Farm Ditch

[gjeppesen]

Hi Guys,

It seems I've been using stormwater modeling programs for too long now and I can't figure out how to do something that should be simple.

Here' the problem. I have a 80 acre project that is currently being farmed. When the property was purchased a few years ago it had a ditch running through it that carried flows from the north to the south. There were a couple of culvert crossings and it meandered through the property. The farmer had to move the ditch to the east and straighten it out in order to better use the property for farming.

The local permitting agency is giving us trouble in that we didn't go through them first (permitting, etc.). However, in our state (Florida), farmers can get away with more than a land developer so our task is to prove that the new ditch does not negatively affect anything upstream or downstream. If we do this, no permit is required.

The ditch has equal or greater capacity than the original ditch per Mannings, but I'm running the risk of sending more water downstream because the ditch is more efficient. I wanted to do a simple calc and not get into something complicated but I'm having a brain freeze....

 

[gbam]

Ok,
Does the new ditch convey the same flow as the original ditch or did the relocation change the watershed boundary? If the flow rates are the same you should be fine for capacity. However, you may need to compare exit velocities so the new ditch will not be anymore erosive than the existing ditch. If it is by your description of more efficient you may have to add mitigation measure to reduce the erosion. Hopefully, the new ditch exits the property at the same location as the old ditch.

 

[gjeppesen]

Yes, same flow. We're covered on erosion. That's not a worry. Yes, ditch exits property at the same location.

 

[gbam]

You can use a couple of XSECs & Manning's for both existing and proposed conditions and evaluate normal depth & velocity to show the minimal change. Call it good.