Wood Fluxes Research Articles

Inter‐basin comparison of wood flux using random forest modelling and repeated wood extractions in unmonitored catchments

AbstractPredicting wood flux (i.e., wood piece number per time interval) or discharge (metre cubes of wood per second) in rivers is crucial for adequate integrated river management that balances risk assessment and ecological improvement. To enhance our understanding of the transport mechanisms of wood and assess their effects in various geographical contexts, it is necessary to conduct inter‐basin comparative studies and preliminary modelling. The wood flux of two river basins was analysed using video monitoring and random forest predictions based on hydrological drivers. The wood flux dynamics of the Ain and Allier rivers were both compared and contrasted. Although there was shared wood transport hysteresis, hourly wood flux, relative critical flow discharges of wood transport and certain hydrological factors exhibited differences between the two river basins. As a next step, the two random forest models, which were trained previously, were applied to predict wood flux and then wood discharge in a third river (the Rhône), in order to estimate a wood volume export, which can be compared with the wood volumes extracted over a series of a few monthly periods in the Génissiat reservoir. By using the random forest modelling, it is possible to estimate the volume of wood on the Rhône river. Despite the absence of any training data, there is a noticeable correlation, however, the estimated volumes were significantly overestimated. To resolve this issue, a correction factor was applied, accounting for disparities in wood recruitment dynamics between the Rhône basin and the training river basins. It was found that high flow events are underestimated, emphasizing the importance of incorporating local annotations and additional parameters in training the random forest model. Accurately predicting wood flux is important for integrated watershed management, but field observations are still lacking for validation and process‐based understanding.

Large in-stream wood yield during an extreme flood (Storm Alex, October 2020, Roya Valley, France): Estimating the supply, transport, and deposition using GIS

During major floods, rivers erode their banks and thus recruit large wood pieces from the riparian zones. There is still a lack of knowledge about the transport of large wood, the volumes involved and the flux distribution, i.e. the large wood connectivity at catchment scale. During storm Alex (October 2020), the French Roya catchment (394 km2) experienced a paroxysmal morphogenic flood involving massive bank erosion. The riparian vegetation was largely recruited, with large wood contributing to logjams and bridge destruction. This paper presents a methodology for volumetric assessment of the large wood fluxes involved. Simple approaches are used to (i) quantify the inputs from stand density data from the national forest inventory and from source areas based on diachronic analysis of active channels highlighting the erosion of 87 ha of wooded areas; and (ii) quantify the volumes deposited via an exhaustive manual digitisation of 16,846 pieces of large wood deposited on 59 km of channels on the Roya and its tributaries. This catchment-scale, large wood connectivity analysis shows that the flood recruited and transported downstream a volume of around 14,000 m3 of large wood (uncertainty range: 7000–29,500 m3). Drone observations of the Roya River mouth in Italy and satellite images showing a raft of driftwood, several km long, drifting off the Roya River mouth in the aftermath of the flood corroborate our findings.

Observer‐bias and sampling uncertainties in riverine wood flux and volume estimation from video monitoring technique

AbstractWood is an integral part of rivers that can have both positive and negative impacts on natural systems and infrastructures. Different techniques have been developed to quantify wood flux or discharge in rivers. Among them, the stream‐side video monitoring technique has proven effective for at‐a‐station wood monitoring with a high temporal and spatial resolution over an indefinite time period. However, the visual annotation of wood pieces in the videos is subject to uncertainties due to observer bias or ‘vision limitations’, and video sampling or ‘time limitations. Vision limitations mean that there are patches in the recorded image that may or may not be considered as wood pieces depending on the judgement of the observer. Time limitations mean that the video record may be sampled to estimate the wood flux rather than completing a census of the full record due to the time‐consuming nature of continuous visual annotation. To assess these uncertainties, six flood events and 13 video segments corresponding to more than 37 days and 64,000 pieces of wood were analysed on two different rivers (Ain and Allier Rivers in France). The results show that while there is a significant difference between observers for the detection of small wood pieces (< 1 m in length), no significant difference exists for the detection of large wood pieces (> 1 m in length). The application of a truncation length (i.e., considering only wood pieces with a size higher than a certain threshold) reduces the piece number uncertainty significantly without resulting in a meaningful change in the total volume of wood. For the time limitation, it is shown that sampling uncertainty depends on wood flux related to water discharge and flood stages (rising versus falling), so a dynamic sampling strategy that depends on flood stage is recommended.

Damming the wood falls.

Rivers historically transported unquantified volumes of driftwood to the ocean. Driftwood alters coastal sediment dynamics and provides food and habitat for diverse organisms. Floating driftwood supports open-ocean organisms. Sunken wood sustains seafloor communities. Centuries of deforestation, flow regulation, and channel engineering have substantially reduced riverine large wood fluxes to the oceans. Here, we use contemporary records of wood flux to reservoirs and coastal regions to estimate the magnitude of potential contemporary global wood fluxes. We estimate that 4.7 million m3 of large wood could enter the oceans each year (the 95% prediction interval range is ~300,000 to 70 million m3). This represents an upper bound for contemporary wood fluxes to the oceans because of wood removal from rivers and reservoirs and a lower bound for historical wood fluxes because of deforestation and river engineering. Substantial reduction of this wood flux likely negatively affects coastal and marine environments.

Video monitoring of in‐channel wood: From flux characterization and prediction to recommendations to equip stations

AbstractWood flux (piece number per time interval) is a key parameter for understanding wood budgeting, determining the controlling factors, and managing flood risk in a river basin. Quantitative wood flux data is critically needed to improve the understanding of wood dynamics and estimate wood discharge in rivers. In this study, the streamside videography technique was applied to detect wood passage and measure instantaneous rates of wood transport. The goal was to better understand how wood flux responds to flood and wind events and then predict wood flux. In total, one exceptional wind and seven flood events were monitored on the Ain River, France, and around 24,000 wood pieces were detected visually. It is confirmed that, in general, there is a threshold of wood motion in the river equal to 60% of bankfull discharge. However, in a flood following a windy day, no obvious threshold for wood motion was observed, which confirms that wind is important for the preparation of wood for transport between floods. In two multi‐peak floods, around two‐thirds of the total amount of wood was delivered on the first peak, which confirms the importance of the time between floods for predicting wood fluxes. Moreover, we found an empirical relation between wood frequency and wood discharge, which is used to estimate the total wood amount produced by each of the floods. The data set is then used to develop a random forest regression model to predict wood frequency as a function of three input variables that are derived from the flow hydrograph. The model calculates the total wood volume either during day or night based on the video monitoring technique for the first time, which expands its utility for wood budgeting in a watershed. A one‐to‐one link is then established between the fraction of detected pieces of wood and the dimensionless parameter “passing time × frame rate”, which provides a general guideline for the design of monitoring stations.

Large wood load fluctuations in an Andean basin

AbstractThe importance of monitoring and analysing wood fluxes in mountain environments is widely recognized. However, there is a lack of information related to the long‐term fluctuations in wood load and associated to the changes in large wood (LW) characteristics. The main aims of the research were to analyse (i) changes in wood characteristics, (ii) fluctuations in wood load, and (iii) the relationship between wood load and sub‐reach settings, proposing an ad hoc roughness index (RI). Repeated field surveys to measure wood load and LW characteristics during a 13‐year period were conducted within a 2.2 km‐long reach of the Rio Toro (Chile), which flows through the Malleco Forest National Reserve that was affected by wildfires in 2002 and 2015. Two spatial scales (i.e. study reach and sub‐reach) were considered. Irrespective of the adopted scale wood load fluctuated considerably, in both number and volume, with significant changes in LW characteristics (i.e. mean diameter and length). Moreover, a clear tendency to the aggregation in wood jams (WJs) was observed within the entire study reach. Based on our results, we conclude that 17 years after the first wildfire wood recruitment has started in the upper parts of the basin. Moreover, Generalized Linear Mixed Models (GLMM) analyses were carried out to investigate the relationship between RI and both number and volume of LW. Both models are characterized by high adjusted R2 of 74.9% and 72.8% for volume and number, respectively. This demonstrated that the RI can characterize the reaches in terms of tendency to deposition and trapping of wood. These results are promising, particularly in improving the knowledge related to potential deposition areas that can control wood load fluctuations, also permitting its management to be improved. © 2020 John Wiley & Sons, Ltd.

Video‐monitoring of wood discharge: first inter‐basin comparison and recommendations to install video cameras

AbstractWood in rivers plays a major role both ecologically and morphologically. In recent decades, due to human activities in the river channels and along the riparian zone, wood obstruction and jamming has exacerbated flooding hazards and infrastructure damage. Therefore, it is necessary to quantify the wood flux and discharge in rivers to improve wood hazard management. Among the various methods for monitoring the wood flux in a river, the streamside videography technique is effective given its high temporal and spatial resolution. Previous work monitored the wood discharge (m3/s) using this technique in the Ain River (France) during three floods (MacVicar and Piégay, 2012), and the same method is implemented on the Isère River (France) to obtain the statistics of wood discharge for two floods. Comparison between the two sites supports the generalization of both the monitoring technique and the link between wood discharge and flood characteristics. We first show that the maximum wood discharge is observed at bankfull discharge, and we confirm the three stage model proposed by MacVicar and Piégay (2012). Additionally, transverse distributions of the number of wood pieces and corresponding wood length appear to be similar for different flood magnitudes on each site. As a technical contribution, the use of the same technique on two sites allows for recommendations on key decisions related to the location and implementation of the equipment. Both statistical and technical contributions can be used by decision makers to implement this monitoring technique, acquire the wood transport parameters, and evaluate the potential wood hazards at local scale or along a river. © 2020 John Wiley & Sons, Ltd.

The pulse of driftwood export from a very large forested river basin over multiple time scales, Slave River, Canada

AbstractThis study presents a case study of large wood transport on the great Slave River in northern Canada with the objective to better understand the processes of and variability in pulsed wood fluxes from large forested catchments. We use a varied approach, integrating field characterization of wood, historical anecdotes, repeat aerial imagery of stored wood, and time‐lapse imagery of moving wood, for a robust analysis and synthesis of processes behind pulsed wood flux, from yearly uncongested export to rare congested wood floods. Repeat monitoring of known sites of temporary storage with new or historic imagery proved to be a very useful tool for constraining wood flux histories. Pulsed wood export on the Slave River is not an artifact of episodic recruitment from major up‐basin disturbances, but rather reflects decadal‐ to half‐century‐scale discharge patterns that redistribute wood recruited from channel migration and bank slumping. We suggest that the multiyear flow history is of paramount importance for estimating wood flux magnitude, followed in declining importance by the yearly sequence of peaks and the magnitude and characteristics of the rising limb of individual floods.

Rules of the road: A qualitative and quantitative synthesis of large wood transport through drainage networks

To effectively manage wood in rivers, we need a better understanding of wood mobility within river networks. Here, we review primarily field-based (and some numerical) studies of wood transport. We distinguish small, medium, large, and great rivers based on wood piece dimensions relative to channel and flow dimensions and dominant controls on wood transport. We suggest further identification and designation of wood transport regimes as a useful way to characterize spatial-temporal network heterogeneity and to conceptualize the primary controls on wood mobility in diverse river segments. We draw analogies between wood and bedload transport, including distinguishing Eulerian and Lagrangian approaches, exploring transport capacity, and quantifying thresholds of wood mobility. We identify mobility envelopes for remobilization of wood with relation to increasing peak discharges, stream size, and dimensionless log lengths. Wood transport in natural channels exhibits high spatial and temporal variability, with discontinuities along the channel network at bankfull flow and when log lengths equal channel widths. Although median mobilization rates increase with increasing channel size, maximum mobilization rates are greatest in medium-sized channels. Most wood is transported during relatively infrequent high flows, but flows under bankfull can transport up to 30% of stored wood. We use conceptual models of dynamic equilibrium of wood in storage and of spiralling wood transport paths through drainage networks, as well as a metaphor of traffic on a road, to explore discontinuous wood movement through a river network. The primary limitations to describing wood transport are inappropriate time scales of observation and lack of sufficient data on mobility from diverse rivers. Improving models of wood flux requires better characterization of average step lengths within the lifetime travel path of a piece of wood. We suggest that future studies focus on: (i) continuous or high-frequency monitoring of wood mobility; (ii) monitoring changes in wood storage; (iii) using wood characteristics to fingerprint wood sources; (iv) quantifying volumes of wood buried within river corridors; (v) obtaining existing or new data from unconventional sources, such as citizen science initiatives, and (vi) creating online interactive data platforms to facilitate data synthesis.

A new methodology for monitoring wood fluxes in rivers using a ground camera: Potential and limits

Ground imagery, which produces large amounts of valuable data at high frequencies, is increasingly used by fluvial geomorphologists to survey and understand processes. While such technology provides immense quantities of information, it can be challenging to analyze and requires automatization and associated development of new methodologies. This paper presents a new approach to automate the processing of image analysis to monitor wood delivery from the upstream Rhône River (France). The Génissiat dam is used as an observation window; all pieces of wood coming from the catchment are trapped here, hence a wood raft accumulates over time. In 2011, we installed an Axis 211W camera to acquire oblique images of the reservoir every 10min with the goal of automatically detecting a wood raft area, in order to transform it to wood weight (t) and flux (t/d). The methodology we developed is based on random forest classification to detect the wood raft surface over time, which provided a good classification rate of 97.2%. Based on 14 mechanical wood extractions that included weight of wood removed each time, conducted during the survey period, we established a relationship between wood weight and wood raft surface area observed just before the extraction (R2=0.93). We found that using such techniques to continuously monitor wood flux is difficult because the raft undergoes very significant changes through time in terms of density, with a very high interday and intraday variability. Misclassifications caused by changes in weather conditions can be mitigated as well as errors from variation in pixel resolution (owing to camera position or window size), but a set of effects on raft density and mobility must still be explored (e.g., dam operation effects, wind on the reservoir surface). At this stage, only peak flow contribution to wood delivery can be well calculated, but determining an accurate, continuous series of wood flux is not possible. Several recommendations are made in terms of maximizing the potential benefit of such monitoring.

Interannual kinetics (2010–2013) of large wood in a river corridor exposed to a 50-year flood event and fluvial ice dynamics

Semi-alluvial rivers of the Gaspé Peninsula, Québec, are prone to produce and transport vast quantities of large wood (LW). The high rate of lateral erosion owing to high energy flows and noncohesive banks is the main process leading to the recruitment of large wood, which in turn initiates complex patterns of wood accumulation and reentrainment within the active channel. The delta of the Saint-Jean River (SJR) has accumulated large annual wood fluxes since 1960 that culminated in a wood raft of >3-km in length in 2014. To document the kinetics of large wood on the main channel of SJR, four annual surveys were carried out from 2010 to 2013 to locate and describe >1000 large wood jams (LWJ) and 2000 large wood individuals (LWI) along a 60-km river section. Airborne and ground photo/video images were used to estimate the wood volume introduced by lateral erosion and to identify local geomorphic conditions that control wood mobility and deposits. Video camera analysis allowed the examination of transport rates from three hydrometeorological events for specific river sections. Results indicate that the volume of LW recruited between 2010 and 2013 represents 57% of the total LW production over the 2004–2013 period. Volumes of wood deposited along the 60-km section were four times higher in 2013 than in 2010. Increases in wood amount occurred mainly in upper alluvial sections of the river, whereas decreases were observed in the semi-alluvial middle sections. Observations suggest that the 50-year flood event of 2010 produced large amounts of LW that were only partly exported out of the basin so that a significant amount was still available for subsequent floods. Large wood storage continued after this flood until a similar flood or an ice-breakup event could remobilise these LW accumulations into the river corridor. Ice-jam floods transport large amounts of wood during events with fairly low flow but do not contribute significantly to recruitment rates (ca. 10 to 30% early). It is fairly probable that the wood export peak observed in 2012 at the river mouth, where no flood occurred and which is similar to the 1-in 10-year flood of 2010, is mainly linked to such ice-break events that occurred in March 2012.

Estimating fluvial wood discharge using time‐lapse photography with varying sampling intervals

ABSTRACTMonitoring large wood (LW: width > 10 cm, length > 1 m) in transport within rivers is a necessary next step in the development and refinement of wood budgets and is essential to a better understanding of basin‐wide controls and patterns of LW flux and loads. Monitoring LW transport with coarse interval (≥ 1 min) time‐lapse photography enables the deployment of monitoring cameras at large spatial and long temporal scales. Although less precise than continuous sampling with video, it allows investigators to answer broad questions about basin connectivity, compare drainages and years,and identify transport relationships and thresholds. This paper describes methods to: (i) construct fluvial wood flux curves; (ii) analyze the effects of sample interval lengths on transport estimates; and (iii) estimate total wood loads within a specified time period using coarse‐interval time‐lapse photography. Applying these methods to the Slave River, a large‐volume (103 m3 s‐1), low‐gradient (10− 2 m km− 1) river in the subarctic (60∘ N), yielded the following results. A threshold relationship for wood mobility was located around 4500 m3 s‐1. More wood is transported on the rising limb of the hydrograph because wood flux declines rapidly on the falling limb. Five‐ and ten‐minute sampling intervals provided unbiased equal variance estimates of 1 min sampling, whereas 15 min intervals were biased towards underestimation by 5–6%, possibly due to periodicity in wood flux. Total LW loads estimated from the 1 min dataset and adjusted for a 15% misdetection rate from 13 July to 13 August are: 1600 ± 200 # pieces, 600 ± 200 m3 and of the order of 1.3 × 105 kg carbon. The total wood load for the entire summer season is probably at least double this estimate because only the second half of the summer was monitored and a large early summer peak freshet was missed. Copyright © 2014 John Wiley & Sons, Ltd.

A structured approach to enhance flood hazard assessment in mountain streams

An evidence-based flood hazard analysis in mountain streams requires the identification and the quantitative characterisation of multiple possible processes. These processes result from specific triggering mechanisms on the hillslopes (i.e. landslides, debris flows), in-channel morphodynamic processes associated with sudden bed changes and stochastic processes taking place at critical stream configurations (e.g. occlusion of bridges, failure of levees). From a hazard assessment perspective, such possible processes are related to considerable uncertainties underlying the hydrological cause-effect chains. Overcoming these uncertainties still remains a major challenge in hazard and risk assessment and represents a necessary condition for a reliable spatial representation of process intensities and the associated probabilities. As a result of an accurate analysis of the conceptual flaws present in the procedures currently employed for hazard mapping in South Tyrol (Italy) and Carinthia (Austria), we propose a structured approach as a means to enhance the integration of hillslope, morphodynamic and stochastic processes into conventional flood hazard prediction for mountain basins. To this aim, a functional distinction is introduced between prevailing one-dimensional and two-dimensional process propagation domains, i.e., between confined and semi- to unconfined stream segments. The former domains are mostly responsible for the generation of water, sediment and wood fluxes, and the latter are where flooding of inactive channel areas (i.e. alluvial fans, floodplains) can occur. For the 1D process propagation domain, we discuss how to carry out a process routing along the stream system and how to integrate numerical models output with expert judgement in order to derive consistent event scenarios, thus providing a consistent quantification of the input variables needed for the associated 2D domains. Within these latter domains, two main types of spatial sub-domains can be identified based on the predictability of their dynamics, i.e., stochastic and quasi-deterministic. Advantages and limitations offered by this methodology are finally discussed with respect to hazard and risk assessment in mountain basins.