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 bridge opening. As the bridge becomes totally drawn, it will switch back to the normal flow condition with submerged bridge deck.
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. Further details on how the value of 1.56 was derived can be found [http://www.tuflow.com/forum/index.php?/topic/1419-1d-loss-coefficients-for-bridge-deck-surcharging/#comment-3673 here].▼
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*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, which means the impact of both abutments and deck are considered in this approach. Therefore, the entry/exit losses are switched off during the pressure flow calculation.
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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.
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When pressure flow occurs a flow regime of "P" is reported in the _TSF layer.
▲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.
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