Flood Load Combinations for Flood Walls

[Nick6781]

I'm designing a solid concrete flood wall. I've looked through FEMA P-55 but still have questions on how to combine different flood load cases.

For one case, I think I need to combine hydrostatic + hydrodynamic + impact.

But how should I combine the loads when there is breaking wave? The breaking wave equation has baked-in hydrostatic pressure based on still water height rather than the (DFE - grade) height. Should I do hydrostatic + breaking wave + impact or just breaking wave + impact?

I'm attaching a screenshot from FEMA P-55.

 

[lexpatrie]

I think if you hunt around there are design examples, perhaps in the Coastal Construction Design Manual.

There are a lot of resources, they just aren't all that easily found.

The flood loads are typically pretty high, especially with the height of stillwater.

 

[Nick6781]

FEMA P-55 is a costal construction manual. There appears to be no examples regarding combining different flood load cases.

 

[Smoulder]

FEMA gives two cases for breaking wave force. One is for dry behind the wall and the other is for standing water behind the wall. You pick the one that matches your situation. The difference between them is beneficial water pressure so be sure it will be there if you take advantage of it.

Your screenshot and also ASCE 7 supp 2 both don't require debris impact combined with breaking wave load.

 

[ENGIRL]

I have never designed flood walls but here is what I can say.
when there is a wave, the water is not static anymore it's dynamic. You can not combine hydrostatic and hydrodynamic together (simply because water can not be still and moving at the same time).
I have done analysis of retaining walls where I checked iced impact + hydrostatic load. Hydrodynamic is usually combined with earthquakes for extreme load cases.

 

[Nick6781]

Supplement 2 to ASCE7-22 says this:

I disagree that wave loads should be combined with hydrostatic and hydrodynamic loads, as wave loads themselves are a combination of hydrostatic and dynamic loads (a different vertical distribution than hydrodynamic loads). It's double counting if we do that.

FEMA P-55 says this:

Again, based on ASCE 7 and FEMA P-55, my understanding is to combine:
1. Hydrostatic + Hydrodynamic + impact
2. Wave pressure + impact

 

[bones206]

I disagree that wave loads should be combined with hydrostatic and hydrodynamic loads, as wave loads themselves are a combination of hydrostatic and dynamic loads (a different vertical distribution than hydrodynamic loads). It's double counting if we do that.

This is also my understanding and I agree that the hydrostatic load component is baked into the breaking wave equations in the code. For breaking wave load combinations, you would not need to separately compute the hydrostatic pressure as you would with other the other loading scenarios.