Date of Award:

5-2014

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Committee Chair(s)

Blake P. Tullis

Committee

Blake P. Tullis

Committee

Michael C. Johnson

Committee

Paul J. Barr

Abstract

A hydraulically undersized control structure (i.e., an emergency dam spillway) could result in water overtopping a dam or riverbanks. To increase hydraulic capacity and reduce flooding risk, nonlinear weirs are being used to replace undersized linear weirs during control structure rehabilitation. The complex geometry of a nonlinear weir creates an infinite number of designs and three-dimensional flow patterns. This study investigates four subjects to further knowledge on two types of nonlinear weir, the piano key and labyrinth.

Weir submergence is a condition when the downstream water level of a weir exceeds the weir crest elevation, and can influence the head-discharge relationship of the structure. The effects of tailwater submergence on laboratory-scale piano key weir head-discharge relationships were evaluated experimentally and compared to previously published data on linear and labyrinth weir submergence. The results of this comparison show that for relatively low levels of submergence, the piano key weir requires marginally less upstream head relative to the labyrinth weir to pass a given flow (<6%). This increase in efficiency was reversed at higher submergence levels.

Staged labyrinth weirs feature multiple weir segments of differing crest elevations, which confine base flows to a subset of the spillway and/or satisfy downstream discharge hydrograph requirements. The flow characteristics of various laboratory-scale staged labyrinth weir configurations were tested. Head-discharge relationships were established, and the accuracy of a head-discharge predictive technique based upon superposition (i.e., calculating the discharge contribution of each weir segment individually and summing) and traditional labyrinth weir empirical data was evaluated. Relative to the experimental results, the superposition technique estimations were generally within ±5% for all configurations tested except at lower headwater depths where maximum estimation errors occurred (maximum of 15%). When discharge was limited to the lower stage weir segment, the predictive discharge errors were up to 20% for some notch configurations.

The influence of linear, labyrinth, and staged labyrinth weir head-discharge characteristics on the outflow hydrograph behavior was evaluated by numerically routing various flood discharges through a fictitious reservoir; peak outflow discharges, the maximum water surface elevation, and the required detention volumes were quantified and are presented for each weir alternative. A staged labyrinth weir can be an effective alternative for modifying (decreasing) the peak outflow hydrograph for frequent events, while increasing discharge (through effective utilization of the reservoir flood-routing detention volume) for higher return period storm events.

For labyrinth weirs in reservoir applications, approach flow perpendicular to the labyrinth weir centerline axis may not be possible in all situations. The head-discharge characteristics of a laboratory-scale 4-cycle, 15° labyrinth weir with a channelized approach flow were evaluated with three different approach flow angles (0°, 15°, and 45°). The experimental data were also compared with the head-discharge characteristics of a prototype labyrinth weir model study that featured significant approach flow angles. For approach flow angles up to 15°, no measurable loss in discharge efficiency occurred. The discharge efficiency reduced by as much as 11% for the 45° approach flow angle case. The skewed approach flow angle produced unique flow patterns in the labyrinth cycles and on the downstream spillway apron.

While all data presented are specific to the weir configurations and geometries tested, these data can be beneficial to the general understanding of nonlinear weirs.

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