Urban Flood Drifters (UFDs): identification, classification and characterisation

Extreme floods threaten lives, assets and ecosystems, with the largest impacts occurring in urbanised areas. However, flood mitigation schemes generally neglect the fact that urban floods carry a considerable amount of solid load. In this study, we define Urban Flood Drifters (UFDs) as loose objects present in the urban landscape that can become mobile under certain flow conditions, thereafter blocking drainage infrastructure and endangering both downstream and upstream communities. Based on 270 post-flood photographic records from 63 major inundations of the past quarter-century across 46 countries, we provide a comprehensive analysis of UFDs and their flood-hazard implications. We show that a variety of vehicles, furniture and a heterogeneous mixture of drifters are present in post-flooding scenarios. Plastic, construction debris and wood (natural or anthropogenic) dominate the statistics of transported drifters in urban floods (with frequencies of roughly 50-60% each), followed by cars (present in 31.5% of post-flood images). Other heavy vehicles are readily observed in post-flood imagery and furniture such as bins, garden sheds or water tanks also appear occasionally, therefore suggesting that they can play a relevant role in extreme floods.Comment: This is an updated version (v2) of a preprint that is currently under revision (29/01/2024

Organised turbulence over mobile and immobile hydraulically rough boundaries

33rd IAHR Congress: Water Engineering for a Sustainable EnvironmentThe present work is aimed at the study of near-bed organised turbulence over mobile and immobile, porous, hydraulically rough boundaries. The bed was permeable and composed of non-cohesive sediments. Two data sets were analysed, characterised by the same u*. The mobile bed data featured generalised sediment transport for all size fractions smaller than the d90. The fixed bed was obtained as result of an armouring process. Comparison of these data sets reveals differences on such parameters of the bursting cycle as the maximum shear stress and the transported momentum. These results point to a reorganization of turbulence, in the near-bed region, when the bed is mobile. The impacts of these reorganization are discussed, namely in what concerns the third order moments of the distributions of the velocity fluctuations

Teildurchgängige Geschiebesammler für sicheren Rückhalt

Geschiebesammler halten oft bei kleineren Hochwassern unnötig Geschiebe zurück. Dadurch wird die öko-morphologische Gewässerkontinuität unterbrochen. Beobachtet werden zudem oft gefährliche, selbsttätige Entleerungen. Durch den Einsatz eines Leitgerinnes im Rückhalteraum von Geschiebesammlern und einem Abschlussbauwerk mit vorgelagertem Grobrechen können die Geschiebedurchgängigkeit bei geringem Abfluss erhöht und der sichere Geschieberückhalt bei Hochwasser verbessert werden

Continuously Fed Density Currents Over Porous Bed

Gravity currents impact significantly the environment and human's life. Understanding their internal structure and their dynamics is fundamental for modelling purposes. In this study we observe experimentally how bed porosity affects the dynamics of density currents. We will focus on density and velocity profiles of currents traveling over porous bed, sinking and entraining water from its cavities. This abstract presents the preliminary results of this study which are relative to density currents travelling over the reference substratum, a smooth impermeable bed. The tests presented will be used as a reference to observe how the water entrained from the bottom cavities can change the structure of the current and are herein used to validate the experimental techniques. Brine water is injected at constant discharge in a 7.5 m long, 0.3 m wide and 0.3 m deep flume while instantaneous velocity and density profiles are simultaneously acquired 3 m downstream of the inlet. Initial excess densities and inlet velocities allow to reproduce gravity flows in sub and super-critical regime

Experimental evaluation of the discharge capacity of flow constrictions by check dams in mountain rivers

Open check dams are built to retain bed load in case of major floods and may have filtering, sieving as well as dosing effects on sediment transport. The combination of an upstream reservoir with an open check dam is designated as bed load trap. These structures are crucial elements for flood protection in mountainous regions as the sediment transport capacity of mountain streams is ample, highly unpredictable and therefore an imminent danger for river dwellers. The contemporary design criteria for open check dams are based on theoretical approaches, laboratory experiments or engineering experience. A multitude of different opening types and shapes were studied and built in the past, mostly based on design parameters which are related to grading curves of the river bed. In a series of laboratory experiments, the opening geometry and position is currently optimized in view of sediment transition for frequent floods in order to ensure sediment dynamics downstream. Herein, the results of a first series of experiments are presented, which were dedicated to the evaluation of the discharge capacity of check dam openings, under the influence of bed load transport on a rough river – bed under supercritical flow conditions. Existing experimental and theoretical formulae for purely hydraulic discharges are reviewed and it is found, that the actually applied value for the orifice coefficient μ are too small. These results may also apply for bridges, which serve in some cases intentionally or unintentionally as open check dams

Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds

Due to a complex three-dimensional flow pattern, the outer banks of river bends are predisposed to erosion. When endangering civil structures, preventing measures to mitigate this erosion are thus required. Vertical ribs at protection walls for scour reduction have been applied to several flood protection projects in mountain rivers; nevertheless, no systematic and intensive study has been presented so far to evaluate their effect. This paper investigates experimentally the effect of vertical ribs, placed as macroroughness elements on the outer vertical wall of a 90° laboratory channel bend. Systematic tests were performed using a wide and coarse grain-size distribution. Scour formation and velocity distribution were assessed in the channel in the presence of a macrorough outer bank, materialized in the laboratory by vertical elements placed on the outer vertical wall along the channel bend. Experiments showed that the macrorough outer bank changed considerably the bed morphology under equilibrium conditions. Maximum scour depth is considerably reduced by the vertical ribs placed at an optimal spacing on the bend outer wall. A considerable grain sorting process occurs across the cross section in the bend, which influences the scour process; differences are observed between situations with and without macrorough banks. The distribution of the time-averaged velocity field across the section shows the influence of the channel rough wall. An optimal macroroughness configuration in terms of scour reduction is discussed and proposed. It was observed that when spacing between vertical ribs is too reduced, these ribs act as uniformly distributed wall roughness, contributing to the width reduction due to the occupation of the cross section and increasing consequently the flow velocity with negative effects in scour reduction

Time evolving bed shear stress due the passage of gravity currents estimated with ADVP velocity measurements

Instantaneous longitudinal velocity fields are derived from the velocity measurements collected with the ADVP. Detailed velocity profiles are the base for the estimation of the bottom shear stress through the logarithmic law of the wall. Bottom shear stress is linked to the erosive and depositive capacity of the gravity current

Effects of Lateral and Vertical Constrictions on Flow in Rough Steep Channels with Bedload

Two-phase flows occurring at flow constrictions such as bridges or open sediment check dams are complex, especially for steep rivers with bedload transport. Dangerous bedload deposition and backwater effects may occur in steep mountain rivers at bridges. In contrast, sediment deposition is desirable at open check dams combined with sediment traps. For design purposes, the discharge and bedload capacity across these flow constrictions must be known. The energy losses, discharge capacity, and bedload transport capacity of vertical and lateral flow constrictions are experimentally studied in a rough, 2% inclined, trapezoidal channel. Both free surface and pressurized flow conditions, as caused by lateral and vertical flow constrictions, respectively, were analyzed because both may occur at bridges and check dams. The experiments demonstrate that the vertical flow constrictions cause a faster increase in the backwater depth, with increasing discharge, than lateral constrictions. The resulting upstream flow conditions can be described by the upstream Froude number, defined as a function of the constriction dimensions (height and width). The bedload transport capacity through the flow constriction decreases with the upstream Froude number. The practical relevance of the findings is illustrated by a design example of flow constrictions at open sediment check dams

Hydromorphodynamic effects of the width ratio and local tributary widening on discordant confluences

River training works performed in the last couple of centuries constrained the natural dynamics of channel networks in locations that include the confluences between tributaries and main channels. As a result, the dynamics of these confluences are currently characterized by homogeneous flow depths, flow velocities, and morphologic conditions, which are associated with impoverished ecosystems. The widening of river reaches is seen as a useful measure for river restoration, as it enhances the heterogeneity in flow depths, flow velocities, sediment transport, and bed substrates. The purpose of this study is to analyze the effects of local widening of the tributary mouth as well as the effects of the ratio between the width of the tributary and that of the main channel on the flow dynamics and bed morphology of river confluences. For that purpose, 12 experiments were conducted in a 70° laboratory confluence. In these experiments, three unit-discharge ratios were tested (q r = 0.37, 0.50, and 0.77) with two width ratios and two tributary configurations. The unit-discharge ratio is defined as the unit discharge in the tributary divided by that of the main channel, measured upstream of the confluence. The width ratio, which is defined as the width of the tributary divided by that of the main channel, was modified by changing the width of the main channel from 0.50 to 1.00 m (corresponding to B r = 0.30 and 0.15 respectively). The tributary configurations consisted of (i) a straight reach with a constant width (the so-called reference configuration) and (ii) a straight reach with a local widening at the downstream end (the so-called widened configuration). During the experiments, a uniform sediment mixture was continuously supplied to both channels. This experimental setup is novel among existing experimental studies on confluence dynamics, as it addresses new confluence configurations and includes a continuous sediment supply to both channels. The experiments were run until the outgoing sediment rate was nearly the same as the incoming; i.e., equilibrium had been achieved. The bed topography and water surface were then recorded in both channels. The results reveal that the width ratio and the locally widened tributary reach influence the dynamics of the confluence. The different width ratios influenced the size of the bank-attached bar at equilibrium, which was wider and longer for B r = 0.15 than for B r = 0.30. Other morphological differences were observed at equilibrium for the different width ratios, such as deeper scour holes and increased penetration of the tributary into the main channel. These differences were attributed to the different values of the ratio between the unit momentum-flux of the tributary and that of the main channel that were noted at equilibrium for the different width ratios. The local widening of the downstream reach of the tributary significantly enhanced the heterogeneity in flow depth, flow velocity, and bed morphology within the widened reach. This heterogeneity contrasts with the homogeneity observed in the tributary without widening (reference configuration). Additionally, the effects of the local tributary widening were limited to the tributary, with minor or negligible effects on the main channel.This study was supported by the Portuguese National Funding Agency for Science, Research and Technology (FCT) and the Laboratory of Hydraulic Construction (LCH) at EPFL in the framework of the Joint Doctoral Initiative IST-EPFL (SFRH/BD/51453/2011)