Experience exchange

[igorce]

Hello,
I need answer for two question
Picture number 1
-Natural slope of the river is bigger than critical slope. Froude number is bigger than 1.0, This tell me Supercritical Flow regime
– Water surface elevation is compute with Supercrtical flow calculation

Picture number 2, same case
– Water surface elevation is compute with Mixed flow calculation

1.(question). WHICH CALCULATION IS CORRECT (1 or 2)?

2.(question). WHICH IS THE SAFETY HEIGHT FROM WATER SURFACE ELEVATION TO LOW CHOR OF THE BRIDGE? (Picture 1 and 2) (I need a few sentences and suggestions which literature you use)

[Anonymous]

igorce,

Here are my thoughts based on a quick review of the water surface profiles:

1. The supercritical water surface profile shows the EGL increasing in the downstream direction. This should not be the case in a natural system, unless energy is being added – such as from a pump. Also, the calculated WSEs appear to be defaulting to critical depth at the majority of the cross-sections.

2. The mixed flow profile indicates the reach impacted by backwater, which increases the calculated water depths and decreases the slope of the water surface. In these situations, the slope of the water surface and stream bed will not be equal, so the natural slope of the river may not provide a good approximation of the flow regime. Therefore, the mixed flow calculations seem more reasonable.

Regarding the difference between the WSEs and bridge low-chord, you should check check design guidance specific to the region where the bridge is located. Design guidance tends to vary with location.

Keith

[Jake G]

igorce,

To build on Keith’s excellent points, here are a couple of additional observations:

(a) Freeboard (safety height) is usually computed on the upstream side of a bridge because that is where debris typically accumulates.

(b) Supercritical flow would only be expected in limited situations, such as a concrete channel or steep bedrock channel. Supercritical or mixed flow should not be used for most natural channels. See, for example, Jarrett (1984):

http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(1984)110:11(1519)

– Jake G.

[igorce]

I agree with Keith M about point one, about unrealistic WSE downstream of the bridge (honestly that part downstream analyzes are not important), but I’m still in doubt about the correct analysis (mixed or supercritical) near the bridge.
Jake G, about your comment in part “b”, you say: Supercritical flow would only be expected in limited situations, such as a concrete channel or steep bedrock channel.
Please read this additional information:
From first Cross section(XS) upstream from the bridge, to third XS downstream from bridge are just how you say: REGULATED CONCRETE CHANEL whit 1% slope (that make me supercritical flow and freoud number more than 1.0).
Now the doubt is:
1. If i make a mixed flow analysis near the bridge, i cant belive on results, becouse i have WSE more than orientation results with equation Q=A*V, but if i make a Supercritical flow analysis than i think i have correct results.
2. Can HEC-RAS combine, mixed flow before regulation(natural zone),and Supercrtical flow in regulation zone (before, in, and after the bridge)?_Honestly i dont belive in this!

PS.
In this case the bridge is part of highway and here i have one more important question:
Did i must make solution for subcritical flow or i can use the slope of the natural terrain?

Thank you for answers and i really appreciate this!

[cameron]

I would not use the supercritical results for bridge design unless I had a really strong compelling reason. If debris ever accumulated on the bridge piers, it would cause a backwater and a hydraulic jump most likely.

Your cross-sections and bridge look very skewed to the direction of flow based on the placement of the other other cross-sections, are you accounting for the Skew? Is the bridge opening aligned with the flow or is it skewed as well? It also appears that the internal cross-sections are the same elevation as cross-section 2 and 3 which can impact the super critical results as it is not a continuous slope through the bridge.

Since this is for a design and you need freeboard, you could just model it as cross-sections (no HEC-RAS bridge) adding blocked obstructions or editing the cross-section to include the piers.

The other option is to switch to a 2D model in HEC-RAS or another such as SRH-2D to develop the required freeboard.

[Author]

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