TUFLOW Forum

# 2d_fcsh - BC_Width

## Recommended Posts

Hi Tuflow

I am a modelling box culverts using a flow constriction “2d_fcsh”.

To model the box culverts I have assigned FC_TYPE= “BC”. This causes the BC_Width attribute to become active. The manual states for BC_Width “The width of one BC culvert barrel in metres. For example, if there are 10 by 1.8m wide culverts, enter a value of 1.8.”

I cannot find an attribute that allows me to enter the number of box culverts? If there was 10 box culverts, as per the in text example, how can I enter this information? What will TUFLOW assume if I enter a box culvert width less than the grid size?

cheers

David

##### Share on other sites

Hi David,

I am not TUFLOW, but be able to answer your first question.

You do not need to enter the culvert numbers. The 2D shape length will tell the total FC length. (The culvert numbers) = (the total FC length)/(The width you entered). Then TUFLOW can correctly calculate the side wall friction.

Tuflow should scale the cell's friction and form loss etc if the banks of culvert width is less than the grid size. I assume so, but not 100% sure. Only TUFLOW can answer this. Anyway, for that case, you'd better to model the BC with 1D approach.

Are you the David Franklin I met 3 years ago?

Cheers,

Eric

##### Share on other sites

Hi David

Good question and thanks Eric for your reply. To clarify, for a BC FC (box culvert flow constriction shape), the number of vertical walls per 2D cell is calculated as the cell size divided by the culvert width (BC_Width attribute) times 2 (ie. 2 side walls per barrel). TUFLOW will increase the wetted perimeter for these cell sides accordingly, thereby adding additional friction due to the vertical walls of the culvert.

At present there is no adjustment of the cell widths or number of vertical walls if the total width of culverts is not exactly the same as the width of 2D cells. For example, if there are eight 3m wide culverts, your cell size is 10m and 3 cells are selected by the FC shape to represent the culverts, the flow width will be 30m (10m*3cells) and the number of vertical walls is 20 (10m/3m*2*3cells). In reality the flow width is 24m and the number of vertical walls is 16, so there will be an overestimate of the flow width and additional wetted perimeter. The pBlockage attribute can be used to reduce the flow width to attain the correct width of 24m by applying a pBlockage of 20% (6/30*100). At present the only way to correct the number of vertical walls is to (inconveniently) change the BC_Width. However, it should be noted that in terms of the hydraulic calculations, having the correct flow width is much more important than correcting the BC_Width to get the exact number of vertical walls. I've made a note to look into automating the adjustment of the cell width and exact calculation of the number of vertical walls as an enhancement to this process.

A good tip when modelling FCs is to have a 2d_po line through the middle of the FC shape (perpendicular to the flow) with "Q_QAH_" for the Type attribute so as to output the flow, flow area and water level over time in the structure. You can then cross-check that you're representation of the flow area is appropriate, especially when the cells selected are at an angle to the grid (ie. are not aligned to the X or Y axis of the grid). Getting the flow area, and therefore the right velocity, is the most important aspect of modelling hydraulic structures as the losses through a structure under outlet controlled conditions are very dependent on achieving the correct change in velocity magnitude and direction as the water contracts into the structure and then expands as it departs.

I hope this makes sense!

Cheers

Bill

##### Share on other sites

Yes Eric it is me, I just PM’d you.

I believe I have found a problem with the description of an attribute in Table 4.19 Flow Constriction Shape (2d_fcsh) of TUFLOW.

If a FC_TYPE of “BC” is selected, the attribute “obvert_or_BC_Height” has a description of ““FC_Type = “BC”: Height of box culvert (m).””

I think this is mislabelled and should read “obvert of box culvert in m above datum”.

If I use a 2d_fcsh flow constriction as a polygon to alter multiple cells and enter an invert level of 22m AHD and an obvert_or_BC_Height of 1.2m, TUFLOW will make ZC point 22m AHD but change the ZU and ZV points too arbitrary high numbers i.e 345.54m AHD. I believe TUFLOW thinks the culvert’s obvert is at 1.2 m AHD, which is underground, and is blocking off any flow from entering the cell.

However, if I enter the obvert_or_BC_Height at 23.2 m, a realistic quantity of water is allowed through and TUFLOW alters the ZU and ZV points to 22m i.e. the invert level.

Can you confirm my understanding is correct?

Cheers

David

##### Share on other sites

Hi David

Good question and thanks Eric for your reply. To clarify, for a BC FC (box culvert flow constriction shape), the number of vertical walls per 2D cell is calculated as the cell size divided by the culvert width (BC_Width attribute) times 2 (ie. 2 side walls per barrel). TUFLOW will increase the wetted perimeter for these cell sides accordingly, thereby adding additional friction due to the vertical walls of the culvert.

At present there is no adjustment of the cell widths or number of vertical walls if the total width of culverts is not exactly the same as the width of 2D cells. For example, if there are eight 3m wide culverts, your cell size is 10m and 3 cells are selected by the FC shape to represent the culverts, the flow width will be 30m (10m*3cells) and the number of vertical walls is 20 (10m/3m*2*3cells). In reality the flow width is 24m and the number of vertical walls is 16, so there will be an overestimate of the flow width and additional wetted perimeter. The pBlockage attribute can be used to reduce the flow width to attain the correct width of 24m by applying a pBlockage of 20% (6/30*100). At present the only way to correct the number of vertical walls is to (inconveniently) change the BC_Width. However, it should be noted that in terms of the hydraulic calculations, having the correct flow width is much more important than correcting the BC_Width to get the exact number of vertical walls. I've made a note to look into automating the adjustment of the cell width and exact calculation of the number of vertical walls as an enhancement to this process.

A good tip when modelling FCs is to have a 2d_po line through the middle of the FC shape (perpendicular to the flow) with "Q_QAH_" for the Type attribute so as to output the flow, flow area and water level over time in the structure. You can then cross-check that you're representation of the flow area is appropriate, especially when the cells selected are at an angle to the grid (ie. are not aligned to the X or Y axis of the grid). Getting the flow area, and therefore the right velocity, is the most important aspect of modelling hydraulic structures as the losses through a structure under outlet controlled conditions are very dependent on achieving the correct change in velocity magnitude and direction as the water contracts into the structure and then expands as it departs.

I hope this makes sense!

Cheers

Bill

Hi Bill,

Thanks for your clarification on how the TUFLOW engine to work out the flow width and the number of BC side walls.

If the BC side walls are calculated in the above way (i.e. cells' size divided by the culvert width), I will think that the unit form loss (for polygon option) and flow length also use the cells' size instead of the actual FC/LFC shape width. I previously did a bridge modelling by using LFC. I used the total form loss divided by the actual bridge width to get the unit form loss. Now, looks I should use total form loss divided by the cells' length to get the unit form loss. This also won't give acurate unit form loss and flow length, just like the flow width and number of side walls for BC without changing BC_Width and applying appropriate pBlockage.

Is it possible for TUFLOW to capture and use the flow length and flow width from the actual FC/LFC shap? Then TUFLOW will also work out correct number of BC side walls and unit form loss. This may be not so important for large scale flood modelling, but may be important for detailed structure modelling.

Cheers

Eric

##### Share on other sites

Thanks David for picking this up. We've managed to reproduce the problem and have identified and fixed a bug for BC types for 2D_fcsh layers (does not affect other types or BC types in 2d_fc layers). The bug is evidenced by a CHECK 2097 message at U and V points indicating that the BC soffit is below the Zpt and the cell side is blocked by TUFLOW preventing flow through the BC FC shape. Also the U and V Zpts in the 2d_zpt_check layer will be set very high to the instability trigger level. Build 2011-09-AF or later will have the bug fix incorporated.

Eric, regarding automating the cell width factors so that the FC shape matches the exact total width of the culverts, unfortunately this is not that straightforward, especially for polygon FC shapes as the flow direction is unknown at the time of input. Probably the only way to do this would be during the simulation by comparing the flow area from a 2d_po line through the culverts from the previous timestep with the exact culvert area and fine tuning the cell width factors to reproduce the exact flow area - not impossible so it's on the list! In the meantime the best option is to use the pBlockage attribute to reduce the cell flow widths accordingly as mentioned above.

If you do need to be very precise about which cells are adjusted, using the original 2d_fc layer is the best option, but this layer is cell size dependent and will take longer to setup.

Thanks again

Bill

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×

×