Clear-water Conditions Research Articles

Closure to “An integrated framework of extreme learning machines for predicting scour at pile groups in clear water condition” by: I. Ebtehaj, H. Bonakdari, F. Moradi, B. Gharabaghi, Z. Sheikh Khozani

The authors of the paper under discussion predicted the scour depth around pile groups in clear water using the extreme learning machine (ELM) neural network. The ELM model performance results were compared with those obtained with support vector machine (SVM) and artificial neural network (ANN). This discussion highlights Baghbadorani, Ataie-Ashtiani, Beheshti's (2018) incorrect calculations in Table 3 and scatter plots in Fig. 1 presented in their discussion. For example, they indicated the overall performance of R2 is −2.5 and MAPE is 67% for the 27 raw data listed in the table. They reported these statistical indices not only according to erroneous calculations but also for merely 27 samples selected arbitrarily and without valid justification from over 1 100 data available from previous studies. Using 27 out of 1 100 data points is evidently not a sound means of evaluating the overall accuracy of the proposed equation. Subsequently, the authors provide information to further clarify their responses to the comments in the discussion paper.

Experimental Comparison of Sacrificial Piles and Submerged Vanes as Scour Countermeasures around Bridge Pier

An experimental study has been conducted on the effectiveness of two types of flow-altering countermeasures placed around a cylindrical pier under clear water condition, which consists of sacrificial piles and submerged vanes. Their arrangements follow the optimal configurations recommended in the published articles with some modification. The temporal evolution of maximum scour depth and its equilibrium amount around the pile without and with two scour countermeasures were recorded; scour depth reduction rates and changes in bed topography were then computed. It can be concluded that pursuant to the result which has been earned from the experiments of this study, submerged vanes had better performances compare with the sacrificial piles in all of investigated aspects. By applying submerged vanes 75% of the scour only occurred in first 5 minute. Presence of submerged vanes played more efficient role in reducing scour depth as subjected scour depth reduction was 17% more than one subjected to sacrificial piles. The length of changes downstream the pier (L) were 12, 13.2 and 6 times of pier diameter (D) corresponding to pier with no protection, sacrificial piles and submerged vanes respectively, which revealed the effectiveness of submerged vanes in reducing the area affected by pier scour. Also the maximum changes of bed level were -6.9 cm to 3.9 cm in unprotected pier test, -4.4 to 3.5 cm in protection by sacrificial piles test and -3 to 3.2 cm in protection by submerged vanes.

On the feedback between water turbidity and microphytobenthos growth in shallow tidal environments

AbstractShallow tidal environments (e.g. bays, estuaries, lagoons) represent one of the most productive ecosystems in the world, and they are threatened by current climate change and increasing human pressure. Monitoring the bio‐morphodynamic evolution of these environments is therefore a crucial task that requires a detailed and holistic scrutiny. The present study aims to investigate the temperature of the water–sediment continuum, its effect on the related microphytobenthos (MPB) growth and the related bio‐stabilization of the bed sediment surface under different water depth and water turbidity conditions. We investigated the vertical energy transfer and the temperature dynamics by applying a 1‐D model to a shallow coastal lagoon. Our results show that the water temperature does not substantially change under different turbidity conditions, whereas the sediment temperature exhibits important changes. Two major factors driving the MPB photosynthetic growth are the sediment surface temperature and the light availability at the sediment bed, which can both be computed using the vertical energy transfer model. We observed that, in general, clear water conditions better promote MPB growth over the entire year. The limiting factor for the photosynthetic process is usually the light availability at the bottom, which increases under clear water conditions. As MPB provides a bio‐stabilizing effect on the bed sediments by producing a biofilm on the sediment surface that reduces sediment resuspension, our results suggest a positive feedback between MPB growth and water column turbidity. Furthermore, MPB growth and the related sediment bio‐stabilization are clearly affected by the seasonal variation of surface sediment temperature and light availability. This seasonal variation of MPB growth rate and surface sediment bio‐stabilization must be considered when studying the long‐term morphodynamic evolution of tidal environments. © 2018 John Wiley & Sons Ltd.

Scour from the interstitial spaces in cobble-bed rivers

The periodic removal of sand from the interstitial spaces between cobbles is extremely important for ecosystem functioning in cobble-bed rivers. In order to flush fine sediments from the interstitial spaces between cobbles in river reaches downstream of dams, specific dam releases known as flushing flows or sediment maintenance flows are utilised. This paper describes the development and calibration of a mathematical model to predict the equilibrium depth of scour of fine sands from between cobbles in terms of applied stream power principles. The model was developed with the aid of physical model experiments and is founded on a stream power model which defines the condition of dynamic equilibrium in a deformed sand-bed river. Calibration was done in the laboratory under clear water conditions and with uniform cobble sizes. The scouring of fine sands in cobble-bed rivers is associated with an increase in absolute bed roughness and an associated decrease in the unit stream power applied along the bed as the cobbles become exposed. When scour ceases, the sand particles on the bed are at the movement threshold and critical conditions exist. In order to establish the relationship between equilibrium scour depth and bed particle characteristics, the power which is required to suspend sand particles under laminar boundary conditions is equated with the turbulent power being applied along the bed.

Study of the Local Scour Around L-Shape Groynes in Clear Water Conditions

River training structures; such as groynes; are efficient engineered methods utilized to protect eroding river banks. But during groynes design stage; the designers faces one of the most significant issues which is the evaluation of the maximum depth of local scour around groynes. This is due to the flow area constriction in the groyne location itself; which in turn cause increasing in local velocity that consequently causes the scour. In the present study, the maximum scour depth around L-shape groyne was computed based on laboratory experiments where different number of groynes and distances between them were used as sort of countermeasures to reduce the scour around the groynes foundations. The results showed clear decreasing in scour depth at increasing number of groynes and also distance between them in the limitation of this study. A new formula based on experimental data was derived to calculate the local scour depth. This formula gave R2 =0.903 which reflecting good agreement to the results.

Effect of submergence ratio of parallel wall on bridge abutment scour

Scour at bridge abutment could be the main cause of bridge failures, leading to increased repair costs and reduced accessibility to roads. Thus, study and research on the prediction of scour at bridge abutment and its prevention is of great importance. The aim of this study was to investigate the effect of submergence ratio of parallel wall on vertical wall abutment scour. Experiments were conducted for different lengths and heights of parallel walls under clear water condition. In all experiments, the flow depth and the abutment length were 16 cm and 8 cm, respectively. Lengths of parallel walls were 4, 6, 8, 10, 12, and 16 cm (0.5, 0.75, 1, 1.25, 1.5, and 2 times of the abutment length, respectively), and parallel wall heights were 4, 8, 12 and 16 cm above sediment level (submergence ratio 25, 50, 75, and 100%, respectively, based on the ratio of the flow depth). By changing the flow pattern around bridge abutment, parallel walls are able to move maximum scour depth away from the upstream nose of the abutment and transfer it to parallel wall nose with less depth. Results of the study indicated that use of parallel wall has the best performance in the submergence ratio of 25%.

Erosion and sedimentation patterns associated with restoration structures of bendway weirs

Bendway weirs (B.W.), known as new and innovative river training structures, are positioned from the outer bank of the river bend and angled upstream toward the flow. In order to investigate the effect of bendway weirs on the sediment process, experiments were conducted in a 90° bend with a relative curvature of 3.3 with and without bendway weirs. Following the installation of bendway weirs with different crest slopes, the scour and sedimentation patterns were evaluated under clear water conditions. To this end, a series of B.W. (7 weirs) with three crest slopes (0, 5, and 10 percent) were installed at the outer bank side in different hydraulic conditions. Erodible bed topography measurements revealed that the maximum scour depth occurred at the tip of the first and last weirs. Moreover, the findings of this study clearly demonstrates the potential of bendway weirs in modifying morphology of river bends and their efficiency in reducing erosion and sedimentation. In general, compared to non-weirs condition, the eroded sediment volume increased by 53, 36, and 15 percent due to the installation of weirs with crest slopes of 0, 5, and 10 percent, respectively. Finally, this study revealed that the maximum scour depth decreased with an increase in the crest slope.

Critical velocities for local scour around twin piers in tandem

Experiments of the local scour around twin piers are carried out under steady clear-water conditions, including 95 tests to observe the influence of the pier spacing and the flow velocity on the local scour characteristics of the twin piers. It is shown that the start of the transition region is synchronous with the sediment transport from the upstream scour hole to the downstream one. The equations for the critical velocities are derived to quantify the velocity range of each of four different scour regions. Finally, a prediction formula of the downstream pier scour depth in the radical-deviation region is established.

Discussion of "an integrated framework of extreme learning machines for predicting scour at pile groups in clear water condition" by: I. Ebtehaj, H. Bonakdari, F. Moradi, B. Gharabaghi, Z. Sheikh Khozani

Discussion of "an integrated framework of extreme learning machines for predicting scour at pile groups in clear water condition" by: I. Ebtehaj, H. Bonakdari, F. Moradi, B. Gharabaghi, Z. Sheikh Khozani

An experimental study of plunge depth in the case of clear water movement

ABSTRACTThe condition in which ambient clear water moves opposite to a density current usually happens in river estuaries during the tide of the sea. The examination of this condition can improve our understanding of plunging phenomenon, which can result in a better estimation of river morphology. In this experimental study, it has been tried to examine the effects of bed slope and clear water discharge on plunge depth in moving clear water condition and to compare these effects with those of stagnant clear water. Fifty four experiments were conducted in a flume with adjustable bed slope. Its length, width, and depth were 9, 0.35, and 0.66 m, respectively. By adjusting the rate of discharge valves, the first 18 experiments were performed under the condition of static clear water, while the others were conducted as clear water was moving opposite to a density current. Results show that, in both static and moving clear water conditions, plunge depth increases as bed becomes steeper. In addition, the discharge of clear water has a direct relationship with plunge depth and moving clear water condition results in higher plunge depths than the static condition. At the plunge point, the dense underflow is critical or subcritical and there is an inverse correlation between densimetric Froude number and bed slope. Finally, empirical equations have been developed to estimate plunge depth.

Protecting bridge piers against local scour using a flow-diversion structure

Previous studies indicate that local scour is a leading cause of most waterway bridge failures during flood events. Using armouring countermeasures such as riprap and gabions is a conventional way of reducing scour around bridge piers, but is very costly and time consuming. As an alternative, a flow-diversion structure is proposed that has a triangular prismatic shape with dimensions much smaller than the actual pier and should be installed upstream of the pier. To assess its performance, experiments were conducted under clear-water scour conditions. After achieving equilibrium bed conditions, the bed profile was measured and the maximum scour depth and volume of the scour hole were determined for each experimental test. The results indicated that the clear distance between the pier and the countermeasure to achieve the maximum reduction in local scour was 1·5 times the pier diameter. For this condition, the proposed countermeasure reduced the maximum scour depth by 38% and the volume of the scour hole was decreased by around 61%. To determine the influence of the countermeasure on flow field around the pier, three-dimensional velocity components were measured at grid points using a micro-acoustic Doppler velocity meter. Analysis of the results indicated that the proposed structure could change both the magnitude and direction of the velocity components upstream of the pier and consequently induce a significant reduction in local scour depth and volume around the pier.

Y-shaped and T-shaped slots in river bridge piers as scour countermeasures

Slots in river bridge piers have proved effective at reducing foundation scour. This paper reports on 33 experiments carried out under clear-water conditions to assess the performance of Y-shaped and T-shaped slots compared with commonly used straight slots. The variables included three flow discharges, five types of slotted piers and a single pier without a slot, two slots partially embedded in the river bed and non-embedded slots. The scour contours and longitudinal and transverse scour profiles were measured. It was observed that, for the Y-shaped slots, the size of the downstream scour was reduced by decreasing the angle between the downstream slot faces. The results also indicated that the Y-shaped slots were more efficient than the T-shaped slots for scour control, while slot efficiency increased with embedment depth. A maximum efficiency of 59% in scour volume reduction was obtained for the best Y-shaped configuration, compared with a 53% reduction for the straight slot. Regarding the maximum scour depth, the straight slot provided a maximum scour reduction of 38%, compared with 33% maximum reduction for the Y-shaped slot.

Physiological changes of submerged macrophytes in response to a floating filamentous green algae bloom in clear-water conditions

The aim of this study was to evaluate the physiological responses of a submerged macrophyte to a floating filamentous green algal bloom in clear-water conditions. Elodea nuttallii was grown with floating Cladophora sp. at four different levels (0, control; 140, 280, 560 g FW/m(2)) in an outdoor experimental system, and its photosynthetic and antioxidant systems were evaluated. The presence of floating Cladophora sp. significantly changed the water environment by decreasing light intensity and increasing dissolved oxygen and the pH value. The photosynthetic parameters of E. nuttallii (e.g. Delta F/F-m',F-v/F-m, total chlorophyll) were higher in the presence of floating Cladophora sp. than in the control at the beginning of experiment. Because of the increasing dissolved oxygen concentration and pH value, the values of these indicators decreased (except for photosynthetic pigments) during the experiment. Compared with E. nuttallii in the control, E. nuttallii growing in the presence of floating Cladophora sp. showed higher malondialdehyde content, catalase activity, and peroxidase activity. The biomass of E. nuttallii was decreased by about 30% in the presence of high biomasses of floating Cladophora sp. (280 and 560 g FW/m(2)). These results suggest that floating Cladophora had complex effects on the biomass of E. nuttallii and that changes in water quality resulting from floating Cladophora sp. may be more important than its direct shading effect.

Experimental investigation of local scour around submerged piles in steady current

Submerged vertical piles are an analogue for a range of subsea structures. In this paper scour around submerged piles with both circular and square cross-section are explored experimentally in steady currents. A wide range of cylinder height to diameter ratio (h/D= 0.1–8) is considered for flow intensities spanning clear-water to sheet flow conditions. Results are presented in terms of the equilibrium scour depth and the time-scale of the scour process. For the cross-sectional shapes considered the experimental results indicate that the equilibrium scour depth reduces with aspect ratio when h/D<∼ 4, whilst for larger aspect ratio the scour depth is similar to that for an infinitely tall cylinder. The amount of reduction in scour depth with aspect ratio for short piles is found to be dependent on the flow intensity, such that the effects of aspect ratio and flow intensity on scour depth are not separable. With respect to the time-scale, the experiments indicate that for a flow intensity corresponding to live-bed conditions, a maximum time-scale occurs for an intermediate aspect ratio; for aspect ratios below this value the time-scale reduces because the equilibrium scour depth reduces, whilst for larger aspect ratios the time-scale reduces because the local transport rate is increased. Alternatively, in clear-water conditions the time-scale reduces continuously with increasing aspect ratio. Empirical equations are derived based on the experimental results so as to better account for aspect ratio in scour design.

Experimental analyses of impact forces on buildings exposed to fluvial hazards

Fluvial processes have a significant impact on buildings and infrastructure and despite their local character they cause annually considerable costs in mountain areas worldwide.Vulnerability studies are based mainly on empirical data from past events whereas laboratory studies investigating the impact forces on buildings are still limited.The paper presents a study on impact forces resulting from complex flow processes on buildings, the influencing factors they depend on and the identification of process parameters they correlate with.A 1:30 physical scale model was built, representing the torrential fan of the Schnannerbach torrent (Austria). Measurement devices, installed on the building structures located on the torrential fan, recorded impact forces induced by a set of fluviatile flood scenarios. The measured impact forces were compared to computation results using recently developed calculation approaches and to measurements resulting from experiments with a simplified, homogeneous scale model set-up with a single wall element in a rectangular flume. The results show a clear correlation between the approaching flow heights and the impact forces on exposed buildings. Compared to clear water conditions, bed-load transport and deposition processes highly influence the impact forces due to sediment accumulations close to the buildings on the torrential fan. The gained insights contribute to a better understanding of potentially damage-causing impacts on buildings resulting from the interaction of flood processes and the exposed built environment.

CFD-DEM Two-Way Coupled Numerical Simulation of Bridge Local Scour Behavior under Clear-Water Conditions

Bridge local scour involves extremely complex interactions among the flow, the sediments, and the pier. Our understanding of the underlying mechanism, especially at the microscale, is far from complete. The newly emerged multiphase numerical simulation overcomes the technical limitations imposed on physical experiments. To advance knowledge of the sediment dynamics under turbulent junction flows, this paper employed a CFD-DEM two-way coupled numerical model to simulate the local scour behavior of uniform spherical mass particles around an oblong pier. In this coupled system, the flow phase was described as a continuum by a detached eddy simulation (DES) model, and motion of the discrete particles was governed by Newton’s laws of motion (DEM). The flow–sediment interactions were directly considered through the momentum exchange between the two phases. The two-way coupled model was shown to successfully capture microscopic sediment dynamics similar to those observed in the literature. Sediment grains in the bed-load layer were shown to be eroded in the form of sliding and saltation. The erosional forces acting on two representative target particles, together with their correspondent motions/trajectories, provide valuable insights into the mechanism that dictates the scour initiation.

Estimation of scour depth at bridges with complex pier foundations using support vector regression integrated with feature selection

This study aims at establishing machine learning models based on the support vector regression (SVR) for estimating local scour around complex piers under steady clear-water condition. A data set consisting of scour depth measurement cases has been collected to construct the prediction models. The data set includes eight influencing factors that consider aspects of pier geometry, flow property, and river bed material. Moreover, to enhance the performance of the SVR model, filter and wrapper feature selection strategies are used. The research finding is that all feature selection approaches can help to improve the prediction accuracy compared with the SVR model that uses all available features. Notably, the feature selection method based on the variable neighborhood search (VNS) algorithm achieves the best performance (MAPE = 21.65%, R2 = 0.85). Accordingly, the prediction model produced by SVR and VNS can be useful for assisting decision makers in the task of structural health monitoring as well as the design phase of bridges.

Numerical study of local scour around bridge piers

Numerical models have more flexibility avoiding the critical limitation in physical ones. The predictive capability of the hydro-morphological model around bridge piers in clear-water conditions have been investigated using mathematical modelling flow 3D 11.2 as there is a lack in using that model. The objective of the study to investigate the effect of cross section of the pier on the predictive capability of the model. Computations has been performed and the results are compared with several sets of experimental data available in the literature. The Van Rijn sediment transport model and (RNG) k-ε model are used. However, this study strongly demonstrates that a 3D hydro-morphological model can effectively predict the scour around piers with different cross section, there is significantly under predicts for the scour depth at the nose of the pier with different pier shapes. It is found that for the circular pier the capability of the model depending on the flow properties not only the geometry of the pier.

Diatom assemblage in the 24 cm upper sediment associated with human activities in Lake Warna Dieng Plateau Indonesia

Lake Warna is a small shallow crater lake on the Dieng Plateau, Central Java, the second highest plateau in the world after Nepal. A 24 cm sediment core was extracted from Lake Warna tore construct environmental changes in the Lake and its catchment from preserved diatom assemblages. Diatoms are microalgae in the Bacillariophyte that have silicious cell walls that can be preserved in sediments. As diatom species are sensitive to water quality changes in the assemblages upcore reflect changes in lake condition. Sediment cores were collected from two sites,sliced at 1 cm intervals for diatom analysis and bulked across 3 cm for 210 Pb radiometric dating. Examination of diatoms in a 24 cm sediment core from Lake Warna reveals clear correlation with human activities in the catchment area over the past 124 years. The record is divided into 2 zones based on sustained changes in the diatom assemblages. The lowest zone Zone I (21–15 cm, estimated 1935–1954) was dominated by Frustulia crassinervia (Brebisson ex W. Smith) Ross, Gomph onema parvulum (Kutzing) Kutzing, Pinnularia valdetolerans Mayama & H. Kobayasi, P. viridis (Nitzsch) Ehrenberg, and Aulacoseira distans (Ehrenberg) Simonson. Zone II (15–0 cm, estimated 1980–2013), the uppermost zone was dominated by P. viridiformis Krammer, P. latevittata Cleve, E. monodonvar.tropica(Hustedt)Hustedt,S.seminulumGrunow,P acrosphaeriaW.Smith. E. zygodon Ehrenberg, P. gibba Ehrenberg, and P. viridiformis Krammer. The 15 cm core from TW3 commenced sediment accumulation 114 years ago and is divided into 2 zones as well. Zone I (15–7 cm, estimated 1901–1981) was dominated by Brachysira brebissonii R. Ross, G. parvulum (Kutzing) Kutzing, E. monodon var. tropica (Hustedt) Hustedt, E. zygodon Ehrenberg and S. seminulum Grunow. Zone II (7–0 cm, estimated 1981–2006) wasdominated by P.viridi for mis Krammer, P.gibba Ehrenberg, Pinnularia viridis(Nitzsch) Ehrenberg, Sellaphora bacillum (Ehrenberg) Mann, S.seminulum Grunow,and Stauroneissp. These assemblages reflect on going acid,clear water conditions for the time represented by thecores. The recent rise in S.seminulum Grunow reflects recent lake eutrophication likely owing to the accelerated agricultural and urban development in the lake’s catchment in recentdecades.

MAXIMUM SCOUR DEPTH AT SUBMERGED SKEWED BRIDGE AND PIER

Maximum local scour depth around pier due to the submerged skewed bridge along with its rectangular pier under clear water condition has been experimentally studied. In present study, twenty four tests were carried out including; six various aligned bridge models with the different angles such as; 0, 5, 10, 15, 22.5 and 30 degree to the presumptive perpendicular line to the flume wall under two flow types of fully and partially submerged bridge above two different median sizes of uniform sediments. However, in the present study, aligned factor in pressure flow is experimentally evaluated, its results are much less than aligned factor in free flow in the same ratio of rectangular pier length to the pier width. Also, the relationship between dependent and independent variables firstly was determined. Thereafter, an equation was acceptably proposed by using dimensional analysis, collected data and multiple linear regressions.