is unclear that how and to what extent the discharge fluctuations influence the delta morphology. In this study, we focus on the morphological response of the Yellow River Delta to the Water‐Sediment Regulation Schemes, which generate impulsive floods and deliver high sediment load within a short time. Tracking the fate of fluvial sediment released by 10 historical events reveals that 51.3%, 19.7%, and 17.8% of fluvial sediment are deposited in the delta front, the subaerial delta and the prodelta, respectively. Hypopycnal and hyperpycnal flows occur alternately during different phases of the regulation schemes, and the latter contribute to 32% of the deposition volume with 10% of the duration. To explore the effects of river discharge schematizations on delta evolution, numerical experiments are conducted to compare models forced by unsteady and constant discharges, with the latter being a common practice in delta morphology modeling. We show that the constant‐discharge simplification fails to capture the highly‐depositional hyperpycnal flows, leading to dramatic underestimation of river mouth depositions. In addition, we demonstrate that including the 3D flow‐sediment interactions are critical in reproducing the correct plume structures, bed surface sediment compositions and deposition patterns. Our study highlights the importance of proper river discharge schematizations and full consideration of flow‐sediment interactions in modeling deltas affected by episodic river floods and high suspended sediment concentrations.

Morphology and origin of liquefaction-related sediment failures on the Yellow River subaqueous delta

Subaqueous slopes of many actively prograding deltas are heavily modified by widespread submarine sediment failures, forming highly complex geomorphic features that undergo continuous evolution even over short time scales. However, the effects of wave-seabed interactions (especially liquefaction) on the evolution of seabed sediment failures are rarely studied. Here, we use side-scan sonar, sub-bottom profilers, multibeam bathymetry, and in situ and laboratory geotechnical tests to map the bottom morphology and sediment structure of the Yellow River subaqueous delta. Our results indicate that: 1) The morphology along the Yellow River subaqueous delta is highly complex and can be categorized as a moderately disturbed area and heavily disturbed area; 2) Heavily disturbed area shows notable lines of evidences of liquefication-related sediment failure and four separate morphological zones (Zones 1–4). Zone 1 consists of differently sized sediment highs, which considered to be scarp or monadnock features. Zone 2 undulates with widely distributed tabular sediment blocks, gullies, and small-scale seafloor irregularities. Zone 3 is characterized by irregular, elongate sediment ridges. Finally, Zone 4 is characterized by relatively flat terrain with some localized highs; 3) Sub-bottom profiles used to visualize failure features in Zone 1 show acoustically continuous high-amplitude parallel or subparallel reflection, and the geomorphic features in this area are mainly caused by erosion. In comparison, Zones 2, 3 and 4 lack internal structures and display chaotic/disturbed signatures in corresponding sub-bottom data, which might be an indication of liquefaction; and 4) A combination of preconditioning and triggering factors lead to sediment failures and the complex morphological features in this area; however, silt liquefaction under storm conditions is the most important controlling factor. Based on this evidence, we propose formation mechanism for the liquefaction-related sediment failure features in this area. Sediment liquefaction forms an initial depression and gully when fine particles are resuspended along the arc-shaped liquefaction boundary as sediment vents and re-solidification of the liquefied sediment leads to a stiffer bed. Gullies are then widened and deepened by the predominant currents, and the disturbed depressions develop into elongated highs because of the denser structure and higher strength following liquefaction. While disturbed by engineering activities, the sediment highs become fragmented due to intense localized erosion, having the potential for downslope transport and the formation of sediment ridges. The geometries and distributions of the sediment failure features on the Yellow River subaqueous delta provide insights into process mechanisms and causative factors, especially when compared with other, similar phenomena reported elsewhere.

River delta eco-morphodynamics under changing scenarios: The case of Lake Turkana, Kenya

In this work we explore the impact that changes in local climate and river hydrology have on the morphodynamics of a river delta, particularly focusing on the evolution of the delta generating at the lake inlet. We investigated the case of the delta in the lower reach of the Omo River in Ethiopia, which flows into lake Turkana, Kenya. The lake is the fourth largest lake and the largest desert lake in the world. This case study is of particular interest because within the last decades three dams have been built within the Omo basin. Among these, the Gibe III dam had a huge impact on the river hydrology and the sediment supply. To quantify changes in land use and river morphology and relate these to climate change and river hydrology we collated a historical dataset by combining information from different satellite sources. We observed that the amount of bare sediments progressively diminished and the biomass became denser compared to the existence of sparse biomass in the past. We argue that this is due to the changes in river hydrology and sediment load imposed by the dam, which increased sediment erosion by deepening the channel. The presence of the dam also increased the low flow level and reduced the peak flow during flood season, reducing the natural oscillations of the water table. In consequence, the possibility to plant for the local population was removed during floods. Our findings also indicate that the hydrology and reduced sediment discharge have changed the downstream (e.g. lake level), and upstream (e.g. mouth bar deposition) boundary conditions controlling the evolution of the delta structure, respectively. As a result, the delta morphology became less dynamic and less complex.

Down‐delta hydraulic geometry and its application to the rock record

ABSTRACTPalaeodischarge estimation is largely undertaken within fluvial settings. However, there are limited palaeodischarge estimates specifically from delta deposits, despite their significance globally. Estimating water palaeodischarges for deltas using catchment‐based approaches developed using data from fluvial settings requires estimating parameters from the rock record (for example, palaeotemperature, palaeoslope and palaeorelief). These may be difficult to determine, leading to under‐estimation or over‐estimation of palaeodischarge values due to differences in process‐form relationships between alluvial rivers and deltas. When a sediment‐conveying fluvial channel enters a standing body of water, delta lobes develop through repeating mouth bar deposition due to flow deceleration, forming a deltaic morphology with distributary channel networks that differ morphologically from those developed in unidirectional flowing alluvial rivers. This study provides empirical relationships determined across five climate regions, using 3823 measurements of distributary channel width from 66 river deltas alongside the trunk river bankfull discharge that feeds into the entire delta, using a hydraulic geometry scaling approach. Empirical relationships are developed from the global delta dataset between bankfull discharge and catchment area (Qb–A), and bankfull discharge and median distributary channel width (Qb–Wmed). These empirical relationships produce very strong statistical correlations, especially between Qb and Wmed, across different climate regions (Qb = 0.34 Wmed1.48, R2 = 0.77). However, both Qb–A and Qb–Wmed relationships have outliers that may be explained by particular hydrological or geomorphic conditions. These new empirical relationships derived from modern systems are then applied to Cretaceous outcrops (Ferron Sandstone and Dunvegan Formation). The comparatively simple scaling relationships derived here produced palaeodischarge estimates within the same order of magnitude as palaeodischarge values previously obtained using existing, more complex approaches. This study contributes to source‐to‐sink investigations by enabling palaeodischarge estimates that intrinsically account for climate impacts on channel geometry at the time of deposition, using measurements of channel width or catchment area of a deltaic outcrop.

First‐Order River Delta Morphology Is Explained by the Sediment Flux Balance From Rivers, Waves, and Tides

AbstractWe present a novel quantitative test of a 50‐year‐old hypothesis which asserts that river delta morphology is determined by the balance between river and marine influence. We define three metrics to capture the first‐order morphology of deltas (shoreline roughness, number of distributary channel mouths, and presence/absence of spits), and use a recently developed sediment flux framework to quantify the river‐marine influence. Through analysis of simulated and field deltas we quantitatively demonstrate the relationship between sediment flux balance and delta morphology and show that the flux balance accounts for at least 35% of the variance in the number of distributary channel mouths and 42% of the variance in the shoreline roughness for real‐world and simulated deltas. We identify a tipping point in the flux balance where wave influence halts distributary channel formation and show how this explains morphological transitions in real world deltas.

Self-weight consolidation process of water-saturated deltas on Mars and Earth

Deltas are water-related sedimentary landforms commonly discovered on Mars and Earth. During the formation of a delta, the self-weight consolidation process reduces its porosity and thickness. Previous studies on the martian consolidation process either adopted the terrestrial consolidation rate or ignored this process altogether. However, the consolidation process may diverge on these two planets due to their different gravity regimes, which may further affect the delta morphology and lithification and the estimation of the delta formation timescale. In this study, we carry out a series of laboratory experiments to determine the basic parameters for modeling consolidation using water-saturated martian sediment simulants. The consolidation process is modeled with Gibson's 1-D consolidation model for deltas with different initial thicknesses (20–150 m) and different types of base drainage conditions (rigid bedrock and loose regolith). The modeling results suggest that for the delta area with fine grains, the settlement amount on Mars is lower than that on Earth, which indicates the martian delta to be looser, more erodible and more easily disturbed than the counterparts on Earth from the effects of the consolidation process alone. Higher delta channelization, more sediment deformations and post-deposition erosions before significant cementation are thus expected on Mars. More cementation/recrystallization is needed for martian deltas to be lithified. By using the simulated final porosity of martian deltas (0.45–0.46) and an updated timescale calculation formula, estimated delta formation timescale is reduced to be 32–44% of the previous results.

SATELLITE OBSERVATIONS REVEAL MORPHOLOGICAL CHANGES IN ASIAN DELTAS FROM 1986 TO 2018

Abstract. Asian deltas are densely populated and richly bio-diversified regions with significant social, economic, ecological and environmental importance across the globe. These low-lying and fertile floodplains support substantial food production. Because of sediment supply reductions, sea-level rising, subsidence, and frequent storm surges, many deltas are rapidly sinking and shrinking, which will threaten survival of millions of populations. It’s of great significance to monitor the delta morphological evolution and understand the relationship between their shoreline migrations and sediment supplies. However, quantifying the delta morphological changes is challenging due to its nature of high dynamics. In this study, we developed a methodology integrated by a time-series algorithm of water frequency and a spatial analysis to track the morphological changes of 11 major Asian deltas from 1986 to 2018.The results show that the deltas of Yellow, Yangtze, Pearl, Red, Mekong, Ganges-Brahmaputra, and Chao Phraya have experienced a net land gain at the total of 1829.3 km2, with China’s three major deltas accounting for 73 percent of all. The deltas of Mahanadi, Irrawaddy, Indus, and Godavari-Krishna have undergone a net land loss at the total of 295.1 km2, of which the Indus River Delta accounted for 65 percent. Most of the shorelines in the Yangtze River Delta were constantly advancing seaward at a rate of over 30 m/year, whereas, many ones in the rest of Asian deltas were significantly being eroded at a rate of over 20 m/year. These changes in delta morphology are closely related to sediment supplies and management policies. These results also suggest that rational wetland reclamation with auxiliary projects of ecological restoration can enhance the deltaic ability of sustainable development.

Slope Break and Avulsion Locations Scale Consistently in Global Deltas

AbstractUnderstanding how deltas respond to changing sea level is crucial as deltas provide important ecosystems, are inhabited by ∼500 million people, and are nexuses of food, energy and economic activity. The response of delta distributary channels to sea‐level rise depends on the geomorphic controls on delta morphology and their scaling relationships. Our data from 105 deltas globally show strong scaling between the upstream distances to slope breaks and to avulsion nodes, and confirm the previously known scaling between backwater and avulsion lengths. The break in slope is proposed to be the principal control on delta development, along with other proposed secondary controls. We identify and discuss the implications of this slope break‐avulsion length scaling, leading to a conceptual model of delta morphology and sedimentology. This model suggests how deltas may respond to future sea level rise and guides interpretation of deltaic deposits in the rock record.

Comparing the Yangtze and Mississippi River Deltas in the light of coupled natural-human dynamics: Lessons learned and implications for management

The Yangtze River Delta (YRD) and Mississippi River Delta (MRD) are two of the largest deltas in the world. Despite similar forcing conditions of fluvial sediment reduction and relative sea-level rise, there are significant differences in deltaic change caused by hydrodynamics and human activities. In this review, we compare the morphological changes of the two deltas at multiple spatial and temporal scales and identify the lessons learned from past engineering and management strategies. While the YRD has seen an increase of subaerial land of ~1500 km2 since 1950, the MRD has lost approximately 5000 km2 of land since the early 1930s. Extensive land reclamation and coastal shoreline embankment in the YRD have led to land gain at the expense of tidal wetland shrinkage. In contrast, flood control, waterway transportation, and petroleum industry in the MRD have resulted in levee construction and canal building, which together with subsidence and sea-level rise, have led to significant wetland loss in the MRD. The tidal forcing in the YRD is much stronger than that in the MRD, which has played a role in redistributing subaqueous sediment back into the delta plain, offsetting the negative effect of fluvial sediment reduction. In contrast with continuous population increase on the YRD, wetland loss and exposure to flooding hazards have resulted in population loss on the MRD. These comparative analyses reveal that nature and human interventions have acted together in shaping the modern deltaic morphology. Using the Driver-Pressure-State-Impact-Response (DPSIR) analysis framework, we propose a conceptual model to provide holistic understanding of the natural-human coupled delta system and management strategies, which could yield broad implications for coping with the future challenges in global deltas.

The Relationship Between Delta Form and Nitrate Retention Revealed by Numerical Modeling Experiments

AbstractRiver deltas display a wide range of morphologic patterns that influence how nutrients interact in channels and wetlands on their way to the coast. To quantify the role of delta morphology on nitrate fate, we simulated reactive nitrate transport over steady base flow conditions for six synthetic, morphologically unique river‐dominated deltas created in Delft3D by varying incoming grain size distributions. We parameterized nitrate removal kinetics using an observed relationship with elevation from Wax Lake Delta (Louisiana, USA). Total nitrate retention across the six synthetic deltas and Wax Lake Delta ranged from 1.3%‐13%, suggesting that these river‐dominated deltas have limited ability to remove nitrate from incoming river water. Nitrate removal is constrained by limited hydrologic connectivity with the areas of greatest nitrate demand, which are found at higher elevation. In these synthetic numerical experiments, the efficiency of nitrate removal is greatest for deltas with more topologic complexity and greater proportions of the delta plain at higher elevation, which are both common characteristics of coarser‐grained deltas. The positive relationship between grain size and nitrate removal may help guide land reclamation projects if project goals include minimizing nitrate export to the sea.

Effects of upstream conditions on digitate shallow-water delta morphology

Digitate deltas consist of one or multiple separate bar fingers, which can form hydrocarbon reservoirs after burial. This paper focuses on digitate shallow-water deltas, which are commonly seen in modern and ancient deposits with similar downstream conditions. Four metrics were adopted to quantify their morphologies, including the average sinuosity, average nondimensional width (average width ratio between the bar fingers and distributary channels), nondimensional delta length (ratio between the delta length and average width of the distributary channels), and number of bar fingers. These metrics measured from 9 modern deposits and 21 Delft3D simulations exhibited wide-ranging values that were strongly affected by the upstream conditions. However, the effects of the upstream conditions remain unclear. The quantitative effects were revealed by performing Delft3D simulations: (1) the average sinuosity is proportional to the sediment cohesion and concentration and inversely proportional to the sand proportion and water discharge, and reaches equilibrium as sediment supply increases; (2) the average nondimensional width is proportional to the sediment concentration, is inversely proportional to the sediment cohesion and water discharge, has an inverse exponential relationship with the sand proportion, and is independent of the sediment supply; (3) the nondimensional delta length has a power-law relationship with the water discharge and is logarithmically related to the sediment supply; (4) the number of bar fingers is proportional to the sand proportion and sediment supply and inversely proportional to the sediment cohesion. Upstream conditions influence channel, mouth bar and levee growth, resulting in various bar finger morphologies. Empirical equations from Delft3D simulations were proven effective in examinations of nine modern deposits and applied to help predict the distribution of a digitate shallow-water delta reservoir. This work improves the fundamental understanding of the upstream controls in digitate shallow-water deltas and may help enhance the inter-well prediction of paralic reservoirs.

Towards an efficient storm surge and inundation forecasting system over the Bengal delta: chasing the Supercyclone Amphan

Abstract. The Bay of Bengal is a well-known breeding ground to some of the deadliest cyclones in history. Despite recent advancements, the complex morphology and hydrodynamics of this large delta and the associated modelling complexity impede accurate storm surge forecasting in this highly vulnerable region. Here we present a proof of concept of a physically consistent and computationally efficient storm surge forecasting system tractable in real time with limited resources. With a state-of-the-art wave-coupled hydrodynamic numerical modelling system, we forecast the recent Supercyclone Amphan in real time. From the available observations, we assessed the quality of our modelling framework. We affirmed the evidence of the key ingredients needed for an efficient, real-time surge and inundation forecast along this active and complex coastal region. This article shows the proof of the maturity of our framework for operational implementation, which can particularly improve the quality of localized forecast for effective decision-making over the Bengal delta shorelines as well as over other similar cyclone-prone regions.

Tracking fluorescent and ferrimagnetic sediment tracers on an energetic ebb-tidal delta to monitor grain size-selective dispersal

Sediment tracer studies use uniquely identifiable particles to track the pathways and fate of individual sand or silt grains in marine environments. These techniques are best applied to assess connectivity between potential sediment sources and sinks, such as between a sand nourishment and an ecologically sensitive area. Significant challenges exist when applying sediment tracing techniques to further understanding of systems with complicated hydrodynamic, sediment, and morphological regimes. Ebb-tidal deltas are highly dynamic coastal environments shaped by the complex interplay of waves and tides, but have been under-explored. In this study, we use dual signature (fluorescent and ferrimagnetic) sediment tracers to simulate the dispersal of dredged sediment placed as a sand nourishment on an energetic ebb-tidal delta (at Ameland Inlet, the Netherlands). After deployment, sediment dispersal and grain size sorting behaviour were monitored via the collection of seabed grab samples and magnetic sampling of sediment transported in suspension. The tracer content within collected samples were put in context with hydrodynamic conditions observed during the study period. Here we show that the use of such dual signature tracers, in addition to novel tracer recovery and analysis techniques, enables the dispersal of sediment to be monitored even in such complex settings and energetic conditions as an ebb-tidal delta. Our observations show that tracers transported in suspension are significantly finer than tracers that accumulated in the seabed. These suggest that preferential transport as a function of grain size is a key process in shaping the morphology of ebb-tidal deltas and thus governing the dispersal of sand nourishments there. The findings of this study and the approach used here provide valuable tools for assessing the baseline conditions of complex coastal environments today, and for planning the interventions which may be necessary in future responses to climate change. Lessons learned from the application of sediment tracers in this study are provided to assist future researchers and practitioners in applying this technique in dynamic coastal environments.

Fan-delta facies architecture, morphological evolution and sediment delivery in the Oligocene Dongying Formation of the Nanpu Sag, Bohai Bay Basin, China

Core, well log and seismic data are used to investigate relationships between fan-delta facies architecture, basin margin morphology and sediment transportation into a deep-lacustrine environment of the Dongying Formation of Nanpu Sag, Bohai Bay Basin. The study interval is subdivided into two fan-delta dominant, third-order sequences of Stratasequence Paleogene Dongying Formation 1 (SQEd 1) (younger) and Stratasequence Paleogene Dongying Formation 2 (SQEd 2) (older). Fan-delta growth occurred in two stages with changing of facies architecture and morphology. During the early stage (SQEd 2), the facies architecture is characterised by smaller and thin, channelised deposits, an extended mouth bar (delta-front sandy deposits) and slump/slides/flood turbidite dominated sandy deposits in the prodelta. However, during the late stage (SQEd 1), the facies architecture is characterised by extended and thicker channelised deposit-dominated area, a local mouth bar (delta-front sandy deposits) and slump/slide-dominated sandy deposits in the prodelta. From the early to late stage, the delta morphology also changed with an upward increase in the delta-plain areal extent and delta-front gradient. Facies architecture variations indicate differences in sediment dispersal. The upward thickening and extending delta plain implies that more sediment was trapped in the proximal area owing to sufficient accommodation space generated by increasing boundary fault displacement. The upward increasing delta-front gradient resulted in differences in the type of prodelta/deep-lacustrine sandy deposits, while grainsize variations of the delta-front deposits affect the prodelta/deep-lacustrine sandy deposits. Boundary fault growth occurred with upward increasing displacement and length with the result that sediment supply outpaced accommodation-space creation and controlled the facies architecture changes. KEY POINTS A two-stage fan delta developed with changing facies architecture is analysed. Facies architecture controlled fan-delta morphology with sediment transport into a deep-lacustrine area. Faults control the facies architecture and sand-rich deposit distribution.