Pier Scour calculations

Dear HEC-USERS

I am performing a scour analysis at a single pier for a proposed bridge and I am confused about the method that should be used to calculate scour for the following scenario:

The pile stem extends below the river bottom (River bottom Elevation is about -7 feet at the pier) to the top of the pile cap at elevation -13 feet and the bottom of the pile cap is at -17.5 feet . Then piles will be constructed below the pile cap, depth of piles to be determined based on this analysis.

Contraction scour depths are minimal and does not expose the pile cap.

Local scour depths (based on the pier stem alone) for the 500 year is about 23 feet resulting in a scour elevation of about -30 feet.

Local scour depths (based on the pile cap alone) for the 500 year is about 33 feet resulting in a scour elevation of about -40 feet.

Considering that the Local scour depths (based on the pier stem alone) for the 500 year is below the pile cap, I am not sure of the correct Case to use for local scour at the pier that is described in fourth edition of HEC-18 to compute the scour depths.

The third edition of HEC-18 said to use the larger of the two scour components: Local scour depths (based on the pier stem alone) or Local scour depths (based on the pile cap alone).

Any comments on the method that I should use, would be appreciated.
Chris Gernat
Chris_Gernat@URSCORP.COM

Chris,

the correct procedure for complex piers is addressed by the fourth edition of HEC-18. You should negelect the methods presented by the third edition. The correct procedure involves superposition of scour depths calculated independently for the individual pier scour components (stem, cap, piles).

For your situation, you will first calculate local scour based on the pier stem (Yscour stem)and the flow depth immediately upstream ofthe pier (Y1).

Then you will calculate scour based on the pile cap (Yscour cap)and an adjusted flow depth Y2(Yscour stem + Y1)/2. The pile cap calculation has two conditions depending upon whether or not the bottom of the cap is exposed by the adjusted flow depth. Based on your 500 year pier stem scour depth of 23 ft, your pile cap will be exposed. Therefore, your calculation for scour depth based on the cap will be Case 1, which is fairly straight forward. Had the cap bottom not been exposed, you would have had to calculate a reduced flow velocity.

Finally you will calculate scour based on the piles (Yscour piles)and a new adjusted flow depth Y3((Yscour stem + Y1)/2 +Yscour cap/2).

The total pier scour is simply the sum of Yscour stem + Yscour cap = Yscour piles

Good luck - the calculation is lengthy and confusing. For 5 million dollars I'll send you a spreadsheet.

Jim Bernard, PE
bernardj@pbworld.com

Chris,

Bernard hit the proverbial 'nail on the head.' Rever to Section 6.4 of HEC-18, 4th ed. All previous editions of HEC-18 are obsolete and do not represent standard practice anymore.

Cheers,
Jeremy
FranzJ@AYRESASSOCIATES.COM

Chris,

thanks for noticing that the adjusted scour depth equations are written incorrectly. They should be:

Y2 = Y1 + Yscour stem/2, and

Y3 = Y1 + Yscour stem/2 + Yscour cap/2

Jim
BernardJ@PBWORLD.COM