High Flow Stages Research Articles

Magnitude of formative flows in stream potholes

Although it is generally recognized that geomorphic work is tied to bedrock channel reshaping, the importance of low vs. high flow stages that cause the most geomorphic impact remains unclear. The objective of the research is to study the concept of “formative flow” in bedrock channels and determine, through morphological studies, if those flows have any impact on sculpted features such as potholes and how this relationship relates to various inputs such as flow stages (magnitude and frequency), shear stress, and sediment size. Here, we studied the distribution of the main pothole typologies and tried to understand why potholes are found along bedrock river channels. Specifically, we examined stream potholes from three locations along the Spanish Central System: Alberche, Tietar, and Manzanares rivers. We conducted the research by taking precise geometric measurements, classifying potholes, analyzing flow magnitude and frequency, and using a two-dimensional (2D) hydrodynamic model to assess key variables in Manzanares river. This research demonstrated that bankfull depths completely cover all pothole typologies in all the analyzed sites but are not sufficient to achieve its formative flow depth (FFD). Using a detailed 2D hydrodynamic model in Manzanares river, we discovered that dimensions of cylindrical potholes are closely related to bankfull discharge and that this depth is connected to FFD. Other potholes, such as erosive-compound and erosive-lateral, are historical remnants, and their shapes are not related to any particular FFD and are likely associated with rare events and catastrophic breaks. A collection of laterals that exhibit FFD near bankfull flows appear to represent a part of the recent evolution of a knickpoint. To summarize, it can be inferred from the findings that the utility of morphological analysis in conjunction with the 2D hydrodynamic model is to examine the fraction of erosional/active features to determine the degree of senescence and/or change in natural conditions in a river reach.

Quantification of shunt fraction using contrast ultrasound and indicator dilution in an in vitro model

Transthoracic saline contrast echocardiography is commonly used to assess intrathoracic shunt flow in vivo. Though the technique has many advantages (safe, simple, repeatable), the measurement technique lacks specificity, and the contrast agent has limited stability. This study sought to determine if the indicator dilution modeling technique could be applied to ultrasound contrast data to quantify shunt fraction and to determine if buoyant force has a significant effect on microbubble pathway determination at a “vascular” bifurcation. A model of the pulmonary circuit was perfused with blood at three distinct flow rates (low, medium and high) over shunt fractions ranging from ∼2–10 %. The buoyancy effect on contrast was quantified using a simplified in vitro model of a vascular bifurcation that had an upper and lower outflow tract where saline contrast formed from carbon monoxide (CO) gas passed through the bifurcation, was collected and quantified. The indicator dilution model was found to have a mean bias of − 3.2 % for the low flow stage, − 2.6 % for the medium flow stage and − 1.4 % for the high flow stage compared to volumetric measurements, suggesting agreement increases with increasing flow rate. Investigations of the buoyant effects revealed that at lower flow rates, contrast bubbles that encounter a bifurcation will favor the upper outflow tract over the lower. However, this effect is reduced by increasing the flow rate two-fold. These data identify that application of indicator dilution theory to contrast ultrasound data and the pathway ultrasound contrast travels in a network of tubules is flow dependent.

Sedimentation of open-framework gravels in alluvial-fan settings: Quaternary Poplar Fan, northwest China

Open-framework gravels (OFGs) form common lithofacies of alluvial fans. However, inferences of their mechanisms of transport and deposition, and of controls on their distribution in alluvial-fan successions, remain difficult, limiting our ability to interpret the environmental significance of such deposits and to predict their distributions in ancient successions. From an applied standpoint, such predictions are important in the development of hydrocarbon reservoirs and remediation of aquifers hosted in alluvial-fan strata. To elucidate our understanding of formative conditions of different types of OFGs, a study has been undertaken on the Quaternary Poplar Fan, in the endorheic Heshituoluogai Basin, China.Based on observations from outcrops and trenches on the Poplar Fan, eight different types of OFGs are identified: these lithofacies types account for 7% of the total deposits of the fan. During high flow stages, OFGs are deposited in association with supercritical sheet-like flow (type 1), unconfined streamflow (type 2), and flow in groundwater-fed channels (type 3). During low flow stages, OFGs are associated with trough cross-stratification (type 4) and sigmoidal cross-stratification (type 5); these types occur over the entire fan. At the fan margins, OFGs are associated with gulleys (type 6), especially at the distal fan toe. Other OFG types are associated with bank collapse (type 7) or intercalated with aeolian deposits (type 8). A model is proposed to account for the distribution of different types of OFGs across the fan, wherein types 1, 2 and 4 seen in the medial fan are dominant. This provides a framework for the identification, prediction and correlation of OGFs in subsurface alluvial-fan successions.

Trees as sentinels of metallic pollution induced by mining along the Odiel River (Southern Iberian Peninsula)

Mining activity is often responsible for the drainage of acid or metal-enriched waters to fluvial systems. The release of metals is especially disturbing due to the toxicity and persistence of these products and their accumulation in the biosphere. Hence, a systematic detection and delimitation of highly polluted floodplains and linkages between pollution and high-flow stages would likely assist the improvement of land management and ease the design of mitigation or rehabilitation measures. Here we test how trees growing in different geomorphic positions along a fluvial system uptake metal during floods and how these uptakes can be documented “a posteriori”. To this end, we apply dendrogeochemical analyses to twenty Pinus pinaster Ait. trees growing on the banks of Odiel River (south-western Spain) as well as to five reference trees growing outside the river channel. In the field, trees were sampled with a large-diameter (1 cm) increment borer. In the lab, tree-ring series were dendrochronologically cross dated and separated into 5-yr blocks, so that wood blocks contained the dates of major floods. Then, Inductively Coupled Plasma Mass Spectrometry (ICPM) was employed to evaluate toxic metal concentrations in trees. Results point to clear correlations between the accumulation of toxic metals and the geomorphic position of trees within the fluvial network. We show that morphological units along a river exert control on toxic metal concentrations in trees, with uptake being much higher in trees located on meander cut banks than in trees growing on point-bar structures. Besides, we detect chemical signatures in trees located farthest away from the main river channel after the largest floods, but not in the aftermath of smaller events. We conclude that tree position is the single-most important determinant for metallic pollution in an environment controlled by fluvial processes, but also find that more studies are still needed to determine linkages with individual floods and interactions of metal uptake in roots via the water table in the river corridor.

Strain Accumulation and Microstructural Evolution During Friction Stir Welding of Pure Magnesium

In this study, the stain accumulation processes of pure Mg material during FSW processes with different rotation speeds are measured and calculated and the microstructure is characterized. Copper foil is inserted as a marker to reveal the flow of pure Mg. The results indicate that the strain accumulation process can be divided into three stages, acceleration stage, high velocity flow stage and decelerate & constant velocity flow stage, respectively. The speed of strain accumulation is relatively low in acceleration stage and high velocity flow stage, while it becomes really high in decelerate & constant velocity flow stage. The higher the speed of rotation, the more severe plastic deformation occurs within the material. The accumulated strain of pure Mg are 12.8 and 14.5 at the rotation speed of 1000rpm and 1500rpm, respectively. Relatively believable equations are established by calculation and derivation. It can be seen from the microstructure that pure Mg material has experienced an obviously grain growth process follows by significantly refinement during the FSW deformation. High temperature field caused by friction is the main reason which leads to the grain growth.

Research of a Spatial Flow of a Low-Flow Stage of a Svd-22 Centrifugal Compressor by Computational Fluid Dynamics Methods Using Super-Computer Technologies

This paper provides the results of the study of a spatial flow in a low-flow stage of a SVD-22 centrifugal compressor of computational fluid dynamics methods using the Ansys CFX 14.0 software package. Low flow stages are used as the last stages of multistage centrifugal compressors. Such multistage compressors are widely used in boosting compressor stations for natural gas, in chemical industries. The flow features in low-flow stages require independent research. This is due to the fact that the developed techniques for designing centrifugal compressor stages are created for medium-flow and high-flow stages and do not apply to low-flow stages. Generally at manufacturing new centrifugal compressors, it is impossible to make a control measurement of the parameters of the working process inside the flow path elements. Computational fluid dynamics methods are widely used to overcome this difficulties. However verification and validation of CFD methods are necessary for accurate modeling of the workflow. All calculations were conducted on one of the SPbPU clusters. Parameters of one cluster node: AMD Opteron 280 2 cores, 8GB RAM. The calculations were conducted using 4 nodes (HP MPI Distributed Parallel startup type) with their full load by parallelizing processes on each node.

Sedimentary characteristics and pattern of distributary channels in shallow water deltaic red bed succession: A case from the Late Cretaceous Yaojia formation, southern Songliao Basin, NE China

Shallow water delta developed during the Late Cretaceous greenhouse climate depositing in the red bed succession of the Yaojia Formation in the Southern Songliao Basin, NE China. The shallow water delta deposits provide an opportunity to understand the influence of high discharge variability in a semi-arid climate on the fluvial patterns, internal sedimentary details, and reservoir quality of distributary channels. The sedimentary process of the shallow water delta in the Yaojia Formation was controlled by a dual-stage process, including a high-flow stage and a low-flow stage within a semi-arid paleoclimate. During the high-flow stage, crevasse channels formed through avulsion of major distributary channels on the delta plain. Subaqueous terminal distributary channels and mouth bars formed at the delta front during progradation of the delta. During the low-flow stage after flood events, crevasse channels were abandoned and frequently exposed becoming reddish in color. This process can be reason for the classification for two main types of distributary channels: major distributary channels and crevasse channels in the delta plain. Sand bodies in distributary channels are distinguished by sedimentary structures indicate critical and supercritical flow conditions. Based on well logs and 3D seismic data, the sand distribution of was analyzed from the sandstone isopach map of the 2nd sand bed in the first member of the Yaojia Formation. The framework sand belts develop coincides with distribution of the major distributary channels. Based on detailed observational and reservoir test data from cores, the distribution of major distributary channels revealed a belt of high-quality reservoirs in the shallow water delta. This study provides a new perspective on the sedimentary pattern of distributary channels in shallow water deltas with high discharge variability, which has important implications for reservoir exploration in lacustrine basins.

Assessment of Suspended Sand Availability under Different Flow Conditions of the Lowermost Mississippi River at Tarbert Landing during 1973–2013

Rapid land loss in the Mississippi River Delta Plain has led to intensive efforts by state and federal agencies for finding solutions in coastal land restoration in the past decade. One of the proposed solutions includes diversion of the Mississippi River water into drowning wetland areas. Although a few recent studies have investigated flow-sediment relationships in the Lowermost Mississippi River (LmMR, defined as the 500 km reach from the Old River Control Structure to the river’s Gulf outlet), it is unclear how individual sediment fractions behave under varying flow conditions of the river. The information can be especially pertinent because the quantity of coarse sands plays a critical role for the Mississippi-Atchafalaya River deltaic development. In this study, we utilized long-term (1973–2013) records on discharge and sediments at Tarbert Landing of the LmMR to assess sand behavior and availability under different river flow regimes, and extreme sand transport events and their recurrence. We found an average annual sand load (SL) of 27.2 megatonnes (MT) during 1973 and 2013, varying largely from 3.37 to 52.30 MT. For the entire 41-year study period, a total of approximately 1115 MT sand were discharged at Tarbert Landing, half of which occurred during the peak 20% flow events. A combination of intermediate, high and peak flow stages (i.e., river discharge was ≥18,000 cubic meter per second) produced about 71% of the total annual SL within approximately 120 days of a year. Based on the long-term sediment assessment, we predict that the LmMR has a high likelihood to transport 4 to 446 thousand tonnes of sand every day over the next 40 years, during which annual sand loads could reach a maximum of 51.68 MT. Currently, no effective plan is in place to utilize this considerably high sand quantity and we suggest that river engineering and sediment management in the LmMR consider practices of hydrograph-based approach for maximally capturing riverine sediments.

With or against the tide: The influence of bed form asymmetry on the formation of macroturbulence and suspended sediment patterns

AbstractThis study examines tide‐dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bed forms. In the Danish Knudedyb inlet, very large primary bed forms remain ebb‐oriented during a tidal cycle while smaller superimposed bed forms reverse direction with each tidal phase. Hydroacoustic in situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bed forms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bed form orientation are aligned, water‐depth‐scale macroturbulence develops in the bed form lee‐sides in the presence of flow separation. Macroturbulent flow structures occur at high flow stages and are coupled to increased amounts of sediment in suspension. When flow and bed form orientation are opposed no evidence of flow separation associated with primary bed forms is found. Sediment‐laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bed forms. The flow structures are advected along the primary bed form stoss‐side (temporary hydraulic lee‐side). The steep primary bed form lee‐side (temporary hydraulic stoss‐side) however, limits transport capabilities beyond the scale of primary bed forms.

A Hydrograph-Based Sediment Availability Assessment: Implications for Mississippi River Sediment Diversion

The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980–2010) suspended sediment yield as affected by different hydrologic regimes to determine actual suspended sediment availability and how this may affect diversion management. A stage hydrograph-based approach was employed to quantify total suspended sediment load (SSL) of the lower Mississippi River at Tarbert Landing during three river flow conditions: Peak Flow Stage (stage = 16.8 m, discharge >32,000 m3 s−1), High Flow Stage (stage = 14.6 m, discharge = 25,000–32,000 m3 s−1), and Intermediate Flow Stage (Stage = 12.1 m, discharge = 18,000–25,000 m3 s−1). Suspended sediment concentration (SSC) and SSL were maximized during High Flow and Intermediate Flow Stages, accounting for approximately 50% of the total annual sediment yield, even though duration of the stages accounted for only one-third of a year. Peak Flow Stage had the highest discharge, but significantly lower SSC (p < 0.05), indicating that diversion of the river at this stage would be less effective for sediment capture. The lower Mississippi River showed significantly higher SSC (p < 0.0001) and SSL (p < 0.0001) during the rising than the receding limb. When the flood pulse was rising, Intermediate Flow and High Flow Stages showed greater SSC and SSL than Peak Flow Stage. Together, Intermediate Flow and High Flow Stages on the rising limb annually discharged 28 megatonnes over approximately 42 days, identifying this to be the best period for sediment capture and diversion.

Avulsion processes at the terminus of low-gradient semi-arid fluvial systems: Lessons from the Río Colorado, Altiplano endorheic basin, Bolivia

The Río Colorado dryland river system in the southeast of the endorheic Altiplano Basin (Bolivia) terminates on a very flat coastal plain at the edge of the Salar de Uyuni, the world's largest salt pan with an area of ca. 12,500km2. Since the Pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods, respectively. At present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the Río Colorado onto the former lake bottom. The present field study of the terminus of the Río Colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth. This bankfull decrease is caused by the combined effects of: (1) extremely low gradient of the lake bottom and, hence, loss of flow energy, and (2) downstream transmission losses due to high evaporation potential and river water percolation through the channel floor. Peak water discharge in seasonal, short-duration rain periods causes massive overbank flooding and floodplain inundation. On satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels. However, field observations show that only one channel is active at low flow stage, and at high-flow stage an abandoned, partially infilled channel may be active as well. The active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain. The rest of the channels which form the divergent network are older sediment-filled abandoned sinuous river courses with multiple random avulsion points. These channel deposits, together with extensive amalgamated crevasse-splay deposits, form an intricate network of fluvial sand deposits. Successive stages of progressively deeper crevasse-channel incision into the floodplain are the result of waning-stage return flow of floodwaters towards the main channel. This results in increased crevasse-channel width, depth and length, and redeposition of eroded lacustrine coastal-plain sediment at the junction of the crevasse channel with the present-day river. A sedimentary model is proposed in which deep river bank incision by return flow in crevasse-channels creates the preferential locus for river avulsion. In a Lowstand Systems Tract with little vertical accommodation increase, the recurring avulsed-river paths result in a thin but laterally extensive network of amalgamated channel-fill, point-bar, crevasse-channel and crevasse-splay deposits. The resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels.

Downstream channel changes on a contracting, anabranching river: The Lachlan, southeastern Australia

Downstream trends in flow, channel morphology, and sediments were investigated over 220 km of the middle and upper alluvial Lachlan River in southeastern Australia. The Lachlan is a perennial river that rises from temperate, humid highlands and flows inland into a semiarid low-gradient plain lacking perennial tributaries. Long daily stage records were used to calculate flow parameters and flood frequencies in single and anabranching channel reaches of the river. Morphological trends were based on cross section and height data compiled from new and water authority data, and from particle size trends constructed from bed and bank sediment samples. Mean and peak flows, channel size, and bed particle size all diminish in a downstream direction. In the partially confined piedmont zone, channel and floodplain morphology feature large flood effects and large channel dimensions that reflect extreme flow variability. On the alluvial plains, the unconfined, single-channel contracts with distance downvalley; and anabranches develop at high flow stages. The development of a full anabranching pattern is associated with large flood volume losses. Diminishing flood volumes, mean flows, and channel dimensions is attributed to storage of flood waters in lakes, floodways, and lagoons and through transmission losses during overbank flows. The frequency of bankfull discharge consistently decreases downstream, and may reflect the long duration and lower frequency of floods in the lower reaches of the river. Particle size of bed sediment fines downstream at rates consistent with those reported from sand-bed rivers elsewhere. This is attributed to a combination of hydraulic sorting and low rates of sediment supply from the catchment.

Characterization of the colloidal organic matter from the Amazonian basin by asymmetrical flow field-flow fractionation and size exclusion chromatography

Colloidal organic matter (COM) collected in small and large watercourse tributaries of the Negro River (Brazil) were fractionated and characterized by an asymmetrical flow field-flow fractionation (AFlFFF) coupled to UV and seven angle laser light scattering (LS) detectors, and by size exclusion chromatography (SEC) coupled with a UV detection. Number and weight average molar masses, weight average gyration radiuses RGw and polydispersity indexes were obtained for each sample in two separate runs under conditions optimized for lower and larger size fractions. The results demonstrate the existence of a decrease of size of the colloidal matter when passing from first order streams to higher order rivers. No significant changes were found in size distributions of samples collected during the low and high flow stages at the same site. The influence of selected pre-treatments such as filtration and reverse osmosis pre-concentration on the size and molar mass distributions was also studied.

Facies model for fluvial systems in the seasonal tropics and subtropics

Research Article| July 01, 2009 Facies model for fluvial systems in the seasonal tropics and subtropics Christopher R. Fielding; Christopher R. Fielding 1Department of Geosciences, 214 Bessey Hall, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0340, USA Search for other works by this author on: GSW Google Scholar Jonathan P. Allen; Jonathan P. Allen 1Department of Geosciences, 214 Bessey Hall, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0340, USA Search for other works by this author on: GSW Google Scholar Jan Alexander; Jan Alexander 2School of Environmental Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, UK Search for other works by this author on: GSW Google Scholar Martin R. Gibling Martin R. Gibling 3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Christopher R. Fielding 1Department of Geosciences, 214 Bessey Hall, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0340, USA Jonathan P. Allen 1Department of Geosciences, 214 Bessey Hall, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0340, USA Jan Alexander 2School of Environmental Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, UK Martin R. Gibling 3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Publisher: Geological Society of America Received: 04 Dec 2008 Revision Received: 23 Feb 2009 Accepted: 25 Feb 2009 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2009 Geological Society of America Geology (2009) 37 (7): 623–626. https://doi.org/10.1130/G25727A.1 Article history Received: 04 Dec 2008 Revision Received: 23 Feb 2009 Accepted: 25 Feb 2009 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christopher R. Fielding, Jonathan P. Allen, Jan Alexander, Martin R. Gibling; Facies model for fluvial systems in the seasonal tropics and subtropics. Geology 2009;; 37 (7): 623–626. doi: https://doi.org/10.1130/G25727A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments, but the deposits of rivers in the sub-humid and semiarid seasonal tropics have been largely ignored. Our observations and data from modern streams and recent deposits in northeastern Australia show how such rivers, with extremely variable discharge, have distinctive deposit characteristics that are substantially different from conventional fluvial facies models. These properties include (1) erosionally based channel-fill lithosomes that exhibit complex lateral facies changes, with (2) abundant, pedogenically modified mud partings, (3) complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies, (4) an abundance of sedimentary structures formed under high flow stage, and (5) an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast-flowing water. We illustrate examples of this fluvial style from the rock record, and set out a new facies model. The recognition of such a distinctive fluvial character, and of changes in character through vertical successions, will aid paleoclimate and reservoir analysis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fish use of stage-specific fluvial habitats as refuge patches during a flood in a low-gradient Illinois stream

A stage-specific habitat classification scheme was developed emphasizing the ecological importance of habitat space as flow refugia for fish. Habitat units were based on planform and channel morphology and the resulting three-dimensional hydraulic patterns that form during floods. Fish were sampled in specific habitat patches using prepositioned areal electrofishing devices at two high-flow stages, near bankfull flow and one-half bankfull flow, during a flood. Fish were also sampled at baseflow immediately following the flood to compare changes in habitat use related to flow stage. During the bankfull stage, fish were found to use floodplain habitat units identified as vegetated point bars and concave-bank benches. During the one-half bank-full stage, fish density and biomass were greater in low-velocity habitat units identified as deflection eddies and expansion eddies than in the high-velocity habitat unit identified as the channel thalweg. Once flow returned to baseflow, fish density and biomass were greater in the main channel habitat units (pools, riffles, and glides) than in lateral habitat units identified as submerged point bars and lateral backwaters. This study provided a framework to quantify flow refugia in low-gradient streams, which is especially important in the restoration of anthropogenically disturbed watersheds that have lost habitat connectivity.

A field test of tubino's (1991) model of alternate bar formation

AbstractThis study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses a physically based non‐linear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bed sediment size and flow stage over a 2 year study period.Both linear and non‐linear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. But these theories do not predict bar development for unsteady flow conditions, which typically occur in natural alluvial channels. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood; (2) a stage of modest bar decay near the peak of the flood; and (3) a stage of non‐linear bar growth during the prolonged falling stage of the flood.Bars developed during the falling limb of a hydrograph, and exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. As flow stage decreased, short, low, fine‐grained bars were superimposed on long, high and coarser‐grained bars that developed under preceding high flow stages. These results suggest that the process of bar formation in artificially straightened natural streams with heterogeneous bed material may occur under different flow conditions and in a different manner than predicted by theoretical models. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes.

Flow and sediment dynamics in a low sinuosity, braided river: Calamus River, Nebraska Sandhills

ABSTRACTThe interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right‐hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left‐hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across‐stream flows are generally stronger than the weak, curvature‐induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune‐covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels.During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at a high flow stage.

Storm Runoff Loading of Nutrients and Pollutants in Upper Rivers.

The mountainous area of an upper river is important as the first site of conversion of rainwater to freshwater in a natural state. To study the sharp changes in storm runoff loading on the steep slopes of mountainous watersheds, detailed observations of three small rivers were conducted, including many runoff loading observations during storm events, regular observation of loading during the high flow stage on dry days for riverwater, and observations of annual total loadings of nutrients and pollutants for rainwater. In the mountainous rivers the concentrations and loadings for many water quality indicators under high flow condition increased as the mean value of observed flows increased in each observation year. The fact that the flow-weighted mean concentrations of almost all water quality indicators were higher than their arithmetic means confirmed the high loadings during the high flow stage. Regression equations for total loading during any storm runoff, ΣLgross/A= b⋅(ΣQgross/A)m, for the three rivers were obtained, and high correlation coefficients were noted for total, particulate and dissolved components of many indicators from 13-14 storm events. The exponent, m, in the regression equations for NO3-N, NO2-N, PO4-P and K for the three rivers exceeded 1.0. These analytical results are considered to confirm increases in nutrient and pollutant loading during storm runoff periods.

Runoff characteristics of COD, BOD, C, N and P loadings from rivers to enclosed coastal seas

It is well known that the three nutrient elements of carbon (C), nitrogen (N) and phosphorus (P), are very important in the eutrophication in enclosed coastal seas. The major source of the C, N and P is the inflow from rivers rather than the release from sediment in the closed sea. In Japan, the environmental quality standards which indicate organic pollution in both river and sea are different from one another; in a river, it is based on BOD and in the sea, on alkaline COD M d . Therefore, these different standards lead to difficulties in obtaining consistent water quality management of the water system from rivers to enclosed coastal seas. Hinuma River observed runoff loading of alkaline COD M d , BOD, C, N and P by weekly observation in 1988 and 1989. When the flow increases, the order of increasing ratio is phosphorus, followed by carbon, and nitrogen. It was found that 88% of phosphorus runs off in the form of particulate phosphorus and 84% of nitrogen runs off in the form of dissolved nitrogen; and though nitrogen runs off in an almost constant concentration, most phosphorus runs off during the high flow stage. The behavior of the particulate phosphorus after flowing in to the enclosed coastal sea is an important factor in eutrophication.

Downstream changes of total and partitioned metal concentrations in the flood deposits of the river Geul (The Netherlands)

The floodplain soils in the Guel basin have unacceptably high levels of pollution,v brought about by metal mining and related industrial activities in the past. Spoil heaps still exist along the Geul river and these are susceptible to erosion and leaching processes. An additional source of metals is formed by erosion of older, locally highly contaminated streambank deposits. These older sediments are polluted as a result of solid waste disposal containing metalliferous ore and tailings in the sand fraction. At present, these sediments function as a major source of heavy metals during high flow stages when streambanks are undermined and suspended sediments are deposited on the floodplains. The flood deposits have a relatively coarse texture, i.e. 70% dry weight in the fraction > 63 um.