International audienceThis study examined the temporal dynamics and longitudinal distribution of wood over a multi-decadal timescale at the river reach scale (36 km) and a meander bend scale (300? 600 m) in the Ain River, a large gravel-bed river flowing through a forested corridor, and adjusting to regulation and floodplain land-use change. At the 36 km scale, more wood was recruited by bank erosion in 1991?2000 than since the 1950s. The longitudinal distribution of accumulations was similar between 1989 and 1999, but in both years individual pieces occurred homogeneously throughout the reach, while jam distribution was localized, associated with large concave banks. A relationship between the mean number of pieces and the volume recruited by bank erosion (r 2 = 0·97) indicated a spatial relationship between areas of wood production and storage. Wood mass stored and produced and channel sinuosity increased from 1993 to 2004 at three meander bends. Sinuosity was related to wood mass recruited by bank erosion during the previous decade (r 2 = 0·73) and both of these parameters were correlated to the mean mass of wood/plot (r 2 = 0·98 and 0·69 respectively), appearing to control wood storage and delivery at the bend scale. This suggests a local origin of wood stored in channel, not input from upstream trapped by preferential sites. The increase in wood since 1950 is a response to floodplain afforestation, to a change from braided to meandering channel pattern in response to regulation, and to recent large floods. We observed temporal stability of supply and depositional sectors over a decade (on a reach scale). Meander bends were major storage sites, trapping wood with concave banks, also delivering wood. These results, and the link between sinuosity and wood frequency, establish geomorphology as a dominant wood storage and recruitment control in large gravel-bed rivers

International audienceThe artificial gravel augmentation of river channels is increasingly being used to mitigate the adverse effects of river regulation and sediment starvation. A systematic framework for designing and assessing such gravel augmentations is still lacking, notably on large rivers. Monitoring is required to quantify the movement of augmented gravel, measure bedform changes, assess potential habitat enhancement, and reduce the uncertainty in sediment management. Here we present the results of an experiment conducted in the Rhine River (French and German border). In 2010, 23 000 m3 of sediments (approximately the mean annual bedload transport capacity) were supplied in a by-passed reach downstream of the Kembs dam to test the feasibility of enhancing sediment transport and bedform changes. A 620-m-long and 12-m-wide gravel deposit was created 8 km downstream from the dam. Monitoring included topo-bathymetric surveys, radio-frequency particle tracking using passive integrated transponder (PIT) tags, bed grain size measurement, and airborne imagery. Six surveys performed since 2009 have been described (before and after gravel augmentation, and after Q2 and Q15 floods). The key findings are that (i) the augmented gravel was partially dispersed by the first flood event of December 2010 (Q1); (ii) PIT tags were found up to 3200 m downstream of the gravel augmentation site after four years, but the effects of gravel augmentation could not be clearly distinguished from the effects of floods and internal remobilization on more than 3500 m downstream; (iii) linear and log-linear relationships linking bedload transport, particle mobility, and grain size were established; and (iv) combined bathymetry and PIT tag surveys were useful for evaluating potential environmental risks and the first morpho-ecological responses. This confirmed the complementary nature of such techniques in the monitoring of gravel augmentation in large rivers

[1] Recent years have seen increased interest in automated methods, utilizing photographs collected with a handheld digital camera, for determining the grain size distribution of coarse river sediments. Such methods are as precise as traditional field methods and have considerable time and cost advantages. Nevertheless, several unresolved issues pertaining to their deployment remain to be addressed. Using data sets collected from seven gravel bed rivers, this paper examines four key issues: (1) the minimum area required to obtain a representative sample, (2) the effect of lower-end truncation on grain size percentiles, (3) the effect of river bed structure such as imbrication and hiding, and (4) the potential benefits of using individual particle measurements rather than the number (or mass) of particles per size class to calculate percentiles. It is demonstrated that sampling areas of between 50 and 200 times that of the largest grain are adequate to achieve percentile errors of <10% (in mm). The appropriateness of lower-end truncation depends on the study aims and sediment properties. It has a limited effect on higher percentiles, except where sand is a major constituent. Understanding the influence of bed structure on image-derived size information is complicated by the absence of error-free benchmarks against which accuracy may be evaluated, but it is likely that other errors are more important. The use of individual particle measurements to calculate percentiles in preference to classified data is shown to have a small, but appreciable, effect on precision. These results will assist practitioners in making appropriate operational decisions to maximize data quality using image-based grain size data capture.

No abstract