Bed Sediment Size Research Articles

Effects of small rock check dams on channel bed, vegetation diversity, and soil properties 10 years after a severe wildfire in forest micro‐catchments of central eastern Spain

AbstractOne of the most common post‐fire management techniques is the construction of check dams in catchments. However, little research has explored how small check dams impact on bed profile, vegetation diversity and soil properties several years after their construction in Mediterranean ephemeral channels draining micro‐catchments burned by severe wildfires. To fill this gap, this study has evaluated the effects of rock check dams installed about 10 years ago in small catchments after a wildfire in Castilla La Mancha (Central Eastern Spain). The changes in channel morphology, plant diversity as well as a large dataset of soil properties have been analysed close to the check dams in comparison to burned but non‐regulated channels. Neither the channel slope nor bed sediment size significantly changed downstream and upstream of the structures in comparison to the non‐regulated channels. Thanks to the unexploited retention capacity, the studied control works are still able to store the mobilised sediments on occasion of the most intense floods. Higher species richness in both upstream and downstream sections (+64%) was detected, while the plant evenness was not altered compared to the non‐regulated channels. Regarding the soil properties, increases in organic matter (over 200%), nutrients (+72% for N and +152% for P) and some cations (+29% for Ca and +86% for Mg) were measured. Overall, the presence of the rock check dams in the burned micro‐catchments, although improving many key plant and soil characteristics, did not sharply change the hydrological, geomorphological and ecological conditions compared with those of the non‐regulated catchments.

Unravelling fluvial versus glacial legacy controls on boulder‐bed river geomorphology for semi‐alluvial rivers in Fennoscandia

AbstractRiver management is founded on predictable self‐organisation between river form and catchment controls in alluvial rivers. However, a substantial proportion of rivers are not fully alluvial. In previously glaciated landscapes, boulder‐rich glacial till influences river channel form and process. Increasing interest in nature‐ and process‐based river restoration requires knowledge of pre‐disturbance natural processes, which does not exist for semi‐ and non‐alluvial rivers in Fennoscandia. We aimed to determine the role of Pleistocene glaciation and subsequent deglaciation versus Holocene fluvial processes in controlling channel form of boulder‐bed rivers in Fennoscandia. We quantified morphological characteristics of northern Swedish boulder‐bed rivers, in which channel morphology was minimally impacted by humans, and used the degree of alluvial signatures to infer fluvial and legacy glacial controls. We conducted surveys of reach‐scale channel geometry, boulder and wood distributions and catchment characteristics for 20 reference reaches (drainage area: 11–114 km2). Reaches ranged in slope from 1% to 8% and were extremely diverse in channel geometry. Rivers showed little self‐organisation at the reach scale; no association exists between channel width and channel slope or bed sediment size. Boulders were rarely clustered into bedforms (e.g., step‐pools) typical of boulder‐bed mountain rivers. Drainage area was positively correlated with channel capacity but not channel width, slope or sediment size. Channel boulder density was best predicted by surveys of terrestrial boulders. Consequently, channel geometry, boulder size and the distribution of boulders were primarily controlled by legacy glacial conditioning rather than current fluvial processes, with some alluvial adjustment of smaller particles within the boulder template. Therefore, restoration of semi‐alluvial rivers should take into account local sediment and geomorphic conditions rather than use management principles built for fully alluvial rivers.

Prediction of Local Scour around Bridge Piers in the Cohesive Bed Using Support Vector Machines

Local scour around bridge piers is one of the most important factors threatening the life of bridges. The three-dimensional highly complicated horseshoe vortex and downflow are known to be the main agents responsible for pier scour. If the bed consists of cohesive sediment, it will add another level of complexity to the pier scour problem. Various approaches have attempted to predict scour depth, but no universal method is available to date. This study presents a prediction of local scour around bridge piers in the cohesive bed using support vector machines (SVMs), a machine learning technique. The maximum scour depth is predicted with seven dimensional variables, including velocity, flow depth, size of bed sediment, pier width, clay content, water content, and bed shear strength. The training and validation of the SVMs are conducted with 197 data from six datasets. Comparisons are made with the training and validation of the adaptive-network-based fuzzy inference system (ANFIS) method. The training of the ANFIS method appears successful, but the validation fails because of overfitting. The predictions with dimensionless variables are compared, and shown to be worse. In addition, the SVMs are found to predict the maximum scour depths better than three existing formulas, gene expression programming (GEP), and a non-linear regression model. The SVMs are applied to two datasets, revealing the importance of the coverage of the training data. Finally, to investigate the contributions of each variable, the mean absolute percent errors (MAPEs) and correlation coefficient are computed by predicting the maximum scour depths by excluding each variable.

Sediment transport modeling in non-deposition with clean bed condition using different tree-based algorithms.

To reduce the problem of sedimentation in open channels, calculating flow velocity is critical. Undesirable operating costs arise due to sedimentation problems. To overcome these problems, the development of machine learning based models may provide reliable results. Recently, numerous studies have been conducted to model sediment transport in non-deposition condition however, the main deficiency of the existing studies is utilization of a limited range of data in model development. To tackle this drawback, six data sets with wide ranges of pipe size, volumetric sediment concentration, channel bed slope, sediment size and flow depth are used for the model development in this study. Moreover, two tree-based algorithms, namely M5 rule tree (M5RT) and M5 regression tree (M5RGT) are implemented, and results are compared to the traditional regression equations available in the literature. The results show that machine learning approaches outperform traditional regression models. The tree-based algorithms, M5RT and M5RGT, provided satisfactory results in contrast to their regression-based alternatives with RMSE = 1.184 and RMSE = 1.071, respectively. In order to recommend a practical solution, the tree structure algorithms are supplied to compute sediment transport in an open channel flow.

Transient versus sustained biophysical responses to dam removal

Dams fragment the longitudinal flux of sediment and obstruct fish passage. To help remediate these impacts, dam removal has become an important tool in the river restoration toolkit. Scientifically, these removals also provide an important experimental design, fixed in time and space, to quantify geomorphic and ecological responses to removal of a disturbance. Using the removal of a 6 m high dam in Massachusetts (USA), we quantify the immediate and subsequent channel and ecological recovery to test whether the immediate ecological/geomorphic responses were sustained or transient. Initial geomorphic adjustments have been generally sustained since removal, especially for bed sediment size, where initial fining and reduction in bed caliber size has been maintained over the course of the study. Sampling of transported passive integrated transponder (PIT) tagged clasts indicates that the median of bedload grain sizes now corresponds to surface median grain sizes below the former dam. Channel cross-sectional area varied irregularly downstream initially but is now spatially uniform. Pool filling, however, has been a transient response as initial aggradation shifted to sediment evacuation. Ecologically, upstream establishment of resident fish species previously restricted to below-dam locations occurred in the first year after removal. Upstream expansion increased upstream species richness and has been maintained throughout the study period, in spite of the variability in presence and abundance of rare and migratory species such as the anadromous Sea lamprey (Petromyzon marinus). In contrast, a reduction in abundance of resident species in dam proximal sites in 2012 appears to be a transient response to abrupt and intense bed movement associated with removal of the dam, as numbers recovered and, in some cases, exceeded pre-dam removal levels. Overall, we found that immediate (within the first 1–2 yr), sustained effects on geomorphology and ecology dominated the response to dam removal in this system, a finding that may be generally applicable to small dam removals in upland catchments.

Riverbed dune morphology of the Lowermost Mississippi River – Implications of leeside slope, flow resistance and bedload transport in a large alluvial river

Dunes are critical for understanding riverine sediment transport, deposition, flow resistance and channel flow processes. Although previous studies have examined the riverbed micromorphology of the Lower Mississippi River in the USA, our knowledge of detailed quantification of dune morphology in this and other large alluvial rivers is still limited. It is also not well understood how dunes in a straight channel reach and a meander bend differ in their characteristics, as well as how dune morphology may have been affected by human activities (i.e., river engineering). In this study, we utilized multi-beam bathymetric measurements over four 1.6–8.0 km long reaches in the Lowermost Mississippi River (LmMR) to analyze riverbed micromorphologic features. Three of the four reaches were located in the upper part of the LmMR between river kilometers (RK) 474–477, 483.6–485.2 and 491.7–493.3, and the other reach was located in the lower part, between RK 120 and RK 128. We analyzed a total of 3258 dunes in these river reaches and found that large dunes were dominant in the LmMR. These dunes were characterized by low mean leeside slope angle (10.8°), indicating that flow resistance caused by dunes should be relatively small. When compared with dunes in the straight reaches, dunes in the meander bends were much larger (1.06 m vs. 0.81 m) and had a higher bed roughness (0.91 vs. 0.68), which may be related to the varied flow velocity. Dune size increased with increasing water depth across the river channel of a straight reach, while it decreased with increasing water depth across the river channel of a meander bend. When compared with the dunes in the lower-river reach, the dunes in the upper-river reach were significantly higher in height and shorter in wavelength, and showed much higher bed roughness (1 vs. 0.68), which may be closely related to the greater riverbed slope and grain size of bed sediment occurred in the upper-river reach, as well as a combined effect of the Old River Control Structure (RK 500) and backwater. These findings indicate the strong impact of turbulent flow, slope and sand source on dune formation and riverbed deposition.

Anisotropy in the Free Stream Region of Turbulent Flows through Emergent Rigid Vegetation on Rough Beds

Most of the existing works on vegetated flows are based on experimental tests in smooth channel beds with staggered-arranged rigid/flexible vegetation stems. Actually, a riverbed is characterized by other roughness elements, i.e., sediments, which have important implications on the development of the turbulence structures, especially in the near-bed flow zone. Thus, the aim of this experimental study was to explore for the first time the turbulence anisotropy of flows through emergent rigid vegetation on rough beds, using the so-called anisotropy invariant maps (AIMs). Toward this end, an experimental investigation, based on Acoustic Doppler Velocimeter (ADV) measures, was performed in a laboratory flume and consisted of three runs with different bed sediment size. In order to comprehend the mean flow conditions, the present study firstly analyzed and discussed the time-averaged velocity, the Reynolds shear stresses, the viscous stresses, and the vorticity fields in the free stream region. The analysis of the AIMs showed that the combined effect of vegetation and bed roughness causes the evolution of the turbulence from the quasi-three-dimensional isotropy to axisymmetric anisotropy approaching the bed surface. This confirms that, as the effects of the bed roughness diminish, the turbulence tends to an isotropic state. This behavior is more evident for the run with the lowest bed sediment diameter. Furthermore, it was revealed that also the topographical configuration of the bed surface has a strong impact on the turbulent characteristics of the flow.

Bedform characteristics and biofilm community development interact to modify hyporheic exchange

The physical and biological attributes of riverine ecosystems interact in a complex manner which can affect the hydrodynamic behaviour of the system. This can alter the mixing characteristics of a river at the sediment-water interface. Research on hyporheic exchange has increased in recent years driven by a greater appreciation for the importance of this dynamic ecotone in connecting and regulating river systems. An understanding of process-based interactions driving hyporheic exchange is still limited, specifically the feedbacks between the physical and biological controlling factors. The interplay between bed morphology and sediment size on biofilm community development and the impact on hyporheic exchange mechanisms, was experimentally considered. Purpose built recirculating flume systems were constructed and three profiles of bedform investigated: i) flat, ii) undulating λ = 1 m, ii) undulating λ = 0.2 m, across two different sized sediments (0.5 mm and 5 mm). The influence of biofilm growth and bedform interaction on hyporheic exchange was explored, over time, using discrete repeat injections of fluorescent dye into the flumes. Hyporheic exchange rates were greatest in systems with larger sediment sizes (5 mm) and with more bedforms (undulating λ = 0.2). Sediment size was a dominant control in governing biofilm growth and hyporheic exchange in systems with limited bedform. In systems where bedform was prevalent, sediment size and biofilm appeared to no longer be a control on exchange due to the physical influence of advective pumping. Here, exchange rates within these environments were more consistent overtime, despite greater microbial growth. As such, bedform has the potential to overcome the rate limiting effects of biotic factors on hyporheic exchange and sediment size on microbial penetration. This has implications for pollutant and nutrient penetration; bedforms increase hydrological connectivity, generating the opportunity to support microbial communities at depth and as such, improve the self-purification ability of river systems.

River Channel Forms in Relation to Bank Steepness: A Theoretical Investigation Using a Variational Analytical Method

Riverbanks vary considerably in anti-scourability and consequently take various profiles. By using an isosceles trapezoid as the generalized form of river channel cross-sections and then incorporating the effects of bank angle into the variational analytical approach developed by Huang and Nanson (2000), this study presents a detailed theoretical investigation of the self-adjustment of alluvial channel forms. It is demonstrated that when alluvial channel flow achieves stable equilibrium, a significant decrease in riverbank steepness leads to a slight decrease in maximum sediment (bedload) discharge, and yet results in a significant increase in optimal channel width and a considerable decrease in optimal channel depth. The hydraulic geometry relations, theoretically derived for bank steepness to vary across a wide range, show that among the multivariant controls, the roles of bed sediment size, channel roughness, flow discharge and sediment (bedload) discharge are independent of bank steepness. While the effects of bank steepness illustrated in the theoretically derived hydraulic geometry relations are highly consistent with the results of threshold theory and previous empirical studies, limitations on using bank angle to reflect the anti-scourability of natural riverbanks are also highlighted.

A 2D hydrodynamic model for shallow water flows with significant infiltration losses

AbstractInfiltration losses may be significant and warrant proper incorporation into mathematical models for river floods in arid and semi‐arid areas, rainfall‐induced surface runoffs in watersheds and swashes on beaches. Here, a depth‐averaged two‐dimensional hydrodynamic model is presented for such processes based on the cell‐centred finite volume method on unstructured meshes, with the full Green‐Ampt equation evaluating the infiltration rate. A local time stepping strategy is employed along with thread parallelization with Open Multi‐processing and high‐performance computing to reduce model run time and therefore facilitate applications for large‐scale processes. The numerical solutions generally agree with the experimental and field‐measured data for typical cases with significant infiltration losses. The case study shows that neglecting infiltration leads to an overestimated discharge hydrograph, which cannot be compensated by means of varied bed resistance as estimated by Manning roughness, and the infiltration parameters play disparate roles in modifying shallow flows compared with Manning roughness. In addition, infiltration affects bed shear stress, which in turn modifies the critical bed sediment size that could be initiated for incipient motion by the flow and therefore needs to be properly accounted for when sediment transport and morphological evolution are to be resolved.

Universal relation with regime transition for sediment transport in fine-grained rivers

Fine-grained sediment (grain size under 2,000 μm) builds floodplains and deltas, and shapes the coastlines where much of humanity lives. However, a universal, physically based predictor of sediment flux for fine-grained rivers remains to be developed. Herein, a comprehensive sediment load database for fine-grained channels, ranging from small experimental flumes to megarivers, is used to find a predictive algorithm. Two distinct transport regimes emerge, separated by a discontinuous transition for median bed grain size within the very fine sand range (81 to 154 μm), whereby sediment flux decreases by up to 100-fold for coarser sand-bedded rivers compared to river with silt and very fine sand beds. Evidence suggests that the discontinuous change in sediment load originates from a transition of transport mode between mixed suspended bed load transport and suspension-dominated transport. Events that alter bed sediment size near the transition may significantly affect fluviocoastal morphology by drastically changing sediment flux, as shown by data from the Yellow River, China, which, over time, transitioned back and forth 3 times between states of high and low transport efficiency in response to anthropic activities.

Incipient conditions of cohesive sediments in oscillatory flows

In this study, the incipient conditions of cohesive sediments with different sizes and water content were experimentally investigated in an oscillatory U-shaped tunnel. A total of 100 experimental runs were conducted with different sediment samples of different yield stresses. The relationship between the incipient condition and the yield stress of bed sediments in oscillatory flows was determined. Further, the effect of yield stress on the incipient velocity of bed sediments and the critical bed shear stress in oscillatory flow were analyzed. The experimental results indicate that the critical Shields parameter of bed sediments in oscillatory flow is exponentially related to the yield stress, which is similar to that in a unidirectional flow. When the bed sediment size is less than 65μm, the incipient condition of cohesive sediment in oscillatory flow is primarily affected by the yield stress; when the sediment size is greater than 500μm, the effect of yield stress on the incipient condition of bed sediment in oscillatory flow is negligible; in addition, when 40.65μm≤D50≤500μm, the bed sediments in oscillatory flow are easier to be transported than in the unidirectional flow in the case where bed sediments exhibited the same yield stress. When D50≤40.65μm, the cohesive sediments in oscillatory flow are more difficult to be transported than that in a unidirectional flow, for cases where cohesive sediments have the same yield stress. Finally, an empirical formula to calculate the incipient condition of the bed sediment with different fluidization levels in oscillatory flows is proposed by introducing the yield stress.

Roles of Bank Material in Setting Bankfull Hydraulic Geometry as Informed by the Selenga River Delta, Russia

AbstractA semi‐empirical bankfull Shields number relation as a function of slope, bed, and bank sediment grain size is obtained based on a field data set that includes the delta of the Selenga River, Russia, and other rivers from around the globe. The new Shields number relation is used in conjunction with continuity, flow resistance, and sediment transport equations to deduce predictive relations for bankfull width, depth, and slope of sand‐bed rivers. In addition, hydraulic geometry relations are obtained specifically for the Selenga River delta. Key results of this study are as follows: (1) bankfull width is strongly dependent on water discharge and is directly related to bank sediment size; (2) bankfull shear velocity is weakly dependent on bed sediment size and is inversely related to bank sediment size; (3) sand‐bed deltas with multiple distributary channels maintain smaller bankfull Shields numbers than is typical of alluvial rivers. This analysis is the first of its kind to include bank sediment size into a predictive bankfull Shields number relation to obtain relations for bankfull hydraulic geometry. The relations presented here can be utilized in morphodynamic models that explore how fluvial and deltaic systems respond to a range of imposed conditions, such as variable base level, sediment, and water supply.

Assessment of riparian vegetation characteristics in Mediterranean headwaters regulated by check dams using multivariate statistical techniques

In mountainous torrents of the Mediterranean environment the riparian vegetation is strongly influenced by the presence of engineering control works, since these structures bring heavy modifications in channel geometry, hydraulic regime and bed sediment size. Previous investigations have shown high linear correlations between physical (section shape, profile slope, specific discharge, surface and subsurface size of the channel bed) and vegetation (development, structure and biodiversity) indicators in headwater channels with check dams of Calabrian (Southern Italy) torrents. Based on these findings, this study applies multivariate statistical techniques (Principal Component Analysis and Partial Least Square Regression) to identify in the same study headwaters new synthetic explanatory variables, representative of the different transects (upstream, downstream or intermediate, compared to the check dam location) and develop predictive models of riparian vegetation characteristics.The Principal Component Analysis has provided a simple parameter (the first Principal Component, explaining about 60% of the total variance), which is able to discriminate the physical and vegetal characteristics of the different transects close to check dams, thus reducing the large number of factors influencing the fluvial processes. Moreover, cover, height and transversal variability of riparian vegetation have a very high influence (loadings over 0.73) on this component, while its biodiversity is correlated to the second Principal Component (loadings over 0.63). The Partial Least Square Regression has shown that it is possible to estimate with fair accuracy (minimum r2 of 0.70) the development, structure as well as transversal variability of the riparian vegetation, starting from the physical features of the channel. These models may be important in the planning steps of new check dams, since their effects on the development and growth of vegetation upstream and downstream can be forecasted before their installation, at least for the quantification of the order of magnitude of the check dam impacts on torrent ecology.

The Relation of Neighbouring Land Use Type and Sedimentary Characteristics of Fluvial Sediments in Tropical Rivers

This study investigates the relationship between land use types and sedimentary characteristics of fluvial bed at three rivers in Selangor, Malaysia; namely Sungai Tengi, Selangor and Langat. The diversity in population density and land development at the adjacent river banks gives associated dominant land use type as forest, agriculture and urban for Tengi, Selangor and Langat, respectively. Field sampling was conducted at consistent spatial locations for Tengi and Selangor, whereas for Langat, the bed sediment was taken at non-uniform spatial points due to limited access. Statistical analysis of the sediment bed mixture shows that forest dominated land gives finer bed sediment size at 650 μm. Correlation analysis proved that there is a relationship between types of land use with the mean sediment size. No distinctive correlation can be observed for sorting, skewness and kurtosis of the sediment with the types of land use.

Morphodynamic model of the lower Yellow River: flux or entrainment form for sediment mass conservation?

Abstract. Sediment mass conservation is a key factor that constrains river morphodynamic processes. In most models of river morphodynamics, sediment mass conservation is described by the Exner equation, which may take various forms depending on the problem in question. One of the most widely used forms of the Exner equation is the flux-based formulation, in which the conservation of bed material is related to the stream-wise gradient of the sediment transport rate. An alternative form of the Exner equation, however, is the entrainment-based formulation, in which the conservation of bed material is related to the difference between the entrainment rate of bed sediment into suspension and the deposition rate of suspended sediment onto the bed. Here we represent the flux form in terms of the local capacity sediment transport rate and the entrainment form in terms of the local capacity entrainment rate. In the flux form, sediment transport is a function of local hydraulic conditions. However, the entrainment form does not require this constraint: only the rate of entrainment into suspension is in local equilibrium with hydraulic conditions, and the sediment transport rate itself may lag in space and time behind the changing flow conditions. In modeling the fine-grained lower Yellow River, it is usual to treat sediment conservation in terms of an entrainment (nonequilibrium) form rather than a flux (equilibrium) form, in consideration of the condition that fine-grained sediment may be entrained at one place but deposited only at some distant location downstream. However, the differences in prediction between the two formulations have not been comprehensively studied to date. Here we study this problem by comparing the results predicted by both the flux form and the entrainment form of the Exner equation under conditions simplified from the lower Yellow River (i.e., a significant reduction of sediment supply after the closure of the Xiaolangdi Dam). We use a one-dimensional morphodynamic model and sediment transport equations specifically adapted for the lower Yellow River. We find that in a treatment of a 200 km reach using a single characteristic bed sediment size, there is little difference between the two forms since the corresponding adaptation length is relatively small. However, a consideration of sediment mixtures shows that the two forms give very different patterns of grain sorting: clear kinematic waves occur in the flux form but are diffused out in the entrainment form. Both numerical simulation and mathematical analysis show that the morphodynamic processes predicted by the entrainment form are sensitive to sediment fall velocity. We suggest that the entrainment form of the Exner equation might be required when the sorting process of fine-grained sediment is studied, especially when considering relatively short timescales.

Evaluating the effects of check dams on channel geometry, bed sediment size and riparian vegetation in Mediterranean mountain torrents

In mountain streams possible negative impacts of check dams on soil, water and riparian vegetation due to check dam installation can be noticed. In spite of the ample literature on the qualitative effects of engineering works on channel hydrology, morphology, sedimentary effects and riparian vegetation characteristics, quantitative evaluations of the changes induced by check dams on headwater characteristics are rare.In order to fill this gap, this study has evaluated the effects of check dams located in headwaters of Calabria (Southern Italy) on hydrological and geomorphological processes and on the response of riparian vegetation to these actions. The analysis has compared physical and vegetation indicators in transects identified around check dams (upstream and downstream) and far from their direct influence (control transects).Check dams were found to influence significantly unit discharge, surface and subsurface sediments (both upstream and downstream), channel shape and transverse distribution of riparian vegetation (upstream) as well as cover and structure of riparian complexes (downstream). The actions of the structures on torrent longitudinal slope and biodiversity of vegetation were less significant. The differences on bed profile slope were significant only between upstream and downstream transects. The results of the Agglomerative Hierarchical Cluster analysis confirmed the substantial similarity between upstream and control transects, thus highlighting that the construction of check dams, needed to mitigate the hydro-geological risks, has not strongly influenced the torrent functioning and ecology before check dam construction. Moreover, simple and quantitative linkages between torrent hydraulics, geomorphology and vegetation characteristics exist in the analysed headwaters; these relationships among physical adjustments of channels and most of the resulting characteristics of the riparian vegetation are specific for the transect locations with respect of check dams. Conversely, the biodiversity of the riparian vegetation basically eludes any quantitative relations with the physical and other vegetal characteristics of the torrent transects.

Entrainment, transport and deposition of sediment by saline gravity currents

Few studies have addressed simultaneously the feedback between the hydrodynamics of a gravity current and the geomorphological changes of a mobile bed. Hydrodynamic quantities such as turbulent and mean velocities, bed shear stress and turbulent stresses undoubtedly govern the processes of entrainment, transport and deposition. On the other hand, the incorporation of entrained sediment in the current may change its momentum by introducing extra internal stresses, introducing thus a feedback process. These two main questions are here investigated. Laboratory experiments of saline gravity currents, produced by lock-exchange, flowing over a mobile bed channel reach, are here reported. Different initial buoyancies of the current in the lock are tested together with three different grain sizes of the non-coherent sediment that form the erodible bed. Results from velocity measurements are combined with the visualization of the sediment movement in the mobile reach and with post-test topographic and photo surveys of the geomorphology modifications of the channel bed. Mean and turbulent velocities are measured and bed shear stress and Reynolds stresses are estimated. We show that the mean vertical component of the velocity and bed shear stress are highly correlated with the first instants of sediment entrainment. Vertical turbulent velocity is similarly related to entrainment, although with lower correlation values, contributing as well to the sediment movement. Bed shear stress and Reynolds shear stress measured near the bed are correlated with sediment entrainment for longer periods, indicating that these quantities are associated to distal transport as well. Geomorphological changes in the mobile bed are strongly related to the impulse caused by the bed shear stress on the sediment. On the other hand, we show that the nature of the grain of the mobile bed reach influences the hydrodynamics of the current which means that a feedback mechanisms between both occurs during the passage of the unsteady gravity current. The signature of this geomorphological changes, which is visible in the form of longitudinal steaks of accumulated sediment downstream the mobile bed, is related to the flow initial buoyancy and to the size of the mobile bed sediment. It is argued that the bed material and near-bed turbulent coherent motion interact and mutually influence each other. The geometry of the front of the gravity currents changes with the incorporation of the sediment, indicating that with the presence of sediment extra energy losses occur in the front of the current.

The Spatial Distribution of Bed Sediments at Krueng Cut Estuary: The Sieve Analysis Approach

This research studied the characteristic of bed sediment in the estuary of Krueng Cut, Banda Aceh. Analyzing the field samples from different site of stations along the channel from upstream to estuary shows different pattern and profile of deposited bed sediment. Therefore, sorting the size of sediment is important spatially because pattern of its deposition and characteristic will illustrate how distribution patterns indicate the nature of the channel. Hence, the study aims to determain the pattern of bed sediment distribution as well as its profile spatially. To implement the intended purpose, the coring method was employed by using purposive random sampling for 15 stations of taken sampling. The sample of sediment was proceeded by using a 2,5 cm diameter of PVC paralon at the depth of 25 cm from the bathymetry base. Sample was analyzed using wet sieve analysis method. The result informed that the dominant of bed sediment size are medium sand found around the mouth of the river, fine sand existed along the mixing area, and very fine sand occurred in the transition zone. Inthe region of coastal dynamic, the sediment was dominated by medium sand size, meanwhile in the area of river dominated flow, the sediment is subject to fine sand size.Thus, the fraction of very fine sand size of sediment was found in the zona of transition.

Impact of hydrogeomorphic processes on ecological functions of brown trout habits

In recent decades, due to river regulations and their impact on river morphology, brown trout populations have been declining along Lar River downstream of Lar Dam located near Tehran, Iran. Considering the recent water scarcity in the country, development of river habitat assessment models seems necessary. Therefore, in this research, an analytically applied approach is adopted to evaluate brown trout habitat by creating a relation among the hydrologic, hydraulic, geomorphic and ecologic processes. After field survey, dimensionless shear stress of the stream flow thresholds including environmental flow, bankfull flow, surface and subsurface sediment flow thresholds was calculated for Lar, Dalichay and Sefidab Rivers using Shields formula. Then, by considering the dimensionless shear stress ranges of the stream flow thresholds, functional flows ranges and duration were calculated together with ecological efficiency of the cross sections. In addition, effects of annual water yield and entrenchment ratio of the cross sections on habitat functionality were also worked out as a result of which an exponential interaction was developed between the dimensionless shear stress and discharge. Results show that an increase in functional flows ranges and duration, together with rising of ecological efficiency, is directly proportional to an increase in median bed sediment size, entrenchment ratio and annual water yield. Therefore, flow regime, cross-sectional geometry, water-surface slope and bed sediment size could be effective on the ecological functions of the brown trout’s life cycle and functionality of river flow.