Sediment Deficit Research Articles

Quantifying the Importance of Ice-Rafted Debris to Salt Marsh Sedimentation Using High Resolution UAS Imagery

Salt marshes are vulnerable to sea-level rise, sediment deficits, and storm impacts. To remain vertically resilient, salt marshes must accrete sediment at rates greater or equal to sea-level rise. Ice-rafted debris (IRD), sediment that has been moved and deposited from ice sheets, is one of many processes that contribute to salt marsh sediment accretion in northern latitudes. On 4 January 2018, a winter storm caused major ice mobilization in the Plum Island Estuary (PIE), Massachusetts, USA, which led to large deposits of ice-rafted sediment. We aimed to quantify the volume and mass of deposited sediment, and evaluate the significance of IRD to sediment supply in Plum Island using pixel-based land-cover classification of aerial imagery collected by an Unmanned Aircraft System (UAS) and a Digital Elevation Model. Field measurements of patch thickness, and the area of IRD determined from the classification were used to estimate annual sediment accretion from IRD. Results show that IRD deposits are localized in three areas, and estimates show that IRD contributes an annual sediment accretion rate of 0.57 ± 0.14 mm/y to the study site. New England salt marsh accretion rates typically vary between 2–10 mm/y, and the average PIE sediment accretion rate is 2.5–2.7 mm/y. Therefore, this event contributed on average 20% of the annual volume of material accreted by salt marshes, although locally the deposit thickness was 8–14 times the annual accretion rate. We show that pixel-based classification can be a useful tool for identifying sediment deposits from remote sensing. Additionally, we suggest that IRD has the potential to bring a significant supply of sediment to salt marshes in northern latitudes and contribute to sediment accretion. As remotely sensed aerial imagery from UASs becomes more readily available, this method can be used to efficiently identify and quantify deposited sediment.

Enhancing the functionality of environmental flows through an understanding of biophysical processes in the riverine landscape

While water resource managers and river scientists recognize the inherent interconnections among hydrology, river structure, biophysical processes and ecological patterns, management of environmental flows still pays insufficient attention to the ecological and geomorphological functionality of particular aspects of the flow regime. Implementation of more natural flow regimes has improved habitat conditions for native species in many moderately impaired rivers but mimicking a natural flow regime in heavily modified riverine landscapes cannot be expected to yield successful ecological outcomes unless such flows trigger functional processes. For example, the restoration of peak flows may not regenerate habitats if the river is starved of sediment or if the river channel is highly confined. High biodiversity is supported when variable flow regimes interact with spatially variable (heterogeneous) river channel and floodplain forms. In contrast, as rivers become homogeneous, biodiversity decreases when these dynamic spatiotemporal interactions are limited by flow alterations, blocked by channel levees, or perturbed by sediment deficit or surplus. Thus, the design of a more natural environmental flow regime without consideration of the implications for sediment transport and implicit recognition of channel–floodplain geomorphology is likely to have limited success in river management and restoration. To enhance the functionality of environmental flows, considerations of physical, biogeochemical, and ecological processes and the inherent heterogeneity of the riverine landscape must be included. A Functional Flows approach enhances the benefits from limited environmental flow allocations by focusing on the ecological and geomorphological functionality of particular aspects of the flow regime, considering geomorphic context, and emphasizing spatiotemporal diversity at key locations in the riverscape, such as adjacent floodplains or tributary junctions. In this paper, we outline and illustrate the concept of Functional Flows using a flow-chain model and provide two case study examples from Australia and the United States, where improvements in channel habitat and reconnection with the floodplain help to achieve the desired functionality of environmental flows.

Significant Human Modification of the Lower Arkansas River Sediment Budget

AbstractAnthropogenic stressors on large rivers have significantly modified riverine sediment transport. However, the impacts of dredging and sand and gravel (S&G) mining are poorly monitored. We quantify these impacts on the lower Arkansas River, USA, where anthropogenic processes are well documented. The construction of dams caused a 98% reduction in suspended sediment discharge (Qss). Since dam construction, fluvially transported suspended sediment (4.4 ± 0.5 Mtyr−1) and suspended sand discharge (Qsand) (1.1 ± 0.1 Mtyr−1) are of the same order as the dredging rate (1.2 ± 0.1 Mtyr−1) and S&G mining rate (1.7 ± 0.1 Mtyr−1). While the modern sediment deficit on the Arkansas River is smaller than in rivers documented in developing countries, dredging and mining significantly alter the sediment balance in this large river, indicating that extraction management is needed worldwide, especially in developing economies, to improve ecosystem sustainability in large rivers.

Sediment deficit and morphological change of the Rhine–Meuse river mouth attributed to multi-millennial anthropogenic impacts

Many delta systems worldwide are becoming increasingly urbanized following a variety of processes, including land reclamation, embanking, major engineering and port constructions, dredging and more. Here, we trace the development of one system, the Rhine–Meuse delta in the Netherlands (RMD) from two natural estuaries (the RME fed by the Rhine river and the HVL fed by the Meuse river) to a densely urbanized delta and the effect human activities have had on its morphology through time. Estuary outlines determined from palaeogeographical and old maps and tidal range at the estuary mouth were used to reconstruct basic estuary parameters. Depth distribution was predicted with a morphological tool. We have determined that the northern estuary, where port activities dominate, shows a stepwise deepening due to dredging for navigation. The southern port shows stepwise shallowing as humans closed off this branch from tidal action. Both estuaries show narrowing and loss of intertidal width over the past five centuries. The total loss of water volume has been -5.5 m3×109 since 1500 AD coinciding with major human intervention in the system, driven by the rapid economic boom during the mid 16th century. This has led to a reduction of intertidal areas and floodplains and long-term sediment shortage resulting in a myriad of problems, including increased flood risk, threats to bank protection and infrastructure and loss of nature areas. These problems will be exacerbated in coming centuries by predicted sea-level rise. Other urban deltas, unrestricted by engineering and dredging innovations which took place more gradually in earlier centuries, are now undergoing rapid changes in mere decades, changes which took hundreds of years in the RMD. Future predictions indicate that by 2050 the RMD will experience the highest loss of sediment in the 3500 year history of the system, despite these changes only occurring vertically (deepening) and not laterally (narrowing or embanking). This shows an urgent need to reconsider sediment management and spatial planning of port expansion in urbanized systems and the fate of such systems under climate change and rising sea-levels.

Long-time impact of a large dam on its downstream river’s morphology: determined by sediment characteristics, pollutants as a marker, and numerical modelling

Many river systems are regulated by dams, which causes an altered flow regime and sediment deficit in the downstream reach. The Rur dam (North Rhine-Westphalia, Germany), constructed from 1900 to 1959, serves as a model example of the impact of a large dam in a European low mountain area on downstream morphology. Today, a new equilibrium incorporates flow regulations, a deficit in suspended sediment supply, and an increased mean sediment diameter downstream of the dam. A hybrid examination of field measurements and numerical modelling shows that the sediment deficit and increased mean sediment diameters downstream of the Rur dam are superimposed by the lithostratigraphy and the sediment supply of tributaries. However, the discharge regulations lead to floodplain decoupling downstream of the dam. Furthermore, the Rur dam functions as a pollutant trap. Overall, more studies on the impact of damming on downstream reaches are needed to classify the impact depending on the river type for sustainable water management.

Boosting riverine sediment by artificial flood in the Yellow River and the implication for delta restoration

Due to accelerated river-derived sediment deficits, global deltas are presently under an increasing risk of being submerged. To address the fundamental need to save Earth's deltas, boosting riverine sediment supply can be considered a reasonable option, but only if the new supply of sediment is prolonged and greater than the opposing forces: sea level rise, delta subsidence and compaction, and ocean energy. In July 2018, an artificial flood with a peak discharge of 3780 m3/s was released from the Xiaolangdi Reservoir, the last reservoir located on the mainstream of Yellow River, China. From July 03 to July 29, 412 million tons (Mt) of sediment was discharged out of the dam, including 240 Mt. that was previously sequestered within the reservoir. During this event, 140 Mt. of the escaped sediment directly reached the estuary. A total of 245 Mt. of sediment was deposited along the lower river channel bed, of which 83.6 Mt. was eroded once again and transported to the sea in the subsequent five months. This dam-released pulse, coupled with channel diversion and tidal shear fronts, nourished the deltaic area. New land was built, with the submarine offshore gaining up to 2.6 m in seafloor shoaling. Sediment impounded behind the dam, if properly released, could thus become a potential savior for sediment-starved deltas.

Effect of alongshore sediment supply gradients on projected shoreline position under sea-level rise (northwestern Portuguese coast)

Shoreline changes are associated with natural and anthropic factors, such as sea-level rise, sediment budget, and beach fills. Shoreline retreat is mainly related to sediment deficit and sea-level rise. Some studies show that by the end of the 21st century some beaches may be lost, with the erosion of sandy beaches becoming predominant across the globe. The main purpose of this work is to model the shoreline evolution for 2050 and 2100 along a sandy coastal stretch in the NW Portugal, considering different scenarios of sea-level rise, to evaluate the influence this parameter and sediment budget have on shoreline displacement, as well as estimate the nourishment volume necessary to hold the shoreline in case of shoreline retreat. For this, we applied the stochastic version of the Random Shoreface Translation Model (RanSTM), in four cross-shore topo-bathymetric profiles, which are representative of alongshore coastal compartments. The overall results indicate that the key drivers in shoreline migration along the coastal stretch are not the same for the different profiles; in areas with a sediment supply deficit, shoreline recession increases, and hence the volume of sand required to hold the shoreline position in those zones is much greater. In contrast, positive sediment budgets can mitigate or reverse the erosion caused by sea-level rise.

Impacts des décisions et actions de protection sur l’évolution morphologique du littoral lacustre de Thonon-les-Bains (France, Haute-Savoie)

The landscape heritage of Thonon-les-Bains’ Lake Geneva shoreline is a key component of the local population's living environment and tourist activity. Since the mid-1990s however, coastal erosion has been observed. With this worrying development, experts were called in to monitor the shoreline’s morphology. The first study (2011-2014) showed that the majority of the shoreline was eroding (60%) at an average rate of 0.40 m.yr and a maximum of 1.5 m. The primary cause is sediment deficit linked to hydroelectric developments in the Dranse catchment area, and sediment extraction at the mouth of the river Dranse on Lake Geneva (Maleval et al., 2016). In light of the study’s findings, the Prefecture refused the extraction company's (Sagradranse) application to continue its activity, and Thonon-les-Bains municipality undertook renaturation work on its shoreline using soft protection methods (plant engineering and removal of hard structures). The latest study (2019), using the same methodology as the first (mainly manual topography and acoustic bathymetry), revealed an encouraging change: between 2014 and 2019, the portion of eroded shoreline decreased to 42%, and the percentage of severely-eroded shorelines decreased from 39% to 5%. On the basis of these latest data, the average annual erosion rate of the Thonon-les-Bains shoreline can be considered as minor (0.13 m), thereby supporting the public authorities’ decisions.

Climatically induced cyclicity recorded in the morphology of uplifting Tihu coastal ridgeplain, Hiiumaa Island, eastern Baltic Sea

Holocene relative sea level changes and palaeogeography of the uplifting Tihu ridge-swale beach system in the functionally tideless Baltic Sea were reconstructed using airborne LiDAR elevation data, ground-penetrating radar surveys, sedimentological proxies, recent models of Fennoscandian uplift due to glacial isostatic adjustment, and GIS-based terrain modelling. Considering that aeolian contribution to the ridge growth was relatively small and uniform along the studied profiles, it was possible to utilize an age estimation method based on terrain elevation and pre-determined RSL curves. The tested relative sea level curves converged around a linear trend during the past 7 ka, the rate of which (3.1–3.2 mm/a) is strongly correlated with relative-to-geoid uplift model output. We found that the Tihu system started to emerge from the sea ~5.4 ka ago. In its older part, it includes a complex of elevated barrier spits where both the isostatic uplift and gradual sediment aggradation due to longshore transport contributed to its nucleation and initial progradation. In its lower (younger) section, a forced progradation “staircase” of palaeocoastlines was formed in a regime of sediment deficit and low-energy hydrodynamic conditions. Analysis of near-cyclic occurrences of beach/foredune ridges and larger ridge sets within the Tihu ridgeplain revealed time intervals of enhanced storminess during 5.4–4.9, 4.8–4.5, 4.6–4.3, 4.1–3.8, 3.7–3.4, 3.2–2.9, 2.7–2.4, and 2.2–1.9 ka BP. Serving as a new independent proxy for past storminess and high sea level events in Estonia, these phases match well with some previous European Atlantic storminess reconstructions for Mid- to- Late Holocene.

Geomorphologic changes around a mid-river bar system at a meandering reach in the lower Yangtze River, China: Impacts of the three Gorges dam (TGD) and human activities

Mid-river bars are important agents that impact the flow and sediment dynamics of rivers and are commonly used for the development of human society. This study focused on the geomorphologic change (riverbed and bar morphology) around a large-scale river bar system in the lower Yangtze River (namely, Heishazhou) that was possibly impacted by the Three Gorges Dam (TGD) and other river engineering works. Relevant data sources including field surveys, hydrological station records, satellite images and historical practice reports were gathered for the investigation. The Heishazhou bar system previously consisted of two major subbars and recently merged as an entirety. Rapid bar growth occurred after the construction and operation of the TGD, leading to the closure of the Middle Channel and the South Channel dominating water conveyance at the reach. With the examination of another three bar systems in the lower Yangtze River, the lower-reach bar systems generally grew due to sediment deposition around bars, which differred from those in the middle reach that generally shrunk. The underlying cause may be that the middle-reach riverbed still supplies sediment to the lower reach. The riverbed at the Heishazhou reach, however, experienced apparent erosion after the construction of the TGD. Combining historical investigations and satellite images, a conceptual description was created to understand the geomorphologic cycle of river bar systems at a meandering reach and found that the formation of an inner-bank bar shall be a major force driving the geomorphologic cycle. Sediment deficits caused by the TGD and human activities such as dikes, bank revetments and riverbed dredging are highly likely to cause a slow-down of the geomorphologic cycle of these river bar systems with distinct mechanisms.

A Simplified Concept to Test Erosion or Sedimentation Potential along a Tidal River in the Ganges-Brahmaputra-Meghna Delta

The Payra is a tidal river with meandering planform in which around 6.5 km long landmass is formed naturally during the last 40 years that is selected for an establishment by the Government of Bangladesh. However, the site experiences intermittent erosion at several locations and poses threat to the newly developed infrastructures. In the present study, a Rouse type simple model is used to estimate the equilibrium profiles of suspended sediment concentration below which the system is likely to be eroded and regarded as “Sediment Deficit System, SDS” and above which the system is likely to be sedimented and regarded as “Sediment Surplus System, SSS”. The model was then tested at the study reach using the measured flow-sediment data complemented by field observation and Landsat images-based planform analysis. The analysis revealed that the residual flow-sediment regime in the study site is representing the “sediment deficit system” that causes continuous threat for erosion throughout the tidal cycles. Therefore, the availability of new landmass should not be the only criterion for the quick selection of a project site, and residual flow-sediment regime needs to consider as an additional parameter for such decision-making process.

Impacts of agricultural expansion on floodplain water and sediment budgets in the Mekong River

In this paper, we address the impact of agricultural expansion on hydrological patterns of water and sediment budget in one of the largest floodplains along the Cambodian Mekong since the 1980s, using field (water level, discharge, sediment, rainfall, and groundwater level) and remote sensing (land use, surface suspended sediment) data, and numerical simulations. Specifically, while both the surface suspended sediment concentration and water level in the Mekong River around Kampong Cham and Neak Luong decreased, the floodplain seasonal water storage increased. In addition, the rate of sediment input from the river to the floodplain was almost constant throughout the studied period. The investigation of the floodplain’s annual sediment budget, however, reveals a significant drop during the analyzed period, mainly due to the decreased sediment trapping rate (66% in the 1980s to 46% in the 2010s). Currently, a good amount of sediment bypass the floodplain and return back to the river. The observed hydrological patterns in the floodplain could have been triggered by the agricultural expansion that has increased surface erodibility (due to removals of primary vegetation) and lowered the land surface elevation (due to groundwater extraction). Despite the well-known impacts of the hydropower dams on the Mekong Delta hydrological conditions, particularly the reduction of sediment reaching the delta due to sediment trapping by dam reservoirs, our observations point to new and more localized driving factors of sediment deficit in floodplain: agricultural expansion. Finally, we used 2D hydrodynamic simulation (Telemac-2D) to visualize the processes of water routing and sedimentation in the floodplain accounting for land cover change since the 1980s. The floodplain hydrology reported in this paper is an unexplored environmental consequence of agricultural expansion in the lower Mekong. Geomorphologically, this study presents a peculiar case of floodplain showing how agricultural expansion can diminish the role of a floodplain as a sediment sink through decreasing the trapping rate.

Modelling the contribution of wind waves to Cap Ferret's updrift erosion

Wind waves breaking at an angle with the shoreline force the drifting of littoral sediments, which is known for contributing to the formation and growth of barrier spits. Intriguingly, increased rates of longshore wave power have also been associated with the erosion of some barrier spits on the updrift margin of tidal inlets. Therefore, a numerical experiment was designed and is presented here, which investigates the possible links between the longshore wave power and the shortening of these elongated coastal barriers. Based on a process-based model, the experiment provides new insights into the forces at play in the redistribution of sediments between a sandspit and its adjacent inlet, respectively the Cap Ferret and the Bay of Arcachon's tidal inlet, in SW France. More particularly, model scenarios were defined that show how combined waves and tide create gradients of residual sediment transport responsible for a sediment deficit at the spit – inlet boundary. The deficit was also found to deepen with increasing longshore wave energy, as if the transfer of sediment from the spit to inlet shoals was accelerated. This physically explains the previously observed retreat of the spit's distal end during periods dominated by the positive phase of North Atlantic Oscillation (NAO) in winter. Indeed, according to model results, higher and/or more oblique waves associated with the positive phase of the NAO are expected to increase the transfer and storage of the drifting sediments to and by the inlet shoals, and this at the expense of the spit. While these conclusions remain valid, we noticed that the sensitivity of model results to the bottom friction enhanced the importance of accurately representing the spatio-temporal distribution of bed roughness when investigating the morphodynamic interactions between real-world tidal inlets and their margins.

Intertwined Eco‐Morphodynamic Evolution of Salt Marshes and Emerging Tidal Channel Networks

AbstractThe formation and development of tidal channels and salt marshes are controlled by complex interactions between hydrodynamics, sediment transport, and vegetation dynamics. Tidal channels affect and, at the same time, are affected by the growth of salt marshes fringing them. The coupled evolution of these morphological units, mediated by vegetation growth, is thus a key ingredient for simulating the behavior of tidal environments. Considering these two factors, we developed a mathematical model to investigate the eco‐morphodynamic evolution of intertidal areas fringing a main channel and of the tidal creeks cutting through them. Model results indicate that vegetation promotes the development of channel networks, leading to more complex channel structures and higher drainage efficiency. Vegetation encroachment influences sediment deposition patterns by trapping sediment in the seaward and middle intertidal areas, while reducing the amount of sediment delivered to landward areas. In the presence of sea level rise, this deficit of sediment enhances the landward‐decreasing trend of the intertidal platform and leads to more isolated vegetation patches. Overall, sea level rise restricts the extension of salt marshes and consequently reduces the effect of vegetation on channel network form and function.

Main Threats in Mediterranean Coastal Wetlands. The Ebro Delta Case

Coastal wetlands are dynamic ecosystems that exist at the interface between land and sea. They represent environments with a great diversity of habitats and communities, high carbon sequestration capacity and a wide range of ecosystem services. In the Mediterranean, the largest coastal wetlands are found in deltaic areas like that of the Ebro River (Spain), which has a coastline length of approximately 50 km, occupying a total area of 325 km2. The Ebro Delta is included in different national and international frameworks for environmental conservation, despite which there are several risks that threaten it. The lack of sedimentary contributions due to the regulation of the Ebro riverbed (irrigation, reservoirs, and hydroelectric power generation) has caused erosion and the retreat of certain sections of its coastline. To this situation of sediment deficit must be added the threat posed by the effects of global change, such as the rise in sea level, the increase in temperature and in the frequency and intensity of storms. This study analyses the particularities of the coastal wetland of the Ebro Delta, identifying the main threats it faces, as well as possible adaptation and mitigation strategies to these changes.

Evaluating adaptation options to sea level rise and benefits to agriculture: The Ebro Delta showcase

Sea level rise (SLR) is threatening low-lying coastal areas such as river deltas. The Ebro river Delta (Spain) is representative of coastal systems particularly vulnerable to SLR due to significant sediment retention behind upstream dams (up to 99%), thereby dramatically reducing the capacity for deltaic sediment accretion. Rice production is the main economic activity, covering 66% of the delta area, and is negatively affected by SLR because of flooding and soil salinization. Therefore, appropriate adaptation measures are needed to preserve rice production. We combined Geographic Information Systems and Generalized Linear Models to identify zones prone to flooding and increasing soil salinity, and to calculate the so-called sediment deficit, that is the amount of sediment needed to raise the land to compensate flooding and soil salinization. We modelled SLR scenarios predicted by the IPCC Fifth Assessment Report, and analysed the economic feasibility (not the technical feasibility) of reintroducing fluvial sediments retained in the upstream river dam reservoirs into the delta plain, which can contribute to maintaining land elevation and rice production with SLR. To do this, the costs of the sediment reintroduction measures and their benefits in terms of avoided loss of rice production income were evaluated with an approximate economic cost-benefit analysis. Results predicted that between 35 and 90% of the rice field area will be flooded in the best and worst SLR scenarios considered (SLR = 0.5 m and 1.8 m by 2100, respectively), with a sediment deficit of 130 and 442 million tonnes, with an associated cost of sediment reintroduction of 13 and 226 million €. The net benefit of rice production maintenance was 24.6 and 328 €/ha. The proposed adaptation measure has a positive effect on rice production and can be considered as an innovative management option for maintaining deltaic areas under SLR.

Beach Monitoring and Morphological Response in the Presence of Coastal Defense Strategies at Riccione (Italy)

The coastal area at Riccione, in the southern Emilia-Romagna littoral region, is exposed to erosive processes, which are expected to be enhanced by climate change. The beach, mostly composed of fine sand, is maintained through various defense strategies, including frequent nourishment interventions for balancing the sediment deficit and other experimental solutions for reducing coastal erosion. Artificial reshaping of the beach and “common practices” in the sediment management redefine the beach morphology and the sediment redistribution almost continuously. These activities overlap each other and with the coastal dynamics, and this makes it very difficult to evaluate their effectiveness, as well as the role of natural processes on the beach morphological evolution. Topo-bathymetric and sedimentological monitoring of the beach has been carried out on a regular basis since 2000 by the Regional Agency for Prevention, Environment and Energy of Emilia-Romagna (Arpae). Further monitoring of the emerged and submerged beach has been carried out in 2019–2021 in the framework of the research project STIMARE, focusing on innovative strategies for coastal monitoring in relation with erosion risk. The aim of this study is to assess the coastal behavior at the interannual/seasonal scale in the southern coastal stretch of Riccione, where the adopted coastal defense strategies and management actions mostly control the morphological variations in the emerged and submerged beach besides the wave and current regime. The topo-bathymetric variations and erosion/accretion patterns provided by multitemporal monitoring have been related to natural processes and to anthropogenic activities. The morphological variations have been also assessed in volumetric terms in the different subzones of the beach, with the aim of better understanding the onshore/offshore sediment exchange in relation with nourishments and in the presence of protection structures. The effectiveness of the adopted interventions to combat erosion, and to cope with future climate change-related impacts, appears not fully successful in the presence of an overall sediment deficit at the coast. This demonstrates the need for repeated monitoring of the emerged and submerged beach in such a critical setting.

Erosion irregularities resulting from series of grade control structures: The Mszanka River, Western Carpathians

Grade control structures (GCSs) are one of the most often used hydraulic structures in a river regulation in mountain catchments. The purpose of their use is to reduce the river bed gradient and prevent downcutting. The aim of this study is to evaluate the influence of GCS on changes in river bed morphology. The study was conducted in a downstream stretch (5.66 km long) of the Carpathian river – the Mszanka, regulated with 25 GCSs. Research has shown that erosion is here a predominant process despite the use of GCSs. The river bed has been cut into bedrock along approximately 40% of a studied stretch. A total of 56 scour holes were identified, which vary substantially in terms of their depth, length, and mechanism of formation. The erosion depth depends mainly on the length of reaches between GCSs and on the water surface gradient. A surprising research discovery was scour holes found upstream of the crests of the GCSs – 16 such scour holes were found. The formation of these scour holes is explained through the drawdown profile characterized by the increasing of the velocity that favored the bed erosion. The studied river is characterized by deficit of sediment. The insufficient sediment supply and imbalance between the river's load and sediment transport are two of the greatest problems affecting the most Carpathian rivers. The novelty of this research is the analysis of such a long and complex reach of mountain river engineered with GCSs in terms of variances in erosion processes. Also a new knowledge presented here is an information about scour holes located upstream of GCSs and explanation of the mechanism of their formation. Results from this research could serve geomorphologists, engineers and ecologists as well as help river managers in decision-making processes when river regulation is planned.

Balance sedimentario del embalse de Ribarroja

El objetivo de este estudio ha sido construir un balance de sedimentos del embalse de Ribarroja, situado en el tramo inferior del río Ebro. Para ello, se han analizado datos continuos de caudal y transporte de sedimentos en suspensión tomados en los principales ríos que desembocan en el embalse, así como en la salida de la presa, para el período 2008-2011. La carga de sedimentos estimada aguas arriba del embalse de Mequinenza fue de 0.66 × 106 t a-1, mientras que la salida estimada fue de 0.04 × 106 t a-1. El aporte de sedimentos proveniente de los grandes afluentes pirenaicos, aunque altamente regulados, Segre y Cinca fue de 0,16 × 106 t a-1, mientras que los ríos intermitentes que fluyen naturalmente desde la Cordillera Ibérica (Matarraña y Algars) suministraron una carga media de 760 t a-1. La carga de sedimentos en suspensión atrapada en los embalses de Mequinenza y Ribarroja para el período de estudio se estimó en 0,78 × 106 t a-1, lo que implica una reducción de ca. 95% de la carga de sedimentos del río. Los resultados ilustran el papel de los embalses en la interrupción de la transferencia de sedimentos en el tramo bajo del Ebro, en un contexto de déficit sedimentario estructural, tras los cambios generalizados de uso del suelo (reforestación) que se produjeron en la cuenca a partir de los años cuarenta del siglo pasado.

Adaptation to climate change in small coastal cities: The influence of development status on adaptation response

In contrast to coastal towns and small urban settlements, small coastal cities (population of between 50,000 and 100,000) may exhibit comparable knowledge and planning infrastructure as larger cities or be similarly connected to research institutions. However, at a global level, there is little statistical data about small cities, and their numbers and locations are thus so far unknown.In this paper, five cases of small coastal cities, two of which are located in the USA - Miami Beach and Milford - and three, in Australia (Mandurah), Bangladesh (Sundarbans), and South Africa (Knysna) respectively, are presented and discussed in view of adaptation to climate change.All cases reported vulnerability to sea level rise, with impacts compounded by increasing storminess (and even hurricanes of increasing intensity), tides, inland flooding and loss of natural habitat. Changes in precipitation resulted in both reduced river flow, leading to sediment deficits and coastal erosion (from the sea) or increased river flow with similar damage (from the land). Adaptation response was most clearly delineated along lines of development status with wealthier, more highly developed cities pursuing more technologically advanced solutions, whilst the two cities of the Global South focused more on governance and capacity building solutions.Also, it was clear from the Knysna case that if a city or municipality lacks the capacity to effectively respond with adaptation measures, local private wealth may dominate and result in haphazard adaptation responses to the detriment of regional adaptation planning efforts.While this paper provides a first step in establishing a richer understanding, it is clear that more case studies across global regions and economic categories are required to deepen our understanding of climate change adaptation planning in small cities.