In-stream Structures Research Articles

SmartWood: field-based analysis of large wood movement dynamics using inertial measurement units (IMUs)

Wood plays an important ecological role in rivers. Yet challenges arise when large wood (LW) is mobilised and transported during floods. Due to a lack of quantitative data, movement behaviour of LW during floods is still not well understood to date. A proof-of-concept study was conducted at three Swiss rivers to test state-of-the-art sensor-tagged logs, so-called “SmartWood” and collect quantitative field-scale data about LW movement behaviour. The experiments utilised innovative inertial measurement units (IMUs), which have been developed at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich and implanted into wood logs (SmartWood) at prototype scale. Each IMU comprised three individual sensors (gyroscope, accelerometer, and magnetometer) and was equipped with an on-board processor, an AA battery (4.35 V), a memory (8 MB), and a Wi-Fi transmitter (100 m) for data transfer. After successful initial verification tests of the sensors, the IMUs were installed into debranched wood logs, measuring 4.35 m in length and 0.33 m in diameter. At the time of the field experiments, each SmartWood-log weighted between 170 and 220 kg, yielding a density of roughly 500 kg∙m−3. At the Limmat, Thur, and Grosse Melchaa Rivers in Switzerland, innovative yet discontinuous data were obtained. Results revealed consistent movement dynamics across all field sites. Specifically, we observed positive yaw movement during transport of SmartWood along the left river bank and negative yaw movement along the right river bank. Furthermore, interactions of SmartWood with channel boundaries, riparian vegetation, and objects (e.g., ferry dock) were registered and quantified, even when the SmartWood-log was transported out of sight of traditional sensing methods. The conducted field experiments enabled the initial testing of SmartWood in the field and exposed critical limitations of the IMUs and software algorithms for the reconstruction and analysis of floating LW dynamics. The gained knowledge and introduced sensing method will benefit the quantitative assessment of LW dynamics in rivers to maintain safety and functionality for instream structures (e.g., considering LW movement dynamics for the robust design of LW retention and guiding structures), but also river restoration projects and numerical models that rely on quantitative field-scale data.

Effect of single vanes on turbulent flow

In response to growing environmental and ecological awareness, eco-friendly in-stream structures such as vanes have been implemented in different parts of the world to enhance stream conditions. FLUENT (ANSYS) was used to perform three-dimensional large eddy simulation to investigate the effect of the vanes permeability rate on flow characteristics. To evaluate the numerical model accuracy, numerical and experimental free surface profiles compared. It is observed that simulated free surface profiles agree reasonably well with measured values. The effect of different permeability rates is obvious in the flow characteristics such as depth-averaged velocity distribution, tip velocity variations, formation of the secondary flows in the flow field, turbulent kinetic energy, and mean kinetic energy contours. On average, maximum velocity values in the flow field is 1,54 times the approach velocity. Tip velocity decreases up to 30,6 % for the 70,0 % permeable vane. Maximum turbulent kinetic energy and mean kinetic energy for the 70,0 % permeable vane decrease up to 58,0 % and 43,3 %, respectively. Generally significant velocity and flow pattern variations around the impermeable vane can be attributed to the local effect of the vane structure and channel cross sectional constriction in comparison to the permeable vanes.

Influence of the in-stream structures and parameters variation on transient storage

In-stream structures are designed to mimic natural topographic features in rivers and have emerged as an effective method for ecological restoration in recent years. However, there is a lack of studies exploring the impact of these structural designs on transient storage characteristics. We implemented a series of weirs and groynes in a stream and conducted conservative tracer experiments to quantify the influence of these structures on solute transport within the stream. Additionally, we utilized the one-dimensional transport with inflow and storage (OTIS) solute diffusive transport model to assess variations in solute transport parameters and characterize transient storage. Our analysis of the OTIS parameters and transient storage index revealed that in-stream structures can enhance the interaction between flowing water in the mainstem area and the transient storage area, consequently improve the stream's transient storage capacity. Notably, weirs had a greater effect on the transient storage characteristics of streams parameters than groynes. Furthermore, the effect of this impact intensified with an increase in the number, spacing, and height of the in-stream structures. However, it was observed that the coupling between the structures decreased simultaneously, resulting in a gradual weakening of the enhancement in transient storage capacity. This study deepens our understanding of how the design factors of in-stream structures influence transient storage characteristics, and provides a theoretical basis for optimizing in-stream structures in ecological restoration projects.

Influence of Riparian Conditions on Physical Instream Habitats in Trout Streams in Southeastern Minnesota, USA

Rivers across the globe experience and respond to changes within the riparian corridor. Disturbance of the riparian corridor can affect warmwater, intermediate, and coldwater streams, which can negatively influence instream physical structure and biological communities. This study focused on assessing the influence of the riparian habitat on instream structure within the Whitewater River, a coldwater stream system within an agricultural watershed in southeastern Minnesota, USA. To understand the influence of the riparian zone on the physical instream habitat, twenty variables (riparian, n = 9; instream, n = 11) were measured at 57 sites across three forks of the Whitewater using a transect method every 10 m across a 150 m reach. We used a modified Wentworth scale approach to assess coarse and fine substrates to describe habitat conditions. Canonical correlation detected significant associations between riparian and instream variables across the river forks, and indicated that wider riparian buffers, more bank grass and shrubs, longer overhanging vegetation, limited bare soil, and more rocks on banks were significantly associated with increased instream cover, high levels of coarse substrates with reduced embeddedness, increased pool habitats, and reduced fine sediments. In contrast, excessive fine sediments, lack of riffle habitat, reduced coarse substrates, and high width to depth ratios indicative of an impaired instream habitat were associated with narrow riparian buffers and high percentages of bare soil on banks. Riparian corridors have the capacity to enhance and protect physical instream habitats and overall ecosystem health when managed properly. Wide, grassy riparian corridors with stable banks, overhanging vegetation, and limited shade from trees should protect and/or enhance the instream physical habitat, providing the structural diversity favored by aquatic communities. We recommend revising the current Best Management Practices to include monitoring for impairments in the riparia, while promoting and developing good land stewardship with private landowners which can be effective in improving river ecosystems in agricultural settings.

The influence of in-stream structures on temperature dynamics via induced hyporheic exchange

In-stream restoration structures, such as woody weirs, are intended to enhance hyporheic exchange, thus altering temperature dynamics in restored reaches. However, the relative effects of various in-stream structure types and streambed settings (i.e. the size, spacing, number and sediment hydraulic conductivity) remain poorly understood. Here, we conducted a field experiment with a channel-spanning weir in a stream to study its impact on hyporheic exchange and streambed and surface water temperatures. We subsequently applied numerical models to explore the effects of height, spacing and number of weirs and sediment permeability on hyporheic exchange and thermal distribution characteristics. Based on the field experiments and numerical models, the results showed that the addition of a weir enhanced hyporheic exchange and produced a characteristic temperature heterogeneity pattern in sediment that varied with height, spacing, number of weirs and sediment permeability. The spatial extent and rate of hyporheic exchange and temperature heterogeneity near the weir primarily increased with weir height and sediment permeability. With increasing spacing, the exchange flux and thermally heterogenous area increased, but the exchange intensity decreased, and the sediment temperature readily stabilized. With increasing number of weirs, the exchange flux first decreased and then increased, and the region exhibiting temperature heterogeneity increased. The interaction between the introduction of in-stream structures introduction and streambed settings must be understood to improve vertical connectivity in rivers, increase the thermal heterogeneity in the hyporheic zone and create localized but potentially valuable thermal refuge areas.

Movements of platypuses around and through instream structures and natural barriers in the Jenolan Karst Conservation Reserve, New South Wales

Severe flooding in early 2020 and 2021 necessitated major desedimentation works at the iconic Blue Lake in the Jenolan Karst Conservation Reserve (JKCR). Movements and behaviour of platypuses were monitored before, during and after these works, using direct observations, remote cameras and environmental DNA (eDNA) sampling. Platypuses were observed along a 3 km reach of the Jenolan River, including the areas where works occurred, although in low numbers. In their use of the available waterways, platypuses negotiated artificial barriers, including a 10 m high dam, two smaller weirs and natural waterfalls and cascades. Overland movements were detected through vegetation tunnels, drainage pipes and culverts, and individuals were seen entering the cave system, where eDNA was also detected. Platypuses responded to the works activity by foraging outside the affected areas but also continued to traverse these areas from time to time. We describe movements around and through instream infrastructure and past natural barriers and report on other species detected by remote cameras. These observations could help planning and deployment of bypasses suitable for movement of platypuses around anthropogenic barriers and provide insights into impediments to dispersal and gene flow within stream systems.

Artificial light at night affects fish passage rates in two small‐sized Cypriniformes fish

AbstractAn increasing presence of instream structures such as weirs, dams, culverts and reservoirs degrades habitats, fragments rivers and blocks fish movements worldwide. Longitudinal river movements are fundamental for many fish species and the most widespread solution to restore longitudinal connectivity is the implementation of different fish passage solutions. Fishway functionality, however, is highly variable. To design a functional fishway, several aspects of the fish's interaction with its environment need to be taken into consideration. Artificial light at night (ALAN) can affect a range of different behaviours in fish, from activity and movement to feeding and predator–prey relationships. In a fish passage setting, fish are exposed to artificial light at night (ALAN) in the form of light pollution, but, sometimes, also as part of the fish passage solution. Although likely highly species specific, the effect of artificial light on fish passage behaviour has been little explored. Here we study the passage behaviour of two small‐sized fish species, European gudgeon (Gobio gobio) and Italian riffle dace (Telestes muticellus), over a scaled deep side notch weir in a hydraulic flume in three different light conditions: daylight, darkness and ALAN. Although both species passed the obstacle at high efficiencies under all light conditions, their passage behaviours were influenced by light, particularly at the higher levels. While ALAN reduced passage success and resulted in delayed passage for gudgeon, riffle dace passed at higher rates under the artificial light compared to night treatment. Both results indicate a risk of negative effects from ALAN on passage performance at real fishways—or movement rates in lit areas of natural streams—for both species. Independent of light conditions, individuals of both species also passed faster after repeated trials, demonstrating learning in a fish passage context.

Evaluating the performance of instream structures for a stream restoration project in Colorado

AbstractMetals pollution and channel disturbance associated with historical mining, land use, and water development degraded aquatic and riparian habitat along the upper Arkansas River near Leadville, Colorado. Stream restoration was conducted for an 8 km reach to improve aquatic habitat and increase trout populations. Instream structures were prescribed to stabilize streambanks, create diverse stream morphology, and provide overhead cover, refuge, spawning, and overwinter habitat for trout. At least 90% of all structures were expected to be stable and functional 3 years after implementation. The objectives of this study were to investigate structure performance by (1) evaluating the integrity and function of instream structures and (2) evaluating the change in residual pool depths (RPD). Annual surveys utilized a rapid assessment procedure to qualitatively rank integrity, erosion, and deposition at each structure (n = 137). Rankings were investigated with ordinal regression to determine if performance varied by structure type and year. Longitudinal profile surveys were conducted annually and used to estimate RPD for 86 pools. The change in RPD was investigated with repeated measures ANOVA to determine if RPD varied between structure types and changed over time. Results suggest that some structures were more prone to failure, with higher rankings observed for boulder toe, log vanes, log toe, and boulder vanes. Pool depths increased during construction, decreased following the first runoff, and then remained relatively stable in subsequent years. Understanding the performance of instream structures from this case study will help inform the design, evaluation, and expectations for future stream restoration projects.

Effects of a large-scale dam structure on upstream and downstream lateral hyporheic exchange and residence time distributions – The Xinglong Water Conservancy Dam, China

Anthropogenic in-stream structures such as large dams can alter stream hydrodynamics and cause river stage fluctuations that have the potential to affect hyporheic exchange flow (HEF) patterns between rivers and groundwater. However, the development of HEF patterns and residence time distribution (RTD) in dam-regulated riverbank aquifers has mostly been studied focusing on areas downstream of large dam structures while upstream areas have often been neglected. In order to investigate the HEF and RTD patterns under the influence of a large dam in upstream and downstream areas simultaneously, groundwater level data was collected from thirteen monitoring wells distributed over three monitoring profiles both upstream and downstream of XingLong water conservancy dam (XLD), in the Han river, China. Based on field observations, a two-dimensional, horizontal numerical model was built to assess the spatiotemporal evolution of lateral hyporheic exchange flow, the spatial extent of the hyporheic zone (HZ) and groundwater RTD under the influence of the XLD. Our results demonstrate that the impact of the XLD is different in upstream and downstream aquifer regions. The storage effect induced by the XLD caused significant groundwater recharge upstream of the dam over long period of model, while significant groundwater discharge occurs downstream of the dam. As the stage of the regulated Han River varies, the HZ along the upstream river boundary significantly enhanced by the dam whereas substantial HEF downstream was observed in response to rising or high river stage. Furthermore, the operation of the XLD resulted in some substantial flow around the dam characterized by short flow path lengths and high flow velocity, which consequently resulted in the development of a significant HZ in the perimeter surrounding the XLD. Residence time distributions showed that the water in the HZ downstream of the XLD is dominated by rejuvenated (younger) groundwater, whereas the HZ upstream of the XLD comprises a mix of older and younger waters. HEF in the immediate vicinity of the dam comprises both aged and rejuvenated discharged groundwater. Our findings emphasize the need to more holistically consider river stage dynamics in river corridors where large dams are operated to better assess and understand the consequences on riverbank storage as well as flow, transport and attenuation patterns in connected floodplain aquifers and their impact on the aquatic environment.

Design, construction, and preliminary hydraulic evaluation of a model rock ramp fishway

AbstractObjectiveNew fish passage structures are frequently designed using information gained from existing structures, particularly those that have been shown to allow passage of the target species. However, this process rarely involves a preinstallation design and testing phase. Developing an apparatus that allows rapid and relatively low‐cost testing of potential fish passage structure designs prior to field installations could reduce the reliance on a “build it, monitor it, does it work?” approach.MethodsTo meet this need, we developed an indoor research flume at Colorado State University’s Foothills Fisheries Laboratory that housed a full‐scale experimental rock ramp fishway.ResultThe slope of the flume can be adjusted (0–10%) and integrates a set of four PIT tag antennas to allow fine‐resolution tracking of fish movements in the fishway. The flume can deliver variable flows (up to 0.082 m3/s) of 9–25°C water. Lessons learned during the design, construction, and initial operation of the flume are presented here.ConclusionThe basic system could be adapted for similar studies in other regions and provides a robust and flexible infrastructure that could be modified for other evaluations of instream structures in lotic systems.

Analysis of fish migration in correspondence with wood and rock-made instream structures

Anthropic exploitation of soil, forests, and water bodies contributes to the modification of river flow regimes and sediment transport dynamics, thus affecting fish habitats and water quality. In the last decades, the increasing awareness of the consequences of such modifications on natural contexts has stimulated researchers and practitioners to provide more innovative solutions aiming at enhancing river biodiversity. Recently developed eco-friendly approaches for river restoration evidenced the necessity of more careful planning and management of natural resources, including the maintenance and enhancement of fish species' habitats. The present study aims at assessing the interaction of different types of wood- and rock-made instream structures with river flow regimes and habitat requirements of many freshwater fish species, accounting for the influence of downstream scour potholes to facilitate fish species migration and spawning. Experimental tests showed that instream structures cause deep scour holes characterized by low flow velocity zones, thus decreasing the required fish swimming power and potentially providing suitable conditions for the migration of freshwater fish species. Using dimensional analysis, empirical relationships were derived to optimize the longitudinal distance between subsequent instream structures, accounting for the effects of various structure geometries and fish biological characteristics. Nevertheless, further investigation is needed to validate the proposed relationships at prototype scale. This study ultimately contributes at providing tools to design river instream structures favorable to fish migration.

Responses of live-bed scour at instream structures to fluvial bedform migration

Migrating bedforms in alluvial rivers can exacerbate scour damage at instream structures and lead to structural failure. This study investigates the interaction between scour at instream structures and bedform migration. Both two-dimensional (submerged weirs) and three-dimensional (uniform and complex piers) structures are considered. Two components constitute live-bed scour depth: the dynamic scour amplification Δds caused by bedforms and the local flow-induced baseline scour depth dsNB. Results show that, for 2D structures, the baseline scour depth is negligible, while the live-bed scour is mainly caused by approaching bedforms. The dynamic scour amplification normalised by the approaching bedform height (Δds/Hb) decreases with the decrease of upstream angle and varies between 0.5 and 2. For uniform 3D structures, the local flow could cause a considerable baseline scour depth, and Δds/Hb varies between 0.25 and 0.5 and decreases with the increase in the flow capacity to damp bedforms. For complex 3D structures, flow pressurisation beneath the pile-cap magnifies Δds/Hb to near 1. In general, Δds/Hb is inversely correlated with ds_NB/y0, where y0 is flow depth. New equations are proposed for estimating the dynamic scour amplification at various instream structures with design rules recommended.

The Grande Ronde Model Watershed: Integrating Science into Restoration Implementation and Adaptive Management.

Watershed conservation groups throughout the Pacific Northwest coordinate and implement watershed and habitat restoration to recover Pacific salmon Oncorhynchus spp. Many watershed organizations struggle with implementing an adaptive management process that integrates monitoring data and the latest science into their restoration programs. We describe the evolution and lessons learned from the Grande Ronde Model Watershed (GRMW), one of the longest running watershed organizations coordinating fish habitat restoration projects. Since 1992, the GRMW has initiated nearly 300 habitat restoration projects and their partners more than 600 projects. These projects have evolved from an opportunistic approach, focusing on small-scale riparian fencing and instream structures to a data driven, collaborative processes for identifying, ranking, and implementing large process-based floodplain projects using the latest science. The GRMW recently developed an adaptive management process to assess restoration goals and priorities, and a multi-scale monitoring program that leverages the extensive data collected by partners, and periodic collection of LiDAR to evaluate past, current, and future restoration projects. These recently developed components, which are based on the collective history of the GRMW, provide important lessons for other watershed restoration organizations. These include partnering with local organizations to collect monitoring data; use of a transparent multi-scale process for prioritizing restoration; development of a stepwise process for design and implementation of priority projects; a formal adaptive management process with a designated lead to use the latest science to modify goals, priorities, project selection, and design; and use of remotely sensed data to assist with multi-scale monitoring of project success.

Experimental and numerical study on the flow characteristics around spur dikes at different length-to-depth ratios

A spur dike is an in-stream structure, one end of which is situated on the bank of a channel, while the other extends into the current. Its primary functions include protecting stream banks and enhancing aquatic habitats. The motivation of this study stems from the hypothesis that a wake parameter, defined as the product of the length-to-depth ratio of spur dikes and the bottom friction factor, serves as the key parameter in characterizing the flow pattern around these structures. The objectives of this study are two-fold: (1) to determine the critical value of a wake parameter for the transition from three- to two-dimensional flow fields around spur dikes, and (2) to examine the bed shear stress distributions around spur dikes with varying length-to-depth ratios. Flow visualization experiments and large-eddy simulations (LESs) were conducted with length-to-depth ratios ranging from 1 to 7.5, corresponding to wake parameter values between 0.005 and 0.075. The experimental and numerical results revealed various flow structures around and in the wake of the spur dike. When the length-to-depth ratio was close to 1, the two- and three-dimensional flow patterns coexisted around the spur dike. However, as the length-to-depth ratio increased, the three-dimensional flow structures diminished and the two-dimensional flow structures became more pronounced. For an length-to-depth ratio of 7.5, the flow pattern showed fully two-dimensional structures featured by an upstream and a pair of downstream recirculation regions with negligible secondary flows, a horseshoe vortex, and a free-surface vortex. A horseshoe vortex system contributed to a significant increase in both mean and root mean square bed shear stresses near the tip of spur dikes, especially when the wake parameter values were low. The upstream recirculation zone was associated with very low bed shear stress at high values of the wake parameter. The results of this study corroborated that the flow around the spur dikes becomes fully two-dimensional when a wake parameter exceeds a critical value of approximately 0.04.

Instream complexity increases habitat quality and growth for cutthroat trout in headwater streams

The extent and availability of suitable habitat is a fundamental factor limiting the abundance of natural populations. In many stream ecosystems, habitat degradation has reduced habitat quality by removing critical habitat features such as pools. We hypothesized that adding pool habitat to streams would increase habitat quality for salmonid fish and improve population productivity. In this study, we used instream structures to add pool habitat to four headwater streams and estimated changes to habitat quality for cutthroat trout ( Oncorhynchus clarkii) across two seasons using a bioenergetic model. Fish populations were monitored over 5 years to evaluate how treatments influenced fish abundance and growth. We found that the proportion of suitable habitat was higher in treatment sections and in artificially created pool habitats. Abundance of young-of-the-year trout was higher in treatment reaches in comparison to controls and the growth of trout across all size classes sampled was higher in treatment reaches. Our results indicate that increasing pool habitat improves habitat quality resulting in increased densities of cutthroat trout and higher fish growth.

Modelling fine scale route choice of upstream migrating fish as they approach an instream structure

This study used pattern-oriented modelling (POM) to investigate the space use and behavioural response of upstream migrating European river lamprey (Lampetra fluviatilis) to the two-dimensional hydrodynamic conditions created by an instream structure (triangular profile gauging weir). Passive Integrated Transponder (PIT) and acoustic telemetry were used to map the spatial-temporal distribution patterns of lamprey as they migrated upstream. Acoustic Doppler velocimetry and computer modelling were used to quantify the hydrodynamic environment. In adherence with the POM methodology, multiple movement models, incorporating increasingly complex environmental feedback mechanisms and behavioural rules were created and systematically assessed to identify which factors might reproduce the observed patterns. The best model was a spatially explicit Eulerian-Lagrangian Individual Based Model (IBM) that included two simple behaviours: 1) tortuous non-directed swimming when in low flow velocity (< 0.1 m s−1) and 2) persistent directed (against the flow) swimming in moderate to high flow velocity (≥ 0.1 m s−1). The POM indicated that flow heterogeneity was an important influence of lamprey space use and that simple behavioural rules (i.e. two separate movement behaviours in response to flow velocity) were sufficient to reproduce the main movement pattern observed: avoidance of flow recirculating regions near the banks. The combination of field telemetry, hydrodynamic modelling and POM provided a useful framework for systematically identifying the key factors (hydrodynamic and behavioural) that governed the space use of the target species and would likely work well for investigating similar relationships in other aquatic species.

Machine learning based assessment of small-bodied fish tracking to evaluate spoiler baffle fish passage design

Fish passage research is important to mitigate the adverse effects of fragmented river habitats caused by waterway structures. The scale at which this research is undertaken varies from small-scale laboratory prototype studies to in-situ observations at various fish passage structures and bottlenecks. Using DeepLabCut, we introduce and evaluate a machine learning based workflow to track small-bodied fish in order to facilitate improved fish passage management. We specifically studied the behaviour and kinematics of Galaxias maculatus, a widespread diadromous Southern Hemisphere fish species. Upstream fish passage was studied in the presence of three different patches of spoiler baffles at an average water velocity of 0.4 m/s. In semi-supervised mode, the fish locations were extracted, and fish behaviour, such as swimming pathways and resting locations, was analysed based on extracted positions and recorded kinematic parameters. Individual fish behaviour and kinematic parameters were then used to assess the suitability of the three different spoiler baffle designs for enhancing fish passage. Using this technique, we were able to demonstrate where different spoiler baffle configurations resulted in significant differences in fish passage success and behaviour. For example, medium-spaced smaller baffles provided more accessible and uniform resting locations, which were required for efficient upstream passage. Results are discussed in relation to fish passage management at small instream structures.

The Influence Research on Nitrogen Transport and Reaction in the Hyporheic Zone with an In-Stream Structure.

The hyporheic zone (HZ) is important for river ecological restoration as the main zone with nitrogen biochemical processes. The engineering of river ecological restoration can significantly change the hydrodynamics, as well as solute transport and reaction processes, but it is still not fully understood. In this study, nitrogen transport and reaction processes were analyzed in the HZ with an in-stream weir structure. An HZ model was built, and three reactions were considered with different design parameters of the weir structure and different permeability characteristics of porous media. The results show that a structure with a greater height on the overlying surface water enables the species to break through deeper porous media. It promotes the mean spatial reaction rates of nitrification and denitrification and results in increased net denitrification in most cases. In addition, increasing the burial depth of the structure leads to the same variation trends in the mean spatial reaction rates as increasing the structure height. Larger permeability coefficients in porous media can enhance flow exchange and increase mean spatial reaction rates. The results can help deepen the understanding of nitrogen transport and transformation in the HZ and optimize the design parameters and location of the in-stream structure.

A national fish passage barrier inventory to support fish passage policy implementation and estimate river connectivity in New Zealand

Fragmentation of river systems is a key driver of freshwater biodiversity loss. Instream structure inventories are an essential component of delivering a strategic approach to restoring river connectivity. We used a range of data sources to collate the best available information on fish passage barriers and instream structures across the whole of New Zealand. To support implementation of new national policy objectives to maintain and improve river connectivity, the structure inventory has been integrated with a new fish passage assessment mobile application that allows river managers and asset owners to ground-truth, update and add records to the database. The data are provided open access and are being used for environmental reporting and to support strategic prioritisation of barrier mitigation actions. We used these data to quantify river connectivity at regional and national scales and demonstrated that at least half of New Zealand's river network is upstream of fish migration barriers. Limitations to our calculation of upstream river connectivity included the unknown influence of unrecorded structures, particularly on private land, missing information within recorded data, the large number of instream structures that have been identified but not yet assessed for their risk to fish passage, and spatial errors in the automated pairing of structures to a digital river network. A critical challenge for river managers is understanding the consequences of these uncertainties in the data for prioritisation and decision-making regarding barrier removal or mitigation.

Massive Riverbed Erosion Induced by Inappropriate Grade Control: A Case Study in a Large-scale Compound Channel

Sediment flux in rivers has dropped globally in the past decades due to the increasing anthropogenic stressors, leading to significant riverbed degradation that may endanger the instream structures, collapse the banks and affect the riverine environment. To stabilise riverbeds, grade control structures (GCSs) have been widely applied in degrading channels. However, further understanding of the impacts of GCS on large-scale compound channels (with a main channel and floodplains) is needed. This paper reports a failure case of grade control in a large-scale compound channel (Shi-ting River, China), including a comparison of the bed profile measured using the structure from motion (SfM) technique before and after the flood season. The impacts of two improperly designed GCS on the bed morphology of the studied river reach are analysed and the reasons for failure of the present case are discussed based on numerical flow modelling. The results of numerical flow modelling and the measured bed profile indicate that the skewed mainstream flow direction in a curved main channel and the local scour downstream of a high drop GCS (3 ∼ 8 m) led to massive erosion on the floodplain near the GCS, significantly increasing the width and cross-sectional area of the main channel near the GCS. Consequently, the flow bypassed the GCS through the eroded area of the floodplain, leading to the dysfunction of the GCS. Due to the GCS dysfunction, a 5-year return period flood caused over 1.3 million m3 of erosion in the studied river reach (length ≈ 3.1 km), inducing severe bed degradation (up to 10 m) and reducing the width/depth ratio of the main channel upstream of the GCS. Through comparing the present study with the documented cases from literature, it is found that the bed degradation induced by GCS failure in compound channels develops much faster than those induced by sediment mining, weir or dam construction and reafforestation. Based on the analysed failure reasons, recommendations are provided for GCS design in large-scale compound channels.