Modeling Approach – Leaky Structure in 2D

[Scott Miller]

I’ve been tasked with simulating an observed stream flood event in which numerous factors contribute to backwater – debris racks, dense in-channel vegetation, foot bridges. The most unconventional factor to model is a slat fence with approx. 1/8 inch gaps between boards and a small gap below sections of the fence. I think the fence might be represented by a structure with a few orifices, or pipes through it. But, I’m seeking suggestions to improve the approach.

[Luis Partida]

Hey Scott, the first thing that comes to mind for me is to model the slat fence as a cross section with lids and either add contraction and expansion losses or add a minor loss to the cross section. This is of course assuming it is perpendicular to flow

[Scott Miller]

Thank you, Luis. Good point about perpendicular flow. The fence is oblique across the floodplain. Overbank flow is partially deflected, and the deflection would vary with stage. I did not give thought to flow direction with regards to a cell face. It seems full momentum ought to help with regard to vectors at the cell face, but, for flood plain modeling, diffusion wave ought to be sufficient. I am not familiar enough with 2D at a code level to knowledgeably switch the equation sets for this situation. What difference might a full momentum might make? How much of a problem would non-perpendicular flow be? Perpendicular flow would be an assumption, and without gauging specific to this fence – it is what it is. Reasonable?

[Luis Partida]

Ah so this is a 2D model…I think the direction of flow in terms of the structure and cell faces can be easily fixed with breaklines. Similar to the methods in Harris county requiring a breakline be placed at the edge of the 1D2D interface which will not actually enforce a cell break but it will orient cell faces perpendicular to the structure. As far as which equation to use, I have always had the understanding that unless you are predicting excessive variances in velocities i.e. a damn breach analysis hence the removal of the convective terms in the simplified equation

[Scott Miller]

QUESTION regarding how flow through gated structure is calculated. I modeled the gap under part of the fence using a gate – fully open when the water is high enough to flow through it. The gate is rectangular, but the topography is trapezoidal. About a third of the gate is obstructed toward the corners. Does RAS take the land surface into account, or is flow through the gate calculated as though the full gate rectangle has flow – based just upon the head and tail waters?

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