Stillwater depth elevation, Ds

[DG57]

In Chapter 5 of ASCE 7, equation 5.4-3 has the stillwater depth elevation, Ds as:
Ds = 0.65(BFE - G)
However, the Coastal Construction Manual has it as:
Ds = (BFE - G).
Does anyone the origin of the 0.65 factor? It makes a BIG difference in calculating the breaking wave on vertical walls.
Thank you.

 

[GC_Hopi]

My understanding is that this is from the definitions used. ASCE 7 use BFE which includes wave height and the Coastal Construction Manual uses design stillwater flood elevation. Its six of one or half dozen of the other. Best to pick a methodology and stick to it.

 

[phamENG]

DG57 - you haven't transcribed the CCM equation properly.

d[sub]s[/sub] = E[sub]sw[/sub] - GS

where:
d[sub]s[/sub] = design still water flood depth (ft)
E[sub]sw[/sub] = design still water flood elevation in ft above datum
GS = lowest eroded ground elevation, in ft above datum, adjacent to a building, excluding effects of localized scour around the foundation

So it isn't BFE - GS. That's an important difference. As you can see from GC_Hopi's post as well as ASCE 7-16 Figure 5.4-1, E[sub]sw[/sub] is generally taken to be 0.65*BFE. So the results are equivalent. Keep in mind, however, that many jurisdictions will not let you design to BFE. Where I am in the Mid-Atlantic, each municipality can set their own minimum freeboard in the local flood ordinance. Most are 3ft. So I would use this equation:

d[sub]s[/sub] = (DFE/BFE)*E[sub]sw[/sub] - GS

where DFE = Design Flood Elevation (BFE + Required Freeboard)

 

[DG57]

Ummm, now I'm confused.
In my case its an ocean front property.
The BFE (by FEMA) is +10.0 NGVD.
The breakaway wall (masonry) is bearing on a concrete grade beam (pile supported). The top of the grade beam is at +12.0 NGVD. The wall is 10-ft high (begins at +12.0 NGVD and ends at +22.0 NGVD).
The DFE for this site is established, not by FEMA but by the Department of Environmental Protection. The DFE is +18.2 NGVD. Erosion is not being considered (i.e. structural slab all around).
So, as I see it, using ASCE 7-16, ds=0.65(18.2-12)= 4.03 ft.
Another engineer doing the peer review, says I'm incorrect and ds=(18.2-12)=6.20 feet.
Who is right?
As I understand your posts, I should be right. Am I missing something?
Needless to say, thank you for taking the time to provide your response.

 

[phamENG]

I would check with the DEP. See if their DFE is based on still water depth or wave crest. If wave crest, you're right. If still water depth, the reviewer is right.

These both assume that your erroded ground surface is the same as the top of your grade beam.

Erosion is not being considered (i.e. structural slab all around)

This doesn't make sense. The presence of a building or a building component has little to nothing to do with erosion. Erosion is generally a macro effect that results in large scale sediment transfer away from an area due to wave action in a storm. Scour would be dependent on the building geometry and, while not so important for the flood loading, it is important for considerations of pile fixity and elevation of the pile head above the soil. And a slab results in some pretty insane scour.