Yee-Meng Chiew

Incipient sediment motion on non-horizontal slopes

The study investigates the threshold condition for the initiation of cohesionless sediment transport on a nonhorizontal streamwise slope. Theoretical study of the stability of a sediment particle lying on a non-horizontal bed slope shows that the critical shear stress is a function of the streamwise bed slope. Experiments conducted with a closed-conduit flow show that the equation derived from force analysis adequately evaluates the critical shear stress of sediments lying on slopes ranging from steep positive to negative (adverse). Tests conducted with a closed-conduit flow avoid the problems associated with those conducted with an open channel flow. In the latter flow condition, uniform flow is often difficult to achieve in a steep channel, and impossible to achieve when the streamwise bed slope is adverse.

Local scour around bridge piers

Local scour around cylindrical bridge piers in uniform, cohesionless sediment was investigated experimentally. Three empirical functions which relate equilibrium scour depth with approach velocity, sediment size and How depth were obtained. The variations of equilibrium scour depth with approach velocity show that the depth of scour decreases at velocities just above the threshold until it reaches a minimum at about twice the threshold velocity. Thereafter, it increases again until a maximum is reached at the transition flat bed condition. At still higher velocity, the equilibrium scour depth decreases due to the formation of antidunes. Both the effects of sediment size and flow depth were found to be independent of the stage of bed particle motion and the approach velocity. Adjustment factors, for the effects of sediment size and flow depth were defined and evaluated.

Incipient sediment motion with upward seepage

This study investigates the effect of upward bed seepage on the critical condition of incipient sediment motion in open channel flow both analytically and experimentally. The critical condition was derived by analyzing the forces acting on a sediment particle lying on a permeable horizontal bed subject to seepage. The ratio of the critical shear velocity with seepage to that without seepage depends on the ratio of theJiydraulic gradient of seepage to its critical value under the quick condition. Experimental results concerning incipient motion of cohesionless uniform sediments in open channel flow show that for a particular size of sediment, the critical shear velocity decreases with increasing seepage velocity. All measurements generally support the theoretically derived expression of the critical shear velocity in the presence of an upward seepage.

Turbulent open-channel flow with upward seepage

Measurements of turbulent open-channel flow subjected to an upward bed seepage were performed in a laboratory flume using a two-dimensional Acoustic Doppler Velocimeter and a minipropeller. The experimental results show that the boundary seepage affects the time-mean streamwise velocity, the rnis values of the velocity fluctuations, the Reynolds shear stress and the bed shear stress in open-channel flow. Along the seepage zone, the mean streamwise velocity increases much more in the surface layer than that in the near-bed region, whereas the turbulent intensities and Reynolds shear stress increase significantly in the near-bed region. The bed shear stress that was computed using the momentum integral equation shows a steady reduction with increasing upward seepage velocity.

Review of seepage effects on turbulent open-channel flow and sediment entrainment

This paper presents a review on the state-of-the-art knowledge of how seepage affects the turbulence characteristics in open-channel flow and its implication on sediment entrainment. Published literature shows that some effects have been intensively examined and the results are well known, such as seepage effects on mean flow velocity distributions. Understanding of the other effects remains rudimentary, such as variations of turbulence intensity and bed shear stresses. In fact, many of these issues remain ambiguous with contradicting inferences and conclusions. For example, the published literature is still not unanimous as to how turbulence intensities, bed shear stresses and bed particle stability change in the presence of seepage. By reviewing literature in this area published over the past 35 years, this paper highlights the main conflicting results and attempts to explain these deviations with certain recommendations.

Parametric study of riprap failure around bridge piers

This paper presents a parametric study on riprap protection around a cylindrical bridge pier with uniform bed sediments. Riprap layers with different characteristic parameters: thickness, cover width, and placement level of the riprap layer; median grain size and density of the riprap stone; and undisturbed approach flow depth were tested under a sequence formation of ripples, dunes and transition flat bed. Observations show that a riprap layer will eventually degrade to a maximum level which is the same as the maximum pier scour depth when the riprap layer is not present. This study proposed a criterion to determine the maximum embedment level and the embedment velocity, i.e., the flow velocity at which the riprap layer has embedded to its maximum level. The experimental results show that variations of the characteristic parameters have no influence on the embedment failure at the upper end of the dune regime. The study also proposes a maximum embedment velocity, which defines a critical flow velocity at which all riprap layers, irrespective of the characteristic parameters will fail.

Houfeng Bridge Failure in Taiwan

Failures of bridges that span a waterway often result from scour and channel instability near the bridge foundations. The Houfeng Bridge, which crosses the Da-Chia River in central Taiwan, collapsed in the 2008 typhoon flood event. On the basis of the historical records and survey of related data just after the collapse of the bridge, a methodology for the assessment of scour depth, including long-term general scour caused by earthquake and impinging jet scour generated by a concrete encased pipeline, is illustrated in the present study. The proposed method provides reasonable estimates for various scour components, which implies that before constructing a new or rebuilding an old bridge, one should use proper methodology and formulas to evaluate the scour potential and improve the bridge design, especially for bridges that are founded around the Houfeng Bridge. This case study also highlights the important effect of long-term general scour on bridge stability. In addition, a lesson is learned from this case study regarding the importance of bridge operation.

Protection of bridge piers against scouring with tetrahedral frames

Abstract Armoring countermeasures, such as riprap stones, are the primary method used to protect bridge piers against scouring; however, these methods have not had definitive success. Recently, flow-altering countermeasures, such as sacrificial piles, have been tested as an alternative to armoring countermeasures. This study investigated the mechanics of an innovative flow-altering countermeasure device, frames in the shape of tetrahedrons that act as a pier-scour countermeasure. Results of measured characteristics for turbulence flow showed that the flow around the tetrahedral frames can be divided into three regions: (1) a deceleration region near the sediment bed; (2) an acceleration region in the middle of water depth region; and (3) a restoration region near the water surface. The velocity magnitudes, turbulent intensities and vorticities decreased in the deceleration region, increased in the acceleration region and reverted to that of the unprotected condition in the restoration region. This pier-scour countermeasure is innovative because of its ability to dissipate energy associated with the downflow and the horseshoes vortex generated around the bridge pier. The scour tests revealed that the frames protected the foundation of bridge piers against scour. The experimental results showed that the percentage reduction of scour depth decreased as the velocity ratio, U/U c , decreased, reaching a value of 50% for the range of parameters tested in this study. Moreover, its efficiency was dependent on the placement density of the frames. The data showed that the frames were more effective when η was larger. However, their influence becomes less significant when η ≥ 0.16.

Suction effects on turbulence flows over a dune bed

This experimental study investigates how suction affects turbulence flow over an immobile, two-dimensional triangular dune. A Laser Doppler Anemometer is used to measure the streamwise and vertical components of velocity over one dune length for three different suction rates at 0, 0.29% and 0.45%, respectively. The response of the mean flow and turbulence velocities to bed suction is explored in both a local sense, through comparison of the time-average velocity, rms values of velocity fluctuations and Reynolds shear stress profiles at five selected sections; and a spatial sense, by producing a set of contour plots over the dune length for all mean and turbulence parameters. The measurements reveal that suction causes the flow to adhere to the bed resulting in reduction of the separation length. The streamwise velocity profile becomes more uniform when compared to that without suction while suction always produces smaller or more negative vertical velocities in the near-bed region. The measurements also show that both the near-bed turbulence intensity and Reynolds shear stresses decrease with suction, which confirms that suction causes the flow within the boundary layer to be less turbulent. An interesting observation is that the flow does not respond to suction in the circulation zone just downstream of the dune crest.

Suction Effects on Bridge Pier Scour under Clear-Water Conditions

AbstractThe experimental study investigates how the application of point suction at different bed locations affects local scour around a cylindrical bridge pier under clear-water conditions. The results show that suction has a profound effect on pier-scour development, and the extent of the influence is related to the location of the suction source. When the suction source is located at or upstream of the pier, the equilibrium scour depth is reduced by up to 50%, with Qs/Qo=2%, where Qs and Qo = suction flow rate and undisturbed approach flow rate, respectively. The data also show that for the suction source that is located downstream of the pier, the equilibrium scour depth is reduced by up to 30% with the same suction flow rate. The extent of pier-scour depth reduction at different locations from the suction source is closely associated with changes to the local effective shear and critical shear velocity. Results of this study show that bed suction may be used as an effective pier-scour countermeasure.