Joel S. Sholtes

Physical context for theoretical approaches to sediment transport magnitude‐frequency analysis in alluvial channels

Theoretical approaches to magnitude-frequency analysis (MFA) of sediment transport in channels couple continuous flow probability density functions (PDFs) with power law flow-sediment transport relations (rating curves) to produce closed-form equations relating MFA metrics such as the effective discharge, Qeff, and fraction of sediment transported by discharges greater than Qeff, f+, to statistical moments of the flow PDF and rating curve parameters. These approaches have proven useful in understanding the theoretical drivers behind the magnitude and frequency of sediment transport. However, some of their basic assumptions and findings may not apply to natural rivers and streams with more complex flow-sediment transport relationships or management and design scenarios, which have finite time horizons. We use simple numerical experiments to test the validity of theoretical MFA approaches in predicting the magnitude and frequency of sediment transport. Median values of Qeff and f+ generated from repeated, synthetic, finite flow series diverge from those produced with theoretical approaches using the same underlying flow PDF. The closed-form relation for f+ is a monotonically increasing function of flow variance. However, using finite flow series, we find that f+ increases with flow variance to a threshold that increases with flow record length. By introducing a sediment entrainment threshold, we present a physical mechanism for the observed diverging relationship between Qeff and flow variance in fine and coarse-bed channels. Our work shows that through complex and threshold-driven relationships sediment transport mode, channel morphology, flow variance, and flow record length all interact to influence estimates of what flow frequencies are most responsible for transporting sediment in alluvial channels.

Effect of Channel Restoration on Flood Wave Attenuation

Stream channel restoration can increase flow storage and energy dissipation of passing flood waves. Elements of restoration design that can enhance attenuation include remeandering, which reduces channel slope and increases channel length relative to the floodplain; restoring channel-floodplain connectivity; and revegetating banks and the floodplain. Reestablishment of floodplain hydraulic function is increasingly a goal of restoration programs, yet the approximate magnitude of possible change to attenuation due to reach-scale restoration remains poorly quantified. We examined the efficacy of channel restoration on flood attenuation using restored reaches and synthetic reaches representing median dimensions of channel restoration projects in North Carolina e.g., 1 km in length . We applied an industry standard dynamic flood routing model UNET in HEC-RAS to route floods in impaired and restored reach models. Floods routed through field-based reach models either exhibited very small increases in attenuation, largely due to assumed increases in floodplain roughness, or a decrease in attenuation. Analysis demonstrated that attenuation of peak discharge is overall most sensitive to channel and valley slope, channel and floodplain roughness, and channel and valley length in decreasing order, but is dependent on flood magnitude. Restoration most impacted floods of intermediate magnitude between 2and 50-year return interval , particularly those confined to the channel under the impaired morphology but able to access the floodplain under the restored morphology. Restoration may rehabilitate a channel’s ability to attenuate small to intermediate floods by augmenting flood access to the floodplain, changing channel geometry, and enhancing channel and floodplain roughness over time. However, our study shows that the predominantly small scale of current channel restoration will provide minimally quantifiable enhancement to flood attenuation. DOI: 10.1061/ ASCE HY.1943-7900.0000294 CE Database subject headings: Channels; Restoration; Wave attenuation; Flood routing; Hydraulic models. Author keywords: Channel restoration; Flood attenuation; Flood routing; One-dimensional hydraulic models.