Prolonged impact of earthquake-induced landslides on sediment yield in a mountain watershed: The Tanzawa region, Japan

To determine for how long a landslide affects sediment discharge, the sediment yields of 15 check-dam basins were compared with the time series of landslide distributions in a mountain basin in the Tanzawa region, central Japan. The distribution of sediment yield was quantitatively estimated from deposition in the sediment pools of check dams. The relationship between the landslide history and sediment discharge in the Nakagawa River basin was examined for an approximately 80-year period. Two major landslide events occurred during this period: the 1923 Kanto Earthquake and the 1972 disaster caused by heavy rainfall. The resulting trend in sediment discharge of the whole basin, estimated using reservoir sedimentation in the Miho Dam at its base, was nearly constant, with high sediment discharge (2897 m³ km^− 2 yr^− 1) in the intervening quarter-century, despite the recovery of vegetation on landslide areas in this period. Comparisons of the landslide distributions resulting from the two disasters, the sediment yields of check-dam basins, and the sediment discharge of the whole basin indicate that recent sediment discharge contains landslide debris that was originated by the Kanto Earthquake that occurred over 80 years ago. Thus, to understand high sediment discharge, it is essential to investigate not only the current basin condition and recent events, but also the landslide history of the basin for at least the previous 100 years.     

Alluvial and colluvial sediment storage in the Geul River catchment (The Netherlands) — Combining field and modelling data to construct a Late Holocene sediment budget

We used a combined approach of a two-dimensional erosion and hillslope sediment delivery model (WATEM/SEDEM) and detailed geomorphological reconstructions to quantify the different components in a sediment budget for the Geul River catchment (southern Netherlands) since the High Middle Ages. Hillslope erosion and colluvium deposition were calculated using the model, while floodplain storage was estimated using field data. Our results show that more than 80% of the total sediment production in the catchment has been stored as colluvium (mostly generated by hillslope erosion), while almost 13% is stored in the floodplain since the High Middle Ages (this situation resembles a capacity-limited system). Model results for the period prior to the High Middle Ages (with a nearly completely forested catchment) show that far less sediment was generated and that most of the sediments were directly transported to the main river valleys or out of the catchment (a supply-limited system). Geomorphological analysis of a large alluvial fan shows the sensitivity of the study area to changes in the percentage of arable land.Our combined field data-modeling study presents an elegant method to calculate a catchment sediment budget for a longer period and is able to identify and quantify the most important sediment storage elements. Furthermore, it provides a valuable tool to calculate a sediment budget while only limited dated fluvial sediment sequences are available.     

Sonication as an aid in cleaning cladoceran remains extracted from sediment cores

Ultrasonic treatment (sonication) of the Szczecin Lagoon sediment samples processed for examination of cladoceran remains is described. As opposed to cladoceran remains from non-sonicated sediment, those extracted from the ultrasound-treated samples were clean, easily stained, clearly visible in microscope slides and identifiable. Sonication is therefore recommended as an aid in rendering cladoceran remains free of adhered particles that obscure the specimens and complicate their identification in some sediments.     

Diatoms and pollen as indicators of water quality and land-use change: a case study from the Oak Ridges Moraine,Southern Ontario,Canada

Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l⁻¹ in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km². Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.     

Dinocyst microlaminations and freshwater "red tides" recorded in Lake Xiaolongwan,northeastern China

We reported a special type of lamination formed in the sediments of Lake Xiaolongwan, northeastern China. The lamination consists of light- and brown-colored laminate couplets in the thin sections. The brown-colored layer is composed mainly of dinoflagellate cysts. The grey-colored layer consists of other organic and siliceous matter (plant detritus, diatoms, chrysophyte cysts) and clastics. Preliminary sediment trap results show that a distinct peak of dinocyst flux occurred in November. The dinocyst flux maximum also corresponds to the peaks of diatom flux and chrysophyte stomatocyst flux. These suggest that "red tide blooms" occur in this freshwater lake. We speculate that the dinocyst flux maximum could be related to autumn overturn due to increased nutrients, and the availability of cysts for germination from the lake bottom. Additionally, it may also reflect increasing dissolved organic matter after leaf fall. An independent chronology derived from ¹³⁷Cs and ²¹⁰Pb shows a good agreement with counted laminations. From the sediment trap data and the independent chronology data, the dinocyst microlaminae appear to be annually laminated, and probably could be called dinocyst varves. Although vegetative (thecate stage) cells of Peridinium volzii and Ceratium furcoides are found in the water samples, it is not possible to relate the dinocysts to these two dinoflagellate species. Based on morphological and ecological analyses, we suggested that they have affinities with species of Peridinium (sensu lato), most probably to P. inconspicuum. Detailed investigations should be carried out to understand the red tide history in this freshwater lake. Annually laminated dinocyst microlayers in freshwater and marine sediments not only provide an uncommon archive for understanding the history of red tides and harmful algal blooms, and why and how certain species periodically bloom over several thousands years, but also provide important records of paleoenvironmental and paleoclimatic changes at seasonal to annual resolution.     

CHARACTERISTICS OF SEDIMENT TRANSPORT ALONG A RIVER REACH WITH A RESERVOIR

Based on long-term measurements at three gauging stations, Toudaoguai, Fugu and Hequ, and one meteorological station, this article discusses the features of discharge (Q) and sediment concentration (Cs) of a river reach of the Yellow River with a reservoir located in the Loess Plateau. The impacts of the local sub-watershed between Toudaoguai and Fugu gauging stations on sediment budget to the Yellow River have been analyzed. In addition, the deposition processes in the Tianqiao Reservoir have been investigated. Results show over 80% of the precipitation that falls in the local subwatershed is unable to contribute to the Yellow River runoff process. It is found that the annualmaximum sediment concentration is usually less than 30 kg/m^3 during flood seasons at Toudaoguai Gauging Station, but the sediment concentration varies dramatically at Fugu Gauging Station. About 35% of the sediment eroded in the sub-watersheds between Toudaoguai and Fugu gauging stationswas produced from the Huangfuchuan sub-watershed which has a drainage area accounting only for 10% of the drainage area between Toudaoguai and Fugu gauging stations. The Tianqiao Reservoir generally has deposition during the summer flood season, and scouring during the non-flood season.On average, over 85% of deposited sediment in the reservoir occurs in the 12 km long lower reservoir reach. The volume of annual deposition in the reservoir mainly depends on the volume of water from the local region between Hequ and Fugu gauging stations.     

FIELD OBSERVATIONS ON BED SEDIMENTS AND SANDWAVES IN CHANGJIANG ESTUARY

A series of advanced instruments were applied to obtain the field data of bed, bed forms and relevant dynamic factors in the 150 km-long reach of the Changjiang River from Jiangyin to Hengsha Island in March 2002. We found that well-sorted fine sand was in the majority of the bed sediment,which had a median diameter of 2φ. Well-developed sandwaves were formed because the grains jumped together on the bed, which was related to the granularity and current velocity during theflood-ebb tidal cycles.     

NUMERICAL SIMULATION OF HEAD-CUT WITH A TWO-LAYERED BED

1INTRODUCTION The rate of gully erosion is dominated by the upstream migration of existing nick-points called headcut.Due to the shape of the headcut,the flow from the upstream channel impinges into the pool of the scour hole and forms a complex three-dimensional flow structure.The turbulent flow deepens the scour hole,transports the eroded material downstream,undercuts the headcut wall and creates gravitational slumping of the gully head material.In reality,the occurrence of a head cut i…     

ESTUARINE AND COASTAL SEDIMENTATION PROBLEMS

This keynote lecture addresses engineering sedimentation problems in estuarine and coastal environments and practical solutions of these problems based on the results of field measurements, laboratory scale models and numerical models. The three most basic design rules are: (1) try to understand the physical system based on available field data; perform new field measurements if the existing field data set is not sufficient (do not reduce on the budget for field measurements); (2)try to estimate the morphological effects of engineering works based on simple methods (rules of thumb, simplified models, analogy models, i.e. comparison with similar cases elsewhere); and (3)use detailed models for fine-tuning and determination of uncertainties (sensitivity study trying to find the most influencial parameters). Engineering works should be designed in such a way that side effects (sand trapping, sand starvation, downdrift erosion) are minimum. Furthermore, engineering works should be designed and constructed or built in harmony rather than in conflict with nature. This ‘building with nature‘ approach requires a profound understanding of the sediment transport processes in morphological systems.