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1D Bridges: Difference between revisions

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==Deck Losses and Pressure Flow==
WhenA waterpressure levelflow reachescondition thecan bridgeoccur deckwhen level,the extrawater energylevel loss can be caused byreaches the bridge deck. Itlevel could result into pressure flow condition where(as water is forceforced through a small opening confined by the bridge abutments and the deck), resulting in additional energy losses. As the bridge becomes totally drawndrowned by the water level, itthe condition will switch back to the normal submerged bridge deck flow condition. withTo submergedaccount for this, once water reaches the height of the bridge deck., the TUFLOW BB bridge tests for either the pressure flow condition or drowned flow condition at every timestep by choosing the flow regime that gives the lower flow:
:*The pressure flow equation is based on the Section 8.3 "All Girders in Contact with Flow (Case II)" of the Hydraulics of Bridge Waterways (Bradley, 1978), with a default deck discharge coefficient of 0.8. This value can be modified using the 1d_nwk HConF_or_WC attribute. Note that the original hydraulic experiment conducted by Liu (1967) used a flume with a pair of bridge abutments and a deck. This means the impact of both abutments and deck are considered in this approach. The entry/exit losses are switched off during the pressure flow calculation to avoid the overestimation of the contraction/expansion losses.▼
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:*For the drowned flow condition, the BB bridge considers extra energy loss caused by the bridge deck/rails using a deck loss coefficient (default = 0.5) in addition to the entry/exit losses. The deck loss coefficient can be adjusted using the WConF_or_WEx attribute or by specifying the LC (energy loss versus height) table. ▼
Once water commences to surcharge the bridge deck, TUFLOW BB bridge tests for pressure flow or drowned flow every timestep by choosing the flow regime that gives the lower flow:
When pressure flow occurs a flow regime of "P" is reported in the _TSF layer. <br>▼
▲*The pressure flow equation is based on the Section 8.3 "All Girders in Contact with Flow (Case II)" of the Hydraulics of Bridge Waterways (Bradley, 1978), with a default deck discharge coefficient of 0.8. This value can be modified using the 1d_nwk HConF_or_WC attribute. Note that the original hydraulic experiment conducted by Liu (1967) used a flume with a pair of bridge abutments and a deck. This means the impact of both abutments and deck are considered in this approach. The entry/exit losses are switched off during the pressure flow calculation to avoid the overestimation of the contraction/expansion losses.
 
▲*For the drowned flow condition, the BB bridge considers extra energy loss caused by the bridge deck/rails using a deck loss coefficient (default = 0.5) in addition to the entry/exit losses. The deck loss coefficient can be adjusted using the WConF_or_WEx attribute or by specifying the LC (energy loss versus height) table.
▲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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