2D numerical flow modeling in a macro-rough channel

A 2D numerical flow model, developed at the Research unit of Hydrology, Applied Hydrodynamics and Hydraulic Constructions at ULg, has been applied to flows in a macro-rough channel. The model solves the shallow water equations (SWE) with a two length scale, depth-integrated k-type approach for turbulence modeling. Data for the comparison have been provided by experiments conducted at the Laboratory of Hydraulic Constructions at EPFL. In the experiments with different non-prismatic channel configurations, namely large-scale cavities at the side walls, three different 2D flow characteristics could be observed in cavities. With the used numerical model features, especially regarding turbulence and friction modeling, a single set of bottom and side wall roughness could be found for a large range of discharges investigated in a prismatic channel. For the macro rough configurations, the numerical model gives an excellent agreement between experimental and numerical results regarding backwater curves and flow patterns if the side wall cavities have low aspect ratios. For configurations with high aspect ratios, the head loss generated by the preservation of important recirculation gyres in the cavities is slightly underestimated. The results of the computations reveal clearly that the separation of turbulence sources in the mathematical model is of great importance. Indeed, the turbulence related to 2D transverse shear effects and the 3D turbulence, generated by bed friction, can have very different amplitude. When separating these two effects in the numerical models, most of the flow features observed experimentally can be reproduced accurately.

High velocity aerated flow on stepped chutes with macro-roughness elements

Stéphanie Joëlle André

Uncontrolled overtopping during flood events can endanger embankment dams. Erosion of the downstream slope and scouring of its base caused by the high velocity and energy of the overflow can indeed lead to breach formation until complete failure. In this context and faced with the important number of overtopped embankment dams to be rehabilitated, since the early eighties, researchers have investigated surface protection solutions for downstream slope. Overlays against erosion such as seeded goetextile or cable-tied cellular concrete blocks, are not sufficient. In fact, they can resist only short events with low discharge and velocity. Solution to overcome more severe overflow lies in overlays which dissipate flow energy along the downstream embankment slope. Conventional steps resulting from Roller Compacted Concrete (RCC) techniques fulfill efficiently this challenge. However, flows over steep stepped chutes are quite complex, characterizing by great aeration, high turbulence and confused wavy free surface. Then, most of hydraulic studies of such flows are performed on physical model. Yet, understanding and definition of flow behaviour and accurate approach to estimate energy dissipation are still lacking. General guidelines of hydraulics of aerated flows over stepped macro-roughness chutes and for optimal design of protection overlay remain confusing. To contribute to reduce these uncertainties, experimental study of flow over stepped chutes equipped with macro-roughness elements is performed in a laboratory gated flume for mild (~ 1:7H : 1V ) and weak (~ 3H : 1V ) chutes. Thus, they are representative of the range of embankment dams and spillways slopes. Three types of stepped macro-roughness overlays are assessed, namely rectangular conventional steps, steps equipped with endsills fixed on their nose over all the flume width and steps equipped with rectangular spaced blocks. Endsills overlays were characterized with different longitudinal distributions whereas blocks overlays consisted in different transverse patterns. Tests were conducted for the three nappe, transition and skimming flow regimes. Results can be extrapolated to 1/5 to 1/15 scaled prototypes using the Froude similarity with negligible scale effects. Flow depth, local air concentration and longitudinal velocities are measured with a double fiber-optical probe. Pressures at macro-roughness faces are taken with piezo-resistive sensors. Sequent depths of the hydraulic jump forced in the stilling basin at the flume base are measured with ultrasound sensors. Thus, this experimental phase of the thesis has allowed: to define flow parameters (regimes, depths, velocity and air concentration distributions, hydrodynamic forces) for tested overlays, to highlight that air-water flow depth is divided into: a rough boundary layer influenced by shear stress and by drag form (macro-turbulence) caused by macro-roughness, a homogeneous aerated layer which represents the main portion of flow involved in energy dissipation mechanism, a free surface layer which must be considered in the side walls design, to stress that energy dissipation is mainly a question of drag losses, to validate indirect method of hydraulic jump for energy dissipation estimation, to estimate relative energy loss for several stepped macro-roughness overlays. Tests finally show that an optimal alternative to dissipate the overflow energy during an overtopping event consists in spaced blocks, with transverse space larger than the width of block and fixed alternately on conventional steps. However, experimental results remain related and limited to their tested domains. Then, in order to provide more general governing equations of aerated flows over macro-roughness stepped chutes, a numerical modeling of two phase flows over conventional stepped flume was performed in collaboration with the Laboratory of Applied Hydrodynamics and Hydraulic Constructions at University of Liège. A quasi-2D numerical model based on the finite volume method was developed. It consists in applying the classical depth-averaged simplified Navier-Stokes equations (viscosity and Coriolis terms neglected) to a 1D incompressible air-water mixture flow over mild and steep slopes with a stepped topography. Self-aeration process is modeled by a transport equation of depth-averaged air concentration whereas turbulent structures are indirectly implemented through the Boussinesq coefficient. This first 1D-approach of semi-theoretical description of aerated flow over steps is tested for a 30o gated stepped flume and a 52° crested spillway laboratory model. This numerical model leads to realistic results regarding mixture depth, mean flow velocity, air concentration and wave amplitudes of the flow free surface. Finally, on the basis of existing protections of embankment dams and previous studies, the present experimental and numerical results contribute to extend the knowledge of high velocity aerated flows over macro-roughness and to provide elements of guidelines to optimize stepped macro-roughness overlays for embankment dams safety.

EPFL2004

Efficiency of Brushwood fences in shore protection against wind-wave induced erosion

Selim Sayah

Shore protection in confined water bodies is of major importance in Switzerland where many lakes suffer severe shore erosion. It occurs mainly in shallow zones due to several reasons such as: the fluctuations of the water level , the wind-wave impact during major wind events, and the increasing effect of incident waves due to solid reenforcement by non-adapted protection measures. In order mitigate this increased erosion and enhance the shore stability by sustainable coastal structures, pioneer and soft measures have been built during the past decades. Although such solutions proved their efficiency, they raised a new challenge for engineers: how to build soft and porous media that take into consideration the hydraulic and hydrodynamic conditions of the project site, and enhance its efficiency in sand trapping and shoreline accretion. By means of experimental and numerical modelling, the performance of a typical soft measure, called brushwood fence, was investigated in hydrodynamic conditions similar to those found in reality. A downscaled model of 1 : 10 was built on a fixed bed, and then tested in a wave tank of 10 m length and 6 m width under incoming perpendicular and regular waves. The response of the structure regarding the transmission coefficient has been analyzed for five dimensionless variables: 1) the relative freeboard of the structure Rc/Hi, 2) its relative height h/Hi, 3) the relative wave number kd, 4) the wave steepness Hi /gT2, and 5) the porosity of the structure p. The analysis of their effect on the performance of the protection structure allowed the establishment of an empirical relationship for the transmission coefficient. It takes into account the geometrical characteristics of the structure as well as the local hydrodynamic conditions. The response of the structure has to be treated separately in relation to its immersion condition. When the structure is submerged, every dimensionless variable has a differentiated effect on the transmission coefficient in comparison to the condition when the structure is emerging. The relative freeboard, wave steepness and the porosity are the three key variables in the response of the structure regarding wave damping. The efficiency of detached brushwood fences built on a movable bed was tested under the action of waves with fixed characteristics. The bed material was made of a granular, fine sand with a diameter d50 = 0.18 mm. The evolution of the shoreline and the transmission coefficient of the structure were determined using a first series of tests. It was demonstrated that time has negative impact on wave damping, at least during the period where the bed in the vicinity of the structure evolves to its equilibrium state. A second series of tests established the effect of the distance S of the detached structure (length B) from the shoreline on its evolution and on the deposited and eroded sand volumes at the leeside of the structure. It was proven that the wave motion through the porous media hinders the formation of significant salients and prevents the formation of tombolos when the structure was very close to the shoreline. Thus, for high ratio B/S = 1.48 no tombolo was formed in line with non porous breakwaters. The deposited and eroded volumes were also evaluated in a third series of tests with the presence of a single gap in the brushwood fences. The effect of the width of the gap on these volumes was analyzed. The results proved that the main deposition area is located between the gap and the shoreline where sediments are transported by the diffracted waves at the edges of the brushwood fences. The highest deposition rate at this location corresponds to the ratio B/S = 0.25. Using the solver of the elliptic mild slope equation developed under Mike 21, the wave field in an enclosed area surrounded by a porous structure was numerically investigated. The calibration of some major variables in the numerical model such as wave breaking and bottom friction parameters, based on the experimental results, proved the adequacy of the selected numerical scheme. The effect of a single and double gap in a linear infinite porous media was afterwards evaluated and diffraction diagrams were built for the enclosed wave field. A rule was proposed to use a relative gap width G/Wmax less then 0.12. The wave field for two gaps is different and significantly influenced by the spacing between the gaps. For low spacing values (Es/G = 2), waves in the middle of the protected area are high along the structure and very low close to the shore. For high spacing values (Es/G = 5), the wave field is also significantly deformed. However, values of Es/G comprised between 3 and 4 seem to be most appropriate since the corresponding wave field is less deformed. The two gaps configuration does not significantly increase the residual total energy behind the structure. It is relatively constant with spacing variations between two gaps and increased slowly with the increase of a single gap width. The experimental observations and the numerical results were successfully applied in Mörigen bordering Lake Biel. The wave fields were calculated numerically behind a series of segmented brushwood fences. Several wind-wave regimes and varying water levels were analyzed to optimize the performance of the porous protection structures.

EPFL2006

Influence of macro-roughness of walls on steady and unsteady flow in a channel

Tobias Meile

High-head storage hydropower plants mainly operate their turbines during periods of high energy demand. The sudden starting and stopping of turbines (hydropeaking) lead to highly unsteady flow in channels and rivers. Besides hydropeaking, other anthropogenic actions and natural events such as sluice gate operations, flushing of reservoirs, debris jam and break up, ice jam and break up, sudden stopping and starting of turbines of runoff river hydropower plants, flashfloods or dambreaks can also cause highly unsteady flow. From an ecological point of view, hydropeaking consists in a non-natural disturbance of the flow regime. Possible mitigation measures aiming to reduce the effect of hydropeaking in a river downstream of the powerhouse can be divided into the installation of detention basins and the improvement of the river morphology. Morphological measures such as macro-roughness at banks might increase the flow resistance as well as the passive retention, which both increase the natural retention capacity of rivers and thus modify the form of the surge wave. In prismatic and nearly prismatic channels, highly unsteady flow conditions can be calculated using the elementary surge wave theory or numerical methods based on the Saint-Venant equations, respectively. For channels with large-scale roughness at the side walls, which may occur by the arrangement of particular morphological measures, no systematic experimental investigations have been done so far on the propagation of surge waves including downstream water-depth and flow velocity. The aim of this research project was to study how macro-roughness elements at the channel banks influence the unsteady flow conditions due to surge waves. 41 configurations of macro-rough banks and various discharges have been tested in a 40 m long flume with a bed slope of 1.14‰. A special experimental setup has been designed which is able to generate surge waves characterized by different discharge ratios. The first step of the experimental investigations focused on the determination of the flow resistance under steady flow conditions caused by large scale roughness elements at the channel banks, namely rectangular cavities (depressions). In a second step, positive and negative surge waves induced at the channel entrance have been tested in the same geometries. Five different discharge scenarios have been considered for each geometry. For comparison, experiments have also been performed in a prismatic reference channel without macro-roughness elements at the banks. The analysis of the experiments for steady flow conditions results in the following conclusions: The total head-loss is governed by different phenomena such as vertical mixing layers, wake-zones, recirculation gyres, coherent structures and skin friction. The flow resistance is significantly increased in the macro-rough configurations due to the disturbance of the bank geometry. Three different approaches have been considered in order to relate the additional, macro-rough flow resistance fMR to the forms of the banks by empirical formulas. The first approach is based on a powerlaw optimization, the second one on a model based on form drag and the third one uses the Evolutionary Polynomial Regression method. By separating the observed flow conditions in a square grooved, a reattachment and a normal recirculating flow type, the developed macro-rough flow resistance formulas are in good agreement with the laboratory experiments. Formulas of the second approach are suggested for practical applications in river engineering. Water body oscillations in cavities have been observed in axi-symmetric macrorough configurations. They lead to water-surface oscillations and transverse velocity components. Peaking excitations with heavy oscillations occur for Strouhal numbers values at 0.42 and at 0.84. The analysis of the experiments with unsteady flow, namely surge waves, shows: The elementary surge wave theory for the prismatic reference configuration including secondary waves and wave breaking could be verified in the laboratory flume. The absolute surge wave celerity Vw and the celerity c are lower for the macro-rough configurations. The decrease of the absolute surge wave celerity Vw is mainly due to the increased flow resistance and lies between 5% and 25% for both, positive and negative waves coming from upstream. Due to the dispersive character, the positive and negative surges from upstream are characterized by a sudden change (front), followed by a progressive change (body) of the water level. The sudden change can be clearly distinguished from the progressive change in the prismatic channel. In the channel with macro rough banks the separation is often hardly visible. The front height decreases between 5% to 25% along the laboratory flume for the prismatic configuration. In the macro-rough channel, the decrease can reach 70%. Behind the wave front of a positive surge, the water level continuous rising to a level which is higher than in the prismatic channel due to the channel bank macro-roughness. The decrease of the height of the surge wave front along the channel could be approached fairly good with an empirical relationship applicable for prismatic and macro-rough configurations. Numerical simulations based on the boundary conditions of the experiments have been done for steady flow tests (2-D simulations). They reveal that the integration of the turbulent stresses is required for the reproduction of the steady flow tests. Test cases with unsteady flow (1-D simulations) show that surge waves in rivers can be computed by integrating the macro-rough flow resistance in the source terms as well as by a modification of the continuity equation taking into account the passive retention.

EPFL2007