Upstream Floodplain Limits 2

[amb2002]

When working on a development in/along a Zone A area, I’ll use HEC-RAS to determine base flood elevations for my site. For some reason, almost every site like this that I work on includes an upstream reach of the Zone A area.

My question is: how do you determine where the upstream limit of the floodplain is located? If I have cross-section and 100 year flow information, I can calc a water surface elevation. I can calc one for the parking lot outside my office. Does that make it a floodplain?

Generally I’ve gone with the idea that once the flow occurs totally within the banks of the channel that is the end of the floodplain. Of course, bank location is somewhat subjective.

I have wondered if some sort of minimum drainage area should play a part in my determination.
While it may exist, I have not found guidance from FEMA’s website.

Does anyone have any suggestions for guidance in this situation?
Thanks

 

[RWF7437]

"When working on a development in/along a Zone A area, I'll use HEC-RAS to determine base flood elevations for my site. .............My question is: how do you determine where the upstream limit of the floodplain is located? If I have cross-section and 100 year flow information, I can calc a water surface elevation. I can calc one for the parking lot outside my office. Does that make it a floodplain?"

No. For regulatory purposes your calculations have no meaning unless and until FEMA, and others, have reviewed and approved them. When that is done the FEMA maps may be revised based on your calculations.

As to the upstream limits of a flood study; the study limits may or may not extend all the way to the upstream limits of flooding. FEMA often cuts off their studies at arbitrary but convenient locations such as political boundaries, locations of minimal human development, or simply where they run out of money.

"I have wondered if some sort of minimum drainage area should play a part in my determination.
While it may exist, I have not found guidance from FEMA's website."

I don't know of any minimum drainage area. The question is; does flooding occur and can it cause significant economic damage or threaten human life ? The only guidance you'll find is in the Federal Law and FEMA's voluminous regulations but all such guidance is general based on the idea that flooding can have bad consequences. Also note that the use of the so called 100 year flood is arbitrary and was the result of legislative compromise in the 1960s.

good luck

 

[amb2002]

I understand your meaning on the arbitrary “limit of (detailed) study” locations. I was more referring to locations where the boundary is not truncated, but rather rounded off and ended at some apparently arbitrary location along the channel (and in the middle of the piece of property slated for development).

I definitely take into account the possibility of the loss of property or human life in the site design. The question is more of a permitting issue: as you noted, regulated floodplains are handled differently than areas where I have simply calculated a 100 year water surface elevation.

However, most of the studies I complete are part of a LOMA or LOMR submittal to FEMA, as the Zone A boundaries are very inaccurate, at least in middle/western Missouri where I work. Therefore my calculations have meaning as they will eventually delineate the effective floodplain.

I could arbitrarily truncate my detailed study at the location of the end of the Zone A area as shown by the currently effective FIRM panel, but that looks bad and raises the question from local regulatory officials: why does the floodplain abruptly end? Which leads me back to trying to find a practical upstream end of the floodplain.

My thought about the drainage area size was based on the definition of Zone B: "An area inundated by 500-year flooding; an area inundated by 100-year flooding with average depths of less than 1 foot or >>>with drainage areas less than 1 square mile<<<; or an area protected by levees from 100-year flooding."

Thanks for your thoughts

 

[RWF7437]

Here are some statements I think are true:
The "100 year" flood is NOT the largest flood that can possibly occur.
The water surface elevations calculated using HEC-RAS are generally considered no more accurate than plus or minus 1 foot.
Flooding can occur for reasons other than excess rainfall; for example, snow melt, blocked culverts or storm drains, debris dams, etc.
Stream banks can be difficult to locate and can change markedly over time.
The probability of the "100 year" flood being equaled or exceeded in the life of a 30 year mortgage is about 24% (Odds: 1 in 4) and in 100 years is about 63% ( Odds: 6 in 10, i.e. more likely than not ).

There is no single scientifically and mathematically correct answer to these questions. They are simply a function of how much risk one is reasonably prepared to accept.

good luck

 

[froude]

amb,

If I understand the main purpose of the question, it is to determine where to end the study, upstream. While I have seen cases similar to the scenario you describe above, I can't say that I've personally had to deal with it.

My thoughts are that a fair place to end your study may be at the upstream end of your property. It will probably mean some additional survey and study of currently unmapped stream, but at least changes to the floodplain as a result of your property will be accounted for. On a FEMA map, it may look "weird" to see a studied zone between two Zone A's...but I have seen it.

I think one option may be to sit down with your local floodplain administrator and figure out where to end the study given the circumstances.

Hope I'm not too far off topic. Good luck!

 

[amb2002]

froude,

You understand the scenario correctly. And when the channel (and the Zone A) runs through the entire property, extending the study to the property boundary is what I usually do.

My difficulty is when the upstream arm of the Zone A area extends into the subject property, but not entirely through it. In other words, the Zone A apparently ends within the property, but the channel, ditch, swale, or however you want to describe it, continues through the property.

I was being a bit facetious in my above post when I mentioned calculating a floodplain in the middle of our office parking lot, but it made me wonder if the type of flooding source should come into play somehow. Thinking about it now, regulatory floodplains shown on FIRM panels are located along what I assume would qualify as “waters of the state.” If I had a determination done by the Corps of Engineers that the waters of the state ended at some location “X” within the property, then that might also be a suitable location to end the flood study?

Thanks for your thoughts

 

[froude]

amb,

I agree w/ your hypothetical scenario...in fact it's funny you mention it because a USACE presenter that i listened to mentioned a very similar situation. I think a USACE delineation might prove that the water feature is "water of the United States," but I don't know if that would govern where the study should end.

I see it this way:

According to FEMA, if a stream is shown on a FEMA map, but there is no SFHA delineated at all, a LOMR isn't required. Around my neck of the woods, local authorities will usually still make us do a flood study (and possibly a LOMR if they desire). I would say that if your local reviewer sticks closely to FEMA standards, that your study can end wherever the effective SFHA ends. However, if your reviewer wants a delineation of the entire portion of stream on your property, then you can probably delineate the new portion of the floodplain (and redelineate the existing portion) based on updated/better hydrology and topo and submit that as a LOMR.

I ultimately believe that the upstream limit of your study is really going to have to be determined by meeting with your local floodplain administrator.

Good luck!

 

[RWF7437]

"According to FEMA, if a stream is shown on a FEMA map, but there is no SFHA delineated at all, a LOMR isn't required. "

If there is no SFHA is it even possible to do a Letter of Map Revision ? What would one be revising ? How can a local administrator require a revision of something that doesn't exist ?

Just wondering.

 

[froude]

I believe that if a community wants to add an SFHA to a stream that currently doesn't have one (which is or isn't helpful depending on the scenario), a LOMR must be filed. Even if there is no effective SFHA.

 

[cvg]

Could be a LOMR or a PMR, either one can be initiated by the floodplain manager to map a new SFHA or revise a SFHA.

Letter of Map Revision (LOMR)
A letter from FEMA officially revising the current FIRM to show changes to floodplains, floodways, or flood elevation. Physical changes include watershed development, flood control structures, etc.

Physical Map Revision (PMR)
A reprinted FIRM incorporating changes to floodplains, floodways, or flood elevations. Because of the time and cost involved to change, reprint, and redistribute a FIRM, a PMR is usually processed when a revision reflects increased flood hazards or large-scope changes.