Low Chord vs Overtopping Storm Event

[mlc3n3]

Hello,

I am in the process of sizing a proposed bridge. PR bridge alignment is around 60 ft upstream of EX bridge. I’m having some issues with storm events near the low chord or overtopping the bridge; mainly the Q5 and Q10.

Below are a few pictures to help. See the plan view, WSP, and rating curves.

At certain bridge lengths I can increase expansion and contraction ratios near the bridge and the Q10 will have a similar shape as the Q5. At other bridge lengths I can’t seem to change the Q10 WSP.

Do you know what is happening with the Q5 or Q10 year storm?
Is this a hydraulic jump after the bridge? Although the water surface is above the critical depth. Any specific reason why the Q10 doesn’t overtop?
Suggestions for an edit or solution?

Plan view:

WSP Q10 + Q5:

Rating curve at upstream bridge x-sec:

[cameron]

what bridge method (energy, pressure, etc..) is being used for the Q5 and Q10? It is important to note that the default is for the HEC-RAS to use the energy grade water surface elevation at bridges and not the actual water surface elevation.

[mlc3n3]

Hi Cameron,

Q5 is using pressure, Q10 is using weir, Q25 is using energy.

Although the Q10 is using a different equation compared to the other storm events, The downstream WSP doesn’t tie into existing conditions. The other storm events do tie in.

Is there an option I should change in general or to help the Q10?

[cameron]

This is where engineering judgement comes into play. For Q10, does it really over top or does it just say that since it is using the energy grade elevation and not water surface elevation. If it is not really overtopping, you could then either switch the program to use the wse instead of energy or you could adjust the minimum weir elevation to be above the Q10 elevation it is using.

Basically (as long as you are comfortable with it), you want the Q5 and Q10 to use the same method which should fix your crossing problem. Remember, the program is just doing what you tell it to do and you need to decide if what it is doing is correct or not.

I do find it interesting that the proposed bridge design has pressure flow at Q5. Are there not design requirements that show you must have freeboard or be able to pass the Q50/Q100?

[mlc3n3]

Ok. I see what you’re saying. The model currently uses upstream water surface elevation instead of energy grade elevation to initiate pressure flow. This is the company protocol and the AR DOT’s preference. However switching it back to energy grade elevation doesn’t change much if anything. Yes, I will try adjusting the minimum weir elevation in a reasonable manner.

Yes, there are normally requirements regarding pressure flow and freeboard. This project is a special case. The PR is replacing a small bridge that is over topped regularly. Based on the magnitude of the storm events and the site characteristics, a very long and perched bridge would be required to meet the requirements. That is out of scope for the city and DOT. So we are aiming for a “no rise.” Its complicated and ugly lol

Thank you for your insight and suggestions!

[Author]

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