Are restored side channels sustainable aquatic habitat features? Predicting the potential persistence of side channels as aquatic habitats based on their fine sedimentation dynamics

Abstract

International audienceThe restoration of side channels (also referred to as abandoned channels, former channels, floodplain channels, or side arms) is increasingly implemented to improve the ecological integrity of river-floodplain systems. However, the design of side channel restoration projects remains poorly informed by theory or empirical observations despite the increasing number of projects. Moreover, feedback regarding the hydromorphological adjustment of restored channels is rarely documented, making it difficult to predict channel persistence as aquatic habitats. In this study, we analyze the spatial and temporal patterns of fine sediment deposition (< 2 mm) in 16 side channels of the Rhône River, France, restored in 1999–2006 by a combination of dredging and/or partial to full reconnection of their extremities and as a by-product of an increase in minimum flow through the bypassed main channels. We develop prediction tools to assess the persistence of restored channels as aquatic habitats, using between five and seven monitoring surveys per channel (spanning 7–15 years after restoration). Observed channel-averaged sedimentation rates ranged from 0 to 40.3 cm·y−1 and reached 90.3 cm·y−1 locally. Some channels exhibited a significant decline of sedimentation rates through time, whereas others maintained rather constant rates. Scouring processes (i.e., self-rejuvenation capacity) were occasionally documented in 15 channels. Six of the 16 studied channels appeared to be self-sustaining. The 10 others accumulated more and more fine sediment deposits after restoration. Parametric modeling of sedimentation rates suggested that among these 10 channels, four have long life-durations (i.e., more than a century), three have intermediate life-durations (i.e., likely between three and nine decades), and three others have short life-durations (i.e., likely between two and five decades). Observed channel-averaged sedimentation rates can be predicted from the frequency and magnitude (i.e., maximum shear stress) of upstream overflow events and the maximum intensity of backflow events (i.e., maximum backflow capacity). These predictors reflect the dominant role of side channel geometry (i.e., morphology of the upstream alluvial plug, slope conditions) in controlling their flooding regime. These models applied successfully to a wide range of channel morphologies and can be used to quantify a priori the likely effects and the sustainability of side channel restoration

Explaining artificial side channel dynamics using data analysis and model calculations

“…17B). An exponential function seems reasonable because the initial bed level change is large and with increasing bed level the sediment supply is expected to decrease until it reaches a bed level height from which much smaller floodplain aggradation rates occur (Riquier et al, 2017;Van Denderen et al, 2018a). The exponential function suggests that this bed level height is reached when the bevel change is between 1.3 and 1.4 m y=0.9(1-e -(x-6.75)/9.9 )+0.5 ( Fig.…”

A characterization of side channel development

“…It is well known that artificial side channels generally aggrade (e.g., Formann, Habersack, & Schober, ; Riquier, Piégay, Lamouroux, & Vaudor, ; Simons et al, ; Van Denderen, Schielen, Westerhof, Quartel, & Hulscher, ). Within one river branch, side channels aggrade at various rates and with various types of sediment (Riquier, Piégay, & Michalkova, ; Riquier et al, ; Van Denderen et al, ). There is a need for a better understanding of the mechanisms and the processes that result in the aggradation of such side channels.…”

Side channel dynamics

“…The flow separation zone captures sediment at the entrance of the channel which in case of a limited transport capacity can enhance the growth of a plug bar (Constantine et al, 2010;Dieras et al, 2013). After the formation of a plug bar, sediment can still enter the channel at the confluence and during water level variations this can lead to the deposition of fines in the channel (Citterio and Piégay, 2009;Riquier et al, 2017). From the moment that the meander is disconnected from the main channel, the filling-in of the channel occurs with fine material during overbank flow conditions (Makaske et al, 2002;Constantine et al, 2010;Toonen et al, 2012).…”

“…This has resulted in more than 20 side channels in the Rhine branches that are connected to the main channel at both their upstream and downstream end ( Figure 1.1 ). It is found that almost all constructed side channels aggrade (Simons et al, 2001;Formann et al, 2007;Riquier et al, 2017). A better understanding of the mechanisms that cause 0 1 aggradation may lead to a more optimal design of side channels and a reduction of the maintenance costs.…”

“…Channels that receive a limited amount of bedload sediment due to, for example, a plug bar or a log jam are then slowly filled with finer sediment that is supplied to the channel during overbank flow conditions (Makaske et al, 2002;Constantine et al, 2010;Toonen et al, 2012). If the channel is still connected to the main channel at the downstream end, backflow can occur in the side channel leading to aggradation even during base flow conditions (Citterio and Piégay, 2009;Riquier et al, 2017). Channels without a blockage at the upstream entrance show deposition of coarse sediment that is spread over the channel until a certain bed level is reached after which fine sediment can be deposited (Makaske et al, 2002;Dieras et al, 2013).…”