Heide Friedrich

University of Auckland · Department of Civil and Environmental Engineering

Large wood (LW) can be transported along a river during floods, increasing flood‐associated hazards, particularly when it accumulates at river‐spanning infrastructures such as bridges and weirs. While most flume studies have explored LW movement with simple wooden elements (dowels), only a few studies have used elements with more complex LW geometr...

Large Wood (LW) transported during floods or channelized mass flows poses a high risk for engineered structures, often leading to significant damage or total failure of the impacted structure. To date little is known about impact magnitudes caused by LW collisions. To better control for such interactions, a better understanding of transport dynamic...

As dunes move along the riverbed, they change in size, shape, and arrangement. This involves sediment fluxes on top of the net downstream motion of the dune field, but how much dune dynamics affect total sediment flux remains unclear. In this study, we obtain high‐resolution and high‐frequency digital elevation models of migrating submerged dunes i...

Plain Language Summary Wave‐generated ripples are a ubiquitous pattern present on seafloors and beaches. Because of their regularity and simple shape, ripples capture the attention of the casual observer and have become the focus of many scientists. When looking at a ripple field in detail, it is possible to observe that crestlines are not necessar...

The cumulative effect of culverts in impeding upstream fish passage is similar to that of large-scale high-head instream structures. The common solution to overcome this impediment is to add ancillary elements, with the goal to lower water velocities and to create discrete low velocity zones (LVZs) for fish to rest. The addition of spoiler baffles...

In the last 30 years, work on large wood (LW) has expanded and matured considerably, and river scientists, managers and practitioners now have a better appreciation of the role of LW in maintaining ecosystems, forming or stabilising riverine landforms, and interacting with river morphodynamics. We have gained a better understanding of the hazards p...

Mobile components of dynamic river systems are varied, and more than fundamental water and sediment interactions must be considered to assess morphodynamic changes. Here, the effects of instream wood and plastic accumulations on the riverbed are modelled and assessed in the presence of a hydraulic structure. We observe a recent tendency towards con...

The processes underlying sediment transport mechanics are still debated. Here, we present novel ensemble particle bedload measurements, obtained with high‐frequency imaging of particle velocities and activity over various bedform migration stages. We show that the bedload particle flux quickly responds to the unsteady non‐uniform flow over the bed....

Turbulent mixing is a key process influencing the dynamics of subaqueous gravity currents. In this study, the evolution of the local turbulent mixing dynamics along a lock-exchange gravity current propagating over a mild slope is statistically investigated by ensemble-averaging and spanwise-averaging 200 large eddy simulation results at two time st...

We performed a three‐dimensional high‐resolution Large Eddy Simulation using a sinusoidal ripple‐like bedform geometry with spacing and amplitude consistent with equilibrium conditions under the prescribed oscillatory forcing. The simulation results are validated qualitatively and quantitatively with previous laboratory experiments and numerical si...

Macroplastics are the primary contributor to riverine plastic pollution by mass, posing a wide range of serious threats for riverine systems, from adversely affecting various life forms within the riverine system, to potentially increasing flood risk, and generally resulting in adverse effects on any livelihoods. Compared to other river-related res...

In this study, the entrainment mechanisms of unconfined turbidity currents that interact with a linear rectangular obstacle will be investigated and compared with confined studies. Laboratory experiments will be performed in a lock exchange basin, where the width allows unconfined and partially unconfined flows, with varying initial current densiti...

Large wood (LW) commonly forms log jams, influencing bed composition and biota via complex flow-sediment-wood interactions. A LW blockage, often induced by river-crossing infrastructure, can strongly alter channel hydraulics and sediment transport patterns, thus inducing a ‘forced’ channel morphology. For studying the deposition of bedload gravel f...

Water-related problems affect several billion people’s lives and represent an annual challenge assessed at multitrillion US dollars, which substantiates their core role in the UN Sustainable Development Goals. Preventing direct and indirect impacts associated with water excess or water scarcity events requires expert judgement based on reliable inf...

Statistical characterisation of turbulence for an unsteady gravity current - Volume 901 - Joë Pelmard, S. Norris, H. Friedrich

Pressures on braided river systems in New Zealand are increasing due to anthropogenic stresses such as demand for irrigation water, braidplain conversion to farmland and invasive vegetation, as well as extreme natural events associated with earthquakes and climate change. These pressures create issues around preserving braided river physical enviro...

The supply of sediment from hillslopes to channels is rarely constant, with discrete events (e.g., landslides) known to transfer large volumes of sediment in geologically-short periods of time, especially in tectonically active areas. Understanding the rates and patterns of subsequent sediment evacuation is important for understanding variability o...

Increasing interest in fish passage solutions past low-head instream structures has led to the development and implementation of new designs with various types of roughness elements within these structures. We know that roughness elements increase the heterogeneity in water velocity by creating a continuous or discrete low velocity zone, which supp...

Wave-generated ripples are macroscopic roughness elements that influence fluid flow and sediment transport. For a major group of ripples (orbital ripples), morphology (height and wavelength) is set by the wave conditions. In natural conditions, where wave forcing is highly variable, ripple morphology is frequently changing. We investigate the rate...

The role of three-dimensionality (3D) in modulating both flow and sediment transport remains poorly understood. 3D bed elevation measurements are difficult to obtain due to irregular dune shapes and submergence. Using photogrammetric tools for topographic reconstruction has become popular in surface studies, yet water refraction makes through-water...

A Structure from Motion (SfM) photogrammetry-based methodology for precise mapping of large wood (LW) accumulations in fluvial systems is presented. The technique may be useful for routine inventory and rapid volume estimation of complex LW structures that tend to divert or obstruct flow in rivers. Our methodology is validated by means of laborator...

Gravel riverbeds naturally present a range of sediment size that can span several orders of magnitude. However, fine sediment (i.e., size <2 mm) additions affect sediment transport and are potentially harmful to the river ecology. Limited research has been done to understand the effects of sand addition on gravel bed topography. For this study, we...

The modification and utilization of rivers in regions where small-bodied diadromous fish are prevalent has largely occurred without fully understanding the migration behaviour of these species. As a result, existing in-stream structures often prevent or restrict migration. Current fish passage design guidance generally focuses on providing average...

Detrending is required to enable robust parameterization of surface roughness by removing large topographic trends from fluvial data. A range of detrending methods exist in the literature; however, there is limited presentation of the effect of these methods on output roughness statistics. Here, the results of non-detrended data compared with flat-...

This work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time‐lapse images of the mobile bed and an automated PIV‐based cross‐correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with man...

Driven by a growing importance to engineered structures, investigating the flow characteristics of turbidity currents interacting with a basal obstruction has become popular over the last three decades. However, research has focused on confined studies or numerical simulations, whereas in situ turbidity currents are typically unconfined. The presen...

Digital elevation models (DEMs) have been increasingly applied in topographic studies in areas such as physical geography and hydraulic engineering. Several methods have been proposed to reconstruct DEMs, including classic close-range stereo photogrammetry and the more novel Structure from Motion (SfM) methodology. Past published studies tend to ap...

Experimental studies of confined turbidity currents interacting with obstacles have become popular over the last three decades; however, little work has sought to quantify the characteristics of unconfined turbidity currents interacting with obstacles. In the present study, unconfined turbidity currents colliding with a rectangular basal obstructio...

Expanding the existing knowledge of the role of roughness on near-bed flow properties is vital to provide deeper insights into the interactions between the flow, sediment transport, morphology and ecology. This study uses four fixed beds, from both the laboratory and the field, to characterise the structure of near-bed flow properties. Dense measur...

By synchronizing data collection, such as photometric and ultrasonic Doppler profiling (UVP) measurement techniques, new insights can be obtained into environmental flows, such as highly dynamic turbidity currents. We introduce a combined experimental setup, which ultimately allows a time reduction in testing programmes, and discuss the measurement...

A novel experimental setup for the laboratory was designed in order to investigate large woody debris accumulations and their influence on hydraulic flow conditions and channel morphology at a river cross-section. Real wood and mobile gravel bedload material were used to simulate morphodynamic interactions in a headwater stream, based on a New Zeal...

Large woody debris (LWD), together with overhanging bank vegetation and large rocks are generally attributed as microhabitat ecosystem units that influence riparian vegetation and sedimentation processes. Especially for hydraulic flow conditions during extreme flood events, when most in-stream material is mobile, it is important to consider interac...

The influence of the grid resolution on the large eddy simulation (LES) of a lock-exchange turbidity current is investigated. The simulations are performed using a finite volume Boussinesq code with a Smagorinsky turbulence model for a range of buoyancy Reynolds numbers, ranging from transitional currents (Reb=1,000) to fully-developed turbulence (...

Surface roughness is a term used in fluvial research without a unanimous definition. Clarification of the term and improved parameterisation is needed in future research. Improvements to the collection of topographic data, using photogrammetry, have provided accurate digital elevation models (DEMs) of field and laboratory gravel-bed patches of vari...

Evidence of downstream fining in sediment size along the length of a gravel bar has frequently been observed. However, there is limited quantitative information on the variation of other roughness statistics. Developments in the acquisition of high-resolution topographic data provide the opportunity for assessing roughness variations along and acro...

In this study we investigate the influence of grid resolution on a near-wall resolved LES model of a lock-exchange particle-driven gravity current. The simulations are performed using the finite volume Boussinesq code SnS with a Smagorinsky turbulence model for a buoyant Reynolds number of 60,000 on 4 grid sizes. According to previous studies, two-...

Traditional sediment transport equations calculate sediment flux from bed shear stress and the equations predict that transport increases nonlinearly with an increase in flow velocity. In a dune field, the dune geometry affects the flow velocity causing accelerating flow over the dune crest and de- and reattachment of the flow downstream of the dun...

Remote sensing of gravel-bed patches and resulting high-resolution digital elevation models (DEMs) allow for the identification of various spatial scales of surface roughness. Thus far, dimensions relating to grain and bedform roughness scales have been determined using semivariograms or equivalent structure/autocorrelation functions. However, it i...

Estimation of bridge scour depth is a common problem encountered by hydraulic engineers involved in the design of major transport infrastructure. While there are standard scour depth estimation methods available, most of them were developed based on uniform pier configurations. Typical floodplain bridge structures in many of the recently constructe...

Stable fluvial armors, shaped by surface coarsening during selective sediment transport, received considerable attention over the years. Stable armoring is important in river engineering studies. For example, a classical problem is the riverbed degradation downstream of a dam. Our knowledge of whether stable armors can develop with limited sediment...

There is a growing consensus that gravel-bed roughness should be parameterized based on bed-surface topography, not only sediment size. One benefit is the possible identification of various spatial scales of surface roughness and evaluation of their respective contributions to flow resistance (and also to bedload transport). The absence of relation...

A novel photometric thresholding process for the study of opaque, sediment-laden flows is presented. The method has been developed to allow photometry and visually intrusive, conventional flow measurement instruments to be used simultaneously. Sediment-laden gravity currents of varying densities were released in a lock-exchange flume containing int...

Temporal entrainment characteristics and mixing processes of sediment-laden turbidity currents interacting with a rectangular obstacle are investigated through lock-exchange experiments. Building on the Morton-Taylor-Turner hypothesis, dependency of temporal entrainment on non-dimensional parameters is examined. Currents of varying density are anal...

The comparability of grain sizes emerging from different methods are discussed, including image-based grain-size analysis. Waterworked gravel-bed surfaces from laboratory and field experiments are analyzed in detail. Grain sizes estimated using freely available object-detection software are compared with grains measured with calipers by hand. On th...

Submerged weirs are river training structures that are used for raising upstream water level, bed stabilization and reducing flow velocity. This paper presents an experimental study of local scour at submerged weirs in sand-bed channels. Two types of tests (coarse sand tests and fine sand tests) were conducted to investigate the effects of sediment...

In situ measurement of grain-scale fluvial morphology is important for studies on grain roughness, sediment transport and the interactions between animals and the geomorphology, topics relevant to many river practitioners. Close-range digital photogrammetry (CRDP) and terrestrial laser scanning (TLS) are the two most common techniques to obtain hig...

Field characterization of gravel-bed surface structure is important for studies on flow resistance, sediment transport and in-stream habitats. The following study presents technical developments that make obtaining high-resolution digital elevation models (DEMs) from gravel beds in the field more efficient and effective. Using close-range digital p...

The process of efficient and effective DEM merging is increasingly becoming more important. To allow DEM analysis for features of different scales, an increase in surface coverage cannot result in reduced measurement resolution. It is thus inevitable that merging individual high-resolution DEMs will become common practice for applications such as h...

The dynamic lift acting on a 100 mm × 100 mm section of a static armor layer during unsteady flow is directly measured in a series of physical experiments. The static armor layer is represented by an artificial streambed mold, made from an actual gravel bed. Data from a total of 190 experiments are presented, undertaken in identical conditions. Res...

The rise of Unmanned Aerial Vehicles for surveying and sensing tasks have created new challenges for quick calibration of sensing systems, which we feel is a critical issue. In this context, calibration is performed often and needs to be achieved as quickly as possible. An approach with minimal user-interaction, which preserves sensing accuracy wou...

Weirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on app...

Grain-scale monitoring of fluvial morphology is important for the evaluation of river system dynamics. Significant progress in remote sensing and computer performance allows rapid high-resolution data acquisition, however, applications in fluvial environments remain challenging. Even in a controlled environment, such as a laboratory, the extensive...

With the rise of affordable processing power and off-the-shelf apparatus supporting 3D imaging, there is a growing need for reliable and fast calibration tools, enabling timely accurate data gathering. When confronted with a choice of camera calibration tools, Zhang's and Tsai's are not only the most cited, but also the most widely available soluti...