Ecosystem carbon stocks across a tropical intertidal habitat mosaic of mangrove forest,seagrass meadow,mudflat and sandbar

Intertidal habitats provide numerous ecosystem services, including the sequestration and storage of carbon, a topic of great recent interest owing to land‐cover transitions and climate change. Mangrove forests and seagrass meadows form a continuum of intertidal habitats, alongside unvegetated mudflats and sandbars, however, studies that consider carbon stocks across these spatially‐linked, threatened ecosystems are limited world‐wide. This paper presents the results of a field‐based carbon stock assessment of aboveground, belowground and sediment organic carbon stock to a depth of 1 m at Chek Jawa, Singapore. It is the first study of ecosystem carbon stocks of both vegetated and unvegetated intertidal habitats in the tropics. Ecosystem carbon stocks were 497 Mg C ha^‐1 in the mangrove forest and 138 Mg C ha^‐1 in the seagrass meadow. Sediment organic carbon stock dominated the total storage in both habitats, constituting 62% and >99% in the mangrove forest and seagrass meadow, respectively. In the adjacent mudflat and sandbars, which had no vegetative components, sediment organic carbon stock ranged from 124–143 Mg C ha^‐1, suggesting that unvegetated habitats have a carbon storage role on the same order of importance as seagrass meadows. This study reinforces the importance of sediment in carbon storage within the intertidal ecosystem, and demonstrates the need to consider unvegetated habitats in intertidal ‘blue carbon’ stock assessments. Copyright © 2015 John Wiley & Sons, Ltd.     

Patterns of Seagrass and Sediment Nutrient Distribution Suggest Anthropogenic Enrichment in Laamu Atoll, Republic of Maldives

Sampling of seagrass cover and sediment nutrients was undertaken in lagoonal habitats of Laamu Atoll (Republic of Maldives) adjacent to three traditional fishing villages (fishing the predominant economic activity for more than 30 yr), three other villages (not traditional landing sites), and four uninhabited islands to determine if chronic input of organic fishing waste from the traditional fishing villages could explain spatial distribution of seagrass cover. Results indicated significantly greater cover of seagrass at the traditional fishing village sites than the other two site groups. Analysis of dried sediments showed sediments at traditional fishing village sites were significantly enriched with phosphorus, though no significant difference in nitrogen was found between groups of sites. These results, together with studies showing that sediment nutrient pools can limit seagrass bed development, suggest that anthropogenic enrichment of lagoonal sediments by fishing waste over generational time scales may have caused substantial proliferation of seagrass beds.     

Effects of discharge,wind, and tide on sedimentation in a recently restored tidal freshwater wetland

Sediment deposition is one of the key mechanisms to counteract the impact of sea level rise in tidal freshwater wetlands (TFWs). However, information about sediment deposition rates in TFWs is limited, especially for those located in the transition zone between the fluvially dominated and tidally dominated sections of a river delta where sedimentation rates are affected by the combined impact of river discharge, wind, and tides. Using a combined hydrodynamic–morphological model, we examined how hydrometeorological boundary conditions control sedimentation rates and patterns in a TFW located in the Rhine–Meuse estuary in the Netherlands. The modelling results show that net sedimentation rate increases with the magnitude of the river discharge, whereas stronger wind increasingly prevents sedimentation. Sediment trapping efficiency decreases for both increasing river discharge and wind magnitude. The impact of wind storms on the trapping efficiency becomes smaller for higher water discharge. The spatial sedimentation patterns are affected by all controls. Our study illustrates the importance of evaluating both the separate and the joint impact of discharge, wind, and tides when estimating sedimentation rates in a TFW affected by these controls. Such insights are relevant to design measures to reactivate the sedimentation process in these areas.     

Impact of pond aquaculture effluents on seagrass performance in NE Hainan,tropical China

The impact of pond aquaculture effluents on the distribution and performance of seagrasses was examined in NE Hainan, tropical China. Samples were taken along transects in three back-reef areas with different extent of aquaculture production in their hinterland. High δ¹⁵N in seagrass leaves and epiphytes (6–9‰) similar to values in pond effluents documented aquaculture as dominant nitrogen source in the back-reefs with decreasing impact with distance from shore. Seagrass species abundance, shoot density and biomass were lower and concentrations of nutrients, chlorophyll and suspended matter were higher at nearshore sites with high and moderate pond abundance than at the control site. High epiphyte loads and low δ³⁴S in seagrass leaves suggest temporal shading and sulphide poisoning of the nearshore seagrasses. Observed gradients in environmental parameters and seagrass performance indicate that the distance from the pond outlets and size of the adjacent pond agglomeration are major determinants of seagrass degradation.     

Fluvial suspended sediment yields over hours to millennia in the High Arctic at proglacial Lake Linnévatnet,Svalbard

Sediment yield can be a sensitive indicator of catchment dynamics and environmental change. For a glacierized catchment in the High Arctic, we compiled and analyzed diverse sediment transfer data, spanning a wide range of temporal scales, to quantify catchment yields and explore landscape response to past and ongoing hydroclimatic variability. The dataset integrates rates of lake sedimentation from correlated varve records and repeated annual and seasonal sediment traps, augmented by multi‐year lake and fluvial monitoring. Consistent spatial patterns of deposition enabled reconstruction of catchment yields from varve‐ and trap‐based fluxes. We used hydroclimatic data and multivariate modeling to examine annual controls of sediment delivery over almost a century, and to examine shorter‐term controls of sediment transfer during peak glacier melt. Particle‐size analyses, especially for annual sediment traps, were used to further infer sediment transfer mechanisms and timing. Through the Medieval Warm Period and Little Ice Age, there were no apparent multi‐century trends in lake sedimentation rates, which were over three times greater than those during the mid‐Holocene when glaciers were diminished. Twentieth‐century sedimentation rates were greater than those of previous millennia, with a mid‐century step increase in mean yield from 240 to 425 Mg km^?2 yr^?1. Annual yields through the twentieth century showed significant positive relations with spring/summer temperature, rainfall, and peak discharge conditions. This finding is significant for the future of sediment transfer at Linnévatnet, and perhaps more broadly in the Arctic, where continued increases in temperature and rainfall are projected. For 2004–2010, annual yields ranged from 294 to 1330 Mg km^?2 yr^?1. Sediment trap volumes and particle‐size variations indicate that recent annual yields were largely dominated by spring to early summer transfer of relatively coarse‐grained sediment. Fluvial monitoring showed daily to hourly sediment transfer to be related to current and prior discharge, diurnal hysteresis, air temperature, and precipitation. Copyright © 2017 John Wiley & Sons, Ltd.     

Sedimentation in arctic proglacial lakes: Mittivakkat Glacier,south‐east Greenland

Several sediment cores were collected from two proglacial lakes in the vicinity of Mittivakkat Glacier, south‐east Greenland, in order to determine sedimentation rates, estimate sediment yields and identify the dominant sources of the lacustrine sediment. The presence of varves in the ice‐dammed Icefall Lake enabled sedimentation rates to be estimated using a combination of X‐ray photography and down‐core variations in ¹³⁷Cs activity. Sedimentation rates for individual cores ranged between 0·52 and 1·06 g cm⁻² year⁻¹, and the average sedimentation rate was estimated to be 0·79 g cm⁻² year⁻¹. Despite considerable down‐core variability in annual sedimentation rates, there is no significant trend over the period 1970 to 1994. After correcting for autochthonous organic matter content and trap efficiency, the mean fine‐grained minerogenic sediment yield from the 3·8 km² basin contributing to the lake was estimated to be 327 t km⁻² year⁻¹. Cores were also collected from the topset beds of two small deltas in Icefall Lake. The deposition of coarse‐grained sediment on the delta surface was estimated to total in excess of 15 cm over the last c. 40 years. In the larger Lake Kuutuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedimentation rates determined for six cores collected from the centre of the lake, based on their ¹³⁷Cs depth profiles, were estimated to range between 0·05 and 0·11 g cm⁻² year⁻¹, and the average was 0·08 g cm⁻² year⁻¹. The longer‐term (c. 100–150 years) average sedimentation rate for one of the cores, estimated from its unsupported ²¹⁰Pb profile, was 0·10–0·13 g cm⁻² year⁻¹, suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100–150 years. The average fine‐grained sediment yield from the 32·4 km² catchment contributing to the lake was estimated to be 13 t km⁻² year⁻¹. The ¹³⁷Cs depth profiles for cores collected from the topset beds of the delta of Lake Kuutuaq indicate that in excess of 27 cm of coarse‐grained sediment had accumulated on the delta surface over the last approximately 40 years. Caesium‐137 concentrations associated with the most recently deposited (uppermost) fine‐grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fine‐grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debris, is the most likely source of the sediment deposited in the lakes. This finding is confirmed by the ¹³⁷Cs concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to those for proglacial material. Copyright © 2000 John Wiley & Sons, Ltd.     

Flocculation: a key process in the sediment flux of a large,glacier‐fed lake

This study investigates the consequences of flocculation for sediment flux in glacier‐fed Lillooet Lake, British Columbia based on density, fractal dimension, in situ profiles of sediment concentration and size distribution, and settling velocity equations presented in the literature. Sediment flux attributed to macroflocs during the late spring and summer accounts for a significant portion of sediment flux in the lake, equivalent to at least one‐quarter of the average annual sediment flux. Fine sediment is reaching the lake floor faster in flocs than occurs if settling as individual grains. This flux varies both spatially and temporally over the observation period, suggesting a link between deposition via flocculation and the properties of bottom sediments. Macrofloc flux increased through June, reached a peak during July, and then declined into August. Macrofloc flux was greatest in the distal end of the first basin, approximately 10 km from the point of inflow. Relatively high excess densities (～0·1 g cm^–3 at 500 µm) for flocs in situ are consistent with a composition dominated by inorganic primary particles. Microlaminations within Lillooet Lake varves have been linked by earlier workers to discharge events, and the action of turbidity currents, emanating from the Lillooet River. While turbidity currents undoubtedly occur in Lillooet Lake, these results demonstrate flocculation as an adjunct process linking discharge, lake level, macrofloc flux, bulk density and microlaminations. In situ measurements of sediment settling velocity in glacier‐fed lakes are required to better constrain flux rates, and permit comparison between flocculation in lacustrine environments with existing studies of estuarine, marine and fluvial flocculation. Copyright © 2009 John Wiley & Sons, Ltd.     

Age accuracy and resolution of Quaternary corals used as proxies for sea level

The accuracy of global eustatic sea level curves measured from raised Quaternary reefs, using radiometric ages of corals at known heights, may be limited by time-averaging, which affects the variation in coral age at a given height. Time-averaging was assessed in uplifted Holocene reef sequences from the Huon Peninsula, Papua New Guinea, using radiocarbon dating of coral skeletons in both horizontal transects and vertical sequences. Calibrated 2σ age ranges varied from 800 to 1060 years along horizontal transects, but weighted mean ages calculated from 15–18 dates per horizon were accurate to a resolution within 154–214 yr. Approximately 40% of the variability in age estimate resulted from internal variability inherent to ¹⁴C estimates, and 60% was due to time-averaging. The accuracy of age estimates of sea level change in studies using single dated corals as proxies for sea level is probably within 1000 yr of actual age, but can be resolved to ≤ 250 yr if supported by dates from analysis of a statistical population of corals at each stratigraphic interval. The range of time-averaging among reef corals was much less than that for shelly benthos. Ecological time-averaging dominated over sedimentological time averaging for reef corals, opposite to patterns reported from shelly benthos in siliciclastic environments.     

Fine scale distributions of porosity and particulate excessPb, organic carbon and CaCO3 in surface sediments of the deep equatorial Pacific

Sediment samples were recovered from the central equatorial Pacific Ocean, sectioned at 1-mm intervals, and analyzed for porosity, organic carbon, excess²¹⁰Pb and CaCO₃. Steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface. Similarly, particulate organic carbon contents are highest at the sediment surface, decrease sharply in the upper 1 cm, and are relatively constant between 1 and 5 cm. CaCO₃ values, on the other hand, are lowest at the sediment surface and increase to a constant value below 5–10 mm depth. At the carbonate ooze sites, excess²¹⁰Pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates. However, extremely high excess²¹⁰Pb activities (> 100 dpm/g) are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing. This apparent contradiction along with the major features of the CaCO₃ and particulate organic carbon profiles can be explained by a particle-selective feeding mechanism in which organic carbon, excess²¹⁰Pb-enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms.     

Floodplain sedimentation: Quantities,patterns and processes

This paper presents the result of measurements of floodplain sedimentation using sediment traps. The study was carried out on two embanked floodplains along the Rivers Rhine and Meuse in The Netherlands during a 3 day flood in January 1993. Raster maps of sediment accumulation were made by interpolating the measurements from the traps using block kriging. The sediment maps show clear patterns in sediment accumulation, together with the estimated interpolation errors. Average sediment accumulation ranges between 0·57 and 1·0 kg m^?2. High sediment accumulation is found on the levees (4 kg m^?2 or more) and on low lying areas (1·6 kg m^?2); sediment accumulation decreases with distance from the main channel. The sedimentation patterns are related to floodplain topography and sediment transporting mechanisms. Sediment transport by turbulent diffusion as well as by convection can be recognized. Also, flood duration and the process of sediment settling out in ponding water in closed depressions are important. The applied method allows comparison of the results with raster-based sedimentation models.     

Sedimentation patterns in the Selenga River delta under changing hydroclimatic conditions

The Selenga River delta (Russia) is a large (>600 km²) fluvially dominated fresh water system that transfers water and sediment from an undammed drainage basin into Lake Baikal, a United Nations Educational, Scientific, and Cultural Organization World Heritage Site. Through sedimentation processes, the delta and its wetlands provide important environmental services, such as storage of sediment‐bound pollutants (e.g., metals), thereby reducing their input to Lake Baikal. However, in the Selenga River delta and many other deltas of the world, there is a lack of knowledge regarding impacts of potential shifts in the flow regime (e.g., due to climate change and other anthropogenic impacts) on sedimentation processes, including sediment exchanges between deltaic channels and adjacent wetlands. This study uses field measurements of water velocities and sediment characteristics in the Selenga River delta, investigating conditions of moderate discharge, which have become more frequent over the past decades (at the expense of peak flows, Q > 1,350 m³ s^?1). The aims are to determine if the river system under moderate flow conditions is capable of supporting sediment export from the main distributary channels of the delta to the adjacent wetlands. The results show that most of the deposited sediment outside of the deltaic channels is characterized by a large proportion of silt and clay material (i.e., <63 μm). For example, floodplain lakes function as sinks of very fine sediment (e.g., 97% of sediment by weight < 63 μm). Additionally, bed material sediment is found to be transported outside of the channel margins during conditions of moderate and high water discharge conditions (Q ≥ 1,000 m³ s^?1). Submerged banks and marshlands located in the backwater zone of the delta accumulate sediment during such discharges, supporting wetland development. Thus, these regions likely sequester various metals bound to Selenga River sediment.     

Sediment yield variability in central Tunisia: a quantitative analysis of its controlling factors

Abstract

This paper refers to the quantification and prediction of the sedimentation rate of 26 hillside-dam reservoirs in Central Tunisia. The objectives of the study are to develop a simple and practical methodology to identify controlling factors of sedimentation, and to propose a regionalization from the study sites. Principal component analysis (PCA) and complementary multi-dimensional statistical methods are used to relate highly variable area-specific sediment yield to hydro-morphometric, lithological, geomorphological and anthropogenic characteristics of catchments. It appears that catchment area is not the main controlling factor of sedimentation in the studied area. The overall slope index, drainage network characteristics and runoff parameters are also important in characterizing sediment yield. Applied to the annual sedimentation rate series, PCA resulted in retaining the first three principal axes, explaining 65% of the total variance. Statistical methods showed that the overall slope index, the total drainage length, the compacity index and the runoff parameters are as important for the sedimentation quantification. This allowed a graphical clustering of the study zone into three distinct groups having similar behaviours: (i) watersheds characterized by high sediment transport rates and high runoff coefficients, (ii) basins distinguished by relatively low values of both flow discharge and sediment transport rates, and (iii) watersheds with an intermediate sediment yield, especially characterized by relatively high relief. In a second step, a multiple regression model including the four characteristic catchment properties was developed, presenting a valuable tool to predict area-specific sediment yield from catchments in central Tunisia. This model shows reasonable efficiency with an absolute prediction error of 81%.

Citation Ayadi, I., Abida, H., Djebbar, Y. & Mahjoub, M. R. (2010) Sediment yield variability in central Tunisia: a quantitative analysis of its controlling factors. Hydrol. Sci. J. 55(3), 446–458.     

Relationships between multibeam backscatter,sediment grain size and Posidonia oceanica seagrass distribution

The use of data, including bathymetry, backscatter intensity and the angular response of backscatter intensity, collected using multibeam sonar (MBS) systems to recognise seabed types was evaluated in the inner shelf of central western Sardinia (western Mediterranean sea), a site characterised by a complex seabed including sandy and gravelly sediments, Posidonia oceanica seagrass beds growing on hardgrounds (i.e. biogenic carbonates) and sedimentary substrates.     

Elevated sedimentation in lake records linked to agricultural activities in the Ishikari River floodplain,northern Japan

The purpose of this study was to examine the historical change in sedimentation rates in lakes that have been impacted by river regulation and agricultural activities in the Ishikari River floodplain. We dated sediment cores using caesium‐137 (¹³⁷Cs) dating and tephrochronology, and we estimated sediment sources from ¹³⁷Cs concentrations in the topsoil of representative land covers. We found that, between 1739 and 1963, the distance between the lake and the main river channel and whether or not the lake was connected to the river affected the sedimentation rates. After 1963, agricultural drainage systems were established in the Ishikari River floodplain. The average sedimentation rate before and after the construction of drainage ditches varied between 1–66 and 87–301 mg cm^–2 a^–1, respectively. The increase in the sedimentation rate after 1963 was caused by the construction of a number of drainage networks, as well as extensive cultivation activity and/or fragmentation of the swamp buffers surrounding the lakes. The ¹³⁷Cs activities at the surfaces of the lake as well as the catchment‐derived ¹³⁷Cs contributions and ¹³⁷Cs inventory in the lake profiles were used to examine the sediment influx from the various drainage areas after the establishment of the drainage system. Our results indicate that the majority of the lake sediments were derived from cultivated areas, and therefore the catchment‐derived ¹³⁷Cs contribution in the lakes was strongly correlated with the sedimentation rate. The ¹³⁷Cs inventory across all of the lake profiles was also significantly greater than the atmospheric fallout. We identified a negative correlation between the ¹³⁷Cs lake profile inventory and the sedimentation rate. This is because the sediment originating from the drainage areas contained low ¹³⁷Cs concentrations, which diluted the overall concentration of ¹³⁷Cs in the lake sediment. Copyright © 2009 John Wiley & Sons, Ltd.     

Depositional environments of well‐sorted detrital limestone from the Minatogawa Formation in the southern part of Okinawa Island,the Ryukyu Archipelago,Japan

Well‐sorted detrital limestone is one of the typical lithofacies of the latest interval of the Pleistocene Ryukyu Group, which is exposed in the Ryukyu Archipelago in southwestern Japan. The depositional environments of the limestone are interpreted to be extremely shallow and to include back‐reef lagoons or moats and subaerial sand dunes. However, detailed micropaleontological analyses have not been performed on this limestone. In this study, the interpretation of the depositional environments and paleo‐water depths was improved by quantitative examination of foraminiferal assemblages for the well‐sorted detrital limestone of the Minatogawa Formation in the southern part of Okinawa Island. Thin sections of limestone collected from the Minatogawa (Horikawa) quarry were subjected to sedimentological and foraminiferal analyses. Comparison with modern foraminiferal distribution within the Ryukyu Archipelago indicates that back‐reef and fore‐reef dwelling foraminifers characterize the fossil assemblages from the well‐sorted detrital limestone (bioclastic grainstone). Three ratios of indicator foraminiferal taxa (ratios of back‐reef to fore‐reef taxa, planktonic foraminifers to Amphistegina lobifera and Amphistegina lessonii, and Calcarina gaudichaudii to other Calcarina species), as well as multivariate analyses suggest that the well‐sorted detrital limestone was deposited in fore‐reef setting shallower than 40 m in water depth. A comparable depth range was reconstructed from the coral assemblage in the associated coral limestone, suggesting that the Minatogawa Formation was deposited in a gently inclined ramp setting with patch reefs and/or fringing reefs. Stratigraphic changes in paleo‐water depth, together with evidence of several unconformities associated with paleosol layers suggest that there were repeated transgressions and regressions, with an amplitude up to several tens of meters, when the Minatogawa Formation was deposited.     

The zoogeomorphology of case-building caddisfly: Quantifying sediment use

Caddisfly (Trichoptera) larvae are an abundant and widespread aquatic insect group characterized by the construction of silk structures, including nets and cases. Case-building caddisfly have the potential to modify the sorting and mobility of sand and fine gravel via: (1) case construction, resulting in altered sediment properties; (2) transporting sediment incorporated into cases over the river bed; and (3) changing the structure of river beds via burrowing activity. To investigate these mechanisms, it is necessary to understand the mass, size distribution and spatial variability of sediment use by case-building caddisfly larvae. We quantified the mineral sediment used by individuals and communities of case-building caddisfly in 27 samples, from three sites on a gravel-bed stream. The mass and size distribution of sediment in individual cases varied between taxa (mass = 0.001–0.83 g, D₅₀ = 0.17–4 mm). The mean mass of sediment used by the caddisfly community was 38 g m⁻² and varied locally. Sediment use was predominantly coarse sand (D₅₀ = 1 mm). 64% of sediment use was attributable to Agapetus fuscipes (Glossosomatidae). Due to within-species variability in case mass, the abundance of most taxa, including A. fuscipes, was only weakly associated with the mass of sediment used by this species, at the river scale. Whilst the caddisfly community used a small percentage of the total sediment available (average 2.99% of the 1–1.4 mm size fraction), A. fuscipes used more fine sediment in their cases at sites where it was more available. Despite variability in local habitat, all sites supported diverse case-building caddisfly communities utilizing mineral sediment. Consequently, geomorphological effects of case-building caddisfly are potentially widespread. The results provide novel insights into the specific grain sizes and quantities of fine sediment used by caddisfly larvae, which represents an important step towards understanding their zoogeomorphic activities. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

High-resolution upper Pliocene to Pleistocene calcareous nannofossil biostratigraphy in Ocean Drilling Program Hole 1146A in the South China Sea

We established a high-resolution calcareous nannofossil biostratigraphy for the late Pliocene–Pleistocene by analyzing a 242 m-thick, continuous sedimentary succession from Ocean Drilling Program Site 1146, Hole A, in the South China Sea (SCS). A total of 14 calcareous nannofossil datums were detected in the SCS succession. They are, in descending order: first occurrence (FO) of Emiliania huxleyi, last occurrence (LO) of Pseudoemiliania lacunosa, LO of Reticulofenestra asanoi, FO of Gephyrocapsa parallela, FO of R. asanoi, LO of large Gephyrocapsa spp., FO of large G. spp., FO of Gephyrocapsa oceanica, FO of Gephyrocapsa caribbeanica, LO of Calcidiscus macintyrei, LO of Discoaster brouweri, LO of Discoaster pentaradiatus, LO of Discoaster surculus, and LO of Discoaster tamalis. The FO of E. huxleyi was not precisely detected due to poor preservation and dissolution of nannofossils in the underlying strata. We refined the previous calcareous nannofossil biostratigraphy in the SCS by identifying Gephyrocapsa species and four evolutionary extinction events of the genus Discoaster. The proposed calcareous nannofossil biostratigraphy correlates with those reported in other terrestrial and marine areas/sites and global benthic foraminiferal δ¹⁸O records. The age–depth curves based on nannofossil biostratigraphy indicate a significant increase in the sedimentation rates at the LO of R. asanoi (0.91–0.85 Ma). The timing of this increase corresponds to reef expansion in the Ryukyu Islands linked to a stepwise increase in Kuroshio Current intensity. This timing is broadly coeval with a sea surface temperature increase of ∼2 °C in the northwestern Pacific due to expansion of the Western Pacific Warm Pool towards the north and south subtropical regions. This can be explained by increased weathering and erosion of terrestrial areas in glacial periods and increased rainfall causing higher sediment transport in interglacial periods, which were both linked to Middle Pleistocene Transition-related climatic changes.     

Sedimentation in small-scale irrigation schemes in Ethiopia: Its sources and management

Numerous irrigation schemes in sub-Saharan Africa (SSA) exhibit excessive sedimentation, resulting in underperformance and high maintenance costs. In the current study, a participatory monitoring program was used to investigate sediment causes and sources, measure the annual sediment load, and monitor desilting campaigns in two small scale irrigation schemes in Ethiopia, Arata-Chufa (100 ha) and Ketar (430 ha), for three years (2016–2018). Sedimentation quantities were huge, where the annual river sediment influx ranged from 220 m³ for the Arata-Chufa scheme to 1,741 m³ for the Ketar scheme. On average 0.3 m³/m of sediment were removed from the main canal for Arata-Chufa costing 794 days of labor per year. In Ketar, sediment quantities were even greater: 1.1 m³/m was removed requiring 3,118 days of labor per year. The sediment influx from the river source amounts to up to 95% for Arata-Chufa and moderately reaches 46% for Ketar, with the remainder of the sediment entering with overland erosion flows. Farmers reported increased sedimentation over time and difficulty paying operation and maintenance fees instead preferring to contribute labor for the desilting campaigns. Sedimentation management is fragile and mainly involves frequent desilting campaigns and unharmonized efforts to reduce overland sediment inflows. Factors contributing to sediment deposition include mild longitudinal bed slopes, the location of the intake, canal layout, and lack of canal banks for protection against surface water inflow in addition to sub-optimal canal operations. Excessive sedimentation is a major challenge resulting in underperformance of numerous irrigation schemes in SSA, and the stakeholders’ lack of awareness of the sources of sedimentation is an underlying factor aggravating sedimentation problems. It is concluded that investigating the sources, extent, and types of sedimentation entering a small-scale irrigation scheme is the basis for reducing maintenance costs and for effective management of sedimentation problems.     

In situ measurements of sediment settling characteristics in floodplains using a LISST‐ST

Due to a lack of data on settling velocities (w_s) and grain size distributions (GSDs) in ?oodplain environments, sedimentation models often use calibrated rather than measured parameters. Since the characteristics of suspended matter differ from those of deposited sediment, it is impossible to derive the w_s and GSD from the latter. Therefore, one needs to measure in situ suspended sediment concentrations (SSCs), settling velocities, effective grain sizes and sedimentation ?uxes. For this purpose we used the LISST‐ST, a laser particle sizer combined with a settling tube. In 2002 (twice) and 2004, we located the LISST‐ST with an optical backscatter sensor and sediment traps in two ?oodplains in The Netherlands: one along the unembanked IJssel River, another along the embanked Waal River. Measurements revealed that the SSC in the ?oodplains varied in relation to the SSC in the river channel. Smaller ?ocs dominated the SSC, while larger ?ocs dominated the potential sedimentation ?uxes. The in situ GSD in the IJssel ?oodplain was signi?cantly coarser than in the Waal ?oodplain, while the dispersed median grain sizes were equal for both ?oodplains. Therefore, the dispersed median grain size was two to ?ve times smaller than the effective one. The in situ grain size exhibited a signi?cant positive relationship with w_s, although the w_s for the largest ?ocs showed high variability. Consequently, the variability in sedimentation ?uxes was also large. In the actual sedimentation ?uxes, and hence in sedimentation models, in situ grain sizes up to about 20 µm can be neglected. In ?oodplain sedimentation models the relation between settling velocity and in situ grain size can be used instead of Stokes's law, which is only valid for dispersed grain sizes. These models should also use adequate data on ?ow conditions as input, since these strongly in?uence the suspended sediment characteristics. Copyright © 2005 John Wiley & Sons, Ltd.     

Runoff and road erosion at the plot and road segment scales,St John,US Virgin Islands

Previous studies have identified unpaved roads as the primary source of erosion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurements. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measured from July 1996 to March 1997 from three plots on naturally vegetated hillslopes, four plots on unpaved road surfaces and two cutslope plots. Sediment yields were also measured from seven road segments with contributing areas ranging from 90 to 700 m². With respect to the vegetated plots, only the two largest storm events generated runoff and there was no measurable sediment yield. Runoff from the road surface plots generally occurred when storm precipitation exceeded 6 mm. Sediment yields from the four road surface plots ranged from 0·9 to 15 kg m⁻² a⁻¹, and sediment concentrations were typically 20–80 kg m⁻³. Differences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed that cross‐slope roads intercept shallow subsurface stormflow and convert this into surface runoff. At the road segment scale the estimated sediment yields were 0·1 to 7·4 kg m⁻² a⁻¹. Road surface runoff was best predicted by storm precipitation, while sediment yields for at least three of the four road surface plots were significantly correlated with storm rainfall, storm intensity and storm runoff. Sediment yields at the road segment scale were best predicted by road surface area, and sediment yields per unit area were most strongly correlated with road segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle‐size analyses indicate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & Sons, Ltd.