2023 |
Do Amaral, F., Trung, T., Pellarin, T., & Gratiot, N. (2023). Datasets Of High-Resolution Water Level And Discharge From The Saigon-Dong Nai Estuary System Impacted By A Developing Megacity, Ho Chi Minh City-ViETNAm. Data In Brief, .
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Le, H., Nguyen, T., Gratiot, N., Deleersnijder, E., & Soares-Frazao, S. (2023). The Multi-Channel System Of The Vietnamese Mekong Delta: Impacts On The Flow Dynamics Under Relative Sea-Level Rise Scenarios. Water, 151(202).
Abstract: The Mekong Delta Has The World'S Third-Largest Surface Area. It Plays An Indisputable Role In The Economy And Livelihoods Of Vietnam And Cambodia, With Repercussions At Regional And Global Scales. During Recent Decades, The Vietnamese Part Of The Mekong Delta Underwent Profound Human Interventions (Construction Of Dykes And Multi-Channel Networks), Which Modified The Hydrodynamic Regime, Especially Cycles Of Field Submersion. In This Study, We First Applied A Full 2D Numerical Hydraulic Model, Telemac-2D, To Examine The Effects Of The Complex Channel And River Networks On The Spatial And Temporal Distribution Of The Flow In The 40,000 Km2 Of The Vietnamese Mekong Delta. Then, Two Scenarios Of Relative Sea-Level Rise In 2050 And 2100 Were Implemented To Simulate The Future Patterns Of Water Fluxes In The Delta. The Results Show That Dykes And Multi-Channel Networks Would Reduce The Inundation Area By 36% And Lessen The Peak Water Level By 15% And The Discharge Over The Floodplains By 24%. Despite This Protection, Under A Relative Sea-Level Rise Of 30 Cm And 100 Cm, The Maximum Flooded Area Could Occupy About 69% And 85% Of The Whole Delta In 2050 And 2100, Respectively.
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2022 |
Nguyen, A., Dao, T., Strady, E., Nguyen, T., Aime, J., Gratiot, N., et al. (2022). Phytoplankton characterization in a tropical tidal river impacted by a megacity: the case of the Saigon River (Southern Vietnam). Environmental Science And Pollution Research, .
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Nguyen, A. T., Nemery, J., Gratiot, N., Dao, T. S., Le, T. T. M., Baduel, C., et al. (2022). Does Eutrophication Enhance Greenhouse Gas Emissions In Urbanized tropical estuaries? Environmental Pollution, 3033.
Abstract: Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO2) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. In this study, we assessed the concentrations and fluxes of CO2, methane (CH4) and nitrous oxide (N2O) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO2, CH4 and N2O at the Saigon River in 2019-2020 were 3174 +/- 1725 μgC-CO2 L-1, 5.9 +/- 16.8 μgC-CH4 L-1 and 3.0 +/- 4.8 μgN-N2O L-1, respectively. Their concentrations were 13-18 times, 52-332 times, and 9-37 times higher than the global mean concentrations of GHGs, respectively. While CO2 concentration had no clear seasonal pattern, N2O and CH(4 )concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated N2O concentrations in this urban area of the estuary. The high concentration of CO2 was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions.
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Nguyen, A. T., Nemery, J., Gratiot, N., Dao, T. S., Le, T. T. M., Baduel, C., et al. (2022). Does Eutrophication Enhance Greenhouse Gas Emissions In Urbanized tropical estuaries? Environmental Pollution, 3033.
Abstract: Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO2) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. In this study, we assessed the concentrations and fluxes of CO2, methane (CH4) and nitrous oxide (N2O) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO2, CH4 and N2O at the Saigon River in 2019-2020 were 3174 +/- 1725 μgC-CO2 L-1, 5.9 +/- 16.8 μgC-CH4 L-1 and 3.0 +/- 4.8 μgN-N2O L-1, respectively. Their concentrations were 13-18 times, 52-332 times, and 9-37 times higher than the global mean concentrations of GHGs, respectively. While CO2 concentration had no clear seasonal pattern, N2O and CH(4 )concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated N2O concentrations in this urban area of the estuary. The high concentration of CO2 was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions.
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2021 |
Camenen, B., Gratiot, N., Cohard, J., Gard, F., Tran, V., Nguyen, A., et al. (2021). Monitoring discharge in a tidal river using water level observations: Application to the Saigon River, Vietnam. Science Of The Total Environment, 761.
Abstract: The hydrological dynamics of the Saigon River is ruled by a complex combination of factors, which need to be disentangled to prevent and limit risks of flooding and salt intrusion. In particular, the Saigon water discharge is highly influenced by tidal cycles with a relatively low net discharge. This study proposes a low-cost technique to estimate river discharge at high frequency (every 10 min in this study). It is based on a stage-fall-discharge (SFD) rating curve adapted from the general Manning Strickler law, and calibrated thanks to two ADCP campaigns. Two pressure sensors were placed at different locations of the river in September 2016: one at the centre of Ho Chi Minh City and one in Phu Cuong, 40 km upstream approximately. The instantaneous water discharge data were used to evaluate the net residual discharge and to highlight seasonal and inter-annual trends. Both water level and water discharge show a seasonal behaviour. Rainfall, including during the Usagi typhoon that hit the megalopolis in November 2018, has no clear and direct impact on water level and water discharge due to the delta flat morphology and complex response between main channel and side channel network and ground water in this estuarine system under tidal influence. However, we found some evidences of interactions between precipitation, groundwater, the river network and possibly coastal waters. This paper can be seen as a proof of concept to (1) present a low-cost discharge method that can be applied to other tidal rivers, and (2) demonstrate how the high-frequency discharge data obtained with this method can be used to evaluate discharge dynamics in tidal river systems. (C) 2020 Elsevier B.V. All rights reserved.
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Nakhle, P., Ribolzi, O., Boithias, L., Rattanavong, S., Auda, Y., Sayavong, S., et al. (2021). Effects of hydrological regime and land use on in-stream Escherichia coli concentration in the Mekong basin, Lao PDR. Scientific Reports, 11(1).
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Nguyen, A., Nemery, J., Gratiot, N., Garnier, J., Dao, T., Thieu, V., et al. (2021). Biogeochemical functioning of an urbanized tropical estuary: Implementing the generic C-GEM (reactive transport) model. Science Of The Total Environment, 784.
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2020 |
Aires, F., Venot, J., Massuel, S., Gratiot, N., Pham-Duc, B., & Prigent, C. (2020). Surface Water Evolution (2001-2017) at the Cambodia/Vietnam Border in the Upper Mekong Delta Using Satellite MODIS Observations. Remote Sensing, 12(5).
Abstract: Studying the spatial and temporal distribution of surface water resources is critical, especially in highly populated areas and in regions under climate change pressure. There is an increasing number of satellite Earth observations that can provide information to monitor surface water at global scale. However, mapping surface waters at local and regional scales is still a challenge for numerous reasons (insufficient spatial resolution, vegetation or cloud opacity, limited time-frequency or time-record, information content of the instrument, lack in global retrieval method, interpretability of results, etc.). In this paper, we use 17 years of the MODIS (MODerate-resolution Imaging Spectro-radiometer) observations at a 8-day resolution. This satellite dataset is combined with ground expertise to analyse the evolution of surface waters at the Cambodia/Vietnam border in the Upper Mekong Delta. The trends and evolution of surface waters are very significant and contrasted, illustrating the impact of agriculture practices and dykes construction. In most of the study area in Cambodia. surface water areas show a decreasing trend but with a strong inter-annual variability. In specific areas, an increase of the wet surfaces is even observed. Ground expertise and historical knowledge of the development of the territory enable to link the decrease to ongoing excavation of drainage canals and the increase of deforestation and land reclamation, exposing flooded surfaces previously hidden by vegetation cover. By contrast, in Vietnam, the decreasing trend in wet surfaces is very clear and can be explained by the development of dykes dating back to the 1990s with an acceleration in the late 2000s as part of a national strategy of agriculture intensification. This study shows that coupling satellite data with ground-expertise allows to monitor surface waters at mesoscale (<100 x 100 km(2)), demonstrating the potential of interdisciplinary approaches for water ressource management and planning.
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Le, H., Gratiot, N., Santini, W., Ribolzi, O., Tran, D., Meriaux, X., et al. (2020). Suspended sediment properties in the Lower Mekong River, from fluvial to estuarine environments. Estuarine Coastal And Shelf Science, 233.
Abstract: The Mekong river is one of the largest rivers in the world, which flows through six countries of Southeast Asia (China, Myanmar, Laos, Thailand, Cambodia and Vietnam). Its hydro-sedimentary regime is changing rapidly, as a consequence of a regional shift of land use (agriculture, road, etc.), damming, sand mining and climate changes, among others. This study assesses the behavior of particles transported in suspension in the Lower Mekong River (LMR), along approximately 1700 km from fluvial to estuarine environments. Suspended sediment properties were estimated, simultaneously with hydrodynamic conditions, during three field campaigns. In addition, further investigations were performed in the laboratory to assess the structures of particles (flocculated or not), their capacity to flocculate (and the impacts on siltation), under a wide range of sediment concentration (20-30,000 mg.L-1). This study confirms that suspended sediment transported in the LMR are predominantly (75% by volume) flocculi (or freshly eroded soils aggregates), with median aggregated particle size in the range 10-20 μm and median settling velocity of the order of 0.01-0.1 μm s(-1). These flocculi are robust under the hydrodynamic conditions (turbulence and suspended sediment concentration – SSC) existing in the LMR. Laboratory investigations reveal the existence of a threshold sediment concentration (400 mg.L-1), beyond which flocculation and sedimentation increase of orders of magnitudes. Thus, concentration that exceeds this threshold might promote the formation of so-called fluid mud layers. Because of the nonlinear response of flocculation and sedimentation with SSC and considering the ongoing changes at a regional scale in the LMR, higher occurrence of fluid mud layers in the fluvial upstream waterbodies might be anticipated, and a lower occurrence in estuaries and alongshore where the concentration decrease. The geomorphology could be impacted, with an over-siltation in dams and an exacerbated erosion of the muddy-mangrove coast.
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Le, H., Lambrechts, J., Ortleb, S., Gratiot, N., Deleersnijder, E., & Soares-Frazao, S. (2020). An implicit wetting-drying algorithm for the discontinuous Galerkin method: application to the Tonle Sap, Mekong River Basin. Environmental Fluid Mechanics, 20(4), 923–951.
Abstract: The accurate simulation of wetting-drying processes in floodplains and coastal zones is a challenge for hydrodynamic modelling, especially for long time simulations. Indeed, dedicated numerical procedures are generally time-consuming, instabilities can occur at the wet/dry front, rapid transition of wet/dry interface and mass conservation are not always ensured. We present the extension of an existing wetting-drying algorithm in two space dimensions and its application to a real case. The wetting-drying algorithm is implemented in Second-generation Louvain-la-Neuve Ice-ocean Model (), a discontinuous Galerkin finite element model solving the shallow water equations in a fully implicit way. This algorithm consists in applying a threshold value of fluid depth for a thin layer and a blending parameter in order to guarantee positive values of the water depth, while preserving local mass conservation and the well balanced property at wet/dry interfaces. The technique is first validated against standard analytical test cases (Balzano 1, Balzano 3 and Thacker test cases) and is subsquently applied in a realistic domain, the Tonle Sap Lake in the Mekong River Basin, where the water level can vary by about 10 m between the dry and the wet season.
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Nguyen, T., N?Mery, J., Gratiot, N., Garnier, J., Strady, E., Nguyen, D., et al. (2020). Nutrient budgets in the Saigon-Dongnai River basin: Past to future inputs from the developing Ho Chi Minh megacity (Vietnam). River Research And Applications, 36(6), 974–990.
Abstract: Ho Chi Minh City (HCMC, Vietnam) is one of the fastest growing megacities in the world. In this paper, we attempt to analyse the dynamics of nutrients, suspended sediments, and water discharges in its aquatic systems today and in the future. The work is based on nine sampling sites along the Saigon River and one on the Dongnai River to identify the reference water status upstream from the urban area and the increase in fluxes that occur within the city and its surroundings. For the first time, the calculated fluxes allow drawing up sediment and nutrient budgets at the basin scale and the quantification of total nutrient loading to the estuarine and coastal zones (2012-2016 period). Based on both national Vietnamese and supplementary monitoring programs, we estimated the water, total suspended sediment, and nutrients (Total N, Total P, and dissolved silica: DSi) fluxes at 137 m(3)year(-1), 3,292 x 10(3)tonSS year(-1), 5,323 tonN year(-1), 450 tonP year(-1), and 2,734 tonSi year(-1)for the Saigon River and 1,693 m(3)year(-1), 1,175 x 10(3)tonSS year(-1), 31,030 tonN year(-1), 1,653 tonP year(-1), and 31,138 tonSi year(-1)for the Dongnai River, respectively. Nutrient fluxes provide an indicator of coastal eutrophication potential (indicator of coastal eutrophication potential), using nutrient stoichiometry ratios. Despite an excess of nitrogen and phosphorus over silica, estuarine waters downstream of the megacity are not heavily impacted by HCMC. Finally, we analysed scenarios of future trends (2025-2050) for the nutrient inputs on the basis of expected population growth in HCMC and improvement of wastewater treatment capacity. We observed that without the construction of a large number of additional wastewater treatment plants, the eutrophication problem is likely to worsen. The results are discussed in the context of the wastewater management policy.
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Noncent, D., Strady, E., Nemery, J., Thanh-Nho, N., Denis, H., Mourier, B., et al. (2020). Sedimentological and geochemical data in bed sediments from a tropical river-estuary system impacted by a developing megacity, Ho Chi Minh City-Vietnam. Data In Brief, 31.
Abstract: Sedimentological and geochemical data were obtained for bed sediments from a tropical estuary environment in Vietnam in October 2014, January 2016, and November 2016. The data include grain-size distribution, percentage of clay, silt and sand, percentage of organic matter, concentration of total particulate phosphorus (TPP), concentration of particulate inorganic phosphorus (PIP), concentration of particulate organic phosphorus (POP), percentage of total nitrogen (TN), percentage of total carbon (TC), trace metals concentrations (V, Cr, Co, Ni, Cu, Zn, As, Mo, Cd, Pb) and major elements (Al, Fe, Mn). Geochemical indexes (Enrichment factor EF and Geo-accumulation Index I-geo) and sediment quality guideline (mean Effect Range Median quotients) were calculated. (C) 2020 The Author(s). Published by Elsevier Inc.
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2019 |
Babut, M., Mourier, B., Desmet, M., Simonnet-Laprade, C., Labadie, P., Budzinski, H., et al. (2019). Where has the pollution gone? A survey of organic contaminants in Ho Chi Minh city / Saigon River (Vietnam) bed sediments. Chemosphere, 217, 261–269.
Abstract: A wide range of persistent organic chemicals, including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), some insecticides, as well as polybrominated diphenyl ethers (PBDEs) and some perfluoroalkyl substances (PFASs) were analyzed in 17 bed sediments collected along the Saigon River and at adjacent canal mouths from upstream to downstream in Ho Chi Minh City (Vietnam). Concentrations were rather low for PAHs, as well as for legacy PCBs and dichloro-diphenyl-trichlorethane and metabolites (DDTs), or below detection limits for several PFASs and all PBDEs measured. Several insecticides (chlorpyrifos-ethyl, and the pyrethroids cypermethrin and lambda-cyhalothrin) displayed rather high concentrations at a few sites within the city. There was no distinct upstream – downstream trend for PAHs, (DDTs) or PCBs. Although adjacent canal sediments tended to be more contaminated than Saigon River sediments, the differences were not significant. Emissions are almost certainly substantial for PAHs, and probably also for other contaminants such as PBDEs and some PFASs. During the dry season, contaminants are presumably stored in the city, either in canals or on urban surfaces. Heavy rainfall during the monsoon period carries away contaminated particle flows into the canals and then the Saigon River. The strong tidal influence in the river channel hinders the accumulation of contaminated particles. Contaminated deposits should accordingly be investigated further downstream in depositional environments, such as the mangrove. (C) 2018 Elsevier Ltd. All rights reserved.
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Besset, M., Gratiot, N., Anthony, E., Bouchette, F., Goichot, M., & Marchesiello, P. (2019). Mangroves and shoreline erosion in the Mekong River delta, Viet Nam. Estuarine Coastal And Shelf Science, 226.
Abstract: 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.
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Brunier, G., Anthony, E., Gratiot, N., & Gardel, A. (2019). Exceptional rates and mechanisms of muddy shoreline retreat following mangrove removal. Earth Surface Processes And Landforms, 44(8), 1559–1571.
Abstract: Probably the largest regular shoreline fluctuations on Earth occur along the 1500 km-long wave-exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large-scale colonization by mangroves, whereas mangrove-colonized areas between banks (inter-bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976-2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to -200 m/yr, and no recovery over the 38-year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave-reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter-bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re-establishment even when there is no sustained long-term deficit in mud supply, as in the case of the Amazon-influenced Guianas coast. (c) 2019 John Wiley & Sons, Ltd.
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Marchesiello, P., Nguyen, N., Gratiot, N., Loisel, H., Anthony, E., Dinh, C., et al. (2019). Erosion of the coastal Mekong delta: Assessing natural against man induced processes. Continental Shelf Research, 181, 72–89.
Abstract: The Lower Mekong Delta Coastal Zone (LMDCZ) is emblematic of the coastal erosion problem facing many tropical deltas. Over the last 3500 years, large river sediment fluxes expanded the delta seaward, and waves and currents formed the Ca Mau Peninsula to the southwest. Since the middle of the 20th century, the LMDCZ is affected by various human activities that include reduction of river fluxes due to damming and sand mining, land subsidence due to groundwater extraction, and reduction of protective coastal mangroves in favor of agriculture and aquaculture. Coastal erosion is observed along many sections of the delta, with a rate of up to 50 m per year in some areas. However, the role of human activities remains difficult to assess because of its complexity. The present modeling study is designed to sort out the contribution of natural hydrodynamic redistribution of sediments from man-induced erosion. The modeling system used is based on CROCO, forced by global reanalyses at the boundaries and at the surface, including wave statistics (required by the sediment transport model). Tides and realistic river forcing are also included. Calibration and validation relies on a combination of in situ and remotely-sensed observations, and laboratory experiments. Once validated, coastal dynamics are investigated by performing sensitivity experiments for both the hydro- and sediment dynamics. The results suggest that while wind is the main factor driving the coastal currents, the sediment dynamics is essentially the result of re-suspension due to wave-induced bed shear stress. The suspended sediments are then redistributed by coastal currents that are not limited to the nearshore zone. Strong seasonality of the process is observed with the northeast winter monsoon being the season of strongest re-suspension and sediment redistribution. The annual sediment budget is characterized by important local disequilibrium, with alongshore patterns that are in agreement with the observed shoreline evolution. The effect of a decrease in river sediment supply is difficult to evaluate because the estuarine zone is still in accretion, apart from the particular case of Go Cong shores. Far from the estuarine zone, subsidence is an additional strong candidate to explain erosion in areas that should naturally be accreting. Synthesizing these results, the study proposes a first attempt at a “taxonomy and geography” of processes along the coastal Mekong delta that can explain the recent observations of shoreline changes and help design protection measures.
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Nguyen, T., Nemery, J., Gratiot, N., Garnier, J., Strady, E., Tran, V., et al. (2019). Phosphorus adsorption/desorption processes in the tropical Saigon River estuary (Southern Vietnam) impacted by a megacity. Estuarine Coastal And Shelf Science, 227.
Abstract: The Saigon River flows through one of the most rapidly growing megacities of Southeast Asia, Ho Chi Minh City (HCMC, > 8.4 million inhabitants). This tidal river is characterized by a tropical monsoon climate, alternating a wet and a dry season. In the last few decades, increased economic and urban developments of HCMC have led to harmful impacts on the water quality of this tidal river, with severe eutrophication events. This situation results from the conjunction of contrasting hydrological seasons and the lack of upgraded sanitation infrastructures: indeed, less than 10% of the domestic wastewater is collected and treated before being discharged directly into urban canals or rivers. This study focuses on P dynamics because this is considered the key nutrient factor controlling freshwater eutrophication. Based on field measurements and original laboratory experiments, we assessed the P levels in the river water and sediments, and investigated P adsorption/desorption capacity onto suspended sediment (SS) within the salinity gradient observed. Field surveys showed a clear impact of the HCMC megacity on the total P content in SS, which increased threefold at HCMC Center, as compared with the upstream values (0.3-0.8 gP kg(-1)). Downstream, in the mixed estuarine area, the Total P was lower than 0.5 gP kg(-1). Laboratory experiments were carried out to characterize the influence of SS concentrations (SS = [0.25-0.9] g L-1 ), salinity (S = [2.6-9.3]) and turbulence (G = [22-44] s(-1)) on the sorption capacity of P onto sediment. The size of sediment particles and their propensity to flocculate were also originally measured with a recently developed instrument: the System for the Characterization of Aggregates and Flocs (SCAF (R)). Under the experimental conditions considered, SS concentrations had the greatest effect on the adsorption of P onto sediment, e.g., P adsorption capacity increased when SS concentrations rose. In contrast, salinity and turbulence had a smaller effect on the adsorption properties of sediments. Among these observed variables, the SS concentration was shown to be the main driver for adsorption capacity of P onto SS within the salinity gradient. We discuss the implication of these findings on understanding P dynamics within a highly urbanized, tropical estuary.
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Nguyen, T., Nemery, J., Gratiot, N., Strady, E., Tran, V., Nguyen, A., et al. (2019). Nutrient dynamics and eutrophication assessment in the tropical river system of Saigon – Dongnai (southern Vietnam). Science Of The Total Environment, 653, 370–383.
Abstract: Saigon-Dongnai Rivers in Southern Vietnam is a complex lowland hydrological network of tributaries that is strongly influenced by the tidal cycles. The increasing economic, industrial and domestic developments in and around Ho ChiMinh City (HCMC) have led to serious impacts on water quality due to lack of appropriate wastewaters treatment. Drinkingwater production is impacted and the large aquaculture production areas may also be affected. We analyzed spatial and seasonal variability of nutrient concentrations (Phosphorus, Nitrogen and Silica) and eutrophication indicators (Organic Carbon, Chlorophyll-a and Dissolved Oxygen) based on bi-monthly monitoring during two hydrological cycles (July 2015-December 2017). Four monitoring sites were selected to assess the impact of HCMC: two upstream stations on the Saigon River and Dongnai River branches to provide the reference water quality status before reaching the urbanized area of HCMC; one monitoring station in the city center to highlight Saigon River water quality within the heart of the megacity; the fourth station downstream of the confluence to evaluate the impact of HCMC on the estuarine waters. This study points to excess nutrients in HCMC's water body with concentrations of NH4+ and PO43- averaging to 0.7 +/- 0.6 mgN L-1 and 0.07 +/- 0.06 mgP L-1, respectively in mean over the monitored period and rising up to 3 mgN L-1 and 0.2 mgP L-1, in extreme conditions. During the dry season, we evidenced that untreated domestic discharges leads to degradation of the Saigon River's water quality with extreme values of algal biomass (up 150 μChl-a L-1) and hypoxic conditions occurring episodically (DO < 2 mg L`(-1)) in the heart of the megacity. Until now, eutrophication in the urban center has had no clear effect downstream because eutrophicwater mass from the Saigon River is efficiently mixed with the Dongnai River and sea water masses during the successive semi-diurnal tidal cycles. (c) 2018 Elsevier B.V. All rights reserved.
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2018 |
Vachaud, G., Quertamp, F., Phan, T. S. H., Tran Ngoc, T. D., Nguyen, T., Luu, X. L., et al. (2018). Flood-related risks in Ho Chi Minh City and ways of mitigation. Journal of Hydrology, .
Abstract: With an ever-growing population of around 10 million inhabitants (officially 7.9 in 2013), Ho Chi Minh City (HCMC) is set to become one of the largest cities in South East Asia and already occupies a major economic role in the area. To accommodate the increasing population, the megacity now stretches out in an urban continuum covering more than 800 square kilometers and is currently growing at a rate of 3.2% per year. If the neighboring provinces around HCMC are included, the total population reaches nearly 18 million people. This paper attempts to describe the interplay between HCMC and flood-related risks and offer some guidelines to deal with inundations. The potential risks of flooding by rain, tsunami and/or dam failure upstream of the city are evaluated and contextualized within the perspective of climate and human-induced environmental changes. The region is highly vulnerable to the combined effects of subsidence and rising sea levels and has already led to serious flooding that may extend spatially before the end of the century. We propose possible preventative solutions to urban flooding using a multi-pronged approach to issues regarding urban development and suggest a redevelopment strategy for major infrastructure projects.
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2017 |
Gratiot, N., Bildstein, A., Anh, T. T., Thoss, H., Denis, H., Michallet, H., et al. (2017). Sediment flocculation in the Mekong River estuary, Vietnam, an important driver of geomorphological changes. Comptes Rendus Geoscience, 349(6-7), 260–268.
Abstract: Over the past several decades, major hydro-sedimentary changes have occurred in both continental and coastal regions of the Mekong Delta, and this has severely impacted coastal erosion. A good characterization of floc properties and of their changes over time is necessary to gain comprehensive understanding and modelling of hydrodynamics and of the associated geomorphological changes. This paper quantifies the influence of sediment concentration, turbulence and differential particle settling on flocculation through field and laboratory investigations of the Mekong estuary. For concentrations lower than 200 mg.L-1, the particles do not exhibit a dynamic response to their environment, while for higher concentrations (up to 3-4 g.L-1), particle size and settling velocity increased by more than one order of magnitude. Flocculation by differential settling has not been sufficiently investigated yet, but this study reveals it as a predominant factor for siltation in quiescent environments such as the ones existing in the inner mangrove fringe. Such results are important to provide realistic simulations of the coastal evolution. (C) 2017 Academie des sciences. Published by Elsevier Masson SAS.
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2016 |
Gratiot, N., & Anthony, E. J. (2016). Role of flocculation and settling processes in development of the mangrove-colonized, Amazon-influenced mud-bank coast of South America. Marine Geology, 373, 1–10.
Abstract: The Guianas coast downdrift (northwestward) of the Amazon River mouth is characterized by the periodic migration of mud banks that originate from the Amazon. The characteristic sizes of these banks as well as their rates of alongshore migration have been estimated from remote sensing. However, the physical mechanisms leading to their displacement are not yet fully understood. The present work is aimed at investigating a number of micro-scale processes involved in the coastal and estuarine dynamics of fine sediments, and expected to occur during the migration of mud banks. The relative magnitudes of flocculation, hindered settling and consolidation have been determined. Sampling of coastal mud was carried out during a field survey in 2001 that focussed on the fluid mud layer in the leading edge of a mud bank in French Guiana. Settling column experiments were conducted under quiescent conditions for various mean sediment concentrations in the range of 2.5-110 g.l(-1), which is typical of mud bank concentrations. The time dependent vertical profiles of suspended sediment concentration were monitored using an optical settling tank equipped with 32 pre-calibrated optical sensors. The corresponding settling velocities were deduced from the equation of the conservation of mass. The results show that the timescales of hindered settling and consolidation processes are much larger than the timescales of mixing mechanisms such as tides or propagating waves. The individual floc settling velocities are too small to counterbalance the turbulent mixing induced by breaking waves. Hindered settling, favoured by wave action and by the high background suspended sediment concentrations, is thus an overarching process in wave-driven mud bank migration. This pervasive hindered settling regime is characteristic of the wave-exposed outer and leading edges of mud banks where active mobilization of mud assures mud bank migration. As a result, consolidation is theoretically precluded. The experiments pointed out, however, that additional flocculation and differential settling should enhance sedimentation during slack water and under low wave conditions. We deduce from this that enhanced settling in the inner,subtidal-intertidal, parts of leading edges of banks during such conditions is important in the temporary sedimentation that generates gel-like fluid mud patches and mud bars. These low-energy inner, leading parts of banks form an accreted substrate colonized by mangroves, which further contribute to enhanced settling and rapid sedimentation. (C) 2015 Elsevier B.V. All rights reserved.
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Gratiot, N., Mercier, B., Legout, C., Wendling, V., Coulaud, C., & Mora, H. (2016). UNIT FOR MEASURING THE SETTLING VELOCITY OF PARTICLES IN SUSPENSION IN A FLUID AND DEVICE COMPRISING AT LEAST SAID UNIT AND AN AUTOMATIC SAMPLER. Munich: European Patent Office.
Abstract: A unit for measuring the falling speed of particles in suspension in a fluid comprises a sealed container (6) having an opening (9) and defining an open compartment (15) comprising the opening and intended to contain the fluid, and a sealed compartment (16), sealing means separating the open compartment from the sealed compartment, and means for measuring the falling speed placed in the sealed compartment and comprising at least three electromagnetic radiation emitters distributed along a longitudinal axis (L) of the open compartment, each emitter being oriented according to a radiation axis crossing the open compartment at different heights along the longitudinal axis, an equal number of receivers distributed along the longitudinal axis, each receiver being placed in the radiation axis of a corresponding emitter, means for controlling the emitters and the receivers, and a system for acquiring data connected to the receivers, the acquired data being used to obtain the falling speed of the particles and the change of same as a function of the height in the open compartment and as a function of time, said change quantifying the flocculation of the particles.
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Nemery, J., Gratiot, N., Doan, P. T. K., Duvert, C., Alvarado-Villanueva, R., & Duwig, C. (2016). Carbon, nitrogen, phosphorus, and sediment sources and retention in a small eutrophic tropical reservoir. Aquatic Sciences, 78(1), 171–189.
Abstract: Rapid urbanization and the absence of efficient water management policies are increasingly degrading the water quality of tropical reservoirs in developing countries. The small tropical reservoir of Cointzio, located in the Trans-Mexican Volcanic Belt, is a warm monomictic water body (surface area = 6 km(2) with short water residence time < 1 year) that is strategic to the drinking water supply of the city of Morelia and to downstream irrigation during the dry season (6 months of the year). The reservoir faces two threats: (a) reduced water storage capacity due to sediment accumulation and (b) eutrophication caused by excess nutrients that likely come from untreated wastewaters in the upstream watershed. Intensive field measurements of water and sediment were conducted in 2009 to characterize the trophic status of the reservoir and to estimate nitrogen (N) and phosphorus (P) sources, total suspended sediment (TSS) (N), (P), and carbon (C) loads, and their accumulation or removal in the reservoir. We found that point sources represent the majority of N and P inputs to the reservoir. The trophic status is clearly eutrophic given the high chlorophyll a peaks (up to 70 A μg L-1) and a long period of anoxia (from May to October). Most of the TSS, C, N, and P were conveyed to the reservoir between June and October during the wet season. The TSS yield from the watershed was estimated at 35 +/- A 19 t km(-2) year(-1), of which more than 90 % was trapped in the reservoir (sediment accumulation rate = 7800 +/- A 2100 g m(-2) of reservoir year(-1)). The export load of C, N, and P downstream at the reservoir outlet was reduced by 31, 46 and 30 % respectively in comparison to the load at the reservoir inlet. This study reveals the effect of climatic seasonality on inputs to tropical reservoirs and accumulation; it also highlights the need to both reduce nutrient input to combat eutrophication and mitigate erosion to maintain the water storage capacity of the reservoir over the long term.
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Wendling, V., Legout, C., Gratiot, N., Michallet, H., & Grangeon, T. (2016). Dynamics of soil aggregate size in turbulent flow: Respective effect of soil type and suspended concentration. Catena, 141, 66–72.
Abstract: The fate of eroded soil particles impacts soil loss, river engineering and aquatic ecosystems. However, little is known about soil aggregate dynamics within the flow just after their detachment from the soil matrix. The relationship between particle size and turbulence has already been studied but few studies analysed the associated effect of particle concentration. The disaggregation/flocculation of three soils, two badland materials and a well developed calcareous brown soil, was studied by using a grid-stirred tank. An isotropic and homogeneous turbulence was generated to focus on the effects of suspended concentration on particle sizes. Increasing the suspended concentration in the range 1-10 g L-1 leads to a decrease of the proportion of the medium size particles and of an increase of the proportion of the smallest particles, as a consequence to enhanced abrasion. The soil aggregates with the largest organic content had the highest strength but were still subject to disaggregation within the turbulent flow, the resulting particle size depending on the suspended concentration. This study demonstrates that soil aggregate characteristics are influenced by concentration, this behaviour being dependent upon the soil type. (C) 2016 Elsevier B.V. All rights reserved.
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2015 |
Doan, P. T. K., Nemery, J., Schmid, M., & Gratiot, N. (2015). Eutrophication of turbid tropical reservoirs: Scenarios of evolution of the reservoir of Cointzio, Mexico. Ecological Informatics, 29, 192–205.
Abstract: This study provides the first numerical simulation of physical and biogeochemical processes in a very turbid and highly eutrophic tropical reservoir of the Trans-Mexican Volcanic Belt in Mexico. The Cointzio reservoir (capacity 66 Mm(3)) is characterized by a lack of water treatment plants upstream and a high content of very fine clay particles. It suffers serious episodes of eutrophication associated with high levels of turbidity and benthic anoxia. Different scenarios for future climate inputs, nutrient inputs and water levels were simulated. The results pointed out the potential negative long-term impact of climate change on this reservoir. In this region, an increase of average air temperature as high as 4.4 degrees C is expected by the end of the century. When coupled with a low water level, this increase could lead to critical conditions with a severe depletion of dissolved oxygen and important algal blooms with chlorophyll a reaching values of up to 94 μg L-1. The calculations indicated that a drastic reduction of nutrient inputs (up to 90%) would be required to significantly reduce chlorophyll a concentrations. If such mitigation measures are adopted, the maximum peak of chlorophyll a would be reduced by 55% after a ten-year period of efforts, with corresponding positive effect on dissolved oxygen concentrations. The main limitation of the chosen numerical approach comes from the assumption of a constant water level in the model. This may be particularly critical for shallow type reservoirs. While this region remains poorly studied, this study brings original information that will help stakeholders to adopt appropriate strategies for the management of their waterbodies that experience critical eutrophication. (C) 2015 Elsevier B.V. All rights reserved.
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Wendling, V., Gratiot, N., Legout, C., Droppo, I. G., Coulaud, C., & Mercier, B. (2015). Using an optical settling column to assess suspension characteristics within the free, flocculation, and hindered settling regimes. Journal Of Soils And Sediments, 15(9), 1991–2003.
Abstract: Instruments able to measure the settling velocity distribution (SVD) and investigate the flocculation behavior of suspensions for a wide range of concentrations and settling regimes are required to understand and model sediment transport in headwater catchments. Such knowledge will improve our water resource management capabilities. An optical settling column, equipped with a vertical array of optical sensors, was used to provide light transmission through a suspension during quiescent settling. A new method to determine the settling velocity and the propensity of suspensions to flocculate is proposed. Its reliability was evaluated based on settling tests for (1) noncohesive sediments, (2) cohesive sediments at medium (similar to 1 g l(-1)) concentration in a natural and deflocculated state, and (3) a cohesive sediment at a very high concentration (similar to 10 g l(-1)). This choice of sediments and concentrations allowed for the assessment of free, flocculated, and hindered settling regimes. The proposed data processing method provides measurements for a range of test conditions. The result showed that different populations of particles with different settling behaviors can be identified within the suspensions. In the case of noncohesive sediments, the proposed method provided SVD similar to those obtained with reference methods. The propensity to flocculate was zero as expected for inert material. The natural cohesive sediment at medium concentration exhibited a large range of SVD (10(-2)-10(-5) m s(-1)) and high propensity to flocculate. These were both reduced with the addition of a deflocculant. Identified particle behaviors were consistent with independent measurement of size distribution, microscopic, and erosion properties. In the hindered regime, a narrow SVD corresponded to the hindered front settling velocity (similar to 10(-4) m s(-1)). An optical settling column was able to provide reliable SVD and an evaluation of the propensity of particles to flocculate. The settling column was able to detect variations of the settling velocities with settled depth, thus highlighting that settling columns with a single measurement point may provide erroneous results by not accounting for the full spectrum of settling depth.
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2014 |
Nord, G., Gallart, F., Gratiot, N., Soler, M., Reid, I., Vachtman, D., et al. (2014). Applicability of acoustic Doppler devices for flow velocity measurements and discharge estimation in flows with sediment transport. Journal Of Hydrology, 509, 504–518.
Abstract: Acoustic Doppler devices (Unidata Starflow) have been deployed for velocity measurements and discharge estimates in five contrasted open-channel flow environments, with particular attention given to the influence of sediment transport on instrument performance. The analysis is based on both field observations and flume experiments. These confirm the ability of the Starflow to provide reliable discharge time-series, but point out its limitations when sediment is being transported. (i) After calibration of the instrument by the Index Velocity Method, the deviation from reference discharge measurements was <20% at the 95% confidence level. (ii) In ungauged conditions at high flows, the Starflow was particularly useful in providing velocity data for approximating measurements of discharge. (iii) However, channel and flume experiments revealed the effects of mobilised sediment on velocity estimates: coarse particles (>= 150 μm) transported by way of saltation or as bedload caused a significant underestimation of velocity by as much as 50%; a slight underestimation (10-15%) was also observed when significant quantities of fine particles (<= 150 μm) were transported in suspension; this underestimation was shown to reach 20-30% when suspended sediment concentrations were very high (c. 50-100 g L-1). (C) 2013 Elsevier B.V. All rights reserved.
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2013 |
Evrard, O., Poulenard, J., Nemery, J., Ayrault, S., Gratiot, N., Duvert, C., et al. (2013). Tracing sediment sources in a tropical highland catchment of central Mexico by using conventional and alternative fingerprinting methods. Hydrological Processes, 27(6), 911–922.
Abstract: Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright (c) 2012 John Wiley & Sons, Ltd.
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2012 |
Anthony, E. J., & Gratiot, N. (2012). Coastal engineering and large-scale mangrove destruction in Guyana, South America: Averting an environmental catastrophe in the making. Ecological Engineering, 47, 268–273.
Abstract: This short communication highlights potential destabilisation of the muddy coast of Guyana, South America, caused by large-scale mangrove destruction. The stability of the coast of Guyana, which is part of one of the world's most extensive mangrove coasts, depends on large mud banks migrating alongshore from the mouth of the Amazon River and on mangrove colonisation of these banks. Under the pressures of economic development, the coastal zone of Guyana is progressively being transformed into agricultural land and aquaculture estates, protected by coastal dikes. These hard coastal defence structures, constructed in recent years, are less effective in dissipating wave energy than mud banks. They also hinder the various processes involved in the consolidation and subsequent mangrove colonisation of these banks, notably by enclosing mature mangrove forests and preventing propagule transport from these forests to mud banks. If unchecked, the progressive breakdown in the mud-bank and associated mangrove system that has led to progradation of the coastal plain of Guyana over the last 5000 years will result in large-scale coastal erosion that can only be countered by further engineering structures at prohibitive costs. The only coastal defence strategy, sound and viable over the long term, with regards to both environmental conservation and cost, consists in restoring a dynamic mud-bank and mangrove system on this wave-exposed coast. (c) 2012 Elsevier B.V. All rights reserved.
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Duvert, C., Nord, G., Gratiot, N., Navratil, O., Nadal-Romero, E., Mathys, N., et al. (2012). Towards prediction of suspended sediment yield from peak discharge in small erodible mountainous catchments (0.45-22 km(2)) of France, Mexico and Spain. Journal Of Hydrology, 454, 42–55.
Abstract: The erosion and transport of fine-grained sediment in small mountainous catchments involve complex processes occurring at different scales. The suspended sediment yields (SSYs) delivered downstream are difficult to accurately measure and estimate because they result from the coupling of all these processes. Using high frequency discharge and suspended sediment data collected in eight small mountainous catchments (0.45-22 km(2)) from four distinct regions, we studied the relationships between event-based SSY and a set of other variables. In almost all the catchments, the event peak discharge (Q(max)) proved to be the best descriptor of SSY, and the relations were approximated by single power laws of the form SSY = alpha Q(max)(beta). The beta exponents ranged between 0.9 and 1.9 across the catchments, while variability in alpha was much higher, with coefficients ranging between 25 and 5039. The broad distribution of alpha was explained by a combination of site-specific physical factors, such as the percentage of degraded areas and hillslope gradient. Further analysis of the factors responsible for data dispersion in each catchment was carried out. Seasonality had a significant influence on variability; but overall, most of the scattering in the SSY-Q(max) regressions was explained by the short-lasting memory effects occurring between successive events (i.e. in-channel temporary storage and remobilization of sediment; antecedent moisture conditions). The predictability of SSY-Q(max) models was also assessed. Simulations of SSY per event and of annual SSY were conducted by using the computed regressions and the measured Q(max). Estimates of SSY per event were very uncertain. In contrast, annual SSY estimates based on the site-specific models were reasonably accurate in all the catchments, with interquartile ranges remaining in the +/- 50% error interval. The prediction quality of SSY-Q(max) relations was partly attributed to the statistical compensation that likely occurred between extreme values over a year; but it also suggests that the complex processes occurring at the event scale were smoothed at the annual scale. This SSY-Q(max) rating appears as a parsimonious predicting tool for roughly estimating SSY in small mountainous catchments. However, in its current form the technique needs further improvement as alpha and beta values need to be better constrained. (C) 2012 Elsevier B.V. All rights reserved.
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Grangeon, T., Legout, C., Esteves, M., Gratiot, N., & Navratil, O. (2012). Variability of the particle size of suspended sediment during highly concentrated flood events in a small mountainous catchment. Journal Of Soils And Sediments, 12(10), 1549–1558.
Abstract: Purpose There is a growing interest in the characterization of the particle size of sediment due to its impact on particle dynamics, especially for connectivity purpose. This study determined the particle size distribution of suspended sediment in a mountainous catchment, with the aim to evaluate the variability of particle size during floods, the main controlling factors, and if indirect information from hillslopes was useful for the interpretation of particle size measured at the catchment outlet. This work involved the development of a measurement protocol. Material and methods Samples were collected automatically from streamwater during flood events using an ISCO 3700 sampler. Five events were analyzed for their particle size distributions using a Malvern Mastersizer 2000. Because the samples were too concentrated, two different protocols were tested to address the errors made during the subsampling step: using a pipette and a home-made device with successive dilution phases. Results and discussion High errors occurred when using a pipette to extract particles within a stirred sample. The maximum errors were reduced from 1,600 to 30 % using the device described within this study. Particles were found to be aggregated at various levels regardless of the discharge they were sampled at. Their size was found to be either variable or stable at the event scale, and statistical analyses revealed that discharge was the factor that best correlated with particle size. The results obtained in this study are in agreement with the few other studies in comparable environments. Some hypothesis are put forward and discussed to explain the positive relationship between particle size and discharge. Input from hillslopes seems to have a measureable effect in this headwater catchment. Conclusions While the need for in situ measurements has long been stressed in lowland rivers, estuaries, and coastal environments, it was shown that the use of an accurate dilution protocol could provide some physically interpretable measurements on the particle size distributions of suspended sediment transported in a mountainous catchment. It also appears that hillslope information has to be considered when studying particle size measured at the catchment outlet.
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Navratil, O., Evrard, O., Esteves, M., Ayrault, S., Lefevre, I., Legout, C., et al. (2012). Core-derived historical records of suspended sediment origin in a mesoscale mountainous catchment: the River Bl,one, French Alps. Journal Of Soils And Sediments, 12(9), 1463–1478.
Abstract: Quantifying suspended sediment fluxes and dynamics across mountains, and identifying the origin of sediment in severely eroded areas, are of primary importance for the management of water resources. This contribution aims to generalise previous results from suspended sediment fingerprinting obtained during 2007-2009 in a mesoscale Alpine catchment (the Bl,one River; 905 km(2)) in France, and to assess variability in sediment sources throughout the second half of the twentieth century. Sediment fingerprinting, based on elemental geochemistry and radionuclide measurements, was conducted on a sediment core collected in an alluvial floodplain at the basin outlet. This technique was combined with hydro-sedimentary time-series to reconstruct the origin of suspended sediment deposited at this location over the last 50 years. Interpretation of sedimentation based on historical hydrological databases corroborates core dating obtained with Cs-137 and Pb-210(xs) activity measurements. Black marls and (marly) limestone sources provided the main fraction of sediment throughout the sequence (40 and 22 %, respectively). However, we also found evidence for the occurrence of major floods carrying large quantities of sediment originating from Quaternary deposits and conglomerates (25 and 16 %, respectively). The variability of sediment sources throughout the sequence may reflect the spatial variability of rainfall within the catchment, which in turn reflects its origin. However, the relatively homogeneous sediment composition throughout the sequence confirms that core-derived information is representative of widespread flood events. These results are consistent with those obtained in previous studies. They also outline the need to take into account the entire grain size range of fine sediment in order to provide an overall picture of sediment sources and transfers within highly erosive catchments. This study also emphasizes the importance of using archival data to validate the results of sediment fingerprinting studies conducted during short contemporary monitoring programmes, and to extend fingerprinting of sediment sources over longer time-scales which include large and widespread floods.
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2011 |
Duvert, C., Gratiot, N., Anguiano-Valencia, R., Nemery, J., Mendoza, M. E., Carlon-Allende, T., et al. (2011). Baseflow control on sediment flux connectivity: Insights from a nested catchment study in Central Mexico. Catena, 87(1), 129–140.
Abstract: In order to assess the extent of sediment connectivity between uplands and lowlands and to quantify the processes of in-channel deposition and remobilization, measurements of suspended sediment fluxes were conducted in a nested rural catchment of the Mexican Volcanic Belt. Data were collected over one year at three upland sites (3 to 12 km(2)) and two downstream stations (390-630 km(2)). Our results show that a structural discontinuity in the catchment (i.e. abrupt slope decrease at the junction between piedmonts and the alluvial plain from 2 to 10% to <0.1%) could be compensated by functional continuity during floods. Direct conveyance of fine sediment to the outlet occurred when a high stream transport capacity was reached. Erosion of the streambed was observed on various occasions and accounted for up to 50% of the flux leaving the catchment during one event. Conversely, temporary in-channel storage was apparent on other occasions, amounting to up to 52% of the flux recorded upstream during one storm. These two distinct behaviours were approximately equally distributed along the rainy season and strongly driven by the extent of coupling between surface and subsurface water. This work indeed highlights the role of baseflow spatial variations in determining the extent of lowland sediment conveyance. Riverbed erosional processes occurred when large differences in pre-event baseflow values (i.e. at least a twofold longitudinal increase) were observed between the 5-km distant lowland stations. Our findings outline the importance of systematically taking into consideration the baseflow parameter in research focusing on fine sediment transport across scales. (C) 2011 Elsevier B.V. All rights reserved.
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Duvert, C., Gratiot, N., Nemery, J., Burgos, A., & Navratil, O. (2011). Sub-daily variability of suspended sediment fluxes in small mountainous catchments – implications for community-based river monitoring. Hydrology And Earth System Sciences, 15(3), 703–713.
Abstract: Accurate estimates of suspended sediment yields depend on effective monitoring strategies. In mountainous environments undergoing intense seasonal precipitation, the implementation of such monitoring programs relies primarily on a rigorous study of the temporal variability of fine sediment transport. This investigation focuses on seasonal and short-term variability in suspended sediment flux in a subhumid region of the Mexican Volcanic Belt. Intensive monitoring was conducted during one year in four contrasting catchments (3 to 630 km(2)). Analyses revealed significant temporal variability in suspended sediment export over various time scales, with between 63 and 97% of the annual load exported in as little as 2% of the time. Statistical techniques were used to evaluate the sampling frequency required to get reliable estimates of annual sediment yield at the four sites. A bi-daily sampling scheme would be required at the outlet of the 630 km(2) catchment, whereas in the three smaller catchments (3-12 km(2)), accurate estimates would inevitably require hourly monitoring. At the larger catchment scale, analysis of the sub-daily variability of fine sediment fluxes showed that the frequency of sampling could be lowered by up to 100% (i.e. from bi-daily to daily) if a specific and regular sampling time in the day was considered. In contrast, conducting a similar sampling strategy at the three smaller catchments could lead to serious misinterpretation (i.e. up to 1000% error). Our findings emphasise the importance of an analysis of the sub-daily variability of sediment fluxes in mountainous catchments. Characterising this variability may offer useful insights for improving the effectiveness of community-based monitoring strategies in rural areas of developing countries. In regions where historical records based on discrete sampling are available, it may also help assessing the quality of past flux estimates. Finally, the study confirms the global necessity of acquiring more high frequency data in small mountainous catchments, especially in poorly gauged areas.
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Navratil, O., Esteves, M., Legout, C., Gratiot, N., Nemery, J., Willmore, S., et al. (2011). Global uncertainty analysis of suspended sediment monitoring using turbidimeter in a small mountainous river catchment. Journal Of Hydrology, 398(3-4), 246–259.
Abstract: A major challenge confronting the scientific community is to understand both patterns of and controls over spatial and temporal variability of suspended sediment dynamics in rivers, as these sediment govern nutriment export, river morphology, siltation of downstream reservoirs and degradation of water quality. High-frequency suspended sediment monitoring programs are required to meet this goal, particularly research in highly erodible mountainous catchments which supply the sediment load of the entire downstream fluvial network. However, in this context, analysis of the data and their interpretation are generally limited by many sources of uncertainty in river monitoring. This paper proposes to estimate the global uncertainty of suspended sediment monitoring using turbidimeter in a small mountainous river catchment (22 km(2); Southern French Alps). We first conducted a detailed analysis of the main uncertainty components associated with the turbidity approach, i.e. a widely used method to continuously survey the suspended sediment concentration (SSC). These uncertainty components were then propagated with Monte Carlo simulations. For individual records, SSC uncertainties are found to be on average less than 10%, but they can reach 70%. At the flood scale, the mean and the maximum SSC uncertainties are on average 20% (range, 1-30%), whereas sediment yield uncertainty is a mean 30% (range, 20-50% depending on the flood considered; discharge error, 20%). Annual specific sediment yield (SSY*) was then 360 +/- 100 t km(-2) year(-1). Uncertainty components associated with the automatic pumping procedure, discharge measurement and turbidity fluctuation at the short time scale were found to be the greatest uncertainties. SSC and SSY uncertainties were found highly site- and time-dependent as they vary significantly with the hydro-sedimentary conditions. This study demonstrates that global uncertainty accounts for only a small part of inter-flood SSC and SSY variability. It outlines the controlling factors of land use, relief, geology and rainfall regime on suspended sediment yields. (C) 2011 Elsevier B.V. All rights reserved.
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Sottolichio, A., Hurther, D., Gratiot, N., & Bretel, P. (2011). Acoustic turbulence measurements of near-bed suspended sediment dynamics in highly turbid waters of a macrotidal estuary. Continental Shelf Research, 31(10), S36–S49.
Abstract: Sediment-turbulence interactions near the bed are still poorly understood in highly turbid estuaries, especially in the presence of fluid mud layers. This results primarily from the difficulty in measuring co-located velocity and suspended sediment concentration (SSC) at sufficiently high rate to resolve small turbulent flow scales. In this paper, we show how a set of commercial acoustic and optical bacicscattering systems known as ADCPs, ADVs and OBSs, can be deployed and used in a complementary way to perform large-scale profilings of tidal current and SSC combined with high-resolution velocity and SSC measurements in the highly turbid near-bed zone. The experiment was done in the Gironde estuary (France) which is well known for its turbidity maximum zone characterized by high SSC values, above 1 g l(-1) near the surface. A first simple inversion method is proposed to convert the backscattered acoustic intensity measured with ADV into SSC data in the highly turbid near-bed zone. Near-bed SSC data from the OBS are used to compensate for the important acoustic sediment attenuation effect at an acoustic frequency of 6 MHz. No a priori knowledge of acoustic backscattering properties of mud suspensions is required with this calibration procedure. We obtain an attenuation coefficient for mud suspensions of 0.28 m(2)/kg at 6 MHz leading to a good agreement between the SSC timeseries from the three ADV receivers and the OBS over the entire tidal cycle. The obtained SSC data are then analyzed with respect to the near-bed velocity, Reynolds shear stress and turbulent kinetic energy (TKE) timeseries in order to identify the relevant sediment transport processes during the tidal cycle. Significant differences in bed shear stress and TICE levels are found between ebb and flood stages with effects on near-bed sediment dynamics. During the ebb, maximum levels of tidal current, bed shear stress and TKE are associated with a reduction of near-bed sediment concentration (from 400 kg m(-3) down to 100 kg m(-3)). Bed liquefaction process is assumed to occur at this moment with the presence of highly concentrated mud layer and a possible lutocline at a distance of less than 20 cm above the bed. During the first 1.5 h of flood, turbulent activity remains moderate. The near-bed flood current is then inhibited very abruptly while a sudden increase in SSC occurred above the bed. Assuming that the ADV is able to estimate relevant turbulent erosion fluxes, the co-located velocity and SSC are multiplied and compared with settling flux measurements made onboard under quiescent water conditions. The mean sediment settling fluxes (averaged over 3 min) increase with SSC and are in relative good coherence with fluxes in quiescent water below the hindered regime, for SSC below 15 g l(-1). Reducing averaging time from 3 min to 30 s allows to increase the range of turbulent fluxes and SSC values, up to 99 g l(-1). At this scale, fluxes keep increasing quasi-linearly at higher SSC, suggesting the inhibition (delay or reduction) of the hindered settling regime as previously shown by Gratiot et al. (2005) from laboratory experiments. However, the 3-min averaged concentration field remained too low to conclude definitively on the effectiveness of such a process. Further analysis conducted at higher SSC regimes and under fully verified equilibrium are necessary. (C) 2011 Elsevier Ltd. All rights reserved.
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2010 |
Anthony, E. J., Gardel, A., Gratiot, N., Proisy, C., Allison, M. A., Dolique, F., et al. (2010). The Amazon-influenced muddy coast of South America A review of mud-bank-shoreline interactions. Earth-Science Reviews, 103(3-4), 99–121.
Abstract: The 1500 km-long coast of South America between the Amazon and the Orinoco river mouths is the world s muddiest. This is due to the huge suspended-sediment discharge of the Amazon River (10(6) x 754 tons yr(-1) +/- 9%) part of which is transported alongshore as mud banks Mud-bank formation is controlled by the physical oceanography of the continental shelf seaward of the Amazon River mouth an initial seafloor storage area for much of the suspended sediment discharged from the river In this area, rapid and sustained fluid-mud concentration and trapping are associated with fresh water-salt water interaction and estuarine front activity on the shelf due to the enormous Amazon water discharge (ca 173 000 m(3)s(-1) at Obidos 900 km upstream of the mouth) Fluid mud is transported shoreward and then along the coasts of the Guianas by a complex interaction of wave and tidal forcing and wind-generated coastal currents The mud banks which may number up to 15 or more at any time are up to 5 m thick, 10 to 60 km-long and 20 to 30 km-wide and each may contain the equivalent mass of the annual mud supply of the Amazon As the banks migrate alongshore their interaction with waves results in complex and markedly fluctuating shorelines that are associated with space- and time-varying depositional bank phases and erosional Inter-bank phases Bank zones are protected from wave attack as a result of wave energy dampening by mud and undergo significant albeit temporary coastal accretion accompanied by rapid mangrove colonization The dampening of waves in bank areas as they propagate onshore is accompanied by the shoreward recycling of mud commonly in the form of individual mud bars These bars progressively undergo desiccation and consolidation and thus constitute a major pathway for rapid and massive colonization by mangroves Erosion by waves propagating across relatively mud-deficient shoreface zones in inter-bank areas can lead to muddy shoreline retreat rates of tens of metres to several kilometres over a few months to a few years accompanied by massive removal of mangroves Notwithstanding the higher incident wave energy on inter-bank shores inter bank shorefaces are permanently muddy due to the pervasive influence of the Amazon muddy discharge Inter bank and transitional bank-to-inter-bank phases are associated with both periodic sandy chenier formation and extreme forms of rotation of rare headland-bound sandy beaches The high mud supply from the Amazon has been the overarching geological control on the Quaternary evolution of the northeastern South American coast having led to the growth of a muddy shelf clinoform at the mouth of the Amazon and more or less important progradation throughout this coast. Net progradation reflects an imbalance in favour of deposition during each mud-bank-inter-bank cycle The high mud supply has presumably blanketed shelf sand deposited by smaller rivers during eustatic lowstand phases The shelf clinoform structure at the mouth of the Amazon and the muddy coastal progradation throughout the coast of the Guianas and into Venezuela provide analogues of the geological record on muddy shorefaces (c) 2010 Elsevier BV All rights reserved
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Duvert, C., Gratiot, N., Evrard, O., Navratil, O., Nemery, J., Prat, C., et al. (2010). Drivers of erosion and suspended sediment transport in three headwater catchments of the Mexican Central Highlands. Geomorphology, 123(3-4), 243–256.
Abstract: Quantifying suspended sediment exports from catchments and understanding suspended sediment dynamics within river networks is important, especially in areas draining erodible material that contributes to the siltation of downstream reservoirs and to the degradation of water quality. A one-year continuous monitoring study of water and sediment fluxes was conducted in three upland subcatchments (3.0, 9.3, and 12.0 km(2)) located within the Cointzio basin, in the central volcanic highlands of Mexico (Michoacan state). Two subcatchments generated high sediment exports (i.e., Huertitas, 900-1500 tkm(-2)y(-1) and Potrerillos, 600-800 tkm(-2)y(-1)), whereas the third subcatchment was characterized by a much lower sediment yield (i.e., La Cortina, 30 tkm(-2)y(-1)). Such disparities in subcatchment behaviours were associated with the presence of severely gullied areas in Huertitas and Potrerillos rather than with rainfall erosivity indices. An adapted classification of hysteretic patterns between suspended sediment concentration (SSC) and discharge was proposed because 42% of flood events contributing to 70% of sediment export were not discriminated by the classical clockwise/anticlockwise typology. This new classification allowed the identification of relationships in the hydrosedimentary responses of successive floods. A stream transport capacity limit was also detected during hydrograph recession phases. Overall, hydrosedimentary processes proved to be seasonally dependent: sediment export was repeatedly limited by the stream transport capacity during the first part of the rainy season, whereas a channel minimum erosivity threshold was frequently reached at the end of the season. (C) 2010 Elsevier B.V. All rights reserved.
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Evrard, O., Nemery, J., Gratiot, N., Duvert, C., Ayrault, S., Lefevre, I., et al. (2010). Sediment dynamics during the rainy season in tropical highland catchments of central Mexico using fallout radionuclides. Geomorphology, 124(1-2), 42–54.
Abstract: Tropical regions are affected by intense soil erosion associated with deforestation, overgrazing, and cropping intensification. This land degradation leads to important on-site (e.g., decrease in soil fertility) and off-site (e.g., reservoir siltation and water pollution) impacts. This study determined the mean soil particle and sediment residence times in soils and rivers of three subcatchments (3-12 km(2)) with contrasted land uses (i.e., cropland, forests, and rangelands) draining to a reservoir located in highlands of the transvolcanic Mexican belt. Calculations were based on rainfall amount and river discharges as well as on fallout radionuclide measurements (Be-7, Cs-137, and Pb-210) conducted on rainfall precipitated samples, soil sampled in the catchments, and suspended sediment collected by automatic samplers in the river during most storms recorded throughout the 2009 rainy season. Calculations using a radionuclide two-box balance model showed that the mean residence time of particles in soils ranged between 5000 1500 and 23,300 7000 years. In contrast, sediment residence time in rivers was much shorter, fluctuating between 50 30 and 200 70 days. The shortest mean residence times were measured in a hilly catchment dominated by cropland and rangelands, whereas they were the longest in an undulating catchment dominated by forests and cropland. Calculation of the Be-7/excess-Pb-210 in both rainfall and sediment allowed gaining insight on sediment dynamics throughout the rainy season. The first heavy storms of the year exported the bulk of the sediment stock accumulated in the river channel during the previous year. Then, during the rainy season, the two steeper catchments dominated by cropland and rangelands reacted strongly to rainfall. Sediment was indeed eroded and exported from both catchments during single heavy storms on several occasions in 2009. In contrast, the agro-forested catchment with gentler slopes exported sediment at a constant and low rate throughout the rainy season. Overall, land cover and flood type clearly proved to exert more control on sediment export than slope steepness and rainfall erosivity. Our results show the priority of stabilising old gully systems to prevent their extension by regressive erosion to cropland and to concentrate the implementation of on-site erosion control measures in cropland and rangeland of the most reactive catchments. (C) 2010 Elsevier B.V. All rights reserved.
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Gratiot, N. (2010). Impact des sédiments cohésifs sur l'écosystème. Habilitation thesis, Université Joseph Fourier, Grenoble, .
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Gratiot, N., Duvert, C., Collet, L., Vinson, D., Nemery, J., & Saenz-Romero, C. (2010). Increase in surface runoff in the central mountains of Mexico: lessons from the past and predictive scenario for the next century. Hydrology And Earth System Sciences, 14(2), 291–300.
Abstract: The hydrological response of a medium scale mountainous watershed (Mexico) is analysed over half a century. The hydrograph separation highlights an increasing surface runoff contribution since the early 1970's. This increase is attributed to land use changes while the meteorological forcing (rains) remains statistically stable over the same period. As a consequence, the intensity of annual extreme floods has tripled up over the period of survey, increasing flood risks in the region. The paper ends with a climatic projection over the 21st century. The decrease of precipitation and the increase of temperature should accentuate the trend engaged since the 1970's by reducing groundwater resources and increasing surface-runoff and associated risks.
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2009 |
Kelle, L., Gratiot, N., & de Thoisy, B. (2009). Olive ridley turtle Lepidochelys olivacea in French Guiana: back from the brink of regional extirpation? Oryx, 43(2), 243–246.
Abstract: The estimated number of olive ridley marine turtles Lepidochelys olivacea nesting annually in 2002-2007 in French Guiana was 1,716-3,257, the highest ever recorded in the country and similar to nesting numbers recorded in neighbouring Suriname c. 40 years ago, where the species has now severely declined. A shift of nesting females from Suriname to French Guiana beaches and improvement of nationwide marine turtle monitoring appear to be the most plausible explanations for the current high level of nesting recorded in French Guiana. The species' nesting status in French Guiana therefore appears less critical than previously documented but ongoing threats suggest the need to reinforce regional conservation efforts in the West Atlantic.
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Proisy, C., Gratiot, N., Anthony, E. J., Gardel, A., Fromard, F., & Heuret, P. (2009). Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data. Continental Shelf Research, 29(3), 632–641.
Abstract: Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as 'opportunistic' since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts. (C) 2008 Elsevier Ltd. All rights reserved.
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2008 |
Anthony, E. J., Dolique, F., Gardel, A., Gratiot, N., Proisy, C., & Polidori, L. (2008). Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana. Continental Shelf Research, 28(6), 813–822.
Abstract: The intertidal topography in the vicinity of the contact zone between a longshore-migrating Amazon-derived mud bank and the muddy terrestrial shoreline in French Guiana was defined from a combination of satellite-based SPOT images, airborne lidar data and high-resolution total station ground surveying of a 75,000 in 2 plot. The three approaches, at different scales, were carried out at different periods. Digital elevation models generated from these three techniques, however, converge in highlighting the topographic micro-scale (centimetre-scale) variability of the mud bank surface while showing meso- to macro-scale features that reflect the dominance of wave activity in mud bank mobilization and attachment to the terrestrial shoreline. These features are bar-like longshore forms that develop in the intertidal zone from the shoreward drift of gel-like mud that accompanies wave damping. The features progressively become consolidated through mud drying out associated with the formation of cracks that are important in mangrove colonization and ecological changes. Fluid-mud accumulations formed from high concentrations of mud trapped in the troughs behind these linear bar forms generate flat featureless surfaces that tend to mask topographic heterogeneity of the mud bank surface. Dewatering of these lower zones by progressive mud consolidation complements tidal water discharge in providing a mechanism for the formation of the numerous channels that dissect the linear bar features, especially in the upper intertidal contact zone with the terrestrial shoreline. This dissection in the upper intertidal zone generates an intricate topography that replaces the original linear bar forms. The innermost bar forms a 'suture' zone with the terrestrial shoreline. Reworking of this bar by high-energy waves may lead to mud dispersal over old terrestrial mangrove substrates, resulting in stifling of mangrove pneumatophores. Mud reworking at the narrow trailing edge of the Mud bank in the subtidal and lower intertidal zones leaves behind a flat bed that will eventually be completely croded by waves in the course of mud bank migration. (C) 2008 Elsevier Ltd. All rights reserved.
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Gratiot, N., Anthony, E. J., Gardel, A., Gaucherel, C., Proisy, C., & Wells, J. T. (2008). Significant contribution of the 18.6 year tidal cycle to regional coastal changes. Nature Geoscience, 1(3), 169–172.
Abstract: Although rising global sea levels will affect the shape of coastlines over the coming decades(1,2), the most severe and catastrophic shoreline changes occur as a consequence of local and regional-scale processes. Changes in sediment supply(3) and deltaic subsidence(4,5), both natural or anthropogenic, and the occurrences of tropical cyclones(4,5) and tsunamis(6) have been shown to be the leading controls on coastal erosion. Here, we use satellite images of South American mangrove-colonized mud banks collected over the past twenty years to reconstruct changes in the extent of the shoreline between the Amazon and Orinoco rivers. The observed timing of the redistribution of sediment and migration of the mud banks along the 1,500km muddy coast suggests the dominant control of ocean forcing by the 18.6 year nodal tidal cycle(7). Other factors affecting sea level such as global warming or El Nino and La Nina events show only secondary influences on the recorded changes. In the coming decade, the 18.6 year cycle will result in an increase of mean high water levels of 6 cm along the coast of French Guiana, which will lead to a 90 m shoreline retreat.
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2007 |
Gratiot, N., & Manning, A. J. (2007). A Laboratory Study of Dilute Suspension Mud Floc Characteristics in an Oscillatory Diffusive Turbulent Flow. Journal Of Coastal Research, , 1142–1146.
Abstract: To further the understanding of mud flocculation the COSINUS project funded a series of laboratory experiments whereby dilute mud suspensions were sheared (between 3.7 s(-1) – about 20 s-1) with nominal concentrations ranging from 200-600 mg l(-1), within a Plexiglas tank, using an oscillating grid. The floc properties were then examined using the LabSFLOC instrument. Results showed that for low concentrations of natural Tamar estuary (UK) mud exposed to high shear, the largest flocs were slightly less than the Kolmogorov eddy size of about 220 pm. The high shear resulted in all flocs having settling velocities of only 0.6 mm s(-1). Lowering the shear for the same mud improved flocculation, and raised the macrofloc settling velocity to 1.8 mm s(-1). This translated into macroflocs constituting 64% of the floc mass, and 80% of the mass settling flux. In contrast, low concentrations of natural Gironde mud displayed significantly faster settling macroflocs at each shear increment than Tamar mud, but the largest flocs were of the same order as the Tamar mud flocs. The suspended matter distribution showed the macroflocs only constituted 20-30% of the particulate mass. However, their faster settling rate transformed the low macrofloc mass into 62% of the settling flux. The removal of organic matter from the Gironde mud resulted in few flocs exceeding 1251 μm in diameter.
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Gratiot, N., Gardel, A., & Anthony, E. J. (2007). Trade-wind waves and mud dynamics on the French Guiana coast, South America: Input from ERA-40 wave data and field investigations. Marine Geology, 236(1-2), 15–26.
Abstract: The South American coast between Brazil and Venezuela is affected by longshore migrating mud banks derived from the fine-grained Amazon sediment discharge. Onshore mud migration prevails over shallow 'bank' areas alternating alongshore with deeper 'inter-bank' areas. The transport on the inner shelf, and attachment to the shoreline, of this migrating mud has been attributed mainly to wind waves. However, the lack of in situ data on waves hampers understanding of the relationship between waves and mud dynamics. A 44-yr record (1960-2004) of the ERA-40 wave dataset generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) was used, in conjunction with field investigations in French Guiana, to define both event-scale and longer-term patterns of mud mobilisation induced by waves. The ratio H-0(3)/T-2, combining wave height H and period T, and the angle of wave incidence alpha, were singled out as the most relevant parameters for describing wave forcing. Typical 'bank' and 'inter-bank' profiles and corresponding mud densities, and a 3-month record of changes in the thickness of the fluid mud layer in an estuarine navigation channel were monitored by echo-sounding from October 2002 to January 2003. An 80-day record of bed-level changes in the intertidal zone was obtained from August to November 2004 using a pressure transducer. The results on the wave regime of French Guiana confirm a distinctly seasonal pattern, and highlight an increase in H-0(3)/T-2 over the 44-yr period related to an increase in trade-wind velocities determined from corresponding trends in Atlantic wind pseudo-stress off the South American coast. Wave forcing over bank areas leads to the liquefaction of a 1-3 m-thick layer of mud that is transported onshore (and alongshore by the longshore component of wave energy). The episodic nature of high wave energy events generally results in the formation of mud bar features from the shoreward mobilisation of gel-like fluid mud. The effect of waves on mud is particularly marked following long periods of low energy, and especially at the onset of the high wave energy season (October to May), when even moderate wave energy events can lead to significant mobilisation of mud. Significant phases of increased wave energy are attended by higher long-term (annual) rates of longshore mud bank migration but the correlation is rather poor between the wave forcing parameter H-0(3)/T-2 and migration rates because stronger wave forcing is generally associated with low angles of wave incidence. This suggests a complementary role of other hydrodynamic mechanisms, such as geostrophic and tidal currents, in longshore mud bank migration. (c) 2006 Elsevier B.V. All rights reserved.
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Kelle, L., Gratiot, N., Nolibos, I., Therese, J., Wongsopawiro, R., & De Thoisy, B. (2007). Monitoring of nesting leatherback turtles (Dermochelys coriacea): Contribution of remote sensing for real-time assessment of beach coverage in French Guiana. Chelonian Conservation And Biology, 6(1), 142–147.
Abstract: Over 4 years, 2001-2004, leatherback-turtle monitoring was conducted on all the potential nesting sites in French Guiana. We estimated minimal leatherback turtle nest numbers of 23,107, 12,229, 13,480, 11,012, respectively. The Awala-Yalimapo Beach, sometimes considered a good estimator of the overall nesting activity for the country, has hosted a significant proportion of the leatherback turtle nests (42% +/- 2%), but this percentage is much lower than formerly described. The relative importance of this nesting site is discussed in light of remote sensing data, suggesting that nest numbers recorded in Awala-Yalimapo may have misrepresented leatherback turtle population trends. Indeed, remote sensing data indicate that the total sandy shoreline available in French Guiana has regularly evolved over the last decades, allowing leatherback turtle nesting attempts out of the scope of monitoring. The importance of a monitoring effort integrating the specific coastal dynamic of the Guianas region is highlighted.
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2006 |
Fiot, J., & Gratiot, N. (2006). Structural effects of tidal exposures on mudflats along the French Guiana coast. Marine Geology, 228(1-4), 25–37.
Abstract: Wetting and drying cycles on intertidal mudflats vary considerably with altitude, modifying the physical characteristics of surface sediment in ways favoring (or not) plant colonization. In this context, sediment properties were investigated by means of laboratory experiments and field surveys on a wide range of fluid to desiccated muds from the highly dynamic coastline of French Guiana. Changes in physical parameters, such as sediment erodability (yield stress), water loss and pore water salinity indicated a long term compaction of mudflats as well as fluctuations related to the successive wetting and drying cycles. Muderacks constituted a spectacular feature representative of the contractional stress. They (re)opened after a few days of dewatering and (re)healed during the subsequent wetting. From the analysis of field data, the trapping of Avicennia germinans propagules in ephemeral mudcracks turned out to be responsible of 95% of the sprouting on the coastal fringe. Thus, desiccation process, usually considered as a typical feature of erosion, revealed herein to be a major mechanism of colonization. This mechanism undoubtedly affects the 1600 km long Amazon coastal system and is believed to exist in many other tropical environments submitted to important siltation. (c) 2006 Elsevier B.V. All rights reserved.
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Gratiot, N., Gratiot, J., Kelle, L., & de Thoisy, B. (2006). Estimation of the nesting season of marine turtles from incomplete data: statistical adjustment of a sinusoidal function. Animal Conservation, 9(1), 95–102.
Abstract: Because of logistical and financial constraints, nest counts of marine turtles are often limited in time and space. To overcome this difficulty, we developed a numerical model that fits the seasonal pattern of marine turtles nesting from complete or fragmented datasets. The duration of the main nesting season, the position and amplitude of its maximum as well as the residual number of nests, outside of the main season are obtained numerically by a least square adjustment. For the seven complete time series at our disposal (Dermochelys coriacea and Lepidochelys olivacea turtles, coast of French Guiana), the model reproduces the seasonal pattern with a correlation of r >= 0.97. When applied on a fragmented dataset, the model accuracy depends on the duration and on the temporal distribution of the monitoring (effort equally distributed during the entire season or concentrated on a part of it only). As a result of this study, we clearly advocate a strategy of monitoring distributed all over the nesting season. Following this recommendation, the model estimates the annual number of nests with a median error lower than 10% when considering only 50 days of monitoring.
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2005 |
Gratiot, N., Michallet, H., & Mory, M. (2005). On the determination of the settling flux of cohesive sediments in a turbulent fluid. Journal Of Geophysical Research-Oceans, 110(C6).
Abstract: [1] In this paper we compare the settling flux of a cohesive sediment mixture measured in a quiescent fluid with that achieved in a turbulent flow. Experiments were performed in a mixing tank. The turbulence produced mechanically by an oscillating grid maintains a stationary, highly concentrated fluid mud layer in which the concentration is almost uniform. In this layer the turbulence decay with increasing distance from the grid is similar to that obtained in clear water. For steady conditions the settling flux of the fluid mud mixture is balanced by the upward turbulent flux, and its value can be determined from the measured depth of the fluid mud layer. At high concentrations ( 10 – 200 g L(-1)), we show that the settling flux in a turbulent fluid is much larger than estimated in a quiescent fluid ( up to two orders of magnitude). Hindering effects in the settling process of cohesive sediments may thus be considerably reduced by turbulence.
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