Ronny Verhoeven

Simulation of sediment transport during flood events: laboratory work and field experiments

Abstract This paper aims at initiating a fundamental understanding of the suspended load transport of river sediment in unsteady flow. Laboratory erosion tests as well as artificial flood experiments are used to evaluate the influence of the transient regime on the transport efficiency of the flow. The erosion experiments reveal that the transport capacity is augmented when the unsteadiness of the flow increases. However, the influence of the transient regime is counteracted by the cohesive properties of the river bed. Field experiments with artificial floods released from a reservoir into a small canal confirm these findings and show a relationship between the friction velocity and the suspended load transport. An appropriate parameter β is proposed to evaluate the impact of the transient regime on the transport of suspended sediment.

Assessment of distributed arterial network models.

The aim of this study is to evaluate the relative importance of elastic non-linearities, viscoelasticity and resistance vessel modelling on arterial pressure and flow wave contours computed with distributed arterial network models. The computational results of a non-linear (time-domain) and a linear (frequency-domain) mode were compared using the same geometrical configuration and identical upstream and downstream boundary conditions and mechanical properties. Pressures were computed at the ascending aorta, brachial and femoral artery. In spite of the identical problem definition, computational differences were found in input impedance modulus (max. 15–20%), systolic pressure (max. 5%) and pulse pressure (max. 10%). For the brachial artery, the ratio of pulse pressure to aortic pulse pressure was practically identical for both models (3%), whereas for the femoral artery higher values are found for the linear model (+10%). The aortic/brachial pressure transfer function indicates that pressure harmonic amplification is somewhat higher in the linear model for frequencies lower than 6 Hz while the opposite is true for higher frequencies. These computational disparities were attributed to conceptual model differences, such as the treatment of geometric tapering, rather than to elastic or convective non-linearities. Compared to the effect of viscoelasticity, the discrepancy between the linear and non-linear model is of the same importance. At peripheral locations, the correct representation of terminal impedance outweights the computational differences between the linear and non-linear models.

The erosion of cohesive mixed deposits: implications for sewer flow quality modelling.

Abstract The movement of sediments in sewers has been widely recognised as being strongly linked with poor sewer flow quality. Currently, empirically calibrated transport capacity relationships are used to estimate the movement of in-sewer sediments. The majority of these relationships were developed and calibrated with laboratory data sets that used uniformly sized granular sediment. In many combined sewers, there are both organic and inorganic sediments. The fine-grained organic sediments are thought to be able to develop considerable cohesive strength under certain conditions. This paper describes the results from laboratory experiments in which the erosion of non-homogeneous in-pipe sediment deposits was investigated. The types of sediment mixtures used ensured that a range of cohesive deposit strengths was developed. The measured bed load and suspended load were compared with transport capacity relationships currently used for sewer design. Significant divergence between the transport capacity predictions and experimental observations was discovered. It was seen that it is unwise to model in-sewer deposits as homogeneous and granular and that it is important to take into account the impact of deposit cohesion.

Fair and sustainable irrigation water management in the Babai basin, Nepal.

This paper attempts to find a strategy to provide year-round irrigation for cultivating three crops per year in the southern plains of the country taking a case study of the Babai basin. Despite having enough flows during the summer for growing rice in total 27,000 ha area, the dry season flows of the Babai river can irrigate only 6,300 ha in winter and 4,000 ha in spring limiting the cropping intensity to 138.50%. It is proposed to irrigate the 7,500 ha southern dry area at the right bank bringing water from a large snow-fed river: the Karnali. Water balance study of the three irrigation regions to be irrigated from the Babai source preserving their existing water rights showed that the year-round irrigation at the west with the proposed arrangement will fall short of only 13.9 million m(3) water volume. At the east side, the head reach area and the tail portion will fall short of 19.4 and 66.4 million m(3) of water to insure a cropping intensity of 250%. The deficits can be fulfilled by means of capturing the excess river water of rainy season in local reservoirs and by making conjunctive use of groundwater. The proposed solution is financially, environmentally and socially viable being a cost effective, user friendly and should be the linchpin towards attaining a sustainable year-round irrigation in the region.

Similarities and differences of pumping conventional and self-compacting concrete

In practice, self-compacting concrete (SCC) is considered as a simple extension of conventional vibrated concrete (CVC) when pumping is concerned. The same equipment, materials, pumping procedures and guidelines used for CVC are applied when pumping SCC. On the other hand, it has been clearly shown that the rheological properties and the mix design of SCC are different than CVC. Can the same pumping principles employed for CVC be applied for SCC? This paper compares the some published results of pumping of CVC with those for SCC. A first striking difference between pumping of CVC and SCC is the flow behaviour in the pipes. The flow of CVC is a plug, surrounded by a lubricating layer, while during the flow of SCC, part of the concrete volume itself is sheared inside the pipe. As a result, the importance of viscosity increases in case of SCC. Due to the low yield stress of SCC, the behaviour in bends is different, but quite complex to study. Due to the lower content of aggregate and better stability of SCC, as it is less prone to internal water migration, blocking is estimated to occur at lower frequency in case of SCC.

The rheology of self-compacting concrete made with Belgian materials

The rheology of fresh concrete and self compacting concrete (SCC) has been studied for several decades to better understand the flowing behaviour of these materials. This is especially important in cases the concrete is pumped into the formwork through pipes. Too stiff concretes, or concretes that segregate are not suitable for pumping operations. These concretes demand too high pressures or will cause blocking during pumping operations. Pumping SCC speeds up the casting process, because the concrete is more fluid and it does not need any compaction. In literature, the rheological behaviour of fresh concrete and fresh SCC is mainly described by the Bingham model. For traditional concrete, this behaviour has been confirmed. However, tests with SCC, made with Belgian materials, indicate in most cases shear thickening behaviour. The Bingham model can not be applied because it causes negative yield stresses. The Herschel-Bulkley equation ( n K γ τ τ & ⋅ + = 0 ) gives positive yield stresses, but it has some other problems. The dimension of the consistency factor “K” is dependent on the exponent “n”. As a result, this dimension is variable and “K” is inappropriate for physical interpretation. The Herschel-Bulkley curve is also forced mathematically to be horizontal for very low shear rates – in case of shear thickening – resulting in an overestimation of the yield stress. The modified Bingham model ( 2 0 γ γ μ τ τ & & ⋅ + ⋅ + = c ), i.e. the Bingham model extended with a quadratic term, correlates the data points as well as Herschel-Bulkley; has constant dimensions and gives a better fit for the yield stress. The degree of non-linearity, expressed by the parameter “n” in Herschel-Bulkley, can be examined with c/μ. This model is also a Taylor development of the second order of the Herschel-Bulkley equation. When examining the results from the rheometer tests, more shear thickening is observed when the slump flow increases. And when the w/p ratio decreases, c/μ increases dramatically.

Influence of Thixotropy on Pressures Required during Pumping of Concrete

Concrete is a very popular construction material, which can be regarded as a suspension of solids of several orders of magnitude, suspended in water. From a rheological point of view, the fresh fluid concrete mostly obeys the Bingham law, showing a yield stress and a plastic viscosity. Due to the presence of cement, a permanent process of coagulation and dispersion occurs, causing the concrete to be thixotropic.This paper describes the influence of thixotropy on the measured pressure losses during high speed pumping. Apparently, the concrete becomes more fluid with increasing discharge, increasing the risk for the loss of internal stability, which can lead to a very low quality product.

Appropriate rehabilitation strategy for a traditional irrigation supply system: a case from the Babai area in Nepal

This paper studies primary canals of three traditional irrigation systems in the southern plains of Nepal. It offers a scientific interpretation of the indigenous technology applied to the systems, which facilitates to use the same channel network for irrigation, drainage and flood management. The flood management technology of the farmers by diverting as much discharge as possible to the field channels results in the reduction of discharge towards the downstream part of the main channel. It is depicted in the simulation study that uses the river analysis program HEC-RAS 4.0. A cascade of weirs is found to be the most cost effective and user-friendly option to upgrade these systems preserving the existing irrigation, drainage as well as flood management functions. This study suggests that the conventional irrigation design principles should be applied very cautiously with full knowledge of the existing socio-institutional setting, hydro-ecological regime and indigenous technology for upgrading any traditional irrigation system successfully. The indigenous flood management technology strengthens the emerging concept that the floods in the Ganges plain are to be managed, not controlled.

In vivo validation of a fluid dynamics model of mitral valve M-mode echocardiogram.

A fluid dynamics model of mitral valve motion during diastolic filling of the left heart is described. Given a pulsed Doppler velocity pattern in the mitral annulus, the radius of circular mitral orifice, the length of leaflets and the end-systolic left ventricular volume, the numerical model predicts the time course of the mitral leaflets during diastole: the mitral valve M-mode echocardiogram. Results obtained by computer simulation have been validated with in vivo data. It is shown that mitral valve flow is essentially a fluid dynamics process of floating mitral valve leaflets with blood flow due to the atrioventricular pressure gradient. In addition, a partial opening of the mitral valve as the initial boundary condition is required to simulate the overshooting of the leaflets during early peak filling. Some back flow is a condition for perfect closing of the native mitral valve. The higher the unsteady character of mitral flow, the less efficient is the opening and closing processes of the mitral valve.

Rheology and Pumping of Self-Compacting Concrete

Self-compacting concrete is a very flowable cementitious material, which does not need external vibration during casting. On the other hand, somewhat surprisingly, pumping of self-compacting concrete requires higher pumping pressures than traditional concrete. This paradox can be fundamentally explained by studying the rheological properties of self-compacting concrete and linking them to pumping operations. This paper describes full-scale pumping tests on self-compacting concrete. The first part deals with the influence of the rheological properties of the concrete on the pumping process, showing that viscosity and shear thickening have a major importance. The second part discusses the influence of pumping on the rheological properties of the concrete, clearly showing a decrease in viscosity due to pumping. Structural breakdown and air content change the rheological properties of the SCC. If structural breakdown dominates the effects of the air content, the yield stress and plastic viscosity will decrease, and the SCC will show a larger tendency to segregate. If the effects of the air content dominate, the yield stress of the SCC will increase, possibly leading to improper filling of the formwork.