SHA Procedures for Computing Scour for Complex Piers

The following procedures are recommended by MDSHA and produce the same results as the methods set forth in HEC-18. Figure 1 illustrates the various options discussed below.

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The Maryland SHA procedure for computing scour at a complex pier is described below. The ABSCOUR User's Manual provides an example of the application of the procedure. As a check, the Maryland procedures were used to compute all the pier scour examples in the FHWA HEC-18 Manual, Fifth Edition, April 2012. The answers from both methods were the same.

Two alternative methods for evaluating pier scour are recommended. Compare the scour computed from both Method 1 and Method 2, and select the method which results in the deepest scour elevation. Use this value as the total pier scour value:

Computing Pier Scour Using Method 1.

Assume contraction scour does not occur. Compute pier scour following the procedure outlined below, using the flow depths and velocities obtained from the water surface model (typically HEC-RAS) and the existing channel bed elevation

Option 1

Option 1 computes local scour for the pier stem only.

  1. Fill in the required information, including the initial flow depth, y1 and the flow velocity V1 as discussed above. Run the program.

  2. If the scour computed by Option 1 is less than the elevation of the top of the footing/pile cap, use this value for the pier scour depth. Then,
    y2 = y1 + (ys)pier
    
  3. If the scour computed by Option 1 is below the top of the footing/pile cap, continue on to Option 2 below. Note that
    (ys)pier = y2 - y1.
    
Option 2

  1. Fill in the information for the footing/pile cap; use the following revised input values for flow depth and velocity.

  2. Set a revised flow depth at an elevation of 1 foot below the top of the footing/pier cap. The total flow depth to this point
    y2 = y1 + (ys)pier cap 
    
    where (ys)pier cap is the pier scour depth between the existing (original) channel bottom and the selected elevation one foot below the elevation of the top of the footing/pier cap.

  3. Compute a new approach flow velocity as
    V2=  V1 * y1 /(y1 + (ys)pier cap / 2)
    
  4. Run the program, and note the computed scour depth. Subtract this computed scour depth from the revised flow depth set in Step 2 above. This determines the scour elevation for Option 2.

  5. If the scour elevation from Step 4 is within the limits of the footing/pile cap use this value for the pier scour. If the scour elevation from Step 4 is below the bottom of the footing/pile cap, go to Option 3.

Option 3

  1. Fill in the information regarding the pile group. Use revised input values for flow depth and velocity as described below.

  2. Set a revised flow depth y3 at an elevation of one foot below the bottom of the footing:

    y3 =  y1 + (ys)pier group
    
    where (ys)pier group is the scour depth measured from the existing(original) channel bottom to the point one foot below the bottom of the footing.

  3. Compute a new approach flow velocity as
    V3=  V1 * y1 / (y1 + (ys)pier group /2)
    
  4. Run the program for Option 3 and obtain the scour depth

  5. Compute the scour elevation for Option 3 as the elevation of the selected point one foot below the bottom of the footing/pile cap (step 2 above) - scour depth (Step 4)

  6. Compare this scour elevation with the scour elevation determined from Method 2. Use the lower scour elevation as the total pier scour elevation.

Computing Pier Scour Using Method 2.

Assume contraction scour does occur. Compute pier scour following the procedure outlined below.

Option 1 for Method 2

Option 1 computes local scour for the pier stem only.

  1. Fill in the required information, including the initial flow depth, y1 and the flow velocity V1 as discussed above. Use the contracted scour bed elevation as the initial bed elevation. Click the run button and note the scour depth computed by Option 1. Subtract this depth from the initial contraction scour bed elevation to obtain the pier scour elevation.

  2. If the pier scour elevation is above the elevation of the top of the footing/pile cap, use this value for the total pier scour. (End computations)

  3. If the scour elevation computed by Option 1 is below the top of the footing/pile cap, continue on to Option 2 below.

Option 2 for Method 2

  1. Fill in the information for the footing/pile cap; use the following revised input values for flow depth and velocity.

  2. Set a revised flow depth(y2) equal to the elevation of the water surface to a point at an elevation of 1 foot below the top of the footing/pier cap. The total flow depth to this point
    y2 = y1 + ys
    
    where y1 is the depth of the contracted scour bed and ys is the scour depth between the contracted channel bottom and the selected elevation one foot below the elevation of the top of the footing/pier cap. (Note: If the contracted channel elevation is already below the bottom of the footing/pile cap, proceed to Option 3)

  3. Compute a new initial flow velocity as
    V2=  V1 * y1 / (y1 + ys / 2)
    
    Where V1 is the initial velocity for Method 2 as defined above and y1 and ys are defined in Step 2 above

  4. Run the program, and note the computed scour depth

Subtract the computed scour depth from the revised flow depth set in Step 2 above. This determines the scour elevation for Option 2.

Option 3 for Method 2

  1. Fill in the information regarding the pile group. Use revised input values for flow depth and velocity as described below.

  2. Set a revised flow depth y3 at an elevation of one foot below the bottom of the footing:
    y3 =  y1 + ys
    
    where y1 is the depth of the contracted channel and ys is the scour depth measured from the contracted channel bottom to the point one foot below the bottom of the footing.

  3. Compute a new approach flow velocity as
    V3 =  V1 * y1 / (y1 + ys /2) 
    
    Where V1 is the initial flow velocity for Method 2 and y1 and ys are defined in Step 1 above.

  4. Run the program for Option 3 and obtain the scour depth

  5. Compute the scour elevation as the elevation of the selected point one foot below the bottom of the footing/pile cap (step 2 above) - scour depth (Step 4)

  6. Compare this scour elevation with the scour elevation determined from Method 1. Use the lower scour elevation as the total pier scour elevation.