Process‐based assessment of success and failure in a constructed riffle‐pool river restoration project

Abstract

Although there is increasing consensus that river restoration should focus on restoring processes rather than form, proven techniques to design and monitor projects for sediment transport processes are lacking. This study monitors bedload transport and channel morphology in a rural, an urban unrestored, and an urban restored reach. Objectives are to compare bedload transport regimes, assess the stability and self‐maintenance of constructed riffle‐pool sequences, and evaluate the impact of the project on coarse sediment continuity in the creek. Sediment tracking is done using radio frequency identification tracers and morphologic change is assessed from repeated cross‐section surveys. Mean annual velocity is used to quantify the average downstream velocity of tracers, defined as the mean overall tracer travel length divided by the total study duration. The channel reconstruction slows down the downstream velocity of particles in the D75 and D90 size classes, but does not significantly change the velocity of particles in the D50 size class or smaller. Surveys show that riffle features remain stable and that pool depths are maintained or deepened, while tracer paths match with what has been observed in natural riffle‐pools. However, the slowdown of coarse sediment and increase in channel slope may lead to future failures related to over‐steepening of the banks and a disruption in the continuity of sediment transport in the creek. This study demonstrates how bedload tracking and morphological surveys can be used to assess river restoration projects, and highlights the importance of incorporating coarse sediment connectivity into restoration design and monitoring.

Plunging Flow and Coherent Turbulent Structures in a Straight Pool‐Riffle

“…This work thus supported numeric modeling of lateral flow convergence by Booker et al. (2001) and tracer studies that show sediment particles taking a relatively narrow path through pools (Milan, 2013; Papangelakis & MacVicar, 2020). Experiments in a straight laboratory channel by B. J. MacVicar and Rennie (2012) further demonstrated that convective deceleration at the head of a pool could induce lateral flow convergence, even in the absence of laterally varying topography.…”

Experiments on restoring alluvial cover in straight and meandering rivers using gravel augmentation

“…Additionally, the high confidence of the models between equilibrium cover and sediment supply rate developed from these experiments suggests that the extent of alluvial cover and bar characteristics in the field can be used to infer the relative sediment supply rate in the field (Nelson et al, 2014), and identify and estimate temporal trends in the sediment supply from upstream. Finally, sediment continuity is an important process for the long‐term success of restored channels (Papangelakis & MacVicar, 2020). Our experiments indicate that continuity is achieved in both the straight and sinuous channel, both during the formation of cover, and once equilibrium is reached.…”

“…Sediment augmentation has been proposed as an alternative intervention for alleviating urban river degradation (Papangelakis & MacVicar, 2020; Papangelakis, MacVicar, & Ashmore, 2019), which relies on feeding alluvial material into the channels, or through artificially emplacing a layer of alluvium on the channel bed. Tests of gravel augmentation have been conducted in alluvial channels to alleviate sediment starvation downstream of dams (Gaeuman, 2014; Gaeuman, Stewart, Schmandt, & Pryor, 2017; Liedermann, Gmeiner, Kreisler, Tritthart, & Habersack, 2018; Sklar et al, 2009), and to enhance the suitability of aquatic habitat (Merz & Setka, 2004; Ock, Gaeuman, McSloy, & Kondolf, 2015; Staentzel et al, 2018; Zeug et al, 2014).…”

Controls of alluvial cover formation, morphology and bedload transport in a sinuous channel with a non‐alluvial boundary

“…Sklar et al, 2009; Gaeuman, 2014; Gaeuman et al, 2017; Liedermann et al, 2018) and have been proposed as alternatives for mitigating urban river degradation (e.g. Papangelakis and MacVicar, 2020; Welber et al, 2020). The experiments presented here can help predict where the alluvial cover is likely to form, its expected trajectory and its final extent and morphology (and, indirectly, hydraulics) given an upstream point source of sediment supply.…”

“…The channel dimensions, planform geometry and hydraulic conditions during the experiments were based on an approximate 1:40 Froude scaling of a constructed reach in Wilket Creek, a small urban gravel‐bed creek in Toronto, Canada (Bevan et al, 2018). Following this scaling, the experimental channel had a bottom width of 0.23 m, a top width of 0.29 m and a depth of 0.05 m. Although the constructed reach in Wilket Creek comprises a set of irregular meanders (Papangelakis and MacVicar, 2020), the experimental channel contained ten symmetrical 90° bends to mimic relatively simple sinuous designs (e.g. Figure 1b).…”