Flood Fence - Questionable calcs

[Pelotoner2]

There are companies providing temporary flood fences around structures to keep rising water out during a storm. These are only for rising water and not for wave action. We have seen these successfully deployed on the news in FL after Hurricane Helene, and have just received s/s shop drawings for one such site. I am questioning the calculations.

See attached. The flood fence is effectively continuous equal leg braces with struts at some spacing (braces and struts are red in the pdf). The issue I have is using the water weight above the horizontal leg to resist the overturning of the brace. In the calcs we received, the anchor tension force required is zero, though they still provided an anchor.

It seems like the rotation would displace the water above the brace, whereby the system rotates. What am I missing?

 

[TRAK.Structural]

Is there a continuous sheet/plate or something in the horizontal plane that lays flat against the ground or just discrete framing at regular intervals?

This one makes my brain hurt a little tbh. My gut says the weight of the water will provide some amount of resistance, but since it's not really a contained system I see your point about the water more or less just displacing with rotation of the fence.

 

[fel3]

Without question, the weight of the water over the base of the flood fence (plus whatever weight the flood fence assembly has) is more than enough to resist overturning from hydrostatic forces, and under these conditions the anchor would see zero tension.

But what about situations where there is little to no water, such as before the flood? As shown, the anchor would be fine for positioning the flood fence and keeping it from moving horizontally (e.g. during installation). The anchor would also help for wind coming in from the right. But if the wind came in from the left, I could see this thing flipping over unless it's got enough weight. I would be inclined to have anchors along both edges of the base to resist wind loads from any direction. Anchorage design would be based on wind loading.

 

[ANE91]

The anchor probably helps keeps the whole thing from floating away or being pushed around laterally.

Are you being asked to seal these?

My take is that they (the manufacturer) have done whatever the bare minimum is to sell a product. As others have pointed out, nothing about this calc approaches reality. I have no doubt that they “work,” some of the time. I just don’t see the point. If you’re on the coast, wouldn’t you be elevated over breakaway walls?

The calc makes sense for only the very specific assumptions stated and in a 2D world where water doesn’t flow or break, as waves.

 

[phamENG]

If you’re on the coast, wouldn’t you be elevated over breakaway walls?

If you're in a house built in the last 10 years, probably. If you're in a 100 year old house, or a 50 year old hospital, police station, fire station, or whatever? No. These are meant to protect existing buildings from floods or storm surge that were not designed for dry or wet flood proofing or elevation above expected floods. Or in areas where the flood maps don't quite represent the full hazard for extreme events.

@Pelotoner2 Your question is valid, but it all comes down to the stiffness of the fence. The water is more than sufficient to hold it in place. If the vertical leg deflects too much and enough water shifts past the base and exerts a downward pressure on the now slanted vertical face, then yes, you'll start to have a problem as it rotates. But unless we know how stiff that is, we can't know how much of a problem that is. The anchor is there to hold it in place to assist with assembly and make sure it doesn't slide around as the winds pick up before the surge comes in.

 

[Compositepro]

For the fence to use the weight of water as ballast to hold it down, there must be no water pressure under it. This requires a good, low-resistance drainage path for any water that leaks under the base edge farthest from the fence wall. This is also the area where you want to form a seal to the ground to stop leakage. This is also the area where there is little weight to create a seal. An inverted cup shape for the base would help, along with a sump pump I side the fence perimeter.

 

[lina124]

Is there a continuous sheet/plate or something in the horizontal plane that lays flat against the ground or just discrete framing at regular intervals?

This one makes my brain hurt a little tbh. My gut says the weight of the water will provide some amount of resistance, but since it's not really a contained system I see your point about the water more or less just displacing with rotation of the fence.

It sounds like you’re asking whether there’s a continuous surface or just spaced framing under the fence. If it’s just framing, the water could flow and shift more freely, which might allow the fence to rotate more easily.

The weight of the water might give some resistance, but since it’s not contained, it’ll probably just move around and not stop the rotation completely. Definitely a tricky one to picture.

 

[XR250]

Wouldn't this have to have a waterproof base? i.e. If water starts seeping under it, won't that negate the pressure of the water above it?

 

[phamENG]

Wouldn't this have to have a waterproof base?

Yes. Not entirely certain how they accomplish it, but it works.

 

[Compositepro]

I'm guessing that in that picture the fence is up against the curb between a roadway and sidewalk.

 

[phamENG]

Possible. But they have plenty of other examples where they clearly are not up against a curb, and some that are. This is just the one that got them all the attention recently. During the hurricanes that affected Tampa Bay a few months ago, these kept the water out of the largest hospital (by number of beds) in Tampa Bay and the only level 1 trauma center in West Central Florida.

These are a proprietary system, though, so nobody should be expecting them to reveal all of their trade secrets. If the OP is looking at an imitator, I'd be cautious and demand testing. But it's not some impossible concept - it's proven tech. AquaFence

 

[lina124]

Wouldn't this have to have a waterproof base? i.e. If water starts seeping under it, won't that negate the pressure of the water above it?

It sounds like you're wondering whether there’s a solid, continuous base under the system or just discrete framing at intervals. If it’s only framing, water would be free to move and shift, which could make it easier for the fence to rotate, your gut makes sense while the weight of the water might provide some resistance, because it’s not contained, the water could just flow and redistribute, making it less effective at preventing rotation, as for the waterproof base, you’re absolutely right. Without one, any water seeping underneath could cancel out the pressure above, undermining the system’s stability. It’s definitely a tricky dynamic to visualize.

 

[Smoulder]

Water seeping underneath equals downward pressure at back edge. Zero uplift at dry edge. No water head there. 1.5 FOS if H=L if no errors trying to do it in head.