Roberto Gaudio

A Cellular Automata model for soil erosion by water

Abstract A Cellular Automata model for soil erosion by water, SCAVATU, was developed. It involves a larger number of states in comparison to the previous models, including altitude, water depth, total head, vegetation density, infiltration, erosion, sediment transport and deposition. The model was applied to the small catchment of the Fiumara Armaconi, Calabria, Southern Italy. First simulations gave encouraging results, even if field erosion data is needed for validation and future calibration and setting of the CA parameters. The model is susceptible to improvement and could represent a valid alternative to classic physically based methods, for the description of complexity through simple local rules.

Local scouring in low and high gradient streams at bed sills

The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two equations are proposed for the estimation of the maximum scour depth and length. The equations are from previous data sets on low-gradient tests and a new one of experimental results on high-gradient cases.

Turbulence in mobile-bed streams

This study is devoted to quantify the near-bed turbulence parameters in mobile-bed flows with bed-load transport. A reduction in near-bed velocity fluctuations due to the decrease of flow velocity relative to particle velocity of the transporting particles results in an excessive near-bed damping in Reynolds shear stress (RSS) distributions. The bed particles are associated with the momentum provided from the flow to maintain their motion overcoming the bed resistance. It leads to a reduction in RSS magnitude over the entire flow depth. In the logarithmic law, the von Kármán coefficient decreases in presence of bed-load transport. The turbulent kinetic energy budget reveals that for the bed-load transport, the pressure energy diffusion rate near the bed changes sharply to a negative magnitude, implying a gain in turbulence production. According to the quadrant analysis, sweep events in mobile-bed flows are the principal mechanism of bed-load transport. The universal probability density functions for turbulence parameters given by Bose and Dey have been successfully applied in mobile-bed flows.

Time evolution of scouring downstream of bed sills

Results of laboratory experiments conducted on time evolution of the scouring phenomenon at bed sills are presented. Starting from previous findings on long-term depth and length of the local scour hole downstream of bed sills, 12 long-duration tests were designed and performed in the Sloping Sediment Duct at HR Wallingford Ltd., using uniform sand. No sediment feeding system was adopted. Video-recordings of the scour holes through the glass-sided duct allowed successive image analysis to obtain water surface and bed profiles over time. Results confirmed the validity of the assessment method of the maximum scour depth, y s, at equilibrium; uncertainties arose about the length of the scour hole, l s. A cyclic phenomenon perturbing the scour hole was also observed and described. The investigation of the time evolution of \\ showed that the scour hole develops rapidly and conditions close to equilibrium are achieved in a short time. The introduction of a morphological time. t s permitted the non-dimensional description of the increasing of y s over time through a unique curve. The short- and the long-term local scour regions are clearly distinguished. An application of the results is shown in a numerical example.

Flow-altering countermeasures against scour at bridge piers: a review

During the past decades, several investigations were conducted to assess the adequacy of countermeasures against local scour at bridge piers. The countermeasures can be broadly classified into two categories: (1) flow-altering and (2) bed-armouring countermeasures. Flow-altering countermeasures, as treated herein, can be classified into four main groups based on their shapes and performances. A comprehensive review of the up-to-date studies on various types of flow-altering countermeasures is presented, placing special emphasis on recently proposed methods while also revisiting studies of former methods. Experimental conditions of different studies under which the tests were performed are critically examined. A preliminary evaluation of the previously conducted experiments indicates that the results of several tests were influenced by side-wall, sediment size, flow shallowness and temporal effects. Experimental results on the combinations of different countermeasures, including their limitations and difficulties in field applications, are also discussed. Finally, suggestions are presented for further research on selected countermeasures.

Combined flow-altering countermeasures against bridge pier scour

This study presents the results of clear-water tests on the following five combined flow-altering countermeasures against bridge pier scour: (1) submerged vanes and a bed sill, (2) slot and sacrificial piles, (3) a collar and sacrificial piles, (4) a slot and a collar, and (5) a bed sill and a collar. Each countermeasure was designed on the basis of the best configuration recommended in the literature. The results clarify that an improper combination of two countermeasures may be less effective than each individual countermeasure; combinations (1), (2), and (3) do not reduce the scour depth significantly with respect to the single countermeasures; combination (3) increases the scour rate during the first few hours with respect to a single collar; combination (4) may also prevent scour intrusion beneath a collar; and combination (5) in the best configuration reduces the scour depth significantly around the collar with respect to maximum scour depth of the unprotected pier, preventing also the scour hole reaching the pier body.

Non-universality of von Kármán's κ in fluvial streams

A distinguished universal aspect of wall-bounded turbulent flows is Theodore von Kármáns universal logarithmic law (log-law) of the wall, describing the time-averaged streamwise velocity variation with distance from the wall. The log-law has a universal von Kármán constant κ governing the velocity gradient. There remain a number of cases of the non-universality of κ in fluvial streams. In particular, it behaves as a variable in flows with low submergence, or if there is bed- and suspended-load transport. This research focuses on the non-universality of κ raising various issues and inviting future research directions.

Countermeasures against Local Scouring at Bridge Piers : Slot and Combined System of Slot and Bed Sill

Results are presented and discussed from two laboratory experimental campaigns specifically designed to investigate the behavior of a slot as countermeasure against local scouring at a smooth circular bridge pier, close to threshold flow conditions of initiation of uniform sediment motion. The investigation was aimed at evaluating the effectiveness of the slot by changing its sinking depth into a sand bed, and assessing the dependence of the scour depth on different dimensionless groups. The results showed that the slot reduces the local scour at pier. The maximum reduction of the scour depth was about 30% (about 70% for both scour area and volume) in the best configurations. A combined countermeasure is also proposed and tested, consisting of a slot and a bed sill placed downstream of the pier and adjacent to it: in the best configuration, the scour reduction in front of the pier reached about 45% on average (with about 80 and 90% for scour area and volume, respectively). This last result shows that a combination of slot and bed sill may be a very effective countermeasure against local scouring at bridge piers.

Effects of Pile Cap Thickness on the Maximum Scour Depth at a Complex Pier

An experimental campaign of 21 long-duration tests on scouring at complex piers of different configurations was performed to investigate the effect of pile cap thickness on the temporal evolution of the maximum scour depth and the development of equilibrium conditions. Two test series were performed with two significantly different pile cap thicknesses over a wide range of pile cap elevations with respect to the expected scour hole and the approach flow depth. In addition, the accuracy of existing methods to calculate the maximum scour depth was evaluated and discussed. The results show that, in general, the thicker the pile cap, the deeper the corresponding scour hole. The increase in scour depth that is attributable to the pile cap thickness depends on the distance between the pile cap and the original bed level. The observed maximum scour depth was greater with the thicker pile cap (with respect to the thinner pile cap) when it was entirely immerged into the flow, partially buried, or totally buried at a small depth from the initial bed level. Depending on the distance between the pile cap and initial bed level, the pile cap thickness may also influence the temporal evolution of the maximum scour depth by increasing or reducing the scouring rate. DOI: 10.1061/(ASCE)HY.1943-7900.0000704.

Evaluation of Flow-Altering Countermeasures against Bridge Pier Scour

In this study, six different types of flow-altering countermeasures against pier scour were evaluated experimentally. The selected countermeasures were submerged vanes, bed sill, transverse sacrificial piles, collar, threading, and pier slot. Laboratory tests were performed in clear-water conditions with flow intensity slightly below the threshold of sediment motion. Tests were designed on the basis of the best configurations recommended in previous studies by different authors to obtain the maximum efficiency in terms of scour depth reduction. Results showed that some countermeasures, which were recommended as highly efficient in the literature, do not perform well under other test conditions; in particular, literature tests carried out with low flow intensity or short duration or in narrow-channel conditions are criticized. The efficiency of bed sill, submerged vanes, and threading was found to be less than 20%, whereas collar, pier slot, and transverse sacrificial piles reduced the maximum scour depth ...