US Army Corps of Engineers

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Boise River Feasibility Study

Ellen Berggren, PMPOutreach Coordinator/ Project Manager

Idaho Governor’s Roadless Commission BriefingJune 10, 2015

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Study Authority

Water Resources Development Act (WRDA) of 1999, Section 41

- Flood control authority

WRDA 2007, Section 4038

- Added ecosystem restoration & water supply authority

- Pre-agreement credit to Sponsor for $500K

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Non-Federal Sponsor

Idaho Water Resources Board (IWRB) Established by Idaho Constitution Appointed by Governor Develops and implements State Water Plans Idaho Department of Water Resources provides

project support and coordination Pays 50 percent of feasibility study costs

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Boise River WatershedAvg. Annual Runoff = 1,940 KAFSystem Storage = 950 KAF

Arrowrock DamReclamationCompleted in 1915Storage = 286 KAF

Anderson Ranch DamReclamationCompleted in 1945Storage = 418 KAF

Lucky Peak DamUSACECompleted in 1954Storage = 264 KAF

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Flood Risk Problems Extensive floodplain development

$10 billion of property at risk

Repeated minor floodingProtection to ~ 3% event from existing system

Largest population center in ID 40% of ID residents live in the Boise River Watershed

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Boise Rr Flood Hydrology

1943 – Flood in Eagle, ID

Annual Peak Spring Discharge Flood Frequencies

FrequencyUnregulated Flow

at Lucky Peak Dam(cfs)

Regulated Flowat Glenwood

(cfs)

10% (1 in 10 chance) 25,200 7,500

5% (1 in 20 chance) 32,300 7,900

2% (1 in 50 chance) 38,000 11,000

1% (1 in 100 chance) 44,100 16,600

0.2% (1 in 500 chance) 59,800 35,000

• Bankfull flow = 6,500 cfs (at Glenwood)• Flood stage = 7,000 cfs (at Glenwood)

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3% chance flood -

8,000 cfs

0.2% chance flood -

35,000 cfs

1% chance flood –

16,600 cfs

0.2% chance flood - Infrastructure at riskover $10 billion*

1% chance flood -Infrastructure at riskover $1 billion*

Source: Ada and Canyon County Hazard Mitigation Plans, 2011 and 2013

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Water Supply Problems

Semi-Arid: 8-12” of precip. per year High Population Growth:

- 4.5% 1990 to 2000- 3.3% between 2000-2010- 4th fastest growing state population

98% of existing DCMI water is from groundwater

Prohibitions on development of additional groundwater sources

Curtailment of junior water rights

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Water Supply Problems

Projected increase in water demand for the 50-year study period (estimated future needs additional 83,000 – 170,000 acre-feet)

Constrained storage system

Climate variability

Interconnected surface and ground water

Uncertain reliability of current and future water supply

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Study Objectives

1) Reduce flood damage and life safety risk

2) Increase operational flexibility and resiliency

3) Increase conveyance

4) Reduce the potential for severe erosion and/or sudden channel migration

5) Increase water supply to meet current and future water demands

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Alternatives

MEASURESALTERNATIVES

A B C DNo

ActionArrowrock Dam Raise X X X Managed Aquifer Recharge X Upgrade Irrigation Headgates X X X Replace Push-Up Dams X X Upgrade Bridges X Controlled Flooding of Pits/Ponds X Temporary Conveyance of Water In Floodplain X X X Flow Split Structure X Enhanced Water ConservationNon-Structural Measures X X

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Arrowrock Dam Raise

Multi-purpose Storage• Flood risk management • Water supply

Potential Maximum Raise • Dam raise up to 74 feet • Up to 270,000 acre-feet increased storage• About 6.5 miles of river inundated

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Maximum Potential Raise • Dam raise - 74 ft. • Reservoir elevation increase - about 70 ft. • Approx 270, 000 acre-feet additional storage.• Approx. 6.5 additional river miles inundated

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Timeline

January 2016 Tentatively Selected Plan Milestone*

February 2016 Draft Feasibility Report / Environmental Impact Statement released for public review

May 2016 Agency Decision Milestone*

December 2016 Final Feasibility Report / Environmental Impact Statement

March 2017 Civil Works Review Board*

June 2017 Chief’s Report*

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Questions?

Lucky Peak Project, 1954