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= 2D Structure Modelling Theory =
ThisThese webinarwebinars by Bill Syme and Greg Collecutt (the TUFLOW DeveloperDevelopers) discussesdiscus accuratethe modellingtheory ofbehind the energy losses and affluxes modelling associated with hydraulic structures.
 
*<u>[https://www.tuflow.com/library/webinars/#structures Webinar Link: Modelling Energy Losses at Structures]</u><br>
*<u>[https://www.tuflow.com/library/webinars/#nov2022_hydraulic_modelling_bridge Webinar Link: 1D, 2D & 3D Hydraulic Modelling of Bridges]</u>
<br>
 
= 2D Bridge Modelling in TUFLOW - Overview =
The TUFLOW 2D solution automaticallyexplicitly predicts the majority of “macro” losses due to the expansion and contraction of water through a constriction, or around a bend, provided the resolution of the grid is sufficiently fine (<u>[http://www.tuflow.com/Download/Publications/Flow%20Through%20an%20Abrupt%20Constriction%20-%202D%20Hydrodynamic%20Performance%20and%20Influence%20of%20Spatial%20Resolution,%20Barton,%202001.pdf Barton, 2001]; [http://www.tuflow.com/Download/Publications/Modelling%20of%20Bends%20and%20Hydraulic%20Structures%20in%20a%202D%20Scheme,%20Syme,%202001.pdf Syme, 2001]; [http://www.tuflow.com/Download/Technical_Memos/Modelling%20Bridge%20Piers%20in%202D%20using%20TUFLOW.pdf Ryan, 2013]</u>). Where the 2D model is not of fine enough resolution to simulate the “micro” losses (e.g. from bridge piers, vena contracta, losses in the vertical (3rd) dimension), additional form loss coefficients and/or modifications to the cells widths and flow height need to be added. 2D flow constriction commands/layers are the recommended approach for this purpose:
==Contraction/Expansion Losses (“Macro” Losses)==
* TUFLOW Geometry Control (TGC) file comand = <font color="blue">Read GIS Layered FC Shape</font> <font color="red">==</font> 2d_lfcsh_...
Loss of energy is caused by the flow contraction during the expansion of water after the vena-contracta inside a bridge section and the flow expansion downstream a bridge. As discussed above, this type of "macro" losses can be explicitly resolved by the TUFLOW 2D solver, provided that a proper turbulence model and mesh size are used. Below is an example of the 2D modelling of flow contraction/expansion at a pair of bridge abutments.
<br>
[[File:FC_Velocity_Example.PNG|600px]] <br>
 
==Pier Losses==
[[File:FC_Velocity_Example.PNG|600px]] <br> [[File:FC_Graph_Example.PNG|500px]]
Piers are usually smaller than the 2D cell size in real-world flood models. Although flexible mesh solver or quadtree refinement can be applied to reduce the local cell size around the pier, it also comes with an expensive computational cost that could significantly increase the simulation time. More practically, the backwater effect of piers can be modelled as sub-grid form losses.
<br>
 
Pier form loss coefficients can be derived from information in publications such as <u>[https://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=1&id=5 ''Hydraulics of Bridge Waterways'' (Bradly, 1978)] or [https://austroads.com.au/publications/bridges/agbt08 ''Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures'' (AUSTROADS, 2019)]</u>. Energy loss estimated from bridge piers or other obstructions, vertical or horizontal, that do not cause upstream controlled flow regimes like pressure flow, are dependent on the ratio of the obstruction's area perpendicular to the flow direction to the gross flow area of the bridge opening, the shape of the piers or obstruction, and the angularity of the piers/obstruction to the flow direction. For example, using Hydraulics of Bridge Waterways (Bradly, 1978) the approach is:
Form loss coefficients are an important input to the flow constriction layers. TUFLOW form loss coefficients can be derived from information in publications such as ''Hydraulics of Bridge Waterways'' ([https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.386.8755&rep=rep1&type=pdf FHA, 1978] or [https://austroads.com.au/publications/bridges/agbt08 AUSTROADS, 2019]).
 
Backwater caused by piers in a bridge constriction is dependent on the ratio of the pier area relative to the gross area of the bridge opening, the type of piers (or piling in the case of pile bents) and the angularity of the piers with the direction of flood flow. The FHA (1978) guidance advises what form loss coefficient should be adopted based on these input parameters.
<ol>
<li>Calculate the ratio of the water area occupied by piers to the gross water area of the constriction (both based on the normal water surface) and the angularity of the piers. These inputs are used to calculate "J" in the FHA documentation.</li>
<li>Use the FHAFigure (1978)7 ''Incremental Backwater Coefficient for Piers'' data to calculate Kp. This is the value which will be entered into TUFLOW as the form loss coefficient.<br>
[[File:FHA_Kp_arrowFHA_Kp_arrow_crop.PNGpng|400px]]
<br>
'''NOTE''': the pier form loss coefficients in Hydraulics of Bridge Waterways are derived based on the cross-sectional averaged velocity through the bridge opening in the absence of piers. It's not necessary to specify a blockage value if a pier form loss coefficient estimated from this method is used.
</li>
</ol>
<li>Digitise <font color="blue">2d_lfcsh_...</font> inputs using either a line or polygon feature. When applying form loss coefficients the best approach is to view the structure as a collection of 2D cells representing the whole structure, rather than being too specific about the representation of each individual cell:
* Thin line features will apply the form loss value to a single row of cells across the waterway. The TUFLOW form loss input should be entered representing the total value (eg. FC = 0.2).
* Polygon features will distribute the form loss between multiple cells across the width of the bridge and across the waterway. Due to this, the TUFLOW form loss input should be entered as the total value per unit width in the direction of flow (eg. FC = 0.2/20m = 0.01).
* Further details are provided below in the Q&A section of this page
 
==Bridge Deck and Rail (Super Structure)==
[[File:fcsh_line.PNG|500px]][[File:fcsh_polygon.PNG|500px]]
When a bridge deck become partially or completely submerged, the deck could generate extra afflux resulting in increased water levels and flood extents upstream of the structure. The flow around the deck is highly 3-dimentional and complexed due to the different deck designs/profiles and/or the occurrence of pressure flow. In 2D SWE solver, depth-varying form loss values are often needed to reproduce the afflux caused by such structure. Due to the complexity of the flow, guidelines on how to set the form loss coefficient for the bridge deck are rare. We have carried out a joint research with QLD TMR (Queensland Department of Transport and Main Roads) regarding how to choose a proper form loss value for the bridge deck (Collecutt et al, 2022). In the research, CFD modelling was conducted to investigate the characteristics of energy loss of a simple bridge with a flat bottomed deck and guardrails.
<br>
[[File:CFD_study.png|600px]]
 
Below are the key findings from the study:
</li>
*The results displayed a characteristic shape for head loss coefficient as a function of downstream water level over the deck thickness (TW/T).
<li> Flow widths (conversely, blockage factors) and flow height should be updated in the <font color="blue">2d_lfcsh_...</font> inputs. This information is used to refine the structures flow area when the model grid resolution isn't fine enough to accurately do so already.
*The head loss (afflux) peaks when the water level is approximately 1.6*T above the bridge soffit, and decays slowly as the bridge becomes progressively drowned out.
<li>The head loss across key structures should be reviewed, and if necessary, benchmarked against other methods (Recorded calibration data, other model solutions). Note that a well-designed 2D model will be more accurate than a 1D model, provided that any “micro” losses are incorporated.</li>
[[File:FormLoss_vs_TWT.png|600px]]
<li>TUFLOW check files should also be reviewed to confirm that the correct form losses are being applied. The <u>[[Check_Files_2d_lfcsh_uvpt | 2d_lfcsh_uvpt_check file]]</u> can be used to review the location of the applied lfcsh and its attributes.</li>
<br>
<li>The flow area through the structure should also be reviewed.If the overall structure flow area is not correct, then the velocities within the structure will not be correct and therefore the energy losses due to the changes in velocity direction and magnitude and additional form losses will not be well modelled.
*The peak loss coefficient value is a function of the ratio of the depth underneath the deck (hB) and the thickness of the deck (T)
* Digitise a Plot Output QA line (<font color="blue">Read GIS PO</font> <font color="red">==</font> 2d_po_...) through the structure from bank to bank, and use this output to cross-check the flow area of the 2D FC cells is appropriate (the QA line will take into account any adjustments to the 2D cells due to FC obverts and changes to the cell side flow widths). </li>
<ol>
{| style="text-align: center;" class="wikitable" width="35%"
! style="background-color:#005581; font-weight:bold; color:white;" width=55%| Deck Height to Thickness Ratio
! style="background-color:#005581; font-weight:bold; color:white;" width=45%| Peak Form Loss Coefficient
|-
| Scenario A (hB/T) = 2 || 0.42
|-
| Scenario B (hB/T) = 4 || 0.28
|-
| Scenario C (hB/T) = 6 || 0.20
|}
</ol>
 
This table can be used to estimate the deck form loss coefficient based on the bridge design (hB/T). The solid portion of the guard rails (blockage * rail depth) can be added to T in addition to the deck thickness to calculate hB/T. For bridge with more complicated designs (e.g. girders), higher form loss might be required due to the higher surface roughness of the bridge.
 
'''NOTE''': This form loss value should not be confused with the value of 1.56 used in the pressure flow approached adopted in <u>[[1D_Bridges | TUFLOW 1D "B" and "BB" bridge]]</u>. TUFLOW 1D bridge pressure flow approach is based on the section 4.13.2 "All Girders in Contact with Flow (Case II)" of ''Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures'' (AUSTROADS, 2019). The original hydraulic experiment conducted by <u>[http://hdl.handle.net/10217/39009 Liu et al (1957)]</u> in a laboratory flume with a pair of bridge abutments and a deck. The flow conditions were similar to orifice flow due to the high blockage ratio caused by the abutments and the deck. When modelling bridges in 2D, the contraction/expansion losses caused by the abutments would be handled explicitly by the 2D solver, so a value 1.56 can lead to duplication of the contraction/expansion losses caused by the bridge abutments.<br>
<br>
 
=TUFLOW 2D Bridge Setup=
= Common Questions Answered (FAQ)=
Traditionally, 2D Layered Flow Constriction (2d_lfcsh) has been used in TUFLOW 2D modelling to specify the depth varying form loss of a bridge structure. As of 2022 release a new GIS layer called 2D BG Shape (2d_bg) has been implemented in order to simplify the model input based on the findings from the joint TMR study.
== What is the difference between using polyline and polygon for 2D bridges and when should I use either approach? ==
Both methods provide options for representing flow surcharging, the pressure flow of bridge decks and eventually submerged bridge flow at higher water levels. During the surcharging of bridge decks, higher energy losses can be specified to simulate the pressure flow. Four flow constriction layers are represented. The lower three layers represents the pier, the bridge deck and the rails. Each layer has its own attributes to specify the blockage and the form loss coefficient. The top (fourth) layer assumes the flow is unimpeded, representative of flow over the top of a bridge. Within the same shape, the invert of the bed, and thickness of each layer can vary in 3D.
When using 2d_lfcsh, the form loss coefficient (FLC) is applied differently when using a line compared to a polygon. The FLC is applied at cell sides (u and v points) as this is where velocities are calculated.
==2D Layered Flow Constriction (2d_lfcsh)==
* When using a polyline, the FLC attribute depends on the type of the polyline:
Four layers are used for 2d_lfsch:<br>
** Thin line (width attribute of zero) - The FLC attribute in the GIS object should reflect the total form loss value for the bridge. A thin 2d_lfcsh line will apply the FLC to a single row of cell sides. As such, thos approach is cell size independent. Thin line lfcsh are the easiest setup, and is the preferable recommended approach.
*'''Layer 1''': Beneath the bridge deck. If bridge piers exist a small form loss is usually expected due to the energy losses associated with the piers. <u>[https://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=1&id=5 ''Hydraulics of Bridge Waterways'' (Bradly, 1978)]</u> can be used to estimate the pier form loss coefficient.
** Thick line (width attribute between zero and 1.5 times the cell size) - The FLC attribute is half of the total loss as the form loss is applied on each cell side of the selected cells. A cell is selected if the polyline intersects the cell crosshair. Caution should be taken when using a "thick" line, due to the fact changes in cell size can trigger a "thick" line to become a "wide" line. If this were to occur the FLC attribute would need to be recalculated to not overestimate losses.
*'''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. The hB/T vs FLC table from the joint research with TMR can be used to estimate the deck form loss coefficient.
** Wide line (width attribute larger than 1.5 times the cell size) - The FLC attribute is a portion of the total loss based on number of cell sides in the predominant direction of flow. The number of cell sides can be checked in the <u>[[Check_Files_2d_lfcsh_uvpt | 2d_lfcsh_uvpt_check]]</u> file. Caution should be taken when using a "wide" line due to the fact changes in cell size can trigger the need to recalculate and define losses.
*'''Layer 3''': The bridge rails. These might be anything from 100% blocked (solid concrete rails) to 10% blocked (very open rails). Sensitivity testing with 100% blockage is recommended as often debris during a flood can be substantial (see images from the Q&A section below). Some form losses would be specified depending on the type of rails and blockage. The solid portion of the rails (pBlockage*L3_Depth) can be added to L2_Depth to calculate hB/T in the table above to estimate the combined form loss coefficient of the L2 and L3.
* When using a polygon, the value entered in the FLC attribute is the total loss per unit length (meters or feet) in the direction of flow. The FLC is applied to all u and v points that fall within the polygon. A polygon lfcsh is mostly used for larger bridges to spread the effect of the FLC across multiple cells. The benefit of this method is that it is cell size independent.
*'''Layer 4''': Flow over the top of the rails - flow is assumed to be unimpeded.
The <u>[[Check_Files_2d_lfcsh_uvpt | 2d_lfcsh_uvpt_check file]]</u> can be used to review the location of the applied lfcsh and its attributes.<br>
<ol>
It is always a good modelling practice to undertake a sensitivity analysis on the applied form losses in the model to check if it makes any difference to the results and/or double check against other methods (hand calculations, other software, CFD modelling), especially if the bridge is anywhere near the area of interest. If calibration data is available, this should take be used to guide the form loss value specification.<br>
[[File:2d_lfcsh_attributes.png | 500px ]]
</ol>
<br>
 
The 2d_lfcsh functions by adjusting the flow width and the form loss of 2D cell faces. The combined blockage across the 4 layers is calculated at each simulation timesteps:
== What form loss coefficient (FLC) values should I use for 2d_lfcsh bridge? ==
<ol>
Firstly, there are several approaches to how FLC values are reconciled over the vertical and this has an important bearing on FLC values for Layers 2 and 3. It is strongly recommended to read Section 3.5.2 of the <u>[https://downloads.tuflow.com/TUFLOW/Releases/2020-10/TUFLOW%20Release%20Notes.2020-10-AA.pdf TUFLOW 2020-10 release notes]</u> before taking note of below.
<br>
[[File:Eq_blockage.png |600px]]
<br>
where<br>
y<sub>i</sub>: the actual depth of water in layer ''i''<br>
y<sub>total</sub>: the total water depth<br>
</ol>
 
The combined form loss coefficient can be estimated using different approaches, which can be individually specified by the 2d_lfcsh Shape_Options attribute, or globally specified by command: <br>
Layered flow constriction layers:
<font color="blue">Layered FLC Default Approach</font> <font color="red">==</font> [ CUMULATE | {PORTION} | METHOD C ] <br>
* Layer 1 - Underneath the bridge deck:
*CUMULATE (Method A): the losses are accumulated as the water level rises through the layers according to the following equation.
** A good starting point is to use the method for estimating form losses from bridge piers. It estimates the form loss coefficient from the ratio of the area occupied by piers and other flow obstructions relative to the gross area of the bridge opening.
<ol>
** If there are no piers or obstructions the FLC should be zero.<br>
[[File:Eq_flc_cumulate.png |450px]]
* Layer 2 - Bridge deck:
</ol>
** Due to the complexity of flow around a bridge deck (different deck designs/profiles; occurrence of pressure flow), there are no hard and fast guidelines on how to set the FLC for the bridge deck. The best approach is to conduct calibration tests if data are available, otherwise draw upon available information on form loss coefficients corresponding to the drag on similar shaped objects. It is always recommended to at least conduct sensitivity tests with different FLC values to ascertain the importance of the FLC values on the results and whether they have a meaningful impact on the modelling investigation.
*PORTION (Method B): the losses are applied pro-rata according to the depth of water in each layer using the equation below.
** FLC values for bridge decks for the CUMULATE, METHOD C and METHOD D approaches would typically be in the range from 0.1 to 0.5 for submerged bridge decks. For the PORTION approach much greater values are appropriate and are dependent on the deck depth to total depth ratio due to the depth proportioning approach used by PORTION. Should pressure flow occur (where the flow for a time becomes upstream controlled as orifice flow somewhat equivalent to flow under a sluice gate), using a vertical coefficient of contraction of 0.8 a form loss value of 1.56 can be derived as recommended in Hydraulics of Bridge Waterways (1978), however, it is not recommended this value is used without calibration or other benchmarking data, and its use should be confined to the PORTION approach only. The loss coefficient is derived from AustRoads, Waterway Design. Refer specifically to Figure 5.18 (pg 47, 1994 edition). The discharge coefficient, Cd, for a surcharging deck is 0.8, as shown highlighted in the attached figure. Assuming that V=Q/(bn*Z) and rearranging the equation given at the bottom of the figure, it arrives at V=Cd* (2g* dh)^(0.5). Where bn is the net waterway width. Also given the loss formula used by the ESTRY engine dh=k*(V^2/2g), the final formula is k=1/(Cd^2). Using a Cd of 0.8 as stated in AustRoads, k=1.5625.
<ol>
<ol>[[File: Bridge_Discharge_Coefficient.png]]</ol>
[[File:Eq_flc_portion.png |430px]]
* Layer 3 - Guard rails:
</ol>
** The advice for Layer 3 to represent guard rails is similar to Layer 2.
*METHOD C: this approach combines the CUMULATE and PORTION approaches by utilising CUMULATE through to the top of Layer 3 and PORTION above Layer 3.
** The blockage attribute can be useful for Layer 3 to adjust the effective flow area due to the degree of debris build up. Sensitivity testing with 100% blockage is recommended as often debris during a flood can be substantial as shown in the image below. The rails on this bridge are recommended for minimising debris build up!
<ol>
[[File:Eq_flc_methodC.png |520px]]
</ol>
All three methods apply a constant form loss value of L1_FLC when the water level is below Layer 2. Above Layer 2, the total form loss coefficient is increased gradually based on the thickness of water in Layer 2 and 3. Due to the depth proportioning approach used in the PORTION approach, larger L2_FLC/L3_FLC values are needed to achieve the same peak form loss coefficient as the other 2 methods. Above Layer 3, the PORTION and METHOD C approaches gradually reduce the total FLC with the increase of the water level, while the CUMULATE method continues to applies the cumulated form loss value. An example, taken from a calibration of a bridge structure from the Iowa River Flood Study is shown below. With water slightly overtopping a bridge deck, a combined form loss coefficient of 0.35 was used to match the observed head loss.
<ol>
{| style="text-align: center;" class="wikitable" width="48%"
!colspan="1" rowspan="2" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Layer
!colspan="1" rowspan="2" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Depth (m)
!colspan="1" rowspan="2" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Blockage (%)
!colspan="3" style="background-color:#005581; font-weight:bold; color:white;"|Form Loss Approach
|-
! style="background-color:#005581; font-weight:bold; color:white;" width=8%| CUMULATE
! style="background-color:#005581; font-weight:bold; color:white;" width=8%| PORTION
! style="background-color:#005581; font-weight:bold; color:white;" width=8%| METHOD C
|-
| 1 || 5.0 || 5 || 0.07 || 0.07 || 0.07
|-
| 2 || 1.5 || 100 || 0.15 || 1.05 || 0.15
|-
| 3 || 1.0 || 50 || 0.13 || 0.70 || 0.13
|}
</ol>
The figure below compares how the form loss value varies with height for the 3 methods.
<br>
<ol>
[[File:FLC_vs_height.png | 600px ]]
</ol>
 
==2D BG Shape (2d_bg)==
2D BG Shape is similar to the Layered Flow Constriction, but has several update to simplify the input based on the findings from the joint study with TMR. The lower three layers have been renamed for clarity.
*'''Pier layer''': Similar to Layer 1 in Layered Flow Constriction.
*'''Deck layer''': The bridge deck.
*'''Rail layer''': The bridge rails. The deck layer and the rail layer are treated as one '''Super Structure''' layer in 2d_bg. A combined form loss coefficient is specified using the 'SuperS_FLC' attribute. The solid portion of the rails (Rail_pBlockage*Rail_Depth) is added to Deck_Depth to calculate hB/T in the table above to estimate the combined form loss coefficient of the Super Structure.
<ol>
[[File:2d_bg_attributes.png | 700px ]]
</ol>
*'''Layer 4''': Above the top of the rails, flow is assumed to be unimpeded.
 
'''Inflection Point''': As shown in the joint study above, the head loss peaks when the water level is approximately 1.6*T above the bridge soffit, and decays slowly as the bridge becomes progressively drowned out. The 'SuperS_IPf' attribute (inflection point factor, default = 1.6) can be used to define the height of the inflection point. The solid portion of the rail layer is also added to the deck thickness to calculate the depth to the inflection point (D<sub>IP</sub>), i.e.:
<ol>
[[File:eq_flc_bg_infection_point.png | 520px ]]
</ol>
The form loss approach is similar to the FLC approach METHOD C, with L2/L3 replaced by a single super structure layer:
<ol>
[[File:eq_flc_bg.png | 480px ]]
</ol>
 
Using the same bridge example with SuperS_FLC of 0.28 and SuperS_IPf of 1.6, D<sub>IP</sub> would be set as 3.2m above the bridge soffit, and the figure further below shows how the form loss value varies with height.
<ol>
{| style="text-align: center;" class="wikitable" width="32%"
!colspan="1" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Layer
!colspan="1" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Depth (m)
!colspan="1" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Blockage (%)
!colspan="1" style="background-color:#005581; font-weight:bold; color:white;" width=8%| Form Loss
|-
| Pier || 5.0 || 5 || 0.07
|-
| Deck || 1.5 || 100 || 0.28
|-
| Rail || 1.0 || 50 || -
|}
 
[[File:FLC_vs_height_bg.png | 600px ]]
</ol>
 
== 2D Bridges Line vs Polygon Layer ==
The form loss coefficient (FLC) is applied differently when using a line compared to a polygon. The FLC is applied at cell sides (u and v faces) as this is where velocities are calculated.<br<
When using a polyline, the FLC attribute depends on the type of the polyline:
*Thin line (width attribute of zero) - The FLC attribute in the GIS object should reflect the total form loss value for the bridge. A thin 2d_lfcsh line will apply the FLC to a single row of cell sides. As such, this approach is cell size independent. Thin line lfcsh are the easiest setup, and is the preferable recommended approach.
* Thick line (width attribute between zero and 1.5 times the cell size) - The FLC attribute is half of the total loss as the form loss is applied on each cell side of the selected cells. A cell is selected if the polyline intersects the cell crosshair. Caution should be taken when using a "thick" line, due to the fact changes in cell size can trigger a "thick" line to become a "wide" line. If this were to occur the FLC attribute would need to be recalculated to not overestimate losses.
* Wide line (only supported for 2d_lfcsh, width attribute larger than 1.5 times the cell size) - The FLC attribute is a portion of the total loss based on number of cell sides in the predominant direction of flow. Caution should be taken when using a "wide" line due to the fact changes in cell size can trigger the need to recalculate and define losses.
The number of cell sides and the assigned FLC value can be checked in the <u>[[Check_Files_2d_lfcsh_uvpt | lfcsh_uvpt_check]]</u> and <u>[[Check Files 2d bg uvpt check | bg_uvpt_check]]</u> files.
<br>
<ol>
[[File:2d_lfcsh_thinline.png|400px]] [[File:2d_lfcsh_thickline.png|400px]]
</ol>
 
For larger bridges that spread across multiple cells, it is recommended to use a polygon layer, which selects all u and v faces falling within the polygon. Caution should be taken when specifying the FLC values for the two different 2d bridge features:
*2d_lfcsh: FLC attribute is the total loss '''per unit length''' (meters or feet) in the direction of flow. The FLC is applied to each face as 'FLC * cell size'
*2d_bg: FLC attribute is still the '''total form loss'''. Instead of converting it to "form loss per meter", the "Deck_Width" attribute is used to automatically distribute the total FLC to the selected faces, i.e. FLC<sub>face</sub> = FLC / Deck_Width * cell size.
[[File:2d_lfcsh_polygon.png|450px]] [[File:2d_bg_polygon.png|450px]]
 
<br>
It is a good modelling practice to check the <u>[[Check_Files_2d_lfcsh_uvpt | lfcsh_uvpt_check]]</u> and <u>[[Check Files 2d bg uvpt check | bg_uvpt_check]]</u> files to confirm the number of faces selected and the FLC values assigned. It is also strongly recommended to undertake a sensitivity analysis on the applied form losses in the model to check if it makes any difference to the results and/or double check against other methods (hand calculations, other software, CFD modelling), especially if the bridge is anywhere near the area of interest. If calibration data is available, this should be used to guide the form loss value specification.<br>
<br>
 
= Common Questions Answered (FAQ)=
== What blockage values should I use for bridge guard rails? ==
The blockage of bridge guard rails can be anything from 100% blocked (solid concrete rails) to 10% blocked (very open rails). In addition, the accumulation of debris during a flood can be substantial as shown in the image below. Sensitivity testing with 100% blockage is recommended.
<br>
[[File:Bridge rail debris.jpg | 500px]]
 
== How to conduct sensitivity test for 2D bridges? ==
General recommendations to cross-check the results are:
* Compare computed affluxes against desktop methods (e.g. Hydraulics of Bridge Waterways, 1978) and/or other software including CFD, especially for unusual bridge designs.
Line 74 ⟶ 195:
* The FLC value applies an energy loss along 1D channels or across 2D cell faces equivalent to FLC*V<sup>2</sup>/2g where V is the 1D channel velocity or the 2D cell face velocity.
* FLC values are often sourced from publications such as Hydraulics of Bridge Waterways or AustRoads (e.g. Kp chart for piers).
* If possible, establish whether the source of the FLC value is based on the approach velocity (the velocity in the absence of piers) or structure velocity (the velocity with area blocked out by the piers) noting that it often isn’t clear or stated.
** If it is the structure velocity, this is usually the velocity at the vena-contracta (point of greatest contraction within the entrance to the structure and therefore highest velocity) - see image below. Bluff or sharp-edged obstructions will have a much more pronounced vena-contracta, and therefore higher velocity compared with a round-edged obstruction.
** FLC values based on the approach velocity will be higher than those based on the structure velocity to achieve the same energy loss.
Line 117 ⟶ 238:
The safest and strongly recommended approach with regard to establishing head losses and therefore flood levels, is to not resolve the obstructions in the mesh but instead model the effects of such obstructions with form (drag) loss coefficients (applied to selected mesh cells) that have been derived from physical testing. This approach has been shown to provide the most consistent results across various mesh resolutions. It also has the added benefit that, by avoiding small cells in the mesh, it will provide much more efficient run times for flow solvers.<br>
 
== How to best convert flow constriction data (2d_fc or 2d_fcsh) into layerednewer flow constrictionformats (2d_lfcsh) formator 2d_bg)? ==
The form loss parameters can be transferred from the flow constriction (2d_fc or 2d_fcsh) to the first layer of the layered flow constriction (2d_lfcsh) or pier layer of the 2d_bg. Definition of the remaining form loss and blockage layer inputs in the 2d_lfcsh should follow the guidance outlined in <u>[[TUFLOW_2D_Hydraulic_StructuresTUFLOW_2D_Hydraulic_Structures_draft#What_form_loss_coefficient_2D_Layered_Flow_Constriction_.28FLC282d_lfcsh.29_values_should_I_use_for_2d_lfcsh_bridge.3F29 | What2D formLayered lossFlow coefficientConstriction]]</u> (FLC)and values<u>[[TUFLOW_2D_Hydraulic_Structures_draft#2D_BG_Shape_.282d_bg.29 should I use for| 2d2D lfcshBG bridge?Shape]]</u> paragraphparagraphs.<br>
When using floating pontoon (type FD in the 2d_fc or 2d_fcsh) different setup might need to be used for different events. For large events when floating pontoon becomes fixed at the top of the supporting piles, standard 2d_lfcsh setup can be used. Smaller events when the pontoon is floating at different heights might require more sensitivity testing of the structure parameters to find out a setup the matches the reality as close as possible.<br>
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== Should I model bridges in the 1D or 2D Domain? ==
The recommended approach depends on if the channel upstream and downstream of the bridge is modelled in 1D or 2D. To preserve the momentum as accurately as possible the bridge should be modelled in the same dimension as the channel, e.g. 1d_nwk bridge if the channels is in 1D and 2d_lfcsh if the channel is modelled in 2D.<br>
 
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