Iacopo Carnacina

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.

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.

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.

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.

Self-Aeration and Friction over Rock Chutes in Uniform Flow Conditions

Interaction between the free surface and the bed material in flow over rock chutes under macroroughness conditions leads to a high air entrainment into the flow. The note reports on an experimental study about air diffusion features in the flow over a long rock chute. Air concentration profiles and water depths over a uniform bed material were measured. An empirical equation for the average air concentration in macroroughness condition for steep slopes is proposed. A new Darcy-Weisbach equivalent friction factor for long chutes as a function of the slope and the relative equivalent depth has also been found.

Coupling acoustic devices for monitoring combined sewer network sediment deposits.

Combined sewer networks display some unique features that are not easily reproducible in the laboratory and have yet to be well understood. The transport of organic and mobile sediments, present in large quantities despite the use of optimal design practices, dramatically changes sewer flow patterns and the interaction between flow and sediment. To understand both the flow patterns and transport features of these complex environments, it is therefore necessary to install instrumentation in situ. For this paper, two distinct acoustic techniques were coupled in order to measure both the sediment interface and velocity. For this purpose, a 2 MHz rotating head acoustic profiler and a 10 MHz acoustic Doppler velocimeter (ADV) were jointly installed to survey the position of sediment deposits and measure velocity patterns. Results were compared with measurements recorded in different environments, where both coarser and finer soft deposits were present. These results typically showed good agreement between the interfaces detected using velocity measurements on coarse deposits, while the presence of soft deposits demonstrated the need for coupling measurements techniques that can correctly assess the sediment interface.

Scour process due to symmetric dam spillways crossing jets

The presence of a dam in a river deeply modifies the natural dynamic process, including sediment transport, debris flow and flow regime occurring both in the upstream and in the downstream branches. Thus a dam cannot be considered as an isolated element in the context of a river, and the study of all the cited phenomena has to be conducted taking its presence into account. In particular, it affects both the global and the local river erosive mechanisms. Moreover, risk management policy considers, among other variables, that in all likelihood toe erosion is the one affecting dam failures. In this perspective, even the local scour phenomenon due to plunging jets assumes a particular importance for all river engineers and has to be carefully taken into consideration in order to prevent structural risks and to correctly design protection measurements downstream of hydraulic structures. Previous studies extensively analysed both scour geometry and flow features due to one single jet. The present paper aims to study both the flow features and the scour hole lengths in the case in which two symmetric crossing jets are used. In particular, experiments were conducted by varying hydraulic and geometric parameters: jet discharge, tailwater depth, jet vertical angle and the angle between the crossing jets. Moreover, it was experimentally proven that the vertical distance of the jet crossing point from the water surface deeply influences the scour hole length. A comparison of scour hole morphology due to an equivalent single jet geometric condition is also proposed, in order to understand under what conditions crossing jets reduce the scour depth.

Sills and gabions as countermeasures at bridge pier in presence of debris accumulations

Transverse structures, such as sills, can reduce local scour at bridge piers. However, in the presence of a sill, the scour hole evolves more rapidly with a debris accumulation, yielding a different behaviour of the countermeasure. This research aims to analyse the efficiency of sills and gabions as countermeasures against local scour in the presence of simplified debris geometry. Experiments were conducted under clear-water conditions with the approach flow velocity set at the threshold of the sediment motion. The temporal evolution and the scour morphology in the presence of smooth sills and gabions were investigated. The efficiency of the countermeasures was assessed by referring to a pilot test with an unprotected pier. Although a delay occurs in the scour development, the main findings reveal an insufficient countermeasure efficiency for large scour development in the presence of large debris accumulation, while gabions were able to further delay the scour process mainly due to larger surface roughness when compared with smooth sills.

Inception Point and Air Entrainment on Flows under Macroroughness Condition

Rock chutes or block ramps are fishway passages with low environmental impact. They also contribute to reaeration of rivers with low dissolved oxygen content, owing to the turbulence enhanced by their three-dimensional macroroughness conditions. This paper analyzes the air entrainment inception in flows over beds in macroroughness condition and the self-aerated flow features of the developing flow downstream of the inception point. Air concentration, inception point locations, and water depth elevations have been measured on two different scaled chutes for slopes ranging between 1V:5.88H and 1V:2.17H. Moreover, two different ogee crest lengths have been tested to assess the role of the inlet conditions on the location of the inception point. New equations have been developed to estimate the location of the point of inception and the respective water depth. Longitudinal variations of the mean air concentration downstream from the inception point have been studied and compared with data from the literature. An expression is presented to estimate the optimum length of the block ramp in natural rivers for maximizing air-water mixing.

Acoustic measurement and morphological features of organic sediment deposits in combined sewer networks

The performance of sewer networks has important consequences from an environmental and social point of view. Poor functioning can result in flood risk and pollution at a large scale. Sediment deposits forming in sewer trunks might severely compromise the sewer line by affecting the flow field, reducing cross-sectional areas, and increasing roughness coefficients. In spite of numerous efforts, the morphological features of these depositional environments remain poorly understood. The interface between water and sediment remains inefficiently identified and the estimation of the stock of deposit is frequently inaccurate. In part, this is due to technical issues connected to difficulties in collecting accurate field measurements without disrupting existing morphologies. In this paper, results from an extensive field campaign are presented; during the campaign a new survey methodology based on acoustic techniques has been tested. Furthermore, a new algorithm for the detection of the soil-water interface, and therefore for the correct esteem of sediment stocks is proposed. Finally, results in regard to bed topography, and morphological features at two different field sites are presented and reveal that a large variability in bed forms is present along sewer networks.