IS FEMA WRONG?

Yes I have seen this before. A restriction in the flow can cause the flood elevation to rise on the upstream side of the restriction. Without review of the data there is no way to determine if the data is correct. If it a problem I would suggest having a flood study performed by an engineer.

Andy

Easily could be due to hydraulics in the stream. You're assuming a hydrostatic situation, but in reality during a flood there are hydrodynamic forces at work. If the higher BFE is on the upstream side of the bridge, I'd probably tend to believe it. Bridges act a small dams, constricting the stream flow and backing up water on the upstream side, especially during a flooding event.

That was the only thing I could think of.

thanks

The hydraulic jump should be visible in the stream profile in the FEMA study that accompanies the FIRM. You are using the steam profile to determine the BFE, aren't you?

JBS

Would the easiest thing not be to cross reference the flood study profile?

That's nice of them to put one on the downstream side, I actually have not seen that before. In the past they would allow one to use the BFE at the upstream side of the bridge for anything downstream of it. Many times this was the best information without large cost to the landowner, but it resulted in unrealistic high BFE. And the general rule was nothing upstream could be based on the bridge BFE, but they did allow us once to use it for a property bordering the bridge upstream.

Study? What study? 🙁

culvert analysis

> Easily could be due to hydraulics in the stream. You're assuming a hydrostatic situation, but in reality during a flood there are hydrodynamic forces at work. If the higher BFE is on the upstream side of the bridge, I'd probably tend to believe it. Bridges act a small dams, constricting the stream flow and backing up water on the upstream side, especially during a flooding event.'

yes, it would be called 'inlet control' if the bfe was higher on upstream side of bridge.
'outlet control' if bfe was on downstream side, in other words, something downstream of the bridge opening creates more backup than the bridge opening itself

Is it really hydraulic jump? Hydraulic jump is the slang or jargon term for supercritical (usually steep) flow to subcritical flow (calms waters run deep or something like that). The hydraulic surface rises going downstream.

I think he may be talking about a backwater situation where the culvert capacity is not the same as the channel capacity therefore the water pools on the upstream side of the railroad which actually is a very handy tool Engineers can use to make a detention effect.

After the 1986 Floods Placer County "reinterpreted" the Rational Method in their drainage standards. They took out the initial time of concentration (usually 10 or 15 minutes) which had the effect of substantially increasing the design Qs. So the required Culvert sizes instantly became much larger. During the next big Pacific Coast Pineapple Express flood event they found out that all that did was send all of the water downstream instantly flooding low lying properties that never flooded before because of the unintentional detention that was designed into the system. Soon after they came out with a thick Stormwater Design handbook which was a more rigorous and balanced approach to the handling of stormwater.

Followed by a blank stare...

That's the typical response I get when I mention them. The study is the document used to prepare the FIRM which lists the BFE rounded to the nearest foot. The stream profile in the study is what the surveyor is supposed to use to determine the BFE, not the FIRM.

JBS

Of course, you're right, Dave. Perhaps I should have noted it as a "sudden change in the HGL" whether up or down.

JBS