I'd like to get opinions of what you would call the BFE for the property (red house) in this image:
I looked through the pile of LOMA's just to the west, and it seems like all of those are based on 40.0 feet. The tiny BFE lines directly south of my property (by the two small bridges) are 40 feet. I have no idea how FEMA came up with 43 feet just to the east of the house and 40 feet north and south of it. The ground elevations at the western end of the 43 foot BFE line is essentially the same (~40 feet) as the ground elevations near my property and the LOMA's to the west. Seems like there should be a vertical BFE line somewhere to help delineate this area better.
I have actually taken this data and created an interpolated surface and that says 40.7 feet. But, I'm not convinced that is right either. I'll probably contact my floodplain administrator for his opinion, but I would like to get input and thoughts from you guys as well.
Thanks
Have you checked the FIS?
Haven't looked at the FIS for this property yet, but I've looked at it for this community before. Not sure how helpful it will be as the property isn't going to fall on any specific cross section. The map above shows a cross section to the NE, but that cross section clearly doesn't influence the area to the NW of the property. FEMA has a 3' difference in BFE between those two areas. Something really strange is going on with the BFE in this spot. There's no obvious topographic or hydrologic reason for a 3' difference that I can see.
LSU map won't give me the BFE there. Just ground elevation and the same BFE contours shown above. I use that website all the time as a rough tertiary check of my ground elevations to ensure I don't have a big bust somewhere in my GPS data.
Obviously this is why there are so many LOMAs. Maybe the 40 is wrong... first check the FIS. Even if you think it won't help, still it's the first place to look.
I was always told the FIS is the most accurate (if you can read the stupid squares on the graph).
I reviewed the FIS and here's what it shows:
Numbers in blue are from the FIS. Keep in mind that all of the LOMAs in the vicinity of E. Riverdale Avenue appear to be based on 40.0, since many of the LAGs are between 40 and 41. So it would appear that FEMA believes that the BFE in this area should be 40.0. I just can't get past the 43' line so close to the 40' line without any real logical way of interpolating between them. So the pink house should be built 3' higher than my red house only 1,200 feet away?? Looks like a poorly drawn FIRM in my opinion.
I would ask the local Floodplain Administrator, since that is his job.
I have already left a message for him. That is who I am relying on in this case. I just wanted to get feedback from the peanut gallery! But this portion of the map would really be screwing with somebody, but I'm not sure who. Red house guy would be happier for insurance purposes, but pink house (assuming he actually rebuilt that high) would be better off in the long run from flooding.
What's going on past the upper right hand corner of the picture? That 43 line cuts off right at the lower peninsula of that Zone X.
I've seen cross section errors on FIRMS. I don't trust them. I only use the FIS.
Rob O'Malley, post: 414592, member: 154 wrote: What's going on past the upper right hand corner of the picture? That 43 line cuts off right at the lower peninsula of that Zone X.
I've seen cross section errors on FIRMS. I don't trust them. I only use the FIS.
The 43 line keeps going on the next panel. It appears the FIS cross sections have been transposed onto the FIRMS correctly. The adjacent profiles just don't jive with each other, so the area in between them is in no-man's land.
I think the 40 is the error. I found it on teh interactive map, and if you zoom around everything is higher BFE to the North and West, and it looks to be diminishing to the South. Your 40s, I think should actually be 42/43.
BTW, those interactive maps are way cool.
Look at the Historical FIRM.
The flooding source for the RED houseÛªs 40 BFE is a tributary Honey Cut Bayou and the flooding source for the PINK houseÛªs 43 BFE is a tributary of the Amite River. I donÛªt know if they took backwater effect into consideration. They may have modeled each flooding source separately and then combined the floodplains?
Hope this helps,
DDSM
Dan B. Robison, post: 414654, member: 34 wrote: Look at the Historical FIRM.
The flooding source for the RED houseÛªs 40 BFE is a tributary Honey Cut Bayou and the flooding source for the PINK houseÛªs 43 BFE is a tributary of the Amite River. I donÛªt know if they took backwater effect into consideration. They may have modeled each flooding source separately and then combined the floodplains?
Hope this helps,
DDSM
I agree with you. I believe that's precisely what happened in this situation. Individual models combined into one floodplain with no designation of those boundaries and no way to intelligently interpolate between them. If you zoom out to a larger area, it's clear that the Amite River would control the flooding in this area, not a 20' wide drainage canal (Honey Cut Bayou). Which is exactly what happened in August. Both the red and pink houses had about 3-feet of water (~43' water surface elevation).
This is a very interesting thread. Your thought process and presentation skills are excellent. I look forward to hearing what your local floodplain administrator has to say. I wonder is there some sort of "tech support" service available directly from FEMA?
Brad Ott, post: 414749, member: 197 wrote: . I wonder is there some sort of "tech support" service available directly from FEMA?
Yes.
First is the local floodplain manager.
Second is your State's Certified Floodplain Manager Association.
Third is the FEMA District headquarters...they usually have PE who will look into special cases, contact the FEMA contractor that made the map, and give you a detailed response.
Then there is the ONLINE CHAT with a FEMA Map Specialist at the Map Service Center.
Hope this helps.
DDSM (CFM)
Brad Ott, post: 414749, member: 197 wrote: This is a very interesting thread. Your thought process and presentation skills are excellent. I look forward to hearing what your local floodplain administrator has to say. I wonder is there some sort of "tech support" service available directly from FEMA?
Thanks. I wear many hats at my firm, including Photoshop spert! As soon as I hear from the floodplain administrator, I'll post back.
I believe the BFE at the point of the peninsula will be the BFE for all of the backwater. The water will not come around the point and climb the hill to a higher BFE. I have done several in the same situation.
andrewm, post: 414995, member: 10888 wrote: Thanks. I wear many hats at my firm, including Photoshop spert! As soon as I hear from the floodplain administrator, I'll post back.
Please do - this is very interesting.
KScott, post: 415013, member: 1455 wrote: I believe the BFE at the point of the peninsula will be the BFE for all of the backwater. The water will not come around the point and climb the hill to a higher BFE. I have done several in the same situation.
Conceptually I agree with you, except in reality there is no hill. Water at 43 ft would (and did) inundate this entire area:
