I am doing a flood certificate for a property that is slightly different from any that I have ever done. The property sets on a point, and is surrounded by the flooding source on three sides. Here comes the tricky part: In the 1,000 feet that the creek travels around the house, the flood elevation drops 15 feet. My gut is telling me to go with the higher elevation, but I've never encountered this before. This is an update of an EC that I did 15 years ago. If I use the higher elevation the house in the flood zone, if I use the average they are fine.
This insurance company is requiring a new certificate because they just published an updated study.
Is there something like a waterfall anywhere in that 1000 feet? Mother Nature doesn't normally like such steep descents unless there is no other choice. This must be a case of occasional downflows being extremely large, as compared to water backing up the creek from a large flood below. Based on the actual topography, where would the water first leave the creekbed and run directly towards the house?
There is that much fall. The creek is down to bedrock, but usually only has a small trickle running through it.
The water could actually get out of the banks at the upper elevation, as well as anywhere along the route we are talking about. The problem is, I have trouble believing that the water could back-up any amount - given that it would simply sheet drain to the lower elevation once it got out of it's banks. I am leaving the flood hypothesizing to FEMA, I was just wondering if anyone knew which elevation FEMA would hold.
I once submitted a LOMA using the average elevation, and FEMA denied it because they used the higher upstream elevation. On the other hand sometimes I think it depends on who is reviewing you. The seem to not be so consistent at times.
What does the FIS profile show?
I was referring to the FIS profile.
You said your experience was that FEMA wanted to use the higher elevation in the past. Isn't varying from that now just wishful thinking?
This property has a unique configuration. I spoke to a mapping specialist and a LOMA is in order.
Good deal. Probably should have started with them first I guess.
I would hold the highest upstream elevation at a point where the stream channel is perpendicular to the structure, per FEMA's rule.
Some of their rules don't seem to make a lot of sense, but we have to go by them.
One is calculating flood openings within 1 foot of grade. In N.C. most openings are air vents, not flood vents. If they are 'permanent' openings (can't be closed), the vents within 1' of grade are usally on the high side (HAG) of the the structure. The openings on the low side may be 8' above grade (LAG), depending on the slope, because they are constructed at the same elevation in the foundation. Now I have to calculate the square inches of the openings on the high side, the side that is within 1' of the adjacent grade. This doesn't make sense, but it is their rule.
Actually, they changed that rule so can count flood vents within 1 foot of grade on the outside, or within 1 foot of the floor on the inside.