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

Csiki, Shane

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Description

Title

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

Author(s)

Csiki, Shane

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Rhoads, Bruce L.

Doctoral Committee Chair(s)

Rhoads, Bruce L.

Committee Member(s)

• Thorn, Colin E.
• Garcia, Marcelo H.
• Parker, Gary

Department of Study

Geography & Geographic InfoSci

Discipline

Geography

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

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

Abstract

The effects of dams on the characteristics of fluvial channels upstream and downstream of 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 about the elevation of the bankfull channel and therefore do not create impoundments upstream that inundate the floodplain under normal flow conditions in the river. Further, the practice of dam removal throughout the United States is largely focused on run-of-river dams. The effects of these types of dams on river geomorphology have received far less attention. The aim of this dissertation research is to examine the effects of run-of-river dams on river channel morphology and sediment characteristics upstream and downstream of four such structures in Illinois. Modeling of hydraulic conditions and flow competence for flows with different recurrence frequencies is also conducted to evaluate the effects of the dams on backwater conditions and on particle mobility upstream and downstream of the dams. The results of this research show that the morphological and sedimentological effects caused by run-of-river dams are relatively minor compared to systematic patterns of upstream sedimentation and downstream erosion typically caused 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 indicate that no pronounced wedge of sediment has accumulated upstream of these structures. Some trapping of fines is evident, but this effect is not substantial enough to produce a distinct morphological signature. Results from this research also indicate that channel degradation downstream of the run-of-river dams is not extensive. Overall, longitudinal profiles do not exhibit abrupt discontinuities from upstream to downstream of the dams. Hydraulic modeling iii indicates that the greatest backwater effect on the upstream flow profiles is found in the immediate vicinity of the structures, with this effect progressively diminishing upstream away from the dams, a result commensurate with known hydraulic impacts created by dams. Modeling of dam removal shows that backwater pools created by the dams are eliminated, but that local topographic variations in the channel bed profile can produce local pools upstream of topographic highs on the channel bed. The flow competence analysis reveals that upstream of the dams competence decreases towards the dams, though trends are spatially variable among sites. The general decline in competence towards the dams suggests the potential exists for bed material to accumulate within the backwater pools at high and medium frequency flows, and that deposited material should fine toward the dams. However, the high competence of low frequency flows could entrain fine particles, flushing accumulated material out of the backwater pools. With the dams removed, spatial variability in, and magnitude of, competence increases, suggesting that dam removal may increase the mobilization of bed material. However, the spatial variability among sites suggests that the patterns of bed material entrainment with removal will also vary, commensurate with substantial variability in local hydraulic, geomorphologic, and sedimentologic conditions in the study reaches at all four sites. The results of this result have important implications for sediment management requirements during dam removals, which often assume that large amounts of sediment can accumulate behind run-of-river dams based on findings from research on impoundment dams. A lack of sediment accumulation upstream of the run-of-river dams in this research, combined with the potential high competence for mobilizing sediment during low frequency flows, suggests that sediment accumulation upstream of run-of-river dams may not be as ubiquitious as previously iv suspected in low-gradient stream environments, such as those in Illinois. This research also illustrates that variability in morphological and sedimentological conditions within each site and among the sites is substantial, and must be accounted for when considering dam removal. Further in-field process based research focusing on hydraulic and sediment transport dynamics across a range of flows at run-of-river dams is needed to more precisely quantify the fluvial processes operating near these structures. Such studies will lead to an improved process-based understanding of the effects of run-of-river dams on fluvial systems and will inform management decisions regarding future removals.

Graduation Semester

2014-08

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Copyright 2014 Shane Csiki

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