Bonneville Spillway Flow Discrepancy

1. Bonneville Spillway Flow Discrepancy TMT – August 4th 2004 Laurie Ebner Hydraulic Engineering – Portland District US Army Corps of Engineers

2. Flow Discrepancy • Flow discrepancy between Bonneville and The Dalles reported in 2003 • Project discharge at Bonneville should be higher than The Dalles (additional inflow) • Discrepancy appeared to correspond to new spill patterns instituted at Bonneville in 2002 • Initially uncertain if discrepancy at Bonneville or The Dalles. • Web data for The Dalles is verified by comparing to the USGS gauge data at The Dalles (just downstream of the project) – “http://waterdata.usgs.gov/nwis/uv?14105700”

3. Bonneville Spillway Gate History • Originally two gate types • 50 foot high gates (Bays 4-15) • 60 foot high gates (Bays 1-3, 16-18) • Bonneville Rating Curve updated in 1967 • In 1972 all gates were modified to 60 foot high gates and all of the gate bottom edges were modified from a flat bottom plate to one with a 45 degree angle to reduce vibration. • The modified gate edges may have changed the flow characteristics and thus the discharge coefficient.

4. Bonneville Spillway Gate History • Discrepancy between actual gate opening and reported gate opening (gate is approximately 4 inches lower than being reported) for openings of 0-2 feet. • Additional measurements need to be taken to determine over what range this error applies.

5. Planned Actions • Develop plan for calibration of each spillway gate – available for review 8/11/2004 • Calibrate/verify the gate opening for each spillway gate • Verify rating curve (especially for small gate openings) • Verify that changes implemented take care of the historical discrepancy in total river flow between The Dalles and Bonneville • Develop strategy to communicate discrepancy so previous published work can be properly interpreted

6. Additional Facts • Large 50 foot vertical lift spillway gates are not typically designed for fine regulation of flow. The uncertainty or range of error at small gates openings defined as less than about 10% of the gate height (5 feet for Bonneville) is large. • There is always inherent uncertainty when regulating a river system. Sources of discharge error at a large project like Bonneville Dam include: • Tributary contributions • Ladder and other miscellaneous flows • At best, a 5% difference between actual and computed discharge at a project would be considered good agreement.

7. Additional Facts • The calibration errors would be significant only when the project was trying to meet a target discharges such as the 75K daytime spill (set for adult fallback purposes) or some other target. Typically, the project discharges through the spillway at the gas cap discharge – at which time the gas concentration in the river downstream determines the spill volume that can be passed. • When total river flows exceed the powerhouse capacity, the flow is discharged through the spillway, increasing the 75K daytime spill. Reducing the error in discharge measurements since the largest error occurs at low spillway discharges.

8. Spill Discrepancy Estimates Data will be updated after completion of spillway gate calibration Assumes a Bonneville Forebay of 74 feet Desired is using actual gate opening requested and coefficient as determined in 1967 model work. Actual is subtracting 0.3 feet from gate opening and using the same coefficients as desired but modified based on gate opening.

9. Recommendations • Modify the measurement system at Bonneville such that the reported flow from Bonneville is closer to the actual flow. • Develop and Coordinate plan to calibrate each spillway gate. • Perform spillway gate calibration. • Verify that corrections for each spillway gate opening account for discrepancy in total river flow between The Dalles and Bonneville. • Develop communication plan for the region

10. Bonneville Gate Openings in Dogs and Feet Source of information is 1967 rating curve analysis. For Your Information