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Form Loss Coefficient for 2d_lfcsh Polygons

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Hi there

I've been reading the 2010 version of the manual with regards to how Form Loss Coefficients (FLC) are modeled if a polygon is used for a 2d_lfcsh layer to represent bridges. Below is an extract of how TUFLOW addresses FLC for lfcsh polygons:

"Polygon: FLC is the form loss per metre length (in the predominant direction of flow). FLC values are not dependent on the flow width, but are on the length of travel in the direction of flow."

Say, with the values below:

  • Below obvert FLC value of 0.2 (typical)
  • Bridge length (perpendicular to direction of flow) is 40m
  • Bridge deck width is 10m

The length (40m) of the bridge is not used and that the FLC value in the "L1_FLC" input box would be 0.2/10=0.02

Is my understanding correct? This would mean that a constant value (0.02) is applied for each cell bounded by the polygon?

Is there a way to check whether the correct FLC values have been applied? edit: found it, was part of the check files

I presume logically the 100% blocked bridge deck would have a L2_FLC value of zero. Does anyone have any guidance regarding L3_FLC values (the rails)?

Thanks

Yau

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Hi Yau,

For your case, the L1_FLC/m is 0.2/10=0.02, the L1_FLC/cell is 0.2/10*cell size.

Instead of setting L2_FLC=0, the manual says that "Layer 2: The bridge deck. This would be 100% blocked and the form loss coefficient would increase due to the additional energy losses associated with flow surcharging the deck"

I have a question for this topic is :

If we use layered flow contractions to model the bridge, how does Tuflow consider the impact of the blockage like the bridge deck and handrail? Will Tuflow consider the blockage impact only by applying form loss coefficient (L1_FLC, L2_FLC, L3_FLC, ) or will Tuflow calculate the pure blockage separately in addition to the form loss? If so, how does Tuflow calculate the pure blockage?

Regards,

Eric

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I presume logically the 100% blocked bridge deck would have a L2_FLC value of zero. Does anyone have any guidance regarding L3_FLC values (the rails)?

With regards to the L2_FLC, I think it should actually be considerably higher than your L1_FLC and not 0 at all.

This is an extract from the TUFLOW manual, in the 2d_lfcsh section:

Layer 2: The bridge deck. This would be 100% blocked and the form loss coefficient would increase due to the additional energy losses associated with flow surcharging the deck.

Furthermore, by default in the 1d_nwk attributes, the default Form_Loss for a "B" (bridge) channel is in the order of 1.56 as through:

Above the underside of the bridge deck (the top of the cross-section) a value of 1.56 is used.

As to the exact values for FLC though, maybe someone else can enlighten this discussion.

I've gone through the FHA's "Hydraulics of Bridge Waterways" manual and still have not been able to determine the exact FLC values to use.

Cheers,

Danny

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Hi

To answer/clarify a few queries from these posts please see below.

Cheers

Bill

Q: "The length (40m) of the bridge is not used and that the FLC value in the "L1_FLC" input box would be 0.2/10=0.02

Is my understanding correct? This would mean that a constant value (0.02) is applied for each cell bounded by the polygon?"

A: Yes, this is correct. For regions, the FLC needs to be divided by the dimension of the bridge in the direction of flow. The advantage of this approach is that the GIS layer is cell size independent, therefore, if you change your 2D cell size you do not have to change the FLC value.

Q: "I presume logically the 100% blocked bridge deck would have a L2_FLC value of zero. Does anyone have any guidance regarding L3_FLC values (the rails)?"

A: No, as Danny correctly points out. If anything, the bridge deck would have a significant FLC. Essentially there are two key attributes to be modelled:

1. The correct flow area (otherwise the calculated velocity is incorrect and given that energy loss is proportional to the velocity squared this is important). TUFLOW computes/uses the correct flow area by taking into account the %blockage.

2. The energy lost from fine-scale features that the 2D solution/grid cannot represent. For a bridge deck, there will be energy lost primarily in the vertical dimension (essentially a 3D problem), so the principle here is to account for this energy loss via a FLC on the bridge deck layer.

Unfortunately there are as yet no hard and fast guidelines (this is somewhat due to the complexity and range of structures that could be modelled using layered FCs). Benchmarking to recommendations from desktop publications such as Hydraulics of Bridge Waterways is often still the best approach where the publication covers the typw of structure being modelled. It would make a great research masters or PhD if anyone's interested!, but in the interim I'll see if we can summarise some recommendations/findings.

Q: "If we use layered flow contractions to model the bridge, how does Tuflow consider the impact of the blockage like the bridge deck and handrail? Will Tuflow consider the blockage impact only by applying form loss coefficient (L1_FLC, L2_FLC, L3_FLC, ) or will Tuflow calculate the pure blockage separately in addition to the form loss? If so, how does Tuflow calculate the pure blockage?"

A: As mentioned above, TUFLOW adjusts the flow area (ie. the area perpendicular to the flow) according to any blockages. So if the bridge deck is Layer 2 and is 100% blocked, once the water reaches the underside of Layer 2, the flow area remains unchanged until the water enters Layer 3. If Layer 3 is 50% blocked, then as the water rises through Layer 3 the flow area will increase by the depth in Layer 3 times half the flow width. TUFLOW also applies any additional energy loss based on the FLC values according to the below as documented in the 2010-10 manual:

• If the water is entirely within Layer 1, the Layer 1 FLC is applied.

• If the water level has reached Layer 2, the value applied is the Layer 1 FLC plus a fraction of the Layer 2 FLC based on the depth of water within Layer 2. For example, if Layer 1 FLC is 0.1 and Layer 2 is 0.5, and the water is 40% of the way up Layer 2, the FLC applied is 0.1 + 0.4*0.5 = 0.3.

• Similarly, if the water level is into Layer 3, the FLC is the Layer 1 FLC plus the Layer 2 FLC plus a fraction of the Layer 3 FLC.

• Once the water level is above Layer 3, ie. is into Layer 4, the FLC is held constant at the sum of the FLCs for Layers 1 to 3.

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Hi, not sure if any guidance/recommendations/findings regarding L2_FLC values has been posted yet by Bill?

I have the same issue trying to accurately model deck and railings blockages in submerged bridges in TF.

So just wondering if anyone has any guidance on the use of such values?

 

Thanks

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Hi fellow modellers,

On 12/16/2011 at 5:44 PM, Bill Syme said:

It would make a great research masters or PhD if anyone's interested!, but in the interim I'll see if we can summarise some recommendations/findings.

Seeing as this was posted in 2011, is there any chance that any recommendations have since been prepared?

Is Danny's suggestion of a 1.56 FLC for layer 2 appropriate?

On 10/20/2011 at 10:02 AM, dannyduong said:

Furthermore, by default in the 1d_nwk attributes, the default Form_Loss for a "B" (bridge) channel is in the order of 1.56 as through:

Quote
Above the underside of the bridge deck (the top of the cross-section) a value of 1.56 is used.

 

Obviously people are coming up with FLCs for their models, I am wondering if they have a standard way of doing this, or are just guessing at numbers to use?

I'm trying to come up with a standard method to model 2d_lfcsh FLCs, so any advice would be much appreciated!

Cheers,

Daniel

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On 9/5/2018 at 10:56 AM, Daniel Dhiacou said:

Is Danny's suggestion of a 1.56 FLC for layer 2 appropriate?

note that this is the total form loss applied using Method A (no longer the default) as the bridge loss approach in 1D.  Section 5.7.2.3 of the 2018-03 version of the manual states, "once the downstream water level is within 10% of the flow depth under the bridge, a bridge deck submergence factor is phased in by applying a correction for submerged decking using a minimum value of 1.5625 (if the specified loss coefficient is greater than 1.5625, this value is applied)".  A derivation of that value follows in the manual

in the past I have subtracted the layer 1 FLC from 1.5625 to get a value for layer 2 when using the "cumulate" method.  However, the value applied will not reach 1.5625 until the top of layer 2 due to the weighting of the FLCs shown in the equation in section 6.12.2.2 of the manual.  It is similarly impossible to select a value for layer 2 that produces a FLC of 1.5625 from the bottom of layer 2 as the value you need to apply to achieve that FLC becomes infinitely large as the depth above the underside of the bridge approaches zero.  I think that if you wanted to reproduce the 1D Method A approach, you'd need layer 2 to represent that part of the flow area "within 10% of the flow depth under the bridge", so that the loss is equal to 1.5625 at the point where the water hits the underside of the bridge deck

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