EVALUATING SELECTED SCOUR EQUATIONS FOR BRIDGE PIERS IN COARSE STREAMBEDS IN NEW YORK

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EVALUATING SELECTED SCOUR EQUATIONS FOR BRIDGE
PIERS IN COARSE STREAMBEDS IN NEW YORK
by

• L.J. Welch, Jr. and G.K. Butch

In cooperation with New York State Department of
Transportation
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Field Conditions

• Turbulent flow

Armored streambed
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Scour is the result of work

• Sf stream force (kg ? m/s2)
• D84 grain size gt84 of armor layer (mm).

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Stream Force

• Sf stream force (kg ? m/s2)
• ? water density (103 kg/m3)
• y1 water depth (m)
• w flow width (1 m)
• V0 flow velocity (m/s).

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Database Statistics
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Model Calibration

• Calculated 19 scour depths measured in 1996 (10
  0.0 m)

MEAN ERROR
1996 MEAN ERROR
(m)
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New York Equation (1972-96)
.
Sf stream force (kg ? m/s2) D84
grain size gt84 of armor layer (mm).
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Relation of Scour Depth to Stream Force and Bed
Material
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Sensitivity Analysis
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FHWA Equation
,

• ys scour depth K1 pier-nose shape
  
• a pier width K2 pier shape flow
  alignment
• y1 water depth K3 streambed
  condition
• Fr1 Froude number K4 armoring by
  bed-material size.

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K4 (modified by Mueller)
If D50 gt 2 mm and D95 gt 20 mm and f(V) gt 0,
otherwise K4 1 D50 median grain size
D95 grain size gt95 percent of armor
layer V0 approach flow velocity V /cD50
approach velocity corresponding to critical
velocity at pier for D50 V /cD95 approach
velocity corresponding to critical velocity at
pier for D95 V cD50 critical velocity for
incipient motion for D50 .
,
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Froehlich Equation

• ys scour depth ? pier shape
• g gravity V0 flow
  velocity
• y0 water depth b pier
  width
• D50 median grain size.

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EstimatedScourvsMeasuredScour
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Summary ofEstimated and Measured Scour
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New Hampshire Study
Field measurements
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New Hampshire Study

• GPR fixed instruments

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SUMMARY

• New York equation - scour a function of stream
  force and bed material - derived from field
  measurements - alternative for coarse
  streambeds - estimates associated with specific
  peak discharges
• Mueller modified-K4 and Froehlich equations -
  less error than FHWA equation in New York study