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  • 1 Water Resources Engineer, State of Utah Division of Water Resources, 1594 W. North Temple, Suite 310, Salt Lake City, UT, 84114, Email: 2 Associate Professor, Utah State University, 8200 Old Main Hill, Logan, UT 84322-8200, Email:


    Aaron C. Spencer, P.E.1, Blake Tullis, PhD2


    Millsite Dam, located in Ferron, Utah is a 115 foot tall, 4150 foot long, zoned earthfill dam. The existing spillway is incapable of passing the inflow design flood (IDF) (31,520 cfs peak), as dictated by Utah State code and NRCS standards. As part of a larger dam safety upgrade project, the spillway control structure will be replaced with a new arced labyrinth spillway (the first in the US), which protrudes into the reservoir. The advantages of using an arced labyrinth spillway are examined in the report, particularly the increased capacity relative to an in-channel labyrinth design and other spillway types. The preliminary Millsite arced labyrinth weir layout was designed, in part, by referencing published hydraulic structure research data. Unique site conditions (e.g., a confined, shallow upstream approach), which varied from the laboratory test conditions, necessitated the use of a physical model (Utah Water Research Laboratory at Utah State University) to confirm the weir design and document the hydraulic performance. Some of the case-specific findings included the influence of varied wall thickness to weir height ratios, abutting the labyrinth into a sloping, overflowable abutment rather than vertical abutment walls, and documenting approach flow velocity profiles for bank protection design.


    Millsite Dam, which was designed and constructed by the Soil Conservation Service (NRCS today) in 1971, is located in Ferron, Utah. The dam is a 115-ft tall, 4150-ft long zoned earthfill dam (see Fig. 1); the 18,000 acre-ft, 435-acre reservoir serves the Ferron Reservoir and Canal Company, and is used primarily for irrigation. The reservoir also provides municipal water for the city of Ferron and industrial supply to the Hunter Power Plant (Pacificorp). The existing spillway consists of a 50-ft long, 60-ft wide duckbill that drains into a straight chute followed by a flip bucket and a 68-ft cliff with a dissipation basin below. The spillway is incapable of passing the flows required by the current design standards of both the State of Utah and the National Resource Conservation Service (NRCS). In a joint project between the Utah Division of Water Resources, NRCS, and the Owners, the spillway will be replaced as part of a larger dam safety upgrade project for the dam. Other upgrades include raising the dam 4 ft (to recuperate volume lost to sedimentation), construction of a downstream stability berm to address unstable liquefiable foundation soils, and extending the outlet works. Key existing and proposed elevation reference data are provided in Table 1.

    Table 1. Millsite Dam Critical Elevations

    Feature Dam Crest Spillway Existing Elevation (NAVD88) 6222.5 ft 6215 ft

    Proposed Elevation (NAVD88) 6226.6 ft 6219 ft

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    Figure 1. Millsite Dam site layout and spillway flip bucket. To increase the spillway capacity without incurring the need to increase the spillway chute width, an arced labyrinth weir was chosen as a replacement for the existing duckbill. Arced labyrinth weir research published by Crookston and Tullis (2012) was used to develop a preliminary design. Variations between the site (shallower, channelized) and laboratory (deeper, 180 converging flow) conditions necessitated the use of a physical model to finalize the design. This paper briefly discusses this process of utilizing current research findings for hydraulic structure design and the importance of understanding the limitations associated with research and design methodologies, and the use of physical modeling to minimize hydraulic performance uncertainties. Spillway Design Considerations The following were some of the key performance requirements and restrictions were the following: 1. The water level cannot exceed the dam crest elevation while routing the PMF through the

    reservoir (Peak inflow of 31,520 cfs). Zero freeboard is permitted during the PMF. 2. No additional freeboard will be provided. 3. Avoid designs which require alterations to the relatively new city-run golf course which

    surrounds the dam and reservoir, or designs that would interfere with or inconvenience patrons, or would negatively alter the aesthetics of the course.

    4. The effect on the discharge from smaller storms with shorter return periods must be accounted for.

    5. Cost-effectiveness, including minimizing operation and maintenance costs. 6. Match the 4 foot raise in the dam crest being done to regain volume lost in the reservoir

    due to sedimentation, 7. Minimize impacts to the aesthetics of the waterfall feature created by the current spillway

    and work within existing topographic restrictions by minimizing the widening and deepening of the downstream end of the spillway chute,

    8. Address the effect of narrow approach conditions on the capacity of the design.

    Millsite Dam


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    Current regulations and standard engineering practice require that spillway designs be capable of passing the Probable Maximum Flood (PMF) resulting from the Probable Maximum Precipitation (PMP) as determined by the National Oceanic and Atmospheric Administrations (NOAA) publication Hydrometeorological Report No. 49 (HMR49). Under the direction of the State Engineer, the Utah Climate Center (Donald T. Jensen) has modified HMR49 to better match conditions in Utah. The resulting studies, USUL (2003) and USUS (1995), provide modified rainfall depths to better match the elevations, mountainous topography, and meteorological characteristics of Utah. According to Utah Code, the runoff determined for the PMF must also be compared to a 100-year event with a saturated watershed, but in this case, the PMF governed. The findings of the hydrology study provided by the NRCS (Nathaniel Todea) are summarized in a separate report. The design storm was a 24-hour PMF with the following characteristics:

    Table 2. Design Storm Characteristics

    Feature Value Watershed Area (sq. mi.) 153

    Rainfall Total (inches) 6.75

    Discharge Volume (ac-ft) 29,164

    Peak Inflow (cfs) 31,520

    The storage volume in the reservoir above the proposed spillway elevation is 18,930 ac-ft. Because the volume of inflow is significantly higher than the storage capacity available, the discharge attenuation was minimal. The spillway must therefore pass a flow nearly equal to the maximum reservoir inflow (~ 30,000 cfs). Due to the relatively large design flows that must pass through the somewhat narrow spillway approach channel (and the spillway chute width is restricted by the narrow width of the cliff and gorge downstream), multiple spillway concepts that could discharge large volumes within a smaller footprint were investigated. Investigated concepts included radial gates, standard and folding Fuse gates (Hydroplus, Inc.), an enlarged duckbill, and finally a labyrinth weir spillway. Maintaining the existing spillway and constructing an emergency fuse plug spillway that drained through a portion of the golf course was also considered. Though each of these concepts held significant promise in the right application, it was determined that the duckbill and the labyrinth are the most favorable options due to their passive operation, cost, no capacity would be lost in the reservoir, and the PMF could be passed without requiring repairs to the control structure. The Folding Fusegate is also a recently developed product and was not yet available in the size required, and it was uncertain whether debris could interfere with the folding support mechanism. Since the duckbill and the labyrinth spillway are functionally similar, cost-effectiveness becomes the primary deciding factor, so a preliminary design and cost estimate was prepared for each option. For channelized approach conditions, the National Engineering Handbook (NEH), Section 14, Chute Spillways, Chapter 2c (Box Inlets) suggests that a reduction in duckbill or box inlet discharge capacity is required when located in a channel. According to the provisions of the NEH, a duckbill capable of passing the required flow exceeds the range of the

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    published design graphs, would require a length of roughly 400 to 500 ft, and the channel width would have to be excavated an additional 100 to 150 ft. This eliminates the duckbill as a feasible option, leaving the labyrinth weir as the selection of choice, due to its higher discharge efficiency.

    Hydraulic Analysis of Labyrinth Spillway

    Labyrinth Design Research Falvey (2003) discusses labyrinth weir design methodologies presented by Lux and Hinchliff (1985), and Tullis et al. (1995); the Tullis et al (1995) method uses a common version of the general weir equation (Eq. 1) to calculate the flow. In Eq. 1, Q is the weir discharge, L is the weir crest length, g is acceleration due to gravity, and HT is the total upstream head in the reservoir; the weir discharge coefficient (Cd) is calculated using empirical relationships. The Tullis et al. (1995) design method was limited to quarter-round crest shapes. Falvey developed a slightly modified spreadsheet-based design method utilizing the Tullis et al (1995) data. The variables and nomenclature associated with labyrinth spillways are as illustrated in Figure 2. Willmore (2004) determined that some of the polynomial coefficients used to calculate the weir


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