Alberto Lamberti

Roll waves simulation using shallow water equations and Weighted Average Flux method

The weighted average flux method is used to represent trough 1-D shallow water equations the development of natural roll waves in rectangular channels. The method is reformulated in such a way that all source terms are incorporated into the local Riemann problem by means of a useful transformation of the equations, maintaining second-order accuracy of the solution. A code based on this method has been developed and results of numerical applications to a Venturi channel and to the attenuation of waves are presented to check for well known situations how accurately the source terms are evaluated. The code is applied to reproduce Brocks experiments on roll waves performed in a rectangular laboratory channel. The numerical solution accurately represents roll waves evolution due to uniform flow instability as well as wave intensity increase due to instability and wave coalescence. A sensitivity analysis of the numerical solution to changes of computational parameters is performed, in order to explain the discrepancies among computed and experimental values.

Flow Field and Morphology Mapping Using ADCP and Multibeam Techniques: Survey in the Po River

An 8-km-long and 250-m-wide reach of the Po River (Italy) was surveyed with two vessels, aiming to investigate the channel morphology and flow field characterizing this major Italian river and to provide data for the calibration of a numerical model. The first vessel was equipped with a multibeam ecosounder (MBES), whereas an acoustic doppler current profiler (ADCP) was located on the second one. Fixed-vessel ADCP measurements were used to preliminarily investigate the turbulence variability affecting the reliability of moving-vessel measurements. To evaluate the friction velocity by using moving-vessel measurements, the vertical variance of velocity profiles as an alternative to logarithmic fitting of ADCP profiles was estimated. An average of 30-m cells of depth-averaged velocity, friction velocity, and surface-bottom velocity angle are presented in maps as the most relevant flow field features. The flow field and channel morphology at bed-form or channel scales showed noticeable correlations (0.9). In ...

Experimental analysis of the impact of dry avalanches on structures and implication for debris flows

Objectives of this work are to experimentally analyse the dynamic impact of dry avalanches on structures and to examine the possibility of extending the results to debris flows; to identify structure efficiency in terms of extension and shape of the deposition area; to numerically validate the adequacy of shallow-water equations in reproducing the impact process and the maximum force on the obstacle; finally to propose an analytical expression for estimating the design force. Experiments were carried out on two dry sandy mixtures flowing down a steep chute and impacting on obstacles of different shape that reproduce check-dams, filter-dams, “houses” and flow-breakers. Effects of mixture composition, obstacle shape and orientation— with respect to mean flow direction—on force values and deposit extension were examined, showing that only check-dams can significantly reduce the kinetic energy and retain the flowing material. Numerical simulations were performed with a 1D code, based on shallow-water equations and the weighted average flux method, to represent forces on an infinite high obstacle (i.e., a wall) in time. Promising results allowed the use of a simple expression directly derived from momentum balance to compute the force exerted by the flow on the structure for given upstream flow depth and velocity. Limitations of the experiments in reproducing impact on defence works in real events are highlighted and suggestions for improving structural design are drawn.

ITALIAN EXPERIENCE ON SUBMERGED BARRIERS AS BEACH DEFENCE STRUCTURES

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

Stone mobility and longshore transport at reshaping breakwaters

Abstract Physical model tests have been performed in two different wave flumes to analyse the threshold of stone movement and quantify the frequency and length of displacements due to head-on wave attacks at a reshaping breakwater. Data on stone movements were obtained from the observation of cumulative displacements at the end of each wave attack and from video records during the attack. Threshold conditions, frequency of movement and displacement length are expressed as function of a suitably modified stability number. A simple model is defined relating longshore transport due to oblique wave attack to stone mobility. The transport model is based on the assumption that movement statistics is affected by obliquity only through the appropriate mobility index and that stones move during up- and down-rush in the direction of incident and reflected waves. Without any calibration, results compare favourably with experimental data available in literature in the range of low mobility conditions where movement statistics was observed. A calibration is provided in order to obtain an accurate transport formula valid in a wide mobility range i.e. for reshaping breakwaters and up to gravel beaches.

Bed-roughness investigation for a 2-D model calibration: the San Martín case study at Lower Paranà

Abstract This paper illustrates how the acoustic Doppler current profiler (ADCP) and single-beam echo-sounder (SBES) recordings can be used for the calibration of existing software to assist in generalizing the morphodynamic processes in large rivers at key sites such as bifurcations and confluences. Calibration of the MIKE21C numerical model by the Danish Hydraulic Institute at the 25-km-long reach of Lower Parana near Rosario (Argentina) is presented. This reach includes two downstream confluences and two bifurcations. The model simulates a 2-D depth-averaged flow velocity and the related sediment fluxes to predict the bifurcation morphodynamics that affects the Parana waterway. To investigate the river channel bathymetry, roughness, flow discharge allocation at bifurcations, suspended sediment concentration and grain size distributions, several instruments were used. These instruments included two ADCPs by Teledyne RDI working at frequencies of 600 and 1,200 kHz, a Sontek ADCP working at a frequency of 1,000 kHz and a SBES. The method to assess suspended sediment concentration and grain size distributions has been previously described. This paper focuses primarily on investigating dune morphology (by means of SBES depth measurements) and friction velocity (by means of ADCP profiling) to determine the river channel bed-roughness. The 2-D model results agree with observed values of bed-roughness, flow velocity and suspended sediment concentration distributions at the investigated sections, known data of water slope and total load of bed sediment are in good agreement with model results.

PROTOTYPE MEASUREMENTS OF THE DYNAMIC RESPONSE OF CAISSON BREAKWATERS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Experimental spatial correlation of wave loads on front decks

The large-scale experiments described herein were carried out at Forschungs-Zentrum Küste (FZK), Hannover (Germany) by a research team of the Universities of Bologna, Edinburgh, Southampton, Plymouth, HR Wallingford and the Coast and Harbor Engineering, Inc. (USA). Wave-induced loads on close-to-prototype jetties were measured. Experimental evidence indicates the presence of force peaks with a short space–time correlation structure, carried by convective processes with a velocity of the order of the wave celerity. The important limitations imposed by the imperfect spatial resolution of pressure transducers on the integrated force are discussed. The finding may be of interest to design laboratory tests involving rapidly-varied processes such as impact waves and the subsequent analysis phase, which may lead to gross errors in the evaluation of the integral process. A guideline to select the cutting frequency is provided.

Numerical modelling of debris surges based on shallow-water and homogeneous material approximations

Abstract The objective of this contribution is to analyse the formation of debris waves in natural channels. Numerical simulations are carried out with a ID code based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behaviour alternatively follows Herschel-BulkJey, Bingham or generalised visco-plastic models. The code is applied to real debris flow events that consisted of a single wave and multiple surges, in particular in the Illgraben catchment (Switzerland) and in the Cortina d’Ampezzo area (Dolomites). Numerical results are presented and compared with available flow depth registrations. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibra...

Effect of Short-Crestedness and Obliquity on Non-Breaking and Breaking Wave Forces Applied to Vertical Caisson Breakwaters

This paper addresses wave forces applied to vertical caisson breakwaters. Design diagrams are proposed to evaluate the reduction of the wave force induced by breaking and non-breaking short-crested oblique waves with increasing horizontal length of the caisson units. Results are derived by testing a model of a typical Italian vertical breakwater with a moderately high rubble mound in the CRF-LSF (Wallingford, UK) wave basin equipped with a multidirectional wave generator. Seven adjacent caisson modules were instrumented with synchronized force transducers. Non-breaking and breaking wave loads are distinguished on the basis of the frequency content. The reduction factor of the load per unit span is derived by the measured spatial correlation, following the argument developed by G. I. Taylor for random movements.