1D Bridges: Difference between revisions
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==Deck Losses and Pressure Flow==
When water level reaches the bridge deck level, extra energy loss can be caused by the bridge deck. It could result into pressure flow condition where water is force through a small opening confined by the bridge
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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:
*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
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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.
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.
=Irregular shaped bridges=
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