Monitoring gravel augmentation in a large regulated river and implications for process‐based restoration

Abstract

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

Special Issue: Gravel Bed Rivers 8

“…These areas are thought to cause the release of large pulses of sediment. Yet another gravel augmentation study was undertaken in the Rhine River (Arnaud et al, 2017). Their study confirmed the positive effects of gravel augmentation in large rivers.…”

“…The river preferentially eroded sediment from floodplains compared with the channel, and this not only promoted valley-wide sediment evacuation, but also facilitated the renewal and differentiation of morphological units, especially in the channel. Yet another gravel augmentation study was undertaken in the Rhine River (Arnaud et al, 2017). As opposed to general incision, gravel augmentation was monitored during a dam-controlled flood with bedload transport measurements, an array of seismometers, and repeat topographic surveys (Gaeuman et al, 2017).…”

Comparison of ground‐based and UAV a‐UHF artificial tracer mobility monitoring methods on a braided river

“…In such environments surveyed ‘in a routine manner’, with no additional tracer seeding being performed, the ES values are conditioned by the recovery rates, and decrease as the number of successive surveys increases (Liébault et al ., ). The lowest ES values (≤0.1) for PIT tag‐equipped tracers were obtained in a survey exhibiting prospection effort over 50 h ha −1 (Depret, ; Arnaud et al ., ; Vázquez‐Tarrío and Menéndez‐Duarte, ), and/or a mean travel distance inferior to 10 m (MacVicar and Roy, ; Nathan Bradley and Tucker, ). Falling somewhere in between these extremes, the surveys using PIT tag‐equipped tracers on the Ain River exhibited ES values that increased with the tracers’ dispersion: 2.98, 5.59, and 6.68 m in 2014, 2015, and 2017, respectively.…”

“…Furthermore, as the time since tracer seeding increases, the surface area needing to be visited increases substantially, and the recovery rate decreases (Piégay et al ., ). This has led scientists to analyse longer‐term tracer dispersion using metrics based on tracer clouds (Haschenburger, ; Arnaud et al ., ), as is done for fine sediment tracking (Milan and Large, ) where particles cannot be individually tracked. Studies have examined the role that the technical specifications of PIT tags play on their recovery rates, including detection range as a function of their immersion, burial situation, or position (Benelli and Pozzebo, ; Chapuis et al ., ; Arnaud et al ., ), orientation and clustering in the antenna sensing field (Lauth and Papanicolaou, , ; Tsakiris et al ., ), the resistance of tracers to abrasion (Cassel et al ., ), and design of the setups (Slaven et al ., ; Cassel et al ., ).…”

Socio-environmental implications of process-based restoration strategies in large rivers: should we remove novel ecosystems along the Rhône (France)?

“…Artificially adding gravel in highly active channel sections was therefore considered as an alternative for sediment supply. It has been tested in situ (23,000 m 3 gravel augmentation in 2010; Arnaud et al 2017) by investigating adverse morphological consequences, notably regarding (i) rapid sediment transfer and threats to downstream infrastructures and (ii) armored bed destabilization and uncontrolled bed incision. The 5-year geomorphic monitoring showed these consequences were avoided.…”