Hyporheic exchanges due to channel bed and width undulations

Abstract

Riffle-pool sequences are fundamental, ubiquitous morphological features of alluvial rivers that are thoroughly studied in general and commonly incorporated into river restoration projects. Most previous investigations on the effect of riffle-pool sequences on hyporheic exchanges focused on solely bed undulation, because that is widely thought to be the defining feature of riffle-pool sequences. However, riffle-pool sequences also have significant width undulations that are vital to riffle-pool sequences morphodynamics, yet relatively few studies exist on the effect of such undulations on hyporheic exchanges.Thus, in this study based on laboratory experiments and numerical simulations, we investigate the effect of bed and width undulations on hyporheic zone characteristics for various ratio of width amplitude to bed amplitude. The variation of hyporheic exchange characteristics (i.e. hyporheic exchange flux and residence time) for different riffle-pool designs were also assessed by creating prescribed topography of synthetic river valleys. The results showed that due to pressure variation along width undulations, the upwelling and downwelling patterns can also be observed for only width undulations in rivers, in the absence of bed undulations, and as width undulation amplitude decreases, the normalized hyporheic exchanges (Q*HZ) increase and normalized median residence time (RT*) decreases. Also, simultaneous channel bed and width undulations result in higher Q*HZ especially when a pool is located in an expansion (aka "oversized" crosssection) and a riffle in a constriction ("nozzle"). Our results suggest that river restoration project that artificially construct wide pools and constricted riffles will achieve maximum Q*HZ and RT*, though they will not be geomorphically self-sustainable.

Hyporheic Exchange in a Meandering Pool‐Riffle Stream

“…In fact, the HE induced by pool‐riffle bedforms and meanders could have similar magnitude and scale as that induced by other mechanisms, such as bars, steps, dams, channel width undulations, etc. (Buffington & Tonina, 2009; Hester & Doyle, 2008; Kasahara & Hill, 2006; Movahedi et al., 2021). This can be associated to the fact that these features generally have relatively large sizes and could strongly influence the surface flow.…”

“…However, the model did not include the pool‐riffle topography. The interactions among surface flow, riffles, and meanders are common and essential to HE processes in pool‐riffle streams, which is thus vital to the studies in redox and nitrogen dynamics in HZ and river design during river restorations (Marzadri et al., 2012; Movahedi et al., 2021; Stonedahl et al., 2013; Tonina, 2005; Trauth et al., 2013). However, a comprehensive study of the hyporheic flow in a pool‐riffle system that considers both various meanders and pool‐riffle bedforms is still missing.…”

Width undulation drives flow convergence routing in five flashy ephemeral river types across a dry summer subtropical region

“…Incidentally, the same GCS has been found to be an important control on river hyporheic exchange rates (Movahedi et al, 2021), so the methodology is relevant beyond just morphodynamics.…”

Engineered hyporheic zones: design and applications in stream health restoration – a review

“…Few studies compared the engineered interventions to natural morphological features (Smidt et al 2015;Morén et al 2017;Hester et al 2018) and reported higher overall flux induced due to engineered structures. Recent studies at bedform scale have highlighted the importance of streambed heterogeneity, bed and width undulations in case of hyporheic response to modification studies Movahedi et al 2021) where the sediment architecture was often more influential than channel morphology in case of highly heterogeneous streambeds . Advancement in computing capabilities enabled numerical simulation and optimization studies of design structures (rock-vanes, weirs) for stream restoration and enhanced nitrogen removal (Khosronejad et al 2018;Liu & Chui 2020).…”