Sediment Accumulation In Reservoirs Research Articles

Predicting reservoir sedimentation using multilayer perceptron – Artificial neural network model with measured and forecasted hydrometeorological data in Gibe-III reservoir, Omo-Gibe River basin, Ethiopia

The estimation and prediction of the amount of sediment accumulated in reservoirs are imperative for sustainable reservoir sedimentation planning and management and to minimize reservoir storage capacity loss. The main objective of this study was to estimate and predict reservoir sedimentation using multilayer perceptron–artificial neural network (MLP-ANN) and random forest regressor (RFR) models in the Gibe-III reservoir, Omo-Gibe River basin. The hydrological and meteorological parameters considered for the estimation and prediction of reservoir sedimentation include annual rainfall, annual water inflow, minimum reservoir level, and reservoir storage capacity. The MLP-ANN and RFR models were employed to estimate and predict the amount of sediment accumulated in the Gibe-III reservoir using time series data from 2014 to 2022. ANN-architecture N4-100-100-1 with a coefficient of determination (R2) of 0.97 for the (80, 20) train-test approach was chosen because it showed better performance both in training and testing (validation) the model. The MLP-ANN and RFR models' performance evaluation was conducted using MAE, MSE, RMSE, and R2. The models’ evaluation result revealed that the MLP-ANN model outperformed the RFR model. Regarding the train data simulation of MLP-ANN and RFR shown R2 (0.99) and RMSE (0.77); and R2 (0.97) and RMSE (1.80), respectively. On the other hand, the test data simulation of MLP-ANN and RFR demonstrated R2 (0.98) and RMSE (1.32); and R2 (0.96) and RMSE (2.64), respectively. The MLP-ANN model simulation output indicates that the amount of sediment accumulation in the Gibe-III reservoir will increase in the future, reaching 110 MT in 2030–2031, 130 MT in 2050–2051, and above 137 MTin 2071–2072.

Effects of Dosing and Mechanical Activation on the Performance of Reservoir Sediment-Cement Composite Cementitious Material

To solve the problem of reservoir sediment accumulation, this paper intends to study the feasibility of preparing reservoir sediment-cement composite cementitious material by partially replacing cement with reservoir sediment. Using the single factor test method, the replacement rate of reservoir sediment and mechanical activation time were selected as variables to explore the effects of water requirement of normal consistency, setting time, mortar fluidity, and mechanical properties of mortar specimens. The results show that with the increase of sediment replacement rate, the water requirement of normal consistency of composite cementitious material paste increases, the setting time increases, the fluidity of mortar decreases, and the mechanical properties of mortar specimens decrease significantly. With the increase of mechanical activation time, the water requirement of normal consistency decreases, the setting time decreases, the fluidity of mortar increases, and the mechanical properties of the mortar specimen increase. The microcosmic results show that mechanical activation can promote the formation of C-S-H gel in reservoir sediment, and fill each other with unreacted reservoir sediment particles, resulting in a denser microstructure. The purpose of this paper is to provide theoretical support for the preparation of reservoir mud-cement composite cementitious material.

Sediment Yield Analysis in Tahtaköprü Dam Basin

The determination of the volume of water to be stored is one of the most important criteria for dam and pond planning and operation. This amount is calculated locally prior to planning studies, and a bathymetric map is created following water storage in the reservoir. Because of sedimentation during the operation period, the volume of the reservoir decreases over time, and this situation appears as a problem in the operation of the water structure. In this study, the reservoir storage volume changes and reservoir sediment accumulation rates between the locally produced map before the construction of Tahtaköprü Dam and the hydrographic survey maps prepared during operation period were investigated by using GIS modules. It was determined that the decrease in the storage volume of the dam reservoir at the maximum water level by 9.15% and at the minimum water level by 51.90% was due to sediment accumulation. The effect of the changes in the land cover and forest area in the basin on reservoir sedimentation was examined and it was found that land cover and forest area in the basin have reduced sediment accumulation in the reservoir. Furthermore, sediment thickness change was mapped to compare the decrease in the storage volume capacity of the reservoir at the maximum water level and at the minimum water level by comparing the digital elevation models of 1962, 1980 and 2014 to support the decision-makers on determining reservoir operation rules in order to benefit water resources effectively within the scope of sustainable management of water resources.

Soil erosion and sediment yield assessment using RUSLE and GIS-based approach in Anjeb watershed, Northwest Ethiopia

Soil erosion is a serious and continuous environmental problem in Ethiopia. Lack of land use planning, environmental protection, over-cultivation, and overgrazing are prominent causes of erosion and sedimentation. This study is conducted in Anjeb watershed located in the Upper Blue Nile Basin, Ethiopia. In this study, the quantity and distribution of soil erosion, sediment delivery ratio (SDR), and sediment yield of the watershed were assessed by employing remote sensing, geographic information system (GIS), and revised universal soil loss equation analysis capabilities. Important data sets of topography, soil, conservations practices, cover management, and rainfall factors were processed and superimposed in GIS analysis, and soil loss rate, SDR, and sediment yield of the watershed were derived. Based on the result found, the watershed was categorized into six classes of erosion: slight (0–5), moderate (5–10), high (10–15), very high (15–30), severe (30–50), and very severe (> 50) t ha−1 yr−1. The estimated average annual soil loss was 17.3 t ha−1 yr−1. The soil loss rate is higher in the steeper and topographically dissected part of the watershed. The average sediment delivery capacity was about 0.122. The result showed that the average sediment yield in the watershed was grouped into classes of low (< 2.5), moderate (2.5–7.5), high (7.5–12.5), very high (12.5–22.5), severe (22.5–40), and very severe (> 40) t ha−1 yr−1. It is found that from a total of 20,125.5 t yr−1 eroded soil over the whole watershed 2254.5 t yr−1 of sediment has been brought and deposited to the channels. Sediment accumulation from the watershed threatens the storage capacity and life span of Anjeb reservoir which is the source of irrigation water downstream. The study provides an insight to planners and resource managers to design and implement practices of watershed management to reduce erosion and enhance land productivity and to minimize the reservoir sediment accumulation.

Assessment of Chimkurgan water reservoir sedimentation processes

Uzbekistan is an inland country with very dry summers and cold winters. The main course of the economy is agriculture. There are two basic kinds of agriculture in terms of irrigation: rainfed and artificial irrigation systems. Therefore, in long dry summers it is reasonable to build water reservoirs to keep enough water resources for vegetation period. Because of sedimentation transport water reservoirs sometimes quickly filled with sediment. Estimating the effects of potential sediment accumulation in reservoirs is an important element in the planning of the dam project. Sediment accumulation in a reservoir may reduce the useful storage of water in this reservoir, change the water quality near the dam, increase flooding level upstream of the dam due to sediment aggradation, influence the stability of the stream downstream of the dam, affect stream ecology in the dam region, etc. This article provides a correlation between the reservoir volume reduction from the bottom to a given level, based on the amount of water flowing into the Chimkurgan reservoir and the river basin monitoring data. Based on the results of observations and calculations, the current state of the reservoir was formed on the basis of GIS tools. In the first step we constructed a flow duration curve, which is the cumulative distribution curve of the stream runoff passing the dam.

Stochastic and analytical approaches for sediment accumulation in river reservoirs

ABSTRACTSediment accumulation in a river reservoir is studied by stochastic time series models and analytical approach. The first-order moving average process is found the best for the suspended sediment discharge time series of the Juniata River at Newport, Pennsylvania, USA. Synthetic suspended sediment discharges are first generated with the chosen model after which analytical expressions are derived for the expected value and variance of sediment accumulation in the reservoir. The expected value and variance of the volume of sediment accumulation in the reservoir are calculated from a thousand synthetic time series each 38 years long and compared to the analytical approach. Stochastic and analytical approaches perfectly trace the observation in terms of the expected value and variability. Therefore, it is concluded that the expected value and variance of sediment accumulation in a reservoir could be estimated by analytical expressions without the cost of synthetic data generation mechanisms.

Ecological influence of sediment bypass tunnels on macroinvertebrates in dam-fragmented rivers by DNA metabarcoding

Sediment bypass tunnels (SBTs) are guiding structures used to reduce sediment accumulation in reservoirs during high flows by transporting sediments to downstream reaches during operation. Previous studies monitoring the ecological effects of SBT operations on downstream reaches suggest a positive influence of SBTs on riverbed sediment conditions and macroinvertebrate communities based on traditional morphology-based surveys. Morphology-based macroinvertebrate assessments are costly and time-consuming, and the large number of morphologically cryptic, small-sized and undescribed species usually results in coarse taxonomic identification. Here, we used DNA metabarcoding analysis to assess the influence of SBT operations on macroinvertebrates downstream of SBT outlets by estimating species diversity and pairwise community dissimilarity between upstream and downstream locations in dam-fragmented rivers with operational SBTs in comparison to dam-fragmented (i.e., no SBTs) and free-flowing rivers (i.e., no dam). We found that macroinvertebrate community dissimilarity decreases with increasing operation time and frequency of SBTs. These factors of SBT operation influence changes in riverbed features, e.g. sediment relations, that subsequently effect the recovery of downstream macroinvertebrate communities to their respective upstream communities. Macroinvertebrate abundance using morphologically-identified specimens was positively correlated to read abundance using metabarcoding. This supports and reinforces the use of quantitative estimates for diversity analysis with metabarcoding data.

Sustainable sediment management options for reservoirs: a case study of Chashma Reservoir in Pakistan

Globally the average annual loss of reservoir capacity is approximately 1%. Pakistan is confronting major issue of sedimentation which is continuously depleting the useful storage of reservoirs. GSTARS3 model was used to determine the rate of deposition and sediment pattern of Chashma Reservoir since its operation. The model was calibrated and validated for bathymetric survey of 2008 and 2012. The results of GSTARS3 were incorporated to a GIS software to visualize sediment accumulation in reservoir. The study reveals that sediment flushing of the Chashma Reservoir can be carried out during flood season at a pond level of 638.15 ft. (194.51 m). However, its negative impact if any on the hydropower generation needs to be analysed. Accordingly, modified operation rules would be required.

Estimates of sediment trapping rates for two reservoirs in the Lake Erie watershed: Past and present scenarios

Sediment accumulation can significantly impact the useful life of dams and the multiple functions served by those dams such as flood attenuation, hydropower, and water supply. However, there is only limited information, and even fewer physical measurements, assessing the rate of sediment accumulation in reservoirs behind dams. Many of the dams within the Great Lakes Watershed were constructed between 100 and 120years ago, and there is reasonable concern that these dams and their associated reservoirs may be reaching capacity with respect to sediment storage. As a reservoir reaches its sediment storage capacity, there are numerous risks. Excess sediment can compromise the water intake for supply systems. Dam failure or removal can potentially allow large quantities of impounded sediment to migrate downstream, negatively impacting fish habitat and water quality. This research investigates the historical function of dams as sediment storage points. Also, this research assesses the effect of anthropogenic influences including land use change and dam construction on sediment yield and accumulation within the Lake Rockwell and Ballville Dam watershed. To better understand the historical and current sediment yield within the Lake Erie watershed, Soil and Water Assessment Tool (SWAT) models of the Lake Rockwell and Ballville Dam watersheds were developed. The resulting model suggests that the average of sediment accumulation rate within Lake Rockwell Dam reservoir varies between the minimum of 1.6 and the maximum of 4.6g/cm2/yr from 1988 to 2007. Within the Ballville Dam reservoir, the rate varies between the minimum of 2.6 and the maximum of 23.2g/cm2/yr from 1980 to 1999.

Watersheds as Natural Infrastructure: New Urban Water Report Analyzes Benefits

More than a decade ago, residents of New Mexico witnessed the devastation caused by the Cerro Grande fire. Perilous fuel loads from decades of fire suppression caused burning to last for two weeks, leaving in its wake a wasteland of destroyed homes, dead trees, and ash. One year after the fire, reservoir sediment accumulation was 140 times higher than the previous 57 years combined. Levels remained significantly elevated for five years, and costs to clean up damage to the water supply ran $17 million. Water managers in the city of Santa Fe, N.M., took heed and invested, alongside the United States Forest Service, in a forest fuel‐reduction strategy to protect its own watershed. This program helped the city avoid an estimated $22 million in potential damage.

Sediment yield and erosion rate estimation in the mountain catchments of the Camastra artificial reservoir (Southern Italy): A comparison between different empirical methods

The sedimentary budget assessment is an important topic for the scientific and social community, because it is crucial to understand the dynamics of orogenic belts and cope with a number of practical problems, such as soil conservation and sediment accumulation in reservoir. Sediment yield or denudation rate estimates in southern-central Italy are generally obtained either by applying simple empirical relationships based on the statistical regression between the geomorphic parameters of the drainage network and the suspended sediment yield measured at the drainage basins outlets, or by using qualitative–quantitative sediment delivery ratio or erosion models. In this work, we carry out a study of catchment dynamics and a sedimentary yield computation of several mountain catchments of the central-western sector of the Basilicata region (southern Italy), which are located upstream of an artificial reservoir. The sediment yield and erosion rate have been computed through both an indirect assessment of the suspended sediment yield, based on the Tu index (mean annual suspension sediment yield), and the application of the RUSLE and USPED empirical methods. The results obtained have been compared with the historical data of sediment accumulation, measured in the artificial reservoir of the Camastra dam where a detailed evaluation of the volumes of historical (i.e. about 40years) sediment storage was collected.The collected dataset represents a basic tool both for the investigation of the morpho-dynamics of a typical mountain catchment of the Mediterranean area and the evaluation of sediment budget related to fluvial and hillslope processes. The 38-year-long record of the sediment storage in the Camastra artificial reservoir located at the outlet of the studied mountain catchments permits the validation of empirical relationships based on the geomorphic and climatic parameters of the drainage basin. Among the three different methods of sediment yield evaluation (Tu index, RUSLE and USPED), the Tu index showed the best prediction ability, although USPED erosion model also furnished a good estimation. As also confirmed by other works, the indirect estimation of sediment yield based on Ciccacci's empirical relationships can represent a good proxy of short-term denudation rates in the Mediterranean areas with geological and geomorphological features similar to the study area but the use of empirical models with increasing complexity such as the USPED erosion model can help to explore the spatial distribution of the sediment yield sectors of the drainage basin, the erosion hot-spots, and the role of landslides in sediment mobilization and hillslope-channel connectivity processes.

Hydroacoustic and spatial analysis of sediment fluxes and accumulation rates in two Virginia reservoirs, USA.

Watershed sediment fluxes and reservoir sediment accumulation rates were analyzed in two contrasting reservoir systems in central and western Virginia. Lake Pelham, located in the Piedmont geologic province, is a human-impacted reservoir with a watershed dominated by agricultural, residential and industrial land uses. Conversely, Lake Moomaw has a largely undeveloped watershed characterized by very steep slopes and forested land use located in the Valley and Ridge province. The Revised Universal Soil Loss Equation (RUSLE) and sediment delivery ratios (SDRs) were used to estimate soil losses in the two watersheds. Bathymetric and sediment accumulation surveys of the two reservoirs were also conducted using a multi-frequency hydroacoustic surveying system. The RUSLE/SDR erosion model estimates 2150kgha(-1)year(-1) for Lake Pelham and 2720kgha(-1)year(-1) for Lake Moomaw, a 410 and 13% increase from assumed pristine (100% forested) land use for the respective basins. Mean sediment accumulation rates of 1.51 and 0.60cmyear(-1) were estimated from the hydroacoustic survey of Lake Pelham and Lake Moomaw, respectively. Overall, Lake Moomaw has relatively low sediment accumulation rates; however, the reservoir is vulnerable to increases in sediment fluxes with further human development due to the steep slopes and highly erodible colluvial soils that characterize the basin. Higher erosion and sediment accumulation rates in Lake Pelham are most likely reflecting the impact of human development on sedimentation processes, where the loss of vegetal buffers and increase in impervious surfaces exacerbates both the surficial soil losses as well as intrinsic stream sediment production leading to the current annual reservoir capacity loss of 0.4%.

Upland sediment supply and its relation to watershed sediment delivery in the contemporary mid-Atlantic Piedmont (U.S.A.)

We use sediment accumulation in ponds and reservoirs to examine upland sediment sources and sinks in the Piedmont physiographic region of Maryland, USA. In zero-order and first-order watersheds, sediment yield is greatest from suburban land cover, followed by agriculture and forest. The idea that sediment yield is small from mature suburban development appears to not be correct. First-order channel enlargement is an important sediment source, causing sediment yield to increase from zero-order to first-order watersheds. Nonchannel sources provide one-third to two-thirds of the upland sediment load.Long-term sediment accumulation in a reservoir at fifth-order indicates that cumulative sediment load from upland areas is reduced by one-quarter by net valley bottom sedimentation. If upland supply exceeds the load delivered from a watershed, sediment must accumulate along valley bottoms. In our study watershed, net sedimentation rate (sedimentation less erosion) averaged over valley bottom area is 2.6mm/y, a value that is similar to independent direct measurements of sedimentation and erosion in a nearby watershed. Evaluation of the relative contributions to sediment mass balance of upland supply, valley bottom sedimentation and erosion, and watershed delivery indicates that, if valley-bottom rates of sedimentation exceed erosion as indicated by recent studies, then the proportion of watershed sediment delivery derived from stream banks is necessarily small.Although sediment yield estimated from stream gage records is similar in magnitude to that from ponds for watersheds smaller than 20km2, sediment yield from reservoir sedimentation is a factor of five larger than that estimated from gage records for watersheds larger than 140km2. This observation confirms that the different methods provide very different estimates of sediment yield. This possibility is reinforced by a sediment yield of 14Mg/km2/y from a gage immediately above a reservoir with a yield of 142Mg/km2/y based on reservoir accumulation.

Carbon Sequestration in a Large Hydroelectric Reservoir: An Integrative Seismic Approach

Artificial reservoirs likely accumulate more carbon than natural lakes due to their unusually high sedimentation rates. Nevertheless, the actual magnitude of carbon accumulating in reservoirs is poorly known due to a lack of whole-system studies of carbon burial. We determined the organic carbon (OC) burial rate and the total OC stock in the sediments of a tropical hydroelectric reservoir by combining a seismic survey with sediment core sampling. Our data suggest that no sediment accumulation occurs along the margins of the reservoir and that irregular bottom morphology leads to irregular sediment deposition. Such heterogeneous sedimentation resulted in high spatial variation in OC burial—from 0 to 209 g C m−2 y−1. Based on a regression between sediment accumulation and OC burial rates (R 2 = 0.94), and on the mean reservoir sediment accumulation rate (0.51 cm y−1, from the seismic survey), the whole-reservoir OC burial rate was estimated at 42.2 g C m−2 y−1. This rate was equivalent to 70% of the reported carbon emissions from the reservoir surface to the atmosphere and corresponded to a total sediment OC accumulation of 0.62 Tg C since the reservoir was created. The approach we propose here allows an inexpensive and integrative assessment of OC burial in reservoirs by taking into account the high degree of spatial variability and based on a single assessment. Because burial can be assessed shortly after the survey, the approach combining a seismic survey and coring could, if applied on a larger scale, contribute to a more complete estimate of carbon stocks in freshwater systems in a relatively short period of time.

Soil erosion and sediment fluxes analysis: a watershed study of the Ni Reservoir, Spotsylvania County, VA, USA

Anthropogenic forces that alter the physical landscape are known to cause significant soil erosion, which has negative impact on surface water bodies, such as rivers, lakes/reservoirs, and coastal zones, and thus sediment control has become one of the central aspects of catchment management planning. The revised universal soil loss equation empirical model, erosion pins, and isotopic sediment core analyses were used to evaluate watershed erosion, stream bank erosion, and reservoir sediment accumulation rates for Ni Reservoir, in central Virginia. Land-use and land cover seems to be dominant control in watershed soil erosion, with barren land and human-disturbed areas contributing the most sediment, and forest and herbaceous areas contributing the least. Results show a 7% increase in human development from 2001 (14%) to 2009 (21.6%), corresponding to an increase in soil loss of 0.82Mg ha(-1)year(-1) in the same time period. (210)Pb-based sediment accumulation rates at three locations in Ni Reservoir were 1.020, 0.364, and 0.543g cm(-2)year(-1) respectively, indicating that sediment accumulation and distribution in the reservoir is influenced by reservoir configuration and significant contributions from bedload. All three locations indicate an increase in modern sediment accumulation rates. Erosion pin results show variability in stream bank erosion with values ranging from 4.7 to 11.3cm year(-1). These results indicate that urban growth and the decline in vegetative cover has increased sediment fluxes from the watershed and poses a significant threat to the long-term sustainability of the Ni Reservoir as urbanization continues to increase.

Sediment distribution pattern in some Iranian dams based on a new empirical reservoir shape function

Sedimentation greatly affects the future performance of reservoirs and thus is a main concern of designers. Reservoir sedimentation is influenced by many parameters, with reservoir shape among the most important. Different empirical and semiempirical methods based on reservoir shape parameters have been previously researched to predict sediment distribution patterns. Our extended investigation over hydrographic maps of 10 large dams in Iran found a strong relationship between reservoir shape and sediment distribution pattern. As a result, we introduced a new method for the prediction of sediment accumulation in reservoirs using a parameter that determines the reservoir shape. This parameter is used to show how accurately reservoir shape can be approximated by its equivalent pyramid. Equivalent pyramid was defined at each reservoir cross section, with the cross sectional area as its base and the distance from the reservoir end as its height. We recommend this new technique to predict long term cumulative distribution of sediment in preliminary design stages for a wide variety of reservoirs with mainly fine-grained sediment particles.

Spatial and isotopic analysis of watershed soil loss and reservoir sediment accumulation rates in Lake Anna, Virginia, USA

Soil erosion and associated sedimentation are a threat to the sustainable use of surface water resources through the loss of volume storage capacity and conveyance of pollutants to receiving water bodies. The RUSLE2 empirical model and isotopic sediment core analyses were used to evaluate watershed erosion and reservoir sediment accumulation rates for Lake Anna, in Central Virginia. A sediment flux rate of 66,000 Mg/year was estimated from the upper basin and land use was determined to be the primary factor contributing to soil erosion. Barren lands and agricultural activities were estimated to contribute the most sediment (>20 Mg/ha/year), whereas forested and herbaceous landscapes were less likely to erode ( 0.30) within the watershed makes this system highly vulnerable to future anthropogenic stressors.

A guided swat model application on sediment yield modeling in Pangani river basin: lessons learnt

The overall objective of this paper is to report on the lessons learnt from applying Soil and Water Assessment Tool (SWAT) in a well guided sediment yield modelling study. The study area is the upstream of Pangani River Basin (PRB), the Nyumba Ya Mungu (NYM) reservoir catchment, located in the North Eastern part of Tanzania. It should be noted that, previous modeling exercises in the region applied SWAT with pre- assumption that inter-rill or sheet erosion was the dominant erosion type. In contrast, in this study SWAT model application was guided by results of analysis of high temporal resolution of sediment flow data and hydro-meteorological data. The runoff component of the SWAT model was calibrated from six-years (i.e. 1977-1982) of historical daily streamflow data. The sediment component of the model was calibrated using one-year (1977-1988) daily sediment loads estimated from one hydrological year sampling programme (between March and November, 2005) rating curve. A long-term period over 37 years (i.e. 1969-2005) simulation results of the SWAT model was validated to downstream NYM reservoir sediment accumulation information. The SWAT model captured 56 percent of the variance (CE) and underestimated the observed daily sediment loads by 0.9 percent according to Total Mass Control (TMC) performance indices during a normal wet hydrological year, i.e., between November 1, 1977 and October 31, 1978, as the calibration period. SWAT model predicted satisfactorily the long-term sediment catchment yield with a relative error of 2.6 percent. Also, the model has identified erosion sources spatially and has replicated some erosion processes as determined in other studies and field observations in the PRB. This result suggests that for catchments where sheet erosion is dominant SWAT model may substitute the sediment-rating curve. However, the SWAT model could not capture the dynamics of sediment load delivery in some seasons to the catchment outlet.

Impact of dams on Yangtze River sediment supply to the sea and delta intertidal wetland response

On the basis of estimates of sediment accumulation in reservoirs, the impact of 50,000 dams on sediment supply and intertidal wetland response in the Yangtze River catchment is examined. The total storage capacity of reservoirs is 200 × 109m3, or 22% of the Yangtze annual runoff. The sediment accumulation rate in reservoirs has increased from ∼0 in 1950 to &gt;850 × 106t/yr in 2003. Although sediment yield has increased with broader soil erosion in the river basin, the total riverine sediment discharge rate shows a strong decreasing trend from the late 1960s to 2003, likely due to dam construction. Consequently, the total growth rate of intertidal wetlands at the delta front has decreased dramatically. A significant relationship exists between intertidal wetland growth rate and riverine sediment supply that suggests riverine sediment supply is a governing factor in the interannual to interdecadal evolution of delta wetlands. Regression analysis of intertidal wetland growth rate and sediment supply shows that intertidal wetlands at the delta front degrades when the riverine sediment discharge rate reaches a threshold level of &lt;263 × 106t/yr. Owing to the construction of the Three Gorges Dam and other new dams, the sediment discharge rate of the Yangtze River will most likely decrease to below 150 × 106t/yr in the coming decades. Therefore unless current management policies are adjusted, drastic recession of Yangtze River delta intertidal wetlands can be expected to occur.

Natural and Human-Induced Variations in Water and Sediment Runoff in the Danube River Mouth

Long-term natural and human-induced variations in water and sediment runoff in the Danube Delta head were studied on the basis of analysis of an extremely long series of observations (1840–2002). A considerable reduction of sediment runoff in the second half of the 20th century related to sediment accumulation in reservoirs was revealed. It was found that human activities had an insignificant impact on the water runoff variations and the period of 1961–2002 turned out to be abnormally water-abundant.