AbstractThis paper, written to celebrate the 60th anniversary of the Journal of Hydraulic Engineering, presents a structured, contemporary approach to scour-depth estimation that matches design ...method practicality to pier flow-field complexity. The approach involves a mix of semiempirical formulation, advanced experimentation aided by new instruments, and computational fluid dynamics (CFD). Highly useful for understanding complex pier flow fields, CFD holds patent promise for substantial use in design estimation of scour depth. Presently, however, CFD’s limited ability to simulate erosion and scour at a pier foundation hampers CFD’s practicality for design estimation of pier-scour depth. The writers reflect back 60 years when Laursen and Toch’s milestone publication provided major insights into pier scour, and when the hot-film anemometer first became available for investigating complex flow fields. Sixty years ago, pier flow fields were thought too complex to measure, or even visualize. The writers indicate where, today, further research into pier flow fields would benefit design estimation of scour depth.
•23 Mother wavelet functions are used to transform input rainfall data.•92 ANN models with transformed input rainfall data are developed.•db2 Wavelet function performed best with Continuous Wavelet ...Transformation (CWT).•db8 Wavelet function performed best with Discrete Wavelet Transformation (DWT).•The DWT gives best result at maximum possible level of decomposition of the input data.
The use of wavelet transformation in rainfall–runoff modeling has become popular because of its ability to simultaneously deal with both the spectral and the temporal information contained within time series data. The selection of an appropriate wavelet function plays a crucial role for successful implementation of the wavelet based rainfall–runoff artificial neural network models as it can lead to further enhancement in the model performance. The present study is therefore conducted to evaluate the effects of 23 mother wavelet functions on the performance of the hybrid wavelet based artificial neural network rainfall–runoff models. The hybrid Multilayer Perceptron Neural Network (MLPNN) and the Radial Basis Function Neural Network (RBFNN) models are developed in this study using both the continuous wavelet and the discrete wavelet transformation types. The performances of the 92 developed wavelet based neural network models with all the 23 mother wavelet functions are compared with the neural network models developed without wavelet transformations. It is found that among all the models tested, the discrete wavelet transform multilayer perceptron neural network (DWTMLPNN) and the discrete wavelet transform radial basis function (DWTRBFNN) models at decomposition level nine with the db8 wavelet function has the best performance. The result also shows that the pre-processing of input rainfall data by the wavelet transformation can significantly increases performance of the MLPNN and the RBFNN rainfall–runoff models.
AbstractThe variation of upstream slope of a submerged weir could significantly alter the local flow pattern and sediment transport. This paper presents an experimental investigation into local scour ...at submerged weirs with different upstream weir slope angles. In this study, 62 tests are carried out in a tilting recirculating flume, including 38 tests using coarse sand and 24 tests using fine sand, to investigate the effects of upstream weir slope on the local scour at a submerged weir. The tests of each sand use four weirs with different upstream weir slopes. For clear-water scour, no upstream scour depth is observed at the sloped weirs, and downstream scour depth is independent of the upstream weir slope. For live-bed scour, a flatter upstream weir slope reduces upstream scour depth for both sands. The results for fine sand show that when the slope is gentler than a critical angle α0, the effects of upstream weir slope on upstream scour depth are minimized, such that the upstream scour depth no longer decreases with decreasing weir slope angle. The existence of upstream weir slope slightly reduces the downstream scour depth when the sediment is transported as bedload. When the sediment transport mode is suspension load, the downstream scour depth is independent of the weir slope. New predictors for the average scour depth including the effects of upstream weir slope are developed. A relationship between the average and maximum scour depth is also proposed.
Live-Bed Scour at Submerged Weirs Guan, Dawei; Melville, Bruce W; Friedrich, Heide
Journal of hydraulic engineering (New York, N.Y.),
2/2015, Letnik:
141, Številka:
2
Journal Article
Recenzirano
AbstractWeirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged ...in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on approach flow intensity (clear-water scour conditions or live-bed scour conditions) and flow regimes (surface-flow regime or impinging-jet regime) over the weir. The fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live-bed scour conditions. This paper develops a measurement and data-processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live-bed scour conditions. The experiments are carried out with uniform sediment in a tilting sediment recirculating flume. Different flow rates and weir heights are used. For all the tests, the flow upstream of the weir is subcritical. Bed elevation changes are measured in the approach flow reach and in the scour zones both upstream and downstream of the weir using a Seatek multiple transducers array (MTA) (SeaTek Instrumentation, Florida). The highly contaminated raw bed-elevation data are filtered. Scour depths and bedform characteristics are extracted in data postprocessing. During live-bed conditions, a scour-and-fill process occurs immediately upstream from the weir in response to periodic approaching bedforms. The influence of the flow regimes on the scour mechanism downstream of the weir is discussed. Based on dimensionless analysis and experimental data, equations for prediction of the scour depth at the weir are proposed.
The 2004 Indian Ocean tsunami and the recent 2011 Japan tsunami have highlighted the need to investigate the interaction between tsunamis and coastal structures. Although some efforts have been made ...to determine tsunami loads on structures, there are discrepancies between the limited number of published design guidelines. This study comprises an experimental investigation of a tsunami bore interaction with an inland structure. Physical modelling of the tsunami bore in the laboratory allowed study of the impact of tsunami bores on a square prism model having different orientations to the flow direction. The use of common geometrical shapes simplifies experiments and increases the reliability of results. The experiments were conducted in a 14m long, 1.2m wide and 0.8m deep wave flume equipped with an automatic gate designed to generate a tsunami bore. Measurements were made of the forces and pressures exerted on the model structure and of the bore heights and velocities. The vertical pressure distribution was measured on what was initially the structure's front wall, with the front wall at 0°, 30°, 45°, 60°, 90° and 135° to its original alignment. A relation between bore velocity and bore height is presented. The measured maximum forces in the stream-wise and upward directions were numerically modelled successfully, and relevant drag coefficients were determined for the structure at different orientations.
•The vertical distribution of the bore induced pressure on the front wall of the structure was measured, at different orientations to the flow direction. In addition, the bore induced forces were measured at the base of the structure.•For bores with fronts sloping at less than 15°, the maximum bore force on the structure is the sum of hydrostatic and hydrodynamic forces.•Values of the total hydrodynamic forces after bore impact suggest drag coefficients ranging from 1.15 to 1.65 for the stream-wise force, for different orientations of the structure to the flow.•For bores with fronts sloping at less than 15°, the upward force is due to buoyancy, and the hydrodynamic uplift force is negligible.
AbstractScouring downstream of submerged weirs is a common problem resulting from the interaction of the three-dimensional turbulent flow field around the structures and the mobile channel bed. This ...paper presents the distributions of flow patterns, bed shear stresses, and turbulence structures in the approach flow and the scour hole downstream of a submerged weir. The experiments were conducted under the clear-water scour condition for an equilibrium scour hole. The experimental results show that the flow structures are considerably changed by the presence of the structure. A large recirculation zone and a flow reattachment region are formed downstream of the submerged weir. Strongly paired cellular secondary flows are observed in the scour hole. The turbulence structures ahead of the recirculation zone govern the dimensions of the scour hole.
AbstractFlooding water in extreme hydrometeorological events may be significantly contracted by bridge abutments and superstructures, leading to severe sediment scour and threatening structure ...stability. A series of large-scale laboratory experiments assessing flood-induced scour processes near bridge abutments are presented, with particular attention paid to the dependence of scour pattern on factors such as deck submergence (vertical contraction), abutment/embankment length (lateral contraction), abutment structure, and general bed mobility, which are analyzed based on post-scour transverse bed profiles and the general bed morphology. It is found that the scour hole’s depth, geometry, location, and corresponding time dependence are intricately affected by the aforementioned factors and demonstrate very complicated behaviors. The current design criterion of HEC-23 is sufficient to prevent disastrous embankment breaching but cannot avoid channel erosion or abutment toe undercutting in severe floods. The location of the greatest scour may migrate with time during floods and is closely related to the abutment structure and length. Furthermore, the existing scour predictors are examined and compared with recent data sets; more improvements are needed to reflect the complex physical mechanism of scour processes. Finally, a new method for estimating sediment loss is proposed.
•The performance of hybrid wavelet static and dynamic models is compared.•Discrete wavelet transform is used to decompose the data.•Hybrid wavelet dynamic models are found as an alternative of most ...commonly used static models.•The db8 wavelet function is found to give best results with both types of models.
In order to predict runoff accurately from a rainfall event, the multilayer perceptron type of neural network models are commonly used in hydrology. Furthermore, the wavelet coupled multilayer perceptron neural network (MLPNN) models has also been found superior relative to the simple neural network models which are not coupled with wavelet. However, the MLPNN models are considered as static and memory less networks and lack the ability to examine the temporal dimension of data. Recurrent neural network models, on the other hand, have the ability to learn from the preceding conditions of the system and hence considered as dynamic models. This study for the first time explores the potential of wavelet coupled time lagged recurrent neural network (TLRNN) models for runoff prediction using rainfall data. The Discrete Wavelet Transformation (DWT) is employed in this study to decompose the input rainfall data using six of the most commonly used wavelet functions. The performance of the simple and the wavelet coupled static MLPNN models is compared with their counterpart dynamic TLRNN models. The study found that the dynamic wavelet coupled TLRNN models can be considered as alternative to the static wavelet MLPNN models. The study also investigated the effect of memory depth on the performance of static and dynamic neural network models. The memory depth refers to how much past information (lagged data) is required as it is not known a priori. The db8 wavelet function is found to yield the best results with the static MLPNN models and with the TLRNN models having small memory depths. The performance of the wavelet coupled TLRNN models with large memory depths is found insensitive to the selection of the wavelet function as all wavelet functions have similar performance.
River training structures are conventionally used to improve river stability and ecological conditions and to protect the upstream instream infrastructures from scour and erosion. A submerged weir is ...a typical river training structure usually built downstream of bridges for grade control. Flow over the weir can cause local scour, destabilizing both the structure and the trained river reach. It is therefore important for safe design to estimate the scour accurately and understand the effects of an upstream bridge pier on local scour at submerged weirs. This study describes experiments on the scour process at a submerged weir with an upstream bridge pier, varying the distance L between the two. In this study, 56 experiments were conducted using uniform coarse sand with a median grain size d50 = 0.85 mm in a sediment‐recirculating flume. For clear‐water scour, the experiments showed that the bedform induced by the pier scour process creates a scour‐and‐fill process upstream of the submerged weir; and the upstream pier can also affect the scour downstream of the submerged weir, through two mechanisms: (i) upstream sediment replenishment and (ii) flow section constriction. The mechanisms (i) and (ii) can reduce and increase the downstream clear‐water scour depth at the submerged weir, respectively. For live‐bed scour, a pier upstream can reduce and increase the scour depth upstream and downstream of the submerged weir, respectively. Based on the experimental data, the dependence of the scour depth at the submerged weir on L is evaluated.
Key Points
Experimental data of the scour at submerged weirs with an upstream circular bridge pier are provided
The effects of bridge piers on the scour process at downstream river training structures (submerged weir as an example) are illuminated
The quantitative impacts of upstream bridge piers on the scour depth at submerged weirs for different separation distances are evaluated
Local Scour at Downstream Sloped Submerged Weirs Wang, Lu; Melville, Bruce W; Guan, Dawei ...
Journal of hydraulic engineering (New York, N.Y.),
8/2018, Letnik:
144, Številka:
8
Journal Article
Recenzirano
AbstractThis paper presents an experimental study of local scour at submerged weirs with different downstream slopes. Thirty-three tests of coarse sand (d50=0.85 mm) and 24 tests of fine sand ...(d50=0.26 mm) were carried out in a sand-recirculating flume. The tests for each sand type used four weirs with different downstream weir slopes (π/6, π/4, π/3, and π/2 to horizontal for the fine sand; π/12, π/6, π/4, and π/2 to horizontal for the coarse sand). The results show that the upstream scour depth is independent of the downstream weir slope β. For both sand types, the downstream weir slope has no effect on the downstream scour depth when it is greater than the upstream slope of the downstream scour hole θ. For β<θ, the downstream weir surface is partly exposed to flow, resulting in a downstream scour depth at the weir surface dw, which can be increased by the increase in either downstream scour depth or θ. In this situation, the upstream water level rises because of the reduced flow cross-section area, and the overflow jet is deflected to be approximately aligned with the downstream weir surface. For the coarse sand tests, when β=π/6, the effect of the rising upstream water level is predominant because β is close to θa (time-averaged value of θ). This results in larger downstream scour depths. When β is considerably smaller than θa, the effect of jet deflection starts to be predominant in the coarse sand tests, generating lower downstream scour depths when U0/Uc<2.6. Those two effects balance each other when 2.6<U0/Uc<3.1 for β=π/12, resulting in the same downstream scour depths as those for a rectangular weir. For the fine sand tests, the jet deflection effect is predominant because the downstream weir slope has limited effect on the upstream water level. A conservative design method accounting for both effects is proposed for the local scour at downstream sloped submerged weirs.
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