Town Lake Dam Replacement Technology Choices

Tempe Town Lake Downstream Dam Replacement

ALTERNATIVES EVALUATION UPDATE

IRS Tempe City Council September 22, 2011

Timeline Alternatives Evaluation – June thru November Project Update with Council – September 22 Phase 1 Validate Concept & Select Dam Technology –

November Phase 2 Design New Dam – August 2012 Acquire Permits – July 2013 Bid and Procure Construction Contract – October 2013 Fabricate New Dam – November 2014 Construction Complete – December 28, 2015*

* Per current agreement with Bridgestone.

Town Lake: Part of a River System

Granite Reef Dam Watershed Map

Town Lake: Part of a River System

Salt River Watershed Schematic Diagram

Tempe Town Lake Dam

Current Level of Flood Protection

Designed to maintain or improve current levels of flood protection – 210,000 cubic feet per second (cfs.)

This is consistent with the rest of the river system.

Maintain Waters of Town Lake Handle Flows from Storms,

Regular Water Sources Return Lake to Normal After

Floods Handle Water from Extreme

Flood Events Smaller Than 210,000 cfs

Meet Regulatory Requirements

Be Safe

Dam Must

1. Maintain or Improve Current Level of Flood Protection 2. Maintain Full Lake Quickly After Flood Event3. Raise, Lower and Operate Reliably at Normal Lake

Levels 4. Be Cost Efficient – Capital, Lifespan, O&M 5. Have Parts Easily Available 6. Be Compatible with Pedestrian Bridge, Existing

Structures7. Meet Regulatory Requirements8. Perform Well in this Climate

Dam Design Criteria

Regulatory Conditions

Regulatory Agency Concerns

U. S. Army Corps of EngineersWater quality, channel conditions, 404 permitting

Flood Control District of Maricopa County Flood control, levee maintenance

Federal Emergency Management Agency (FEMA) Flood control, levees

Arizona Department of Water Resources Dam Safety

Flood Control Criteria

Flood Control District of Maricopa County:

Use 200-Year Discharge for Design as it’s Close to Capacity of Area Bridges (like Mill Ave, Rural Rd)

West Dam 200-Year Discharge: 204,000 cfs

East Dam 200-Year Discharge: 207,000 cfs

Tempe Town Lake Capacity Discharge: 210,000 cfs

Current Alternatives Evaluation

About 20 Dam Options Studied:

Radial (tainter) Gates Bascule or Bottom-Hinged Leaf

Gates Inflatable Rubber Dams

(water and air-filled) Ogee Crest Weirs Labyrinth Weirs Many Styles of Fuse Plugs Several Styles of Pneumatically-

Operated Hinged Crest Gates (Obermeyer)

Hydraulic Hinged Crest Gates Dyrhoff Rubber Dams (Sumitomo) Vertical Lift Gates Swing Gates Fusegates (Hydroplus) Earth Embankment/Fuseplug Several Styles of Mixed-type Spans Cable-Operated Hinged Crest Gate Others

Mixed Type Alternatives:Concrete Sections and Gates

Different technologies in the different spans between the existing piers

Concrete dams in outer spans and gates in inner spans Gate section atop concrete section each span

Concrete Section Concrete SectionGate SectionGate Section

Gate SectionConcrete Section

Gate SectionConcrete Section

Gate SectionConcrete Section

Gate SectionConcrete Section

Results of Restricted Flow

Results of alternative that requires additional piers

Results of Restricted Flow

Results of Mixed Type Alternative that Impedes Two Spans

Earthen (Fuseplug) Embankment

Typically located within an auxiliary spillway channel. Zoned earth and rockfill embankment with a sloping impervious core

Designed to wash out in a predictable and controlled manner.

Alternative not carried forward due to uncertain performance, potential to increase upstream flooding, inability to maintain lake after storm and permitting issues

Pros Cons

Lower cost for initial constructionCannot catch end of flood to reestablish lakeCannot reliably meet flood protection/flood impact criteria

Permitting uncertain

Long reconstruction time after flood event

Extensive economic impacts after large flows

Earthen (Fuseplug) Embankment

1. Obermeyer Crest Gates

2. Fusegates

3. Hinged Crest Gates

4. Inflatable Rubber Dams

Alternatives Carried Forward

1

3

2

4

Inflatable Rubber Dams

Rubber body fixed to a reinforced concrete foundation Inflated by pumping air or water until height is reached Deflated by allowing the air or water inside the rubber

body to escape

Inflatable Rubber Dams

Pros ConsRelatively low construction cost Performance in site environment uncertain

Known operation Long term cost may by highCan catch end of flood to reestablish lake Manufacturer support and warranty uncertain

Does not require additional piers Vulnerable to vandalism

Lake could be maintained during construction

Fusegates (Hydroplus)

Consists of a bucket, a base, and an intake well connected to a chamber in the base Designed to breach and wash out when overtopped

Pros ConsDoes not require additional piers Long term costs may be high

SimpleCannot catch end of flood to reestablish lake

Reliable Permitting requirements uncertain

Lake can be maintained during construction

Fusegates (Hydroplus)

Obermeyer Crest Gates

Steel gate panels supported on their downstream side by inflatable air bladders Water elevation maintained by controlling the pressure in the bladders

Pros ConsDoes not require additional piers Higher construction cost than rubber dam

SimpleBladders have similar limitations as rubber dam

ReliableLake can be maintained during constructionCan catch end of flood to re-establish lake

Obermeyer Crest Gates

Hinged Crest Gates (Hydraulic Operation)

Steel gate panels hinged at the bottom Operated with overhead pier-mounted hydraulic cylinders

Pros ConsSimple Would require additional piers

Reliable May require lake to be drained to construct

Durable Complicated operation and maintenanceCan catch end of flood to reestablish lake Would require periodic opening

Less vulnerable to vandalism than rubber dams

Hinged Crest Gates (Hydraulic Operation)

Steel gate panels hinged at the bottom Operated with wire rope electric motor hoist

Hinged Crest Gates (Electric Operation)

Pros ConsSimple Would require additional piers

Reliable May require lake to be drained to construct

Durable Complicated operation and maintenance

Can catch end of flood to re-establish lake Would require periodic opening

Less vulnerable to vandalism than rubber dams

Hinged Crest Gates (Electric Operation)

Dam Budget

Minus $367,000 Gannett Fleming Contract for

Dam Technology Analysis

Preliminary Capital Cost Estimates: $25 - $35 Million

Includes Design, Materials, Construction

Does Not Include East Dam or Pump System

Finance Options Include: Use Remaining $3,633,000 Seek Voter Approval for Bond

Authorization for Dam Replacement Capital Costs

Sale of City Properties Combinations

Dam Financing

Public Meeting on Oct. 17, 6 p.m. Tempe Center for the Arts

Failure Modes and Effects Analysis

Third Party Review Gannett Fleming, Staff Will

Recommend Dam Technology Review Financing Options Bring Selected Dam System

to Council in November

Next Steps

Want more information? Visit www.tempe.gov/lake and

Click on Town Lake Dam Replacement in the Blue Box