Modeling tidegates in Extran

I am trying to simulate tidegates with SWMM EXTRAN. I am using the I2 card to specify an outfall with a tide gate. I am also using the J cards to input tide stages and times at the outfalls with tidegates. I have a few questions on how the tidegate option works.

1. The tidegate prevents backflow, does this mean that flow is discharged only when the tide is below the level of the tidegate?
2. What controls the opening and closing of the tidegate?
3. Should head losses due to the tidegate be included or are the minimal?

Any other ideas on using a tidegate would be appreciated.

A tide gate applies a head (usually the elevation of the receiving water body) to the discharge side of an outfall while preventing reverse flow. Basically, the discharge rate is based on the head differential between the upstream and downstream sides of the gate.

1. Flow is discharged through a tide gate out of the system only when the hydraulic grade line at the outfall is greater than the head in the tide water. That is, a submerged outfall can still discharge if the head in the sewer system at the outfall is greater than the head in the receiving water.
2. The tide gate works as indicated above. It opens or closes in accordance with the head difference between the outfall junction and the tidal tail water.
3. A head loss is computed by the program for a weir with tide gate, as indicated on page 156 of the Extran User's Manual. For "ordinary" tide gates, at the end of a pipe, properly the head loss should be incorporated into the outfall pipe roughness, although most users probably don't bother. Whether or not it's important for your simulation could be determined by a sensitivity analysis.