Road Crown and Finished Floor Elevation Next to Wetland? 1

[1SEngineer]

I am reviewing a site in North Florida...

The site is in a FEMA X(unshaded) zone.

There is a river passing by the house on one side, about 1,000 feet away. On that side, the nearest flood zone is AE, which is about 500 feet from the site. The BFE is 5.0 feet.

There is also a wetland on the other side of the property. It's about 300 feet from the site. The survey notes that the wetland is at 9.0 feet.

The existing grade at the site is an average of 10.0'. So, the site elevation is only 1 foot higher than the wetland.

So, the question I have is, what would be an ideal road crown and finished floor elevation (for habitable space) considering that the site is close to a wetland?

I know there needs to be at least 1 foot of freeboard from he BFE. With the exiting grade at 10.00 feet, this is not an issue. However, how should the wetland be dealt with? In reality, the flood elevation is higher than the FEMA BFE.

Can anyone point me to information...codes, website, papers, books, etc...on this topic?

Thank You.

 

[phamENG]

Is there a drainage path from the wetland to the river that doesn't involve the building site? If so, I wouldn't worry too much about the wetland. You say the flood elevation is higher than the FEMA BFE - what data do you have for that? If you have more accurate data, then by all means use it - especially if it results in a safer and more resilient structure.

BFE +1'? Wow. Where I am Design Flood Elevation is BFE +3'. Look at local ordinances for flood zone overlay districts, flood resiliency districts, things like that. They'll often set minimum elevations relative to both BFE and grade for finished floor. Most cities near me are BFE+3' or 3' above grade in the overlay districts, whichever is higher. Outside of the overlay districts you just have to get code minimum fall from the building to ensure positive drainage. Otherwise you can have a slab right on grade.

 

[1SEngineer]

Pham:

I do not see a drainage path from the wetland to the river.

Regarding the "flood elevation" comment, the wetland's SHW is at 9.0 feet. I'm seeing this as the real BFE as stormwater has nowhere to go.

BFE + 1' is the case here.

Even at BFE + 3' we'd be at...5' + 3' = 8'. This is lower than the current grade.

The wetland, in this case, is affecting the drainage and the finished floor elevation must account for this excess runoff.

In this case the road crown should be higher than the wetland by at least a foot. The floor of the habitable space should be 18 inches above the crown.

 

[jgailla]

If the wetland is at EL 9, then seasonal high groundwater is at EL 9 or above. You need your base and subbase up out of the seasonal high. The road should be at EL 11 unless there is other information.
I would have an FFE at minimum EL 12, 3 feet above the wetland elevation.

 

[1SEngineer]

jgailla:

Can you point me to a code or standard that says the the FFE should be 3' above the wetland?

Thank you.

 

[jgailla]

It's not a code and it is very dependent on your location and hydrology.

The way I read your situation, the road needs to be at 11 to stay up our of the groundwater. I assumed that your FFE would need to be higher based on drainage patterns. Local codes are different. Your particular conditions could lead to a much different answer. For instance, if the wetland is caused by a perched water table that is separated from your site, your site groundwater is much lower, and there is no hydraulic connection, an FFE at 12 could be too conservative.

I've practiced in north Florida, but it's been a while since I set an FFE, so I can't remember the code. There are many locations where my original assumptions are correct and many locations where they are not. Once you get into the bluffs the local drainage is more important than the BFE.