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Dunes form where winds blow over a bed of mobile sediment grains — conditions that are common in our solar system. On Earth, dunes abound in arid continental interiors and along sandy coastlines. Dune fields have also been recognized on Venus, Mars, Saturn's moon Titan, and Pluto. In response to the different boundary conditions and other environmental forcings, dunes adopt a rich diversity of shapes, sizes, and behaviors. Thus, people around the globe and over centuries have developed a rich vocabulary to describe dunes and their complexity. As a result, existing dune nomenclature often includes redundant terms with differing definitions across scientific communities. Previous studies have endeavored to link dune shape to environmental forcing, usually by means of correlation. Although instructive, correlation-based classifications can be misleading if not based on underlying mechanics or if dune morphogenetic classes are not uniquely defined. Here, we synthesize existing dune terminology and use the last two decades of research on dune morphodynamics to propose three complementary dune classification schemes based on: (1) descriptive dune geomorphology, (2) morphodynamic processes, and (3) fluid mechanics and physics of sediment transport. The first classification relates dune types to geomorphic setting, the presence or absence of vegetation or obstacles, and dune shape (including planform shape, and cross-sectional symmetry or asymmetry). Dune classes can be further subdivided where the direction of sand transport is known independently. The second classification relates dune types and shapes to bed properties (sand-covered vs partially starved bed) and wind forcing (directional variability or the relative strengths and directions of wind modes) that together influence dune dynamics (growth, migration, elongation) and select the dominant processes by which dunes are shaped and oriented relative to the resultant transport direction. The third classification relates, for different planetary environments, the range of possible dune sizes, from minimum to maximum wavelength, to fluid flow regime (rough or smooth) and the response of sediment flux, which influence the coupling between sand bed topography, fluid flow, and sediment transport. These characteristic lengths are useful scales for comparative geomorphology. The three classification schemes provide complementary information. Together, they form a unified framework for geomorphologists, sedimentologists, geographers, physicists, and others to describe windblown sand dunes on Earth and beyond through their shape, dynamics, and size as a response to winds and boundary conditions. [ABSTRACT FROM AUTHOR]

Chirp profiles collected on the southern Brazilian shelf were combined with sedimentological and hydrodynamic data to characterize shelf geomorphology, focusing on the development of outer shelf bedforms and their genetic links with major oceanic currents. Outer shelf sediment accumulations are interpreted as two scales of bedforms, such as dunes and sand ridges. In the São Paulo Bight, cuspate bedforms are bounded landward by a geomorphological scarp. The sector between Florianopolis and Mostardas also exhibits frequent asymmetric bedforms, whereas the inner- middle shelf is covered by semitransparent sheets. The southern Rio Grande Cone is characterized by stratified deposits covering vast outer shelf areas. Surficial sediment samples reveal a dominantly muddy composition and relatively high carbonate contents. Hydrodynamic data indicate a major influence of the Brazil Current over the São Paulo Bight. The prolific bedform outer-shelf environment is assumed to be mostly relict formed in a transgressive shallow-water setting. The sand ridges are believed to have evolved from a moribund stage characterized by morphological degradation to a relict stage, with preservation enhanced by fine-grained winnowing, coarse sediment armoring, and early cementation. Bedform migration seems to be occurring on the São Paulo Bight due to the presence of a nearly permanent meander of the Brazil Current. Over most of the inner shelf, the northward advection of sediments mainly led by the Brazilian Coastal Current has formed extensive fine-grained deposits. The occurrence of a relict shelf scarp may have conditioned the formation of the outer-shelf bedform environment. The vast spatial distribution of bedforms over most of the outer shelf seems comparable with other shelf areas, such as the south African shelf, dominated by the influence of unidirectional oceanic currents. Dune dimensions are lower on the Brazilian shelf, due to their relict character, the geomorphological configuration of the margin, and the scarcity of sand sources. • Outer shelf along the southern Brazilian margin covered by bedforms • Inner-to-middle shelf advection of fine sediments driven by Brazilian Coastal Current • Two scales bedforms (sand ridges and very large dunes) in a relict to moribund stage • Bedform preservation aided by fine-grained winnowing by the Brazil Current, armouring and cementation • Prolific bedform environment comparable with the south African shelf [ABSTRACT FROM AUTHOR]

Proglacial areas in the European Alps and other high-elevation mountains are currently undergoing rapid change due to global warming. Because of rising temperatures, glaciers and glacier forefields are subjected to increased melting and associated sediment export. This observation is increasingly important with respect to high-elevation geomorphological and ecological dynamics, emerging natural hazards and mitigation efforts, and hydropower plant management. It is therefore crucial to analyze the factors and feedback mechanisms governing sediment production, transport, and deposition in these rapidly changing areas. In this study, we investigated the sediment dynamics of a proglacial area located in the Eastern Italian Alps over the period 1969–2021 with the aims of: i) identifying the areas of sediment production; ii) quantifying volumes and rates of bedload sediment transport; and iii) determining the relative contribution of glacial export and fluvial erosion to the total sediment budget. We found that i) apart from glaciers, moraines and fluvial channels have been the most important sediment sources, albeit with substantial differences in terms of connectivity and thus supply rates; ii) the volumes and rates of sediment erosion varied by one order of magnitude (between tens and hundreds of mm per year), and were generally higher along the channel network; and iii) for a relatively shorter time interval between 2005 and 2021, the relative contribution of glacial bedload input with respect to the total sediment budget ranged between 34 % and 37 %, whereas 45 % to 59 % was derived from lateral fluvial erosion. Only a relatively small sediment volume was generated by net channel bed incision. These results imply that most of the sediment released from the proglacial area of the Sulden glacier is progressively transferred to the downstream sector of the channel network, with volumes that range between 931 and 1017 tons yr−1 km−2. These values are in the typical range of sediment export volumes from glaciated basins and highlight the high dynamicity of this region of the Alps. In general, our results confirm the complexity – in terms of spatial and temporal variability – of Alpine proglacial systems and highlight the need to systematically study these areas on a wide spatial and temporal scale, since the information provided by single locations or individual sectors of the sediment cascade, may not be adequate for understanding the dynamics acting in the entire proglacial regions. [Display omitted] • Proglacial areas change rapidly and supply sediment to the downstream valleys. • Moraines and channels are the most important sediment sources. • Glacial and fluvial erosion contribute between 35 % and 50 % to the sediment budget. • The proglacial area of the Sulden glacier is dominated by sediment transport. • In the Sulden region, sediment export may increase in the near future. [ABSTRACT FROM AUTHOR]

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

Uranium (U) isotopes can be used to estimate the comminution age of sediments , i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N = 64) includes U isotopic measurements for many of the world's largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios > 1, with clay-size fractions (<4 µm) being less depleted in 234U (higher 234U/238U) than corresponding silt-size fractions (4–63 µm). In contrast, sediments derived from multi-lithological basins or draining sedimentary rocks are typically characterized by (234U/238U) ratios <1, with clays generally exhibiting more depleted 234U signatures than silts. Taken together, these observations suggest that the formation of secondary clay minerals in soils from basins draining mostly igneous, metamorphic is accompanied by partial incorporation by recoil injection of 234U initially released during weathering processes, possibly from U-rich minerals, such as sphene or apatite. Instead, in multi-lithological catchments draining sedimentary rocks, we propose that the erosion of recycled sediments having experienced several cycles of weathering, possibly over glacial-interglacial timescales, could explain the much lower (234U/238U) ratios observed in clay-size fractions. While no direct relationships can be identified between sediment (234U/238U) ratios and lithology, weathering intensity, climatic or geomorphic parameters in corresponding river basins, we show that the catchment size probably plays an important role in controlling the distribution of (234U/238U) in river sediments, through its direct influence on the sediment residence time. Finally, a multiple regression analysis of our data, combining various environmental parameters for the lithology, climate and geomorphology of studied river basins, indicates predicted (234U/238U) values that are very similar to measured values (with R2 ∼ 0.8). This finding provides further support for the usefulness of (234U/238U) ratios in the sedimentary record for reconstructing past landscape changes and their effect on sediment transport and residence time in river basins. [ABSTRACT FROM AUTHOR]

The upper third member of Eocene Dongying Formation (Ed 3 u) is considered to be one of the most important hydrocarbon-rich intervals in Nanpu Sag, Bohai Bay Basin, China. Through integrated analysis of the drilling cores, wire-line logs, and high-quality three-dimensional (3-D) seismic data, the sedimentary characteristics of Ed 3 u were analyzed by utilizing the seismic geomorphology approach in the double slope systems of Laoyemiao transverse anticline, Nanpu Sag. The Ed 3 u, a third-order sequence, can be subdivided into three fourth-order sequences (from base to top as SS1, SS2, and SS3). The fan delta front and sublacustrine fan have been identified from well-based analysis of the sedimentary facies and core examinations reveal that the sublacustrine fan deposits were dominated by six types of resedimented lithofacies, namely, slides, slumps, sandy debris flows, muddy debris flows, turbidity currents, and hyperpycnal flows. Stratal slices were obtained from 3-D single-frequency seismic volumes to map the sediment dispersal characteristics and document the evolution of the depositional systems constrained by the double slope break zones of Laoyemiao transverse anticline. The unique structures of the double slope break zones and the variation in the lake level associated with the system tracts of the third-order sequence jointly control the dispersal patterns of the sand bodies and the spatial-temporal evolution of the depositional systems. Based on the above analysis, the depositional evolution model of Ed 3 u was established. The proposed facies-controlled model and sediment transport model can be used to accurately predict the favorable reservoir sandstones. • Analysis lithofacies and origin of sublacustrine fan. • Investigate the controls of double slope break zones over the distribution of sedimentary systems and sand bodies. • Exhibit the spatial-temporal evolution of fan delta front and sublacustrine fan in the systems tract. • Establish the depositional model from source to sink under the control of the transfer zones and slope systems. [ABSTRACT FROM AUTHOR]

Patterns of sediment yield have been widely investigated across a variety of scales, but few of these studies have been carried out in urban watersheds. The design of this study represented a unique opportunity to use high-frequency sensor data to compare sediment yields across spatial and temporal scales in an urbanized watershed. Near real-time turbidity and discharge data were collected continuously for four years at five stream gages over three nested spatial scales in the highly impervious 14.2-km2 Dead Run watershed, located in Baltimore County, Maryland, USA. Monthly suspended sediment yields varied by over two orders of magnitude across all stations. Two mid-watershed stations on tributaries to the mainstem of Dead Run are located only about 500 m apart and about 500 m upstream of their confluence, yet displayed observable differences in the timing of their highest sediment yields. Average annual suspended sediment yield was higher at the headwater station with older development (78.2 t/km2/yr) than at the headwater station with more recent development (47.0 t/km2/yr). Average annual yields increased with watershed size from the headwater stations to the mid-watershed stations (87.0 t/km2/yr and 60.4 t/km2/yr). Additionally, yields at the station farthest downstream (81.7 t/km2/yr) were slightly larger than the yield calculated from the sum of both mid-watershed stations (74.6 t/km2/yr), which together represented about 80% of the upstream drainage area. Several additional patterns highlighting spatial heterogeneity of the system were observed. The ratio of sediment yields between the headwater stations increased steadily over time while the runoff ratio remained almost constant. Yields at one mid-watershed station were consistently higher than yields at the other mid-watershed station, despite similarity in runoff totals. Upstream watersheds show year-to-year variation in sediment yield. In contrast, yields at the farthest downstream station show minimal variation. This observation is consistent with the possibility that internal storage and remobilization tend to modulate downstream yields even with spatial and temporal variation in upstream sources. Observable inconsistencies in sediment yields between subwatersheds at monthly time scales suggest that there is a substantial degree of heterogeneity in terms of sediment response to large storm events. Despite significant internal variability, the overall range of sediment yield values at each site was consistent with values reported for multiple urban mid-Atlantic Piedmont watersheds. Comparison with global values indicates that sediment yields from Dead Run are in the same general range as the 25% quartile of urban sediment yields. • Urban sediment data collected from 5 gages at 3 nested scales for 4 years • Temporal patterns of sediment yield differ from temporal patterns of runoff. • Dominant sources of sediment vary spatially in a 14 km2 highly impervious catchment. • Downstream site exhibits lower variation in annual sediment yield than tributaries. • Storage has a substantial role in the sediment dynamics of a small, urban watershed. [ABSTRACT FROM AUTHOR]

The MARSSIM landform evolution model has been used successfully over the past three decades to simulate sediment transport on the surface of Mars. The model relies on an advection–diffusion relation that has been used to describe the difference in scale efficiency and profile concavity in landscapes. Beyond this, MARSSIM has also been used to simulate groundwater flow, eolian transport, and impact cratering. We first describe the history of the model in the published literature, followed by an in-depth analysis of the physical theory on which it is based. We review the successes and limitations of the current model and how these have been addressed. Other parts of the model included in the source code have been rarely used, so we also investigate new applications in these areas. Finally, we propose a modification to the model that would allow for an integrated system of precipitation, infiltration, and groundwater flow, and provide a practical example on a simulated cratered terrain similar to those found in past MARSSIM studies. We find that the introduction of an integrated model has noticeable effects on erosion patterns within the simulation domain compared to cases of either complete infiltration or complete runoff. • MARSSIM is a powerful computational tool for understanding early Mars hydrology. • Model parameterizes an advection–diffusion relation to simulate sediment transport. • Modification of current model allows groundwater infiltration from runoff. • Addition of infiltration represents distinct landform evolution scenario for Mars. [ABSTRACT FROM AUTHOR]

Abstract Megaclast-dominated fluvial systems have been largely ignored by researchers. Yet megaclasts exert an important control on channel geomorphology, flow behaviour and sediment transport, and such systems are probably far more common than this neglect would suggest. Despite the explicit identification of megaclasts as the products of cataclysmic flow, observation of modern fluvial systems confirms that megaclasts can be entrained by flows that lie within the normal flow regime. The pattern of fining along the megaclast-dominated Sabeto River of western Viti Levu, Fiji reveals a strong negative exponential relationship between particle size and distance downstream, and a rate of downstream fining comparable with that of other gravel-bed rivers. The downstream fining pattern is straightforward and step-like discontinuities in the fining gradient are absent. This is suggestive of a sedimentary system only minimally disrupted by the supply of sediment from tributaries or the reworking of material from sources such as hillslopes and alluvial terraces. There is evidence, however, of anomalously coarse sub-populations at two sites. These coarser components must pre-date the other deposits at the sites. We speculate that they represent the products of earlier and higher-magnitude events. The bed sediments may thus be composite features, made up of individual components each transported by a specific event of particular magnitude. Our work confirms the relationship between the rate of downstream fining and particle size. This may arise because the distance over which individual particles are transported is likely to be size dependent. It is also possible that abrasion may be more effective in the case of larger particles than smaller ones, with the result that the coarser fraction of the river's bed sediment fines downstream more rapidly than the finer fraction. There is a long-standing debate about the relative roles of abrasion and sorting in downstream fining. Our data reveal little evidence for a downstream increase in the roundness of the river's gravels, implying that abrasion is likely to play a minor role in the fining process in this system. Highlights • Megaclast-dominated rivers occur widely, but have been largely ignored by researchers. • Megaclasts may be entrained by normal flows and not solely by catastrophic events. • Megaclast downstream-fining rates are comparable with those of other gravel-bed rivers. • There is a close relationship between bed-sediment size and rate of downstream fining. • Bed-sediment assemblages consist of discrete components, each transported by individual events. [ABSTRACT FROM AUTHOR]

Abstract The adjustment of a gravel bed during cycles of sediment storage was analyzed for a flume experiment, designed to study the effects of episodic sediment supply regimes. As the bed aggraded and degraded due to changes in sediment feed, it developed a complex topography with bedforms at different scales (e.g., pebble-clusters, bars). Sediment inputs increased bedload rates and promoted large changes in bed morphology. Without an external supply, the bed armored, which increased bed stability, counteracted bed degradation, and decreased bedload rates. Fully-mobile gravel (2–8 mm) was the only fraction that exhibited a deficit over the experiment. Preferential storage of coarse grains caused the development of a sediment wedge upstream and downstream fining on the bed surface. The storage of sand was also significant (60 % of feed) probably because of infiltration (e.g., in the upstream wedge) and hiding effects promoted by bed armor and structures. Size-selective transport caused a cumulative increase in bed slope, which was likely to affect sediment transport capacity. The large variability in bedload rates for same levels of bed storage indicated that sediment availability and bed state were the main controls on sediment transport regime. We provide empirical evidence that cycles of aggradation and degradation, caused by changes in sediment feed upstream, condition bed composition and topography, and the rate and texture of sediment output. Highlights • Aggradation-degradation cycles condition sediment transport rates. • Partial mobility promotes coarse sediment storage upstream and downstream fining. • Sand storage is significant within lag coarse deposits. • Differences in sediment availability cause hysteresis in bedload rates. [ABSTRACT FROM AUTHOR]

Abstract During the transition from the last glacial into the Holocene interglacial, the atmospheric circulation in the North Atlantic area changed from a glacial mode, influenced by the presence of large ice sheets, into a system more similar to that of the present day. This change had implications for wind, temperature and precipitation patterns that can be traced in geological archives. Here, we focus on reconstructing past wind directions over southern Scandinavia, as inferred from the net sediment transport direction of inland dunes in south-central Sweden. We analysed single dunes and dune fields along a 540-km-long S-N transect with LiDAR (Light detection and ranging) derived DEMs (digital elevation models) to determine their geomorphological type and the directional mode of the wind that led to their formation. Luminescence ages of sand drift and dune formation were compiled from previous studies, allowing us to build a chronology of sand drift after the deglaciation of south-central Sweden. The majority of the dunes appear to have been formed by westerly or northwesterly winds and stabilised ∼10.5–9.0 ka. Although this is rather close in time to the local deglaciation for the northern sites, it is long after the disappearance of ice from the more southern sites. The coeval dune stabilisation over such a large geographical area suggests that regional environmental conditions played an important role. It is likely that an unstable climate in conjunction with low groundwater tables during the early Holocene locally delayed vegetation from stabilising the dunes. Highlights • Palaeowind directions are inferred from stabilised inland dunes, Sweden. • W and NW winds are the most abundant palaeowind directions. • Dune stabilisation primarily occurred 10,500 to 9000 years ago. • Prolonged dune activity is thought to have been caused by unstable climate. [ABSTRACT FROM AUTHOR]

Abstract During the second half of the twentieth century, in the central part of the Călimani Mountains, the mining activities for sulfur-rich ore extraction and processing produced significant landscape changes. Sediment transfers across the affected catchments started during the mining period and are still ongoing today, more than two decades after the cessation of the mining activities in 1997. Subsequently, neither rehabilitation works nor erosion control measures were undertaken to stabilize the spoil heap deposits and natural reforestation. Also, material consolidation was impeded by the highly unstable talus slopes. Moreover, little is known about the frequency of the hydrogeomorphic processes responsible for sediment transfers and their consequences. Dendrogeomorphic investigations, grain-size analyses, and topographic surveys were carried out to document the history of hydrogeomorphic activity, the volume of material stored behind the retention dams, and the rate of sediment accumulation. Tree-ring analysis allowed us to reconstruct ten debris flow events in the natural environment, as well as in mining disturbed environment for the period 1931–2017 and 1970–2017, respectively. Triggering of debris flows appears related to spring and summer precipitation originating from short duration convective systems or long-lasting rainfalls. In these remote mining areas, the -reconstructed frequency of debris flows (return intervals of 8.1 and 4.7 years) might be explained by the rainwater accumulation on the spoil heap platform and the availability of sediments, which flow down the talus and then follow the stream channels to reach one of the sediment retention reservoirs (7.4–8.4 cm y−1). Highlights • Debris flows in undisturbed vs. anthropogenically disturbed sites were assessed • Past events with return intervals of 8.1 respectively 4.7 years were reconstructed • Sediment accumulation rates within the reservoirs varied from 7.4 to 8.4 cm yr−1 • Debris flows threaten tailing ponds, infrastructure and human activities [ABSTRACT FROM AUTHOR]

Abstract Nearshore bars are important morphologic features associated with intermediate and dissipative natural beaches. Bars impact the direction, magnitude, and patterns of sediment transport in the nearshore. They serve as a buffer against extreme and meso-scale events. In this review article, we investigate remotely-based observations, specifically near-Earth and satellite imagery, which have been used to investigate nearshore bars. Several recent advances in technology and techniques allow the remote measurement of bar width and height, beach slope, shoreline orientation, and bar count. Video monitoring imagery is presently the most popular method to derive these data. However, spatial prediction models using satellite imagery can also provide reliable bar morphodynamic information. Highlights • Synthesis of remote sensing techniques used to identify bar characteristics. • Remote sensing allows for longer temporal and larger spatial scale studies. • Video monitoring is the most popular method used to study bar characteristics. • Reliable bar morphodynamic data can be obtained from satellite imagery. [ABSTRACT FROM AUTHOR]

Abstract The impact of wind waves and salt marsh erosion on sediment transport dynamics of macro-tidal estuarine systems has been examined using a numerical model. Morphological changes associated with salt marsh erosion facilitate the propagation of waves to the marsh edges, and increase the resuspension and export of sediments from the estuary. Results also highlight the impact of tidally-modulated changes in wave action upon sediment transport dynamics throughout a tidal cycle and from neap to spring tide. In particular, wave action at the landward margin of the estuary is limited by the shallow water depth, and tidally induced increments in water depth only cause a modest increase in wave shear stresses. At the seaward side of the domain, the impact of waves is limited by the deeper water, and tidally induced water level variations have a more significant control on wave shear stresses. The outcomes contribute new insights that are important for the sediment budget of macro-tidal estuaries facing climate change and have implications for the long-term morphological evolution of estuarine wetlands. Highlights • Enhanced sediment export from estuaries caused by wind waves • Enhanced salt marsh erosion caused by wind waves • Salt marsh erosion due to morphological changes of estuaries • Wave impact changes with varying water depth [ABSTRACT FROM AUTHOR]

Regular sediment inputs are required for deltas to maintain their surface elevation relative to sea level, which is important for avoiding salinization, erosion, and flooding. However, fluvial sediment inputs to deltas are being threatened by changes in upstream catchments due to climate and land use change and, particularly, reservoir construction. In this research, the global hydrogeomorphic model WBMsed is used to project and contrast ‘pristine’ (no anthropogenic impacts) and ‘recent’ historical fluvial sediment delivery to the Ganges-Brahmaputra-Meghna, Mahanadi, and Volta deltas. Additionally, 12 potential future scenarios of environmental change comprising combinations of four climate and three socioeconomic pathways, combined with a single construction timeline for future reservoirs, were simulated and analysed. The simulations of the Ganges-Brahmaputra-Meghna delta showed a large decrease in sediment flux over time, regardless of future scenario, from 669 Mt/a in a ‘pristine’ world, through 566 Mt/a in the ‘recent’ past, to 79–92 Mt/a by the end of the 21st century across the scenarios (total average decline of 88%). In contrast, for the Mahanadi delta the simulated sediment delivery increased between the ‘pristine’ and ‘recent’ past from 23 Mt/a to 40 Mt/a (+77%), and then decreased to 7–25 Mt/a by the end of the 21st century. The Volta delta shows a large decrease in sediment delivery historically, from 8 to 0.3 Mt/a (96%) between the ‘pristine’ and ‘recent’ past, however over the 21st century the sediment flux changes little and is predicted to vary between 0.2 and 0.4 Mt/a dependent on scenario. For the Volta delta, catchment management short of removing or re-engineering the Volta dam would have little effect, however without careful management of the upstream catchments these deltas may be unable to maintain their current elevation relative to sea level, suggesting increasing salinization, erosion, flood hazards, and adaptation demands. [ABSTRACT FROM AUTHOR]

Abstract Tsunamis can be responsible for coastal geomorphic crises. Extensively inundated coasts can suffer widespread and dramatic changes in their morphology due to sediment erosion, transport, and deposition processes. In this paper, we investigate the tsunami-induced morphological changes on coasts prone to inundation and how sediment transport is controlled by both tsunami and sediment characteristics. We focus on a tsunami-prone coast in the NE Atlantic, Tangier-Morocco, that experienced the 1755 Lisbon tsunami. We use a coupled hydrodynamic and morphodynamic numerical model, together with high-resolution digital elevation and sedimentary models, to simulate the impact of tsunamis on the morphology and sediment distribution of Tangier Bay. Due to the uncertainty on the source and effects of the 1755 event, we consider two 1755-like tsunamigenic scenarios that have the potential to cause different levels of inundation. Results show that the coastal zone of Tangier can undergo substantial changes in morphology and sediment distribution following the tsunami impact. For the selected scenarios, the volume of sediments mobilized, at both the offshore and onshore zones of the Tangier site, ranges from 30,000 to 200,000 m3. Moreover, the presence of mobile sediments and conditions favoring tsunami shoaling lead to an increase in wave impact on the coastal configuration. Conversely, the presence of the breakwaters helps to protect the Tangier coast by limiting the extent of the affected area. The results of this study suggest that the assessment of tsunami-induced coastal impact should include the effect on morphology. Highlights • Morphological changes are a critical component of tsunami coastal impact assessment. • Tangier coast is prone to inundation and geomorphic changes from tsunamis in the SWIM. • 1755-like earthquake scenarios cause different impacts along the Tangier coast. • Major erosion occurs at the wadi mouths of the Tangier bay. • Tangier breakwaters help reducing the tsunami-induced impact on the coastal morphology. [ABSTRACT FROM AUTHOR]

Computer-based coupled soilscape-landform evolution models provide an avenue for estimating the erosion potential of a landform over geological timescales. They also provide tools to understand soil-landform relationships in the context of both field and modelled pedogenesis processes. In this study, we used the State Space Soil Production and Assessment Model (SSSPAM) and CAESAR-Lisflood landform evolution model to simulate the evolution of a natural catchment where soil erosion rates have been closely monitored over the last 20 years. Both models predict comparable erosion rates to that of field measurements and produce similar catchment scale geomorphological patterns. Rock content data measured in the field along two transects were compared with SSSPAM simulations at the same representative points. The results showed that SSSPAM produced similar rock content variation along the respective transects. The findings demonstrate 'SSSPAM's ability to predict catchment scale erosion and surface soil distribution. The insights demonstrated here provide confidence in the SSSPAM model for Soilscape predictions for natural, agricultural and post-mining landform assessment. [Display omitted] • SSSPAM and CAESAR-Lisflood models were used to simulate the evolution of a natural catchment. • Both models produced catchment erosion rates comparable with field observations. • SSSPAM reproduced surface rock content variation on 2 transects observed in the field. • SSSPAM also revealed a possible relationship between area, slope and soil grading. • This study demonstrates SSSPAM's ability to predict erosion and surface soil distribution. [ABSTRACT FROM AUTHOR]

Submarine canyons, channels and gullies are conduits that transport sediments across shelf-slope margins to deep water. In South Taranaki Basin, an increase in sediment supply through the Miocene resulted in progradation and significant steepening of the slope system. Previous studies have identified numerous sediment conduits developed within this system, however their morphology and morphometric relationships with the depositional slope have not been considered. Here we apply seismic geomorphology to establish the statistical relationships between the metrics of sediment conduits at three stratigraphic intervals between which the slope gradient progressively increased. In the early-Middle Miocene, sinuous upper Moki Formation channel complexes with an average width of 1.1 km developed on a slope with an average gradient of 0.2o, routing sediment from south to north. By the late-Middle Miocene, the slope began to prograde rapidly, concurrent with a regional reorientation of the slope to the northwest, on which the lower Mount Messenger Formation canyon networks developed with a slope gradient of 0.4–1.0o. At shallow slopes of less than 0.5o, canyon morphometrics (mean width 6.6 km) are 1.8–4.7 times larger than on related upper slopes with gradients steeper than 0.5o (mean canyon width of 2.7 km). This significant shift in morphometrics occurs abruptly across the clinoform toe line. Rapid Late Miocene slope progradation resulted in the development of steep clinoform slope surfaces up to 9o, into which linear upper Mount Messenger Formation gully complexes incised. The mean gully width throughout the Middle to Late Miocene interval decreased from 1.3 km to 1.0 km as the slope gradient became steeper. This study documents how the morphology and morphometrics of sediment conduits on the South Taranaki Basin slope system changed through time in relation to changes in depositional slope gradients. • Seismic geomorphology used to quantify the Miocene sediment conduit morphometrics. • Sinuous channel complexes in the upper Moki interval. • Large canyon networks dominate lower the Mount Messenger interval. • Numerous linear gully complexes represent the upper Mount Messenger interval. • Increase sediment supply resulted in basinward progradation slope system. [ABSTRACT FROM AUTHOR]

This study presents an analysis of variable sediment erosion and deposition, and their the complex mechanism in natural heavy silt-carrying rivers in the Ningxia Plain Reaches of the Yellow River (NPRYR). The aims of this study are to examine the sediment scouring and siltation laws, along with the evolution trend of fluvial facies in the NPRYR. Four small-scale typical reaches (R 1 , R 2 , R 3 , and R 4 ) were divided firstly based on river regimes and geological conditions of the river beds. And then with ground-truth data from 1993 to 2015, a cross-section morphology method was used to calculate the average fluvial facies coefficient ζ a of the typical river reaches, and also the sediment siltation weight T during the different time periods. In this study, with application of mathematical statistics and regression analysis, the main channel, beach, and whole reach were used as the basic units in order to identify the relationships of the responses among the suspended sediment concentration per unit discharge ( ξ ), sediment deposition per unit water weight ( W ), and variations of the fluvial facies coefficient (△ ζ a ) in different units. This analysis is for establishing the prediction method for the sediment erosion and deposition, along with the evolution trends of fluvial facies. Our results show a logarithmic mapping relationship between the ξ and W , and significant correlation between T and △ ζ a . Therefore, a quick analysis and prediction of the silt sedimentation, and the trends of fluvial facies variations in the small-scale river reaches under a single water and sediment condition are feasible with our approaches presented here. The critical incoming sediment coefficient ξ cs for the scouring and siltation balance in the R 1 to R 3 reaches were 0.0051 kg·s·m −6 , 0.0055 kg·s·m −6 , and 0.0049 kg·s·m −6 , respectively, while the R 4 reach shows an overall continuous siltation trend. The fluvial facies coefficient of the R 1 to R 4 reaches show an overall decreasing trend. Our results provided robust theoretical support for analyzing the sediment erosion and siltation, and the river regime evolution in the NPRYR. Our study also indicates a need for future research with an aim to further understand the evolution of erosion and siltation in heavy silt-carrying rivers. [ABSTRACT FROM AUTHOR]

Examination of large scale, alluvial estuarine morphology and associated time evolution is of particular importance regarding management of channel navigability, ecosystem, etc. In this work, we analyze morphological evolution and changes of the channel-shoal system in the Changjiang Estuary, a river- and tide-controlled coastal plain estuary, based on bathymetric data between 1958 and 2016. We see that its channel-shoal pattern is featured by meandering and bifurcated channels persisting over decades. In the vertical direction, hypsometry curves show that the sand bars and shoals are continuously accreted while the deep channels are eroded, leading to narrower and deeper estuarine channels. Intensive human activities in terms of reclamation, embankment, and dredging play a profound role in controlling the decadal morphological evolution by stabilizing coastlines and narrowing channels. Even though, the present Changjiang Estuary is still a pretty wide and shallow system with channel width-to-depth ratios >1000, much larger than usual fluvial rivers and small estuaries. In-depth analysis suggests that the Changjiang Estuary as a whole exhibited an overall deposition trend over 59 years, i.e., a net deposition volume of 8.3 × 10 8  m 3 . Spatially, the pan-South Branch was net eroded by 9.7 × 10 8  m 3 whereas the mouth bar zone was net deposited by 18 × 10 8  m 3 , suggesting that the mouth bar zone is a major sediment sink. Over time there is no directional deposition or erosion trend in the interval though riverine sediment supply has decreased by 2/3 since the mid-1980s. We infer that the pan-South Branch is more fluvial-controlled therefore its morphology responds to riverine sediment load reduction fast while the mouth bar zone is more controlled by both river and tides that its morphological response lags to riverine sediment supply changes at a time scale >10 years, which is an issue largely ignored in previous studies. We argue that the time lag effect needs particular consideration in projecting future estuarine morphological changes under a low sediment supply regime and sea-level rise. Overall, the findings in this work can have implications on management of estuarine ecosystem, navigation channel and coastal flooding in general. [ABSTRACT FROM AUTHOR]

Continuity of sediment transfer through glacial systems is essential to maintain subglacial bedrock erosion, yet transfer at temperate glaciers with overdeepened beds, where subglacial fluvial sediment transport should be greatly limited by adverse slopes, remains poorly understood. Complex multiple transfer processes in temperate overdeepened systems has been indicated by the presence of large frontal moraine systems, supraglacial debris of mixed transport origin, thick basal ice sequences, and englacial thrusts and eskers. At Svínafellsjökull, thrusts comprising decimetre-thick debris-rich bands of stratified facies ice of basal origin, with a coarser size distribution and higher clast content than that observed in basal ice layers, contribute substantially to the transfer of subglacial material in the terminal zone. Entrainment and transfer of material occurs by simple shear along the upper surface of bands and by strain-induced deformation of stratified and firnified glacier ice below. Thrust material includes rounded and well-rounded clasts that are also striated, indicating that fluvial bedload is deposited as subglacial channels approach the overdeepening and then entrained along thrusts. Substantial transfer also occurs within basal ice, with facies type and debris content dependent on the hydrological connectedness of the adverse slope. A process model of transfer at glaciers with terminal overdeepenings is proposed, in which the geometry of the overdeepening influences spatial patterns of ice deformation, hydrology, and basal ice formation. We conclude that the significance of thrusting in maintaining sediment transfer continuity has likely been overlooked by glacier sediment budgets and glacial landscape evolution studies. [ABSTRACT FROM AUTHOR]

Channel bars are a major depositional feature in alluvial rivers and their morphodynamics has been investigated intensively in the past several decades. However, relatively less is known about how channel bars in alluvial rivers respond to river engineering and regulations. In this study, we assessed 30-yr morphologic changes of 30 large emerged bars located in a 223 km reach of the highly regulated Lower Mississippi River from Vicksburg, Mississippi, to the Mississippi-Atchafalaya River diversion. Landsat imagery and river stage data between 1985 and 2015 were utilized to characterize bar morphologic features and quantify decadal changes. Based on bar surface areas estimated with the satellite images at different river stages, a rating curve was developed for each of the 30 bars to determine their volumes. Results from this study show that the highly regulated river reach favored the growth of mid-channel and attached bars, while more than half of the point bars showed degradation. Currently, the mid-channel and attached bars accounted for 38% and 34% of the total volume of the 30 bars. The average volume of a single mid-channel bar is over two times that of an attached bar and over four times that of a point bar. Overall, in the past three decades, the total volume of the studied 30 bars increased by 110,118,000 m 3 (41%). Total dike length in a dike field was found mostly contributing to the bar volume increase. Currently, the emerged volume of the 30 bars was estimated approximately 378,183,000 m 3 . The total bar volume is equivalent to ~ 530 million metric tons of coarse sand, based on an average measured bulk density of 1.4 t/m 3 for the bar sediment. The findings show that these bars are large sediment reservoirs. [ABSTRACT FROM AUTHOR]

Reservoirs behind dams act as deposition sites for much of the sediment being transported by rivers. As a result, the downstream river flow can be well below the transport capacity for bed-material. This promotes bed erosion and other geomorphic changes over some length of river located immediately downstream from a dam. These adjustments have been characterized for the Trinity River, TX, downstream of Livingston Dam. Field measurements and results from a 1D numerical model define a 50–60 river kilometer segment of river undergoing bed erosion as the transport capacity for bed material is reestablished. Consequences of this erosion include lowering of the channel bed, reduction in the sediment volume of channel bars, coarsening of sediment on bar tops, steepening of channel banks, and reduction in lateral migration rates of river bends. Repeat surveys of the river long profile reveals that 40 yr of dam closure has produced up to seven meters of channel-bottom incision downstream of the dam, transforming an initially linear profile into a convex-up long profile. The model output matches this observed change, providing confidence that calculated estimates for spatial and temporal changes in bed-material sediment flux can be used to explore the long-term signature of dam influence on the geomorphology of a sand-bed channel. Measurements of channel geometry, profile, lateral migration, and grain size of the lower Trinity River with distance downstream define both the trend and expected variability about the trend associated with the disruption to the bed-material load. [ABSTRACT FROM AUTHOR]

This study presents measurements of blowout topography obtained with annual terrestrial laser surveys carried out over a three-year period at a single, large bowl blowout located in the Provincelands Dunes section of Cape Cod National Seashore, in Massachusetts. The study blowout was selected because its axis is aligned with northwest winds that dominate the region, and because it was seemingly interacting with a smaller saucer blowout that had recently formed on the southern rim of the primary feature. Assuming that blowouts enlarge both horizontally and vertically in response to the wind regime, the objectives of the study were to determine both the amount of horizontal growth that the blowout experiences annually and the spatial patterns of vertical change that occur within the blowout. Changes to the blowout lobe surrounding the feature were also determined for areas with sparse enough vegetation cover to allow laser returns from the sand surface. The results show that the blowout consistently expanded outward during the three years, with the greatest expansion occurring at its southeast corner, opposite the prevailing winds. The most significant occurrence was the removal, in the first year, of the ridge that separated the two blowouts, resulting in a major horizontal shift of the southern rim of the new combined blowout. This displacement then continued at a lesser rate in subsequent years. The rim also shifted horizontally along the northwest to northeast sections of the blowout. Significant vertical loss occurred along the main axis of the blowout with the greatest loss concentrated along the southeast rim. On the lobe, there were large areas of deposition immediately downwind of the high erosion zones inside the blowout. However, there were also small erosion areas on the lobe, extending downwind from eroding sections of the rim. This study shows that: 1. blowouts can experience significant areal and volumetric changes in short periods of time; 2. significant changes may occur relatively suddenly when adjacent blowouts combine into a single feature; and 3. the sediment transport paths are highly controlled by the topography. The joining of two blowouts not only creates a new larger feature, but it also releases large amounts of sediment that are then distributed across the landscape downwind, creating a potential for major changes to a landscape over the longer term. [ABSTRACT FROM AUTHOR]

This study is the first to simultaneously compare flow and sediment transport through a blowout and over an adjacent foredune, and the first study of flow within a highly sinuous, slot and cauldron blowout. Flow across the foredune transect is similar to that observed in other studies and is primarily modulated by across-dune vegetation density differences. Flow within the blowout is highly complex and exhibits pronounced accelerations and jet flow. It is characterised by marked helicoidal coherent vortices in the mid-regions, and topographically vertically forced flow out of the cauldron portion of the blowout. Instantaneous sediment transport within the blowout is significant compared to transport onto and/or over the adjacent foredune stoss slope and ridge, with the blowout providing a conduit for suspended sediment to reach the downwind foredune upper stoss slope and crest. Medium term (4 months) aeolian sedimentation data indicates sand is accumulating in the blowout entrance while erosion is taking place throughout the majority of the slot, and deposition is occurring downwind of the cauldron on the foredune ridge. The adjacent lower stoss slope of the foredune is accreting while the upper stoss slope is slightly erosional. Longer term (16 months) pot trap data shows that the majority of foredune upper stoss slope and crest accretion occurs via suspended sediment delivery from the blowout, whereas the majority of the suspended sediment arriving to the well-vegetated foredune stoss slope is deposited on the mid-stoss slope. The results of this study indicate one mechanism of how marked alongshore foredune morphological variability evolves due to the role of blowouts in topographically accelerating flow, and delivering significant aeolian sediment downwind to relatively discrete sections of the foredune. [ABSTRACT FROM AUTHOR]

Rivers can experience sudden pulses of sediment, from human and natural erosion processes, that can accumulate in the bed. Abundant studies have examined the sources and dynamics of sediment pulses, and problems caused by these pulses, particularly flooding, avulsions, and habitat simplification. Much less has been written about what managers can do about sediment pulses, and that is the purpose of this review. The first option for managers is to do nothing, and this decision can be informed by many case studies and by theory on the propagation and character of sediment pulses (their diffusion, translation, and celerity). Doing nothing should be informed by the secondary effects of sediment pulses on channels including; widening, avulsions, and tributary interactions. If managers decide that something needs to be done about the sediment, they have four options: (1) reducing the sediment supply at source, (2) trapping sediment in the channel (3) accelerating sediment transport through a reach, and, (4) directly extracting sediment. The most common of these actions is undoubtedly to reduce the supply at source, but there are few examples of the consequences of this for sediment pulses. There are even fewer examples of trapping, accelerating and extracting sediment. All of these options have great potential for managing sediment pulses, however, they also have the potential to trigger incision of tributaries and of the channel behind the passing sediment wave. Overall, the literature equips managers to understand the dynamics of sediment pulses, but it does not yet equip them to confidently manage these geomorphic events. [ABSTRACT FROM AUTHOR]

The Colorado River delta is a dramatically transformed landscape. Major changes to river hydrology and morpho-dynamics began following completion of Hoover Dam in 1936. Today, the Colorado River has an intermittent and/or ephemeral channel in much of its former delta. Initial incision of the river channel in the upstream ∼50 km of the delta occurred in the early 1940s in response to spillway releases from Hoover Dam under conditions of drastically reduced sediment supply. A period of relative quiescence followed, until the filling of upstream reservoirs precipitated a resurgence of flows to the delta in the 1980s and 1990s. Flow releases during extreme upper basin snowmelt in the 1980s, flood flows from the Gila River basin in 1993, and a series of ever-decreasing peak flows in the late 1990s and early 2000s further incised the upstream channel and caused considerable channel migration throughout the river corridor. These variable magnitude post-dam floods shaped the modern river geomorphology. In 2014, an experimental pulse-flow release aimed at rejuvenating the riparian ecosystem and understanding hydrologic dynamics flowed more than 100 km through the length of the delta’s river corridor. This small artificial flood caused localized meter-scale scour and fill of the streambed, but did not cause further incision or significant bank erosion because of its small magnitude. Suspended-sand-transport rates were initially relatively high immediately downstream from the Morelos Dam release point, but decreasing discharge from infiltration losses combined with channel widening downstream caused a rapid downstream reduction in suspended-sand-transport rates. A zone of enhanced transport occurred downstream from the southern U.S.-Mexico border where gradient increased, but effectively no geomorphic change occurred beyond a point 65 km downstream from Morelos Dam. Thus, while the pulse flow connected with the modern estuary, deltaic sedimentary processes were not restored, and relatively few new open surfaces were created for establishment of native riparian vegetation. Because water in the Colorado River basin is completely allocated, exceptional floods from the Gila River basin are the most likely mechanism for major changes to delta geomorphology for the foreseeable future. [ABSTRACT FROM AUTHOR]

Check dams are transversal structures built across morphologically-active streams in mountainous regions. These structures have been used widely in torrent-hazard mitigation for over 150 years. Thousands of them are regularly maintained by stream managers and torrent-control services. The stabilization role of these structures is well known, i.e. they durably constrain the stream-bed through the creation of vertical and planar fixed points. What is not yet clear is to what extent check dams influence bed-load transport: How do peak solid discharge or flood-transported volume change when check dams are added to a reach? To address these questions, long-lasting small-scale experiments were conducted in a 4.8-m-long flume with either one, three or no structures. The results show that the addition of structures creates independent compartments in the bed level, which have a strong influence on bed surface armouring and stream morphodynamics: the consequence is that instantaneous transport intensities are unchanged, but peak solid discharge occur more often and for shorter duration. This results in the same total transported volume over the long term, but reduced volume for a single transport event. It reaffirms the observation of pioneering authors of the mid-19th and early 20th century who conceptualized the possible sediment transport regulation function of check dams: in addition to stabilizing the stream-bed, check dams influence bed-load transport through a buffering effect, releasing frequently and in small doses what, in their absence, would be transported abruptly en masse during rare extreme events. [ABSTRACT FROM AUTHOR]

Digital elevation models (DEMs) built from repeated topographic surveys permit producing DEM of Difference (DoD) that enables assessment of elevation variations and estimation of volumetric changes through time. In the framework of sediment transport studies, DEM differencing enables quantitative and spatially-distributed representation of erosion and deposition within the analyzed time window, at both the channel reach and the catchment scale. In this study, two high-resolution Digital Terrain Models (DTMs) derived from airborne LiDAR data (2 m resolution) acquired in 2005 and 2011 were used to characterize the topographic variations caused by sediment erosion, transport and deposition in two adjacent mountain basins (Gadria and Strimm, Vinschgau - Venosta valley, Eastern Alps, Italy). These catchments were chosen for their contrasting morphology and because they feature different types and intensity of sediment transfer processes. A method based on fuzzy logic, which takes into account spatially variable DTMs uncertainty, was used to derive the DoD of the study area. Volumes of erosion and deposition calculated from the DoD were then compared with post-event field surveys to test the consistency of two independent estimates. Results show an overall agreement between the estimates, with differences due to the intrinsic approximations of the two approaches. The consistency of DoD with post-event estimates encourages the integration of these two methods, whose combined application may permit to overcome the intrinsic limitations of the two estimations. The comparison between 2005 and 2011 DTMs allowed to investigate the relationships between topographic changes and geomorphometric parameters expressing the role of topography on sediment erosion and deposition (i.e., slope and contributing area) and describing the morphology influenced by debris flows and fluvial processes (i.e., curvature). Erosion and deposition relations in the slope-area space display substantial differences between the Gadria and the Strimm basins. While in the former erosion and deposition clusters are reasonably well discriminated, in the latter, characterized by a complex stepped structure, we observe substantial overlapping. Erosion mostly occurred in areas that show persistency of concavity or transformation from convex and flat to concave surfaces, whereas deposition prevailingly took place on convex morphologies. Less expected correspondences between curvature and topographic changes can be explained by the variable sediment transport processes, which are often characterized by alternation of erosion and deposition between different events and even during the same event. [ABSTRACT FROM AUTHOR]

Despite widespread recognition that landforms are complex Earth systems with process-response linkages that span temporal scales from seconds to millennia and spatial scales from sand grains to landscapes, research that integrates knowledge across these scales is fairly uncommon. As a result, understanding of geomorphic systems is often scale-constrained due to a host of methodological, logistical, and theoretical factors that limit the scope of how Earth scientists study landforms and broader landscapes. This paper reviews recent advances in understanding of the geomorphology of beach-dune systems derived from over a decade of collaborative research from Prince Edward Island (PEI), Canada. A comprehensive summary of key findings is provided from short-term experiments embedded within a decade-long monitoring program and a multi-decadal reconstruction of coastal landscape change. Specific attention is paid to the challenges of scale integration and the contextual limitations research at specific spatial and/or temporal scales imposes. A conceptual framework is presented that integrates across key scales of investigation in geomorphology and is grounded in classic ideas in Earth surface sciences on the effectiveness of formative events at different scales. The paper uses this framework to organize the review of this body of research in a ‘scale aware’ way and, thereby, identifies many new advances in knowledge on the form and function of subaerial beach-dune systems. Finally, the paper offers a synopsis of how greater understanding of the complexities at different scales can be used to inform the development of predictive models, especially those at a temporal scale of decades to centuries, which are most relevant to coastal management issues. Models at this (landform) scale require an understanding of controls that exist at both ‘landscape’ and ‘plot’ scales. Landscape scale controls such as sea level change, regional climate, and the underlying geologic framework essentially provide bounding conditions for independent variables such as winds, waves, water levels, and littoral sediment supply. Similarly, a holistic understanding of the range of processes, feedbacks, and linkages at the finer plot scale is required to inform and verify the assumptions that underly the physical modelling of beach-dune interaction at the landform scale. [ABSTRACT FROM AUTHOR]

High-resolution topographic surveys using LiDAR and multibeam sonar can be used to characterize and quantify fluvial change. This study used repeat surveys to explore how topographic change, fluvial processes, sediment budgets, and aggradation and incision rates vary across spatial scales and across two contrasting decadal flow regimes in a regulated gravel/cobble river. A novel method for quantifying digital elevation model uncertainty was developed and applied to a topographic change detection analysis from 2006/2008 to 2014. During this period, which had four modest ~ 3–5 year floods, most sediment was laterally redistributed through bank erosion and channel migration. Erosion primarily occurred in the floodplain (97,000 m 3 ), terraces (80,000 m 3 ), and lateral bars (58,000 m 3 ); while deposition occurred in the adjacent pools (73,000 m 3 ), fast glides (48,000 m 3 ), and runs (36,000 m 3 ). In contrast, significantly higher magnitude and longer duration floods from 1999 to 2006/2008 caused sediment to be displaced longitudinally, with the upstream reaches exporting sediment and the downstream reaches aggrading. The river maintained floodplain connectivity during both periods, despite different processes dominating the type of connectivity. Larger floods promoted overbank scour and avulsion, while smaller floods promoted bank erosion and lateral migration. This study explores and illustrates how the geomorphic response to contrasting flood regimes in a nonuniform river is highly dependent on which landforms are controlling hydraulics. [ABSTRACT FROM AUTHOR]

Measurements of wave transformation, beachface cobble transport and shoreline morphology change were undertaken over a 14 month period at Fatato Island, a cobble and sand ‘motu’ on the southeast rim of Funafuti Atoll, Tuvalu. Results document significant cobble transport and beach profile change on Fatato during a year in the absence of any significant high energy wave events. Changes in beachface volume were measured on 11 across-shore beach profiles, and show marked variations of + 7.1 and − 7.01 m 3 between profiles and between survey periods. Net volumetric beachface change over the 14 month experiment was approximately zero, indicating a balance between erosion and deposition and net conservation of sediment volume on the ocean shoreline. Cobble movement was measured using Passive Integrated Transponder (PIT) tags inserted in 300 randomly selected clasts (mean b-axis = 75 mm) on the mid-beachface. Tagged particles were re-surveyed after 6 and 14 months, achieving 76% recovery, and identify large excursions of clasts on the beachface and dominant southwest alongshore transport, with an average net movement of 68 m after 14 months. A simple agent based probabilistic model was developed to simulate cobble movement and explore the coupling between hydrodynamic and shoreline processes. Model simulations reveal: predominant south-westerly cobble transport is driven by mean wave conditions associated with easterly trade winds, and; long-period swell events that approach the reef from the south drive episodic north-easterly transport reversals and significant onshore transport that can strand particles on the upper beach. Results present the first observations of morphodynamic behaviour of an atoll motu and highlight the highly dynamic nature of motu shorelines under mean energy conditions. Such results challenge conventional perspectives that motu are storm adjusted landforms that require extreme events to stimulate geomorphic change and inform future projections of island change. [ABSTRACT FROM AUTHOR]

Suspended sediment transport is an important contributor to ecologic and geomorphic functions of streams. However, it is challenging to generalize predictions of sediment yield because it is influenced by many factors. In this study, we quantified the relevance of natural controls (e.g., geology, catchment physiography) on suspended sediment yield (SSY) in headwater streams managed for timber harvest. We collected and analyzed six years of data from 10 sites (five headwater sub-catchments and five watershed outlets) in the Trask River Watershed (western Oregon, United States). We used generalized least squares regression models to investigate how the parameters of the SSY rating curve varied as a function of catchment setting, and whether the setting modulated the SSY response to forest harvesting. Results indicated that the highest intercepts ( α ) of the power relation between unit discharge and SSY were associated with sites underlain primarily by friable rocks (e.g., sedimentary formations). The greatest increases in SSY after forest harvesting (up to an order of magnitude) also occurred at sites underlain by the more friable lithologies. In contrast, basins underlain by resistant lithologies (intrusive rocks) had lower SSY and were more resilient to management-related increases in SSY. As such, the impact of forest management activities (e.g., use of forested buffers; building of new roads) on the variability in SSY was primarily contingent on catchment lithology. Sites with higher SSY, or harvest-related increases in SSY, also generally had a) lower mean elevation and slope, b) greater landscape roughness, and c) lower sediment connectivity. We used principal component analysis (PCA) to further explore the relationship between SSY and several basin physiographic variables. The PCA clearly separated sites underlain by friable geologic units from those underlain by resistant lithologies. Results are consistent with greater rates of weathering and supply of sediment to headwater streams in catchments with more friable lithologies, and limited sediment supply in catchments underlain by resistant lithologies. We hypothesize that a similar framework may aid in predicting the overall SSY of a catchment as well as its susceptibility to increases in SSY following forest harvesting. [ABSTRACT FROM AUTHOR]

This paper presents the results of morphodynamic modelling and analysis of onshore and offshore sandbar migration based on a depth-integrated approach. The coastal flow was modeled using the Boussinesq equation and the morphological evolution was modeled using the suspended sediment transport equation and bed load formulae based on the instantaneous velocity and acceleration. The proposed model was applied to the accretive and erosive conditions and the model reproduced the onshore and offshore sandbar migration and the formation of a berm around the shoreline reasonably. An analysis of the computed results revealed the following. (i) The vertical flow velocity can affect the suspension time of the sediments considerably and the bottom evolution. (ii) The suspended load is the main contributor to the morphological changes in terms of the quantity and quality, regardless of the accretive or erosive conditions. (iii) Regardless of accretive or erosive conditions, in terms of the time-average, the instantaneous flow velocity and acceleration-based bed load models always yielded an offshore and onshore direction sediment flux, respectively, except in the swash zone. On the other hand, the suspended sediment flux calculated by the advection-diffusion equation results in the sediment transport in either direction depending on the flow field. [ABSTRACT FROM AUTHOR]

Morphodynamic equilibrium is a widely adopted yet elusive concept in the field of geomorphology of coasts, rivers and estuaries. Based on the Exner equation, an expression of mass conservation of sediment, we distinguish three types of equilibrium defined as static and dynamic, of which two different types exist. Other expressions such as statistical and quasi-equilibrium which do not strictly satisfy the Exner conditions are also acknowledged for their practical use. The choice of a temporal scale is imperative to analyse the type of equilibrium. We discuss the difference between morphodynamic equilibrium in the “real world” (nature) and the “virtual world” (model). Modelling studies rely on simplifications of the real world and lead to understanding of process interactions. A variety of factors affect the use of virtual-world predictions in the real world (e.g., variability in environmental drivers and variability in the setting) so that the concept of morphodynamic equilibrium should be mathematically unequivocal in the virtual world and interpreted over the appropriate spatial and temporal scale in the real world. We draw examples from estuarine settings which are subject to various governing factors which broadly include hydrodynamics, sedimentology and landscape setting. Following the traditional “tide-wave-river” ternary diagram, we summarize studies to date that explore the “virtual world”, discuss the type of equilibrium reached and how it relates to the real world. [ABSTRACT FROM AUTHOR]

Changes in land use can affect local geomorphology and sediment dynamics. However, these impacts could conceivably lead to changes in geomorphological processes beyond the area of land use change, thereby evidencing a geomorphic connectivity in the landscape. We conduct a numerical modelling experiment, using the CAESAR landscape evolution model, to investigate the extent and nature of such connectivity in the River Swale basin. Six simulations are run and analysed. Two of these are reference simulations, where the basin has a hypothetical total grassland cover or total forest cover. In the other four simulations, half of the basin is subjected to either deforestation or reforestation during the simulation. Simulations are analysed for temporal trends in sediment yield and for spatial trends in erosion and deposition across the basin. Results show that deforestation or reforestation in one half of the basin can indeed affect the geomorphology of the other half, thus implying a geomorphological connectivity across the basin. This connectivity is locally very high, with significant morphological impacts close to where de- or re-forestation occurs. Changes are observed both downstream and upstream of the areas where the land use changes occurred. The impacts are more pronounced in the downstream direction and are still apparent in the basin scale sediment yields, as deforestation of half the basin can increase decadal sediment yields by over 100%, whilst reforestation of half the basin can lead to 40% decreases. However, our results also indicate a reverse connectivity whereby erosion and deposition in upstream headwaters and tributaries can, for the first time, be conclusively attributed to land use changes several kilometres downstream, due to alterations in the valley floor base level resulting from incision and alluviation. [ABSTRACT FROM AUTHOR]

Coastlines where waves consistently approach at highly oblique angles experience anti-diffusional behavior, causing perturbations to grow seaward and form sand waves, capes, and spits. Coasts where waves approach at low offshore angles experience the opposite: perturbations diffuse and the coastline remains (or becomes) smooth. In this paper, by coupling a 2-D large-scale coastline evolution model to a spectral wave model, we show that anti-diffusional behavior is also possible in low-angle wave climates if the nearshore wave field is altered by complex bathymetry. In model simulations, low-angle waves refract over local shoals, creating a convergence in alongshore sediment flux behind the shoals that coalesces into small ‘minor capes’. Depending on wave height and period, shoreline features take 80–400 years to reach an equilibrium cross-shore relief of 1–1.5 km over an alongshore distance of ~ 20 km. The modeled equilibrium time scale is consistent with analytically-determined characteristic shoreline diffusion time scales, and the modeled cross-shore relief and aspect ratio are similar to observed ‘minor capes’ along the U.S. Atlantic Coast where offshore bathymetric anomalies have previously been linked to shoreline change patterns. Better understanding of the links among nearshore bathymetry, wave transformation, and alongshore sediment transport is critical to understanding shoreline change patterns at the local level and how they fit into broader, regional-scale behavior. [ABSTRACT FROM AUTHOR]

Understanding the sediment transport in oscillatory flows is essential to the investigation of the overall sediment budget for coastal regions. This overall budget is crucial for the prediction of the morphological change of the coastline in engineering applications. Since the sediment transport in oscillatory flows is dense particle-laden flow, appropriate modeling the particle interaction is critical. Although traditional two-fluid approaches have been applied to the study of sediment transport in oscillatory flows, the approaches do not capture the interaction of the particles. The study of the motion of individual sediment particles and their micromechanics (e.g., packing and contact force) in oscillatory flows is still lacking. In this work, a parallel CFD–DEM solver SediFoam that can model the inter-particle collision is applied to study the granular micromechanics of sediment particles in oscillatory flows. The results obtained from the CFD–DEM solver are validated by using the experimental data of coarse and medium sands. The comparison with experimental results suggests that the flow velocity, the sediment flux and the net sediment transport rate predicted by SediFoam are satisfactory. Moreover, the micromechanic quantities of the sediment bed are presented in detail, including the Voronoi concentration, the coordination number, and the particle interaction force. It is demonstrated that the variation of these micromechanic quantities at different phases in the oscillatory cycle is significant, which is due to different responses of the sediment bed. To investigate the structural properties of the sediment bed, the correlation of the Voronoi volume fraction and coordination number is compared to the results from the fluidized bed simulations. The consistency in the comparison indicates the structural micromechanics of sediment transport and fluidized bed are similar despite the differences in flow patterns. From the prediction of the CFD–DEM model, we observed that the coordination number in rapid sheet flow layer is larger than one, which indicates that a typical particle in the sediment layer is in contact with more than one particles, and thus the binary collision model commonly used in two-fluid approaches may underestimate the contact between the particles. [ABSTRACT FROM AUTHOR]

This study describes channel changes following completion of the Provo River Restoration Project (PRRP), the largest stream restoration project in Utah and one of the largest projects in the United States in which a gravel-bed river was fully reconstructed. We summarize project objectives and the design process, and we analyze monitoring data collected during the first 7 years after project completion. Post-project channel adjustment during the study period included two phases: (i) an initial phase of rapid, but small-scale, adjustment during the first years after stream flow was introduced to the newly constructed channel and (ii) a subsequent period of more gradual topographic adjustment and channel migration. Analysis of aerial imagery and ground-survey data demonstrate that the channel has been more dynamic in the downstream 4 km where a local source contributes a significant annual supply of bed material. Here, the channel migrates and exhibits channel adjustments that are more consistent with project objectives. The upstream 12 km of the PRRP are sediment starved, the channel has been laterally stable, and this condition may not be consistent with large-scale project objectives. [ABSTRACT FROM AUTHOR]

Hybrid 2D/one-line shoreline models are becoming increasingly applied over the mesoscale (101–102 years; 101–102 km) to inform coastal management. These models typically apply the one-line theory to simulate changes in shoreline morphology based on littoral drift gradients calculated from a 2DH coupled wave, flow, and sediment transport model. However, the key boundary conditions needed to effectively apply hybrid 2D/one-line models and their applicability beyond simple planform morphologies are uncertain, which can potentially comprise coastal management decisions. To address these uncertainties, an extensive numerical modelling campaign is carried out to both assess the sensitivity and calibrate an advanced hybrid 2D/one-line model (MIKE21) against six variables in three different sandy coastal system morphologies: (a) a simple planform morphology with a gentle sloping profile, (b) a simple planform morphology with a steep sloping profile, and (c) a complex planform morphology. The six variables considered include nearshore discretisation, bathymetry, bed friction, sand grain diameter, sand porosity, sediment grading, and the weir coefficient of hard defence structures. Five key conclusions are derived from the sensitivity testing and calibration results. First , the optimal boundary conditions for modelling shoreline evolution vary according to coastal geomorphology and processes. Second , specifying boundary conditions within physically realistic ranges does not guarantee reliable shoreline evolution predictions. Third , nearshore discretisation should be treated as a typical calibration parameter as (a) the finest discretisation does not guarantee the most accurate predictions, and (b) defining a discretisation based on process length scales also does not guarantee reliable predictions. Fourth , hybrid 2D/one-line models are not valid for application in complex planform morphologies plausibly because of the one-line theory assumption of a spatially invariable closure depth. Fifth , hybrid 2D/one-line models have limited applicability in simple planform morphologies where the active beach profile is subject to direct human modification, plausibly due to the one-line theory assumption of a constant time-averaged coastal profile form. These findings provide key theoretical insights into the drivers of shoreline evolution in sandy coastal systems, which have practical implications for refining the continued application of shoreline evolution models. • The hybrid 2D/one-line shoreline modelling approach is tested in three coastal morphologies. • Optimal boundary conditions correspond to observed coastal morphology. • The nearshore discretisation in shoreline models should be treated as a calibration parameter. • The hybrid 2D/one-line shoreline modelling approach is limited to simple planform morphologies. [ABSTRACT FROM AUTHOR]

Although distally steepened carbonate ramps have been studied by numerous researchers, the processes that control the development of these carbonate systems, including tectonics, differential carbonate production along the ramp profile, or antecedent physiography of the slopes, are an ongoing discussion. We use a stratigraphic forward model to test different hypotheses to unravel controls over distally steepened ramp development, referenced to the well-known Upper Miocene Menorca carbonate ramp (Spain). Sensitivity tests show that distally steepened ramps develop under complex interaction among accommodation, carbonate production and sediment transport parameters. Ramp slope initiation is favoured by still stands and falls of sea-level, in a setting with high-frequency sea-level fluctuations with amplitude between 20 m and 40 m. Low-frequency and higher amplitude sea-level fluctuations of about 115 m tend to form models with no significant slope development. The impact of antecedent slope on the geometry of ramps is determined by the paleoslope inclination, with flat to subhorizontal paleosurfaces resulting in ramps that mirror the antecedent slope. In contrast, steeper paleosurfaces tend to result in ramps with well-defined slopes. Our models, therefore, show that the ramp profile becomes more influenced by the depth constraints on the carbonate sediment producers than by the geometry of the underlying topography as the inclination of the paleosurface increases. The presented models also show that seagrass-dominated shallow carbonate production tends to result in steep slopes due to the low-transport characteristic imposed by seagrass trapping. This steepness can, however, be altered by the introduction of high transport sediment grains from deeper carbonate producers, which fill the slopes and more distal sections of the ramp profile. [ABSTRACT FROM AUTHOR]

Wave climate and Pacific basin coastal behaviour associated with El Niño Southern Oscillation (ENSO) is understood at a reconnaissance level, but the coastal response to different central Pacific (CP) versus eastern Pacific (EP) flavours of ENSO is unknown. We show that CP ENSO events produce different patterns of directional wave power to EP ENSO along the southeast Australian shelf and southwest Pacific region, because of significant variability in trade-wind wave generation. The modulation of the trade wind wave climate during CP ENSO has thus far been neglected in existing coastal process studies. We also show that coastal change between CP and EP ENSO cannot be inferred from shifts in the deepwater wave climate. This is because variability in trade wind wave generation is masked in deepwater by the persistence of high power extra-tropical waves that have reduced impact on nearshore processes due to high wave refraction. Morphodynamic modelling in a headland-bay beach indicates that CP ENSO leads to higher coastal erosion potential and slower post-storm recovery than EP ENSO during an El Niño/La Niña cycle. We show that the alongshore variability in beach morphological type can be used to model the static equilibrium planform response for each ENSO phase. Results indicate that shoreline response to ENSO in most headland-bay beach coasts is not as simple as the existing paradigm that (anti-) clockwise rotation occurs during El Niño (La Niña). Our methods provide a second-order approach to project coastal response and predict the discrete shoreline rotations for ENSO flavours. [ABSTRACT FROM AUTHOR]

A laboratory experiment was carried out to study sediment transport dynamics occurring in the swash zone of a coarse-sandy beach built in a large-scale wave flume. Hydro- and morpho-dynamic as well as sediment transport data were collected using sensors mounted on a scaffold rig deployed in the lower swash zone close to the moving bed. The high resolution of near-bed data permitted quantitative evaluation of suspended and sheet flow contributions to the total sediment transport. Although sheet flow sediment fluxes were higher than suspended fluxes, the vertically integrated suspended sediment load overcame the sheet flow load during uprush and it was on the same order of magnitude during backwash. The observed cumulative sediment transport was generally larger than the morphological changes occurring shoreward of the rig location implying either an underestimation of the offshore sediment transport or an overestimation of the onshore fluxes obtained from concentration and velocity profile data. Low correlations were found between net swash profile changes and runup parameters suggesting that local hydrodynamic parameters provide little or no predictability of accretion and erosion of an upper beach which is near equilibrium. The balance between erosion and deposition induced by individual swash events brought a dynamic equilibrium with small differences between the profiles measured at the start and at the end of the run. [ABSTRACT FROM AUTHOR]

This study advances mechanistic interpretation of predictability challenges in hydro-geomorphology related to the role of soil moisture spatial variability. Using model formulations describing the physics of overland flow, variably saturated subsurface flow, and erosion and sediment transport, this study explores (1) why a basin with the same mean soil moisture can exhibit distinctly different spatial moisture distributions, (2) whether these varying distributions lead to non-unique hydro-geomorphic responses, and (3) what controls non-uniqueness in relation to the response type. Two sets of numerical experiments are carried out with two physically-based models, HYDRUS and tRIBS+VEGGIE+FEaST, and their outputs are analyzed with respect to pre-storm moisture state. The results demonstrate that distinct spatial moisture distributions for the same mean wetness arise because near-surface soil moisture dynamics exhibit different degrees of coupling with deeper-soil moisture and the process of subsurface drainage. The consequences of such variations are different depending on the type of hydrological response. Specifically, if the predominant runoff response is of infiltration excess type, the degree of non-uniqueness is related to the spatial distribution of near-surface moisture. If runoff is governed by subsurface stormflow, the extent of deep moisture contributing area and its “readiness to drain” determine the response characteristics. Because the processes of erosion and sediment transport superimpose additional controls over factors governing runoff generation and overland flow, non-uniqueness of the geomorphic response can be highly dampened or enhanced. The explanation is sediment composed by multi-size particles can alternate states of mobilization or surface shielding and the transient behavior is inherently intertwined with the availability of mobile particles. We conclude that complex nonlinear dynamics of hydro-geomorphic processes are inherent expressions of physical interactions. As complete knowledge of watershed properties, states, or forcings will always present the ultimate, if ever resolvable, challenge, deterministic predictability will remain handicapped. Coupling of uncertainty quantification methods and space-time physics-based approaches will need to evolve to facilitate mechanistic interpretations and informed practical applications. [ABSTRACT FROM AUTHOR]

Numerical landscape evolution models were initially developed to examine natural catchment hydrology and geomorphology and have become a common tool to examine geomorphic behaviour over a range of time and space scales. These models all use a digital elevation model (DEM) as a representation of the landscape surface and a significant issue is the quality and resolution of this surface. Here we focus on how subtle perturbations or roughness on the DEM surface can produce alternative model results. This study is carried out by randomly varying the elevations of the DEM surface and examining the effect on sediment transport rates and geomorphology for a proposed rehabilitation design for a post-mining landscape using multiple landscape realisations with increasing magnitudes of random changes. We show that an increasing magnitude of random surface variability does not appear to have any significant effect on sediment transport over millennial time scales. However, the random surface variability greatly changes the temporal pattern or delivery of sediment output. A significant finding is that all simulations at the end of the 10,000 year modelled period are geomorphologically similar and present a geomorphological equifinality. However, the individual patterns of erosion and deposition were different for repeat simulations with a different sequence of random perturbations. The alternative positions of random perturbations strongly influence local patterns of hillslope erosion and evolution together with the pattern and behaviour of deposition. The findings demonstrate the complex feedbacks that occur even within a simple modelled system. [ABSTRACT FROM AUTHOR]

The Meaípe-Maimbá embayed beach (MMEB) on the south-east coast of Brazil has been subject to anthropogenic pressures since the 70's. In this study we discuss the adequacy and contribution of the parabolic planform model to determine the planform and variability of the MMEB, taking into consideration variation in wave conditions. The role of different controlling conditions on the planform variability is analyzed, as well as the morphological and planform mobility. MMEB exhibited a new configuration in response to the construction of a harbor, which interrupted the longshore sediment transport. After four decades, three particular morphodynamic sectors have been recognized along the beach. The central sector is more exposed to normal wave incidence and cross-shore processes predominate. The northern and southern sectors are influenced by wave diffraction processes around the headlands and port, respectively. In the northern sector, the presence of secondary headlands and inner islands imposed a geomorphological control on beach morphology and coastal processes. The use of the parabolic planform model provided useful insights for the assessment of potential planform mobility, since the decadal shoreline evolution combined with beach profiles and sediment characteristics allowed understanding of the beach mobility processes and supported the interpretation of modeling results. [ABSTRACT FROM AUTHOR]

The trend and episodes of erosion at Great Kills Park, part of the National Park Service, are products of the origin of the park's location and the impact of a negative sediment budget. Management response to the impacts of erosion is somewhat limited by National Park Service philosophy, but some options remain because the negative sediment budget is a product of barriers to sediment transport updrift of the Park. The erosional situation is exacerbated by an exposure of an antecedent backmarsh feature within the Park that is affecting the inshore pattern of incident waves. Recent monitoring data indicate planform conformity to an updrift log-spiral erosional bluff that extends downdrift to a site with beach and dune features. Despite a general net negative sediment budget in the profile, the dune feature is increasing in volume as it shifts inland, fitting models of foredune development. Seasonal monitoring of the topography records that storms cause a stepwise inland displacement of an updrift portion of the Park and a more linear displacement in the downdrift portion. Among the options consistent with management policy responding to the negative sediment budget issue is an opportunity to work within the scale of the small sediment transport cell and backpass sediment toward the updrift margin of the cell. [ABSTRACT FROM AUTHOR]

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974. This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts. [ABSTRACT FROM AUTHOR]

Using a dam removal on the Ashuelot River in southern New Hampshire, we test how a sudden, spatially non-uniform increase in river slope alters sediment transport dynamics and riparian sediment connectivity. Site conditions were characterized by detailed pre- and post-removal field surveys and high-resolution aerial lidar data, and locations of erosion and deposition were predicted through one-dimensional hydrodynamic modeling. The Homestead Dam was a ~ 200 year old, 4 m high, 50 m wide crib dam that created a 9.5 km long, relatively narrow reservoir. Following removal, an exhumed resistant bed feature of glaciofluvial boulders located 400 m upstream and ~ 2.5 m lower than the crest of the dam imposed a new boundary condition in the drained reservoir, acting as a grade control that maintained a backwater effect upstream. During the 15 months following removal, non-uniform erosion in the former reservoir totaled ~ 60,000 m 3 (equivalent to ~ 9.3 cm when averaged across the reservoir). Net deposition of ~ 10,700 m 3 was measured downstream of the dam, indicating most sediment from the reservoir was carried more than 8 km downstream beyond the study area. The most pronounced bed erosion occurred where modeled sediment transport increased in the downstream direction, and deposition occurred both within and downstream of the former reservoir where modeled sediment transport decreased in the downstream direction. We thus demonstrate that spatial gradients in sediment transport can be used to predict locations of erosion and deposition on the stream bed. We further observed that bed incision was not a necessary condition for bank erosion in the former reservoir. In this characteristically narrow and shallow reservoir lacking abundant dam-induced sedimentation, the variable resistance of the bed and banks acted as geomorphic constraints. Overall, the response deviated from the common conceptual model of knickpoint erosion and channel widening due to dam removal. With thousands of dams likely to be considered for removal or repair in the coming decades, this study helps to advance predictions of the geomorphic response to dam removal and contributes to a broader understanding of the variability in both style and timing of fluvial responses to disturbances. [ABSTRACT FROM AUTHOR]