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Bridge Loss Coefficients

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I have built a 1d HEC-RAS model of a bridge structure and have run various flows through the model in order to generate the afflux through the structure. I would like to generate a bg table of loss coefficents using the outputs from the HEC-RAS model which I will then use in an ESTRY model.  I understand that to calculate the K values, I need to use the upstream and downstream water levels to determine the head loss. However, I am a little unsure as to what to do next....!  Could anybody kindly point out the next step.....


I usually use the Hydraulics of Bridge Waterways to calculate my loss coefficients, but it appears that this method is significantly under predicting the afflux at this 3-arch bridge.


I look forward to any pointers...


Kind regards,


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Hi Laura


The K value should be set equal to the 2*g*dh/V^2 where g is gravity (9.81), dh is water level drop and V^2 the velocity squared.  Note that by default TUFLOW does not adjust the loss coefficient for bridges by the approach/departure velocities as is done (by default) for culverts, as the effect of the approach/departure velocities are usually taken into account in the derivation of bridge losses (as is the case for Hydraulics of Bridge Waterways and may or may not be the case for HEC-RAS depending on whether you specify "no flow" zones on the sides of your cross-sections upstream/downstream of the bridge).  You can have TUFLOW adjust the loss according to the approach/departure velocities by specifying the A flag (ie. a BA channel).  For more info, please see Section Adjustment of Contraction and Expansion Losses of the 2010 TUFLOW Manual.


This post may also be useful:  http://www.tuflow.com/forum/index.php?showtopic=1259.




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