Longhorn Dam Labyrinth Weir Conceptual Evaluation

Longhorn Dam Labyrinth Weir Conceptual Evaluation Prepared for:

Austin Energy June 2013

Prepared by: FREESE AND NICHOLS, INC.

10814 Jollyville Rd., Bldg. 4, Ste 100 Austin, Texas 78759 512‐617‐3137 TBPE Firm F‐2144

Longhorn Dam Labyrinth Weir Conceptual Evaluation Prepared for:

Austin Energy

June 2013

DRAFT This document is released for the purpose of interim review under the authority of Dustin Mortensen, P.E., Texas No. 100000 on June 21, 2013. It is not to be used for construction, bidding or permit purposes. Freese and Nichols, Inc. Texas Registered Engineering Firm F‐ 2144

Prepared by: FREESE AND NICHOLS, INC.

10814 Jollyville Rd., Bldg. 4, Ste 100 Austin, Texas 78759 512‐617‐3137 TBPE Firm F‐2144

AU412421

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy

TABLE OF CONTENTS 1.0 PROJECT OVERVIEW .................................................................................................................................. 1 1.1 Background ................................................................................................................................................. 1 1.2 Purpose ......................................................................................................................................................... 1 1.3 River Flow Rates ....................................................................................................................................... 1 2.0 GATED STRUCTURE .................................................................................................................................... 4 2.1 2.2 2.3 2.4 2.5

Original Design .......................................................................................................................................... 4 Gate Operation........................................................................................................................................... 6 Gated Spillway Rating Curve ............................................................................................................... 9 Historical Performance of Existing Dam ..................................................................................... 10 Gate Rehabilitation Project Summary .......................................................................................... 12

3.0 FIXED WEIR ................................................................................................................................................. 14 3.1 3.2 3.3 3.4 3.5 3.6

Fixed Weir Overview ........................................................................................................................... 14 Labyrinth Weir Examples .................................................................................................................. 15 Proposed Labyrinth Configuration at Longhorn Dam .......................................................... 17 Labyrinth Weir Rating Curve ........................................................................................................... 19 Labyrinth Performance ...................................................................................................................... 21 Backwater Analysis .............................................................................................................................. 25

4.0 LABYRINTH WEIR ASSESSMENT ....................................................................................................... 26 4.1 4.2 4.3 4.4 4.5

Lake Levels ............................................................................................................................................... 26 Hike and Bike Trail ............................................................................................................................... 26 Cesar Chavez Street at Lamar Boulevard .................................................................................... 26 Existing Structures and the 100‐Year Floodplain ................................................................... 27 Debris and Sedimentation ................................................................................................................. 27

5.0 STAKEHOLDER MEETINGS ................................................................................................................... 29 5.1 5.2 5.3 5.4 5.5 5.6

Public Works ........................................................................................................................................... 29 Parks and Recreation ........................................................................................................................... 29 Watershed Protection Department ............................................................................................... 30 Austin Water Utility ............................................................................................................................. 31 Lower Colorado River Authority .................................................................................................... 31 Lake Brazos Dam Site Visit ................................................................................................................ 31

6.0 ENVIRONMENTAL AND PERMITTING CONSIDERATIONS ..................................................... 32 6.1 Environmental Permitting ................................................................................................................ 32 6.2 TCEQ Dam Safety ................................................................................................................................... 32 i

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy 6.3 State Historic Preservation Office .................................................................................................. 32

7.0 OPINION OF PROBABLE CONSTRUCTION AND OPERATION COSTS ................................. 34 7.1 Gate Rehabilitation Project ............................................................................................................... 34 7.2 Labyrinth Weir Project ....................................................................................................................... 34 7.3 Life Cycle Costs ....................................................................................................................................... 35 8.0 CONCLUSION ............................................................................................................................................... 36 8.1 Comparison .............................................................................................................................................. 36 8.2 Conclusion ................................................................................................................................................ 37 Table of Figures Figure 1: Longhorn Dam Existing Site Plan ................................................................................................... 5 Figure 2: Cross Section of Dam and Stilling Basin through Lift Gate ................................................. 6 Figure 3: Downstream side of automated bascule gate. .......................................................................... 7 Figure 4: Downstream view of lift gate 3. ...................................................................................................... 8 Figure 5: Gate operation occurs on this sidewalk shared with the Hike and Bike Trail ........... 9 Figure 6: Gated Spillway Rating Curve – Best Case Scenario ............................................................. 10 Figure 7: Historical Normal Levels with Releases from Two Turbines at Tom Miller Dam . 11 Figure 8: Historical Lady Bird Lake Levels compared to Best Case Gate Operation ................ 12 Figure 9: Plan View of Typical Labyrinth Weir ........................................................................................ 14 Figure 10: Lake Brazos Dam shortly after construction ....................................................................... 15 Figure 11: Lake Townsend Labyrinth Weir under Construction (Schnabel, 2013) ................. 16 Figure 12: Elmendorf Dam ................................................................................................................................ 16 Figure 13: Rendering of Labyrinth Weir at Longhorn Dam ................................................................ 17 Figure 14: Labyrinth Weir at Longhorn Dam with cycles connected to existing bridge piers ....................................................................................................................................................................................... 18 Figure 15: Labyrinth Weir at Longhorn Dam with cycles not connected to existing bridge piers ............................................................................................................................................................................. 18 Figure 16: Rating Curve for Gated Structure and Labyrinth Weir ................................................... 19 Figure 17: Historical Lady Bird Lake Levels vs. Predicted Rating Curves .................................... 20 Figure 18: Fractal Labyrinth Weir ................................................................................................................. 21 Figure 19: Historical Dam Performance and Predicted Labyrinth Performance ...................... 22 Figure 20: Observed and Predicted Lake Levels for November 2004 Flood ............................... 23 Figure 21: Observed and Predicted Lake Levels for September 2010 Flood (Tropical Storm Hermine) ................................................................................................................................................................... 23 Figure 22: Observed and Predicted Lake Levels during typical LCRA Releases from One Turbine....................................................................................................................................................................... 24 Figure 23: Observed and Predicted Lake Levels during typical LCRA Releases from Two Turbines .................................................................................................................................................................... 24

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Table of Tables Table 1: Discharge for Various Storm Events .............................................................................................. 2 Table 2: Historical Events with Discharge Greater than 25,000 cfs (USGS Gage 08158000, Colorado Rv at Austin) ........................................................................................................................................... 3 Table 3: Key Features of Gate Rehabilitation Conceptual Design .................................................... 13 Table 4: Lake Levels at Various Storm Events .......................................................................................... 20 Table 5: Gated Structure Repair, Operation and Maintenance Item Costs ................................... 34 Table 6: Labyrinth Weir Construction and Operation Costs .............................................................. 35 Table 7: Life Cycle Costs ..................................................................................................................................... 35 Table 8: Attributes of Alternatives ................................................................................................................. 36 Appendices Appendix A ‐ Backwater Figures Appendix B ‐ Flood Prone Areas Identified by Parks and Recreation Appendix C – Labyrinth Weir Opinion of Probable Construction Cost Appendix D ‐ Survey Points Appendix E ‐ Rating Curves Appendix F – Project Schedules

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EXECUTIVE SUMMARY Longhorn Dam was constructed with a gated spillway in 1960 to create a cooling pond for Holly Power Plant. Austin Energy has operated and maintained the dam since construction. Now, with the decommissioning of Holly Power Plant, Longhorn Dam will have no functional relationship to Austin Energy and power generation. Nevertheless, Austin Energy continues to operate and maintain the dam and gates. The dam is now in need of numerous repairs and upgrades to improve the reliability of the dam and spillway gates. A conceptual design to repair the dam, replace the gate operators and make improvements to the channel and river banks was previously prepared by Freese and Nichols, Inc. (FNI). Austin Energy prioritized the repairs in the conceptual design and decided that $10 million in repairs were dam operation and maintenance priorities. Prior to designing the proposed improvements, Austin Energy requested that FNI evaluate the option to construct a fixed weir at Longhorn Dam in lieu of repairing the gates and operators. It was anticipated that a fixed weir would reduce the operations and maintenance requirements of the dam. This report presents the conceptual evaluation of a labyrinth spillway as a fixed weir and replacement to the gates at Longhorn Dam. A labyrinth spillway is a passive structure that uses a trapezoidal‐shaped weir wall geometry (plan view) to increase the weir length in a channel and thus increase discharge. The labyrinth weir has benefits over the gated structure; these benefits are directly related to the elimination of the gates. The largest benefit is that the labyrinth does not require personnel to operate the lift gates during storm events or to verify that the automated bascule gates are functioning properly. Another benefit is predictable lake levels during storm events. The historical record indicates that lake levels have risen several feet higher than predicted by river models due to spillway gates not being timely opened. Operating spillway gates during storm events to maintain the desired lake levels is challenging and requires frequent gate adjustment. The labyrinth weir will eliminate higher than anticipated lake levels caused by difficulties in proper gate operation. The labyrinth weir will also prevent water from being wasted, which has occurred when the gates release more water than planned causing the lake level to drop below normal pool. There are drawbacks to the labyrinth weir at Longhorn Dam. During major floods similar to those that occurred in 1997 and 2002, the labyrinth weir will cause the lake level to be approximately two feet higher than it would be if the spillway gates were operated properly and timely. Historically the Longhorn Dam gated system has not approached this level of control. Another drawback of the labyrinth weir is that debris will not flow over the dam as easily as it does through the gates. The labyrinth weir

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy will likely require debris removal, especially following storm events, although this could be mitigated with a debris boom upstream. The opinion of probable construction cost for the labyrinth weir is $15.0 million and the cost to repair the gated structure is $10 million. Based on a 50‐year design life and 4% interest rate, the present value of the gated structure is estimated at $20,000,000 and the present value of the labyrinth at $17,040,000. Longhorn Dam is a very important asset in need of improvements to properly serve its recreational purpose. The labyrinth weir is a feasible replacement for the gated spillway. The selection of this alternative over the gate repairs must consider if the advantages of reduced operational costs and more predictable lake levels outweigh the disadvantages of higher lake levels during flood events and debris removal.

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1.0

PROJECT OVERVIEW

1.1

BACKGROUND

Longhorn Dam is owned by Austin Energy and creates Lady Bird Lake. The dam was constructed in conjunction with the Holly Power Plant to create a cooling pond for the plant. Now that the plant has been de‐commissioned, Austin Energy is in the position of maintaining the dam solely for the purpose of creating Lady Bird Lake. Austin Energy is charged with operating and maintaining the nine spillway gates at the dam which are used to regulate the lake level and pass flood flows. The gates have a history of poor operation and are not reliable. In 2012, Freese and Nichols (FNI) completed a conceptual design to rehabilitate the dam and make improvements to the gate operation (Freese and Nichols 2012). The proposed gate improvements would result in a dam that is more reliable and easier to operate. The dam was previously operated and maintained by personnel at the Holly Plant. Because the Holly Plant has ceased operation and is no longer staffed by Austin Energy personnel, ongoing efforts of gate operations and maintenance at Longhorn Dam have been complicated. In an effort to minimize or eliminate the need for gate maintenance and operations, Austin Energy requested that FNI conduct a feasibility study of a fixed weir at Longhorn Dam as an alternative to the currently planned modifications to the existing gates and hoists.

1.2

PURPOSE

This study developed a conceptual design of a fixed weir at Longhorn Dam and evaluated the feasibility of the concept. The purpose was to evaluate lake levels with the labyrinth weir, identify stakeholders who would be affected and develop budgetary opinions of probable cost. The conceptual design is preliminary and intended to provide information to help decide how to proceed with improvements to Longhorn Dam. The purpose of this report is to provide a summary of the concept and highlight attributes and design considerations.

1.3

RIVER FLOW RATES

Daily flows through Longhorn Dam are largely controlled by Lower Colorado River Authority (LCRA) releases from the Highland Lakes dams and especially Tom Miller Dam. There are two turbines at Tom Miller Dam through which LCRA typically releases water. During storm events, flow through Longhorn Dam comes from releases from Tom Miller Dam flood gates as well as flood flows from area creeks.

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy LCRA has estimated flow rates at Longhorn Dam due to various storm events (Halff Associates 2003). Discharge from the turbines as well as the various storm events are shown in Table 1. Table 1: Discharge for Various Storm Events

Discharge Event (cfs) One Turbine 1,750 Two Turbines 3,500 2‐year 14,564 5‐year 24,567 10‐year 29,945 25‐year 50,050 50‐year 90,046 100‐year 90,361 500‐year 366,912 Lake Travis and Mansfield Dam significantly reduce flooding in the City of Austin by storing floodwaters in the lake and releasing them in a controlled manner. LCRA releases from Mansfield Dam are regulated by the U.S. Army Corps of Engineers (USACE). The regulations typically require LCRA to limit flows near Longhorn Dam to 30,000 cfs. Releases of floodwater from Lake Travis may last from a few days to a few weeks. Historical records show that average daily flow rates due to releases from Mansfield Dam have exceeded 25,000 cfs for extended periods on a number of occasions. Table 2 shows that there have been 141 days where the average daily flow rate was above 25,000 cfs since Mansfield Dam was completed in 1942. These flows occurred in nine different years. The 2010 storm is not included in Table 2 because the average daily flow in 2010 only reached 15,900 cfs according to the USGS.

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Table 2: Historical Events with Discharge Greater than 25,000 cfs (USGS Gage 08158000, Colorado Rv at Austin) Maximum Total Average Daily Average Daily Year Days Flow Rate Flow Rate 1957 32 34,072 37,900 1959 4 31,125 35,400 1977 4 30,200 31,500 1987 11 27,291 31,700 1992 49 29,635 35,500 1997 16 27,400 28,500 2002 6 26,166 26,800 2004 4 26,450 27,100 2007 15 25,953 26,700 Flood events that began in December and ended after January 1 are included with the following year.

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2.0

GATED STRUCTURE

2.1

ORIGINAL DESIGN

Longhorn Dam is located on the Colorado River in the City of Austin approximately 1.5 miles east of IH‐ 35. The dam is approximately 1,240 feet long and consists of a 506‐foot wide gated spillway with an earthen embankment on each side of the spillway. The gated spillway supports a four lane bridge for Pleasant Valley Road and the Hike and Bike Trail. Figure 1 shows a plan view of the dam. Longhorn Dam was constructed in 1959 to create a cooling water supply for the Holly Power Plant. The dam also improved the reliability of the water intakes to Seaholm Power Plant and the Thomas C. Green Water Treatment Plant by replacing a rock weir located upstream of South First Street. The original design of the dam included plans for a dam without the highway bridge as well as with the highway bridge. The original study found that a highway bridge could be constructed across the river as part of the dam for much less than the cost of a separate bridge (Brown & Root 1958). There are no hydroelectric generating facilities at the dam, although there have been several studies regarding hydroelectricity at Longhorn Dam, the most recent in 2011 (Toohey 2011). The dam has seven, 50‐foot wide by 13‐foot tall manually operated lift gates and two, 50‐foot wide by 9‐ foot tall automated bascule gates. The spillway gates discharge into an 85‐foot long concrete bottomed stilling basin. The stilling basin has an end sill that maintains 7 feet of water behind the gates to dissipate energy. Figure 2 shows a cross section of the dam and stilling basin.

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Figure 2: Cross Section of Dam and Stilling Basin through Lift Gate

2.2

GATE OPERATION

The two bascule gates were designed to operate automatically and maintain the lake near elevation 428.25 ft‐msl without intervention from operating personnel. Unfortunately, the gates are no longer able to perform this function well (Freese and Nichols 2010). The bascule gate operators are an outdated mechanical and hydraulic system that requires frequent adjustments. The controls consist of an elaborate system of valves, cables and pulleys. Adjusting the gate requires trial and error movements of the cables and valves. The moving parts are worn and difficult to stabilize, causing the gate to require additional adjusting. When this occurs, personnel must visit the dam several times a day to adjust the gates. With the Holly Power Plant closing, personnel will be located further from the dam and frequent visits to the dam will become more difficult with corresponding time delay to the gate operations.

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Figure 3: Downstream side of automated bascule gate.

When the river flow exceeds the bascule gates’ capacity, personnel must visit the dam and open the lift gates. The need to open these gates frequently occurs after normal working hours and with limited warning. The lift gates have a long history of poor operation, failing to open or close properly. Currently, the guide wheels on Gate 1 have broken off the side of the gate, and the gate is jammed in the slot, rendering the gate inoperable. The lift gates tend to open unevenly and rack in the gate guides. Fortunately, each time a gate has been racked; Austin Energy has been able close the gate before the lake level dropped significantly. Lifting cable failures are also common, making gate operations impossible until the cable is repaired. The lift gates are opened with an electric hoist that lifts from one side of the gate which contributes to the gate operating difficulties.

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Figure 4: Downstream view of lift gate 3.

Determining when and how much to open the lift gates requires trial and error. The amount gates are typically opened is based on the experience the Holly Power Plant personnel have gained over the years. Once a gate is opened, the lake level is monitored and if it begins to rise or fall more than desired, additional adjustments are made. The lift gates are operated with controls located at each pier. In order to access the controls, personnel must open the door on the pier. The open door blocks the sidewalk, which is also the Hike and Bike Trail. The roadway is a few feet from the pier doors. A concrete barrier and chain‐link fence have been installed to protect personnel from motorists.

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Figure 5: Gate operation occurs on this sidewalk shared with the Hike and Bike Trail

2.3

GATED SPILLWAY RATING CURVE

The spillway was designed to maintain the full capacity of the river channel without materially increasing the water level upstream over that which would prevail prior to the dam construction. The nine gates have the discharge capacity to maintain a constant lake level up to approximately 42,000 cfs. At this point, the tailwater below the dam is approximately equal to the lake level and the lake level is controlled by the river level more than the spillway gates. The automated bascule gates were designed to allow the dam to pass more than 7,500 cfs with the lake at the normal pool elevation of 428.25 ft‐msl without opening a lift gate. Once this flow rate is exceeded, the manual lift gates must be opened to prevent the lake from rising. The original designers assumed that due to the river control by Tom Miller Dam and Mansfield Dam there would be adequate time provided to raise the lift gates for any flood (Brown & Root 1958).

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Figure 6 shows the rating curve assumed for the dam. This rating curve is used to model water surface elevations through the City due to flood events. The rating curve assumes that the lake level at Longhorn Dam does not rise until flows exceed the gate capacity. This rating curve is the best case scenario assuming perfect gate operation and, as discussed in the following section, is unlikely to occur during storm events.

Gate rating curve assumes all gates are functional and opened in a timely manner.

Figure 6: Gated Spillway Rating Curve – Best Case Scenario

2.4

HISTORICAL PERFORMANCE OF EXISTING DAM

The gated spillway rating curve shown in Figure 6 is based on the best case scenario for gate operation. The rating curve is presented based on the assumption that all of the gates are fully functional and opened just enough to anticipate increased flows but not too much to waste water by lowering the lake level. Operating the gates in this manner is challenging. The original specifications for the automated bascule gates allowed for the lake to fluctuate within plus or minus 3 inches of normal pool and allowed the lake to rise six inches before the bascule gates are fully opened (Brown & Root 1959). Figure 7 shows typical fluctuations of the lake level with LCRA generating through two turbines. The figure shows that 10

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy the bascule gate generally causes the lake to fluctuate one foot per day and that the lake is frequently above 429 ft‐msl. Replacing the bascule gate controls with a modern system will improve this performance but will still allow the lake to fluctuate approximately six inches.

Figure 7: Historical Normal Levels with Releases from Two Turbines at Tom Miller Dam

The manual lift gates would not typically be opened until the river flows exceed the bascule gate capacity. As seen in Figure 7, the lift gates would generally not be opened until after the lake reaches 429 ft‐msl and if the flow rates continued to increase. Due to the flash flood nature of the Colorado River and the limited storage volume in Lady Bird Lake, determining when to open lift gates is challenging. LCRA shares river flow predictions with Austin Energy and notifies Austin Energy of gate operations at Tom Miller Dam. This coordination with LCRA helps with the gate operations but there is still uncertainty to flood timing. The river flow rate can increase in a few hours from a volume easily handled by the bascule gates to a volume requiring five lift gates to be open. Additionally, lake levels often rise during non‐working hours, extending the response time for a gate operator. Now that personnel are no longer stationed at Holly Power Plant 24‐hours a day, the response time may increase even more.

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Historical lake level records from November 13, 2001 through December 13, 2012 were reviewed In order to determine how the dam has performed in the past. In Figure 8, each dot represents one hour during the time period mentioned above, and each dot’s position indicates the river flow rate and lake level elevation during that hour. The figure shows that the lake elevation has frequently exceeded the elevation predicted by the rating curve for the gates.

Figure 8: Historical Lady Bird Lake Levels compared to Best Case Gate Operation

2.5

GATE REHABILITATION PROJECT SUMMARY

Longhorn Dam is in need of numerous repairs and upgrades to improve the reliability of the dam and spillway gates. The existing condition of the dam is documented in a 2010 inspection report (Freese and Nichols 2010). Following the inspection, FNI prepared a conceptual design to improve gate operations and reliability and provide enhancements to the downstream channel, river banks and the Hike and Bike Trail. The opinion of probable construction costs for the conceptual design was $14.2 million (Freese and Nichols, April 2012). After reviewing the conceptual design, Austin Energy prioritized the repairs and

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy determined that $10 million in repairs were dam operation and maintenance priorities. The key repairs prioritized by Austin Energy are summarized in Table 3 along with the benefits of these features. Table 3: Key Features of Gate Rehabilitation Conceptual Design Repair Benefit Replace bascule gate controls

Bascule gate will maintain normal lake level in tighter range than with existing system

Replace lift gate hoists

Lift gates will open and close evenly, reducing likelihood of gates becoming stuck or the cables coming loose.

Construct hoist platform on piers

Hoist platform will move operations away from the public sidewalk reducing Hike and Bike Trail closures.

Recoat bascule and lift gates

Gates will be protected from corrosion for many years.

Provide second set of stop logs

Duration of construction will be reduced.

Provide remote monitoring.

Allow remote observation of the dam to verify performance and security.

Replace electrical system

Provides second power feed to operate gates in case of power failure. Replaces obsolete electrical parts.

The conceptual design for the gate improvements replaced the controls for the automated bascule gate with modern electronic controls. The new controls would be expected to reduce the daily fluctuations in lake level to approximately six inches. The conceptual design for the lift gates replaced the lift gate hoists that contribute to the gate racking and becoming stuck. The lift gates would remain manually operated, requiring personnel to respond to floods and determine when to open the lift gates. While not included in the conceptual design, the manual controls could be located at a remote location, which would eliminate the travel time needed to reach the dam and open the gates. It would not, however, reduce the amount of time required to recognize that the gates should be opened.

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3.0

FIXED WEIR

3.1

FIXED WEIR OVERVIEW

A fixed weir is an overflow structure with no moving parts. It can consist of a concrete wall, a rounded spillway, or a high spot in a channel. Since there are no moving parts, it is “fixed” in position, as opposed to a gated structure. Fixed weirs have several advantages to gated structures. They do not require operation during storm events eliminating human and mechanical error and their maintenance requirements are typically less. The chief advantage of gated structures is discharge capacity. Discharge from a dam is related to flow area. Gated structures increase flow area by opening gates, thus increasing the area below the gate to pass flow. Gated structures can maintain a nearly constant lake level for a wide range of discharges. Because fixed weirs do not have moving parts, the only way to increase the flow area at a given weir, and thus the discharge, is for the lake to rise. For a given channel width, a gated structure will typically have a higher discharge capacity than a straight fixed weir. Labyrinth weirs have been developed in order to obtain the benefits of fixed weirs without sacrificing the discharge capacity of gated structures. Labyrinth weirs are linear weirs that appear folded in plan‐ view to increase the crest length for a given width. A plan view of a typical labyrinth weir section is shown in Figure 9. A labyrinth weir is able to pass large discharges at relatively low heads compared to traditional weirs of equal width. Each triangle shape of the labyrinth weir is called a cycle.

Figure 9: Plan View of Typical Labyrinth Weir

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3.2

LABYRINTH WEIR EXAMPLES

As a result of their hydraulic performance and geometric versatility, labyrinth weirs have been placed in streams, canals, rivers, ponds and reservoirs as headwater control structures, energy dissipaters, flow aerators, and spillways. Labyrinth weirs are well suited for spillway rehabilitation where aging infrastructure, dam safety concerns, freeboard limitations, and a revised and larger probable maximum flow have required increased spillway capacity. Labyrinth weirs have been constructed since as early as the 1950s. Many of them were constructed at existing dams as a replacement to gated spillways. FNI designed Lake Brazos Dam labyrinth weir, shown in Figure 10 as a replacement to a gated spillway on the Brazos River near Waco, Texas. The innovative labyrinth weir design combined reuse of the existing dam site with an unconventional spillway configuration and yielded substantial reduction in construction cost and time. Following construction, the project received numerous awards for its innovative design approach and successful construction (Vasquez et al. 2007).

Figure 10: Lake Brazos Dam shortly after construction Lake Townsend Dam impounds the primary water supply for the City of Greensboro, North Carolina (Figure 11). A 300‐foot wide labyrinth weir was chosen to replace the existing, deteriorating concrete gated spillway to provide additional spillway capacity and a low maintenance structure for the City (Schnabel 2013). This project was also an award‐winning rehabilitation project.

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Figure 11: Lake Townsend Labyrinth Weir under Construction (Schnabel, 2013) The San Antonio River Authority constructed a labyrinth weir on Elmendorf Lake in 1995 to replace a gated structure built in 1973. The 12‐foot tall weir is 350 feet wide and has 1,700 linear feet of weir (Figure 12).

Figure 12: Elmendorf Dam

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3.3

PROPOSED LABYRINTH CONFIGURATION AT LONGHORN DAM

A labyrinth weir could be constructed at Longhorn Dam on the existing stilling basin. A rendering of the labyrinth weir is shown in Figure 13. The labyrinth weir consists of approximately 17 cycles and 2,200 linear feet of wall. Two different configurations have been developed, each with advantages. The first configuration is shown in Figure 14. The cycles are connected to the existing bridge piers. The second configuration is shown in Figure 15. The cycles do not connect with the existing bridge piers. The configuration that connects to the piers may simplify construction by making it easier to dewater the work areas. The other configuration has approximately 100 more feet of weir length and avoids modifying each pier to connect with the labyrinth weir. The benefits of each configuration should be evaluated if design of the labyrinth weir continues. A 10‐foot wide by 10‐foot tall slide gate has been included in both configurations. The slide gate is not needed to pass flood flows but can be used to lower the lake for hydrilla control or other reasons. The current labyrinth concept removes all nine gates and provides a sluice gate to lower the lake for maintenance. Another option would be to replace eight of the gates and leave one of the existing gates to lower the lake for maintenance. The remaining gate would require repairs to improve reliability. If all nine gates are removed, the gate piers which currently support the mechanical equipment would be available for other uses, such as widening the Hike and Bike Trail.

Figure 13: Rendering of Labyrinth Weir at Longhorn Dam 17


Figure 14: Labyrinth Weir at Longhorn Dam with cycles connected to existing bridge piers

Figure 15: Labyrinth Weir at Longhorn Dam with cycles not connected to existing bridge piers

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3.4

LABYRINTH WEIR RATING CURVE

The labyrinth weir was sized to maintain the existing rating curve as much as possible. The crest of the labyrinth weir was set at elevation 427.6 ft‐msl, approximately 8 inches below the current normal pool level of 428.25 ft‐msl. This crest level was set so that the lake would be 428.25 ft‐msl when two turbines are discharging at Tom Miller Dam. The labyrinth rating curve is shown in Figure 16 along with the best case scenario gate rating curve. Table 4 shows flow rates, lake levels and the difference between the gated spillway lake elevations and the labyrinth weir lake elevations for the various storm events. Lake elevations for events up to and slightly beyond the 25‐year event would be somewhat higher with the labyrinth weir than with the gated spillway. The lake levels for the gated and labyrinth spillways would be nearly equal approximately midway between the 25‐year and 50‐year storm events. Thereafter, the reservoir elevations would be the same for both the gated spillway and the labyrinth. The rating curves shown in the following figures for the existing structure assume that all gates are functional and opened in a timely manner to manage flows. The labyrinth rating curve assumes that it is free from debris which would reduce the weir efficiency and cause increased lake levels.

Gate rating curve assumes all gates are functional and opened in a timely manner. Labyrinth rating curve assumes the weir is free from debris and sediment.

Figure 16: Rating Curve for Gated Structure and Labyrinth Weir 19

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Table 4: Lake Levels at Various Storm Events

Increased Gated Labyrinth Headwater Due Spillway Weir to Labyrinth Discharge Headwater Headwater Weir Event (cfs) (ft‐msl) (ft‐msl) (ft) 2‐year 14,564 428.3 429.3 1.0 5‐year 24,567 428.3 430.0 1.7 10‐year 29,945 428.3 430.3 2.0 25‐year 50,050 430.7 431.7 1.0 50‐year 90,046 439.2 439.2 0.0 100‐year 90,361 439.2 439.2 0.0 Figure 17 shows the historical lake level compared to the labyrinth weir rating curve. The figure shows that the lake level has frequently exceeded the elevation predicted by the labyrinth spillway rating curve. While the true flood performance of the existing structure must include inefficiencies in gate operation due to maintenance and operational limitations, the labyrinth spillway would be expected to more closely follow the predicted rating curve because the labyrinth passively responds during flood events.

Figure 17: Historical Lady Bird Lake Levels vs. Predicted Rating Curves 20

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy There are design modifications that could be made to the labyrinth to mitigate the head rise. The crest could be lowered even more, but that would also lower the normal pool level. The angle of the labyrinth cycles could be reduced, which would increase the length of each cycle and the overall weir length. A third option would be a fractal labyrinth weir. A fractal labyrinth weir would consist of constructing a small labyrinth alignment out of steel plate to act as the overflow crest along the top of the large labyrinth walls (Figure 18). The fractal labyrinth could be constructed on the entire large labyrinth or only on a few of the cycles. Fractal labyrinths have not been implemented in major civil projects, but a similar concept has been laboratory tested with promising results for lower heads (Erpicum, et al. 2011). The fractal labyrinth could potentially reduce the labyrinth head rise up to the 25‐year event. A significant modeling effort would be required to evaluate the impact of the fractal labyrinth. The steel plate could potentially be subject to more damage than the concrete labyrinth, increasing maintenance obligations.

Figure 18: Fractal Labyrinth Weir

3.5

LABYRINTH PERFORMANCE

In order to evaluate the historical performance of the gated structure and the predicted performance of the labyrinth weir, FNI prepared visual comparisons of the expected labyrinth performance when it is subjected to available historical flows. The first visual comparison incorporates more than ten years of historical data. Figure 19 shows a simulation of what the lake levels would have been with the proposed

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy labyrinth weir, given the historical flows at the dam. The figure also shows the actual lake levels during this time. Note that given these historical flow rates, the labyrinth would have reduced a number of peak lake levels and eliminated events where the lake fell below normal pool.

Figure 19: Historical Dam Performance and Predicted Labyrinth Performance

An examination of the data shown in Figure 19 for significant flood events and typical operation is shown in Figure 20 through Figure 23. Figure 20 shows that the highest lake level during the November 2004 flood would have been reduced by 2.8 feet with the labyrinth in place. The figure also shows that between November 26 and December 2, the lake level with the labyrinth would have been approximately 1.5 feet higher than what actually occurred with the gated spillway. The performance of the labyrinth was also compared to the Tropical Storm Hermine flood of September 2010 and the labyrinth would have reduced the peak lake elevation for that storm event as well (Figure 21). The performance of the labyrinth was also compared to the historical performance when LCRA is generating power at Tom Miller Dam. Figure 22 and Figure 23 show the dam performance for one and two turbine turbines releasing at Tom Miller Dam. There has historically been a 6 to 15 inch fluctuation in lake levels between generation releases and no generation releases. The fluctuation with the labyrinth will be similar.

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Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Peak lake level would not have occurred with labyrinth in place.

Higher lake level would have occurred with labyrinth in place.

Figure 20: Observed and Predicted Lake Levels for November 2004 Flood

Peak lake level would not have occurred with labyrinth in place.

Figure 21: Observed and Predicted Lake Levels for September 2010 Flood (Tropical Storm Hermine)

23


Figure 22: Observed and Predicted Lake Levels during typical LCRA Releases from One Turbine

Figure 23: Observed and Predicted Lake Levels during typical LCRA Releases from Two Turbines

24


3.6

BACKWATER ANALYSIS

Section 3.4 showed that the labyrinth weir increases the lake level at Longhorn Dam by up to 2 feet compared to the best case gate operation. Section 3.5 showed that the labyrinth weir will reduce peak lake levels during storm events by eliminating the need to operate gates. Section 3.5 also showed that once there has been enough time to properly adjust the gates, the gated structure will maintain lower lake levels than the labyrinth weir. These analyses were based on the lake level at Longhorn Dam. In order to determine what impact the elevated lake levels at the dam may have upstream, the City’s HEC‐ RAS effective models were used. These models assume that the lake level remains constant at Longhorn Dam until approximately 40,000 cfs. As mentioned previously, this is not typically the case. Flows for the 2‐year event to the 100‐year event were modeled, along with steady flow rates of 25,000 and 30,000 cfs representing the large historical floods shown in Table 2 that had extended periods of relatively steady high flows. Results of the modeling are shown in Appendix A. Observations from the backwater analysis will be discussed in later sections.

25


4.0

LABYRINTH WEIR ASSESSMENT

4.1

LAKE LEVELS

The labyrinth weir will result in less fluctuation in lake levels than the gated structure has had historically. As seen in Figure 8 the lake has dropped below normal pool many times due to improper gate operation. This represents lost water that LCRA must release to refill Lady Bird Lake. The labyrinth weir will reduce peak lake levels during storm events but will cause a longer duration of higher lake levels.

4.2

HIKE AND BIKE TRAIL

The Hike and Bike Trail and the new Boardwalk are located on Lady Bird Lake. Austin Energy surveyed many locations along the river to obtain elevations for the Hike and Bike Trail. The survey points are included in Appendix D. The City’s Public Works Department provided a copy of the Boardwalk drawings which included the elevations of the Boardwalk. The backwater analysis showed that with best case gate operations the Boardwalk will be above the 25‐year storm event lake level and submerged by the 50‐ year event. A small section of the Boardwalk will be submerged by the 25‐year storm event with the labyrinth weir. Results from the backwater analysis are shown in Appendix A. For the labyrinth weir, low spots on the existing Hike and Bike Trail will be inundated during the 5‐year event, while under best case gate operation conditions these low spots would be inundated during the 10‐year event. Due to the uncertainty with current gate operation, the Hike and Bike Trail may be inundated earlier than the 10‐year event.

4.3

CESAR CHAVEZ STREET AT LAMAR BOULEVARD

There is a low spot on West Cesar Chavez Street beneath the Lamar Boulevard Bridge. This area has historically flooded and caused Cesar Chavez to be closed. FNI searched for records indicating when and for how long the roadway has been closed but did not find any data. Discussions with the City of Austin Watershed Department (Watershed) indicated that the flooding may be due to the lake backing water into the culvert in this location. Reportedly, Watershed now blocks the culvert during large floods to prevent lake water from flowing from the lake into the street and then pumps local drainage from the roadway.

26

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy The backwater analysis showed that Cesar Chavez may flood beginning in the 5‐year event with the labyrinth weir and in the 10‐year event with the gated structure. The analysis with the steady flow rates representing extended floodwater releases from Mansfield Dam indicated that Cesar Chavez would not flood with the existing structure as long as the gates are operated correctly but may flood with the labyrinth. There may be engineering methods available to prevent the flooding of Cesar Chavez, such as the installation of a backflow preventer and sump pump in the culvert. The backwater analysis also indicated that constrictions in the river between South First Street and South Lamar Boulevard increase the water level. These constrictions include the bridge piers and possibly remnants of an old rock dam and sediment that accumulated behind the rock dam. Removal of the rock dam remnants and sediment may reduce the likelihood of flooding at West Cesar Chavez Street and Lamar Boulevard.

4.4

EXISTING STRUCTURES AND THE 100‐YEAR FLOODPLAIN

The labyrinth weir and gated structure will perform similarly for the 50‐year and 100‐year storm events. The labyrinth will not increase the lake levels at these events. Watershed provided finished floor elevations of structures along the river. The lowest structure is Joe’s Crab Shack on Riverside Drive. The backwater analysis indicated that Joe’s Crab Shack would not be inundated by either the gated structure or the labyrinth weir at the 25‐year event but would be inundated during the 50‐year event with either structure.

4.5

DEBRIS AND SEDIMENTATION

While some debris washes over labyrinth weirs, large debris such as trees can become caught in the cycles. Debris buildup can reduce the labyrinth weir discharge capacity. Debris will need to be cleaned from the labyrinth weir. A debris boom could be installed upstream of the dam to collect debris and prevent it from accumulating on the labyrinth weir. An access road could be provided downstream of the labyrinth to facilitate debris removal from the downstream side. The renderings shown in Figure 13 through Figure 15 show a debris boom upstream of the dam. Figure 13 also includes an access road on the downstream side. Watershed Field Operations Division currently cleans debris from the lake and removes more than 200 tons of trash from the lake annually. There are already several debris booms in the lake that capture debris upstream and reduce the amount reaching Longhorn Dam. While removing debris from the

27

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy debris boom or from the labyrinth would increase the lake cleaning effort, it would also have the environmental benefit of removing the trash from the lake and not allowing it to pass downstream. Sedimentation can reduce labyrinth weir discharge if significant sediment accumulates in the cycles. Lady Bird Lake has not experienced significant sedimentation. No issues with sediment accumulation are anticipated but the possibility of sedimentation should be evaluated.

28


5.0

STAKEHOLDER MEETINGS

Lady Bird Lake is a valuable asset to the downtown area. Hence, there are many parties who have an interest in the lake. Austin Energy and FNI held meetings with many of the stakeholders to identify concerns regarding the labyrinth weir.

5.1

PUBLIC WORKS

A meeting was held with Pirouz Moin, Supervising Engineer, Infrastructure Management Group of the Public Works Department on March 22, 2013 to discuss the labyrinth weir’s impact on the Pleasant Valley Bridge. The two labyrinth configurations were presented. Mr. Moin preferred the option that did not connect the weir to the existing piers. He highlighted the need to consider differential movement between the existing structure and the labyrinth weir. Changes in water surface elevation during storm events for both structures were discussed. Mr. Moin did not see any major issues with the labyrinth related to his group. At the time of the meeting, the backwater analysis had not been performed and the potential flooding at Cesar Chavez Street and Lamar Boulevard had not been identified and was not discussed in the meeting. Mr. Moin also asked about the impact the changed water levels may have on the Boardwalk that is currently under construction. He put us in touch with David Taylor, Boardwalk Project Manager, who provided copies of the Boardwalk drawings. Mr. Walker asked if the mechanical equipment would still be needed at the dam if the labyrinth weir were built. During planning on the Boardwalk, public works looked into using the dam to widen the Hike and Bike Trail. He expressed interest in using the dam for the Hike and Bike Trail if it would not interfere with the dam mechanical equipment.

5.2

PARKS AND RECREATION

A meeting was held with Charles Vaclavik and Ricardo Solis, division managers of the Parks and Recreation Department (PARD) along with several other PARD personnel on March 29, 2013. The labyrinth weir concept was presented and discussed. The historical performance of the existing dam was discussed including the quick lake rise during a flood due to timing of gate operations. Similarly, the performance of the labyrinth weir was discussed including the reduction in peak water surface elevation and longer duration of elevated lake levels for certain events. The potential for a lower normal pool if a labyrinth weir was built was also discussed. PARD indicated that vendors on the lake may be impacted

29

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy by a lowered normal pool level. The vendors have floating docks and the level change may affect the dock access. PARD was concerned about the impact to the Hike and Bike Trail during construction at the dam. FNI explained that modifications to the gates will require significant work from the bridge while the labyrinth construction would likely have less impact since most of the labyrinth construction will take place below the bridge. We also discussed that if the labyrinth is constructed and the existing gate hoist equipment is removed, there is a possibility that the piers could be used to improve the Hike and Bike Trail over the dam in the future. Following the meeting, PARD identified a few areas along the Hike and Bike Trail that have flooded in the past. These areas were at Auditorium Shores between the Stevie Ray Vaughn Statue and the Pavilion on the south side of the river and the entire trail from Waller Creek to Festival Beach on the north side of the river as well as the peninsula at Holly Power Plant. Maps of the flood prone areas provided by PARD are included in Appendix B. The backwater analysis in Appendix A shows the storm events that impact these areas.

5.3

WATERSHED PROTECTION DEPARTMENT

Several meetings were held with Watershed to discuss the labyrinth weir. Kevin Shunk, (Floodplain Management, Flood Early Warning System), provided finished floor elevations for several structures along the river. Once the backwater modeling was complete, FNI met with Mr. Shunk and Eduardo Acosta to discuss the results. The drainage criteria manual requires stormwater runoff peak flow rates for the two, ten, twenty‐five and one‐hundred year frequency storms to not cause increased inundation of any building or roadway surface. There was discussion that the drainage criteria manual was developed mainly for uncontrolled streams. Permitting the labyrinth weir will require some discussion within Watershed to determine how to apply the drainage criteria manual for the project. The baseline water surface in the model is based on the best case scenario gate operations. Since the best case scenario is not representative of the historical operations, a different baseline may be needed for this project. Mr. Shunk indicated that Watershed’s Field Operations Division cleans the lake. No coordination has taken place with this division to discuss the potential debris on the labyrinth.

30


5.4

AUSTIN WATER UTILITY

FNI evaluated Austin Water Utility (AWU) as a stakeholder to the project. AWU has water and wastewater lines that cross beneath the lake as well as manholes located on the shore. Since the difference in water surface elevations is only two feet at small events and there is no difference for the 50‐year and larger events and the lake has historically exceeded the predicted lake levels with the labyrinth weir, no impact to the Utility is anticipated. Further consultation in the future may be required.

5.5

LOWER COLORADO RIVER AUTHORITY

A meeting was held with LCRA regarding the labyrinth on April 8, 2013. FNI and Austin Energy introduced the concept of a labyrinth weir for Longhorn Dam. LCRA was enthusiastic about the project. LCRA is required to maintain minimum flow rates at downstream river gages. The bascules gates can be unpredictable and complicate LCRA’s river operations. When the bascule gates cause the lake to drop below normal pool, LCRA must release additional water to fill the lake before the bascule gates release the flow. As a passive structure, the labyrinth weir will be more predictable than the bascule gates and will not allow the lake to drop below normal pool.

5.6

LAKE BRAZOS DAM SITE VISIT

Representatives from Austin Energy met with Ricky Garrett, City of Waco Director of Utility Services on April 17, 2013 to discuss their experience with the labyrinth weir at Lake Brazos Dam. Mr. Garrett discussed the history of Lake Brazos Dam and the reasons for converting the gated spillway to a labyrinth weir. The gates were unreliable and frequently failed rendering the lake non‐existent for extended periods. The City of Waco has been happy with the labyrinth weir performance. Debris collects on the weir and has been removed five times in six years. The dam was scheduled to be cleaned in May 2013. Following the visit with Mr. Garrett, the group visited Lake Brazos Dam to observe the weir and debris.

31


6.0

ENVIRONMENTAL AND PERMITTING CONSIDERATIONS

6.1

ENVIRONMENTAL PERMITTING

An environmental site assessment was performed as part of the conceptual design for the gate repair. The assessment searched for items that could impact the project or require additional permitting. No sites such as former underground storage tanks and other facilities that may have contaminated the soil were found that have the potential to have caused conditions of environmental contamination within the project limits. No mapped historical or cultural places were found within the project limits, therefore no impacts are expected to historical or cultural places due to the project construction. No critical environmental features or threatened or endangered species were located in the project area. No impacts to the project are anticipated based on the environmental study. It is anticipated that the proposed labyrinth modifications can take place under U.S. Army Corp of Engineers Section 404 Nationwide Permit which allows maintenance to existing dams. The City of Austin tree protection ordinance will apply to this project. Trees larger than 8 inches in diameter are protected and may not be removed unless the Watershed Protection and Development Review Department (WPDRD) has issued a permit for the removal. There are a few protected trees in the project area. Coordination with WPDRD will occur as part of final design. WPDRD may require mitigation, including the planting of replacement trees, as a condition of application approval. The project is located on City parkland. Coordination with PARD will occur to coordinate easements for work areas, material storage sites and access roads.

6.2

TCEQ DAM SAFETY

The Dam Safety Program of the Texas Commission on Environmental Quality (TCEQ) monitors and regulates dams in the State of Texas. Texas Administrative Code requires that dam owners submit final construction plans and specifications to TCEQ for review and approval before commencing construction or repairs to an existing dam. The final design will be subject to TCEQ approval and will be in accordance with TCEQ criteria.

6.3

STATE HISTORIC PRESERVATION OFFICE

The Antiquities Code of Texas was passed in 1969 and requires that the Texas Historical Commission staff review any action that has the potential to disturb historic and archeological sites on public land.

32

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy Actions that need review under the Antiquities Code of Texas include any construction program that takes place on land owned or controlled by a state agency or a state political subdivision, such as a city or a county. Projects that require review include: 

Reservoirs constructed by river authorities and water districts;



Construction of recreational parks or the expansion of existing facilitates by city governments;



Energy exploration by private companies on public land; and



Construction by a city or county government that exceeds 5 acres or 5,000 cubic yards, whichever comes first. If the activity occurs inside a designated historic district or affects a recorded archeological site, it needs to be reviewed, regardless of project size.



Rehabilitation or demolition of a building owned by a state agency or university that is listed or eligible to be listed in the National Register of Historic Places (NRHP).

The State Historic Preservation Office (SHPO) was consulted in the conceptual design for the gate repair project. The dam is not listed as a State Archeological Landmark (SAL) or in NRHP. However, the dam may be eligible for listing in the National Register of Historic Places (NRHP). If the project does not require a permit from the Corp of Engineers, a review by SHPO will not be required. However, a need for a Corp of Engineers permit is anticipated, therefore an evaluation of NRHP eligibility would need to be performed and a thorough evaluation of the project would need to be made by SHPO during final design. The project will likely require a permit from the Texas Historical Commission.

33


7.0

OPINION OF PROBABLE CONSTRUCTION AND OPERATION COSTS

7.1

GATE REHABILITATION PROJECT

The gated structure has served the City for over 50 years and is in need of repairs. As discussed in Section 2.5, after reviewing the gate rehabilitation conceptual design, Austin Energy prioritized the repairs and determined that $10 million in repairs were dam operation and maintenance priorities. Once the dam has been repaired, there will be ongoing operation and maintenance (O&M) costs. Austin Energy reports that the gates have historically been recoated every ten years. This frequency is unusual as 25 to 30 years is more typical for spillway gates. A 15‐year frequency for gate coatings was selected for further cost analysis. The remote monitoring and control system for the gates will also need to be updated every 15 years. Operation of the gates will require that trained personnel be available 24 hours a day to operate the lift gates in storm events and verify proper operation of the automated bascule gates regularly. Austin Energy estimated the need for six trained personnel assigned halftime to the dam and provided labor costs for these personnel. The estimated costs to repair, operate and maintain the gated structure are shown in Table 5. Table 5: Gated Structure Repair, Operation and Maintenance Item Costs Item Frequency Cost

7.2

Engineering Design

One Time

$600,000

Rehabilitation Construction

One Time

$10,000,000

Gate Coatings

15‐years

$1,500,000

Control System Upgrades

15‐years

$100,000

O&M Labor Costs

Annual

$360,000

O&M Expense Costs

Annual

$125,000

LABYRINTH WEIR PROJECT

Design of the labyrinth weir will require the use of computer and physical models to further evaluate the labyrinth performance and refine the labyrinth discharge rating curve. The study will also verify the gated structure rating curve. These studies will increase the duration of the design over the gate rehabilitation project. The opinion of probable construction cost for the labyrinth weir is $15.0 million; details of the probable cost are included in Appendix D. The cost includes a service road to access the downstream side of the labyrinth and to drive behind the entire width of the labyrinth. This road can be

34

Longhorn Dam Labyrinth Weir Conceptual Evaluation Austin Energy used to facilitate debris removal. As an alternate, this access road could be eliminated and debris removed with a crane on Pleasant Valley Bridge but would impact traffic. The elimination of the access road would reduce the probable construction by approximately $1 million. Schedules for the two alternatives are included in Appendix F. The labyrinth weir will have small annual operational cost. The slide gate should be exercised annually to verify that it functions. Debris will need to be removed on an as‐needed basis. The estimated costs to construct and operate the labyrinth weir are shown in Table 6. Table 6: Labyrinth Weir Construction and Operation Costs Item Frequency Cost Engineering Design One Time $1,500,000 Initial Cost One Time $ 15,000,000 Debris Cleaning Annual $20,000 Exercise Gate and Miscellaneous Observation Annual $5,000

7.3

LIFE CYCLE COSTS

The present value and the equivalent annual worth of the gated structure and the labyrinth weir were calculated for a 50‐year life cycle and 4% interest rate and are shown in Table 7. Based on the assumed operation and maintenance costs, the labyrinth weir is less expensive than the gated structure. Table 7: Life Cycle Costs Gated Structure Present Value $20,000,000 Equivalent Annual Worth $930,600

35

Labyrinth Weir $17,040,000 $793,100


8.0

CONCLUSION

8.1

COMPARISON

Longhorn Dam is in need of improvements. Both the gate rehabilitation and the labyrinth weir provide many benefits over the existing condition. Table 8 summarizes the attributes for each of the two configurations of Longhorn Dam as discussed in this preliminary investigation. Table 8: Attributes of Alternatives Alternative

Factors

Gate Improvements

Labyrinth Weir

Initial Engineering and Construction Cost

$10,600,000

$16,500,000

50‐year Present Value

$20,000,000

$17,040,000

Flood Operation

Requires personnel at any time of day to operate gates

No operation required

Debris Removal

None

Approximately once per year

Peak Lake Levels

Lake rises quickly during storm events due to delay in gate operation

Instant and passive flood response significantly reduces peak lake levels for storm events

Lake levels during extended flood events

After period of gate adjustment, lake level at dam can be near normal pool

The lake level will remain elevated during extended flood events or flood releases from upstream dams

Lake level is a function of inflow and gate opening configuration, which leads to significant variation

Lake level is only a function of inflow leading to consistent and predictable lake levels

Wasted Water

Gates can allow lake to drop below normal pool

Lake will not drop below normal pool

Maintenance

Requires regular maintenance and adjustment

Minimal

Hike and Bike Trail

Removes most operational requirements from the public sidewalk reducing Hike and Bike Trail closures

May allow the piers to be used to support the Hike and Bike Trail

Lake level reliability

36


8.2

CONCLUSION

This report presented a conceptual design for a labyrinth weir at Longhorn Dam, evaluated the hydraulic feasibility and identified stakeholders to the project. The analysis found that the labyrinth weir would improve the operational requirements of Longhorn Dam by eliminating the need for gate operations. Peak lake elevations during flood events would likely be reduced by eliminating the gate operations. The lake level would remain elevated during high river flows. Additional analysis and evaluations will be required to determine if a labyrinth weir is appropriate at Longhorn Dam. Items to consider when evaluating the labyrinth weir have been discussed previously and include: 

Environmental permitting including historic considerations,



Management of river trash and debris,



Reduced ability to lower the level of Lady Bird Lake for upstream maintenance or hydrilla control,



Impact of increased lake level on Hike and Bike Trail,



Impact of increased lake level on West Cesar Chavez Street

The information presented in this report should be considered conceptual in nature. The replacement of an existing gated spillway with a labyrinth weir is an involved project requiring detailed analysis and design.

37


REFERENCES Brown & Root, Inc., (August 1958). Report on Low‐water Dam: Prepared for City of Austin, Texas. Brown & Root, Inc., (February 9, 1959). Low Water Dam Contract Drawings. Brown & Root, Inc., (1959). Low Water Dam Colorado River contract No. 80, Legal Notice, Specification, Proposal, Contract & Bond Form. Toohey, Marty. (May 25, 2011). “Company proposes hydropower plant at Longhorn Dam”, Austin American‐Statesman, accessed May 29, 2013, http://www.statesman.com/news/news/local/company‐ proposes‐hydropower‐plant‐at‐longhorn‐da‐1/nRbLr/ Schnabel (2013). Lake Townsend Dam ‐ Dams and Levees Portfolio. www.schnabel‐eng.com. Accessed March 4, 2013. Vasquez, V., Boyd, M., Wolfhope, J., and Garret, R. (2007). “A labyrinth rises in the heart of Texas.” Proceedings of the 28th Annual USSD Conference. 813‐826. Halff Associates, Inc., (October 2003). “Flood Damage Evaluation Project: Information Paper: Lower Colorado River Basin Study ‐ Phase I Problems, Needs and Opportunities,” Report and Appendices A and B, prepared for Lower Colorado River Authority and U.S. Army Corps of Engineers. Erpicum, S. and Laugier, F.D.R. and Pirotton, M. and Boillat, J.L. and Reverchon, B. and Schleiss, A.J. (2011). Development of a new concept of Piano Key Weir spillway to increase low head hydraulic efficiency: Fractal PKW, Labyrinth and Piano Key Weirs: PKW 2011, CRC Press/Balkema. Freese and Nichols, Inc. (April 19, 2012). Longhorn Dam Modernization Conceptual Design Report, Austin, Texas. Freese and Nichols, Inc. (December 6, 2010). Longhorn Dam Comprehensive Facility Review, Austin, Texas.

38


Appendix A Backwater Figures

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REDBUD TRAIL

MOPAC EXPY

CONFLUENCE W/ BARTON CREEK

S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

2‐YEAR STORM WATER SURFACE PROFILES EXISTING DAM PROPOSED LABYRINTH

440

POINTS ALONG NORTH TRAIL POINTS ALONG SOUTH TRAIL 435

TOM MILLER DAM

430

LONGHORN DAM

ELEVATION (FT‐MSL)

445

425

Note: Water surface profile for existing dam assumes instant operation of gates in response to flood flows according to the "best case " rating curve.

420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

DISTANCE UPSTREAM FROM LONGHORN DAM (MI.)

A2

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

5‐YEAR STORM WATER SURFACE PROFILES EXISTING DAM PROPOSED LABYRINTH

440

POINTS ALONG NORTH TRAIL POINTS ALONG SOUTH TRAIL

TOM MILLER DAM

435

430

LONGHORN DAM


445

425


420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A3

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

440

10‐YEAR STORM

430

WATER SURFACE PROFILES

425

TOM MILLER DAM

435

LONGHORN DAM


445

EXISTING DAM PROPOSED LABYRINTH POINTS ALONG NORTH TRAIL


POINTS ALONG SOUTH TRAIL

420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A4

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

TOM MILLER DAM

440

435

25‐YEAR STORM

430

425


LONGHORN DAM


445

EXISTING DAM PROPOSED LABYRINTH Note: Water surface profile for existing dam assumes instant operation of gates in response to flood flows according to the "best case " rating curve.

POINTS ALONG NORTH TRAIL POINTS ALONG SOUTH TRAIL

420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A5

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

TOM MILLER DAM

440

435

50‐YEAR STORM

430

425


LONGHORN DAM


445

EXISTING DAM PROPOSED LABYRINTH POINTS ALONG NORTH TRAIL



420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A6

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

440

STEADY 25,000 CFS

430


425

EXISTING DAM

TOM MILLER DAM

435

LONGHORN DAM


445

PROPOSED LABYRINTH POINTS ALONG NORTH TRAIL Note: Steady flow conditions may develop during extended flood releases from Lake Travis at Mansfield Dam.


420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A7

REDBUD TRAIL

MOPAC EXPY


S. LAMAR BLVD

UNION PACIFIC RR

455

S. 1ST ST.

INTERSTATE 35

S. CONGRESS AVE.

460

450

440

STEADY 30,000 CFS

430


425

EXISTING DAM

TOM MILLER DAM

435

LONGHORN DAM


445

PROPOSED LABYRINTH POINTS ALONG NORTH TRAIL

Note: Steady flow conditions may develop during extended flood releases from Lake Travis at Mansfield Dam.


420 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0


A8


Appendix B Flood Prone Areas Identified by Parks and Recreation

P

ST R

Y ST S AB I NE ST N IH 35 S B

NIT

A

O

WL N

Yes No y !

Water Access Area Creek Centerlines City of Austin Parks

YS T

YR

LE

VA L

D

O RT

LE A

SA

NT

TI FO N N I RD N RD

B LV

D

SA S AB N AS T

ON

TH

AN

Problem Flood Areas (Plus Red Bud Isle-not shown)

LBL Trail Flood Areas

ST

E OL Parker TOR FS T

Lady Bird Lake Trail

Legend

ER

This product is for informational purposes and may not have been prepared for or be suitable for legal, engineering, or surveying purposes. It does not represent an on-the-ground survey and represents only the approximate relative location of property boundaries. This product has been produced by the Parks and Recreation Department for the sole purpose of geographic reference. No warranty is made by the City of Austin regarding specific accuracy or completeness.

SP

G

DR

ON

R

OE D

AK

S LAKESHOR E

ER IV D R

LO N

O

PI L N NE

N

IV E

Roy G. Guerrero Colorado River Park

ER

DO L D

R ED

EL

LU

BU

FS T

Gillis

Blunn Creek

L

HI

TRI

ON R OE A E AN ST B ONH NI E TE R ST

RL N

WO LTO R

Edward Rendon Sr. Park at Festival Beach

EM

ST

ST

AW S

ARY

DE

KE

South

WATER LO Austin TR L O

RD

DE

LC

UR TO

RD K INN E RD Y

DI ST E

WA LL

AM AV OU N RAB E T B

PA R

B RO

ST

B

CL

WM

OE

RI VE D

B

S

NR

E OD AV

Kealing

I RS R

ST

E

R

MO

ST

5S N IH 3

RD

S

W

Lott

PA R

IL

IDE R AS H AV BY E FO RD ST HE TH E

Town Lake Rowing Town Center Lake Rowing Center Town Lake Rowing RI VE R Center

Butler Park

LI ET ST

N C ORBIN GA L LN FO VI R C L LL K GA AV T S N EL R E N LN NE KI NN S5 R AV TH E YA LA E S 4T ST S 6T V H E H M ST ST AR BL JE ST VD SS AC S T IE Y S LN 7 S 6 TH TH ST WI S ST LS 2N B O OU DS EU N S L C T D S3 S1 T AV ID RD I N AV ST E E ST ST S S 3R 2 N EW ND D TO N ST ST S CO E NG RVA S T ST ESS AVE

TR A

JU

W2 ND ST

TO OM RD E Y

E 1 3TH ST

M

GS RD

B

R IN

Zilker

F RD

E

Boggy WO ROS E Creek E E 9T 11T HS HS E 8 T S E 10T T TH E 1 ST H H 1 T H ST ST 165 S T E 10 TH ST E 6T E 7 T TH E 5TH S T H S T E 8TE 9 T H S H E4 ST Parque E 3 TH S T R E6 Zaragoza D E T E C 2ND S T E 5 H ST E SA S T TH RC ST E3 HA RD VEZ S ST T E 2 E ND S T 3R D HO ST L HA LY S T S KE LL Metz ST

5S

SP

IV

E 8T E E 7T H S T H S T 8T H E 6T H S T ST E 4T H ST

H3

LOU NE F

SI

D

R

C O LU

M BUS DR

Lamar Beach at Town Lake

NW AV O O D E

R

D

TH H E 20 ST H ST E 1 8 T H ST E 17T ST 1 E 6TH ST H E 14T ST H T 3 1 E EW N H ST Y ORK E 12T AV E

E 20T ST

R

Old Bakery

KE

Center

BAR TO N

D

W W 1 11 TH S 0 T W 9 TH ST TH S T W 8T Treaty H Oak Square W 6 ST T W3 W 5T H S T RD H ST ST

HILL S AV I DE E

O TF

BA YL O ST R NL AM AR B

OP AC M N

S TRA

W1 7TH W 15TH W 16TH ST ST ST

N A ST SAL I A ST LE ON AL ST

Austin Rowing Dock

Pease

C OM

EX

10T

N IH S V R 35 D N I H SB S VR 3 5 D NB

W

12T H S ST H WTW 10T 9T H ST W 9T H ST HS T W 5T W 6 Texas T H HS ST T Rowing

HS T

GU A DA C OL O R AD LAVACLUPE S CON A ST T O G RE SS A S T VE

Haskell House

W

LV D

B

7T

PY N

W

SI

DE

05 April 2013

MS

0

600 1,200 Feet

±

434

T

OS

T

NS

T

HS

TH

Yes

D

No

AM

ST

AR B 43 4

ST

BA

NG ZI

TO O

ME

YR

D

B

ST

RT E RO

Umlauf Sculpture Garden

N O S RT NG B A P RI D S R

BE

430

L

EE

B SP A R RI TO RD NG N S

RD

ST ER

434

RT ON LV D

R D N

EW

BA RT O

DR

D

LI A M

ST

LVD

W5 TH

NL 8 42

Butler Shores at Town Lake

ZILKER R

City of Austin Parks

NL

ST W 5T H

BLV

Feet

Water Access Area

AR

ST

600

OR

432 434

450

y !

D ST

43 2 432

0

RD

AN

WIL

43

IE

GS

LOU NEFF RD

432

Problem Flood Areas-West End

300

BA YL

SE

S L AM AR B S L AM LVD A R SB LE BLVD EB S LA NB AR MA RB TO LVD ND R

2 43 8 42

IN

434

BAY L

ST

RO

430

SS

SP R

432


W 2N N

430

AM

RD

JE

Zilker


150

Treaty Oak Square

42 8

Barton Creek

ST

SH WA L

0

43 4

BA RT O

ELEVATION

ST

HA RT HA

W7

43 0

NE FF

ST

428

ST

LO U

Legend

TH

43 2

NB

COLUMBUS DR

W9

OR

DR W N LEE C IR

LO

WI NF

NC

428

434

ER ST

DA VE

SE RVE RD

8T

BL A

434

434

PR E

E T R C L I FF L

T LS

PR ES SL

DA VE AN HL HI G W LY EST NN ST PO WE ST L L

PA U

428

SSL ST ER

PA C

MO

430

R

LA N

W4 TH ST

Z

P Y YS NB B

RE

E AV

EN E C TER R N ATU D

OA K

NB PY EX

T EX N PY S MO B PA C

NT AS AT LA N

43 4 43 2

ST

V CL Z UB IL HO KE US R E RD

CH

Lamar Beach at Town Lake

VD BL

K

R

DR

LA

Red Bud Isle

430

SA

T IN

CE

US

W

42 8

W

0 43

SV

RD

W3 RD ST

43 2

0 43 428

STEPHEN

8 42 0 43 4 43

DR

M

FA

Texas Rowing Center

EX P X C A E O P AC M P S MO S

Y

PY EX

43 4

RD

S

AC OP

Zilker Nature Preserve

O TF RA

P EX A C SB OP RD S M SV

ST

E AL

ST

N STI

0

ST LAKE CLIFF CT

AU

43

P

LN

0

H

T 428

BR O

L AK E

43

2 43

E

5T

TH

HS

1Redbud 0T Isle Inset HS W1 T 1 TH ST W HA 9TH LF W ST REDBUD TRL 10TH ST

434

M RA

C

W

9T

8 42

0 43

NB

8

N

42

VA

W6

West 432 Austin

430

C PA

Austin High Tennis Center

W

W

430

432

R

434

430

Austin Rowing Dock

T

MO

SD

HS

TO

N RA

Johnson Creek

43 2

430

TE

2

8T

Z AVE

VE

43

Veterans

W

CH

Eilers (Deep Eddy)

RA VE

FRA N AV CIS E

CESA R

FO ST E

Shoal Beach at Town Lake

428

42 8 432

W

428

RI V

ER S

Auditorium Shores at Town Lake

ID

ED

R

Butler Park

05 April 2013

MS


0

250 Feet

500

±

SVRD N IH 35

S ST

ST ER WA LL

ST

HA S

Z

434


ED

ED

AL VIS TA T AVE A

DR

M

A

Blunn Creek

AL A

R CO

NB

SB

EA S

B IE

HE S R RN A RD ND E

DR

RD

W AV NI N E G

NE

MA S AN

N IH

35 S VRD BR U NB SHY ST

SB N IH

SA BI NE ST

ST

RAI

N IH

N IH 35 NB

ST

NASH

NY

BIC KL ER

RAVINE R HI D SS LL MELI A LN SI DE AVE

EVA DR ST

City of Austin Parks

G EC

LI FF

Norwood Tract at Town Lake

432

5 S IH 3 B N SVRD35 NB S IH 5 S B S IH 35 S IH 3 SB S VRD

EVA

VRD

RED R 434

ST NE Y RAI

AVE ESS NG R S CO

ST

S2 ST

S3

RD

35 S

IVE R ST

ST

ITY

H ST ST S1

T

BO ULD

ND

DR

Water Access Area

D NB

SU N

SI DE

ST

5 SVR

B

T

CE

No

y !

N IH 3

RD S

Problem Flood Areas-Central

CIRCLE

VE R

R TE


H ST

E

W ELIZABE T

N ST

TH

IBSO

RI

AV E

LN

DEM Y

LOW

KEL ST L

3 5 SV

ACA

W JA ME ST S WG

E

A LE GR NDE E AV

NE L L IE ST

EL IZA BE TH ST JEW ELL ST

SPE N

LA M

ST

T Nicholas Dawson

Yes

WIL

BIE

N IH

TH S

ST

EA

SS

434


Sanchez

2

S5

L AM

430

W

S 3 RD S

RI VE R ST

43

0 43

ME

432

4 43

JA

432

W

4 43

IN A VE

East Bouldin Creek

430

T

Waller Beach at Town Lake

AY W AV O E OD

ST RD S3

ST

LL S

N IH 35 SB

N RD

D

434

OA K

SR

DR

DR

ING

E ID RS

D AWSO

VE RI

SPR

428

SKI

NE Y

W

BAR TO N

DRI

DAV IS ST

Town Lake Rowing Center Town Lake Rowing Center Town Lake Rowing 42 8 Center

DR

ST

RE TA M ST A EB ON ST Y

West Bouldin Creek

Waller Creek

DE

PO

2

Legend ELEVATION

VRD

43

D ST

35 S

ER SI

ST

E AS T AVE

IV

CH A VE Z

E

R

SA R

E 3R

H ST

Sir Swante Palm

T AV

W

DAWSON RD

DANI EL

Mary Dawson

E CE

RI V ER ST

R

E 2N D ST

RE D

Auditorium Shores at Town Lake

E 2N D ST

TRI N

ED

ZOS BR A ST ZOS ST

434

ST Shoal Beach at Town Lake

Butler Park

Dougherty Arts Center

AVE Z

E 4T

E 3R D ST

B RA

SID

R CH

CO N

ER

E SA

0 43

S L AMA R L B VD LEE B SB AR TO ND R

Butler Shores at Town Lake

E

WC

43 2

WR IV

W 2N D E ST 2N D ST

0

SS A V

43

G RE

432

428 432

434


432

05 April 2013

434

MS

0

250 Feet

500

±

NA

430

ND

EZ

IEW

HA SK

ST

SR RD

JES

SE E

SEG

OV

IA S

NP L VAL EASA LEY NT RD

SAB ST A

S ST

ST T OS

S AB

RN

AS

T

AL E

T

Legend ELEVATION 428 430 432

Holly Shores at Town Lake

EL L

434


ST 432

RIV

DS

N PL E VAL ASAN LEY T RD

T

EZ

LLA N

LY S

T

HAV

SAN

Metz

RC

E 2N

S AN

ST

ER BUR YS

PED E

NT

CLA

M IL DR

ST

CA

ESA

JU L IU ST S

ERV

RA

T ED S

LYN N

RIV

RD EN

EC

428

430

Yes

434

ER

GA

HO L

LYN N ST

NA SH H

ST

ST

S AL IN ST A

AV E

ON Y TH

T

AN

EL L

ST

ST

MA

LS

T

HA SK

LY S

LYN N ST

HO L

EN

CO

D NB 5 SVRB N IHI3H 35 NB N H 35 S NI

ST EA

Waller Beach at Town Lake

WA LLE R

ST

GAR D

No

ERV

IEW

T

y !

Manuel and Robert Donley Park

Water Access Area City of Austin Parks

ST

Norwood Tract at Town Lake

E

432


R IV

428

43 2

432

428

S ER ID

DR

43

43 0

4

430

E

428

434

RD

AS AN TV AL SP

V C R IO L OW ET NL N

R

LE

LA D

DR NF OR NI

IR

N

TD

Central Maintenance Complex at Town Lake

LE Y

DL N IR

432

Lakeshore at Town Lake

YB

D

43 2

43 2

434

DR

EK TIN

N KE RL YA L

BE

PA R

RO

TO W LA KEC

Problem Flood Areas-East End

E


ON

Roy G. Guerrero Colorado River Park

42 8

WA T CI ERL T Y OO LN

EL M

VE D

ID R

S ST

T ES R C

E DR

RS

D

I NE

TRICT DR

RI

LO R

GA

S

L

DR

Peace Point at Town Lake

NC RE

UP

T

E DIS R HO

TO W

R LA N

DD

ST

ES

434

S LAKESHORE BL VD

E

LL A D R IR E

UR

VA L

432

428

D SI ER IV DR

MH

NY

2

International Shores at Town Lake

LO M DR A

S IH 3 5 SVRD NB

S IH 35 SVRD SB S I H 35 S B

E

OM DR A

TT Y DR JO

430

ER

BE

434

4 30

E

0

P IN

434

43

SU N EL

OO D AV LAN E

D

LN

GAN

T ER

TA Y

W

432 428

43

LU

430

434

REA

RI D VE R RS I

R ED ID R RSE D VEID RI S EVER I E R

Blunn Creek Park

Longhorn Shores at Town Lake

432

E


05 April 2013

MS

0

250 Feet

500

±


Appendix C Labyrinth Weir Opinion of Probable Construction Cost

Austin Energy Longhorn Dam Preliminary Opinion of Probable Construction Cost Longhorn Dam Labyrinth Weir ITEM 1 2 3 4 5 6 7 8 9 10 11 12 13 13 14 15 14

DESCRIPTION Mobilization Temp. Erosion and Sediment Control Site Preparation and Restoration Lift Gate/Hoist Removal Care Of water Flood Insurance Labyrinth Weir Spillway Slab (3 foot thick) Labyrinth Weir (3 foot thick walls) Concrete Apex Piers Concrete Training Walls/ Pier Modifications Debris Boom Access Ramp Access Drive Behind Labyrinth Additional Foundation Slab Behind Bascule Gates Outlet Gate Handrail Embankment Drain Pipe

QUANTITY UNIT 1 LS 1 LS 1 LS 1 LS 1 LS 1 LS 4,000 CY 6,000 CY 600 CY 1 LS 550 LF 1 LS 1300 CY 980 CY 1 LS 1 LS 1 LS SUBTOTAL:

UNIT PRICE $ 1,058,000 $ 75,000 $ 100,000 $ 250,000 $ 1,200,000 $ 60,000 $ 400 $ 850 $ 800 $ 75,000 $ 600 $ 400,000 $ 400 $ 400 $ 250,000 $ 41,000 $ 50,000

PROJECT SUBTOTAL: CONTINGENCY:

OPINION OF PROBABLE CONSTRUCTION COST

~26%

TOTAL $ 1,058,000 $ 75,000 $ 100,000 $ 250,000 $ 1,200,000 $ 60,000 $ 1,600,000 $ 5,100,000 $ 480,000 $ 75,000 $ 330,000 $ 400,000 $ 520,000 $ 392,000 $ 250,000 $ 41,000 $ 50,000 $ 11,890,000 $ 11,890,000 $ 3,110,000

$ 15,000,000

THIS DOCUMENT IS RELEASED FOR PURPOSE OF INTERIM REVIEW. IT IS NOT TO BE USED FOR CONSTRUCTION OR BIDDING PURPOSES.

Prepared by Freese and Nichols, Inc. 6/21/2013


Appendix D Survey Points

Austin Energy SURVEYING 2526 Kramer Ln Bldg. E Austin TX 78758-4007 1-512-505-7183 Wednesday, May 15, 2013 2:45:08 PM PROJECT: \\Kramer3\Survey\Surveying\HIKE&BIKE TRAIL SHOOTS 5-14-2013.pro -------------------------------------------------------------------------------

Point Coordinates Listing FORMAT

-

WEST POSITIVE LAT LONG IN DECIMAL DEGREE

PT# LATITUDE LONGITUDE Elev Name -----------------------------------------------------------------------------1 30.252534291 97.740769389 430.63 GD 2 30.252667285 97.740596028 428.63 TOPWATER 3 30.252216609 97.740828702 434.11 PARKING LOT 4 30.252769559 97.741119026 437.87 PATIO 5 30.261354281 97.747342391 431.11 TRAIL 6 30.261364772 97.747339430 429.00 TOPWATER 7 30.261274656 97.747141271 430.86 TRAIL 8 30.262441033 97.749103028 431.47 TRAIL 9 30.262676478 97.749549873 430.61 TRAIL 10 30.262737220 97.749583191 430.23 TOPROCK 11 30.262746759 97.749582574 428.71 TOPWATER 12 30.262805644 97.749931085 430.65 TRAILTOP 13 30.262883541 97.750183914 430.19 TRAIL 14 30.262994515 97.750532940 430.63 TRAIL 15 30.263253915 97.751156217 432.44 TRAIL 16 30.263388586 97.751820995 434.65 TRAIL 17 30.263778547 97.752469411 432.37 TRAIL 18 30.263832294 97.752422993 428.86 TOPWATER 19 30.263978532 97.753395432 435.88 TRAIL 20 30.264268106 97.755176605 436.29 TRAIL 21 30.265191153 97.757224116 428.71 TOPWATER 22 30.266002982 97.755469881 447.70 STEPS-100FLOOD 23 30.266212010 97.755775002 433.00 TRAIL 24 30.266348489 97.756175346 432.54 TRAIL 26 30.266446816 97.756252431 431.94 MANHOLE 27 30.266451488 97.756247316 431.06 FLOWLINE 28 30.266310672 97.756163568 429.01 WATER EDGE 29 30.266452189 97.755365044 441.68 COA BRASS CAP 30 30.265990854 97.755067104 435.76 TRAIL 31 30.265009205 97.752369384 428.78 TOPWATER 32 30.264198293 97.750286293 432.78 TRAIL 33 30.264289096 97.750597795 432.50 TRAIL 34 30.270332016 97.765928609 428.77 TRAIL 35 30.270305007 97.765911136 429.64 TRAIL 36 30.270341835 97.765982296 430.79 TRAIL 37 30.270260260 97.765996991 428.93 TOPWATER 38 30.272001870 97.768865476 430.88 TRAIL 39 30.271554570 97.767987206 431.38 TRAIL 40 30.271083922 97.767333331 431.99 TRAIL 41 30.269136697 97.762640714 433.20 TRAIL 42 30.260833457 97.742371933 432.23 TRAIL 43 30.260672182 97.742453922 429.92 DOCK

44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

30.260512663 30.260557416 30.253304936 30.255007269 30.252494773 30.252464421 30.252447892 30.252524840 30.252414325 30.250931518 30.250581705 30.250312580 30.249942989 30.249101055 30.248749013 30.248369389 30.248340644 30.250622235 30.250625595 30.250575816 30.244738580 30.244514469 30.244076989 30.244101151 30.245343107 30.246063247 30.246430586 30.246465291 30.257720801

97.741830687 97.741557783 97.738495525 97.739862665 97.737448214 97.737472039 97.737485463 97.737555622 97.737393241 97.734717255 97.733561912 97.732835691 97.732045777 97.730054305 97.728838213 97.727815986 97.727829351 97.715423291 97.716035598 97.716127662 97.716507037 97.716485861 97.716514240 97.716525271 97.722504528 97.722195471 97.720699998 97.720676732 97.743123847

435.52 451.58 431.14 431.49 430.43 429.35 428.63 430.49 431.15 431.08 432.27 430.62 432.00 431.80 430.29 430.06 428.67 432.15 432.28 428.67 432.35 434.48 431.84 428.72 433.36 441.19 432.35 431.81 432.55

BLDG FLOOR DECK TRAIL TRAIL TOPBOATRAMP BOATRAMP TOPWATER TRAIL TRAIL TRAIL TRAIL TRAIL TRAIL TRAIL TRAIL BOATRAMP TOPWATER TRAIL TRAIL TOPWATER TRAIL TRAIL TRAIL TOPWATER TRAIL TRAIL TRAIL TRAIL TRAIL


Appendix E Rating Curves

LABYRINTH WEIR DESIGN No Approach Velocity PROJECT: PROJECT NO.

Longhorn Dam 1

TIME: DATE:

FLOOD CRITERIA:

PMF

BY:

14:51:55 31-May-13

DGM

USER INPUT Max. Res Crest el. Floor el. Spillway width Apex Width No. of cycles Magnification

Zr Zc Zf Ws 2a n L/W

454.0 427.6 403.0 50.0 2 2 4.95

ft ft ft ft ft

CHECK ON RATIOS Ld/B = 1.60 USE FEWER CYCLES Ho/P = 1.08 WARNING! H/P > 0.7! L/W RATIO IS OK Note: Ld/B must be