100 Year Flood Plain???

Old406Kid

Old406Kid

Legend
How does a person determine what the 100 year flood plain is?
I have a bridge that crosses a year around creek that is in need of repair which would be the easier route but I would prefer to put in a new bridge 100' downstream where the span would be @30% less. One of the first criteria mentioned for the new location is that the bottom of the bridge would need to be
3' above the flood plain level.
I pulled up a FEMA map but it showed no data available for my location.
Any help would be greatly appreciated.
 
Try floodsimple.com?

My county website shows floodplain info. Maybe your does too?
 
In the wake of the terrible flooding of BC and WA in November of 2021 (and recent massive flooding in other parts of the world), I seem to recall opinions that our current estimates of 100 year flood levels are obsolete, given accelerated climate change. So maybe FEMA is in the process of updating its information. Are there other agencies in your area that could give you advice?
 
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Zak said:
Federal/FEMA: https://msc.fema.gov/portal/home

State of Washington: https://gis.ecology.wa.gov/portal/apps/webappviewer/index.html?id=7779e901b22340f8892c8dcb1181a677 (turn on the "Legend" and look for "1% annual chance (Floodplain)"
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I started with the FEMA portal and just tried the Ecology site which brough up a map but unfortunately got this.
"No digital data"
I guess I'll be making a few phone calls tomorrow. This property is at the end of the line as far as developed property and is Forest Service land beyond in a sparsely populated county of eastern WA.
 
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The FEMA maps will only give you the areal extent of the floodplain for various flood return intervals, e.g. 2 year, 10 year, 100 year floodplain. But that’s mostly for major rivers. What you need is the flood height for a 100 year flow for wherever it is you want your bridge, and it sounds like it’s not a major river but a smallish stream maybe. This is usually done by surveying the channel upstream and downstream of your bridge site. Then you determine your catchment for that point, and run it through the HEC RAS model to get the 100 year flow quantity and height at your bridge site. Then you can make sure the bottom chord of your new bridge is the required height above that water surface elevation.
Probably not what you wanted to hear.
 
With climate change, it’s not a bad idea to add some extra capacity under stream crossings.
 
Matt B said:
The FEMA maps will only give you the areal extent of the floodplain for various flood return intervals, e.g. 2 year, 10 year, 100 year floodplain. But that’s mostly for major rivers. What you need is the flood height for a 100 year flow for wherever it is you want your bridge, and it sounds like it’s not a major river but a smallish stream maybe. This is usually done by surveying the channel upstream and downstream of your bridge site. Then you determine your catchment for that point, and run it through the HEC RAS model to get the 100 year flow quantity and height at your bridge site. Then you can make sure the bottom chord of your new bridge is the required height above that water surface elevation.
Probably not what you wanted to hear.
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Yes, smallish year around stream that comes out of the ground a couple of miles upstream of me.
The sad part is that it doesn't fluctuate much more than 6-8 inches yearly and has been in the family since 1944.
Seems like a lot of headache but...
 
I would work with whatever agency needs to approve the bridge. I’m a hydrogeologist by training so water flowing on top of the ground is beyond my expertise. ;) I’ve found that even though I thought that I understood the regulations, those who approve the permit have the ultimate say.
 
Fwiw even if you get a fema map you need to really look at it and possibly need to amend it. I was building a trail bridge (actually not building it as our bids came in over the funding we have) and the fema maps missed the creek entirely and showed the floodplain on a bluff 30 feet above the creek. We had to pay to get the property surveyed and had to petition to change the fema map.
 
Suspect you need to pay a lisenced consulting hydrologist to estimate the flood plain elevation at your site and generate a report.. Much easier if the creek has a gage, where historical flow records are available, and if you have some sort of site survey data. Possible from other records, but if you need something official..

Check what documentation requirements are. Agree historical data are losing ability to predict flood intervals.
 
Matt B said:
The FEMA maps will only give you the areal extent of the floodplain for various flood return intervals, e.g. 2 year, 10 year, 100 year floodplain. But that’s mostly for major rivers. What you need is the flood height for a 100 year flow for wherever it is you want your bridge, and it sounds like it’s not a major river but a smallish stream maybe. This is usually done by surveying the channel upstream and downstream of your bridge site. Then you determine your catchment for that point, and run it through the HEC RAS model to get the 100 year flow quantity and height at your bridge site. Then you can make sure the bottom chord of your new bridge is the required height above that water surface elevation.
Probably not what you wanted to hear.
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Exactly what I would of said. Built up areas will have detailed studies with elevations determined, more semi-rural areas a modeled line on a map, and rural areas with small streams will have no study at all. I did 10-20 of these cross section/hec raz (I used quick 2) projects a year when I was practicing. I would contact a local surveyor/engineer to discuss details and costs. Most likley they have worked with the planners, bridge designers, ect on these projects before and can help not only with this flood plain analysis but the other shoreline permitting tasks as well. First step is always get a check list of needs for your project from the building official, sounds like you have done that. Next steps usually require professionals.
 
Thanks to all that have responded.
I was hoping for an easy answer but that doesn't look to be. :(
 
Old406Kid said:
Thanks to all that have responded.
I was hoping for an easy answer but that doesn't look to be. :(
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I don't know how far along you are or how comfortable you are with this. Approved projects are public record, maybe the planner/building official would walk you through an approved project file to see exactly what they want. Also, the officials are not supposed to recommend someone, but maybe ask what firms (plural) have submitted recently, or have been good to work with in the past.
 
Tom Butler said:
I don't know how far along you are or how comfortable you are with this. Approved projects are public record, maybe the planner/building official would walk you through an approved project file to see exactly what they want. Also, the officials are not supposed to recommend someone, but maybe ask what firms (plural) have submitted recently, or have been good to work with in the past.
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Thanks Tom, not sure what I'll do at this point as it sounds like it will be more than I can justify spending.
That said, I'll be making some 'inquiry' calls this week.
What really sucks is that the majority of the 30 acres is on the other side of the creek.
 
Old406Kid said:
Thanks Tom, not sure what I'll do at this point as it sounds like it will be more than I can justify spending.
That said, I'll be making some 'inquiry' calls this week.
What really sucks is that the majority of the 30 acres is on the other side of the creek.
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Good luck with your project. Ask lots of questions, that parts free. A good resource is the guys who build them. Maybe you could find out who's done one in your area recently and ask them about your project.
 
Ummm... Didn't you ask the same/similar question a year or so back?
 
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