Modeling culvert with an orifice plate on the upstream

[Alik]

Hi all,

I want to model a culvert that has an orifice plate at the inlet of the culvert on the upstream to restrict entering flow.

My idea is to model the culvert with its original size and define the ineffective flow for the cross station near the inlet of culvert on the upstream as a function of size of the orifice plate size, and then set it to the permanent ineffective flow. The Hec-RAS manual suggests a 1:1 contraction rate for the upstream in order to find the effective flow.

Another idea is modeling two culverts in series. The first culvert will have a size of the orifice plate with 1 foot length, and the second culvert with its original size will be added to the first culvert with no distance between them.

Are both of these approaches correct? Or you have another idea to model the plate?

Thanks!

[Anonymous]

I think the modelling it as two separate culverts. To do this, I think you need one or two cross sections between the culverts. But the cross sections can match the shape of the culvert (you will have to add a lid to the cross section). You will also have to give some thought to making sure the entrance and exit loss coefficients are all appropriate.

[Alik]

Thanks Jarvis,

I agree with you that we need to be very cautious when we model two culverts in series. I am just wondering if that ineffective flow concept works fine or not. I still think that is why ineffective flows have been defined in HEC-RAS. Or at least it can be close to what really happens behind a restricted culvert. I am surprised there are not many examples of modeling these culverts despite they should be popular for designers.

[Anonymous]

The ineffective flow option is important in order to correctly compute the contraction losses before the flow enters the culvert, and yes, the infective flow areas may need to take into account that the culvert opening is very small.

The culvert computations have a specific equation to compute the entrance loss as the water enters the culvert. I would think that for this to be correct, you would need to have the upstream face of the culvert be the reduced size. Although perhaps you could simulate it by adjusting the entrance loss coefficient. Adjusting the entrace loss coefficient may be something you would want to do anyway.

[Alik]

Exactly. The entrance loss coefficient is really important. However, defining the coefficient precisely is rather hard. According to HEC Ras manual for concrete box culverts with sharp edges, entrance head loss of 0.5 should be good. I used this number for modeling the culvert.

[Anonymous]

I know this is old but I wanted to note that I think this would only work for non-overtopping flows. Otherwise you would be putting the overtopping flow from your orifice back into your cross-sections with lids and recomputing culvert and over topping flows for the actual Culvert. If you are concerned about overtopping then you could do a split flow where your overtopping section is a normal cross section and model your orifice either as a culvert or section with lid where the orifice cant be overtopped.

[Anonymous]

Just create the entire model of culverts using cross sections with lids, this is the best way to represent what you have described imo

[Author]

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