Foundation Design For Scour

[RFreund]

I am trying to better understand the foundation design requirements for Scour.
According to the IDOT manual you would design your foundation based on Q100 flood and "check" the design based on Q500. By "check" they mean using a reduction factor = 1.0.
My questions are:
Is this correct? My example uses IDOT, but I would be glad to hear general comments.
If a flood level less than Q500 causes a water topping event, would you use this as your check? What limit states need to be checked in this event?

Thanks!


EIT

http://www.HowToEngineer.com

 

[OSUCivlEng]

Here in Oklahoma I design for three scenarios. First, I check the foundation with the 100 year scour and the 100 year stream pressure (water surface at Q100), this would be the Design Flood in Section 2.6.4.4.2. Second, I check the foundation with the 100 year scour and a dry stream. Finally I check the foundation with the Q500 scour and the 500 year stream pressure (water surface at Q500), this would be the Check Flood in Section 2.6.4.4.2. The first two cases I check with Strength I,III,IV, V and Service I, which is probably overkill but it is done quickly with analysis software. For the 500 year event I only check with Extreme Event II per section 2.6.4.4.2. and 3.4.1.

You can read in section 2.6.4.4.2 about if you should use the overtopping flood versus the 100 year or 500 year event. For the design flood it is supposed to be the more severe of the 100 year or overtopping. The check flood can be the 500 year event or an overtopping event of lesser recurrence interval. My take on the check flood is you are trying to prevent the bridge from being washed away in a 500 year event.