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Lih_Chong

Doubling of losses at 2d_lfcsh objects?

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I have modelled two small bridges over a 2d concrete channel using 2d_lfcsh objects. The bridges are simply concrete slabs placed over the channel, with no piers.

I have specified a layer 1 loss (L1_FLC) of 0.01, a L2_FLC of 1.56 and a L3_FLC of 1.56.

I opened the lfcsh_uvpt_check layer, and the relevant invert/obvert levels and blockages (0% for L1 and 100% for L2 and L3) are all correct, however, the losses have been doubled (e.g. L1_FLC = 0.02, L2_FLC and L3_FLC = 3.12)... is TUFLOW reading and treating the layer's attributes correctly? Should the losses not be as specified in the 2d_lfcsh layer?

cheers

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Hi Lih

The most likely reason you are seeing the changed values is that you've used a polygon for the layered FC. For polygons, the FLC value's units are loss per metre length so that the layer is not dependent on cell size. This is done because the FLC is dependent on the length in the direction of flow between computation points. You can force the FLC value to not be loss per metre by using a negative value. For more info, please see the extract from the TUFLOW manual below.

Cheers

Bill

Extract from Table 4.17 in the 2008 TUFLOW manual:

Note: So that this attribute is independent of 2D cell size it has different treatment depending on the object it is attached to as follows:

• Line: For thin lines, the FLC value is applied to the cell sides unchanged. For thick (whole cell) lines, the FLC value is divided by two (two cell sides in the direction of flow). For wide lines the FLC value is divided by the number of cells across the line (ie. the line’s width divided by the cell size) and applied to all cell-sides.

• 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.

However, if a negative FLC value is specified, the absolute value is taken and applied unadjusted to all cell-sides affected by the shape. Note, this is not cell size independent, therefore if the 2D cell size is changed, this attribute also needs to be changed.

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