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Howard A. Hanson Dam being constructed

Dam Specifications

Height Above Bedrock:  235 ft (71.6 m) Length at Crest: 450 ft (137.2 m)

Total Length, Including Spillway and Abutments: 675 ft (205.7 m)

Thickness:  Base 960 ft (292.6 m), Crest 23 ft (7 m)

Concrete Spillway:  Capacity 107,000 cubic ft per sec (3030 cubic m per sec)  

2 Spillway Tainter Gates (each):  45 x 30 ft (13.7 x 9.1 m)

Outlet Tunnel, Concrete Lined, Horseshoe Shape: 19 ft wide x 900 ft long

2 Tunnel Tainter Gates:  10 x 12 ft (3.05 x 3.7 m)

Reservoir Length:  7 miles (11.3 km) Green River, 4 miles (6.4 km)

North Fork Reservoir Capacity:  106,000 acre-ft (130,753,000 cubic m)

Frequently Asked Questions

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A1:  The Corps constructed a grout curtain and initial improvements to the drainage tunnel in 2009, and these measures underwent rigorous testing in 2010. Tests showed that the repairs completed to date have improved the control of seepage through the area of concern.  The Corps initiated a dam safety study to determine how best to permanently repair the dam.  The Corps determined that additional interim measures were needed to restore the dam to a safe condition and provide full flood storage capacity during the time the study is being finalized and approved.  Vertical and horizontal drains into the drainage tunnel to further improved the drainage collection system through the right abutment. Repairs to further protect the dam in extreme flood events requiring use of the spillway began construction in late 2011, including the addition of rock protection on the upstream dam face and a new log boom system in the reservoir and were completed in 2012. The Corps finished a detailed dam safety study in 2014 to ensure all significant and credible risks have been evaluated.  The final report by the Corps has also verified that the risk reduction measures completed between 2010 and 2012 have further reduced the dam’s overall risk by an order of magnitude.

A2:  In January 2009, the Corps became concerned after the discovery of two sinkholes, muddy seepage and high flow rates through the right abutment after the reservoir held a pool of record in 2009.  To date, engineers have excavated the depressions, installed additional monitoring equipment, conducted sub-surface exploration, and conducted tests while water was held at the traditional summer conservation pool elevation. Scientists and engineers agree that the two depressions formed for reasons that do not significantly affect the safety of the dam’s operation. Repairs, including a grout curtain and drainage tunnel improvements, have resulted in better control of seepage through the area of concern.  Data from sub-surface exploration and new instrumentation provided engineers and scientists a much better idea of what went on inside the dam’s abutment and has provided the basis for determining what repairs needed to be made. Increased instrumentation data and other new information allow the Corps to make better decisions about flood storage, enabling them to see and react quickly to any future signs of concern.

A3: The Corps has confidence that the completed repairs have restored the dam, and operations have returned to its designed capacity.  The Corps is in the process of finalizing its assessment of seepage failure risk now that all proposed repairs are complete.  However, even though Howard Hanson Dam has returned to full operational capacity, in partnership with a functioning levee system downstream, the dam does not eliminate all risks of flooding. The dam and levees only reduce the risk of flooding resulting in a 140-year flood protection level.

A4: All repairs were completed by the end of 2012 and no further repairs or upgrades are scheduled at this time.  Our highly experienced team continues to monitor and assess the dam’s performance with state of the art technology with a commitment to public safety.  Recently, rainfall and flow rate during the February 2012 flooding was among the top ten events in Howard Hanson Dam’s history, but many people may not have realized the potential impact because the dam functioned according to design. Without Howard Hanson Dam holding back flows, estimated flows on the Green River would have been near 22,000 cubic feet per second, nearly double the levee system’s maximum design capacity of 12,000 cubic feet per second measured in Auburn, Wash.
A5:  Yes.  The combination of the grout curtain and the additional drainage improvements in the right abutment are effective risk reduction measures.  Monitoring completed during the 2010 conservation pool showed that seepage is more effectively controlled now through the area of highest concern in the right abutment.  The Corps also improved the drainage of the right abutment by installing drains that more effectively direct seepage into the drainage tunnel.
A6:   We have confidence in the project’s capabilities and operated the dam to its design capacity this past flood season. However, the dam has never held a pool above 1,189 feet in elevation, and in the event we experience a pool above that level, a thorough monitoring plan is in place.  Testing the new drainage systems was conducted up to the summer reservoir level (elevation 1,167 feet), and additional monitoring is done on any pools held by the dam.  While the dam’s purpose is to reduce the risk of flooding, it does not eliminate those risks and residents living in affected areas should analyze individual risks and construct an appropriate emergency plan.
A7: Following a record high level of water behind Howard Hanson Dam in January 2009, the Corps of Engineers became concerned after discovery of two depressions on the right abutment, increased water levels in groundwater monitoring wells, and the appearance of sediment-laden water entering the abutment drainage tunnel. To date, engineers have excavated the depressions, installed additional monitoring equipment, made improvements to the abutment and conducted tests while water was held at the traditional summer conservation pool elevation. Scientists and engineers agree that the two depressions formed for reasons that never significantly affected the safety of the dam’s operation.  We constructed a grout curtain to address the seepage in the right abutment that was a concern and added drainage wells to direct water into an existing drainage tunnel.
A8: The Corps of Engineers has provided to King County and the local communities a variety of possible mapped flood scenarios, and local emergency management officials have used that information to develop response and recovery plans.  The scenarios that the Corps has provided represent only a few of the virtually limitless possibilities for what may occur in a record storm event or events, and all residents in the valley are urged to plan and prepare for flooding as a prudent precaution.  While the dam’s purpose is to reduce the risk of flooding, it does not eliminate those risks and residents living in affected areas should analyze individual risks to construct an appropriate emergency plan.
A9:  The Corps has operated the dam since 1962 to reduce the risk of exceeding channel capacity on the Green River at Auburn.  The Corps will continue to operate this way.  The dam is kept empty during the flood season except during flood events that would otherwise cause the Green River to exceed downstream channel capacity. In these cases, water is stored behind the dam.  At full design capacity of the dam, the risk of downstream flooding has an annual probability of 1 in 140 of occurring.  Even at full capacity, there is a possibility of the area experiencing a storm or storms that exceed the dam’s capacity.  Flooding in the Green River flood plain will continue to be a risk independent of the dam’s successful operation.  Public safety remains our number one priority.  We encourage local residents and businesses to contact local emergency managers and work with them to determine the best path for being prepared for any contingency.
A10:

The right abutment has seeped since its construction. Howard Hanson Dam's seepage has always been highly monitored and mitigated, allowing for safe operations for nearly 50 years, which in turn allowed the development of the Green River Valley.  The record high flood storage pool in January 2009 resulted in several internal and external changes to the right abutment never before observed that may be symptoms of internal erosion within the right abutment. The sediment-laden water in the drainage tunnel, high water levels within the right abutment and the depressions that were discovered after the January pool of record alerted the Corps to a new situation. Well construction and drainage tunnel improvements were complete on schedule in November 2011, which was the beginning of flood season in the Pacific Northwest.   No additional work is required at present to reduce the risk associated with seepage in the right abutment.  The Corps continues to closely monitor dam operation and will continuously reassess the pool capacities as conditions change and after careful deliberation may change the capacity of pool storage.

A11:  Howard Hanson Dam has never presented immediate danger to people and property below the dam.  However, the Corps urges citizens to contact their local emergency managers and work with them to determine the best path for being prepared for any contingency. Public safety is our number one priority.

A12:  The Corps is in frequent communication with county and city officials and emergency managers. The Corps urges Green River Valley residents to be in contact with those agencies to ensure preparedness for any contingency.  Any emergency messages regarding evacuation will come from those entities.  For preparedness information for your community, you may want to begin with King County’s portal to local emergency management organizations:
http://www.kingcounty.gov/safety/FloodPlan/GreenRiverValley.aspx

Or their Preparedness website:http://www.kingcounty.gov/safety/prepare/residents-business.aspx

A13: Since the flood of 2009, the Corps has installed more than 30 piezometers, which measure water levels within the right abutment.  This instrumentation, combined with on-site visual monitoring, provides a real time determination of water levels, and any undesirable material movement in the right abutment.  The Corps has also installed weir boxes within the drainage tunnel to collect and measure the volume of water coming from the various vertical and horizontal drains in the drainage tunnel. Transducers that measure water level with the weir boxes were installed so that the flow through the weir boxes can be automatically calculated and displayed real time.  The Corps also installed automated turbidity meters in some weirs to alert scientists to any sudden movement of material through the drains.  To date we have seen no unusual turbidity readings.