Closely spaced cabins; creating water rise/contributing to flooding?

[dylansdad]

I hope I worded the subject to adequately convey my question. I was contacted by a board of directors (AHJ) who are concerned about recent flooding events to some of the cabins located in a fairground area (see attached). Some of the cabin owners occasionally demolish/re-build (as per their right under certain guidelines)as generations pass/ownership passes. Older generation cabins were conventionally framed, 2 to 3 feet above ground, but newer generation owners prefer the slab on grade system. The board is concerned that the newer slab on grade cabins will cause a water rise/flood exacerbation because water is not allowed to "pass under" the cabins in the areas prone to flooding. These cabins are spaced very close together, say around 3'. In my mind this has nothing to do with increasing water rise whether the water passes "under" or "around". In my mind, my solution would be to find some of the cabins that don't flood, place a foundation/block wall system and slab on grade there. I would appreciate any comments on this. Thanks.

 

[phamENG]

I think it'll depend on whether that's just a flood zone or if it's classified as a flood way. If it's simply an area where water rises in heavy rain or tidal events, then I don't think it would cause a significant issue. If it's in a flood way, then it'll be an issue either way.

Raised foundations with flood vents/openings are designed to minimize the differential head on either side of the wall, not alleviate flow issues. If water is flowing such that you get some sort of ramp up due to the obstruction caused by the house, then an 8"x16" hole every few feet in the solid wall isn't going to change things (but it might make some neat water-cannon like effects under the house as it slams into piers a few feet away...yikes). In that case, any new cabins should be built on open foundations if they will be rebuilt at all (which they shouldn't, but that's an argument for another place and time, I guess).

 

[dylansdad]

Thanks for the reply, phamENG, I concur with your assessment. BTW, the shaded area in the attachment in the OP is in Flood Zone A, Special Flood Hazard Area, no BFE determined.

 

[phamENG]

I'm going to second guess myself a little bit here. How large is the flood zone? If the area of the houses is small compared to the flood zone, it'll have little impact. If the volume displaced by the houses is large in comparison to the overall retention of the flood zone, however, you may get a measurable increase in flood heights.

 

[TerryScan]

You may wish to check the construction regulations for the cabins within the flood zone. In an undetermined A Zone (no BFE) I thought one needed to determine a site specific BFE by either simplified or detailed methods.

A simplified method would be contour interpolation of the mapped flood zone boundaries nearest the structure (from FEMA documents such as "Managing Floodplain Development in Approximate Zone A Areas".)

One would then typically be required to construct the floor system above that BFE.

 

[dylansdad]

TerryScan, not sure about any regulations (have not asked, going for site visit Monday and will see) regarding building in a flood zone in this area. The county in which this is located has adopted flood zone regulations and I am fairly certain (but will find out)they would pertain to this situation. My main question was, and I might simplify it as asking how many marbles can I drop in a pond before I notice a water rise and phamENG hit on it at 19:34. The flood zone area is large and the amount of cabins located within the area would be very low in comparison. Thanks for the comments.

 

[swazimatt]

the fairground may still operate fine (depending on your assessment) but surely there are insurance risks to the cabin owners. If the new cabins are built knowingly below flood level in a flood zone the insurers may use this as a reason not to pay for flood damage

 

[zelgar]

In addition to your rules, the owners of the cabins may need to contact the appropriate State agency if they're constructing within the 10-year floodplain or in the floodway. New construction will probably be restricted if it is in the floodway (as this is within the channel of the floodwaters). Flood insurance is another factor that your cabin owners may need to consider.

 

[dylansdad]

As a follow up, I got with our County Floodplain Administrator, as he said that the county adopted a Flood Damage Prevention Ordinance which requires the establishment of BFE for elevation certificate purposes along with the floor to be 2' above. Flood venting is required for elevated SOG.

 

[phamENG]

Flood venting is required for elevated slabs on grade? What? How is that even possible?

 

[dylansdad]

Mispoke, flood ventilation required for enclosed elevated floors.