Impact of low-head dams on bedload transport rates in coarse-bedded streams

Colm M Casserly; Jonathan N Turner; John J O'Sullivan; Michael Bruen; Craig Bullock; Siobhán Atkinson; Mary Kelly-Quinn

doi:10.1016/j.scitotenv.2020.136908

Impact of low-head dams on bedload transport rates in coarse-bedded streams

Sci Total Environ. 2020 May 10:716:136908. doi: 10.1016/j.scitotenv.2020.136908. Epub 2020 Jan 24.

Authors

Colm M Casserly¹, Jonathan N Turner², John J O'Sullivan³, Michael Bruen³, Craig Bullock⁴, Siobhán Atkinson⁵, Mary Kelly-Quinn⁵

Affiliations

¹ School of Civil Engineering, UCD Dooge Centre for Water Resources Research and UCD Earth Institute, University College Dublin, Dublin 4, Ireland; School of Geography and UCD Earth Institute, University College Dublin, Dublin 4, Ireland. Electronic address: colm.casserly@ucdconnect.ie.
² School of Geography and UCD Earth Institute, University College Dublin, Dublin 4, Ireland.
³ School of Civil Engineering, UCD Dooge Centre for Water Resources Research and UCD Earth Institute, University College Dublin, Dublin 4, Ireland.
⁴ School of Architecture, Planning and Environmental Policy, and UCD Earth Institute, University College, Dublin, Dublin 4, Ireland.
⁵ School of Biology and Environmental Science, and UCD Earth Institute, University College Dublin, Dublin 4, Ireland.

PMID: 32069694
DOI: 10.1016/j.scitotenv.2020.136908

Abstract

This paper presents an empirical study that uses the movement of RFID tracers to investigate the impacts of low-head dams on solid transport dynamics in coarse-bedded streams. Here we report on the influence of two structures located in Ireland's South-East, both of which indicate that particles greater than the reach D₉₀ can be carried through and over low-head dams. This observation suggests that both structures may have reached a state of 'transient storage' as hypothesized by previous research. However, when the data were reinterpreted as fractional transport rates using a novel application of existing empirical relations, we observed patterns consistent with supply-limited conditions downstream. Expanding on existing conceptual models and mechanisms, we illustrate how a system may continue to exhibit supply-limited conditions downstream without the need for a net attenuation of sediment to occur indefinitely. We propose that once a transient storage capacity has been reached, the system then enters a state of dynamic disconnectivity where the long-term average sediment flux equals that under reference conditions, but now with the amplitude and wavelength of these sediment fluctuations having increased. We hypothesize that the time-lag associated with the reduced frequency of events competent enough to move bedload over the structure accounts for the time necessary to complete the 'fill' phase of the transient storage dynamic; a process that will continue until both the fill and flow thresholds are again met to allow the system to reenter the 'scour' phase. This model reconciles how a system may exhibit a sediment deficit for time intervals longer than those experienced under reference conditions. As water and sediment are the drivers of channel morphology and associated habitat units, the impact a structure has on a channel's sediment regime should therefore form part of any assessment regarding the prioritization of barriers for removal or remediation.

Keywords: Connectivity; Hydromorphology; RFID tracers; Run-of-River; Sediment; Weir.