Wood Loads Research Articles

Wildfire and the patterns of floodplain large wood on the Merced River, Yosemite National Park, California, USA

We quantified floodplain large wood load (m3 wood/ha) and spatial distribution on the Upper Merced River in Yosemite National Park, California, USA. The upstream portion of the study area includes a recently burned section of the Merced River corridor and the downstream portion contains floodplain with undisturbed forest, facilitating investigation of the effects of wildfire on floodplain large wood. We used measurements of wood load and spatial distribution to test hypotheses regarding floodplain wood dynamics in the channel versus the floodplain and in burned versus unburned portions of the study area. The median wood load on the Merced River floodplain, as measured along numerous transects, is 259 m3/ha overall, with non-significant differences between burned (median 196 m3/ha) and unburned (median 277 m3/ha) portions of the floodplain. We found that jams can occur across the entire width of the floodplain. Burned wood pieces are present throughout the study area in the channel but are largely absent from unburned portions of the floodplain, despite the occurrence of overbank flows since the wildfire. A greater proportion of large wood is within logjams in burned portions of the floodplain. We infer that wood recruited to the channel via bank erosion moves readily downstream within the channel, whereas wood moving from the channel onto the floodplain concentrates near the margin of the main channel or within secondary channels and depressions on the floodplain, leading to the formation of long, narrow logjams.

Flow forming of wood irradiated with electron beam ― Molding load of irradiated wood and mechanical properties of the molded material

In the flow forming technique of wood, a wood block is flowed into metal dies to mold the material into a three-dimensional complex shape. The purpose of this study was to investigate the effect of molding load and the mechanical properties of the molded material in the case that wood as a raw material was irradiated with electron beam (EB). The EB-irradiated wood board was impregnated with thermosetting resin and was subsequently molded into the material by adding pressure and heating in a closed metal die. It was found that the molding load of the impregnated wood was decreased with increasing the EB absorbed dose. The mechanical properties of the molded material were evaluated using modulus of elasticity (MOE) and modulus of rupture (MOR) in a three-point bending test. With increasing EB dose, MOR decreased greatly, while MOE decreased slightly. The EB irradiation on raw wood made it possible to mold the material at low load, though higher dose irradiation caused larger decreases in the mechanical properties.

Logjams as a driver of transient storage in a mountain stream

AbstractWe use four stream segments along a wood‐rich, pool–riffle mountain stream in the Southern Rockies of Colorado, USA to examine how spatial variations in wood load and variations in discharge during and after the snowmelt peak flow influence the magnitude of surface and subsurface transient storage. Segments range in complexity from a single channel with no large wood to an anabranching channel with closely spaced, channel‐spanning logjams. Discharges at which transient storage was assessed range from base flow to snowmelt peak flow. To explore these relations, we used 10 geomorphic variables representing channel morphology and bed substrate, four wood‐related variables representing wood load and associated backwater storage, and two measures of skewness from instream and bulk electrical conductivity breakthrough curves during tracer tests. Instream curves reflect surface and subsurface transient storage, whereas bulk curves primarily represent subsurface transient storage. Higher values of skewness indicate greater retention, and we used the values here as a metric of increased transient storage. Although limited sample size restricts the power of our results, our findings suggest that stream segments with greater instream large wood loads have more and larger pools, greater storage of fine sediment and particulate organic matter, and higher values of skew from instream conductivity. The results also suggest that the presence of instream wood, rather than changes in channel morphology associated with wood, is the most important driver of transient storage. This implies that river management designed to foster transient storage should focus on retaining instream large wood. We did not find significant correlations between geomorphic or wood‐related variables and the skew estimated from bulk conductivity, which may reflect the relatively thin alluvium present in the field area and the prevalence of surface transient storage in this system.

Loading of stream wood following the 2016 Chimney Tops 2 Wildfire: Great Smoky Mountains National Park, Tennessee, USA

AbstractLarge wood drives physical and ecological processes in river systems, but the relative roles of continuous individual tree mortality versus episodes of mass mortality in wood recruitment are not well understood. Here, the 2016 Chimney Tops 2 wildfire, a rare severe wildfire in the eastern United States, was used as a case study for examining the role of mass tree mortality in wood loads of streams in Great Smoky Mountains National Park (GSMNP), Tennessee. Wood surveys were conducted on four reaches in two drainage systems affected by the fire to ascertain the total frequency and volume of burned and unburned wood pieces and jams. Relative to unburned wood, the Chimney Tops 2 fire resulted in a major influx of burned wood to the study reaches, with large percentages of individual pieces (average of 47% by frequency, 48% by volume) and jams (average of 72% by frequency, 93% by volume) exhibiting burn marks. Burned wood pieces had a larger diameter than unburned pieces (p = .065). Results suggest that rare mass‐mortality events like the Chimney Tops 2 wildfire could play a major role in wood dynamics of mountain streams in humid forested regions.

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.

Large wood load and transport in a flood‐free period within an inter‐dam reach: a decade of monitoring the Dyje River, Czech Republic

AbstractThis study examined the large wood (LW) load and transport during the non‐flood period (2009–2018) following major floods that occurred in 2002 and 2006 within the inter‐dam reach of the Dyje River (Czech–Austrian border). The LW load was examined in 36 river corridor segments scattered within the reach in the 2009–2018 period. Two whole‐reach surveys (2011 and 2019) on LW frequency and distribution were conducted, and the export of LW to the downstream reservoir was analysed between June 2013 and December 2018.In the period of non‐flood discharges, the recruitment and depletion of LW were highly variable processes in space and time, leading to a considerable change in the total LW quantity. Whereas the total number of LW pieces decreased, the total LW volume increased because of the increasing dimensions of newly recruited pieces. The annual variability in the quantity of newly recruited pieces was better explained by the variation in the maximum annual discharges (y = 41.043ln(x) + 3.2737, R2 = 0.5352) than by the variability in the number of days with wind gusts >17.2 m/s (y = 1.5004x + 82.096, R2 = 0.118). The land use change with the abandonment of human settlements after World War II and the progressive expansion of forest was the major historical factor driving the increased recruitment of LW to the river corridor.While the 2006 (>100‐year RI) flood brought approximately 1,250 LW pieces to the reservoir, the 2013 (1.5‐year RI) flood delivered 45 pieces. The long‐term average monthly input of LW to the reservoir was 7.7 pieces. The exceptional low‐magnitude flood of 2013, which occurred at the beginning of the monitoring period, was shown to be a threshold above which the number of LW pieces that floated to the dam significantly increased. © 2020 John Wiley & Sons, Ltd.

Experimental assessment of failure criteria for the interaction of normal stress perpendicular to the grain with rolling shear stress in Norway spruce clear wood

The anisotropic material behavior of wood, considered as a cylindrically orthotropic material with annual rings, leads to several different failure mechanisms already under uniaxial stresses. Stress interaction becomes important in the engineering design of structural elements and is often predicted by failure criteria based on uniaxial properties. The prediction quality of failure criteria has been assessed with longitudinal shear stress interaction, though less is known on rolling shear stress in interaction with stress perpendicular to the grain. The study aims at investigating the corresponding mechanical behavior of Norway spruce (Picea abies) clear wood by validating failure envelopes for stress combinations in the cross-sectional plane, based on experimental investigations. For this purpose, a test setup that controls the stress interaction and loading of clear wood along pre-defined displacement paths needed to be developed. Experimentally defined failure states could then be compared to failure surfaces predicted by the phenomenological failure criteria. Material behavior was quantified in terms of stiffness, strength, and elastic and post-elastic responses on dog-bone shaped specimens loaded along 12 different displacement paths. A comparison with failure criteria for two nominal compressive strain levels showed that a combination of failure criteria would be required to represent the material behavior and consider the positive effect of compressive stresses on the rolling shear strength. The findings of this work will contribute to studying local stress distribution of structural elements and construction details, where stress interactions with rolling shear develop.

Evaluating floodplain organic carbon across a gradient of human alteration in the boreal zone

River corridors play an important role in the carbon cycle as sites of carbon transport, storage, and transformation. Floodplain soil organic carbon (OC) and dead, downed large wood (LW) are two of the largest OC stocks in rivers. Human modifications of river corridors, such as damming and floodplain land-use change, have likely modified floodplain OC storage and retention through removal of LW and potential reduction of OC concentrations in soils. However, the effect of human alterations on floodplain OC storage is poorly understood. We measured floodplain soil OC and downed LW loads on three rivers in northern Sweden that display a gradient in the degree of human alteration. The Muddus River is located in a national park and is unaltered. The Vindel River is free-flowing but has been modified via logging and other land-use changes within the floodplain. The Ume River is dammed along its length and has also experienced floodplain logging and land-use change. We used statistical models to determine which factors are associated with differences in mineral soil OC and LW among rivers with different degrees of human alteration. We find the highest mineral soil OC concentrations on the unaltered Muddus River (mean ± standard error (SE) = 3.70 ± 0.59%; median = 3.81%), with lower soil OC along the Vindel (mean ± SE = 1.44 ± 0.22%; median = 0.72%) and Ume (mean ± SE = 2.47 ± 0.44%; median = 1.12%) Rivers. The Muddus River also has the highest downed LW loads (mean ± SE = 22.25 ± 6.99 m3 ha−1) compared to the Vindel (mean ± SE = 3.10 ± 1.26 m3 ha−1) and Ume (mean ± SE = 7.26 ± 3.53 m3 ha−1) Rivers. Variations in soil OC and downed large wood loads indicate that damming may reduce floodplain OC in these boreal systems through reducing lateral channel-floodplain connectivity and longitudinal connectivity. Logging and other land-use changes likely reduce OC inputs to the floodplain surface through removal of organic matter and LW. Further research is needed to elucidate the impact of human modifications on floodplain OC across diverse regions and to inform river restoration efforts to enhance floodplain OC storage.

The effects of longitudinal variations in valley geometry and wood load on flood response

AbstractWe use field measurements and airborne LiDAR data to quantify the potential effects of valley geometry and large wood on channel erosional and depositional response to a large flood (estimated 150‐year recurrence interval) in 2011 along a mountain stream. Topographic data along 3 km of Biscuit Brook in the Catskill Mountains, New York, USA reveal repeated downstream alternations between steep, narrow bedrock reaches and alluvial reaches that retain large wood, with wood loads as high as 1261 m3 ha−1. We hypothesized that, within alluvial reaches, geomorphic response to the flood, in the form of changes in bed elevation, net volume of sediment eroded or aggraded, and grain size, correlates with wood load. We hypothesized that greater wood load corresponds to lower modelled average velocity and less channel‐bed erosion during the flood, and finer median bed grain size and a lower gradation coefficient of bed sediment. The results partly support this hypothesis. Wood results in lower reach‐average modelled velocity for the 2011 flood, but the magnitude of change in channel‐bed elevation after the 2011 flood among alluvial and bedrock reaches does not correlate with wood load. Wood load does correlate with changes in sediment volume and bed substrate, with finer grain size and smaller sediment gradation in reaches with more wood. The proportion of wood in jams is a stronger predictor of bed grain‐size characteristics than is total wood load. We also see evidence of a threshold: greater wood load correlates with channel aggradation at wood loads exceeding approximately 200 m3 ha−1. In this mountain stream, abundant large wood in channel reaches with alluvial substrate creates lower velocity that results in finer bed material and, when wood load exceeds a threshold, reach scale increases in aggradation. This suggests that reintroducing small amounts of wood or one logjam for river restoration will have limited geomorphic effects. © 2020 John Wiley & Sons, Ltd.

EFFECT OF WORKING RANGE ON PRODUCTIVITY AND COSTS OF HARVESTING MACHINES IN A EUCALYPTUS STAND

Wood harvesting should be planned to reduce environmental impacts by minimizing machine traffic, increase productivity and reduce costs. In this context, the aim of this study was evaluate the effect of working range on operational performance of a harvester and forwarder in a Eucalyptus saligna stand under a clear cutting regime. The study was carried out in Paraná State, Brazil, in a cut‐to‐length system in cutting and wood extraction operations in two working ranges: T1 - width of 12 m with a cut of four planting lines; and T2 - width of 18 m with a cut of six planting lines. A time and motion study was performed to determine work cycle times, productivity, production costs, and machine traffic, with working ranges compared by the t-test (α = 0.05) for independent samples. The results showed that the wood processing and loading elements consumed the longest operating cycle time in cutting and wood extraction. The harvester machine presented higher productivity (61.05 m³ PMH0-1) in the T1 working range, while the forwarder was superior (48.32 m³ PMH0-1) in the T2 working range. Regarding the wood harvesting system, it was observed that the T2 working range enabled a reduction of 1% in production costs, which is important when considering the large scale production of the company, while there was a 33.4% reduction in traffic. Therefore, an increase in machines’ working range can provide operational and environmental benefits to wood harvesting operations in forest plantations.

Drivers of variability in large wood loads along the fluvial continuum of a Mediterranean intermittent river

AbstractAlthough in‐channel and floodplain large wood (LW) has been recognized as an important component of lotic ecosystems, there is still limited knowledge on the recruitment, mobility and retention of LW in rivers with an intermittent hydrological regime. In this study, we analysed the LW characteristics and related reach‐scale variables of 22 reaches in a Mediterranean intermittent river (Evrotas, Greece) in order to identify predictors of in‐channel and floodplain LW distribution. Our results indicated high downstream variation in LW volumes in the fluvial corridor (0.05–25.51 m3/ha for in‐channel LW and 0–30.88 m3/ha for floodplain LW). In‐channel and floodplain LW retention was primarily driven by the hydrological regime of the studied reaches (i.e. perennial or non‐perennial) with higher volumes of LW observed in perennial sections. The width of the riparian corridor was an important predictor of LW storage at the reach scale. Non‐perennial reaches had a disproportionally larger number of relatively small‐diameter living trees at the expense of mature trees with larger diameters typical for riparian stands functioning as LW recruitment areas in perennial reaches. The smaller dimensions of in‐channel LW in non‐perennial reaches, coupled with the dominance of loose LW pieces, implies frequent LW transport during ordinary flood events. Nevertheless, overall low LW retention in the fluvial corridor under non‐perennial flow regime predicts low volumes of mobilized LW. In contrast, the recruitment of relatively long and large‐diameter LW from mature riparian stands in perennial reaches, together with additional LW stabilization by banks, bed sediments, living trees or other LW pieces decreases the potential for further LW transport. © 2020 John Wiley & Sons, Ltd.

Modelling headwater channel response and suspended sediment yield to in-channel large wood using the Caesar-Lisflood landscape evolution model

Forest management activities have the potential to significantly modify large wood loads in riparian areas, and thus the availability of material for recruitment into the stream channel, hence it is important that forest management practices take account of the role wood supplied from riparian areas play in the development of channel structure and sediment retention capacity. In this study the cellular landscape and river reach process model CAESAR LISFLOOD (C-L) was used to simulate the effect of in-channel obstructions created by large wood on the distribution of erosion and deposition within a small headwater channel and suspended sediment yield at the catchment outlet. The process model was parameterised and calibrated against an observed discharge and suspended sediment record. Following calibration, 12 discharge and large wood obstruction scenarios were carried out to model erosion and suspended sediment transport over a timber harvesting cycle. The inclusion of obstructions into the model runs to simulate in-channel LW pieces substantially reduced suspended sediment yields in all three discharge scenarios and increased sediment storage along the channel. The modelled location of channel bed degradation and aggradation generally coincided with the pattern of sediment storage along the channel bed demonstrating that C_L was predicting the pattern of erosion and deposition in the channel reasonably well. Modelled erosion rates were extremely low for the respective discharge and large wood model scenarios, with the highest rates ranging from 0.02 t ha−1 year−1 to 0.1 t ha−1 year−1 for the no large wood model over the 22-year period. These findings reinforce the importance of maintaining sufficient volume of standing wood in buffer strips alongside small headwater channels to ensure ongoing recruitment of large wood to the channel network in forests managed for timber production.

Long‐term large wood load fluctuations in two low‐order streams in Southern Chile

AbstractThe eco‐hydrogeomorphic significance of large wood (LW) and its potential for increasing downstream hazards during extreme floods have been widely recognized. We used LW data collected for a 10‐year period from the two low‐order streams of Pichún (Pi) and Vuelta de Zorra (VZ) in Southern Chile to (a) determine if the abundance and dimensions of individual LW pieces change with time, (b) quantify wood load fluctuations during the 10‐year period, and (c) assess the role of LW recruitment from the riparian forests to explain wood load fluctuations during the study period. Nine years after the first survey, the number of LW pieces in Pi and VZ diminished by 60 and 40%, respectively. Despite the reduction in these numbers, in Pi, the LW dimensions did not change significantly during the study. In VZ, the dimensions exhibited statistically significant differences, despite being within the same class. In both catchments, the LW load fluctuated during a 10‐year period, but the drivers of change differed. Although tree toppling was the recruitment mechanism responsible for LW in both stream cases, the high wood load measured in Pi at the beginning of the study suggested massive tree recruitments before the first survey, followed by wood exports which were higher than inputs in the subsequent 10‐year period. In VZ, LW load decreased during the first 9 years (mean annual rate of ~9.2 m3 year−1) and then increased by ~12.1 m3 year−1 in year 10. At VZ, the inputs consisted of single trees that were recruited from the riparian area and by upstream flotation, while exports occurred by downstream fluvial transport. Wood inputs and exports occurred asynchronously and led to LW load fluctuations at decadal and annual intervals. Land management and tree species thus exert a major influence on wood inventory and budget in streams. © 2020 John Wiley & Sons, Ltd.

Does variable channel morphology lead to dynamic salmon habitat?

AbstractStream channel morphology forms the template upon which hydraulic aspects of aquatic habitat are created, yet spatial and temporal variability in habitat imposed by changing morphology is not well understood. This paper presents a conceptual model linking sediment supply patterns to spatial and temporal variability in channel form and aquatic habitat. To evaluate this model, change over time in three habitat variables is quantified using a 2D hydrodynamic modeling approach. A 45‐year record of topographic data from Carnation Creek, a catchment in coastal British Columbia, is used for the flow modeling. Using the Nays2DH modeling platform, water depths and velocities are simulated in eight channel segments located at different positions relative to locations of historical colluvial input using seven flow levels ranging from 3% to 400% of mean annual discharge (0.02 to 3.31 m s ). Results indicate that habitat availability changes through time as a result of sediment supply‐driven changes to channel morphology and wood loads, but patterns in habitat vary as a function of dominant channel segment morphology. Spatial and temporal variability in morphology also influences the relationship between habitat availability and river discharge, leading to non‐stationary habitat‐discharge rating curves. When habitat areas are predicted by applying these curves to daily flow series spanning annual dry seasons, over 50% of the variance in cumulative seasonal habitat area can be explained by year‐to‐year changes in channel morphology and wood loading, indicating that changing morphology is an important factor for driving temporal habitat variability. This variance is related to the morphological variability of a channel segment, which in turn is associated with the segment position relative to zones of colluvial input. Collectively, these results suggest that variability in habitat is impacted by channel morphology, and can be evaluated partly on the basis of a channel's sediment supply regime. © 2019 John Wiley & Sons, Ltd.

Standing dead trees contribute significantly to carbon budgets in Australian savannas

Previous estimates of greenhouse gas emissions from Australian savanna fires have incorporated on-ground dead wood but ignored standing dead trees. However, research from eucalypt woodlands in southern Queensland has shown that the two pools of dead wood burn at similar rates. New field data from semiarid savannas across northern Australia confirmed that standing dead trees comprise about four times the mass of on-ground dead wood. Further, the proportion of total woody biomass comprising dead wood increases with decreasing fire frequency and a decreasing proportion of late dry season (August to December) fires. This gives scope for increasing the carbon stock in the dead wood pool with a reduced fire frequency. Following a previously published approach to quantify total dead wood loads in savannas, new and previously collected data on tree stand structures were used across the whole savanna zone to quantify dead wood loads in equilibrium with historic fire regimes. New parameters are presented for calculating dead wood dynamics including dead trees in Australia’s savannas.

Wood process domains and wood loads on floodplains

AbstractDowned large wood on floodplains creates similar geomorphic and ecological effects as wood in the active channel, but has been the subject of fewer geomorphic studies. I propose floodplain large wood process domains that are distinguished based on recruitment source at the reach to river‐length scale. Wood recruited to the floodplain can be autochthonous (individual or mass recruitment from floodplain forest), fluvially transported, or transported from adjacent hillslopes via mass movements that come down the valley side slopes or down the main channel. Fluvially transported wood can be further distinguished as being deposited: within the channel and subsequently accreted to the floodplain; marginal to the channel; on the floodplain during overbank flow; or on tributary fans. The mechanism of wood recruitment to a floodplain influences the spatial distribution of the wood across the floodplain and the proportion of wood pieces within jams, which in turn influences geomorphic and ecological effects of the floodplain wood. Using published studies of floodplain wood load for unmanaged river corridors, I hypothesize that the climate‐controlled balance between forest primary productivity and decay rates of downed wood is the first‐order control on floodplain large wood loads. Disturbance regime and wood recruitment mechanism are second‐order controls on wood load and primary controls on the spatial distribution of large wood. Understanding of floodplain large wood can be applied to quantifying the effect of large wood on river corridors; river restoration; paleoenvironmental inferences; and estimation of organic carbon stock in river corridors. © 2019 John Wiley & Sons, Ltd.

Investigation of temperature fields in the air environment above wood subjected to thermal degradation

The paper deals with the visualization of thermal fields above the sample body while observing thermal loading of wood. Via holographic interferometry, thermal fields were visualized in a non-contact manner and the values were recorded in 0, 3, 7 and 15 minutes or earlier in case of degradation. In real time, the ongoing processes were recorded in the thermal boundary layer above the surface of sample bodies in dimensions of 43 mm × 40 mm × 10 mm of beech wood in three different directions: longitudinal, radial, tangential cut. The temperatures of isothermal curves above the heated samples and coefficients of heat transfer α were determined by a quantitative analysis of holographic interferograms. The heat transfer coefficient α ranged within 4.6-7.4 W/m2K. The simulation of heat transfer for the selected samples was prepared by the Fluent programme. Consequently, the measured values from the experiments were compared with the calculated values. There is a correlation between the measured and calculated values.

A two-dimensional lattice model for simulating the failure and fracture behavior of wood

A two-dimensional lattice model consisting of truss elements was developed in this study to characterize the failure behavior under arbitrary loading and fracture behavior of wood. The characteristic of this model is to use the integral analytical procedure to determine the elastic stiffness and ultimate strengths of bonds based on energy and strength criteria, respectively, instead of using the trial-and-error method adopted by the current scholars in the field of timber. To prove the feasibility of the lattice model, the tension test, compression test, shear test, and off-axis test on Korean pine were carried out, and two types of fracture tests of Pinus pinaster were cited. The properties of the truss elements were determined from the basic mechanical parameters of wood. The simulated results from using the proposed lattice model were compared with the experimental data, and good agreements including deformation and stress–strain curves were observed. The lattice model in this paper can accurately simulate the failure and fracture behavior of heterogeneous wood, and this method can promote the use of lattice models in the wood mesoscopic and microscopic fields.

Effect of instream logs on bank erosion potential: a flume study with a single log

It is well established that riparian trees reduce bank erosion. However, fallen trees can increase bank erosion as the flow is deflected and accelerated between the log and its root-plate and the bank face, increasing the boundary shear stress and eroding the bank. This erosion contributes to hydraulic variability but can also be a concern in restoring wood loads in rivers. In this first quantitative study into this phenomenon, we develop a theoretical hydraulic model to estimate the near-bank velocity around a single log using principles of continuity and energy loss for a range of log characteristics (morphology, angle, distance from the bank) and flow characteristics. Flume experiments support the theoretical model and suggest that a single log can almost double the near-bank velocity. The fractional channel area taken up by the log (blockage ratio) and the distance between the log and the bank are key factors governing the near-bank velocity. The smaller the angle of the log with the bank the lower the erosion rate, but also the further downstream the erosion extends. As the morphology of the log changes after falling into the channel, bank erosion potentially develops quickly, but slows over time; and migrates downstream.

Wood controls on pool spacing, step characteristics and sediment storage in headwater streams of the northwestern Cascade Mountains

Wood influences channel morphology in headwater streams by creating steps and pools in the longitudinal profile and trapping sediment. We used field data from 32 headwater stream reaches in the northwestern Cascade Mountains to test the hypotheses that 1) larger diameter classes of large wood (LW, defined as pieces with diameter>10cm and length>0.5m) are critical step-keying materials in headwater channels despite narrow channel widths (1–4m), 2) wood-keyed steps trap more sediment than clast- and root-keyed steps, and 3) the negative relationship between LW frequency and the distance between pools observed elsewhere in large streams extends to headwater streams. We found that the frequency of step key pieces peaked in the 20–40cm diameter class. Similarly, 40–100cm diameter pieces were disproportionately associated with key-piece function compared to their overall association with step formation (wood with diameter<10cm was an important step-keying material in channels of width<2m). Steps keyed by wood were significantly more likely to store sediment than clast- or root-keyed steps. In contrast to previous work that did not detect a relationship between wood loading and pool habitat in step-pool channels, pool spacing ranged between ∼17 and 1.5 channel widths with an apparent relationship described by a negative exponential function of LW frequency (RMSE=2.85 channel widths/pool), although the range of LW frequency and the functional relationship were different than those of lower gradient channels. Additionally, linear and nonparametric models demonstrate that sediment storage and step and pool characteristics are related to wood loading and stand history with important distinctions based on channel width. These results confirm the importance of maintaining riparian buffers sufficient to provide functional wood to headwater streams.