Bed Level Research Articles

Experimental study on local scour at complex bridge piers under steady currents with bed-form migration

Local scour at pile-supported bridge foundations in coastal and offshore areas are an essential factor to engineering design and evaluation. This study experimentally investigates live-bed local scour at complex bridge piers under the influence of bed-form migration. The results show that the variation of the mean scour depth as a function of flow intensity ratio U/Uc (Uc= threshold velocity for incipient motion) depends on the pile-cap elevation and is significantly different to single piers. Instantaneous bed level, cross-correlation analysis, and power spectra show that bed-forms approaching an exposed pile group may be damped, leading to mean bed level fluctuation that are slightly smaller than the bed-form heights. In contrast, when the pile-cap is completely buried by the original bed but can be exposed during scour, approaching bed-forms tend to keep their individual features and cause larger scour fluctuation at pile-cap's leading edge. The enlargement is due to the concurrence between the enhanced (or weakened) scouring capacity and the decreased (or increased) upstream sediment supply. The scour predictor proposed by previous studies is extended to live-bed range and performs better than existing predictors.

Remote bed-level change and overwash observation with low-cost ultrasonic distance sensors

Few datasets exist of high-frequency, in situ measurements of storm overwash, an essential mechanism for the subaerial maintenance of barrier islands and spits. Here we describe a new sensor platform for measuring bed-level change and estimating overwash inundation depths. Our MeOw (Measuring Overwash) stations consist of two ultrasonic distance sensors, a microprocessor board, and a camera and are capable of withstanding the impacts of large storm events, can be left unattended to collect data for months to years, and are relatively inexpensive. With the exception of the camera, the MeOw stations are built with all open-source hardware and software. Herein we provide complete instructions for manufacturing the MeOw stations and present observations from a single MeOw station for a three-month (2019) deployment on a frequently overwashed section of Smith Island, VA. The MeOw stations captured three large storm events over the course of the deployment (Hurricane Dorian, Tropical Storm Melissa, and a November nor’easter), as well as several high-tide events. Based on our interpretation of the raw data, bed-level changes occurred throughout the deployment from both storm and non-storm overwash, but were particularly large during Tropical Storm Melissa where initial accretion of approximately 0.15 m was followed by 0.77 m of erosion over three days. The maximum overwash inundation depth occurred during the nor’easter and measured approximately 0.83 m. The variability in bed level over the course of our experiment highlights the importance of in situ high frequency bed-level measurements for constraining overwash inundation depths. MeOw stations are ideally suited for measuring storm overwash — or any process that necessitates tracking bed and water level elevations at high frequency during harsh conditions.

A Two‐Layer Numerical Model for Coastal Sheet‐Flow Sediment Transport

AbstractSheet‐flow sediment transport is an important topic for coastal sediment research. In this study, a two‐layer numerical model is proposed for coastal sheet‐flow sediment transport. The bottom pick‐up layer, in which sediment motion is usually treated as bedload transport, is resolved in an empirical way, so no bedload model is required. For the suspension layer, the model solves the turbulent diffusion equation for sediment concentration, while the hydrodynamic part is described by Reynolds‐averaged Navier‐Stokes equations. The damping of turbulence due to concentration gradient and the hindered settling effect due to particle‐particle interactions are both incorporated in the present model. A predetermined erosion depth is taken as an input for determining the no‐motion sand bed level. This two‐layer model is capable of describing the detailed spatial‐temporal variations of flow velocity and sediment concentration within the sheet‐flow layer, and the model's fidelity is comparable to that of some two‐phase‐flow models. A large campaign of model validation is carried out to demonstrate that the models works well for wave‐driven sheet‐flow sediment transport.

An experimental study of a volatiles distributor for solid fuels chemical-looping combustion process

A novel concept called volatiles distributor (VD), with the purpose to achieve an even distribution of volatiles over the cross-section of a fluidized-bed and better contact between volatiles and bed materials, has been investigated. The concept could be useful for chemical- looping combustion, as well as other solid fuel conversion processes in fluidized-beds. An experimental study of the VD in a circulating fluidized-bed (CFB) cold-flow model was conducted under different fluidization velocities and flows of simulated volatiles. In the reference case without VD, a local plume of volatiles is formed and the maldistribution becomes more pronounced at higher fluidization velocity in the range from 1 m/s to 4 m/s. Conversely, higher fluidization velocity gives a more even volatiles distribution in the presence of VD. The relative standard deviation of volatiles horizontal distribution decreases from 131% in absence of VD to 22% in presence of VD at the fluidization velocity of 4 m/s. There is no significant effect of volatiles flow rate on VD performance at a fluidization velocity 1 m/s. As the fluidization velocity and volatiles flow rate increase, the bed level inside VD is lowered and the volatiles inside the VD become less diluted, because less air from the main fluidization passes through the VD.

The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study

Phragmites australis is a globally distributed wetland plant. At the mouth of the Mississippi River, P. australis on natural levees of the network of distributary channels appears to increase the flow in the deep draft navigation channel, which, in turn, may reduce the sedimentation and benefit the navigation dredging. For several years, P. australis has been dying in the Mississippi River’s Bird’s Foot Delta, which appears to be shortening the distributary channels and increasing the lateral flow from the remaining portions. A conceptual model based on D-FLOW FM was applied to calculate channel sedimentation in a series of idealized deltaic systems to predict the consequences of P. australis dieback and other factors that diminish the delta complexity, such as sea-level rise and subsidence, on sedimentation in the distributary channels. Channel complexity in each system, which was quantified with an index ranging from 0 to 10 that we developed. Model results indicate that sedimentation was insensitive to the channel complexity in simple deltas but was sensitive to the channel complexity in complex deltas, such as the current Mississippi River Delta with extensive P. australis. Channel sedimentation remains stable from 0 until the channel complexity index reaches 6. In more complex deltas, the sedimentation decreases rapidly as the channel complexity increases. The sedimentation is also affected by waves, river discharge, sediment concentration, grain sizes, and bed level. River managers in Louisiana may benefit from new models based on bathymetric data throughout the Bird’s Foot Delta; data on the effects of the P. australis belowground biomass on bank erodibility across a range of current velocities; and data on the effects of P. australis stem density, diameter, and height on the lateral flow across a range of river stages and tidal stages to help them decide how much to respond to Phragmites dieback. Options include increased navigation dredging, increased restoration of the channel complexity via a thin layer of sediment deposition on natural levees and the planting of more salt-tolerant vegetation on natural levees.

Computing the sediment and ensuing its erosive activities using HEC-RAS to surmise the flooding in Kulfo River in Southern Ethiopia

Purpose The change in sediment transport phenomenon and morphological characteristics of Kulfo River in the southern part of Ethiopia is estimated using one-dimensional hydraulic modelling. The purpose of this study is to predict erosion and sedimentation using hydrological engineering center-river analysis system (HEC-RAS) model. Design/methodology/approach Geometrical survey data of 2005 and 2019 were used to assess the impact of flood depth with 100 years of return per period on the morphology of the river. The bed and bank materials at selected sites of the river were sampled to estimate the grain size using manning roughness coefficient. Discharge and suspended sediment concentration were sampled thrice per day for a stretch of three months to develop a rating curve. Findings HEC-RAS model indicates that flood depth with 100 years return period had a significant inundated area during 2019 in comparison to 2005 demonstrating a temporary change in the morphology of the river. Acker and White method in HEC-RAS was used extensively to calculate the sediment load and subsequently calibrated. In the upper reach of the study area, there was aggradation and degradation, whereas the only degradation was noted in the middle to the downstream reach of the channel. Seasonal flood during peak flow due to a rise in bed level is most prevalent during the aggradation period. Originality/value Comparison of a flood depth inundating the catchment and sediment deposition has been intricately analyzed by using HEC-RAS model.

Experimental Studies and Analysis on Mobilization of the Cohesionless Sediments Through Alluvial Channel: A Review

Entrainment of river bed particles by turbulent flow is a core matter of study in river hydrodynamics. It is of great interest to river engineers to evaluate the shear stress for initiating river bed motion. The main objective is to calculate transport rates for bed load, to predict changes in bed level which are scoured or aggraded and to design a stable channel. Forces acting upon the particle especially fluid forces which give a major role in the incipient motion of the particle on the rough boundary. For calculation generally use shield’s diagram but some other modified methods and approaches are discussed. Modeling criteria are discussed for the hydraulically smooth and rough boundary depending on Reynolds number. In the past, experimental studies on tractive shear stress have been done by many researchers but consideration of lift force to analyze the movement of sediment is very limited. For suspended load transport, a detailed analysis of lift force is required. Based on the study it has been observed that shear stress depends on channel slope not only due to gravitational force but also many other factors like drag force, lift force, friction angle, fluctuations, velocity profile, etc. Complete analysis of these factors provides slope dependency over shear stress. To improve past studies, some factors have been discussed, to give a more correct force balance equation. This is very difficult task to analyze more and more variable’s dependency on the slope. Consideration of the possible number of variable holds complete analysis of experimental study. This paper also reviews the effect of particle Reynolds number and relative submergence over critical shield stress. Doi: 10.28991/cej-2021-03091700 Full Text: PDF

Field Observation and Numerical Modelling for Morphological Response in the Macro-tidal Beach

This study investigated the sediment transport and bed level change in the macro-tidal beach using field observation and numerical modelling. The study area is Sinduri beach, located on the west coast of Korea, which developed a large scale multi-sand bar with a tidal range of 5 to 7 m. The multi-sand bar is destroyed in the high-wave conditions (winter) and developed in the low-wave conditions (summer), which have different characteristics from the wave-dominated beach. Therefore, this study measured the wave level, current, and bed level change using ADCP (Acoustic Doppler Current Profilers) and VRS-GSP (Virtual Reference Station Global Positioning Systems) to investigate the mechanism of the multi-sand bar formation. Then, this study simulated sediment transport and bed level change, coupled with the Delft3D-FLOW and Delft3D-WAVE model. The numerical results show that sediment transport and bed level change dominantly occurred under the shore-normal wave condition and tend to develop a multi-sand bar. However, the destruction of the multi-sand bar was not reproduced. Delft3D-FLOW model is based on the two-dimensional depth-averaged shallow water equation and it is not applicable to simulate the wave-induced current near the seabed. Also, the aeolian sediment transport by north-west wind can not involve in the numerical simulation. Keywords: wave, tide, macro-tidal beach, Delft3D, sediment transport

Influence of suction and collar on reducing local scouring in cylindrical pier

Bridges are among the most important structures in river engineering. One of the significant causes of the destruction of bridges is local scouring around the piers. In the present research, collar and suction system both employed the upstream of the pier against local scouring. In separate experiments, the collar models were examined in different dimensions and the suction system was considered at different depth levels. Then, the combined collar-suction model was tested. The results showed that the suction system at the bed level (Z/h = 0) with the suction rate of (Qs/Q = 1.7%) had 51% efficiency; the 3D-length, 2D-width collar yielded 59% efficiency; and the combined suction-collar model had 94% efficiency in reducing local scouring of the pier by decreasing down-flow velocity, as well as the Reynolds Shear Stresses (RSSs) and Turbulence Kinetic Energy (TKE).

Introducing ring collars and effective spiral threading elevation for cylindrical pier scour control

In this study, laboratory experiments are conducted to assess the effects of a set of novel ring-type collars and prevalent spiral threading around a circular pier on scouring in a clear-water flow regime in non-cohesive bed materials. Also, numerical simulations are conducted to reveal the flow field in the scour hole around a pier with spiral threading. Various angles and spacings of the ring collars are considered to find the best configuration. Moreover, four test scenarios of spiral threading with different elevation levels are considered to investigate the optimum upper bound of spiral threading elevation. Results show that the ring collars (Angle = 45˚, relative spacing = 4/3 of pier diameter) are the best configuration and reduce the scour depth by 29% according to the scour depth around a plain pier. This configuration of ring collars reduces maximum scour depth three times better than the best configuration of single spiral threading (suggested by previous research). Furthermore, the ring collars reduce the scour rate while the spiral threading has no considerable effect on the scour rate. The spiral threading under the initial bed level plays a major role in reducing local scour.

Small-Scale Morpho-Sedimentary Dynamics in the Swash Zone of a Megatidal Mixed Sand–Gravel Beach

On mixed sand–gravel beaches, impacts from gravel- and cobble-sized grains—mobilized by the energetic shorebreak—limit the utility of in situ instrumentation for measuring the small-scale response of the beach face on wave period time scales. We present field observations of swash zone morpho-sedimentary dynamics at a steep, megatidal mixed sand–gravel beach using aeroacoustic and optical remote sensing. Coincident observations of bed level and mean surficial sediment grain size in the swash zone were obtained using an array of optical cameras paired with acoustic range sensors. Lagrangian tracking of swash-transported cobbles was carried out using an additional downward-oriented camera. The principal objective of the study was to investigate linkages between sediment grain size dynamics and swash zone morphological change. In general, data from the range sensor and camera array show that increases in bed level corresponded to increases in mean grain size. Finer-scale structures in the bed level and mean grain size signals were observable over timescales of minutes, including signatures of bands of coarse-grained material that migrated shoreward with the leading edge of the swash prior to high tide berm formation. The direction and magnitude of cobble transport in the swash varied with cross-shore position, and with the composition of the underlying bed. These results demonstrate that close-range remote sensing techniques can provide valuable insights into the roles of cobble-sized versus sand-sized particle dynamics in the swash zone on mixed sand–gravel beaches.

Free Discharge of Subsurface Drainage Effluent: An Alternate Design of the Surface Drain System in Pakistan

In Pakistan, many subsurface (SS) drainage projects were launched by the Salinity Control and Reclamation Project (SCARP) to deal with twin problems (waterlogging and salinity). In some cases, sump pumps were installed for the disposal of SS effluent into surface drainage channels. Presently, sump pumps have become dysfunctional due to social and financial constraints. This study evaluates the alternate design of the Paharang drainage system that could permit the discharge of the SS drainage system in the response of gravity. The proposed design was completed after many successive trials in terms of lowering the bed level and decreasing the channel bed slope. Interconnected MS-Excel worksheets were developed to design the L-section and X-section. Design continuity of the drainage system was achieved by ensuring the bed and water levels of the receiving drain were lower than the outfalling drain. The drain cross-section was set within the present row with a few changes on the service roadside. The channel side slope was taken as 1:1.5 and the spoil bank inner and outer slopes were kept as 1:2 for the entire design. The earthwork was calculated in terms of excavation for lowering the bed level and increasing the drain section to place the excavated materials in a specific manner. The study showed that modification in the design of the Paharang drainage system is technically admissible and allows for the continuous discharge of SS drainage effluent from the area.

Lithological control on scour hole formation in the Rhine-Meuse Estuary

River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake.In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works.

Evaluating the Structure from Motion Technique for Measurement of Bed Morphology in Physical Model Studies

The selection of instrumentation for data acquisition in physical model studies depends on type and resolution of data to be recorded, time frame of the model study, available instrumentation alternatives, availability of skilled personnel and overall budget of the model study. The available instrumentation for recording bed levels or three-dimensional information on geometry of a physical model range from simple manual gauges to sophisticated laser or acoustic sensors. In this study, Structure from Motion (SfM) technique was applied, on three physical model studies of different scales and study objectives, as a cheap, quicker, easy to use and satisfactorily precise alternative for recording 3D point data in form of colour coded dense point cloud representing the model geometry especially the river bed levels in the model. The accuracy of 3D point cloud generated with SfM technique were also assessed by comparing with data obtained from manual measurement using conventional surveying technique in the models and the results were found to be very promising.

Dense upflow fluidized bed (DUFB) solar receivers of high aspect ratio: Different fluidization modes through inserting bubble rupture promoters

A fluidized bed of Geldart-A particles is promoted as heat transfer fluid in the tubular solar receivers of solar towers. A pressure-driven upward particle flow affects the hydrodynamic flow structure and properties of the fluidized bed. Experiments involved a tube of 0.05 m internal diameter but of very high height/diameter ratio (>120), representative of the future solar receiver and of numerous chemical reactors. Solid circulation fluxes and aeration velocities were varied. Configurations of a bare tube and a tube with bubble rupture promoters were compared. In the bare tube, freely bubbling is transformed into axi-symmetric slugging at a bed level of ~1 m. With bubble rupture promoters, freely bubbling prevails to about a bed level of 3 m, and a turbulent fluidization mode develops higher up the tube (a more chaotic two-phase system with elongated and unstable “gas voids” and “dense solid clusters”), without axi-symmetric slugging detected. Experimental results for both tube configurations were assessed and compared with CFD predictions by the Euler n-fluid code, NEPTUNE_CFD. A good agreement of bed properties was obtained for slug/void frequencies and solids volume fraction in both tube configurations. BRPs moreover enhance the bubble through flow of the fluidizing gas, thus limiting the visible bubble flow rate and bubble sizes while increasing the gas/particle contact, and hence important in designing multi-tube chemical reactors. Whereas slugging limits the heat transfer from the tube wall to the suspension at ~200 W/m2K, the presence of BRPs maintains a heat transfer coefficient in excess of 600 W/m2K.

Hydrodynamic and Sediment Transport Simulation at The Port of The Electric Steam Power Plant Adipala and Serayu Estuary, Central Java Province, Indonesia

The coastal area of Cilacap Regency in Indonesia has an important role in increasing activity, economy and ecosystem. The research location is in the southern coastal area of Java and directly faces the Indian Ocean. This can lead to dynamic marine phenomena and cause various kinds of sedimentation problems. To analyze the phenomena that occur in Cilacap seawater, a numerical model is applied to obtain optimal results. MIKE21 software with Hydrodynamic and Sand Transport modules were used in this study to analyze the flow patterns and sediment transport that occurred. The model was built with Cilacap waters as the boundary and was made within 30 days. The result obtained is the model validation using the RMSE method shows a value of 16.33% so that the model is included in the good category. The average direction of the current pattern from the results of the model leads to the west and northwest. The current velocity at ebb is greater than at tide. The most significant bed level changes occurred at the mouth of the Serayu River and the east side of the breakwater.

Morphodynamic Equilibria in Short Tidal Basins Using a 2DH Exploratory Model

AbstractA depth‐averaged (2DH) exploratory model is developed to identify morphodynamic equilibria in short mesotidal inlet systems with arbitrary planform geometries. The water motion is forced by an M 2 tidal constituent at the seaward entrance and is described by the depth‐averaged shallow water equations, whereas the depth‐integrated suspended sediment concentration follows from a diffusion equation, taking into account local inertia, horizontal eddy diffusion and topographically induced diffusive effects, erosion, and deposition. Based on a scaling analysis, it follows that the fine sandy bed evolution is dominated by the depth‐integrated diffusive sediment transport. The depth‐integrated advective contributions are one order smaller and therefore neglected. This observation also allows for the neglect of the advective terms in the governing equations. The associated morphodynamic equilibria are directly identified based on a continuation approach. By means of the exploratory model, the morphodynamic equilibria are studied in basins with a planform geometry characterized by width variations as a function of the distance to the seaward boundary. The model results show that in the case of a sufficient degree of widening in the landward direction, the equilibrium bed level exhibits significant lateral structures, characterized by shallow zones and deeper channels. The first channel bifurcation, as observed in many short tidal inlet systems, is forced by the planform geometry of the basin, and the associated physical mechanisms are explained. Furthermore, two mechanisms inducing asymmetric morphodynamic equilibria are investigated, of which the effect of an asymmetric basin planform seems to be dominant over that of the Coriolis force.

Variability of sediment processes around a tidal farm in a theoretical channel

Sediment transport plays a crucial role in coastal ecosystems and is one of the least known parameters in high energy sites. The recent development of tidal energy projects challenges the scientific community to better understand this natural phenomenon and the interactions with tidal turbines. Using MIKE21/3 software from DHI, a benchmark for sediment transport model was developed with simulations with and without a tidal farm in idealised two and three-dimensional tidal channels. Results reveal that a 2D approach is sufficient for regional scale morphological assessments, however 3D models allow for a closer examination of influences around the tidal farm. Differences in calculating sediment transport rates based on approaches after Engelund & Fredsøe and Van Rijn formulae illustrate the degree of uncertainty in modelling sediment transport rates. Results for the Engelund & Fredsøe models also show that they are sensitive to mesh resolution in equilibrium conditions and are more stable in non-equilibrium conditions for the bed level change rates. Tests have also demonstrated that models using Engelund & Fredsøe formulae are more sensitive to physical characteristics than models based on Van Rijn, accentuating the necessity to evaluate sediment transport formulae with field data before making a choice of model.

Impact of Sedimentation on Water Seepage Capacity in Lake Nakuru, Kenya

Accumulation and deposition of sediments in waterbody affect the seepage capacity that could lead to improper water balance and results in the water level rise. This study analysed the influence of sedimentation on seepage capacity in Lake Nakuru and the impact of sediment characteristics to the water seepage and the flow rate formation at the lake bed level. The study was performed by sampling and analysing the sediment cores from two locations in the lake. The sediment hydraulic properties, i.e., moisture and porosity, particle sizes, and hydraulic conductivity, were determined using the oven-drying method, sieve analysis, hydrometer analysis, and falling head tests, respectively. The results showed that the lake sediment sample from location P1 had an average ratio of 39.38% for silty soil, 34.00% for clayey sediment, and 26.63% for fine-sand sediment particles with the maximum permeability coefficient of 3.37 ∗ 10 − 5 cm/s, while the one from location P2 had an average ratio of 63.17% for sand, 20.17% for fine particles, and 16.67% for gravels with the maximum permeability coefficient of 0.010793 cm/s. The hydraulic conductivity of sediment sample from location P1 and P2 increased along the core depth. This could lead to the rise of water level due to the decreases of water movement induced from the sediment cementation in the top layers under the waterbody. Sedimentation affects Lake Nakuru water volume and water balance; hence, there is a need to control the inflow of sediment resulting from anthropogenic activities in the watershed.

Diagnostic Lobectomy for Bethesda III Thyroid Nodules: Pathological Outcomes and Risk Factors for Malignancy.

An indeterminate thyroid nodule fine-needle aspiration (FNA) presents a unique dilemma. We evaluated our institution's experience with Bethesda III thyroid nodules, including the risk of malignancy (ROM) of these nodules removed for diagnostic lobectomy and radiologic and clinical risk factors for malignancy. Retrospective chart review. San Antonio Military Medical Center (SAMMC; 483 bed Military Treatment Facility and Level 1 Trauma Center). We identified all patients with a Bethesda III thyroid FNA at our institution from 2010 to 2018 and determined which nodules were removed. The final histological diagnosis was recorded. Whether or not age, gender, body mass index (BMI), race, nodule size, margin regularity, rate of nodule growth, vascularity, internal calcifications, family history of thyroid cancer, personal history of radiation, and history of repeated AUS/FLUS on FNA had a significant impact on malignancy risk was evaluated with chi square and rank sum Wilcoxon tests. 492 patients had 1 or more AUS result. 52% (258/492) underwent repeat FNA. This resulted in Bethesda II or III in 90% (232/258). In 10% (26/258), the repeat FNA resulted in a higher Bethesda grade. 183 lobectomies were performed on the side containing an AUS nodule. The malignancy percentage was 38.3% (70/183). Age less than 30 was the only variable showing statistical significance for increased risk of malignancy (P = .04). The ROM of nodules characterized as AUS/FLUS on FNA may be higher than expected. Age may be a better predictor of malignancy than repeat FNA.