Stefano Pagliara

Temporal scour evolution at bridge piers: effect of wood debris roughness and porosity

Large wood debris transported by floods affects the scour morphology at bridge piers, thus increasing the bridge failure potential. The characteristic size and shape of the riparian vegetation includes various roughness and permeability conditions of the debris surface. The interaction between two-dimensional flow and rough debris accumulations increases the shear stress, the turbulence and consequently affects the scour evolution process at bridge piers. An experimental study on the bridge pier clear-water scour evolution in the presence of wood debris was conducted at the PITLAB research centre, University of Pisa, Italy. A debris accumulation is characterized by roughness, shape and porosity. Flow intensities range from 65 to 100% of the threshold velocity and included up to 18% of the total flow area. Flow depths were varied from 2.67 to 5.67 times the pier diameter. The effects of wood debris roughness and porosity were analysed in terms of scour temporal evolution and scour morphology.

Uncertainties and trends in extreme rainfall series in Tuscany, Italy: effects on urban drainage networks design

Sound basin management at urban or greater scale needs reliable design storm definition. A statistical analysis is carried out on extreme annual rainfall series for durations of 1, 3, 6 and 12 hours occurring at two gauges in Tuscany, Italy. Kendalls test is applied to the extremal series to detect a definite increasing (or decreasing) trend. A special form of an ARIMA model is also fitted to the series to quantify possible linear trends and their respective significance. Results show a clearly increasing trend at shortest duration at both gauges, no trend at longer ones. Time evolution of design storms for all durations and return periods up to 25 years is derived and analysed based on Gumbel distribution. Applications are presented concerning impacts of uncertainties for the design of urban drainage networks.

Flow resistance in large-scale roughness condition

This paper presents an experimental investigation of the flow resistance in long chutes with a slope of up to 9% and large-scale roughness. Rock chutes generally have steep slopes and the presence of large rocks induces great resistance to the flow. The chute bed was characterized by different sizes of crushed stones with protruding boulders. The proposed relationship correlates the flow resistance in terms of the Darcy–Weisbach friction factor with and without protruding boulders. The proposed logarithmic law is a function of the relative submergence, the slope, and the boulder concentration. A comparison of the data calculated using the proposed logarithmic law with those from earlier experiments shows a reasonable agreement in the tested experimental range.

Rock grade control structures and stepped gabion weirs: Scour analysis and flow features

Rock and stepped gabion weirs are peculiar hydraulic structures that received relatively little attention in technical literature. Nevertheless, they can be successfully used for river restoration instead of traditional hydraulic structures. They have the advantage of being elastic structures and to preserve the natural environment. They can easily adapt to the in situ conditions and can be effortlessly modified according to the different hydraulic or geometric conditions which can occur in a natural river. The present study aims to analyze the effects of their presence on flow pattern and on the scour hole occurring downstream. The analysis involved scour processes, hydraulic jump types, stilling basin morphology and flow patterns. Two different hydraulic jump types were distinguished and classified. It was shown that the flow regime deeply influences the scour process, which evolves much more rapidly when a Skimming Flow regime takes place. Empirical relationships are proposed to evaluate maximum scour depth, maximum axial length, and non dimensional axial profiles.

Influence of large woody debris on sediment scour at bridge piers

Abstract Large woody debris (LWD) reduces the flow area, deviate the flow and increases the velocity in correspondence of the bridge pier, therefore increases the maximum scour hole depth and accelerates sediment removal. Logs and drifts accumulated on bridge piers are of different dimensions. According to logs characteristics and river morphology, drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream. This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour. The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department, University of Pisa, Italy. Drift accumulation was characterized by different relative longitudinal lengths, flow area occlusions, length of longitudinal drift and downstream planimetrical positions relative to the pier center. The experimental investigation has been carried out in clear-water conditions. Several pier sizes, channel widths and sediment materials have been tested. Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier. Finally, data were compared with previous literature findings, which highlight the effect of the downstream extension of drift accumulation on bridge pier scour. New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour, both in terms of relative maximum scour and temporal scour evolution.

Effect of Stilling Basin Geometry on Clear Water Scour Morphology Downstream of a Block Ramp

The scour mechanism downstream of a block ramp in clear water conditions is quite a complex phenomenon that depends on several parameters. Majors role are played by ramp configuration, hydraulic conditions (downstream tailwater level and discharge), material granulometry, and stilling basin geometry. Previous studies analyzed both the scour phenomenon and the effects of all the parameters involved except the last one (i.e., the case in which the ramp has the same width as the downstream stilling basin) and the case of symmetrically expanding stilling basins. This last basin configuration represents an optimal equilibrium between the necessity to dissipate energy and to create a natural pool for the biological species, thus it has a prominent ecological value. The present paper aims to assess the effect of both the width and length of the downstream stilling basin on scour features and flow pattern in clear water conditions. Different scour morphology types are distinguished and classified according to hyd...

Influence of Wood Debris Accumulation on Bridge Pier Scour

This note deals with the influence of debris accumulation on scour around bridge piers. Clear-water experiments in different hydraulic conditions have been carried out with three wood debris shapes: rectangular, triangular, and cylindrical. A wide range of debris thickness and width were studied in order to determine their influence on the maximum scour hole depth temporal evolution. The ratio of the pier diameter to the channel width was varied between 0.05 and 0.12 with total bridge contractions up to 20%. A proposed relation presents a simple design procedure to predict the increase in scour depth, which mainly depends on the flow contraction due to the debris accumulation.

Scour downstream of J-Hook vanes in straight horizontal channels

J-Hook vanes are grade control structures used to stabilize the riverbed. This paper aims to investigate the behaviour of J-Hook vanes as a grade-control structure in straight rivers. Scour downstream of J-Hook vane structures like other grade-control structures depends on the shape of the structure and the river hydraulic conditions. The purpose of this study is classifying the scour geometry and predicting the main scour parameters such as the scour depth, length, width, and the ridge height and length downstream of the J-Hook vanes in straight rivers. Experiments were carried out in a horizontal channel. For each length of the structure, three heights in different hydraulic conditions, including densimetric Froude numbers, water drops, and opening ratios, were tested. Results show that the densimetric Froude number, the drop height, and the height of the structure are the key parameters to form and classify the scour. Equations have been derived using dimensional analysis and experimental data to predict the maximum scour depth, the maximum length of the scour, the maximum scour width, and the maximum height and length of the dune. All the experiments were conducted in clear water conditions.

Influence of Tailwater Depth and Pile Position on Scour Downstream of Block Ramps

Scour control downstream of hydraulic structures is an important topic in hydraulic engineering. Block ramps or rock chutes are often used to control scour downstream of hydraulic structures and have the peculiarity to be ecofriendly. Although these structures assure great energy dissipation, the rapid passage from supercritical to subcritical flow at the toe results in a scour hole with geometric parameters that have to be evaluated in order to avoid foundation problems. For this reason, the analysis of the scour process and the comprehension of the hydrodynamic mechanisms on which it is based are extremely important. In this paper, the results of systematic experimental tests are shown that analyze both the influence of the stilling basin tailwater depth and the ramp toe stabilizing structures, for both uniform and nonuniform channel bed materials. In fact, block ramps are generally stabilized by inserting piles or micropiles at the toe. The upper edge level of piles or micropiles was found a relevant parameter for the scour hole geometry. Simple novel relationships that account for tailwater depth, pile position, and bed material gradation are developed to evaluate the main lengths of the scour hole, in the case in which a free hydraulic jump in a mobile bed occurs. These simple relationships give engineers helpful instruments in block ramp design.

Scour of Clear Water Rock W-Weirs in Straight Rivers

AbstractScour characteristics downstream of rock W-weir structures were studied in a horizontal straight channel and under clear water conditions. For each structure, different hydraulic conditions were considered, including densimetric Froude numbers, water drops, and tailwaters. Results showed that the tailwater depth plays an important role in predicting the scour parameters. Dimensional analysis was used to derive equations to predict the maximum scour depth and length and the maximum dune height and length. Selected tests were conducted with an open W-weir. In this case, results show that the maximum scour depth decreases and its location shifts downstream with respect to classical W-weirs. Three types of scour morphology were classified: Type A, characterized by one scour hole and two ridges; Type B, with two scour holes and two ridges; and Type C, characterized by only one scour hole and one ridge.