Article

The Evolution of Gravel Bed Channels After Dam Removal: Case Study of the Anaconda and Union City Dam Removals

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The Evolution of Gravel Bed Channels After Dam Removal: Case Study of the Anaconda and Union City Dam Removals

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Abstract

The Anaconda and Union City Dams on the Naugatuck River in Connecticut were removed in February and October 1999. A detailed study of the sites prior to removal was undertaken including sediment testing and predictions of upstream channel formation post-dam removal. The 3.35-m-high timber crib/rock fill spillway of the Anaconda Dam partially breached during a storm prior to the dam's scheduled removal allowing a portion of the impounded sediment to move down through the river system. This event changed the removal plans and the remainder of the spillway was removed under an emergency order in the course of 4 days. The Union City Dam, a 2.44-m-high timber crib/rock fill dam capped with concrete and stone, was removed on schedule. A portion of the impounded sediment was removed by mechanical means during the deconstruction of the structure. The evolution of the two upstream channels post-project provided unique challenges and valuable insights as to what kind of channel transition can be expected in gravel bed river systems after a low head dam has been removed. This paper describes the initial engineering analysis and design, the subsequent removal of the two dams, and compares observations on the transition of the upstream channels following dam removal to the initial engineering predictions and other models. The relatively steep gravel bed channels evolved in a predictable manner, except where anthropogenic barriers (sanitary sewer, rock weir) interrupted.

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... The extent to which high flow periods can re-suspend and transport sediment past the dam will depend on the system hydraulics and sediment characteristics. Observations of run-ofriver dams in various geomorphologies have found varied sediment trapping patterns including: minimal sediment accumulation upstream of the dam Csiki and Rhoads 2014;Lindloff 2003;Roberts et al. 2007), as well as sediment accumulation in the impoundment (Wildman and MacBroom 2005;Orr and Koenig 2006) or sediment accumulation further upstream (Cheng and Granata 2007). Sawaske and Freyberg (2012) compared sediment dynamics among 12 small dam removals from highly sediment-impacted systems across the northern U.S. ...
... Applying the TEL/PEL classification system, Ashley et al. (2006) likewise found removal of the Manatawny Creek Dam did not significantly redistribute sediment contaminated with polycyclic aromatic hydrocarbons (PAHs), PCBs, and heavy metals. In contrast, sediment analysis of the Naugatuck River revealed contamination with polycyclic aromatic hydrocarbon compounds (PARH); the sediment was removed to a landfill prior to the Union City Dam removal (Wildman and MacBroom 2005). Cantwell et al. (2014) demonstrated that passive samplers are effective for measuring dissolved organic contaminants, which was established in comparison to observations from sediment traps. ...
... Low sediment transport. Impoundment bed gravel and sandy pebbly sediment, estimated transport \1 % Sandusky River, OH Cheng 2007 See references for study details: Burroughs et al. (2009, Chaplin et al. (2005), Cheng and Granata (2007), Doyle et al. (2005), Rumschlag and Peck (2007), Wildman and MacBroom (2005) Tools for predicting channel evolution Predicting channel evolution is a critical step in understanding the risk from dam removal. Estimating sediment volume movement is important for understanding how post-dam hydrology will influence sediment transport potential, which determines contamination potential, changes to stream habitat, and sediment buildup near stream infrastructure such as water intakes. ...
Article
Full-text available
We review and build on a growing literature assessing small dam removal outcomes to inform future dam removal planning. Small dams that have exceeded their expected duration of operation and are no longer being maintained are at risk of breach. The past two decades have seen a number of small dam removals, though many removals remain unstudied and poorly documented. We summarize socio-economic and biophysical lessons learned during the past two decades of accelerated activity regarding small dam removals throughout the United States. We present frameworks for planning and implementing removals developed by interdisciplinary engagement. Toward the goal of achieving thorough dam removal planning, we present outcomes from well-documented small dam removals covering ecological, chemical, and physical change in rivers post-dam removal, including field observation and modeling methodologies. Guiding principles of a dam removal process should include: (1) stakeholder engagement to navigate the complexity of watershed landuse, (2) an impacts assessment to inform the planning process, (3) pre- and post-dam removal observations of ecological, chemical and physical properties, (4) the expectation that there are short- and long-term ecological dynamics with population recovery depending on whether dam impacts were largely related to dispersion or to habitat destruction, (5) an expectation that changes in watershed chemistry are dependent on sediment type, sediment transport and watershed landuse, and 6) rigorous assessment of physical changes resulting from dam removal, understanding that alteration in hydrologic flows, sediment transport, and channel evolution will shape ecological and chemical dynamics, and shape how stakeholders engage with the watershed.
... Pizzuto concluded that sediment fills are likely to erode even during low flows if the fills are composed of sand or cohesive silt and clay. More recent examples with the same or similar results in upstream conditions observed by Pizzuto (2002) include the works of Wildman and MacBroom (2005), Wilcox et al. (2014), and Randle et al. (2015). ...
... A study by Kibler et al. (2011) on a small gravel-filled dam (>5 m) found that post-dam removal channel adjustment can vary as a result of the presence of diverse sediment size in the reservoir bed. In a cohesive sediment bank channel evolution model, Wildman and MacBroom (2005) found that incision steepens the banks to a critical threshold, which leads to mass failure, aggrading the bed with substantial sediment flux. Under a coarse-grained situation, bank failure occurred at a lower critical height, resulting in less sediment removal (Wildman and MacBroom, 2005). ...
... In a cohesive sediment bank channel evolution model, Wildman and MacBroom (2005) found that incision steepens the banks to a critical threshold, which leads to mass failure, aggrading the bed with substantial sediment flux. Under a coarse-grained situation, bank failure occurred at a lower critical height, resulting in less sediment removal (Wildman and MacBroom, 2005). This indicates that for small dams of varying sizes (about 4-15 m), sediment load and post-dam removal stream channel adjustment is site specific and is very dependent on local site conditions (Tullos et al., 2014). ...
Article
Post-dam removal geomorphologic adjustment of a stream channel has been documented in the scientific literature at watershed, hillslope, and laboratory scales. Hillslope-scale studies in channel cross sections are most common and add significant value in the dam-removal literature. This study examines geomorphic stream channel adjustment following dam removal at the hillslope scale under natural climatic conditions. A sedimentfilled silt fence dam (1 m tall, 12.65 m wide) was removed in three stages, and the width and depth of the upstream developing channel was monitored at six transects for 15 months. Headcut retreat and changes in channel sinuosity were also recorded. After the silt fence dam was removed, channel development was initiated by headcut formation, which migrated upstream at a rate of 4 cm/d for about 10 months and then gradually reached attenuation. The channel progressed through four distinct stages: Stage 1 (Initial conditions); Stage 2 (Downcutting)-wide, shallow, meandering channel incised to a maximum depth of 0.52 m, and sinuosity decreased; Stage 3 (Floodplain development)-upon reaching base level, surface runoff began to meander within the channel, widening it through bank slumps and erosion; and Stage 4 (Quasi-equilibrium)-channel development reached dynamic (quasi-) equilibrium with only minor widening at downstream transects (maximum width of the incised channel reached 0.46 m), accompanied by sediment aggradation. The stages of upstream channel development and headcut retreat pattern in this study are consistent with the findings of other studies at the laboratory and watershed scales, indicating that channel development after dam removal is scale independent.
... Perhaps the most common occurrence of sediment-wave evolution is during river restoration projects involving the removal of flow barriers. Although previous studies have shown that river response to flow-barrier removal varies considerably, a common response is the incision of the sediment wave behind the structure (Bednarek, 2001;Pizzuto, 2002;Doyle et al., 2003;Ahearn and Dahlgren, 2005;Wildman and MacBroom, 2005;Burroughs et al., 2009;Im et al., 2011). The particle-size distribution of the sediment wave has been found to be important in predicting how the fill is likely to evolve once the barrier is removed. ...
... The release of fine-grained material from the sediment wave following flow-barrier removal has been documented in instigating fine-grained sedimentation downstream, damaging habitat or increasing flood risk by reducing flow conveyance (Bednarek, 2001;Hart et al., 2002;Doyle et al., 2003;Ahearn and Dahlgren, 2005;Im et al., 2011). Generally, however, such impacts only last for short durations with biodiversity often improving in the vicinity of numerous barrier-removal studies (Bednarek, 2001;Wildman and MacBroom, 2005;Burroughs et al., 2009;Im et al, 2011). ...
... The similarity of our findings with the previous work (e.g. Pizzuto, 2002;Wildman and MacBroom, 2005;Burroughs et al., 2009) suggests that rapid channel widening because of bed-material wave dispersion (A) Sediment aggradation is apparent both upstream and downstream of the wave as its apex decreases in amplitude with time; this is a fully dispersive wave. (B) The wave migrates in its entirety along the bed retaining both its wavelength and amplitude; this is a fully translative can be expected within the many British gravel-bed rivers that are scheduled for flow-barrier removal unless riverbanks are adequately reinforced. ...
Article
Kentchurch Weir, a low-head weir on the river Monnow, Wales, was demolished in August 2011, releasing a sediment wave that had formed behind the structure for at least a century. We surveyed channel topography and bed-material composition through a 1.5-km long reach prior to weir removal and then periodically over a 2-year period. The fill material was finer than the ambient bed material with all particles mobilized by bankfull flows. Rapid degradation of the 1460-m3 sediment fill in the previously impounded reach occurred as bed material appeared to disperse downstream, consistent with other studies of sediment waves in gravel-bed rivers. The riverbed profile was gradually smoothed through the study reach by degrading the elevated fill as a migrating knickpoint and aggrading the channel bed and bars immediately downstream of the former weir location. Extensive bank erosion was evident in the previously impounded reach with up to 10 m of widening following a single flow event, increasing channel width by more than 20%. Mitigation measures to enforce the riverbanks have been required as the gradual dispersion of the sediment wave continues to force flow diversion towards the riverbanks. The evolution of sediment stores behind flow obstructions follows that of sediment waves and theory available to describe wave evolution should do much to improve management efforts that seek to minimize channel widening following weir removal. Copyright © 2014 John Wiley & Sons, Ltd.
... American Association of State Highway and Transportation Official (AASHTO) (2005) Run-of-river dam Run-of-river structures are dams that create reservoirs with small storage capacity and do not alter the river's flow regime Stanley and Doyle (2002) A constructed barrier in a river where the river inflow normally overflows from behind the dam from one side of the waterway to the other. A runof-river dam has limited short-term storage capacity American Association of State Highway and Transportation Official (AASHTO) (2005) Run-of-the-River dam is a manmade structure which is built across a river or stream for the purposes of impounding water where the impoundment at normal flow levels is completely within the banks and all flow passes directly over the entire dam structure within the banks, excluding abutments, to a natural channel downstream Geomorphological and riparian vegetation responses following a low-head dam removal 3 to bank collapse (Wildman and MacBroom 2005). Particularly, a flood plain development or extension has been often shown in several years after removal. ...
... Flow regime may be restored with slight changes by adjusted river geomorphology Sediment diameter can be restored or coarser than pre-dam condition (e.g.Doyle et al. 2003b;Ahn et al. 2012). Altered flow regime and sediment diameter can cause some change for transport rate (e.g.Wildman and Macbroom 2005; Burrough et al. 2009) Water quality can be restored without retention of flow. Physical habitat can be improved similar to pre-dam condition with increasing diversity of flow structures (e.g.Im et al. 2011). ...
... Reconnected longitudinal corridor can improve fish movement (e.g.Ahn et al. 2012) Bed slope can be restored with slight changes (e.g.Burrough et al. 2009). Channel shape and sand/ gravel bar can be newly formed by the channel evolution following a low-head dam removal (e.g.Pizzuto 2002;Doyle et al. 2003b;Wildman and Macbroom 2005) ...
Article
More and more low-head dam structures have deteriorated in recent years. Unlike the large dam, the geomorphological and ecological impacts of a low-head dam removal have not been quantified with insufficient monitoring data on pre- and post-removal. Therefore, this study intends to identify the low-head dam removal impacts on geomorphology and riparian vegetation based on previous studies. The characteristics of stored sediment in the impoundment play a critical role for geomorphological responses on a low-head dam removal creating a knickpoint and promoting a headcut migration. These geomorphological changes often form a new floodplain and create enough room for riparian vegetation establishment. The river geomorphology after a low-head dam removal can be a state of quasi-equilibrium within about a decade. In this state, a newly formed floodplain tends to be colonized by riparian vegetation in many low-head dam removal cases. After a decade to several decades, the riparian vegetation in the floodplain often develops to tree plants.
... Despite an emerging conceptual basis for the effects of dam removals, this field continues to lack the empirical information that is needed to verify these hypotheses, calibrate preexisting models for use with dam removal, and generate novel insights into the effects of dam removal (Bushaw-Newton et al., 2002;Doyle et al., 2002;Graf, 2003;Hart et al., 2003). Qualitative observations on the effects of dam removal exist for several dam removal case studies (American Rivers et al., 1999;Smith et al., 2000), and several quantitative studies exist on the effects of dam removal on fluvial geomorphology (Evans et al., 2000;Wohl and Cenderelli, 2000;Bushaw-Newton et al., 2002;Chaplin, 2003;Wildman and MacBroom, 2005), aquatic insects (Bushaw-Newton et al., 2002;, and fish (Hill et al., 1994;Kanehl et al., 1997;Bushaw-Newton et al., 2002). While these studies provide unique insights into the outcomes of dam removals, many were relatively short in time duration (i.e., 1-2 years post-dam removal), and the empirical information on the effects of dam removal is still very limited (e.g., Graf, 2003;Doyle et al., 2005). ...
... As sediments were eroded adjacent to the dam and transported downstream, the process of sediment fill incision progressed upstream. Our onsite experience suggested that most of the sediment fill incision occurred rapidly, within hours to days, similar to observations by Wildman and MacBroom (2005). Each stage in the removal process pushed sediment fill incision progressively farther upstream, eventually reaching the upstream boundary of the original impoundment. ...
... Similarly small percentages of reservoir sediment fill mobilization were found by Evans et al. (2000) in an Ohio dam failure (9-13%) and Doyle et al. (2003) in a Wisconsin dam removal (8-14%). The amount of sediment to be mobilized with a dam removal is an important consideration during the planning and decision making process in removing dams Randle, 2003;Rathburn andWohl, 2003, Wildman andMacBroom, 2005). In situations where sediment transport downstream from dam removals is undesirable, sediment removal or management can add considerably to the expense of removing dams. ...
Article
Although dam removal has been increasingly used as an option in dam management, and as a river restoration tool, few studies provide detailed quantitative assessment of the geomorphological response of rivers to dam removal. In this study, we document the response of the Pine River, Michigan, to the gradual removal of Stronach Dam. In 1996, prior to the initiation of removal, 31 permanent cross-sectional transects were established in the 10-km study area. These transects were surveyed annually during the course of the removal (1996–2003) and for the three years following removal (2004–2006). Dam removal resulted in progressive headcutting of sediments in the former impoundment, extending upstream 3.89 km of the dam. Over the course of the 10 years since dam removal was initiated, a net total of 92 000 m3 of sediment erosion occurred. The majority of sediments stored in the former reservoir remained in place, with only 12% of the estimated reservoir sediment fill being eroded. Approximately 14% of the net erosion was deposited within the stream channel 1 km downstream of the dam location, with the remainder being transported further downstream or deposited in the floodplain. Sediment fill incision resulted in a narrower and deeper channel upstream, with higher mean water velocity and somewhat coarser substrates. Downstream deposition resulted in a wider and shallower channel, with little change in substrate size composition. Counter-intuitively, water velocity also increased downstream because of the increased slope that developed. Prior to removal, bedforms in the former impoundment were dominated by runs but are showing signs of restoration toward reference conditions. Continuing changes in river geomorphology are evident even three years following removal and are likely to occur for years to come.
... Pizzuto concluded that sediment fills are likely to erode even during low flows if the fills are composed of sand or cohesive silt and clay. More recent examples with the same or similar results in upstream conditions observed by Pizzuto (2002) include the works of Wildman and MacBroom (2005), Wilcox et al. (2014), and Randle et al. (2015). ...
... A study by Kibler et al. (2011) on a small gravel-filled dam (>5 m) found that post-dam removal channel adjustment can vary as a result of the presence of diverse sediment size in the reservoir bed. In a cohesive sediment bank channel evolution model, Wildman and MacBroom (2005) found that incision steepens the banks to a critical threshold, which leads to mass failure, aggrading the bed with substantial sediment flux. Under a coarse-grained situation, bank failure occurred at a lower critical height, resulting in less sediment removal (Wildman and MacBroom, 2005). ...
... In a cohesive sediment bank channel evolution model, Wildman and MacBroom (2005) found that incision steepens the banks to a critical threshold, which leads to mass failure, aggrading the bed with substantial sediment flux. Under a coarse-grained situation, bank failure occurred at a lower critical height, resulting in less sediment removal (Wildman and MacBroom, 2005). This indicates that for small dams of varying sizes (about 4-15 m), sediment load and post-dam removal stream channel adjustment is site specific and is very dependent on local site conditions (Tullos et al., 2014). ...
Article
Full-text available
Post-dam removal geomorphologic adjustment of a stream channel has been documented in the scientific literature at watershed, hillslope and laboratory scales. Hillslope scale studies in channel-cross-sections are most common, and add significant value in the dam-removal literature. This study examines geomorphic stream channel adjustment following dam removal at the hillslope scale under natural climatic conditions. A sediment-filled silt fence dam (1 m tall, 12.65 m wide) was removed in three stages and the width and depth of the upstream developing channel was monitored at six transects for fifteen months. Headcut retreat and changes in channel sinuosity were also recorded. After the silt fence dam was removed, channel development was initiated by headcut formation which migrated upstream at a rate of 4 cm/day for about ten months and then gradually reached attenuation. The channel progressed through four distinct stages: Stage 1 [Initial conditions]; Stage 2 [Downcutting] -wide, shallow, meandering channel incised to a maximum depth of 0.52 m, and sinuosity decreased; Stage 3 [Floodplain development] -upon reaching base level, surface runoff began to meander within the channel, widening it through bank slumps and erosion; Stage 4 [Quasi-equilibrium] -channel development reached dynamic (quasi) equilibrium with only minor widening at downstream transects [maximum width of the incised channel reached 0.46 m] accompanied by sediment aggradation. The stages of upstream channel development and headcut retreat pattern in this study are consistent with other studies at the laboratory and watershed scales indicating that channel development after dam removal is scale independent.
... Various studies have empirically supported this model in its broad outlines (e.g., Burroughs et al. 2009;Cannatelli et al. 2004;Cheng and Granata 2007;Draut et al. 2011;Evans 2007;MacBroom 2009;Major et al. 2008;Major et al. 2012;Neave et al. 2009;Rumschlag and Peck 2007;Sawaske and Freyberg 2012;Wildman and MacBroom 2005). Not all channel responses to dam removal will perfectly fit Doyle et al.'s model as local environmental conditions, the amount and character of sediments contained in reservoirs, and the method chosen for dam removal vary by river. ...
... Morphological responses can also be influenced by sediment influxes, local geology, and the amount of large woody debris situated in the channel (Draut et al. 2011). Because alluvial channel geometry is controlled by sediment grain size and caliber, this can also influence adjustment pathways (Wildman and MacBroom 2005). Evans (2007) revised Doyle et al.'s CEM based on channel response following the failure of the IVEX Dam. ...
Article
Over the past 30 years, fluvial geomorphologists have sought to understand the historical trajectory of channel adjustments after disturbance by developing channel evolution models (CEMs). These models use a combination of quantitative data and qualitative indicators to describe the spatial progression of channel evolution. This article surveys the historical development of CEMs, how they function, and what types of CEMs are best suited for different geomorphic settings. It describes the logic underlying classic CEMs, and their subsequent application to predict channel response following dam removal. After this, it recounts the emergence of multi‐pathway CEMs, which have been used to capture the evolutionary trajectories in complex fluvial systems (e.g., anastomosing rivers). Lastly, it argues that a state‐and‐transition framework offers an appropriate modeling template to document channel evolution in a wide variety of fluvial systems, including those originally modeled using classic CEMs. A state‐and‐transition approach sees rivers as composed of interrelated process and form (morphological) states. They identify under what conditions a river channel would undergo a transition from one formal state to another (e.g., from a single‐threaded to braided planform). They can also be used to understand the form‐process dynamics underlying subtle morphological transitions that produce changes in a river's biogeomorphological structure without triggering wholesale river metamorphosis.
... Downstream of dams, intense scour can occur at the base of structures as the stream reach degrades in order to re-establish sediment load through erosion of bed and bank materials (Bollaert and Schleiss, 2003; Doyle and Harbor, 2003). The river response to RORD removal depends upon the amount of sediment stored behind the impoundment, which can range from negligible to complete infilling (e.g. Wildman and MacBroom, 2005; *Correspondence to: K. H. Costigan, School of Environment and Natural Resources, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Ave, Wooster, Ohio 44691, USA. E-mail: Costigan.12@osu.edu; ...
... Draut et al., 2011; Brenkman et al., 2012) and post-dam conditions (e.g. Doyle et al., 2003b; Wildman and MacBroom, 2005; Major et al., 2008; Pearson et al., 2011) of large dam removal projects. However, much less is known about pre-dam conditions in general or what the effects of removing a RORD might have on a sand-bed river. ...
Article
Run-of-the-river dams (RORDs) comprise the vast majority of dams on river systems and are commonly removed as a part of stream restoration strategies. Although these dams are routinely removed, few studies have documented the geomorphological responses of sand-bed rivers to the removal of RORDs. We examined the response of a large sand-bed river located in South-Central Kansas, USA, to the installation and removal of a dam that is installed annually for seasonal recreational purposes. Channel adjustments were tracked using cross-sections sampled over the course of 7 months as the dam was installed and subsequently removed. Multivariate spatiotemporal analysis revealed emergence of channel stability when the dam was in place for most cross-sections, except for those immediately adjacent to or at great distances from the dam. Our results provide an approximation for how sand-bed rivers respond to RORD construction and removal and are useful for guiding management decisions involving preservation or restoration of connectivity. Results of this study suggest that sand-bed rivers are resilient and recover quickly when transient RORDs are removed. Copyright © 2014 John Wiley & Sons, Ltd.
... Evaluating the impacts of in-stream structures on geomorphic dynamics along European rivers as part of river restoration efforts has further evolved as a legal requirement under the European Water Framework Directive (EU, 2000), and is one of the major issues in river basin management plans, such as in the Danube River Basin (ICPDR, 2009). One measure to restore river continuity and channel morphology which has become increasingly popular in recent decades is dam removal (Bushaw-Newton et al., 2002;Wildman and MacBroom, 2005;Burroughs et al., 2009;Kibler et al., 2011;Tullos et al., 2014). The commonly-held view of geomorphic river recovery after dam removal is that the channel will again incise and remove the sediment accumulated in the reservoir, and in this way restore pre-dam conditions (Poff and Hart, 2002). ...
... A, B and E sites exhibited an up-to-downstream increase in channel depth indicating downstream incision. At category C sites we observed an up-to-downstream decrease in channel depth, indicating upstream incision after dam removal followed by the presence of a deeply incised channel in the upstream reaches (Wildman and MacBroom, 2005;Burroughs et al., 2009). However, at many other locations geomorphic responses to dams and their removal were more complex as they are governed by dam interactions and channel protection measures as well as related legacy effects on river sediment dynamics. ...
Article
Full-text available
Dams represent one of the most dominant forms of human impact upon fluvial systems during the Anthropocene, as they disrupt the downstream transfer of water and sediments. Removing dams restores river continuity and channel morphology. Both dam construction and dam removal induce geomorphic channel responses that often require the installation of channel protection structures. Although such measures are well-established in river engineering, little is known about their interactions or legacy effects on river sediment dynamics, channel morphology and riverine habitats. This study investigated the legacy effects of small dams and their removal on bed sediment and channel morphology in two small mixed-load streams in Austria using field mapping and DEM-based geomorphometric channel analyses. At active dams, results showed increases in channel slope (1.76-13.88%) and depth (0.1-2. m) as well as in dominant bed sediment grain size. At some dam removal sites without channel protection structures, we observed balanced channel slope conditions and decreases in channel depth (0.5-1.9. m) as well as homogeneous bed sediment textures. At sites exhibiting channel engineering, bed and bank protection structures inhibited geomorphic response to dam removal, thereby preserving dam-induced channel conditions. However, at numerous locations geomorphic responses to dams and their removal were observed to be more complex as they are governed by dam interactions and feedback processes that are further influenced by the proximity of other dams and related hydro-geomorphic conditions.
... The new CEM, referred to as the Doyle CEM in this paper, represented an important step forward in understanding the geomorphic effects of dam removal, but site-specific characteristics of the Wisconsin dams have prevented its broad application. The rivers used to develop the Doyle CEM were dominated by sand size sediments, prompting modifications for a dam removal CEM specific to gravel bed channels (Wildman and MacBroom 2005). An upstream migrating headcut has been documented as a primary feature of channel formation following dam removals in which the reservoir was confined laterally and channel formation was primarily longitudinal (Doyle et al. 2002(Doyle et al. , 2003aPizzuto 2002). ...
... The similarity of findings in Wisconsin and Virginia illustrates the need to modify CEMs to explicitly incorporate area hydrology. The Doyle CEM was modified to incorporate seasonal hydrology and vegetation growth in the process of channel evolution toward quasi-equilibrium, while also considering the previous CEM modifications and the channel evolution data (Pearson et al. 2011;Wildman and MacBroom 2005;Evans 2007;Straub 2007). The slow draining nature of the reservoir allowed for measurements of channel evaluation at cross sections emerging under different seasonal flows [denoted by α, β, δ, or γ in Fig. 8(h)]. ...
Article
A slow draining reservoir on the U.S. East Coast was monitored to identify the processes governing channel evolution upstream of a dam removal. Channel evolution was documented through cross section surveys, sediment size analysis, discharge measurements, and visual assessments of vegetative growth. The reservoir drained slowly, allowing for an analysis of channel evolution and identification of the morphometric parameters defining the path and time required for a channel to reach dynamic equilibrium. Channel evolution was a multidirectional process, and evolving channel reaches actively migrated laterally while alternating between aggradation and degradation. Channel formation was dominated primarily by the hydrologic regime at the time of dam removal and secondarily by the ability of vegetation to establish and stabilize the channel form. The importance of seasonal site hydrology over the minimum time required for channel evolution and the process through which the channel evolves indicates the complexity of channel formation within the first year following dam removal. Existing channel evolution models (CEMs) were modified and a new CEM that explicitly incorporates local hydrology and vegetative growth in the channel evolution process is presented. The modified CEM is applied to dam removals in Illinois, Virginia, and New Hampshire to illustrate its application beyond the immediate study area. In all cases, the seasonal flows were a dominant factor over the time frame and process of channel evolution. The CEM can be used to improve predictions of the length of time needed for a channel to evolve and the magnitude of channel change during the evolutionary process, such that it can be used to aid in planning and facilitating dam removals in similar regions. DOI: 10.1061/(ASCE)HY.1943- 7900.0000526. (C) 2012 American Society of Civil Engineers.
... Understanding outcomes of dam removals has a broader context than simply helping dam owners and stakeholders make decisions. Better knowledge of how river systems respond to sediment pulses introduced by dam removals informs assessments of hazards from natural dam failures; sediment injections from natural processes such as landslides (Hoffman and Gabet 2007), wildfires (Shakesby and Doerr 2006), and volcanic eruptions Doyle, Stanley, and Harbor (2003) Anaconda (CT) Naugatuck 3.3 11 900 Sand, gravel Sudden Wildman and MacBroom (2005) Stronach (MI) Pine 3.6-5 800 000 70% sand, 30% gravel Phased Burroughs et al. (2009) Munroe Falls (OH) Cuyahoga 3.7 Uncertain 70% sand, 20% mud, 10% gravel Phased Rumschlag and Peck (2007); Peck and Kasper (2013) Homestead (NH) Ashuelot 4 Uncertain Sand Sudden Gartner, Magilligan, and Renshaw (2015) Merrimack Village (NH) Souhegan 4 62 000 95% sand Sudden Pearson, Snyder, and Collins (2011); Santaniello, Snyder, and Gontz (2013) IVEX (OH) Chagrin 7.4 236 000 Mud Sudden Evans et al. (2000); Evans (2007) Savage Rapids (OR) Rogue 12 150 000 70% sand, 30% gravel Sudden Bountry, Lai, and Randle (2013) Milltown (MT) Clark Fork 12.8 5.5 million ‡ Sand Sudden § Evans and Wilcox (2014) Marmot (OR) Sandy 15 750 000~50% gravel, 50% sand Sudden Major et al. (2012) Elwha (WA) Elwha 32 4.9 million 47% mud, 53% sand and gravel Phased Randle et al. (2015); Warrick et al. (2015) Condit (WA) White Salmon 38 1.8 million 60% sand, 35% mud, 5% gravel Sudden Wilcox, O'Connor, and Major (2014); Colaiacomo (2014) Barlin ( (Pierson and Major 2014); behavior of legacy sediment (James 2013;Merritts et al. 2013); and restoration activities such as gravel augmentation (Sklar et al. 2009). More than 87 000 dams taller than 1.8 m have been catalogued in the United States (Figure 13.1; US Army Corps of Engineers 2013), and more than 1100 have been removed intentionallymost since 2000 (American Rivers 2014). ...
... Reservoir geometry can affect rates and magnitudes of sediment evacuation. In several reservoirs having small aspect ratios (width/length) or in those where lateral erosion could swing unobstructed, erosion rapidly evacuated large percentages (≥40%) of impounded sediment after full base-level fall was achieved (Figure 13.3) (e.g., Wildman and MacBroom 2005;Epstein 2009;Pearson, Snyder, and Collins 2011;Major et al. 2012;Sawaske and Freyberg 2012;Bountry, Lai, and Randle 2013;Wilcox, O'Connor, and Major 2014;Collins et al. 2014;Warrick et al. 2015). By contrast, for reservoirs in which lateral erosion and landslides were limited, transport capacity diminished longitudinally, or the rate of base-level fall was strongly moderated, the rate and quantity of sediment evacuation were also limited (e.g., Evans et al. 2000;Doyle, Stanley, and Harbor 2003;Cheng and Granata 2007;Straub 2007;Burroughs et al. 2009). ...
... Reservoirs may contain toxic substances, and the release of contaminants in the accumulated sediment are a major concern for local communities [Wildman and MacBroom, 2005], especially after the calamitous 1973 removal of Fort Edward Dam on the Hudson River released PCB-contaminated sediment [Shuman, 1995]. Because many reservoirs were created decades or centuries ago, neither the original topography nor the composition of accumulated materials may be well known. ...
... Former rapids, bedrock outcrops, older dams, and other preexisting structures may exist beneath reservoir sediment [Harris and Evans, 2014;Wilcox et al., 2014;Gartner et al., 2015]. Besides potentially adding to materials being transported downstream and creating hazards, such structures, if exposed, could affect upstream/downstream connectivity and modify reservoir responses to dam removal [Wildman and MacBroom, 2005 [Evans et al., 2013]. This may slow reservoir sediment erosion Zunka et al., 2015] and inhibit fish passage and navigation. ...
Article
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Dam removal is widely used as an approach for river restoration in the United States. The increase in dam removals—particularly large dams—and associated dam-removal studies over the last few decades motivated a working group at the USGS John Wesley Powell Center for Analysis and Synthesis to review and synthesize available studies of dam removals and their findings. Based on dam removals thus far, some general conclusions have emerged: (1) physical responses are typically fast, with the rate of sediment erosion largely dependent on sediment characteristics and dam-removal strategy; (2) ecological responses to dam removal differ among the affected upstream, downstream, and reservoir reaches; (3) dam removal tends to quickly reestablish connectivity, restoring the movement of material and organisms between upstream and downstream river reaches; (4) geographic context, river history, and land use significantly influence river restoration trajectories and recovery potential because they control broader physical and ecological processes and conditions; and (5) quantitative modeling capability is improving, particularly for physical and broad-scale ecological effects, and gives managers information needed to understand and predict long-term effects of dam removal on riverine ecosystems. Although these studies collectively enhance our understanding of how riverine ecosystems respond to dam removal, knowledge gaps remain because most studies have been short (< 5 years) and do not adequately represent the diversity of dam types, watershed conditions, and dam-removal methods in the U.S.
... In some cases, partial or complete infilling by fine sediment occurs behind the dams, whereas in others minimal sedimentation can be detected and the bed material upstream is relatively coarse. For example, little or no sediment accumulation was observed upstream of a dam in New Hampshire prior to removal (Lindloff, 2003), while up to 1 m of accumulation was noted upstream of a dam in Connecticut (Wildman and MacBroom, 2005). The dam in Connecticut was 2 m high, and the quantity of sediment stored within the upstream pool was less than originally expected for a dam of that height. ...
... Field studies have documented upstream migrating knickpoints following removals of dams in Connecticut, Pennsylvania, and Wisconsin (Pizzuto, 2002;Wildman and MacBroom, 2005;. At the site of St. John's Dam (Sandusky River, Ohio), degradation progressed to 3 km upstream of the dam following removal. ...
Article
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.
... When the headcut propagated upstream across the observation region through the lowest part near the left wall, the unconfined flow gradually gathered in the low-elevation areas of the initial fluctuated bed during drawdown, forming an anabranching pattern. The narrowed corridor of flow was incised as scour paths (Wildman and MacBroom 2005), and other unchanged area was exposed upon water surface as scattered bars (Fig. 6b). Because of the big difference of bed elevation between the scour path of headcut migration and other scour paths, the erosion rate had a correspondingly huge gap. ...
... The formation of bars by headcut has been observed in the natural fluvial rivers. Wildman and MacBroom (2005) found that bars formed upstream of the removal dam as a consequence of the rapid water level drop and the accompanying headcut in the field study. The upstream migration of headcut followed the path of least resistance and created an incised channel on the impounded sediment with defined thalweg, making other area free from the incision outcrop as bars. ...
Article
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Bars are widely thought to be large sediment bodies formed under sediment supply. Nevertheless, little has been known on how they form in sediment scouring processes. In this study, we carried out a flume experiment to study the formation of bars without sediment supply. The experiment was divided into two stages. In the first stage, the discharge was successively increased; in each discharge step, the disturbance from upstream and downstream boundaries had little influence on the flow. We observed that no bars formed in this stage. In the second stage, we kept a small discharge at the flume inlet. In this stage, the bars emerged from the fluctuated bed topography with millimeter-scale bed forms by headcut initiated from the outlet. As the headcut migrated upstream, the accompanying undercut gradually forced the unconfined flow run into the low-elevation zone, lowering the water level and inducing the outcrop of regions free from the incision (i.e., bars). At the end of the experiment, a relatively stable topography formed under the joint effect of the upstream migrating headcut, the following undercut after the headcut and the lateral erosion on the emerged bars. The requirements for the formation of bars and the distinctive characteristics of the bars induced by headcut were investigated. This study shows that bars can form in a scouring process under appropriate conditions, and the headcut may be one of the precipitating factors for the formation of bars in natural rivers.
... Understanding outcomes of dam removals has a broader context than simply helping dam owners and stakeholders make decisions. Better knowledge of how river systems respond to sediment pulses introduced by dam removals informs assessments of hazards from natural dam failures; sediment injections from natural processes such as landslides (Hoffman and Gabet 2007), wildfires (Shakesby and Doerr 2006), and volcanic eruptions Doyle, Stanley, and Harbor (2003) Anaconda (CT) Naugatuck 3.3 11 900 Sand, gravel Sudden Wildman and MacBroom (2005) Stronach (MI) Pine 3.6-5 800 000 70% sand, 30% gravel Phased Burroughs et al. (2009) Munroe Falls (OH) Cuyahoga 3.7 Uncertain 70% sand, 20% mud, 10% gravel Phased Rumschlag and Peck (2007); Peck and Kasper (2013) Homestead (NH) Ashuelot 4 Uncertain Sand Sudden Gartner, Magilligan, and Renshaw (2015) Merrimack Village (NH) Souhegan 4 62 000 95% sand Sudden Pearson, Snyder, and Collins (2011); Santaniello, Snyder, and Gontz (2013) IVEX (OH) Chagrin 7.4 236 000 Mud Sudden Evans et al. (2000); Evans (2007) Savage Rapids (OR) Rogue 12 150 000 70% sand, 30% gravel Sudden Bountry, Lai, and Randle (2013) Milltown (MT) Clark Fork 12.8 5.5 million ‡ Sand Sudden § Evans and Wilcox (2014) Marmot (OR) Sandy 15 750 000~50% gravel, 50% sand Sudden Major et al. (2012) Elwha (WA) Elwha 32 4.9 million 47% mud, 53% sand and gravel Phased Randle et al. (2015); Warrick et al. (2015) Condit (WA) White Salmon 38 1.8 million 60% sand, 35% mud, 5% gravel Sudden Wilcox, O'Connor, and Major (2014); Colaiacomo (2014) Barlin ( (Pierson and Major 2014); behavior of legacy sediment (James 2013;Merritts et al. 2013); and restoration activities such as gravel augmentation (Sklar et al. 2009). More than 87 000 dams taller than 1.8 m have been catalogued in the United States (Figure 13.1; US Army Corps of Engineers 2013), and more than 1100 have been removed intentionallymost since 2000 (American Rivers 2014). ...
... Reservoir geometry can affect rates and magnitudes of sediment evacuation. In several reservoirs having small aspect ratios (width/length) or in those where lateral erosion could swing unobstructed, erosion rapidly evacuated large percentages (≥40%) of impounded sediment after full base-level fall was achieved (Figure 13.3) (e.g., Wildman and MacBroom 2005;Epstein 2009;Pearson, Snyder, and Collins 2011;Major et al. 2012;Sawaske and Freyberg 2012;Bountry, Lai, and Randle 2013;Wilcox, O'Connor, and Major 2014;Collins et al. 2014;Warrick et al. 2015). By contrast, for reservoirs in which lateral erosion and landslides were limited, transport capacity diminished longitudinally, or the rate of base-level fall was strongly moderated, the rate and quantity of sediment evacuation were also limited (e.g., Evans et al. 2000;Doyle, Stanley, and Harbor 2003;Cheng and Granata 2007;Straub 2007;Burroughs et al. 2009). ...
... dam removals release trapped sediment and organic matter from the former reservoirs, reversing to some extent the geomorphic and sedimentary effects of dam emplacement and long-term operation (Foley, Bellmore, et al., 2017). Upstream of a former dam site, dam removal followed by natural fluvial erosion commonly triggers knickpoint migration and fluvial incision through reservoir sediment, causing sediment export from the former impoundment (e.g., Collins et al., 2017;Doyle et al., 2002;Major et al., 2012;Pizzuto, 2002;Wildman & MacBroom, 2005). Downstream from the dam site, river response to a pulse of reservoir sediment is commonly anticipated to include aggradation, channel widening, filling of riverbed pools, and bedsediment fining (Cui et al., 2016;Downs et al., 2009;Pizzuto, 2002), changes generally confirmed by field studies to date (e.g., Major et al., 2012;Pearson et al., 2011;Tullos et al., 2014;Wang & Kuo, 2016;Wilcox et al., 2014;Wohl & Cenderelli, 2000). ...
... Journal of Geophysical Research: Earth Surface MacBroom, 2005;Zunka et al., 2015). The Elwha River dam removals differed from some others in that the reservoirs were partially rather than entirely filled with sediment ( Figure 12b). ...
Article
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Understanding river response to sediment pulses is a fundamental problem in geomorphic process studies, with myriad implications for river management. However, because large sediment pulses are rare and usually unanticipated, they are seldom studied at field scale. We examine fluvial response to a massive (~20 Mt) sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States, in an 11-year before-after/control-impact study of channel morphology and grain size. We test the hypothesis that for a given flow magnitude, greater geomorphic change occurs under sediment-rich conditions than under sediment-starved conditions. Channel response to flow forcing was significantly different during the sediment-pulse peak, 1–2 years after dam removal began, than earlier or later. During peak sediment supply our hypothesis was supported; major geomorphic change occurred under low flows and unit stream power ≤60 W/m². However, by 4–6 years after dam removal began, rates of geomorphic change and sensitivity to stream power had decreased substantially such that our hypothesis was no longer unequivocally supported. These findings are consistent with a two-phase conceptual model of dam-removal response, involving a transport-limited state followed by a more supply-limited state. From comparisons with other dam removals and natural sediment pulses, we infer that the longevity of sediment-pulse signals in gravel-bed rivers depends upon gradient, river discharge, valley morphology, and sediment grain size. Stream power associated with substantial geomorphic change varies with sediment supply, such that assigning a general threshold stream power to gravel-bed rivers may be untenable.
... Various studies described the restoration of fish habitats or freshwater food webs following the removal of dams [32][33][34][35]. Some other papers were more focused upon changes in the stream channel morphology which occurred after dam removal, and on potential consequences for habitats [36][37][38][39][40][41][42][43]. ...
... The barriers are located in streams characterized by diverse structural connectivity. In the Tâmega sub-basin, the majority of connected patches are linked to extremely low (423, 90.4%) or very low (38,8.1%) dPC connector values ( Table 2). ...
Article
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Dams provide water supply, flood protection, and hydropower generation benefits, but also harm native species by altering the natural flow regime, and degrading the aquatic and riparian habitats. In the present study, which comprised the Douro River basin located in the North of Portugal, the cost-benefit assessment of dams was based upon a balance between the touristic benefits of a dammed Douro, and the ecological benefits of less fragmented Douro sub-catchments. Focused on four sub-catchments (Sabor, Tâmega, Côa and Corgo), a probabilistic stream connectivity model was developed and implemented to recommend priorities for dam removal, where this action could significantly improve the movement of potadromous fish species along the local streams. The proposed model accounts for fish movement across the dam or weir (permeability), which is a novel issue in connectivity models. However, before any final recommendation on the fate of a dam or weir, the connectivity results will be balanced with other important socioeconomic interests. While implementing the connectivity model, an inventory of barriers (dams and weirs) was accomplished through an observation of satellite images. Besides identification and location of any obstacles, the inventory comprised the compilation of data on surrounding land use, reservoir water use, characteristics of the riparian gallery, and permeability conditions for fish, among others. All this information was stored in a geospatial dataset that also included geographical information on the sub-catchment drainage network. The linear (drainage network) and point (barriers) source data were processed in a computer program that provided or returned numbers for inter-barrier stream lengths (habitat), and the barrier permeability. These numbers were finally used in the same computer program to calculate a habitat connector index, and a link improvement index, used to prioritize dam removal based upon structural connectivity criteria. The results showed that habitat patch connectivity in the Sabor, Tâmega and Côa sub-catchments is not dramatically affected by the installed obstacles, because most link improvement values were generally low. For the opposite reason, in the Corgo sub-catchment, obstacles may constitute a relatively higher limitation to connectivity, and in this case the removal of eight obstacles could significantly improve this connectivity. Using the probabilistic model of structural connectivity, it was possible to elaborate a preliminary selection of dams/weirs that critically limit stream connectivity, and that will be the focus of field hydraulic characterization to precisely determine fish movement along the associated river stretches. Future work will also include the implementation of a multi-criteria decision support system for dam removal or mitigation of the critical structures, as well to define exclusion areas for additional obstacles.
... Geomorphic impacts of dams and their removal may 3 further include channel bed and bank erosion affecting infrastructure such as roads and 4 bridges ( Kondolf et al., 2014) which often is countered by the installation of channel 5 protection measures (e.g., Gregory, 2006;Simon & Rinaldi, 2006). The presence of such 6 stabilization systems has been shown to minimize sediment releases and downstream impacts 7 of dam removals (e.g., Wildman & MacBroom, 2005), further influencing the trajectory of 8 geomorphic response to dam construction or removal as they impose new boundary 9 conditions ( Major et al., 2017). ...
... especially in systems with a long and diverse management history ( Doyle et al., 2005;Poeppl 1 et al., 2015Poeppl 1 et al., , 2017. Wildman and MacBroom (2005), for example, investigated channel evolution after multiple dam removals in the Naugatuck River in Connecticut (U.S.) and 3 observed curtailed headcutting and reduced sediment loss due to the presence of an upstream 4 grade control in the form of a weir dam. As many rivers are impacted by multiple dams, in the 5 context of dam removal and river restoration efforts, studies on the cumulative effects of their 6 removal are desirable ( Foley et al., 2017). ...
Article
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Dam removal can generate geomorphic disturbances, including channel bed and bank erosion and associated abrupt/pulsed release and downstream transfer of reservoir sediment, but the type and rate of geomorphic response often are hard to predict. The situation gets even more complex in systems which have been impacted by multiple dams and a long and complex engineering history. In previous studies one-dimensional (1-D) models were used to predict aspects of post-removal channel change. However, these models do not consider two-dimensional (2-D) effects of dam removal such as bank erosion processes and lateral migration. In the current study the impacts of multiple dams and their removal on channel evolution and sediment delivery were modeled by using a 2-D landscape evolution model (CAESAR-Lisflood) focusing on the following aspects: patterns, rates, and processes of geomorphic change and associated sediment delivery on annual to decadal timescales. The current modeling study revealed that geomorphic response to dam removal (i.e. channel evolution and associated rates of sediment delivery) in multiple dam settings is variable and complex in space and time. Complexity in geomorphic system response is related to differences in dam size, the proximity of upstream dams, related buffering effects and associated rates of upstream sediment supply, and emerging feedback processes as well as to the presence of channel stabilization measures. Modeled types and rates of geomorphic adjustment, using the 2-D landscape evolution model CAESAR-Lisflood, are similar to those reported in previous studies. Moreover, the use of a 2-D method showed some advantages compared to 1-D models, generating spatially varying patterns of erosion and deposition before and after dam removal that provide morphologies that are more readily comparable to field data as well as features like the lateral re-working of past reservoir deposits which further enables the maintenance of sediment delivery downstream.
... Channel responses to dam removal vary considerably depending on channel characteristics and sediment regimes (Doyle et al. 2003;Wildman and MacBroom 2005), and stem from complex adjustment processes between channel aggradation and degradation Cooper 2013). Responses to dam removal can be immediate to longer term Doyle et al. 2003), although increasing evidence suggests that many river responses can occur within months, not years (Grant and Lewis 2015). ...
... This finding runs counter to other studies that have documented a short-term decrease in particle size at downstream reaches following dam removal (e.g., Thomson et al. 2005). In some gravel-bed rivers, the transport rate of sediment exceeds the sediment-supply rate; thus, all but the coarsest material was rapidly removed in the former impounded areas 5 years following four lowhead dam removals in Connecticut, USA (Wildman and MacBroom 2005). Finer-grained sediments flushed downstream can expose riffles in the former impoundment (Egan 2001). ...
Article
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Dam removal is an increasingly common river restoration option, yet some of the mechanisms leading to ecological changes remain unquantified. We assessed relationships between riffle structure and benthic macroinvertebrate and fish assemblages 2 years after a lowhead dam removal in Ohio, USA. Hydrogeomorphic, water-chemistry, and biotic surveys were conducted at seven study riffles at six time intervals from spring 2014 through summer 2015. The density and diversity of macroinvertebrates and fish were significantly different over time, largely as a function of season (lowest densities in early spring, greatest in summer). Macroinvertebrate, but not fish, assemblage composition was different by time but not riffle. Although hydrogeomorphic characteristics (e.g., streamflow velocity, substrate size) were linked to shifts in macroinvertebrates and fish, chemical water-quality parameters (e.g., dissolved oxygen, nutrient concentrations) were also implicated as potential biotic drivers. Our results indicate that riffle habitat development can be an important mechanism related to restoring sensitive species and biological diversity following dam removal. Electronic supplementary material The online version of this article (10.1007/s10661-018-6716-1) contains supplementary material, which is available to authorized users.
... 644 dams in the United States alone since 1999 (63 and 51 in 2012 and 36 2013 respectively) [American Rivers, 2014]. Although there are many reasons for retirement, 37 including safety issues [Pearson et al., 2011], river restoration is one of the common goals pursued 38 when a dam is removed [Wildman and MacBroom, 2005; Cantelli et al., 2007; Sando and Lambing, 39 2011; Major et al., 2012]. 40 ...
Article
Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: a) how does an initial channel excavated into the deposit evolve, and b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly-sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection-like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width and bed grain size distribution, that when solved numerically, describe the observed channel processes.
... In the next 50-100 years, a large number of dams will have lost such high amounts of their capacity that dam removal will become an increasingly important issue [9]. The impacts of dam removal on mountain rivers is related mostly to sediment transport and river dynamics and has to be analysed from existing examples and laboratory experiments [10]. Since only a very small proportion of sediment can be removed from dam reservoirs by excavation, river systems will have to cope with artificially high inputs of sediment in the post-dam era. ...
Article
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Most dams worldwide are constructed in mountainous upstream parts of river basins with the most active sediment transport rates and largest discharge variability. Precise knowledge of river resilience to dams is lacking due to absence of interdisciplinary approaches to dam planning and their long term impacts. A toolbox to analyse basin-wide resilience of impounded rivers is suggested taking into account climate change.
... Unfortunately, despite the rapid growing number of weir and dam removals, scientific knowledge on the post-removal rivers is still fairly poor. Up to now, most of the researches related to weir or dam removals are based on limited field survey and post-removal monitoring e.g. 2), 3), 6), 7) , the results obtained from which are generally site-specific and problem dependent. ...
Article
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This paper presents an experimental study on the impacts of weir removal on the upstream channel dynamics in non-uniform sediment beds. It is found that weir removal promotes local scour and sandbar development in upstream channels, which are two dominant processes to be considered prior to any removal action. On the other hand, the mean sediment size and the geometric standard deviation of sediment mixtures are two governing and practical parameters in characterizing the sediment bed non-uniformity when local scour and bar development are concerned. Sand ribbons are observed in non-uniform beds due to sediment sorting and exert great influences on the bar system and the channel morphology.
... Although monitoring studies of dam removals are becoming more common (Bushaw-Newton et al., 2002;Doyle et al., 2003;Wildman and McBroom, 2005;Cheng and Granata, 2007), empirical knowledge of the effects of dam removal is still limited. Furthermore, most observations and conceptual models tend to focus on the transient effects of dam removal, the shorter-term patterns of upstream sediment mobilization and downstream sediment storage. ...
Article
We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi-quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D50) is increased slightly by dam construction, but D50 remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer-grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well-vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long-term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long-term geomorphic changes.
... Within the U.S., over 1000 dams have been decommissioned since 1912 [American Rivers, 2013] to address economic, environmental, safety, and regulatory concerns [Graf, 2002]. Nearly all have been of small dams less than 10 m high on low-gradient rivers, many with modest physical consequences but nevertheless highly visible [Doyle et al., 2003a[Doyle et al., , 2003bWildman and MacBroom, 2005;Pearson et al., 2011;Sawaske and Freyberg, 2012;Collins et al., 2013;DeGraff and Evans, 2013]. Dam removals, particularly of large dams such as Condit that impound voluminous sediment, create opportunities to understand the processes by which rivers reconnect after several decades of interrupted movement of water, sediment, and aquatic life [Service, 2011]. ...
Article
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Condit Dam on the White Salmon River, Washington, a 38-m-high dam impounding a large volume (1.8 million m3) of fine-grained sediment (60% sand, 35% silt and clay, 5% gravel), was rapidly breached in October 2011. This unique dam decommissioning produced dramatic upstream and downstream geomorphic responses in the hours and weeks following breaching. Blasting a 5-m-wide hole into the base of the dam resulted in rapid reservoir drawdown, abruptly releasing ~1.6 million m3 of reservoir water, exposing reservoir sediment to erosion, and triggering mass failures of the thickly accumulated reservoir sediment. Within 90 minutes of breaching, the reservoir's water and ~10% of its sediment had evacuated. At a gaging station 2.3 km downstream, flow increased briefly by 400 m3 s-1 during passage of the initial pulse of released reservoir water, followed by a highly concentrated flow phase —up to 32% sediment by volume—as landslide-generated slurries from the reservoir moved downstream. This hyperconcentrated flow, analogous to those following volcanic eruptions or large landslides, draped the downstream river with predominantly fine sand. During the ensuing weeks, suspended-sediment concentration declined and sand and gravel bedload derived from continued reservoir erosion aggraded the channel by > 1 m at the gaging station, after which the river incised back to near its initial elevation at this site. Within 15 weeks after breaching over 1 million m3 of suspended load is estimated to have passed the gaging station, consistent with estimates that > 60% of the reservoir's sediment had eroded. This dam removal highlights the influence of interactions among reservoir erosion processes, sediment composition, and style of decommissioning on rate of reservoir erosion and consequent downstream behavior of released sediment.
... 2002, Stewart 2006, Cannatelli & Curran 2012. Uran syvyys ja uuden jokiuoman muoto vastaavat pääpiirteissään patoa edeltäviä olosuhteita, vaikka joissain tapauksissa uoman syveneminen voi jopa ohittaa alkuperäisen tason (Wildman & MacBroom 2005, Amos 2008, Engle ym. 2013, Wilcox ym. ...
... Having studied channel behavior downstream of the Elwha dam sites will present an unprecedented opportunity to understand fluvial adjustment not only to a century-long loss of sediment supply but also, in the future, to its restoration. Although scientists have had valuable opportunities to study fluvial response to removal of dams less than 15 m high (Bushaw-Newton et al., 2002;Pizzuto, 2002;Wildman and MacBroom, 2005;Major et al., 2008;Schenk and Hupp, 2009), much remains to be learned about channel response to dams and their removal, indicating the need for landscape-scale case studies before, during, and after dam removal (Grant, 2001;Doyle et al., 2002;Pizzuto, 2002;Graf, 2003). ...
Article
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Like many rivers in the western U.S., the Elwha River, Washington, has changed substantially over the past century in response to natural and human forcing. The lower river is affected by two upstream dams that are slated for removal as part of a major river restoration effort. In preparation for studying the effects of dam removal, we present a comprehensive field and aerial photographic analysis of dam influence on an anabranching, gravel-bed river.Over the past century with the dams in place, loss of the upstream sediment supply has caused spatial variations in the sedimentary and geomorphic character of the lower Elwha River channel. Bed sediment is armored and better sorted than on the naturally evolving bed upstream of the dams. On time scales of flood seasons, the channel immediately below the lower dam is fairly stable, but progresses toward greater mobility downstream such that the lowermost portion of the river responded to a recent 40-year flood with bank erosion and bed-elevation changes on a scale approaching that of the natural channel above the dams. In general, channel mobility in the lowest 4 km of the Elwha River has not decreased substantially with time. Enough fine sediment remains in the floodplain that – given sufficient flood forcing – the channel position, sinuosity, and braiding index change substantially. The processes by which this river accesses new fine sediment below the dams (rapid migration into noncohesive banks and avulsion of new channels) allow it to compensate for loss of upstream sediment supply more readily than would a dammed river with cohesive banks or a more limited supply of alluvium. The planned dam removal will provide a valuable opportunity to evaluate channel response to the future restoration of natural upstream sediment supply.
... Dam removals involving natural river erosion or the rapid release of impounded sediment can have negative effects on downstream benthic habitats and biota from sediment transport (Thomson et al., 2005;Orr et al., 2008). By contrast, dam removals involving sediment excavation had no significant downstream sediment aggradation (Wildman and MacBroom, 2005;Plante et al., 2009). Thus, potential negative effects on downstream fish and habitat after removal of Hemlock Dam were apparently mitigated by removing much of the sediment behind the dam. ...
Article
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Dam removal is an increasingly practised river restoration technique, and ecological responses vary with watershed, dam and reservoir properties, and removal strategies. Moderate-sized dams, like Hemlock Dam (7.9 m tall and 56 m wide), are large enough that removal effects could be significant, but small enough that mitigation may be possible through a modified dam removal strategy. The removal of Hemlock Dam in Washington State, USA, was designed to limit channel erosion and improve fish passage and habitat by excavating stored fine sediment and reconstructing a channel in the former 6-ha reservoir. Prior to dam removal, summer daily water temperatures downstream from the dam increased and remained warm long into the night. Afterwards, a more natural diel temperature regime was restored, although daily maximum temperatures remained high. A short-lived turbidity pulse occurred soon after re-watering of the channel, but was otherwise similar to background levels. Substrate shifted from sand to gravel–cobble in the former reservoir and from boulder to gravel–cobble downstream of the dam. Initially, macroinvertebrate assemblage richness and abundance was low in the project area, but within 2 years, post-removal reaches upstream and downstream of the dam had diverse and abundant communities. The excavation of stored sediment and channel restoration as part of the dam removal strategy restored river continuity and improved benthic habitat while minimizing downstream sedimentation. This study provides a comparison of ecological effects with other dam removal strategies and can inform expectations of response time and magnitude. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
... These observations are consistent with those reported by Pearson and Pizzuto (2015), who calculated a comparable ratio of 25% for the reservoir of a low-head dam (height = 2.5 m) in a slightly different context (i.e., a larger river with a similar slope, a lower energy and finer bedload sediments). On the other hand, other studies highlighted a low amount of sediment in reservoirs (Wildman and MacBroom, 2005;Roberts et al., 2007;Csiki and Rhoads, 2014) but in different contexts. In addition, one must consider three essential factors to explain this variability in the amounts of bedload sediment stored in reservoirs: (i) the age of the reservoir and the volume of annual bed material supply, (ii) the presence and operation of devices likely to result in emptying the reservoir, and (iii) the combined effect of multi-weir series. ...
Article
Restoring active bedload transfer in human-impacted rivers has received increasing attention in recent years, notably in response to the European Water Framework Directive (WFD), which requires that the continuity of rivers not be disturbed by anthropogenic features such as dams or weirs. The Bocq River (233 km²), a moderate-gradient stream in Wallonia, Belgium, has a hydraulic resource that was formerly largely exploited with 74 weirs (up to 2.3 m high) along 43 km. We examined the effects of seven old abandoned weirs on bedload transport for three different types of weirs (defined by the presence and position of the sluice gate system). First, the volume estimates of bedload stored in reservoirs indicated that, despite their old age, the reservoirs were not completely filled (between 25 and 50% filled compared to the reservoir volume capacity) and did not evolve very much since 1990. Second, the grain size analysis of bed material upstream, downstream and within the reservoirs, and the direct measurements of sediment transport (slag particles and PIT-tagged pebbles) demonstrated that bedload continues to be transported out of the reservoir, even though the selective trapping of coarser elements was observed within the reservoir. Particles in the range of the median can pass over the crest of weirs, but the coarser elements tend to remain in the reservoirs. This trapping effect is mitigated when the weir has open or collapsed flushing gates that facilitate bedload transfer. This indicates that weirs act as leaky barriers that allow bedload to pass through, although the individual geomorphic setting plays a primary role in determining the local sediment continuity. These findings suggest that river connectivity is less impacted than initially thought and is likely to increase over time as old weirs gradually fall into disrepair. This needs to be acknowledged when planning barrier removal projects.
... A key factor when considering or implementing dam removal involves the potential effects of trapped sediment on geomorphic and ecosystem function (Heinz Center, 2002, 2003Poff and Hart, 2002;Whitelaw and MacMullan, 2002;Doyle et al., 2003a). A number of studies have investigated a river's geomorphic response to dam removal using morphometric techniques (e.g., Pizzuto, 2002;Wildman and MacBroom, 2005;Pearson et al., 2011;Cannatelli and Curran, 2012;Bountry et al., 2013); yet, only a handful of dam-removal projects have concomitant sediment monitoring and analysis of sediment loads and fates during and after removal (e.g., Ahearn and Dahlgren, 2005;Riggsbee et al., 2007). The study of sediment dynamics associated with the August and November 2000 removal of a low-head dam on the Manatawny River, Pennsylvania, showed little increase in sediment transport with the dam breaching but increased transport during a large runoff event following removal (Johnson et al., 2001). ...
Article
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The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21 ± 3 million m3 of sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000 ± 3,400,000 tonnes of transported sediment, with 1,100,000 and 7,100,000 t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000 ± 80,000 t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292 m3/s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300 mg/L and gauged bedload transport as large as 24,700 t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000 t in year 1 and 5,400,000 t in year 2. Of the total 6,300,000 ± 3,200,000 t of suspended-sediment load, 3,400,000 t consisted of silt and clay and 2,900,000 t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000 ± 360,000 t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of < 2-mm bedload was estimated to be on the order of 1.5 Mt.
... It is also possible that the further development of the Krzczonówka channel will not occur in line with the above model. Wildman and MacBroom (2005) have shown that in gravel bed channels with a large gradient, failure of the loose, coarse grain banks occurs with less sediment mass as in the case of cohesive banks and channel capacity exceeds the sediment supply rate, so that bed aggradation due to bank collapse is little or none. Further investigations in the Krzczonówka stream will make it possible to assess the manner and time of channel adjustment to a reduced base level. ...
Article
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Channel response to dam removal is still poorly understood, as there is a lack of monitoring data. A small dam in the gravel bed Krzczonówka Stream was lowered in 2014 as the first in the Polish Carpathians. The paper describes the direction and magnitude of channel changes after the check dam lowering against the backdrop of slow changes in the riverbed occurring over a period of several decades. Geomorphologic mapping and geodetic measurements started in 2013 and were repeated in 2014. Archived cartographic sources were used to identify channel morphology in the past. After the studied check dam had been partially lowered, a flood occurred and caused movement of sediment from the reservoir into the channel downstream. Debris filled pools and artificial riffles were created in 2013—the largest deposition occurred just below the dam. The channel width also increased in this area. The channel reach upstream from the dam was incised. Additional gravel supply is limited because of a sequence of drop structures just upstream of the studied reach. Long-term channel evolution after dam lowering depends on flood events and the availability of material for fluvial transport.
... 2002, Stewart 2006, Cannatelli & Curran 2012. Uran syvyys ja uuden jokiuoman muoto vastaavat pääpiirteissään patoa edeltäviä olosuhteita, vaikka joissain tapauksissa uoman syveneminen voi jopa ohittaa alkuperäisen tason (Wildman & MacBroom 2005, Amos 2008, Engle ym. 2013, Wilcox ym. ...
Technical Report
Link: https://jukuri.luke.fi/handle/10024/547478 The report (in Finnish) collects scientific knowledge on damming and dam deconstruction effects on ecology. The report creates an example of usable ecological criteria for dam removal in Finland and includes a presentation for calculating the net present value of small scale hydropower and effects on outdoor recreation values after dam removal. Tämän selvityksen A-osiossa kerättiin yhteen tieteellisissä julkaisuissa esitettyä tutkimustietoa sekä virtavesien patoamisen että padon poistamisen ekologisista vaikutuksista. Vesien patoaminen on heikentänyt merkittävästi virtavesien habitaattien ja lajiston monimuotoisuutta. Patoaltaissa ovat virtavesiä suosineet lajit vähentyneet tai jopa kadonneet, ja niitä ovat korvanneet yleisemmät ja allasmaisempia olosuhteita suosivat lajit. Selvityksen B-osiossa on esitetty esimerkinomaisesti kotimaisten patojen poistamisen ekologisen kriteeristön laatiminen hyödyntäen olemassa olevia aineistoja. Työn tavoitteena oli kohdistaa vaellusesteiden purkaminen ensimmäisenä niihin vaellusesteisiin ja vesistöihin, joilla saavutettaisiin mahdollisimman suuri ekologinen hyöty vaelluskalojen lisääntymisen kannalta. Työssä keskityttiin patojen poistamisen ekologiseen näkökulmaan ja kohdelajiksemme valikoitui taimen. Ekologista selvitysosuutta täydennettiin esimerkeillä pienien voimalaitoksien nettonykyhyödyn taloudellisesta kannattavuusarvioinnista sekä virkistyskäyttöarvon arvioinnilla.
... However, the design procedure for a pipeline crossing often does not involve a rigorous estimation of the hydrological and hydrodynamic conditions [3]. The pipes buried initially might become exposed as a result of long-term river bed degradation, which increases the risk of pipeline damage caused by currents, waves, vibration, and human activities [4][5][6]. The encasement method, where pipes are completely encased in a rigid jacket (e.g., reinforced concrete), has increasingly been used to protect the buried pipelines from unpredictable loads and risks. ...
Article
Full-text available
The effects of exposed pipe encasements on the local variation of hydrodynamic and sediment conditions in a river channel are examined. Laboratory experiments are performed to assess the response of water level, flow regime and bed deformation to several representative types of concrete encasements. The experimental conditions considered are: three types of exposed pipe encasements exposed on the bed, including trapezoidal shape, circular-arc shape and polygonal shape, and three sets of discharges, including annual discharge, once-in-3-year flood, and once-in-50-year flood. Our experiments show that: (1) the amount of backwater definitely depends on the encasement geometric shape and the background discharge; (2) smaller discharges generally tend to induce local scour of river bed downstream of the encasement, and the order of sensitivity of bed deformation to the encasement geometric shape is trapezoidal > circular-arc > polygonal; (3) comparatively speaking, the polygonal encasement may be considered as a suitable protective structure for pipelines across alluvial rivers, with relatively modest effects on the local hydrodynamic conditions and bed stabilization.
... Few studies provide evidence of incision progressing below the pre-dam river bed. However, this did occur at Union City Dam, Connecticut (Wildman and MacBroom, 2005), where deep incision occurred due to an exposed sanitary sewer pipe with rock riprap that caused local downstream scour. Once the pipe feature failed (five years post removal), the headcut progressed upstream about 0.5 m below the original river bed. ...
Article
Full-text available
Managers make decisions regarding if and how to remove dams in spite of uncertainty surrounding physical and ecological responses, and stakeholders often raise concerns about certain negative effects, regardless of whether these concerns are warranted at a particular site. We used a dam-removal science database supplemented with other information sources to explore seven frequently raised concerns, herein Common Management Concerns (CMCs). We investigate the occurrence of these concerns and the contributing biophysical controls. The CMCs addressed are the following: degree and rate of reservoir sediment erosion, excessive channel incision upstream of reservoirs, downstream sediment aggradation, elevated downstream turbidity, drawdown impacts on local water infrastructure, colonization of reservoir sediments by nonnative plants, and expansion of invasive fish. Biophysical controls emerged for some of the concerns, providing managers with information to assess whether a given concern is likely to occur at a site. To fully assess CMC risk, managers should concurrently evaluate site conditions and identify the ecosystem or human uses that will be negatively affected if the biophysical phenomenon producing the CMC occurs. We show how many CMCs have one or more controls in common, facilitating the identification of multiple risks at a site, and demonstrate why CMC risks should be considered in the context of other factors such as natural watershed variability and disturbance history.
... Existing studies of sedimentary and geomorphic response to dam removal indicate that rates of geomorphic change and sediment export can decline fairly rapidly (within 1-2 years) following the disturbance caused by reservoir-sediment erosion and transport during and after dam removal Foley et al., 2017a). Geomorphic adjustment upstream of a former dam site commonly includes knickpoint migration and channel incision through the reservoir sediment deposit (Doyle et al., 2003;Wildman and MacBroom, 2005;Major et al., 2012;Bountry et al., 2013;Randle et al., 2015;Wang and Kuo, 2016). The introduction of sediment pulses to river channels commonly results in bed aggradation as the introduced sediment pulse translates downstream as a discrete wave, disperses in place or evolves through a combination of translation and dispersion (Lisle et al., 2001;Cui et al., 2003a;Sklar et al., 2009). ...
Article
Full-text available
Dam removal provides a valuable opportunity to measure the fluvial response to changes in both sediment supply and the processes that shape channel morphology. We present the first study of river response to the removal of a large (32‐m‐high) dam in a Mediterranean hydroclimatic setting, on the Carmel River, coastal California, USA. This before‐after/control‐impact study measured changes in channel topography, grain size, and salmonid spawning habitat throughout dam removal and subsequent major floods. During dam removal, the river course was rerouted in order to leave most of the impounded sediment sequestered in the former reservoir and thus prevent major channel and floodplain aggradation downstream. However, a substantial sediment pulse occurred in response to base‐level fall, knickpoint migration, and channel avulsion through sediment in the former reservoir above the newly rerouted channel. The sediment pulse advanced ~3.5 km in the first wet season after dam removal, resulting in decreased riverbed grain size downstream of the dam site. In the second wet season after dam removal, high flows (including a 30‐year flood and two 10‐year floods) transported sediment >30 km downstream, filling pools and reducing cross‐channel relief. Deposition of gravel in the second wet season after dam removal enhanced salmonid spawning habitat downstream of the dam site. We infer that in dam removals where most reservoir sediment remains impounded and where high flows follow soon after dam removal, flow sequencing becomes a more important driver of geomorphic and fish‐habitat change than the dam removal alone.
Article
Topographic variation within fluvial systems is essential for providing a mosaic of physical habitats and supporting the dynamic hydraulic, geochemical, and biological processes that determine both aquatic and riparian ecosystem function. In highly-modified rivers through both urban and rural settings, the physical heterogeneity of alluvial channels has been diminished by anthropogenic activities. As riparian areas are increasingly under pressure from agricultural and urban development, identifying the geomorphic controls on physical heterogeneity through these environments is critical. In this study, we use the bed coefficient of variation (CV) extracted from a high-resolution bathymetric LiDAR survey as a dimensionless metric for topographic variation and physical heterogeneity over 100 km of the Boise River corridor that spans an urban-rural gradient. Our CV results for both the streambed and channel demonstrate that the average topographic variation of reaches in urban areas is 22–25% lower than reaches located in rural areas along the same river. While these results initially support the application of the urban stream syndrome hypothesis, CV values had similar magnitudes in both urban and rural reaches suggesting there is a dominant control on topographic variation that was not directly related to urban land use. Analysis of CV values relative to normalized levee width indicates that the causative driver of morphologic simplification in the channel was lateral constraints from levees. In the Boise River, topographic variation increased linearly with normalized levee widths that ranged between 50% and >300% of the average channel width. Further, topographic variation was maximized in reaches where flow expansion during high discharge inundated between 1 and 2 times the average channel width (approximately 65–70% of the available floodplain). Our simple and objective watershed-scale approach leverages high-resolution topography data to identify reaches of high physical heterogeneity for river conservation, as well as help guide environmental flow releases in managed rivers.
Article
The ability to predict the effects of dam removal in highly sediment-filled systems is increasingly important as the number of such dam removal cases continues to grow. The cost and potential impacts of dam removal are site-specific and can vary substantially depending on local conditions. Of specific concern in sediment-impacted removals is the volume and rate of reservoir deposit erosion. The complexity and potential accuracy of modeling methods used to forecast the effects of such dam removals vary substantially. Current methods range from predictions based on simple analysis of pre-dam channel geometry to sophisticated data-intensive, three-dimensional numerical models. In the work presented here, we utilize data collected from past dam removals to develop an additional tool for predicting the rate and volume of sediment deposit erosion. Through the analysis of sediment, discharge, deposit, removal timeline, channel, and watershed data, in conjunction with post-removal monitoring data from a wide range of dam removal projects, some significant trends in the evolution of reservoir deposits following dam removal can be seen. Results indicate that parameters such as median grain size, level of cohesion, spatial variability of the deposit, and removal timeline are among the most influential factors in determining the rate and volume of sediment erosion. By comparing local conditions of dams and reservoirs slated for removal with those of past removals, we hope that predictions of the rate and volume of sediment deposit erosion can be usefully constrained.
Article
Dams are known to create discontinuities in river flow and sediment transport, with ensuing effects on the fluvial geomorphology of dammed systems. While the effects created by large impoundment dams are well documented, less is known about the influence of small run-of-river dams (common across the eastern United States) on river geomorphology. Recent emphasis on dam removal has focused on run-of-river structures, highlighting the need for improved understanding of the geomorphological effects of these types of dams. This research examines spatial variation in channel morphology and bed sediment character upstream and downstream of four run-of-river dams in Illinois. Results show that the four dams do not create major discontinuities in channel morphology or sediment character. Silt/clay content of bed material at the four sites is higher upstream of the dams than downstream, but this size fraction generally is a minor component by weight of the sediment samples collected. Although percentages of sand are generally higher upstream of the dams and gravel percentages are generally higher downstream, not all of these differences are statistically significant. Longitudinal profiles through the dams and changes in channel depth upstream and downstream of the dams indicate that no major accumulations of sediment have occurred behind the dams. Analysis of 137Cs in sediment cores at two sites shows no evidence of long-term fine sediment storage. Apparently, these dams are not acting as major sediment traps, nor do these structures produce substantial downstream channel erosion. Variability in spatial patterns of channel morphology and sediment characteristics among the sites suggests that local site-specific factors have an important influence on geomorphological responses. Because of this variability, the findings of this study indicate that information on site-specific conditions should be an important consideration in the removal planning process for run-of-river dams.
Article
Base-level lowering of reservoirs impounding upstream sediment supply triggers a series of channel evolution steps such as degradation, lateral erosion, and redeposition that can dramatically alter the reservoir landscape and decouple the relationship between stream power and sediment supply. Many case studies exist for small dam removals with a few years of sediment storage or dam breaches triggering instantaneous large sediment releases. However, quantitative information for a controlled drawdown initiating erosion of a large sediment deposit is rare. We investigate reservoir sediment response to the phased and concurrent drawdown of two reservoirs on the Elwha River, Washington, USA, during the largest dam removal in history by measuring changes in reservoir topography and channel morphology as a function of base-level lowering, river discharge, and cohesion.
Article
The ability to understand and predict the impacts of dam removal in river systems is important, especially as dam decommissioning is becoming increasingly popular. In this study, we document the morphological and sediment impact of the removal of Chijiawan Check Dam in May 2011; a 13-m-high dam located on a coarse-grained, steep mountain river channel in Taiwan. An estimated 0.2 million m3 of sediment had accumulated within the impoundment before its removal. Longitudinal and bankfull cross-sectional surveys and a detailed sediment textural survey were undertaken along a 3.2-km study reach of the Chijiawan Creek between 2010 and 2012. A rotating knickpoint with migration rates of up to 22 m/day was observed along the study reach, following dam removal. The rate and character of channel change, associated with the dam removal, appear to be driven as much by channel morphology and distance from the dam as by the hydrology variability. Our results suggested that relatively small amounts of sediment were eroded during the first 3 weeks following dam removal because of low discharge conditions. However, after 1 and 15 months, 10 and 75% of the sediment that had accumulated within former impounded was eroded, respectively. Sites near the former dam had a sediment texture that reflected the transport of released sediment, and this suggested that basin-wide sediment processes exerted a strong influence. The removal of Chijiawan Dam offers unique insight on how sediment processes can drive river channel responses to sediment pulses may vary with discharge and sediment load, in areas subject to remarkably high flows and sediment loads. Copyright © 2015 John Wiley & Sons, Ltd.
Article
Most geomorphology studies of dam removals have focused on sites with appreciable quantities of stored sediments. There is great interest in channel responses to sediment releases because of potential effects on aquatic and riparian habitats and human uses of these areas. Yet, behind many dams in the Northeast U.S. and other regions of the world only minor accumulations of sediment are present because of small impoundments, run‐of‐river dam design and management (inflow ≈ outflow), low watershed sediment yield, and/or channel beds dominated by coarse sediment and/or bedrock. The two lowermost dams on the Penobscot River in Maine, United States, removed in 2012–2013, exemplified those conditions. Great Works and Veazie dams were about 6 and 10 m high, respectively. Pre‐project geophysical surveys showed coarse substrates dominated the reservoir beds and little sediment was stored in either impoundment—functions of reach geology, late Quaternary history, and upstream dams. Repeat cross‐section surveys in each impoundment, as well as the upstream and downstream reaches, were completed from 2009 to 2015 to evaluate channel morphology responses to the removals. Bed‐sediment grain size and turbidity were also measured to characterize changes in bed texture and suspended sediment. Pre‐ and post‐removal survey comparisons confirmed the expectation that bed elevations, channel shapes, and channel positions would not change substantially. Changes were often within, or close to, our estimated random measurement error. Our study shows that large‐scale physical changes are likely to be minimal when impoundments storing relatively little sediment are removed from erosion‐resistant streambeds. Many dams eligible for removal have these characteristics, making these observations an important case study that is largely unrepresented in the dam removal literature.
Article
Dam removal continues to emerge as a viable option in river management, yet little is known about the effects that a release of associated reservoir sediment may have on the riverine system downstream. An armored riverbed, commonly seen downstream from dams may have a complicating effect on the response of the channel to high inputs of fine sediment. It is hypothesized that an influx of fine sediment can affect the downstream bed by enhancing the transport of gravel fractions or in-filling the pore spaces in the surface gravel of the bed. When reservoir sediment is flushed downstream as part of a dam removal, the sediment is input to an armored bed condition. Upon the sudden input of fines, does the armored bed break or remain intact? If broken, then engineers must account for not only the reservoir sediment flushing downstream, but also the newly broken armor layer and associated substrate. A sediment feed flume was used to examine the response of an armored bed to a sudden influx of sand. Four separate runs were conducted, each consisting of three phases: the first to obtain a dynamic equilibrium, the second to establish an armor layer, and the third phase to simulate a sudden flush of fine grained sediment, defined as less than or equal to 2 mm, onto an armored bed surface downstream from a dam. The four runs tested two sediment transport rates against two flow rates. Results show the armor layer being broken and mobilized during the third phase of each run. An increase in the total sediment transport rate is recorded and both sand and gravel are transported out of the flume. The remaining bed significantly fills in with sand regardless of flow rate or sediment feed rate. Distinctive patterns between Runs 1and 3 (lower flow rate) and between Runs 2 and 4 (higher flow rate) exist when the bed both armors and breaks. The results indicate that the flow rate has more control over the amount of sediment being mobilized and transported downstream while the sediment feed rate controls the level of sand deposition on the surface. Microtopography, imbrication, and clusters are observed to varying degrees in the armored bed, which may prove, with further research, to have a greater influence on the response of how the armor bed breaks.
Article
Full-text available
Drainage improvement of watersheds draining from the Manitoba escarpment has been based on channel designs which result in nearly straight canals with uniform gradients and trapezoidal cross-sections. These conditions are often associated with environmental degradation, such as loss of fish spawning and rearing habitat, increased erosion and siltation, and changes in water conveyance patterns that can cause property damage. In mobile bed streams, the geometry of the "natural" stream channel in profile and in cross section is determined by the geology of the basin, the extent of the tributary drainage area, and the position of a stream reach in the hierarchical order of the drainage network. Regional stream channel characteristics can be determined from channel surveys in sample reaches that are selected throughout the basin. Selected aspects of the "natural" stream geometry can then be mimicked in rehabilitation schemes that enhance or create physical stream characteristics in the uniform drainage canals that are significant biologically and hydraulically. Experimental rehabilitation and enhancement works have been constructed in Wilson Creek and Mink River. The works have re-stabilized the channels and created new pool and riffle habitats for fish spawning and rearing.
Conference Paper
The Naugatuck River and its tributaries once supported abundant anadromous fish runs. These runs ceased during the industrial revolution due to the construction of numerous run-of-the-river dams and poor water quality. Recent and anticipated water quality improvements now justify a parallel effort to restore anadromous fish runs to the Naugatuck River basin. This watershed-wide restoration project focuses on fish passage at eight obsolete industrial dams on the Naugatuck and Mad Rivers, ranging in height from 1.2 to 6 meters. Multiple fish passage alternatives were considered prior to recommending that four of the eight dams be fully or partially removed, an instream by-pass channel, rock ramp fishway and a fish ladder will be constructed around three others. The final dam is still under study and it is anticipated that dam removal will be recommended. To date, four dams have been removed. This paper will focus on the fish passage alternative analysis prepared for this project and the issues that arose during design and construction of the first four dam removals.
Article
We examined channel response following the removal of low-head dams on two low-gradient, fine- to coarse-grained rivers in southern Wisconsin. Following removal, channels eroded large quantities of fine sediment, resulting in deposition 3–5 km downstream. At one site (Baraboo River), upstream changes were rapid and included bed degradation, minimal bank erosion, and sediment deposition on channel margins and new floodplain. Sand was transported through the former impoundment and temporarily deposited downstream. At the second site (Koshkonong River), head-cut migration governed channel adjustments. A deep, narrow channel formed downstream of the head-cut, with negligible changes upstream of the head-cut. Fluvial changes were summarized in a conceptual channel evolution model that highlighted (1) similarities between adjustments associated with dam removal and other events that lower channel base-level, and (2) the role of reservoir sediment characteristics (particle size, cohesion) in controlling the rates and mechanisms of sediment movement and channel adjustment.
Article
Dams are integral to U.S. riverine ecosystems. The number of dams being considered for removal is increasing because of concerns about structural deterioration, safety, and environmental quality. Conflicting values and regulatory issues complicate dam-removal decisions, but a step-by-step decisionmaking process can help identify possible effects of a dam's removal on the environment, the local and regional economy, and surrounding communities.
Article
Approximately 400 million cubic feet of channel sediments have been delivered to the Mississippi River from the Obion-Forked Deer River system in the last 20 years. The discharge of sediment from these channelized networks in West Tennessee varies systematically with the stage of channel evolution. Variations in yields over time reflect the shifting dominance of fluvial and mass-wasting processes as the networks adjust to lower energy conditions. Maximum bed-material discharges occur during the initial phases of degradation (Stage III). In contrast, yields of suspended-sediment peak during the threshold stage (Stage 1V: large-scale mass wasting) as sediments are delivered from main-channel banks and tributary beds. Suspended-sediment yields then decrease as aggradation (Stage V) becomes the dominant trend in the main channels, but remains relatively high through restabiliza-tion (Stage VI) because of continued degradation and widening in the tributaries. Bed-material discharges decrease from the degradation stage (III) to Stage V, and increase again during restabiliza-tion (Stage VI) because secondary aggradation increases gradients and incipient meandering serves to rework bed sediments. This secondary maxima in bed-material discharge is analogous to those described previously as complex, or oscillatory, response. The trends of sediment production and transport described from these rejuvenated networks are in agreement with experimental and theoretical results of earlier investigations.
Article
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths. The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.
Anadromous Fish Enhancement and Restoration
  • S Gephard
  • G Hamley
  • J Ravita
  • Williams Jr
Gephard, S., Hamley, G., Ravita, J., Williams Jr., B., 1994. Anadromous Fish Enhancement and Restoration. Connecticut Department of Environmental Protection, Fisheries Division.
Dam removal and sediment management in dam removal research status and prospects. Proceeding of The Heinz Center Dam Removal Research Workshop
  • T J Randle
Randle, T.J., 2002. Dam removal and sediment management in dam removal research status and prospects. Proceeding of The Heinz Center Dam Removal Research Workshop. Washington, DC. Schumm, S.A., Harvey, M.D., Watson, C.C., 1984. Incised Channels, Morphology, Dynamics and Control. Water Resour-ces Publications, LLC, Littleton, CO.
Hydrology and sediment management for small dams. Course notes for dam removals and nature-like fishways
  • L A Wildman
Wildman, L.A., 2002. Hydrology and sediment management for small dams. Course notes for dam removals and nature-like fishways, University of Wisconsin, presented at Waterville, NH.
Dam Removal Success Stories ASCE Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities The complex decision making process for removing dams
  • American Rivers K David
  • D Graf
American Rivers, Friends of the Earth, Trout Unlimited, 1999, Dam Removal Success Stories, Washington, DC, 114 pp. ASCE Task Committee on Guidelines for Retirement of Dams and Hydroelectric Facilities, 1997. American Society of Civil Engineers, Washington, DC, 222 pp. Baish, S.K., David, D., Graf, W.L., 2002. The complex decision making process for removing dams. Environment 44 (4), 20 – 31(
Bald Eagle Attracted to Waterbury, Naugatuck River Fish Entice Birds
  • R Gambini
Gambini, R., 2004. Bald Eagle Attracted to Waterbury, Naugatuck River Fish Entice Birds. Waterbury Republican-American, Waterbury, CT, p. B1 (February 10).
Phase I Design Report, Anadromous Fish Restoration, Naugatuck River Basin, Pre-pared for State of Connecticut
  • Milone
  • Macbroom
  • Inc
Milone and MacBroom, Inc., 1998. Phase I Design Report, Anadromous Fish Restoration, Naugatuck River Basin, Pre-pared for State of Connecticut. Cheshire, CT.
Geologic Features of the Connecticut River Valley
  • R Jahns
Jahns, R., 1947. Geologic Features of the Connecticut River Valley, Massachusetts, U.S. Geologic Survey Water Supply Paper, vol. 996. (Washington, DC).
Dam Removal, A New Option for a Assessment of rock ramp fishways. NWS Fisheries Research Institute and the Cooperative Research Center for Freshwater Ecology. US Corps of Engineers Channel Stability Assessment for Flood Control Projects
  • New Century
  • Dc Washington
  • G A Thorncraft
  • J Harris
The Aspen Institute, 2002. Dam Removal, A New Option for a New Century, Washington, DC. Thorncraft, G.A., Harris, J., 1996. Assessment of rock ramp fishways. NWS Fisheries Research Institute and the Cooperative Research Center for Freshwater Ecology. US Corps of Engineers, 1994. Channel Stability Assessment for Flood Control Projects., EM 1110-2-1418, Washington, DC.
Friends of the Earth, Trout Unlimited, 1999, Dam Removal Success Stories
  • American Rivers
Dam removal and sediment management in dam removal research status and prospects
  • Randle
Stream corridor restoration, principles, practices and processes. U.S. Department of Agriculture: Flint, R.F., 1930
  • FISRWG