Patterns Of Sediment Storage Research Articles

Shoreline change of the Mekong River delta along the southern part of the South China Sea coast using satellite image analysis (1973-2014)

The Mekong River delta is the world’s third largest delta, consequence of a favourable morphosedimentary setting, high sediment supply and rapid growth during the Holocene. Analysis of Landsat satellite images from 1973 to 2014 shows that nearly 70 % of the 160 km - long South China Sea shoreline of the delta has strongly eroded. This trend represents a reversal of the massive, long-term Holocene progradation that characterized this part of the delta. Erosion and land loss along the South China Sea coast are not related to lobe switching, but more likely to decreasing river sediment supply and variations in patterns of sediment storage in the delta that appear to be due to human-induced modifications. These include fluvial sediment trapping by dams, enhanced subsidence due to massive groundwater abstraction, and riverbed aggregate extractions. Shoreline erosion is further exacerbated by the replacement of protective mangroves by shrimp farms. This erosion constitutes an additional hazard to the future integrity of a mega-delta already considered particularly vulnerable to subsidence, and to future large-capacity dams.

Postglacial sediment budget of Chilliwack Valley, British Columbia

ABSTRACTThe paraglacial reworking of glacial sediments by rivers and mass wasting is an important conditioning factor for modern sediment yields in mountainous catchments in formerly glaciated regions. Catchment scale and patterns of sediment storage are important influences in the rate of postglacial adjustment. We develop a quantitative framework to estimate the volume, sediment type, and fractional size distribution of legacy glacial materials in a large (1230 km2) watershed in the North Cascade Mountains in south‐western British Columbia, Canada. Chilliwack Valley is exceptional because of the well‐dated bounds of deglaciation. Interpolation of paleo‐surfaces from partially eroded deposits in the valley allows us to estimate the total evacuated sediment volume. We present a chronology of sediment evacuation from the valley and deposition in the outlet fan, based on infrared stimulated luminescence (IRSL) and14 C dating of river terraces and fan strata, respectively. The effects of paraglacial sedimentation in Chilliwack Valley were intensified through a major fall in valley base‐level following ice retreat. The steepened mainstem valley gradient led to deep incision of valley fills and fan deposits in the lower valley network. The results of this integrated study provide a postglacial chronology and detailed sediment budget, accounting for long‐term sorting of the original sediments, lag deposit formation in the mainstem, deposition in the outlet fan, and approximate downstream losses of suspended sediment and wash load. The mass balance indicates that a bulk volume of approximately 3.2 km3of glacial material has been evacuated from the valley. Copyright © 2012 John Wiley & Sons, Ltd.

A water and sediment budget for the lower Mississippi–Atchafalaya River in flood years 2008–2010: Implications for sediment discharge to the oceans and coastal restoration in Louisiana

Summary The Mississippi is the largest riverine system in North America and one of the most engineered rivers in the world. The challenges of studying the Mississippi River are due to its complex sediment–water dynamics and the multi (and often competing) uses for its resources. Flood control and navigation are primary factors that control how the river is managed. A third factor is the use of river resources, namely water and sediment, for nourishing the degrading coastal wetlands of the states of Louisiana and Mississippi. As such, these factors must be fully considered and coordinated while investigating and developing techniques to harness the sediment resources of the River for coastal restoration. This paper presents a detailed suspended sediment budget analysis for the lowermost Mississippi and Atchafalaya River systems for the flood years of 2008, 2009, and 2010. Data were derived mainly from Federal and State of Louisiana measurements of water discharge and suspended sediment load at (1) monitoring stations along the river channel and (2) boat-based measurements made during specific project studies at natural passes and man-made channel diversions. The present study was focused on flood years 2008–2010 to (1) minimize the influence on the budgets of a historical decline in sediment loads carried by the river as observed by previous investigators and (2) take advantage of recent improvements in the monitoring network. The results show that both the Mississippi and Atchafalaya distributary pathways were efficient at sequestering suspended sediments, particularly the larger (sand) size fraction. Approximately 44% of the total Mississippi + Red River suspended load (80% of the sand) reaching Old River Control structures split between the distributaries was sequestered upstream of the Gulf of Mexico by overbank storage and channel bed aggradation. Increases in bed aggradation in the Mississippi distributary are linked to a loss of stream power associated with man-made and natural exits upstream of the Gulf of Mexico. This further decreased the water and suspended sediment load reaching the deep water Gulf distributary exits in FY08–10 to 46% (water), 19% (total suspended load), and 1.4% (suspended sand). These patterns of sediment storage and delivery have major implications for channel dredging and river sediment diversions planned for Louisiana coastal restoration.

Dynamic sedimentary environments of an Arctic glacier-fed river estuary (Adventfjorden, Svalbard). I. Flux, deposition, and sediment dynamics

The spatial patterns of the transport, settling, and deposition of sediments were studied in a small Arctic glacier-fed river estuary (Adventfjorden, west Spitsbergen, Svalbard). Salinity and temperature, suspended solids concentration, and particle fluxes in the water column were measured at seven stations situated on the tidal flat formed at the river mouth, the delta slope, and along the fjord axis down to the fjord mouth. Turbidity currents were measured in situ using a current meter combined with a turbidity meter. The sediments collected with a Niemisto gravity corer were x-rayed and analyzed for granulometric composition. The results of the major sampling campaign undertaken in July 2002 were supplemented with winter hydrological data from January 1996. Since the river is frozen in winter, no fresh water or fluvial sediment supply is observed in the fjord water column. A hypopycnal plume of brackish water (1.5 m thick) extends in summer as far as 0.8 km from the river mouth. The highest concentration (826 mg l −1) and vertical flux (over 1000 g m −2 day −1) of suspended solids were noted at the edge of the tidal flat and over the upper slope of the delta. Both concentrations and the solid particle flux decreased as distance from the river mouth increased. Local peaks of turbidity in the intermediate and deep water layers in the central fjord indicated the presence of hyperpycnal plumes. Tidal flat sediments are rhythmically laminated by the tidally controlled resuspension/redeposition cycles and occasionally eroded by catastrophic events such as intense storms or winter ice cover scouring. The steep inclination of the bottom of the slope (15–19°) promotes the gravity-driven processes of sediment transport (debris flows). Frequent resuspension and redeposition events can be traced on x-radiographs and in the granulometric composition of the sediment cores collected on the slope. Turbidity currents are observed throughout the fjord, although their intensity and frequency of occurrence change as distance from the river mouth increases. The near bottom hyperpycnal flows transport the finest sediment over the slope and into the low energy environment of the central basin of Adventfjorden. The current study shows that the patterns of sediment storage and sedimentary dynamics in a glacier-fed river estuary are driven by gravity flows and turbidity currents rather than by the patterns of the vertical sedimentation of suspensions in the water column.