Swale size for water redirection

This is a small scale project. I need to keep water from broad siding the shop when it rains or snow melts. the shop is down hill from where I'd put some on contour swales. It's floor is cracking and sinking and it's only a few years old. I had great success with the hugel swale project keeping our yard dryer, so I want to create some swales again. The question is, if I just use a sturdy hoe that only creates a small swale about 6-10 inches wide and perhaps 5 inches deep, how long will that small swale be effective? It is a wooded area in Indiana with average rain and snow for the area. Should I get rocks in these swales? The lack of sunlight doesn't promise much for a hugel swale, but does anyone recommend a bigger size for water diversion swales? I am concerned for the root structure of the poplar stand I will be digging these swales. Thanks for any input!

Hi George:

I guess I would design the diversion ditch to handle the amount of water in an "extreme" event that you want to divert from your given watershed.

So "watershed area" x "amount of precipitation in extreme event that needs to be diverted" = size.

http://permaculture-and-sanity.com/pcarticles/permaculture-earthworks-and-swales.php