The transport of wood in rivers during floods is an important process that underlies differences in habitat and morphology between water courses and regions. Quantitative data are needed to properly address management objectives and balance wood budgets. In this study we use a streamside video camera to detect wood passage and measure quasi‐instantaneous rates of wood transport in the Ain River, France. The objectives are to verify the procedure, describe the relation between wood transport and discharge, and construct and validate a wood budget for the reach upstream of the camera. Verification of the procedure includes tests of detection frequency, wood velocity, and piece size. A log base two transformation is proposed to classify wood by piece length. It was found that a wood transport threshold occurs at approximately two thirds of the bankfull discharge. Wood transport follows a positive linear relation with discharge up to the bankfull discharge but is both more variable and less sensitive to discharge when the floodplain is inundated. Transport rates are approximately four times higher on the rising limb of the hydrograph than on the falling limb. Wood transport estimates from a three‐stage rating curve are two to 10 times higher than those from a wood budget using local and aerial surveys of upstream dynamics. Future work should address uncertainties related to wood diameter measurements, sampling length and frequency, and antecedent floods. Copyright © 2012 John Wiley & Sons, Ltd.

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Wood plays an important role in stream ecology and geomorphology. Previous studies of wood in rivers have quantifi ed spatial distributions but temporal dynamics remain poorly documented. The lack of such data is related to limitations of existing methods, especially when applied to large rivers. Five techniques are fi eld-tested to assess their utility for quantifying the temporal dynamics in rivers: repeated high-resolution aerial surveys, the measurement of wood physical characteristics as proxies for 14 C dating, passive and active radio frequency identifi cation (RFID) tags, radio transmitters, and video. The spatial distribution of wood is surveyed using aerial imagery with a resolution fi ner than 0·10 m. The estimation of temporal trends by repeated aerial-based surveys needs to consider vegetation growth and hiding. Wood residence times can be calculated using 14 C analysis, but the assessment of wood physical characteristics including decay status and wood density offers a cheaper, if less accurate, alternative. Wood resistance to penetration is tested but results are not signifi cant. Radio transmitters are reliable for multi-year (~5 year) surveys and can be detected at 800 m. Passive RFID tags are limited by a read range of 0·30 m but are reliable for longer term (>5 year) studies. Active RFID tags combine a moderate read range (10-300 m) and low cost with in-fl ood detection but require more testing. Video monitoring of wood passing on the surface of a river is successfully implemented. For a single fl ood on the Ain River (France), wood transport rates are an order of magnitude higher on the rising limb of the hydrograph than on the falling limb. Overall, the techniques improve the ability to gather the data needed to understand wood transfer processes and calibrate budgets of wood in rivers.

[1] Riffle pools are fundamental units of many gravel bed rivers. Considerable debate exists as to an appropriate hydrodynamic model for this bed form type. Sampling designs in previous studies have not always anticipated the degree of spatial variability of flow parameters in riffle pools nor considered the effect of the nonuniform boundary on hydrodynamics. The objective of this paper is to detail the distribution of mean velocities and turbulence intensities in a forced riffle pool. While limited by the lack of lateral velocity measurements, the data set is of high quality and includes measurements during a bankfull flood. Key observations include the perturbation of velocity profiles from what is observed in uniform flow, lateral convergence and divergence of flow, and the generation of turbulence away from the channel boundary. In the thalweg of the pool head, velocities are high near the water surface and low near the bed. Turbulence intensities are relatively high near the bed and may be significant for sediment entrainment. Higher mean velocities occur over the side bar and could indicate that sediment is routed around the thalweg. The midpool is characterized by the lateral constriction of flow and a shear zone downstream of the forcing element. The pool tail has very high velocities and turbulence intensities near the bed. A velocity reversal occurs in the pool tail at bankfull flow but does not occur in the pool head. A consideration of deceleration and acceleration as a result of vertical expansion and contraction is shown to explain many of the key observations.

Since the earliest use of this technology, a growing number of researchers have employed passive Radio Frequency Identification (RFID) transponders to track sediment transport in gravel rivers and coastal environments. RFID transponders are advantageous because they are inexpensive, durable and use unique codes that allow sediment particle mobility and displacement to be assessed on a clast-by-clast basis. Despite these advantages, this technology is in need of a rigorous error and detection analysis. Many studies work with a precision of~1 m, which is insufficient for some applications, and signal shadowing can occur due to clustering of tagged particles. Information on in-field performance is also incomplete with respect to burial and submergence, especially for different transponders and antennae combinations. The objectives of this study are to qualify and quantify the factors that influence the detection zone of RFID tracers including antenna type, transponder size, transponder orientation, burial depth, submergence and clustering. Results of this study show that the detection zone is complex in shape due to a set of lobes in the detection field and provide a better understanding of transponder detection shape for different RFID transponder/antenna combinations. This study highlights a strong influence of clustering and submergence, but no significant effect of burial. Finally we propose standard operating procedures for tagging and tracking in rivers and coastal environments.

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Watershed urbanization and stormwater management (SWM) alter the hydrologic processes of rivers. Although differences have been documented in channel morphology and sediment yield pre-and posturbanization, little is known about how the modified hydrology affects grain-scale bedload transport dynamics. This study aims to characterize the bedload sediment transport regime of three rivers with different hydrologic settings: rural, urban with no SWM, and urban with peak-shaving SWM. The rivers are "semi-alluvial," characterized by an alluvial layer over a cohesive till. Bedload transport was monitored using tracer stones over 3 years. Hydrograph characteristics of the streams fit with what is expected in urban and SWM systems, and the rural stream has an episodic transport regime typical of gravel-bed rivers. Entrainment thresholds are not detectably impacted by the semi-alluvial bed cover, but travel lengths of grains relative to their size are longer than in alluvial gravel-bed streams. Downstream displacement rates of particles up to the D 90 are accelerated in the urban river due to more frequent mobilization rather than increased event-based travel lengths and may explain channel enlargement. SWM decreases the mobility and travel lengths of particles below those in the rural system, which is combined with channel narrowing, and the loss of bed forms suggests a shift toward a competence-limited transport regime. This new regime is a result of reduced shear stresses that are insufficient to transport coarse material. This study presents empirical evidence of the effects of watershed urbanization and SWM on bedload transport and provides recommendations for process-based river management strategies.Plain Language Summary Most studies on the effects of urbanization on river systems focus on linking the extent of urbanization to channel form or total sediment yield. Few studies apply process-level sediment transport theory to urban environments. This study compares how coarse sediment (bedload) is transported through three streams: a rural river, an urban river with no stormwater management, and an urban river with stormwater management. The glacial geology of the area introduces a unique sediment composition (semi-alluvial) that increases the transport distance of mobilized bedload. The speed of bedload sediment displacement was increased in the urban stream because of more frequent floods, which can explain the enlarged channel dimensions observed. Although stormwater management was successful at reducing the displacement speed of coarse sediment, it resulted in a narrow channel, coarser bed material, and a loss of natural bed variation important for aquatic habitats. Results from this study can be used to improve urban river management strategies that aim to reduce erosion and promote ecologic health.

.[1] Multiple hypotheses have been advanced to explain the occurrence of pools in gravel bed rivers. These hypotheses were developed without a hydrodynamic model of how open channel flow is affected by pools, and it is not clear why and when the flow phenomena they describe might occur. Laboratory experiments are warranted to improve our understanding of how a gradual convective deceleration and acceleration of the flow, without flow separation, redistributes flow and turbulence in an open channel. Experiments are conducted in a 1.5 m wide flume with a 0.25 m deep, 7.29 m long straight pool, entry and exit slopes of 5 , vertical side walls, and gravel sediment (D 50 ¼ 9.9 mm). Three-dimensional velocity components are recorded at 50 Hz using Nortek Vectrinos. Velocity and Reynolds stress profiles in the channel centerline agree with previous results in nonuniform flow and include increased Reynolds stress during deceleration and high velocity near the bed during acceleration. Lateral flow convergence occurs where depth is increasing, which demonstrates that convergence is induced during flow deceleration and does not require a lateral flow constriction. Turbulence during deceleration is characterized by sweeps angled toward the sidewall of the channel, an effect that could lead to the formation of a nonuniform pool depth through lateral gradients in the deposition of mobile sediment. A conceptual model of pool hydrodynamics is proposed that includes increased turbulence, near-bed acceleration, and lateral flow convergence as linked aspects of convective deceleration and acceleration due to depth changes in the pool.