Hand modification to catch basin produces the same spread output value

Good morning. We have placed a catchment by picking points. We analyzed our intake spacing by checking the spread report. The spread value was too high. We then modified the catchment, by hand, and spaced the intakes closer together to come up with a lower spread value. We reanalyzed and checked the spread report. The spread value computed did not change. Does the program not recognize manual modifications to catchments? Do we need to synchronize the drawing before reanalyzing? Thanks for any help that can be sent our way.

  • Manual modification is supported but several other things could be in play. Some possibilities are:

    • Double check the Utility Properties > Physical to confirm the road and gutter slopes are correct.  There are some situations where the cross-slopes as extracted from the surface will fail and you will get zero for the road cross-slope. If this is the case, then first edit the MicroStation properties to turn off the setting of "Use Road Cross-slope" This will prevent the zero cross-slope from getting extracted all over again when you run computations. Then return to the utility properties and input the correct road cross-slope. Thus, the computed spread will be more correct.
    • If all the properties are correct then you may be running into physical limitations of the ground and the inlets. When the longitudinal slopes for the two catch basis are very flat and when the inlets are grates only (no curb opening) the spread on the upstream inlet can be so great that the the water never gets a chance to fall in grate one and thus the bypass remains unchanged.  In such case, rather than reducing the spacing, you may have to insert additional inlets.
    • If the offending spread is at a sag inlet, you may have to add flanking inlets near the edges of the sag ponding to reduce the spread at the center inlet.

    Robert Garrett
    Senior Consultant

    www.envisioncad.com