Investigating the impact of low-head dams on sediment transport dynamics in gravel-cobble streams

Sediment connectivity, though typically viewed as subsidiary to concerns surrounding fish passage, serves an important role in a functioning riverine ecosystem, with both substrate stability and particle size distribution acting as key determinants of benthic community structure and spawning habitat. However, despite more than a decade of pressure to restore stream continuity under the Water Framework Directive (WFD), there have been very few empirical studies on the impact that low-head dams (i.e. weirs) have on bed and suspended sediment conveyance, and little progress in the development of replicable quantitative methodologies for doing so. In this study we explore these knowledge gaps through field investigations of three gravel-cobble streams in southeast Ireland using RFID technology to investigate bedload connectivity, and integrated high-resolution monitoring of turbidity and discrete suspended sediment sampling to establish above dam vs, below dam patterns of suspended sediment conveyance.

Suspended sediment inputs and outputs over a range of flow conditions (above baseflow) reveal elevated sediment flux at the downstream station (below dam) compared to that coming into the reach (above dam). These observations are indicative of a local source of sediment between monitor­ing stations. Here we suggest that as sediment inputs became exhausted before peak discharge, the structure’s impounded zone (typically considered a depositional area) becomes the dominant source of sediment to the downstream reach. We argue that if sediment trapped behind the structure is available for transportation during high flow events, the system must be trapping sediment under lower flows, which is consistent with field observations.

Results for bedload connectivity and tracer transport over low-head dams demonstrate that particles exceeding the reach D₉₀ can be carried through and over these structures, which is consistent with what has been reported from the US. This observation suggests that both structures may have reached a state of ‘transient storage’ as hypothesized by other authors. However, RFID tracer data when reinterpreted as fractional transport rates using a workflow based on existing empirical relations, indicate patterns consistent with supply-limited conditions downstream, demonstrating conflicting lines of evidence between the event-scale tracer movement and long-term sediment regime. Utilizing our empirical data and additional observations collected from a stationary RFID antenna mounted on a weir crest, we expand on existing models and mechanisms to show how a system may continue to exhibit supply-limited conditions downstream without the need for a net attenuation of sediment to occur indefinitely.

These results indicate that low-head dams may continue to alter the hydrosedimentary processes of fluvial systems long after dam construction and any hypothetical storage capacity has been reached. Though the impact low-head dams have on sediment disconnectivity to the downstream reach is likely to be variable and relatively localized, we hypothesize that the magnitude of any supply-limitation experienced downstream is predominantly a function of both dam height and the structure’s propensity to become drowned out under high flows.