ToC, peak flow changes with drainage ditch relocation

[pinkpig]

Suppose I have a square shape pasture subcatchment (2 square miles) and it currently drains to a ditch long its border. Now, I am taksed to relocate this ditch to the basin center.

Since the TOC will be decreased (probably not by exactly one half, but I am sure it will be decreased), so the peak flow under same return period will be increased.

In order to make the new ditch handle the same level of runoff flow (or the same return period), the new ditch needs to be larger than the existing ditch.

Does this sound reasonable to you? I have a hard time to explain this to the local governing authority. They think the new ditch will be the same or very similar size as the old one.

 

[TerryScan]

If the subcatchment in question is the ONLY contribution to the ditch on its border, then I would imagine the ToC for the "upper" half of the subcatchment to the ditch to be shorter creating a higher peak. But how is one getting the "lower" half of the subcatchment to ditch? Is it being regraded toward the ditch? or will there be other lateral ditches or pipes to get it to the relocated ditch? If that is the case, the ditch may be larger just to be deep enough to accommodate the necessary grades.

 

[pinkpig]

Yes, the entire basin is very flat, and after relocation, if needed, the lower half will be regraded but this is not my task. (actually a little bit flooding at lower half is not a big concern, since the entire basin is for pasturing only, no residence, not commerce,on industrial,...)

Thanks for confirming that the ditch will need to be larger. -PG.

 

[Drew08]

I think this concept would be easy to show using the kinematic wave method in EPA SWIMM rather than the SCS unit hydrograph method. In this first option the watershed width would be the length of the channel, in the second option, the watershed width would be 2x the length of the channel. The single watershed is essentially bisected into two separate drainage areas.