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2D_fcsh Bridge loss coefficient

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I’m attempting to model a large bridge in the 2D domain by using a 2d_fcsh_layer, however I’m unsure on some of the fields within the table. I have included a picture of the bridge to demonstrate the design. Any help answering the following questions would be gratefully received:

1. Is it right to assume the blockage field is a percentage of the total area of the bridge to the total area of the piers? i.e The total area between the piers is 135m2 and the total area of the piers is 12.48m2, therefore the blockage would be 9.2%.

2. How is the FLC_below_Obvert calculated? The Manual does provide a dynamic head equation, but I would really appreciate some guidance due to the complexity of the bridge design.

Thank you,



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Hi Sam

Yes, your interpretation of the blockage % is correct. This has the effect of reducing the flow area, and subsequently increasing the velocity (and losses).

Regarding appropriate values of additional form (energy) losses, this is still very much an area of investigation. It is complicated as most 2D schemes will represent losses due to changes in velocity direction and magnitude (eg. expansion of flow downstream of a structure), but, of course, cannot represent fine-scale losses such as around piers (due to the cell size being too coarse), and from any effects in the vertical dimensional (being only 2D) - hence the need to add additional form losses. It is therefore important to scrutinise your results in terms of the amount of dynamic head loss that is occurring (ie. look at the velocity though the bridge cells, work out V2/2g, and compare this with the drop in water level across the bridge. If you don't have calibration data (normally the case), compare the head drop with that obtained using other desktop methods or models. Generally, some additional form loss is required as the 2D scheme without any additional form losses usually underestimates the overall losses, especially if there are piers and/or the bridge deck is surcharged. As a very general guide, usually additional form losses for below the bridge deck are in the range from 0.1 to 0.5, and higher once the bridge deck is surcharged. Sensitivity testing of parameters is also strongly recommended.

We are carrying out testing on hypothetical bridge arrangements to hopefully establish some guidance on typical parameters. Once this is published/available, we'll post it on this forum.

I hope this is of some assistance.



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