Hydraulic and geomorphological effects of run-of-river dams

Abstract

The practice of dam removal has received increasing attention as a consequence of maintenance and liability concerns related to the advanced age of many of these structures. Most dams that have been removed thus far are small run-of-river structures. As the number of removals of run-of-river dams increases, it is crucial to understand the effects that these structures have on river geomorphology and sedimentology while in place and how rivers respond to removals so that possible responses to future removals can be anticipated and predicted. This paper reviews current knowledge related to the influence of run-of-river dams on the hydraulics and geomorphology of rivers and suggests types of studies that need to be undertaken to address gaps in current knowledge. Compared to studies of large impoundment dams, field investigations of channel morphology and sedimentology upstream and downstream of run-of-river dams are few and limited in geographic scope. Available studies indicate that the response of rivers to the long-term existence of run-of-river dams is variable both in terms of upstream sediment storage and downstream channel erosion. Future research should focus on how geomorphological responses of rivers to run-of-river dams vary with geographical context and on integration of process-based field studies, numerical modeling and experimental investigations to determine the influence of these dams on flow structure, sediment transport, and patterns of channel erosion and deposition.

Run-of-River hydropower and salmonids: potential effects and perspective on future research

“…The creation of headponds inundates riparian areas and fundamentally interrupts the NFR in reaches immediately upstream of low-head dams by reducing flow variability, velocity, and turbulence and increasing the deposition of fine sediment (Csiki and Rhoads 2010;Butler and Wahl 2011). Such changes in physical habitats can lead to impacts on riverine ecosystems by reducing water quality and altering the abundance, richness, and composition of periphyton, invertebrate, and fish assemblages (Santucci et al 2005;Mueller et al 2011;Anderson et al 2014).…”

Ship‐lock–induced Surges in an Impounded River and Their Impact on Subdaily Flow Velocity Variation

“…In comparison with preimpounded conditions, the increase in water volume leads to reduced flow velocities and increasing residence times (Friedl and Wüest, 2002). This has several implications, including direct changes in the sediment dynamics (Verstraeten and Poesen, 2000;Csiki and Rhoads, 2010) as well as indirect effects on water quality due to changing dissolved oxygen and nutrient concentrations and turnover rates (Friedl and Wüest, 2002;Santucci et al, 2005;McGinnis et al, 2006) and changing habitat conditions for organisms (Junk et al, 1989;Biggs et al, 2005).…”

Planning and implementing small dam removals: lessons learned from dam removals across the eastern United States

“…ICF Consulting (2005) defined small dams as structures not exceeding 50 feet, run-of-river dams as structures where river inflow flows over the entire barrier across the entire waterway and has limited storage capacity, and defined low-head dams as structures where the hydraulic head (head water to tail water) is less than 25 feet. Csiki and Rhoads (2010) emphasized run-of-river dams have little storage capacity and result in impounded water contained within the banks of the natural bed. Tschantz and Wright (2011) described low-head dams as constructed to allow the flow to pass directly over the entire dam structure, as generally 3-5 m in height, and constructed to raise water level for industrial and municipal water supplies as well as for recreation.…”

“…For many historic low-head dams constructed as mill dams throughout the eastern U.S., dam height can exceed the channel banks upstream. As a result, mill dams can disrupt flow across an entire valley bottom, leading to sediment deposition throughout the historic floodplain, and, therefore, having different sediment dynamics than run-of-river dams (Csiki and Rhoads 2010).…”

“…This recommendation is in contrast to a sediment management discussion from an ecological perspective in Bednarek (2001) Brune (1953) estimated sediment trapping to be 75-100 % for storage capacity [10 % of annual flow, while sediment trapped is estimated to be 30-55 % when a reservoir stores 1 % of average annual flow. However, Csiki and Rhoads (2010) note that little is known about sediment trapping in run-of-river dams. Because run-of-river dams generally have low hydraulic residence times, they do not necessarily trap significant sediment.…”