The impact of run-of-river dams on channel morphology and sedimentation

dam removal;fluvial geomorphology;run-of-river dam;applied river management

Csiki, Shane

关键词: dam removal;    fluvial geomorphology;    run-of-river dam;    applied river management;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/50434/Shane_Csiki.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The effects of dams on the characteristics of fluvial channels upstream and downstreamof these structures have mainly focused on the effects of large impoundment dams. However,many dams in the United States are run-of-river structures, which do not extend in height aboutthe elevation of the bankfull channel and therefore do not create impoundments upstream thatinundate the floodplain under normal flow conditions in the river. Further, the practice of damremoval throughout the United States is largely focused on run-of-river dams. The effects ofthese types of dams on river geomorphology have received far less attention. The aim of thisdissertation research is to examine the effects of run-of-river dams on river channel morphologyand sediment characteristics upstream and downstream of four such structures in Illinois.Modeling of hydraulic conditions and flow competence for flows with different recurrencefrequencies is also conducted to evaluate the effects of the dams on backwater conditions and onparticle mobility upstream and downstream of the dams. The results of this research show thatthe morphological and sedimentological effects caused by run-of-river dams are relatively minorcompared to systematic patterns of upstream sedimentation and downstream erosion typicallycaused by impoundment dams. Minimal sediment storage is occurring upstream of the four runof-river dams examined in this study. Longitudinal profiles and trends in channel depth indicatethat no pronounced wedge of sediment has accumulated upstream of these structures. Sometrapping of fines is evident, but this effect is not substantial enough to produce a distinctmorphological signature. Results from this research also indicate that channel degradationdownstream of the run-of-river dams is not extensive. Overall, longitudinal profiles do notexhibit abrupt discontinuities from upstream to downstream of the dams. Hydraulic modelingiiiindicates that the greatest backwater effect on the upstream flow profiles is found in theimmediate vicinity of the structures, with this effect progressively diminishing upstream awayfrom the dams, a result commensurate with known hydraulic impacts created by dams. Modelingof dam removal shows that backwater pools created by the dams are eliminated, but that localtopographic variations in the channel bed profile can produce local pools upstream oftopographic highs on the channel bed.The flow competence analysis reveals that upstream of the dams competence decreasestowards the dams, though trends are spatially variable among sites. The general decline incompetence towards the dams suggests the potential exists for bed material to accumulate withinthe backwater pools at high and medium frequency flows, and that deposited material should finetoward the dams. However, the high competence of low frequency flows could entrain fineparticles, flushing accumulated material out of the backwater pools. With the dams removed,spatial variability in, and magnitude of, competence increases, suggesting that dam removal mayincrease the mobilization of bed material. However, the spatial variability among sites suggeststhat the patterns of bed material entrainment with removal will also vary, commensurate withsubstantial variability in local hydraulic, geomorphologic, and sedimentologic conditions in thestudy reaches at all four sites.The results of this result have important implications for sediment managementrequirements during dam removals, which often assume that large amounts of sediment canaccumulate behind run-of-river dams based on findings from research on impoundment dams. Alack of sediment accumulation upstream of the run-of-river dams in this research, combined withthe potential high competence for mobilizing sediment during low frequency flows, suggests thatsediment accumulation upstream of run-of-river dams may not be as ubiquitious as previouslyivsuspected in low-gradient stream environments, such as those in Illinois. This research alsoillustrates that variability in morphological and sedimentological conditions within each site andamong the sites is substantial, and must be accounted for when considering dam removal. Furtherin-field process based research focusing on hydraulic and sediment transport dynamics across arange of flows at run-of-river dams is needed to more precisely quantify the fluvial processesoperating near these structures. Such studies will lead to an improved process-basedunderstanding of the effects of run-of-river dams on fluvial systems and will inform managementdecisions regarding future removals.