1D Bridges: Difference between revisions
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=Introduction=
The following section looks at bridges using the 1D component of TUFLOW. Reference should also be made to Section 5.7.2 of the <u>[https://downloads.tuflow.com/_archive/TUFLOW/Releases/2018-03/TUFLOW%20Manual.2018-03.pdf 2018 TUFLOW Manual]</u>. For information on bridges in the 2D domain
'''Example of a bridge that could be modelled in 1D'''▼
1D bridge channels do not require length, Manning's n, divergence or bed slope (they are effectively zero-length channels in terms of conveyance) and rely on a reasonable estimate of energy losses associated with re-expansion of water after the vena-contracta (entrance losses), expansion of water downstream (exit losses), pier losses, bridge deck and guard rail losses. Other factors include accounting for occurrence of bridge deck pressure flow and the effects of bridge skew and multiple bridges (shielding effects of an upstream bridge on a downstream bridge). <br>▼
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[[File:Photo 04-12-2014 13 16 25.jpg|border|400px]]▼
Care must be taken when choosing the approach to modelling the bridge and setting appropriate loss values. ▼
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London, UK (pht: Rohan King)▼
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▲Care must be taken when choosing the approach to modelling the bridge and setting appropriate loss values. <br>
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=1D Bridge Channel Types=
TUFLOW offers two 1D bridge channels, B and BB. BB channels were introduced for Build 2016-03-AA and are a more advanced solution than B channels, which are retained for legacy models. By default, BB channels are superior to B channels as they:<br>
:*adjust the entrance and exit losses every timestep according to the approach and departure velocities (in the same manner as for other structures such as culverts); and
:*automatically simulates pressure flow conditions. The only loss coefficients required to be specified for BB channels are those due to piers, and the bridge deck when it is submerged and not under pressure flow.
:*<u>[https://www.tuflow.com/Download/Presentations/2012/2012%20Aust%20Workshops%20-%20TUFLOW%20Modelling%20Bends,%20Structures%20and%20Obstructions.pdf 1D and 2D Modelling Bends, Structures and Obstructions]</u>
:*<u>[https://www.tuflow.com/Download/Publications/Modelling%20of%20Bends%20and%20Hydraulic%20Structures%20in%20a%202D%20Scheme,%20Syme,%202001.pdf Modelling of Bends and Hydraulic Structures in a Two-Dimensional Scheme]</u>
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<li style="display: inline-block;"> [[File:2d_Channel_to_1d_Bridge_to_1d_Channel.JPG|thumb|none|307px|2D channel to 1D bridge to 1D channel]] </li>
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▲'''Example of a bridge that could be modelled in 1D'''
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▲[[File:Photo 04-12-2014 13 16 25.jpg|border|400px]]
▲<br>
▲London, UK (pht: Rohan King)
▲<br>
=Loss Theory=
▲1D bridge channels do not require length, Manning's n, divergence or bed slope (they are effectively zero-length channels in terms of conveyance) and rely on a reasonable estimate of energy losses associated with re-expansion of water after the vena-contracta (entrance losses), expansion of water downstream (exit losses), pier losses, bridge deck and guard rail losses. Other factors include accounting for occurrence of bridge deck pressure flow and the effects of bridge skew and multiple bridges (shielding effects of an upstream bridge on a downstream bridge). <br>
==Contraction/Expansion Losses==
Energy loss is caused by the flow contraction due to the expansion of water after the vena-contracta inside a bridge section and the flow expansion downstream of the bridge. The contraction/expansion loss coefficients (or the entry/exit loss coefficients) can be specified for TUFLOW 1D BB bridge using the following attributes:<br>
:*
:*
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[[File:Structure_Contraction_Expantion_Losses.png|600px]]
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The entry/exit loss coefficients are not used in the legacy B bridge. Instead, the bridge opening ratio is used to obtain the backwater coefficient Kb value from Figure 6 "Backwater coefficient base curves" of Hydraulics of Bridge Waterways (Bradley, 1978).
==Pier Losses==
Energy loss estimates 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 FHA (1978) the approach is:
<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>
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When pressure flow occurs a flow regime of "P" is reported in the _TSF layer.
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For the legacy B channels, the deck loss coefficient was fixed at a value of 1.5625, which is derived from the discharge coefficient in Hydraulics of Bridge Waterways of 0.8 (1.56 = 1/0.8^2) to approximate pressure flow conditions. Whilst this is reasonable when the bridge deck experiences pressure flow, it will over-estimate the losses once the bridge deck starts to drown out and flow returns fully to downstream controlled.<br>
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=Irregular shaped bridges=
In the UK arch shaped bridges can often be seen on waterways, whereas in Australia these types of structures are quite uncommon. Modelling an irregular shaped bridge utilises the hydraulic properties elevation-width (CS/HW) type cross section and an irregular type culvert. <br>
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[[File:Irregular_culvert_2.jpg|border|400px]]
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London, UK (pht: Rohan King)<br>
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=Check Files=
The table below highlights some of the commonly used check files when reviewing 1D bridges. The full list of TUFLOW check files can be found [[TUFLOW_Check_Files | here]].
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Any further questions please email TUFLOW support: [mailto:support@tuflow.com?Subject=TUFLOW%201D%20bridges%20help support@tuflow.com]
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