Reduced Sediment Supply Research Articles

Rapid Loss of Tidal Flats in the Yangtze River Delta since 1974.

As the home to national nature reserves and a Ramsar wetland, the tidal flats of the Yangtze River Delta are of great significance for ecological security, at both the local and global scales. However, a comprehensive understanding of the spatiotemporal conditions of the tidal flats in the Yangtze River Delta remains lacking. Here, we propose using remote sensing to obtain a detailed spatiotemporal profile of the tidal flats, using all available Landsat images from 1974 to 2018 with the help of the Google Earth Engine cloud platform. In addition, reclamation data were manually extracted from time series Landsat images for the same period. We found that approximately 40.0% (34.9–43.1%) of the tidal flats in the study area have been lost since 1980, the year in which the tidal flat area was maximal. The change in the tidal flat areas was consistent with the change in the riverine sediment supply. We also found that the cumulative reclamation areas totaled 816.6 km2 and 431.9 km2 in the Yangtze estuary zone and along the Jiangsu coast, respectively, between 1974 and 2018. Because of reclamation, some areas (e.g., the Hengsha eastern shoal and Pudong bank), which used to be quite rich, have lost most of their tidal flats. Currently, almost 70% of the remaining tidal flats are located in the shrinking branch (North Branch) and the two National Nature Reserves (Chongming Dongtan and Jiuduansha) in the Yangtze estuary zone. Consequently, the large-scale loss of tidal flats observed was primarily associated with reduced sediment supply and land reclamation at the time scale of the study. Because increasing demand for land and rising sea levels are expected in the future, immediate steps should be taken to prevent the further deterioration of this valuable ecosystem.

Influence of Floods, Tides, and Vegetation on Sediment Retention in Wax Lake Delta, Louisiana, USA

AbstractSediment is the most valuable natural resource for deltaic environments because it is required to build new land. For land building to occur, sediment must be retained in the delta instead of being transported offshore. Despite this, we do not know what controls sediment retention within a delta. Here we use a calibrated numerical model of Wax Lake Delta, Louisiana, USA to analyze sediment retention for different riverine flood magnitudes, tidal amplitudes, and vegetation extents. Our results show that as riverine flood magnitude increases, areally averaged vertical accretion increases from 0.33 to 2 cm per 60‐day flood, but sediment retention decreases from 72% to 34%. For the uniform vegetation characteristics considered, the buffering effect, defined as the reduction of sediment flux onto the islands in the presence of vegetation, reduces the sediment flux onto the islands 14 to 22% on a fully vegetated delta. When sediment is transported onto the islands, vegetation enhances retention, which we refer to as the trapping effect, by ~10%. But, this does not offset the buffering effect, and vegetation decreases vertical accretion and retention in the delta up to 6% (or ~0.5 cm per 60‐day flood). We suggest that vegetation will increase sedimentation only when trapping compensates for buffering. Finally, greater tidal amplitude at higher discharges enhances vertical accretion by ~0.5 cm per 60‐day flood compared to smaller tidal amplitudes. These results provide insight on the mechanisms behind coastal systems growth, and suggest how sediment diversions might be operated more efficiently in deltas with reduced sediment supply.

Changes to long-term discharge and sediment loads in the Vietnamese Mekong Delta caused by upstream dams

The discharge and sediment loads of the Mekong River (MR) have been significantly altered because of natural processes and anthropogenic activities. We examined the impacts of dams in the MR basin on changes to the long-term discharge and sediment loads from upstream of dams in China (e.g., Jiuzhou) to the Vietnamese Mekong Delta (VMD) at daily, monthly, and annual scales over a 55-yr period (1961–2015) using various statistical methods. Results revealed sediment trapping in the Lancang cascade dams. To clarify the consequences of the reduced MR sediment load on morphological changes in the VMD, bathymetric data measured in 2014 and 2017 were examined along 100 km of the Tien river. The results show significantly reduced suspended sediment loads at all stations in the lower MR, but the sediment loads at Jiuzhou was increased. Thus, the reduced sediment load is caused not by a reduced sediment supply from the upper MR basin but by sediment trapping in the Lancang cascade dams. The sediment was reduced by 74.1% in the VMD; 166.7 ± 33.3 Mt/yr occurred in the predam period and 43.1 Mt/yr occurred in 2012–2015, with 40.2% caused by six mainstream dams in the Lancang cascade. Therefore, the Tien river in the VMD was severely incised, with an incision rate of −0.5 m/yr in 2014–2017. Upstream development has caused large-scale morphological changes in the VMD. Sand mining was responsible for a maximum of 14.8% of the annual riverbed incision in the VMD, while the remainder was caused by upstream hydropower dams.

The use of small-Unmanned Aerial Systems for high resolution analysis for intertidal wetland restoration schemes

Coastal and estuarine wetlands provide a range of important ecosystem services, but are currently being damaged and degraded due to human activities, reduced sediment supply and sea level rise. Managed realignment (MR) is one approach used to compensate for the loss of intertidal habitat, however saltmarshes in MR sites have been recognised to have lower biodiversity than natural environments. This has been associated with differences in the physical functioning including the sediment structure, reduced hydraulic connectivity, and lower topographic variability such as the abundance of intertidal creek networks. Intertidal morphology, including creek networks, play an important role in supporting and regulating saltmarsh environments through the supply of sediment, nutrients and water, and in draining intertidal marshes. However, there is a lack of empirical data on the formation and evolution of topographic features and variability in saltmarsh environments. This is likely to be due to creek networks in natural marshes already being in a state of quasi-equilibrium, making MR sites an ideal environment to investigate creek development. However, traditional remote sensing techniques (such as LiDAR) tend to be relatively expensive, infrequent and at a coarse resolution meaning small, but important (cm-scale), changes are often missed. This study advances the ability to detect these small scale changes by demonstrating the suitability and potential applications of using the emerging photogrammetric method Structure-from-Motion (SfM) on images taken using a small-Unmanned Aerial System (sUAS). Three surveys from a rapidly changing, near-breach site were taken at the Medmerry Managed Realignment Site in July 2016, September 2017 and July 2018. A suitable degree of confidence was found between the modelled surface and independent check points (vertical root-mean-square-errors of 0.0245, 0.0704 and 0.1571 for 2016, 2017 and 2018 respectively). DSMs of Difference (DoD) analysis was performed to evaluate elevation change, with areas experiencing up to 85 cm of accretion between 2016 and 2018. However, when considering the error associated with both surveys, between 2016 and 2017, only 34.39% of the survey area experienced change above the level of detection (LoD). In contrast, 76.97% experienced change greater than the LoD between 2017 and 2018. Stream order analysis classified the creek networks into five orders in 2016 and four orders in 2017 and 2018, with 2016 having a higher abundance (291 in 2016 compared to 117 (2017) and 112 (2018)) and density (0.44 m/m2 in 2016 compared to 0.27 m/m2 in both 2017 and 2018) of creek networks. These results provide an innovative high resolution insight into the evolution of restored intertidal wetlands, and suggest that SfM analysis of images taken using a sUAS can be a useful tool with the potential to be incorporated into studies of MR and natural saltmarsh sites. sUAS analysis can, therefore, advance the management of these environments to ensure the provision of ecosystem services and to protect against future anthropogenic activity, sea level rise and climate change.

Depocentre transfer in the Lingdingyang estuary: Interferences from natural and anthropogenic forcings

Estuarine morphodynamic processes caused by the combined effects of global and coastal environmental processes and human activities significantly influence the sustainable development of estuaries. Based on the bathymetric and boundary data extracted from multi-year digital charts for the Lingdingyang estuary (LE), South China, and in situ observations of the main hydrological stations, the spatiotemporal variations of depocentres in the LE and possible contributing factors are analysed. The results indicate that the spatial evolution of the morphology of the LE features shoal-deposition and channel erosion, and that the temporal evolution (1955–2015) presents a general pattern of alternating erosion and sediment deposition. A pattern of multi-depocentres exists in the LE, and the evolution of the depocentres can be divided into three general stages: i) a stage dominated by natural processes (1955–1989), ii) a stage dominated by highly-intensive reclamation (1989–2006), and iii) a stage dominated by large-scale channel dredging or sand mining (2006–2015). The transfer path of depocentres in the eastern and middle shoals traversed mainly from a north-westerly to south-easterly direction, while the transfer path of depocentres in the western shoal shows a clockwise spiral transfer feature. The large reduction of sediment supply from upstream river after the 1980s, resulted in an overall decrease in the mean annual sediment deposition volume in the LE from ~0.61 million m3/year (1989–1998) to ~0.29 million m3/year (2012–2015). The mean annual sediment deposition rate of the LE over time (i.e. 1955–2015) fluctuates in an oscillating manner rather than a linearly declining trend. Large to extreme flood (e.g. flood events of ‘94.6’, ‘98.6’ and ‘08.6’) led to higher sediment deposition rates and larger deposition volumes in the LE during the periods 1989–1998 and 2006–2008. The position of the four outlets were significantly pushed outwards by highly-intensive reclamation between 1989 and 2006, thus resulting in the bifurcation of runoff (in the Jiaomen and Hengmen outlets) and the formation of new artificial ‘outlets’ in the new channels. These great variation of the outlets induced the depocentres to transfer southeast or southwest during the period 1989–2006. A large number of temporary depocentres formed as a result of large-scale sand mining, were located in the middle shoal from 2006 to 2015. In addition, the enhancement of tidal dynamics combined with the great reduction of sediment supply from upstream river, actuated the continued transfer of the depocentres in the eastern, middle, and western shoals to the northwest or northeast after 2006. It is expected that our findings can provide a scientific foundation for port engineering construction, channel development, and the use of the tidal flat zone in the LE. Our conclusions provide a reference for other estuaries that are subject to similar anthropogenic activities.

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.

Tectonic processes, variations in sediment flux, and eustatic sea level recorded by the 20 Myr old Burdigalian transgression in the Swiss Molasse basin

Abstract. The stratigraphic architecture of the Swiss Molasse basin, situated on the northern side of the evolving Alps, reveals crucial information about the basin's geometry, its evolution, and the processes leading to the deposition of the siliciclastic sediments. Nevertheless, the formation of the Upper Marine Molasse (OMM) and the controls on the related Burdigalian transgression have still been a matter of scientific debate. During the time period from ca. 20 to 17 Ma, the Swiss Molasse basin was partly flooded by a shallow marine sea striking SW–NE. Previous studies have proposed that the transgression occurred in response to a rise in global sea level, a reduction of sediment flux, or an increase in tectonically controlled accommodation space. Here, we readdress this problem and extract stratigraphic signals from the Burdigalian molasse deposits that can be related to changes in sediment supply rate, variations in the eustatic sea level, and subduction tectonics. To achieve this goal, we conducted sedimentological and stratigraphic analyses of several sites across the entire Swiss Molasse basin. Field investigations show that the transgression and the subsequent evolution of the Burdigalian seaway was characterized by (i) a deepening and widening of the basin, (ii) phases of erosion and non-deposition during Lower Freshwater Molasse (USM), OMM, and Upper Freshwater Molasse (OSM) times, and (iii) changes in along-strike drainage reversals. We use these changes in the stratigraphic record to disentangle tectonic and surface controls on the facies evolution at various scales. As the most important mechanism, rollback subduction of the European mantle lithosphere most likely caused a further downwarping of the foreland plate, which we use to explain the deepening and widening of the Molasse basin, particularly at distal sites. In addition, subduction tectonics also caused the uplift of the Aar massif. This process was likely to have shifted the patterns of surface loads, thereby resulting in a buckling of the foreland plate and influencing the water depths in the basin. We use this mechanism to explain the establishment of distinct depositional settings, particularly the formation of subtidal shoals wherein a bulge in relation to this buckling is expected. The rise of the Aar massif also resulted in a reorganization of the drainage network in the Alpine hinterland, with the consequence that the sediment flux to the basin decreased. We consider this reduction in sediment supply to have amplified the tectonically controlled deepening of the Molasse basin. Because the marine conditions were generally very shallow, subtle changes in eustatic sea level contributed to the formation of several hiatuses that chronicle periods of erosion and non-sedimentation. These processes also amplified the tectonically induced increase in accommodation space during times of global sea level highstands. Whereas these mechanisms are capable of explaining the establishment of the Burdigalian seaway and the formation of distinct sedimentological niches in the Swiss Molasse basin, the drainage reversal during OMM times possibly requires a change in tectonic processes at the slab scale, most likely including the entire Alpine range between the Eastern and Central Alps. In conclusion, we consider rollback tectonics to be the main driving force controlling the transgression of the OMM in Switzerland, with contributions by the uplift of individual crustal blocks (here the Aar massif) and by a reduction of sediment supply. This reduction of sediment flux was likely to have been controlled by tectonic processes as well when basement blocks became uplifted, thereby modifying the catchment geometries. Eustatic changes in sea level explain the various hiatuses and amplified the deepening of the basin during eustatic highstand conditions.

Evaluation of depth erosion as a major issue along regulated rivers using the classification tool Valmorph for the case study of the Lower Rhine

ABSTRACTDepth erosion often occurs along rivers. Erosional tendencies are mainly caused by river training works, course shortenings, flood protection and reduced sediment supply due to dams and weirs. Balanced sedimentation, transport and erosion processes are of utmost importance for the connectivity of rivers and floodplains as well as for human water uses. In order to classify the level of bed incision, investigations were carried out for the case study area of the Lower Rhine in Germany, Europe’s most used waterway, using the hydromorphological classification tool Valmorph. It was developed to capture, quantify and evaluate hydromorphological conditions and changes. Comparative conditions for an evaluation have to be derived predominantly from type-specific historical quantitative data analyses. This paper analyses the Valmorph indicator ‘mean bed level changes’. Data relating to the heights of the river bed levels between the years 1896–2010 were investigated. The classifications show an immense reduction from the previously extremely high erosion rates. Additionally, the monitoring identifies cause–effect relationships between incision rates and implemented sediment management measures. The findings are shown for various temporal and spatial resolutions, reflecting the development of the depth erosion. The knowledge obtained can be used for river basin management, for example for designing large-scale measures to restore river ecosystems with sediment deficits and for monitoring their success.

Late Quaternary evolution and stratigraphic framework influence on coastal systems along the north-central Gulf of Mexico, USA

Coastal systems in the Gulf of Mexico are threatened by reduced sediment supply, storm impacts and relative sea-level rise (RSLR). The geologic record provides insight into geomorphic evolution thresholds to these forcing mechanisms to help predict future barrier evolution in response to climate change. This study synthesizes ∼2100 km of geophysical data, 700 + sediment cores, and 62 radiocarbon dates to regionally map two lowstand sequence boundaries, multiple ravinement surfaces and fourteen depositional facies demonstrating stratigraphic and antecedent topographic influences on coastal evolution. The Mississippi-Alabama (MSAL) barriers are anchored by a marine isotope stage (MIS) 5e section of Dauphin Island coupled with an MIS 2 surface gradient change. Sand for the modern MSAL barriers were largely sourced through Holocene transgressive ravinement of relict valley fill deposits, providing up to 300 × 106 m3 of sand. Mud-filled MIS 2 tributaries correspond to areas of repeated storm breaching or tidal inlets.A Holocene geomorphic evolutionary model was created for Petit Bois and Dauphin Islands, highlighting RSLR rates, changes in sediment supply and the antecedent geologic framework. As the MIS 2 surface was flooded, tidal/wave scour supplied sand to migrating marine shoals. These transgressing shoals converted drowned paleovalleys to estuaries ∼9ka BP. Islands formed in their modern positions ∼6ka BP, when sediment supply was high and RSLR rates were 2 mm/yr. Between ∼4ka-1750 CE, islands prograded from reduced RSLR rates of 1-0.4 mm/yr and sufficient sand supply from alongshore/inner shelf sources. Currently, the islands experience 3.74 mm/yr of RSLR and reduced sediment supply, resulting in barrier degradation.

Historical changes in hydrology, geomorphology, and floodplain vegetation of the Willamette River, Oregon

AbstractHistorical trends in hydrology, geomorphology, and floodplain vegetation provide fundamental contexts for designing future management of large rivers, an area of fluvial research extensively informed by studies of historical channel dynamics. Changes in hydrology, channel structure, floodplain forests, and large wood were documented for the 273‐km main stem of the Willamette River from 1850 to present. Reduced sediment supply and frequency and magnitude of floods have decreased channel mobility and incised channels, leading to fewer gravel bars, islands, and side channels. Human alteration of channel morphology, vegetation, and bank hardening has exacerbated channel simplification caused by reductions in floods, sediment supply, and inputs of wood. A substantial number of floodplain channels reoccupied remnants of previous active channels inundated during recent floods, demonstrating functional but often forgotten role of historical geomorphic structure in modern floodplains and flood processes. In most reaches, area of floodplain forests in 1990 was only 10% to 25% of the area of forests in 1850. Abundance of wood in the wetted channel was generally greater in reaches with higher abundances of floodplain forests. Future trajectories will be influenced by legacies of the historical river but increasingly will reflect evolution of a new river shaped by human development, changing climate, and emerging hydrogeomorphic and vegetation processes. Understanding historical characteristics and anticipating future rates and patterns of ecosystem change provide fundamental contexts for restoring biophysical processes and structure in a large floodplain river.

Origin and Evolution of Stacked Cut-and-fill Structures On the Southwestern Margin of the Ulleung Basin, East Sea (Japan Sea)

AbstractMultichannel seismic reflection profiles and well data were analyzed to unravel the origin and depositional history of stacked cut-and-fill structures (CF) in the southwestern margin of the Ulleung Basin, East Sea. The CFs in a syn-compressional megasequence are characterized by discontinuous, low-amplitude, chaotic, and transparent seismic reflections. They display both U-shaped and V-shaped morphologies along their lengths. The CFs are thickest and widest at the shelf edge and taper both landward and basinward. Based on the stratigraphic position, the CFs are found in three depositional sequences (DS8–DS10). Thirty CFs have been identified that range from 0.5 to 8.8 km in width and 58 to 453 m in thickness (assuming a 2000 m/s seismic velocity of the sediments). The larger and numerous CFs occurred in the middle depositional sequence. Seismic characteristics, spatial distribution, hundred-meter-scale incisions, and high gamma-ray responses indicate that the CFs were caused by submarine canyons. During the period of DS8, small CFs were formed locally on the shelf margin, and were little influenced by the deformation of the thrust-fold (Dolgorae Thrust Belt). Extensive and numerous CFs in DS9 developed in the oversteepened shelf margin, where uplift of both the thrust-fold and anticline (Gorae V structure) occurred simultaneously, and where a large volume of sediment was supplied. During the period of DS10, a general decreasing pattern in the occurrence and dimensions of the CFs resulted from waning tectonic activity of the thrust-fold that reduced sediment supply. Consequently, this study suggests that variation in contractional tectonic activity and sediment supply, associated with the back-arc closure of the East Sea, mainly controlled the evolution of the CFs rather than eustasy.

Coalesced Delta-front Sheet-like Sandstone Bodies from Highly Avulsive Distributary Channels: The Low-accommodation Mesa Rica Sandstone (Dakota Group, New Mexico, U.S.A.)

AbstractLow-accommodation deltaic systems are often challenging to interpret due to their condensed, low-gradient nature, which often results in extensive, sheet-like sandstone bodies. As a result, detailed studies of such systems are scarce, and their internal depositional architecture is still poorly understood. We analyze one such system, the Cenomanian deltaic Mesa Rica Sandstone (Dakota Group), which was deposited in the Western Interior Seaway, in east-central New Mexico, USA. A > 20-km-long escarpment, subparallel to the main delta progradation direction, allows a detailed analysis of facies distribution, depositional architecture, and the spatial extent of stratigraphic surfaces. Results reveal an arrangement of laterally variable shallowing-upward facies successions with three depositional cycles preserved. The first cycle is characterized by deltaic sheet-like sandstone bodies that are consistently overlain by sand-filled amalgamated distributary-channel deposits. The two successive cycles record a progressive reduction of sediment supply into the basin. Vertical and lateral relationships between facies associations and architectural geometries allow the recognition of regional key stratal surfaces, incised-valley fills, and the presence of lagoonal deposits at a sub-regional scale.The Mesa Rica deltaic system represents a river-dominated delta with multiple distributary channels. The sheet-like delta-front sandstone bodies are interpreted as the result of the combined effect of high sandy-sediment supply and low accommodation. The latter acted as an accelerator for autogenic depositional mechanisms such as mouth-bar deposition and abandonment, and for the highly avulsive character of distributary channels. After deposition, minor wave reworking facilitated lateral sand redistribution and favored bioturbation. This study demonstrates that sheet-like delta-front sandstone geometries from low-accommodation systems can be formed without the dominance of wave redistribution processes. This cautions against interpretations of amalgamated shoreline systems based solely on apparent sandstone geometries, without taking into account the preservation potential and postdepositional modification of primary deltaic characteristics.

Development of an eco-friendly bio-based granular geomaterial for the environmental restoration of Ichkeul Lake, Tunisia

Ichkeul Lake in northern Tunisia is a UNESCO World Heritage site. During the 1990s, its role as a nesting site for birds migrating between Europe and Africa became less prominent due to the increase in salinity due to the inflow of seawater from the Bay of Bizerte. This seawater inflow with increasing saline concentration was enhanced by lake-bed subsidence due to a reduction in sediment supply as a result of reservoir development. This phenomenon occurred by decreasing lake-bed elevation, which was accelerated by the draining from Ichkeul Lake to the Bay of Bizerte, with the resuspension of lake-bed sediments due to wind disturbance. In this study, self-medicating granular geomaterial with in situ microorganisms was proposed to raise or maintain the lake-bed elevation. This geomaterial can enhance urease-producing bacterial activity in the lake and accelerate calcite precipitation onto the surface of the lake bed, thus improving the benthos’ living conditions. Further, nitrogen resources are released from this geomaterial, which enhances the lake ecosystem by promoting aquatic plant growth. Filed column exposure tests were conducted to evaluate the proposed method’s effectiveness. Results were as expected. Therefore, this new geomaterial should help restore the function of Ichkeul Lake as a habitat for migratory birds while also recovering lake-bed elevation and aquatic plant conditions.

Mangroves and shoreline erosion in the Mekong River delta, Viet Nam

The question of the rampant erosion of the shorelines rimming the Mekong River delta has assumed increasing importance over the last few years. Among issues pertinent to this question is how it is related to mangroves. Using high-resolution satellite images, we compared the width of the mangrove belt fringing the shoreline in 2012 to shoreline change (advance, retreat) between 2003 and 2012 for 3687 cross-shore transects, spaced 100 m apart, and thus covering nearly 370 km of delta shoreline bearing mangroves. The results show no significant relationships. We infer from this that, once erosion sets in following sustained deficient mud supply to the coast, the rate of shoreline change is independent of the width of the mangrove belt. Numerous studies have shown that: (1) mangroves promote coastal accretion where fine-grained sediment supply is adequate, (2) a large and healthy belt of fringing mangroves can efficiently protect a shoreline by inducing more efficient dissipation of wave energy than a narrower fringe, and (3) mangrove removal contributes to the aggravation of ongoing shoreline erosion. We fully concur, but draw attention to the fact that mangroves cannot accomplish their land-building and coastal protection roles under conditions of a failing sediment supply and prevailing erosion. Ignoring these overarching conditions implies that high expectations from mangroves in protecting and/or stabilizing the Mekong delta shoreline, and eroding shorelines elsewhere, will meet with disappointment. Among these false expectations are: (1) a large and healthy mangrove fringe is sufficient to stabilize the (eroding) shoreline, (2) a reduction in the width of a large mangrove fringe to the benefit of other activities, such as shrimp-farming, is not deleterious to the shoreline position, and (3) the effects of human-induced reductions in sediment supply to the coast can be offset by a large belt of fringing mangroves.

Decadal morphological evolution of the mouth zone of the Yangtze Estuary in response to human interventions

AbstractThe morphology of the Yangtze Estuary has changed substantially at decadal time scales in response to natural processes, local human interference and reduced sediment supply. Due to its high sediment load, the morphodynamic response time of the estuary is short, providing a valuable semi‐natural system to evaluate large‐scale estuarine morphodynamic responses to interference. Previous studies primarily addressed local morphologic changes within the estuary, but since an overall sediment balance is missing, it remains unclear whether the estuary as a whole has shifted from sedimentation to erosion in response to reduced riverine sediment supply (e.g. resulting from construction of the Three Gorges Dam). In this paper we examine the morphological changes of two large shoals in the mouth zone (i.e. the Hengsha flat and the Jiuduan shoal) using bathymetric data collected between 1953 and 2016 and a series of satellite images. We observe that the two shoals accreted at different rates before 2010 but reverted to erosion thereafter. Human activities such as dredging and dumping contribute to erosion, masking the impacts of sediment source reduction. The effects of local human intervention (such as the construction of a navigation channel) are instantaneous and are likely to have already resulted in new dynamic equilibrium conditions. The morphodynamic response time of the mouth zone to riverine sediment decrease is further suggested to be >30 years (starting from the mid‐1980s). Accounting for the different adaptation time scales of various human activities is essential when interpreting morphodynamic changes in large‐scale estuaries and deltas. © 2019 John Wiley & Sons, Ltd.

Morphological changes and human impact in the Entella River floodplain (Northern Italy) from the 17th century

In this article the morphological changes undergone by the Entella River (Northern Italy) over the last four centuries has been investigated. The historical analysis has allowed reconstruction of fluvial evolution and shoreline dynamics and demonstrated their relationship to human disturbance over a very long period compared to most previous studies in Italy and Europe. A set of 12 historical and current maps and aerial photos, ranging from the 17th century to the present, were entered into a Geographic Information System (GIS) in order to calculate four morphological parameters (i.e., channel length, width, sinuosity and centreline shifting) and the distance of the shoreline.Modification of the Entella River over the 360-years period 1656–2016 include: i) a reduction in channel length by 128 m (3%); ii) a mean narrowing of 56 m (46%); iii) a decrease in total sinuosity from 1.10 to 1.05, with a variation in river pattern, from sinuous to straight; iv) a total shifting of the centreline of 30 m. This evolutionary trend is consistent with most of the previous studies on Italian and European rivers. Conversely, the total channel shortening and the recent phase of substantial morphological stability, with a slight increase in channel width and length, seems to be in contrast with the results of other studies.We correlated the channel planform changes and the regression of the shoreline, at least until the latter 20th century, to the reduction in sediment supply produced by the morphological modifications due to human intervention. At the beginning of the 19th century and, later, from the 1950s to the end of the 20th century, channelization, channel diversion, land-use changes and coastal defences accounted for this supply change to the Entella River and its floodplain.Furthermore, our findings reveal that human disturbance has contributed to increased flood risk in the plain, through the progressive reduction in width of the riverbed and the increasing urbanization along its riverbanks. At the same time, channelization seems to have a negative effect not only on the morphological evolution of the river, but also in terms of prevention and flood risk reduction within the floodplain.

Groundwater discharge dynamics into a salt marsh tidal river

Exchange of groundwater is an important transfer mechanism for nutrients and pollutants between coastal aquifers and surface waters. Constraining such exchange in salt marshes - where biological productivity and biogeochemical cycling rates are among the highest of all coastal ecosystems – is vital for understanding ecosystem function and vulnerability. Here, we quantify groundwater discharge into the tidal Duplin River from the adjoining salt marsh near Sapelo Island, Georgia using high spatial and temporal resolution field measurements of radon-222. Field campaigns occurred for several weeks each summer during 2013, 2015, and 2016. Spatial surveys reveal a general increase in radon activity upstream through the Duplin River, which may result from either higher groundwater discharge or lower mixing rates in the headwaters. To distinguish between these possibilities, we use a radon mass balance model to determine groundwater input. We find that groundwater discharge (normalized to inundated marsh surface area) to the headwaters average 5.1–5.8 cm3/cm2 marsh/day across all three field campaigns, which are comparable to those to the main channel (averaging 6.0–6.5 cm3/cm2 marsh/day across all three field campaigns). Our work reveals a positive relationship between aerial extent of marsh inundation and groundwater discharge into the Duplin River. Discharge is generally maximal during falling tide, reflecting a hydraulic gradient driver, but also is significant prior to high tide, indicative of sediment compression as a driver of groundwater inputs. Constraining the relationship between marsh inundation and resulting groundwater dynamics is an integral aspect to assessing how salt marsh circulation processes may respond to intensifying inundation (from reduced sediment supply, subsidence, and/or rising sea levels).

Tracking shoreline erosion of “at risk” coastal archaeology: the example of ancient Siraf (Iran, Persian Gulf)

Successful heritage management requires a robust comprehension of the threats facing archaeological sites, at both current and future timescales. Siraf, on the Persian Gulf of Iran, is a site of national and international importance whose history stretches back to the Sassanid period (224–652 AD). In the present context of global change (drought and relative sea-level rise) and anthropogenic impacts (coastal artificialization and reduced sediment supply), the city's waterfront archaeology is undergoing significant erosion. Nonetheless, at present, the processes leading to the loss of Siraf's in situ archaeological remains are still poorly understood, including the rates, timing and drivers of coastal erosion. Here we use Landsat images to monitor shoreline changes along the Siraf coastline between 1973 and 2016. We spatially quantify the causes and impacts of surface changes along 244 transects. The results demonstrate that coastal erosion is responsible for widespread and archaeologically significant damage, with 48% of the studied transects showing erosion during the period 1973–2016. We elucidate significant temporal variations in the data and, most notably, demonstrate that continued drought since the early 2000s has impacted upon sediment supply to coastal areas, severely accentuating erosion. For instance, between 2003 and 2016, 70% of transects recorded shoreline retreat with worrying implications for the waterfront archaeology of Siraf. These quantitative results provide invaluable spatial information regarding the causes and impacts of erosion upon Siraf's waterfront heritage, in addition to furnishing a template for the protection of the city's internationally important cultural heritage.

Morphodynamic model of the lower Yellow River: flux or entrainment form for sediment mass conservation?

Abstract. Sediment mass conservation is a key factor that constrains river morphodynamic processes. In most models of river morphodynamics, sediment mass conservation is described by the Exner equation, which may take various forms depending on the problem in question. One of the most widely used forms of the Exner equation is the flux-based formulation, in which the conservation of bed material is related to the stream-wise gradient of the sediment transport rate. An alternative form of the Exner equation, however, is the entrainment-based formulation, in which the conservation of bed material is related to the difference between the entrainment rate of bed sediment into suspension and the deposition rate of suspended sediment onto the bed. Here we represent the flux form in terms of the local capacity sediment transport rate and the entrainment form in terms of the local capacity entrainment rate. In the flux form, sediment transport is a function of local hydraulic conditions. However, the entrainment form does not require this constraint: only the rate of entrainment into suspension is in local equilibrium with hydraulic conditions, and the sediment transport rate itself may lag in space and time behind the changing flow conditions. In modeling the fine-grained lower Yellow River, it is usual to treat sediment conservation in terms of an entrainment (nonequilibrium) form rather than a flux (equilibrium) form, in consideration of the condition that fine-grained sediment may be entrained at one place but deposited only at some distant location downstream. However, the differences in prediction between the two formulations have not been comprehensively studied to date. Here we study this problem by comparing the results predicted by both the flux form and the entrainment form of the Exner equation under conditions simplified from the lower Yellow River (i.e., a significant reduction of sediment supply after the closure of the Xiaolangdi Dam). We use a one-dimensional morphodynamic model and sediment transport equations specifically adapted for the lower Yellow River. We find that in a treatment of a 200 km reach using a single characteristic bed sediment size, there is little difference between the two forms since the corresponding adaptation length is relatively small. However, a consideration of sediment mixtures shows that the two forms give very different patterns of grain sorting: clear kinematic waves occur in the flux form but are diffused out in the entrainment form. Both numerical simulation and mathematical analysis show that the morphodynamic processes predicted by the entrainment form are sensitive to sediment fall velocity. We suggest that the entrainment form of the Exner equation might be required when the sorting process of fine-grained sediment is studied, especially when considering relatively short timescales.

Hydrological response of a large-scale mountainous watershed to rainstorm spatial patterns and reforestation in subtropical China

This paper aims to investigate the hydrological response of a large-scale (8973 km2) mountainous watershed to different rainstorm spatial patterns and reforestation. Based on 32 years of observations, measurements of 184 rainstorm events and 125 sediment-producing events with complete hydrographs were analyzed. The K-means clustering method was used to classify the spatial patterns of rainstorm events in accordance with their event-based spatial rainfall characteristics. The 184 rainstorm events were classified into four spatial patterns, among which the spatial features differ significantly: (I) Spatial Pattern I (SPI) includes rainstorms with a low amount of cumulative areal rainfall (27.4 mm), the highest spatial variability (0.986), and the highest frequency; (II) Spatial Pattern II (SPII) includes rainstorms of high spatial variability (0.759) and the largest amount of local maximum daily rainfall (106.8 mm); (III) Spatial Pattern III (SPIII) includes rainstorms with a medium amount of cumulative areal rainfall (58.7 mm) and low spatial variability (0.362); and (IV) Spatial Pattern IV (SPIV) includes rainstorms with the largest amount of cumulative areal rainfall (117.2 mm) and the lowest spatial variability (0.313). Vegetation cover in the upper Du watershed was significantly improved after the implementation of the Grain-for-Green project. The average area-specific sediment yields (SSY) for the four SPs were 15.4, 65.5, 55.8, and 286.2 t km−2 before reforestation and decreased to 6.0, 59.3, 43.7 and 89.9 t km−2, respectively, after reforestation. ANOVA (analysis of variance) indicated that reforestation resulted in a significant reduction in runoff coefficient under SPIII and SPIV and a significant reduction in SSY under SPI and SPIV. A hysteresis analysis suggested that the proportion of events with a clockwise loop increased from 64.9% before reforestation to 82.1% after reforestation and that complex loops became less common during 2000–2010 under SPIV, thereby implying a reduced sediment supply.