Physical Model of Spillway and Reservoir Debris · PDF fileSpillway and Reservoir Debris...

Physical Model of Spillway and Reservoir Debris InteractionUSSD 2017

4/4/2017Kent Walker PE

Funded by USBR Dam Safety Office

Research Goals

• Estimate change in reservoir WSE with debris

• Estimate discharge capacity reduction with debris

• Index of photographs for use during risk assessments

Literature ReviewL:W ratio or H:W ratioBridge & pier interactionLog propertiesChanges to reservoir head or flowGate operations (0 or 100% open)

Variables• Reservoir

– WSE, shape (long skinny vs. circular), location of spillway

• Spillway– Gate count, gate geometry, gate

opening, approach geometry, approach velocity, pier design

• Debris– Length, diameter, density, branched

vs. pole, irregularity, with or without rootball, quantity and timing of debris flux

– Docks, boats, cars, buildings, propane tanks, etc.

Reclamation’s Inventory• Uncontrolled = ~60%

– ~30% ogee crest– ~20% drop inlet– ~20% chute– ~10% side channel– ~20% other (bathtub, culvert,

tunnel, orifice, etc.)• Controlled = ~40%

– ~60% ogee crest– ~10% weirs– ~10% tunnel inlet– ~20% other (siphon, fuseplug,

side channel, orifice, overfall)

Model design, radial gate ogee crest• 1:18 Froude Scale model • Radial gate

– 2 gate design – Little or no redundancy

Model design, morning glory• 1:18 Froude Scale model

– Relatively small diameter with short drop– Uncontrolled drop inlet with piers

Dimensionless Gate Index• Combines reservoir head,

gate / orifice opening, proximity to surface into 1 term

• Ratio of vertical gate orifice opening to the reservoir head above the ogee crest

• Consistently provides trends that describe tests

• Can be estimated during floods or risk assessments• GI=Go/HR

Go= vertical orifice opening

HR = Head above ogee crest in reservoir.

GI=0.47

Gate Index

GI=1.44 GI=0.70 GI=0.50 GI=0.11

Note: Typically, a GI > 0.8 indicates free flow under the gate

Test materials

Single dowel Log Results

• Trend shows increasing ability to pass as dowel log diameter increases

• Increasing ability to pass as gate index increases • Similar results between different flow rates

Single dowel log with rootball

Cluster of 5 logs• Total logs passed (out of 100 logs)

Large mat tests, GI > 0.8 (uncontrolled)

Large mat tests, GI=0.7

Large mat tests, GI=0.55

Large mat tests, GI=0.36

Greater efficiency at GI = 0.36?

• Streamlines approach varies due to debris and high WSE, results in higher gate discharge coefficient

• Removes clear water vortices near the gate

Videos• Gate Index = 0.36

• Gate Index = 0.70

Changes to WSE relative to Clearwater initial WSE

WSE change statistics

Flow rate change relative to clear water gate rating curve (see notes)

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Slide 22 notes: 

The reduction values are based on taking the final WSE from the with debris condition and interpolating 

a flow rate from a clear water rating curve for each gate setting above. This does not indicate that 

closing the gates gives greater capacity, but that a smaller gate index value will result in a lower impact 

to both changes in WSE and flow rates with debris present. 

Slide 23 notes: 

With clear water, the WSE for a fixed flow rate continues to decrease as the gates are opened to 

uncontrolled flow (100% open). However, there appears to be a stabilization in the WSE for the with 

debris condition once the gates are open to a certain amount. Each data point was taken at a fixed flow 

rate of 10,300 cfs, but the value of the gate opening is different (right hand axis) which leads to a 

different initial WSE for each gate setting.  

Gate index Gate opening 

Initial mean Head above crest (clear water), ft 

Final mean Head above crest (with debris), ft 

Maximum rise in WSE, ft 

0.36  47%  26.5  26.1  0.02 

0.55  57%  20.7  22.5  2.6 

0.71  68%  19.3  21.7  4.5 

1.10  100%  18.2  21.8  5.1 

Head over crest at 10,300 cfs (see slide notes)

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Analysis• Limited tests to date, only 1 flow and 1 spillway

configuration with 5 iterations of each. Results have been very dependent on gate index

• Uncontrolled flow (GI>0.8)– Average WSE rise of 3.5 ft, max = 5.1 ft– Average reduction to discharge capacity of 30%, max = 35%

• Low gate index values (GI=0.36)– Average WSE drop of 0.4 ft, max rise of 0.02 ft – Average increase to discharge capacity of 2%, max drop of

0.6%• Not uncommon for risk assessments to estimate a

40-60% reduction in discharge capacity which may overestimate impact

Upcoming Tests

• Additional flow rate (6,800 cfs) with the existing ogee crest gated spillway

• Installation of Morning Glory spillway and testing• One additional installation of a gated spillway and

one additional morning glory spillway design.

Questions?

Kent Walker PEkgwalker@usbr.gov

303.445.2151