Showing papers on "Siltation published in 2016"

Physically based soil erosion and sediment yield models revisited

Abstract: A plenty of models exist for study of the soil erosion and sediment yield processes. However, these models vary significantly in terms of their capability and complexity, input requirements, representation of processes, spatial and temporal scale accountability, practical applicability, and types of output they provide. The present study reviews 50 physically based soil erosion and sediment yield models with respect to these factors including shortcomings and strengths. The literature generally suggests the use of models like SWAT, WEPP, AGNPS, ANSWERS and SHETRAN for soil erosion and sediment studies. Most of the developed soil erosion and sediment yield models are capable of simulating soil detachment and sediment delivery processes at hillslope scale; a limited development was found in the field of reservoir siltation and channel erosion processes. The study proposes a guideline for selection of an appropriate model to the reader for a given application or case study. The future research suggested to improve the simulation and prediction capability of physically based soil erosion and sediment yield models, and should focus on incorporation of improved global web based weather database, inclusion of sediment associated water quality and gully erosion simulation module, and improvement in reservoir siltation and channel erosion simulation processes.

Land claim and loss of tidal flats in the Yangtze Estuary

Abstract: Tidal flats play a critical role in supporting biodiversity and in providing ecosystem services but are rapidly disappearing because of human activities. The Yangtze Estuary is one of the world's largest alluvial estuaries and is adjacent to the most developed economic zone in China. Using the Yangtze Estuary as a study region, we developed an automatic algorithm to estimate tidal flat areas based on the Land Surface Water Index and the Normalized Difference Vegetation Index. The total area of tidal flats in the Yangtze Estuary has decreased by 36% over the past three decades, including a 38% reduction in saltmarshes and a 31% reduction in barren mudflats. Meanwhile, land claim has accumulated to 1077 km(2), a value that exceeds the area of the remaining tidal flats. We divided the Yangtze Estuary into Shanghai and Jiangsu areas, which differ in riverine sediment supply and tidal flat management patterns. Although land claim has accelerated in both areas, the decline in tidal flat area has been much greater in Jiangsu than in Shanghai because of abundant supplies of sediment and artificial siltation in the latter area. The results highlight the need for better coastal planning and management based on tidal flat dynamics.

Relative Contribution of Rill/Interrill and Gully/Channel Erosion to Small Reservoir Siltation in Mediterranean Environments

Abstract: Reservoir siltation because of water erosion is an important environmental issue in Mediterranean countries where storage of clear surface water is crucial for their economic and agricultural development. The high density of gully systems observed in Mediterranean regions raises the question of their contribution to reservoir siltation. In this context, this study quantified the absolute and relative contributions of rill/interrill and gully/channel erosion in sediment accumulation at the outlet of small Tunisian catchments (0·1–10 km2) during the last 15 years (1995–2010). To this end, a fingerprinting method based on measurements of caesium-137 and total organic carbon combined with long-term field monitoring of catchment sediment yield was applied to five catchments in order to cover the diversity of environmental conditions found along the Tunisian Ridge and in the Cape Bon region. Results showed the very large variability of erosion processes among the selected catchments, with rill/interrill erosion contributions to sediment accumulated in outlet reservoirs ranging from 20 to 80%. Overall, rill/interrill erosion was the dominant process controlling reservoir siltation in three catchments whereas gully/channel erosion dominated in the other two catchments. We identified the presence of marly gypsum substrates and the proportion of catchment surface covered by soil management/conservation measures as the main drivers of erosion process variability at the catchment scale. These results provided a sound basis to propose guidelines for erosion mitigation in these Mediterranean environments and suggested to apply models simulating both rill/interrill and gully/channel erosion in catchments of the region. Copyright © 2015 John Wiley & Sons, Ltd.

Distribution of Artisanal and Small-Scale Gold Mining in the Tapajós River Basin (Brazilian Amazon) over the Past 40 Years and Relationship with Water Siltation

Abstract: An innovative remote sensing approach that combines land-use change and water quality information is proposed in order to investigate if Artisanal and Small-scale Gold Mining (ASGM) area extension is associated with water siltation in the Tapajos River Basin (Brazil), containing the largest small-scale gold mining district in the world. Taking advantage of a 40-year period of the multi-satellite imagery archive, the objective of this paper is to build a normalized time-series in order to evaluate the influence of temporal mining expansion on the water siltation data (TSS, Total Suspended Solids concentration) derived from previous research. The methodological approach was set to deliver a full characterization of the ASGM expansion from its initial stages in the early 1970s to the present. First, based on IRS/LISSIII images acquired in 2012, the historical Landsat image database (1973–2001) was corrected for radiometric and atmospheric effects using dark vegetation as reference to create a normalized time-series. Next, a complete update of the mining areas distribution in 2012 derived from the TerraClass Project (an official land-use classification for the Brazilian Amazon) was conducted having IRS/LISSIII as the base map with the support of auxiliary data and vector editing. Once the ASGM in 2012 was quantified (261.7 km2) and validated with photos, a reverse classification of ASGM in 2001 (171.7 km2), 1993 (166.3 km2), 1984 (47.5 km2), and 1973 (15.4 km2) with the use of Landsat archives was applied. This procedure relies on the assumption that ASGM changes in the land cover are severe and remain detectable from satellite sensors for decades. The mining expansion area over time was then combined with the (TSS) data retrieved from the same atmospherically corrected satellite imagery based on the literature. In terms of gold mining expansion and water siltation effects, four main periods of ASGM activities were identified in the study area: (i) 1958–1977, first occurrence of mining activities and low water impacts; (ii) 1978–1993, introduction of low-budget mechanization associated with very high gold prices resulting in large mining area expansion and high water siltation levels; (iii) 1994–2003, general recession of ASGM activities and exhaustion of easy-access gold deposits, resulting in decreased TSS; (iv) 2004 to present, intensification of ASGM encouraged by high gold prices, resulting in an increase of TSS.

Diagnosing problems of fine sediment delivery and transfer in a lowland catchment

Abstract: Fine sediment (here defined as fine sand, silt and clay) is a serious management problem in lowland rivers because of alterations to river channels, floodplains and the wider landscape. The multi-scale, complex and stochastic nature of sediment production, delivery and transport processes complicates the diagnosis of fine sediment sources, pathways and impacts. The hydromorphological assessment framework developed by the REFORM project offers a flexible approach to investigate fine sediment pressures. In this study, the framework was applied to a lowland river impacted by excess fine sediment (River Frome, Dorset, UK) to investigate likely sources and timing of sediment production, the segment-scale capacity of the river to transport sediment, and the reach-scale geomorphological response of the river. Land use mapping and agricultural census records suggest that intensive cultivation of cereals and high livestock numbers during the second half of the twentieth century are the probable causes of fine sediment production and that a lack of any significant riparian buffer zone facilitates delivery of fine sediment to the river network. Sediment budget modelling indicates that transport capacities for gravel and sand are low along the river, which is supported by field observations of compacted gravel/sand beds covered with algae. Analysis of historical maps reveals that the river has responded to the increase in fine sediment over the last 40–50 years with channel narrowing and an increase in sinuosity, as fine sediment is trapped and stabilised by aquatic vegetation in the channel margins. Interactions between hydrodynamic forces, sediment supply and vegetation are driving progressive adjustment of the River Frome, and are the key to its holistic future management.

Sediment trapping in the Changjiang Estuary: Observations in the North Passage over a spring-neap tidal cycle

Abstract: Water current, salinity, and suspended sediment concentration (SSC) were measured at three anchored boat sites along the North Passage (NP) of the Changjiang Estuary over a spring-neap tidal cycle, in order to study sediment trapping and siltation in the estuary. Pronounced stratification was observed during the late flood tide and the following early ebb tide, along with an advancing and retreating salt wedge, whereas strong vertical mixing occurred during the late ebb when the effect of the salt wedge faded. Therefore, the SSC in the flood-ebb tidal cycle tended to be asymmetric. In the upper reach of the NP, the seaward advective near-bed sediment transport dominated the total near-bed sediment transport, whereas in the middle reach of the NP, the landward tidal pumping component dominated. Accordingly, a notable convergent near-bed residual sediment transport was generated near the middle reach. Because the convergence of residual sediment transport in the region of a salt wedge is generally recognized as sediment trapping, convergent near-bed residual sediment transport is the cause of the high sedimentation rate in the NP.

Spatial and temporal variability of the rainfall erosivity factor in Northern Algeria

Abstract: Erosion, sediment transport, and deposition result in agricultural soil degradation, dam siltation, and significant and costly damage. In Algeria, despite a decrease in total rainfall, especially in the central and western parts of the country, dam siltation and erosion are on the rise according to recent studies by the National Agency of Dams and Transfers. Developing a map of the spatial variation of rainfall erosivity will provide a powerful tool for land and dam managers. Erosivity can be quantified using the R factor from the Universal Soil Loss Equation. The purpose of this work is to develop a model for estimating rain erosivity based on the modified Fournier index (MFI), longitude, altitude, and mean maximum daily rainfall, and then to produce a map of the spatial distribution of erosivity in Northern Algeria. The R factor reaches a maximum value of roughly 905 MJ mm ha−1 year−1 in the Jijel area of Mediterranean Eastern Algeria and a minimum value of 37 MJ mm ha−1 year−1 in the southern portion of the study area. The study of the temporal evolution of annual rainfall erosivity index (R) and I max over 30 min showed that R recorded a negative trend and I max recorded no trend.

Ecological degradation of a meandering river by local channelization effects: a case study in an Austrian lowland river

Abstract: Anthropogenically induced siltation has serious effects on micro-habitat diversity and thus on aquatic organisms in lotic systems. The present study deals with the impact of siltation on the macroinvertebrate community in the River Lafnitz, south-eastern Austria. Our aim was to examine various ecological parameters in a meandering river stretch. The results significantly show (1) a generally lower taxa diversity, (2) a dominance of tolerant taxa such as Chironomidae and a clear reduction of sensitive taxa such as Ephemeroptera, Plecoptera, Trichoptera (EPT-taxa) and (3) overall lower abundances and biomass in sandy fractions. The present case study highlights various processes of fine sediment sources and sinks. While in most cases, fine sediment input is locally caused by erosion of adjacent terrestrial areas, even small-scale channelization of meandering rivers leads to subsequent trapping of huge sandy fractions especially in morphologically natural river sections. This inconspicuous but steady process is a risk to biodiversity and masks serious ecological degradation. Suitable management and restoration measures for anthropogenically silted rivers are therefore strongly recommendable. In summary, the study underlines the sensitivity of meandering systems and its dependence on catchment scale degradation and questions the reversibility of human impacts.

RUSLE and SDR Model Based Sediment Yield Assessment in a GIS and Remote Sensing Environment: A Case Study of Koga Watershed, Upper Blue Nile Basin, Ethiopia

Abstract: Soil erosion and the subsequent sedimentation are the major watershed problems in Ethiopia. Removal of top fertile soil, siltation of Koga irrigation reservoir, clogging of irrigation canal by sediment and reduction of irrigated land are the major threat of Koga watershed. Hence, this study was attempted to assess and map the spatial distribution of sediment yield of Koga watershed in a GIS and remote sensing environment. Sediment yield is dependent on factors of soil erosion such as rainfall erosivity, soil erodibilty, land use land cover (C and P) and topography (LS) and sediment delivery ratio of the drainage basin to the total amount of sediment yield by sheet and channel erosion. RUSLE framed with GIS and Remote sensing technique was therefore employed to assess the amount of soil loss existed in KW. Main stream channel slope based sediment delivery ratio analysis was also carried out. Soil map (1:250,000), Aster DEM (30 × 30 m), Thematic Mapper (TM) image (30 m × 30 m) of the year 2013, thirteen years (2000-2013) rainfall records from four rain gauge stations and topographic map (1:50,000) were the major data used. The estimated mean annual SY delivered to the out let of KW was found to be 25 t ha-1year-1. Most critical sediment source areas are situated in the steepest upper part of the watershed due to very high computed soil loss and sediment delivery ratio in this part. It could be therefore difficult to attain the intended goal of Koga irrigation reservoir positioned at lower part of the watershed. Sustainable land management practices have to be conducted in the upper part of the watershed by taking each stream order as a management unit to increase the storage capacity, and/or lessen the transportation capacity of the watershed. Proper drainage construction and stream bank stabilization via vegetative cover have to widely implement to safely dispose the eroded sediment.

Small dam reservoir siltation in the Atlas Mountains of Central Morocco: analysis of factors impacting sediment yield

Abstract: Since the construction of the first dam, siltation phenomenon has always preoccupied every hydraulic engineer. The problem is widely studied on large dams than on smaller ones even if it has higher impact. Actually, building small dams might be a good option to cope with climate change and improve community resilience in arid areas such as our study area in Morocco. So, well-planned small dams depend immediately on a good understanding of siltation phenomenon linked to water erosion process on the catchment. As known, this process is related to many factors including: rainfall’s intensity, runoff, slope, lithology, and vegetation cover upon the whole watershed. The objective of the current study is to identify the most influencing factors that control sediment yield variability in catchment areas. Data of 24 small dams located in western High Atlas and Anti-Atlas Mountains in Morocco are collected and analyzed. Different tools are used, to extract catchment attributes for each reservoir, such as digital elevation models, satellite images, topographic and lithological maps. During a field survey, unavailable sediment yield data are estimated using approximate method. Based on Pearson’s correlation and multiple regressions, the relationship between sediment yield and catchment characteristics is studied. The first analysis of Pearson’s correlation shows that sediment yield is positively linked to different factors as: catchment area, catchment’s shape, drainage density, global slope index, erodibility and sinuosity’s degree of the stream. The second analysis of multiple regression with stepwise method shows that only the drainage density, erodibility and global slope index were included in the suggested model with acceptable coefficient of determination (R 2 = 0.85). The results and outcomes of this study will be used to generate thematic maps that will be used as “decision support tool” to better plan small dam projects and mainly to minimize erosion sensitive areas.

Soil cracking effects on hydrological and erosive processes: a study case in Mediterranean cultivated vertisols

Abstract: Shrink–swell soils, such as those in a Mediterranean climate regime, can cause changes in terms of hydrological and erosive responses because of the changing soil water storage conditions. Only a limited number of long-term studies have focused on the impacts on both hydrological and erosive responses and their interactions in an agricultural environment. In this context, this study aims to document the dynamics of cracks, runoff and soil erosion within a small Mediterranean cultivated catchment and to quantify the influence of crack processes on the water and sediment supplied to a reservoir located at the catchment outlet using water and sediment measurements at a cultivated field outlet as baseline. Detailed monitoring of the presence of topsoil cracks was conducted within the Kamech catchment (ORE OMERE, Tunisia), and runoff and suspended sediment loads were continuously measured over a long period of time (2005–2012) at the outlets of a field (1.32 ha) and a catchment (263 ha). Analysis of the data showed that topsoil cracks were open approximately half of the year and that the rainfall regime and water table level conditions locally control the seasonal cracking dynamics. Topsoil cracks appeared to seriously affect the generation of runoff and sediment concentrations and, consequently, sediment yields, with similar dynamics observed at the field and catchment outlets. A similar time lag in the seasonality between water and sediment delivery was observed at these two scales: although the runoff rates were globally low during the presence of topsoil cracks, most sediment transport occurred during this period associated with very high sediment concentrations. This study underlines the importance of a good prediction of runoff during the presence of cracks for reservoir siltation considerations. In this context, the prediction of cracking effects on runoff and soil erosion is a key factor for the development of effective soil and water management strategies and downstream reservoir preservation. Copyright © 2016 John Wiley & Sons, Ltd.

Hydropedology and soil evolution in explaining the hydrological properties of recharge dams in arid zone environments

Abstract: The effects of anthropogenic activities on soil evolution due to siltation and the impact on hydraulic properties of the vadose zone on the augmentation of the aquifer recharge are investigated for the Al-Khoud dam in Oman. Inside the dam reservoir and in areas adjacent to the embankment, downstream, 33 pedons (of depths between 1.5 and 2 m) were excavated and studied during 2011 and 2012. Soil analysis revealed that the subsoil’s physiochemical properties of the study area are continuously changing due to damming, i.e., alteration of the natural runoff, intensified sedimentation, and infiltration. Variation of hydropedological properties caused by the geotechnical construction is evident in a distinct vertical stratification of texture of accrued sediments and almost an order of magnitude drop in saturated hydraulic conductivity (K s ) of the dam bed. Correspondingly, spilling of ponded water over the dam crest occurs more frequent and therefore increases the potential hazards of flooding of the downstream recharge area. Some fine particles of the suspended load carried to the reservoir by the feeding wadi migrate vertically downward, driven by seepage, into the originally coarse matrix of the parent soil and cause clogging of large pores (with time, hard pans in the subsurface are developing) even without visible cake formation on the soil surface. Development of hard pans was also discovered in pedons at depths close to 1 m. This is attributed to presence of a pedogenic carbonate derived from the parent rock and formed by precipitation of dissolved salts due to a vertical upward moisture evaporation to a hot and dry bed surface during prevailing dry bed periods of dam operation. K s measured downstream of the dam was relatively high (6 m/day) and was three times higher than the average value inside the reservoir (2.1 m/day), ranging there between 0.01 and 3.96 m/day, but less than at the upstream site outside the reservoir.

Calculating the sediment budget of a tropical lake in the Blue Nile basin: Lake Tana

Abstract: . Soil erosion decreases soil fertility of the uplands and causes siltation of lakes and reservoirs. However, very little data exists to quantify accurately the impact of sediment on lakes in tropical monsoonal areas in the African highlands. Lake Tana is one of these lakes in Ethiopia. The objective of this study is to quantify the sediment budget for Lake Tana watershed with limited observational data. To overcome these limitations we use the Parameter Efficient Distributed (PED) model that has shown to perform well in the Ethiopian highlands. PED model parameters are calibrated using daily discharge data and sediment concentration infrequently measured for establishing sediment rating curves for the major rivers. The calibrated model parameters are then used to predict the sediment budget for the period 1994–2009. Sediment retained in the lake is calculated from two bathymetric taken 15 years apart and the sediment leaving the lake is based on measured discharge and observed sediment concentrations. Results show that annually on average 34 Mg/ha/year of sediment is removed from the gauged part of the Lake Tana watersheds. Depending on the up scaling method, 14 to 32 Mg/ha/year is transported from the watershed of which 82% to 96% (with the upper estimate more likely) is trapped on the floodplains and in the lake.

Vulnerability and Impact of Climate Change Processes on Water Resource in Semi-Arid Areas: In Essaouira Basin (Morocco)

Abstract: Since the 50s, many changes are observed in the global climate namely; the warming of the atmosphere and ocean decreased the extent of snow and ice, and rising sea levels. In addition to population pressure, the Maghreb countries suffer from drought, which accentuates the phenomenon of desertification; soil erosion and salinity, leading to salinization and depletion of water resources. In Morocco, climate change causes soil erosion as a result of desertification resulting in siltation of dams, worsening of salinization soles and water resources. The Essaouira Basin the subject of this study focuses on the Atlantic coast of Morocco in the southeast of the city of Essaouira. Climate aridity and intensive exploitation due to uncontrolled pumping for irrigation have caused a drastic decrease in the piezometric level of the aquifers in Essaouira basin, and have seriously degraded groundwater quality. This basin is characterized by a semi-arid climate with average annual rainfall of around 300 mm/year and temperatures average 20 °C. Considering the importance of the Essaouira aquifer in the supply of the region in groundwater, a study was done to quantitatively and qualitatively assess the impact of climate change on water resources in the area. Aquifers characterizing this area are Cenomanian-Turonian of the upstream and downstream Plio-Quaternary, separated by the Tidzi diaper. The piezometric maps pelvis were made (90-09). Geochemistry shows that these present Sodium-chloride facies. The electrical conductivity ranges from 700–7000 μS/cm. The concentration of 18O; 2H; 3H and 14C were measured, a local meteoric right close to the global meteoric right characterizing ocean precipitation was determined and the age of groundwater is determined. The Essaouira Basin is very vulnerable and sensitive to climate change because its charge is entirely dependent on rainwater.

Modelling Soil Erosion and Sedimentation in the Oued Haricha Sub-Basin (Tahaddart Watershed, Western Rif, Morocco): Risk Assessment

Abstract: This study evaluates the annual loss of soil in the sub-basin of Oued Haricha (Tahaddart basin, Western Rif, NW Morocco). The integration of revised (RUSLE) and modified (MUSLE) soil loss empirical equations of Wischmeier and Smith in combination with GIS permits the modelling of soil erosion at the scale of parcels. The characteristics of precipitation and runoff, the soil properties, the culture system and the current working practices of soil in the sub-basin of the Oued Haricha are collected from local data. The digital terrain model is used to generate topographic factors. The combination of different RUSLE factors shows that the annual soil is 62.72 t/ha/year and corresponds to an average level of risk. The total losses calculated by MUSLE method are valued at 221,468 t/year. The rates of loss due to linear erosion are 82,652 t/year. These soil losses represent 20.33% of the total losses, and confirm that the losses on the slopes outweigh the losses due to the river system. Sedimentation module shows that the areas of high erosion (greater than 200 t/ha/year) are concentrated in the reliefs with average and high slope and occupy 38% of the total area. The deposition areas occupy the centre of sub-basin and constitute 9.12% of the total area. These deposits were concentrated on the edges of major rivers and the outlet of the sub-basin and contributed to siltation of the April 9, 1947 dam.

Soil erosion vulnerability under scenarios of climate land-use changes after the development of a large reservoir in a semi-arid area

Abstract: Climate and land-use/cover changes (LUCC) influence soil erosion vulnerability in the semi-arid region of Alqueva, threatening the reservoir storage capacity and sustainability of the landscape. Considering the effect of these changes in the future, the purpose of this study was to investigate soil erosion scenarios using the Revised Universal Soil Loss Equation (RUSLE) model. A multi-agent system combining Markov cellular automata with multi-criteria evaluation was used to investigate LUCC scenarios according to delineated regional strategies. Forecasting scenarios indicated that the intensive agricultural area as well as the sparse and xerophytic vegetation and rainfall-runoff erosivity would increase, consequently causing the soil erosion to rise from 1.78 Mg ha−1 to 3.65 Mg ha−1 by 2100. A backcasting scenario was investigated by considering the application of soil conservation practices that would decrease the soil erosion considerably to an average of 2.27 Mg ha−1. A decision support system can assi...

Modeling sediment concentration and transport induced by storm surge in Hengmen Eastern Access Channel

Abstract: Sediment is an important factor for excavation, dredging and maintenance of Hengmen Eastern Access Channel in Pearl River Estuary. As storm surge is considered as an important role in determining sediment re-suspension and transport, as well as creating landforms in the areas of estuary and coast, along with the storm surge disaster damage in Pearl River Estuary is one of the most serious events in China, reasonable simulation of storm-induced sediment concentration, transport and channel siltation in Hengmen Eastern Access Channel is of much significance. Based on the feasibility condition of less research on numerical simulation of storm-induced sediment concentration and transport, especially channel siltation in the Pearl River Estuary, using a curvilinear grid, a nested and coupled model which combines typhoon model, hydrodynamic model (Delft3D-FLOW), wave model (Delft3D-WAVE) and sediment transport model (Delft3D-SED) was set up for the region of Pearl River Estuary. After a series of model verifications, which showed that this coupled model performed well to reflect the characteristics of the typhoon field, tidal currents, wave height, storm surge, distribution of suspended sediment in the studied region, the model was applied to study the storm-induced sediment concentration and transport in Hengmen Eastern Access Channel. Through simulation of one extra tropical storm surge process with this coupled numerical model, the storm-induced sediment concentration and transport in Hengmen Eastern Access Channel were studied, and the storm-induced erosion and deposition were further discussed. Results showed that the effect of storm surge on sediment concentration, transport and siltation was significant. Under the influence of storm surge, the velocity and bed stress around Hengmen Eastern Access Channel increased significantly, which led the re-suspension and transport of sediment, and thus, the higher sediment concentration and more channel siltation occurred. By running this coupled model, the simulated results can be employed in the optimum decision making of Hengmen Eastern Access Channel Regulation Project.

Accelerated erosion in a watershed in the southeastern region of Brazil

Abstract: An understanding of erosive processes and the washing away of sediments to watersheds is an essential tool for decision makers planning water resource use. This study assessed the potential for surface runoff due to natural attributes together with land use/land cover to highlight the potential for accelerated erosion in the Araras River Watershed (352.77 km2) at a 1:50,000 scale. The analytic hierarchy process was used with the data provided to combine geoenvironmental attributes (soil, rock, water, relief and land use/land cover) that trigger erosive processes. Just over 51 % of the basin area presented an average potential for surface runoff, while 76.5 % presented a low to average potential for accelerated erosion. Despite this, upstream areas used for water collection for Araras city show a medium to high potential for surface runoff and accelerated erosion, reducing water infiltration and recharge, and resulting in the silting of reservoirs and water quality damage.

Dams Siltation and Soil Erosion in the Souss–Massa River Basin

Abstract: Siltation of 26 large and small dams, spatially distributed in the Souss–Massa basin, has been investigated The disparity observed in the reservoir’s silting reflects the variability of each watershed’s physical parameters that control the water erosion These parameters include in particular lithology, topography, climate, vegetation, hydrology, and human actions

How are anatomical and hydraulic features of the mangroves Avicennia marina and Rhizophora mucronata influenced by siltation

Abstract: This article provides significant data in the debate on whether siltation might have a negative impact on the hydraulic functioning of two widespread mangrove tree species Avicennia marina and Rhizophora mucronata. Elevated sediment addition, or siltation, within mangrove ecosystems is considered as being negative for trees and saplings, resulting in stress and higher mortality rates. However, little is known about how siltation influences the hydraulic functioning of mangrove trees. Comparing two mangrove tree species (Avicennia marina Vierh. Forsk. and Rhizophora mucronata Lam.) from low and high-siltation plots led to the detection of anatomical and morphological differences and tendencies. Adaptations to high siltation were found to be either mutual among both species, e.g., significant smaller single leaf area (p A.marina = 0.058, F1.38 = 3.8; p R.mucronata = 0.005, F1.38 = 8.7; n = 20 × 20) and a tendency towards smaller stomatal areas (p A.marina = 0.131, F1.8 = 2.8; p R.mucronata = 0.185, F1.8 = 2.1, n = 5 × 60), or species-specific trends for A. marina, such as higher phloem band/growth layer ratios (p = 0.101, F1.8 = 3.4, n = 5 × 3) and stomatal density (p = 0.052, F1.8 = 5.2, n = 5 × 4). All adaptations seemingly contributed to a comparable hydraulic conductivity independent of the degree of siltation. These findings indicate that silted trees level off fluctuations in their hydraulic performance as a survival mechanism to cope with this less favourable environment. Most of the trees’ structural adaptations to cope with siltation are similar to known drought stress-imposed adaptations.

Decadal trends and causes of sedimentation in the Inner Mongolia reach of the upper Yellow River, China

Abstract: Using hydrological and sediment data, this study investigated decadal trends in sediment erosion/deposition in the Inner Mongolia reach of the upper Yellow River. The calculated yearly sediment erosion/deposition show that the reach was dominated by aggradation, degradation, and aggradation successively in three periods with the years around 1961 and 1987 as break-points. By constructing relations between water discharge and sediment load, the contributions of key factors to the changes in sediment erosion/deposition in the reach were quantified. Results show that the sediment retention behind the main stem dams, the increase of natural runoff, and the decrease of sediment inputs from tributaries and upstream watershed were the main factors causing the transition from aggradation during 1955–1961 to degradation during 1962–1987. The reduction of natural runoff, the decrease of sediment retention behind dams, and the rise of sediment supply from tributaries were the key causes of the reversal from degradation in 1962–1987 to aggradation in 1988–2003. Water diversion has played an important role in the long-term aggradation of the Inner Mongolia reach. The main stem dams had functioned to alleviate siltation after 1961, but their effects on siltation reduction had been gradually diminishing since the 1990s. Copyright © 2015 John Wiley & Sons, Ltd.

Tidal River Management (TRM) and Tidal Basin Management (TBM): A case study on Bangladesh

Abstract: Bangladesh is the biggest delta of the world. Construction of numbers of polders is one of the flood resilient approach. But the presence of coastal polders de-linked the flood plain. The siltation in river causes riverbeds to become higher than the adjacent crop lands, and vast area under the polders became permanently water logged rendering large tract of land uncultivable. The current practice is temporarily de-poldering by cutting embankment. This is a natural water management process with very little human interventions but it needs strong participation and consensus with a great deal of sacrifice by the stakeholders for a specific period (3 to 5 years or even more)[1]. An attempt has been made to study the phenomena of tidal basin management reviewing some secondary data and processes involved in successfully operated tidal basins of Bangladesh. And preliminary laboratory experiments are carried out to precisely look into the suspended sediment transport. With varying outflow discharge and sediment supply, the transport processes are investigated. 3D sediment transport model developed using openFOAM has good agreement with experimental result and can be used to better understand effectiveness of tidal basin management.