Effects Of Sediment Supply Research Articles

Sediment Supply Effects in Hydrology‐Sediment Modeling of an Alpine Basin

AbstractIn mountain river basins, sediment availability on hillslopes and in channels is key to predict the sediment response to hydrological forcing. However, quantification of sediment availability and its variability in time is challenging, because sediment supply is often strongly stochastic and dominated by mass wasting. In this paper, we introduce a variable landslide sediment supply as a function of topography, hydrology, and hillslope activity in the hydrology‐sediment model TOPKAPI‐ETH. We use the model to analyze the dynamics of sediment storage in a mesoscale pre‐Alpine basin. We simulate a range of transport‐ and supply‐limited conditions to quantify the variability of suspended sediment concentrations and load, and the seasonal dynamics of sediment storage. We show that supply limitation dampens the natural variability of the hydrological and sediment transport processes, and therefore reduces the scatter of the suspended sediment rating curve. By comparing the model results with observations, we demonstrate that alternation of low and high sediment availability favors sediment load variability at the outlet. The temporal dynamics of sediment storage depends on the hillslope activity, and the balance between sediment supply by landslides and evacuation by runoff. When the export flux is comparable or greater than the recharge flux, the storage shows seasonal fluctuations with a maximum in late winter or spring, and sediment starvation in summer. By representing the dynamics of both inputs and outputs of the sediment storage, the proposed model provides a physically based tool to isolate the effect of transport‐ and supply‐limited conditions in basin sediment response.

Effect of sediment supply on geometry of wave ripples: preliminary results of flume experiment

ウェーブリップルの形状への堆積物供給の影響を調べるための造波水路実験を行った．実験では，水理条件がほぼ同一で，上方より降らせる堆積物供給の速度のみを3段階に変化させることでその影響を調べた．堆積物供給速度が比較的小さい実験では，ウェーブリップルの形状は維持されたまま積み重なっていた．堆積物供給速度が大きくなるにつれてウェーブリップルは平坦化され，その領域は広くなった．今回の実験の水理条件および堆積物の条件（水深：0.3 m，波の周期：1.0 s，波の平均波高：76-78 mm，堆積物粒径：0.20 mm）の下では，砂床上昇速度が32 mm/min以上の実験でウェーブリップルの平坦化が観察された．今回の実験は，ウェーブリップル形状は堆積物供給の影響を受けないというこれまでのウェーブリップル葉理形成モデルにおける仮定が，堆積物供給速度が大きい場合には必ずしも成り立たないことを示唆している．

Effect of Sediment Supply on Morphodynamics of Free Alternate Bars: Insights from Hydrograph Boundary Layer

Sediment supply plays an essential role in river morphology. However, the specific impact of sediment supply on river morphology is not apparent. According to the hydrograph boundary layer (HBL) concept, upstream riverbed changes caused by the imbalance between sediment supply and the capacity can propagate only a limited length and have a negligible effect on the riverbed beyond such a short length. We performed a two-dimensional morphodynamic calculation to test the concept of HBL, which was proposed under a one-dimensional simulation, meaning that the concept of HBL is still valid for plane changes in river morphology. We employed an unsteady flow with equilibrium or constant sediment supply in a straight, modeled gravel-bedded channel with an unerodible bank to simulate alternate bar morphodynamics. The results show that regardless of the sediment supply condition, the alternate bar features formed downstream of the HBL are considerably similar. This suggests that sediment disturbance at the upstream end has a negligible effect on the mobile-bed dynamic processes, including alternate bar formation and development downstream of the HBL.

Effect of Sediment Supply on Cyclic Fluctuations of the Disequilibrium Ratio and Threshold Transport Discharge, Inferred From Bedload Transport Measurements Over 27 Years at the Swiss Erlenbach Stream

AbstractBedload transport in natural channels typically shows large fluctuations even for constant hydraulic forcing. It has been shown that the threshold shear stress for initiation of motion depends, for example, on the bed morphology and on channel slope and that it may be influenced by flood history effects. This study is based on a unique data set of continuous field observations of bedload transport acquired during 27 years with impact plate measurements and involving more than 500 sediment‐transporting flood events. A disequilibrium ratio is defined which represents the observed bedload mass per event divided by the bedload mass calculated via a bedload transport equation integrated over the time of a flood event. The disequilibrium ratio varies in a cyclic behavior around equilibrium conditions defined by its median value. The mean duration of a cycle depends on upstream sediment supply or availability on the bed, and longer cycles are associated with a larger active layer on the bed. A memory effect is evident regarding the temporal evolution of both disequilibrium ratio and bed state as reflected by threshold transport conditions, determined from the observations as the critical discharge at the start and at the end of a bedload‐transporting event. The disequilibrium ratio and the threshold transport discharge are inversely correlated with each other, likely providing a feedback mechanism governing the fluctuations around an equilibrium state. Accounting for the memory effect, that is, for the bed state after the previous event, improves the prediction of bedload transport for the following event.

The Effect of Episodic Sediment Supply on Bedload Variability and Sediment Mobility

AbstractThe effect of sediment supply on sediment mobility is analyzed for a poorly sorted (0.5–64 mm) experimental bed. Water discharge was held constant over a sequence of seven runs, and 300 kg of sediment was supplied during each run in different magnitudes and frequencies. In runs with constant feed bedload transport rate increased gradually. In contrast, runs that received large sediment pulses showed pronounced increases in bedload rate as the bed surface got finer, followed by monotonic declines as the bed surface coarsened. We studied the temporal scales of bedload fluctuations by means of sample autocorrelation coefficients and the rates of decrease in bedload fluctuation with sampling time scale. The significant trends caused in bedload rate by large occasional sediment pulses increased long‐term autocorrelation in bedload rate time series relative to runs with constant feed. Bed evolution and local changes in sediment storage caused multiple scales of variability in bedload rate, which increased autocorrelation and caused long‐term persistence in bedload series over periods with a nearly constant mean. The scaling statistics of bedload transport fluctuation depended on grain size, and those for total bedload were similar to those for fine gravel (2–8 mm), which was fully mobile and dominated bedload transport. Grain size dependence of bedload fluctuation was not affected by changes in sediment feed because water discharge and sediment texture were held constant.

The effects of sediment supply and concentrations on the formation timescale of martian deltas

We used a comprehensive 2-D and 3-D basin-filling morphodynamical model to explore the effects of changing hydrologic and sedimentary conditions on delta formation under martian conditions. Spatial and temporal deposition of sediment into a body of standing water was simulated using event-based time stepping (e.g. seasons). River and sediment discharge were calculated using the Darcy–Weisbach equation for width- and depth-averaged flow velocity and well-known and tested terrestrial sediment transport predictors modified for application to martian flows. A range in sediment grain sizes was determined from both landers and orbiting spacecraft data in order to include grain sizes typical of Mars and yet representative of the martian deltas. Several different scenarios of seasonal river discharge and sediment supply were explored to encompass a range of possible formation conditions.With the ability to track sediment as it is transported throughout the basin, it is apparent that non-negligible amounts of material are moved beyond the delta-forming region. This loss of sediment increased with river discharge, increasing the gap between the calculated and modeled formation timescales. These results have important implications for the formation timescales of the deltas and the small number of identifiable deltas on Mars today. Formation timescales that consider only the supply of sediment to the system and do not take into account factors affecting deposition and delta formation are underestimates of the actual amount of time needed to form the deposits. The lack of identifiable deltas in many paleolakes is likely a result of significant sediment transport beyond the channel mouth, causing the expected delta deposit to be draped throughout the basin.

Controls on aeolian construction in the Mojave Desert – distinguishing the effects of sediment supply, availability, and mobility

Controls on aeolian construction in the Mojave Desert – distinguishing the effects of sediment supply, availability, and mobility

The impact of different sediment concentrations and sediment transport formulas on the simulated floodplain processes

Summary Overbank sedimentation is an important process in river floodplain ecosystems and is a component of the floodplain geomorphologic evolution. The impact of suspended sediment supply on floodplain processes is still unclear because sediment deposition can be influenced by many factors. We quantified the effect of sediment supply (suspended sediment) and transport formulas on simulated floodplain processes using a coupled two-dimensional hydrodynamic and sediment transport model (MIKE21C). Erosion and deposition depths, net sedimentation depth and total volume were quantified based on the last time step of the simulation period. The MIKE21C model was validated by comparing simulated water surface elevations to those from a one-dimensional hydrodynamic model. We compared the sediment transport model simulated suspended sediment concentrations (SSCs) to measured concentrations at a gage station. Erosion and deposition processes were simulated using five hydrograph scenarios as a function of high and low SSC and two sediment transport equations, Van Rijn (1984) and Engelund and Hansen (1967) . A specific location could be an erosional or depositional zone at different time steps of the simulation. Thus, floodplain deposition is a discontinuous function of river discharge and varies spatially and temporally over the floodplain. Large flows with high SSC were more effective for floodplain deposition than lower discharges, which dominantly caused sediment scour. Coupled hydrodynamic and sediment transport models that account for feedback processes between topography and hydraulics should be given first preference for future floodplain restoration projects. From a restoration perspective, larger flows are required for greater floodplain deposition rates and maintenance of dynamic processes. The Engelund and Hansen (1967) equation simulated higher transport rates than the Van Rijn equation (1984) . For future studies, transport equations should be selected based on the study objectives and field characteristics. The current model might be used to analyze the impact on floodplain processes from altered SSC due to Libby Dam operation.

マスバランス論に基づく堆積システム累重・堆積盆埋積様式の類型化（その２）：2次元変化と砕屑物供給の影響

マスバランス論に基づく堆積システム累重・堆積盆埋積様式の類型化（その２）：2次元変化と砕屑物供給の影響

Stratigraphic architecture and evolution of the continental slope system in offshore Hainan, northern South China Sea

We investigate the evolution of the stratigraphic architecture of two siliciclastic northern South China Sea continental slope systems with distinct structural styles, including rift-transform and rift segments. Our results reveal that the stratigraphic patterns of the slope system in the Yinggehai basin are mainly characterized by progradational slope clinoforms, which result in a rapid shift of shelf edges seaward and southeastward due to a high rate of sediment supply. In contrast, in the eastern Qiongdongnan area, the slope margins show a vertical stacking pattern or a slight shift seaward, characterized by dense gravitational faults, rollover structures, and slump deposits. The associated stratigraphic architecture in slope clinoforms is chaotic. Reactivation of pre-existing strike-slip faults in the Yinggehai area and extensional faults in the Qiongdongnan area resulted in the formation of initial slope clinoforms during the Miocene post-rifting stage. Dextral movement of the northeast marginal fault (No. 1 fault) since the Early Pliocene resulted in the formation of a deep embayment and early slope clinoforms along the No. 1 fault in the Yinggehai basin. The formation of initial slope clinoforms is closely related to movement along the underlying basement fault in the eastern Qiongdongnan area. Although the development of the slope architecture is influenced by tectonic movement and eustasy, the primary control of the geometry of slope clinoforms is through the effects of sediment supply. Plentiful sediment supply leads to rapid progradational slope clinoforms at a large distance of progradation in the Yinggehai and adjacent areas between the two basins. Sediment-starved margins occur in the eastern Qiongdongnan area because of a very wide shelf and insufficient sediment supply.

Holocene sedimentary evolution and palaeocoastlines of the Lower Khuzestan plain (southwest Iran)

The Holocene sequence of the Lower Khuzestan plain in southwest Iran has been investigated in the context of coastal evolution and relative sea-level change. A literature review about the coastal evolution of the Shatt-el Arab region with respect to relative sea-level changes is provided. The sedimentary succession in undisturbed hand-operated cores and temporary outcrops is described and facies are identified on the basis of lithology, sedimentary structures and macrofossils. Four main sedimentary environments are interpreted from the Holocene sedimentary record of the plain: brackish tidal flat, clastic coastal sabkha, brackish–freshwater marsh and fluvial plain. The study of the vertical and spatial distribution of sedimentary facies, their environmental interpretation especially with respect to the relationship to tide levels is done for five different zones in the plain. Chronological control is provided by radiocarbon dates. On the basis of this analysis, palaeogeographical reconstructions of the Lower Khuzestan plain, and the northern extension of the Persian Gulf were made for different points in time between 8000 and 450 cal BP. This study shows that during the early and middle Holocene, the Lower Khuzestan plain was a low-energy tidal embayment under estuarine conditions. During the initial sea-level rise of the early Holocene, the coastline rapidly transgressed across the shelf, and drowning of a major valley resulted in the development of extended tidal flats. Deceleration of sea-level rise after approximately 5500 cal BP, together with probably more arid conditions, allowed coastal sabkhas to extend widely and to aggrade while the position of the coastline remained relatively stable. Continued deceleration of sea-level rise initiated the progradation of the coastline from ca. 2500 cal BP. The effect of sediment supply by the rivers became more important than the effect of relative sea-level rise. The Karun megafan developed under a descelerating rate of sea-level rise, controlling the avulsive shifting of the rivers Karkheh and Jarrahi and their loci of sediment input. Moreover, in this study it is suggested that a Holocene RSL highstand, above the present-day sea level, as suggested by previous workers, did not occur.

“Hummocky” bedform and its internal structure in an oscillatory-flow circular flume experiment : effects of aggradation

Hummocky cross-stratification (HCS) is one of the most common and widely recognized structures in ancient storm-dominated sequences. It has been thought to be formed in shallow marine environments during high-energy storm events. There is little knowledge of sedimentary processes of it because modern shelf observation during storms is limited and experimental approaches had never succeeded in forming HCS. U-shaped wave duct was commonly used in forgoing experimental studies. This is well-suited to generating oscillatory flow but not to supplying sediments because it consists of a closed duct.New circular flume used in this study was designed for observation of sedimentary processes with sediment supply under long-period oscillatory flow as high-energy wave in storm event. This flume can generate flow by paddle movement and can keep hydraulic condition constant even in bed aggradation.Three runs with different rates of sediment supply were conducted. Quartz sands with the mean diameter of 0.3 mm are used. Period of oscillatory flow is 15 sec and maximum near-bed velocity is 80 cm/sec. It was observed that the development of hummocky bedform was suppressed under aggrading conditions with the rate of 1-3 mm/min. This rate is not exceptional in modern shelf. In the run with temporal change of supply rate from 0 to 1 mm/min, HCS-like structure was formed. Upward flattening laminae were observed in each bed, which is common in ancient rock record of HCS. This result suggests that upward flattening laminae in HCS might be formed with increasing of sediment supply. Although further confirmation that the effect of sediment supply is not an artifact of experimental design is needed, this new criteria for sediment supply rate contributes to better reconstruction of ancient marine sedimentary processes.

EFFECTS OF SEDIMENT SUPPLY AND GRAIN SORTING ON LOW-WATERCOURSE FORMATION IN CHANNEL WITH ALTERNATE BARS

Effects of sediment supply and grain sorting on the formation process of low-watercourse in a straight channel with alternate bars are investigated by means of flume tests using uniform and non-uniform sediment. Results are summarized as follows:(1) The low-watercourse is shallow and unstable when the bed level does not vary by sediment supply, whereas the deep and stable low-watercourse is formed when the bed degradation progresses by no sediment supply.(2) The emerged bars in the non-uniform sediment bed are formed with grain sorting, and their upper and lower part mainly consists of coarse grains and fine grains, respectively. These emerged bars are more stable than those in the uniform sediment bed, and contribute to the stability of the low-watercourse.(3) The meandering wavelength of the low-watercourse increases considerably with progression of the bed degradation, and is about 2-3 times the mean wavelength of the alternate bars.

Case Study: New Zealand Bridge Scour Experiences

Details of three case studies of scour damage for New Zealand bridges are presented. These cases cover ranges of bed materials, flood magnitudes, and river morphologies, and illustrate a range of scour processes occurring at bridge foundations. The case study details facilitate evaluation of proposed bridge scour calculation methodologies. Such a proposed methodology for the assessment of scour depths at a bridge site is illustrated for the presented case studies. The resulting scour predictions highlight the value of a judicious use of the proposed methodology, the effect of sediment supply and transport balance considerations for engineering projects, aspects of river morphology to be considered in bridge foundation design, and that the combination of various components of scour needs to be considered when assessing bridge scour.

Comment on “Effects of hydraulic roughness on surface textures of gravel‐bed rivers” and “Effects of sediment supply on surface textures of gravel‐bed rivers” by John M. Buffington and David R. Montgomery

Comment on “Effects of hydraulic roughness on surface textures of gravel‐bed rivers” and “Effects of sediment supply on surface textures of gravel‐bed rivers” by John M. Buffington and David R. Montgomery

Aeolian saltation transport rate: an example of the effect of sediment supply

The channel of the Whitewater River near Palm Springs, California, is an excellent location to assess the effect of sand supply and mobility on the quantity of material moved by aeolian saltation. Saltation fluxes at the site taken by Sharp (1964) were compared to the fluxes that would have been produced under ideal conditions for the spectrum of winds inferred for the site. The discrepancy between the two fluxes is, in general, consistent with whose observed elsewhere for similar surfaces. Variations are linked to runoff in the Whitewater River and, hence, the supply of fresh sand.