Tuflow and estry manual Version 3

3.14.41D Hydraulic Structures

Hydraulic structures in the 1D domains are modelled by replacing the momentum equation with standard equations describing the flow through the structure. The basic structures available are listed below and described in the following sections.

A channel is flagged as a hydraulic structure using the Channel_Type attribute (see Table 4 .10) as described in Table 4 .9. Except for culverts, a structure has zero length, ie. there is no bed resistance.

3.14.4.1Adjustment of Contraction and Expansion Losses

As of Build 2005-05-AN, a new feature allows the energy losses, associated with the contraction and expansion of flow lines into and out of a structure, to be automatically adjusted according to the approach and departure velocities in the upstream and downstream channels. This is particularly important where:

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  There is no change in velocity magnitude and direction as water flows through a structure. Examples are:
  
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    A clear spanning bridge over a stormwater channel where there are no losses due to any obstruction to flow until the bridge deck becomes surcharged.
    
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    Flow from one pipe to another where the pipe size remains unchanged and there is no significant bend or change in grade.
    
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  There is a slight change in velocity that does not warrant application of the full entrance and exit loss.
  

For bridges and culverts, if Structure Losses is set to ADJUST, the entrance and exit losses are adjusted according to the following equations:

As the entrance and exit losses are combined as one coefficient for a bridge, the entrance and exit loss adjustments are proportioned one-third / two-thirds respectively.

As can be seen, as the strucuture velocity approaches the approach and/or departure velocities, the loss coefficient approaches zero. While, when the approach and/or departure velocity approaches zero (ie. water is leaving/entering a large body of water), the loss coefficients approach their full value.

The selection of the upstream and downstream channels on which to base the approach and departure velocities is as follows:

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  The upstream channel is taken, as the channel that has a positive flow direction into the structure that’s invert is closest to that of the upstream invert of the structure. If no channel exists, no adjustment of losses is made. Note that the upstream channel must be digitised so that it has the same positive flow direction to that of the structure.
  
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  The downstream channel is selected on a similar basis to that for the upstream channel.
  
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  The selected upstream and downstream channels are listed in the .eof file for cross-checking (search for “Primary Channel”).
  

TUFLOW has no requirement of a minimum loss coefficient value for stability, and therefore allows the adjusted coefficient to approach zero. This allows the correct modelling of structure losses when the structure causes no disturbance to flow, or when one pipe discharges into another of identical size, grade and alignment.

The adjustment of loss coefficients does not apply to:

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  Any bend or additional loss for a culvert entered using the Form_or_Bend_Loss attribute in the 1d_nwk layer. This coefficient can be used to apply additional losses (eg. pit or bend losses) that are not affected by changes in the relativity of the approach/departure and structure velocities.
  
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  Any additional loss coefficient component of a bridge entered using the Form_or_Bend_Loss attribute in the 1d_nwk layer. This coefficient can be used to apply additional losses (eg. pier losses) that are not affected by changes in the relativity of the approach/departure and structure velocities.
  

If Structure Losses is set to FIX, the loss coefficients are not adjusted.