we had a stupid guy
build an earthen dam
on his property above ours...ok
then it finally broke in a big storm
completely washing out our drive way
and that was a 1/4 mile walk to home...
we had to park one car on one side of the fissure,
and park the other car on the other side
and switch cars all the time just to come and go.
i almost was killed inside the friggan culvert
trying to remove all the debris clogging it up
all because of that stupid bastid....i wanted to beat him
within an inch of his life afterwards...let me tell ya.
the answer to your question is....
you build the swale ....
while its raining
with a sharp as a knife shovel .....
the moving water will explain how deep
to dig it... and you go about three inches deeper
than you'd think because afterwards it fills in with silt
and lawn... (sod)...
then the next time you get a huge rain you watch it work
and adjust it if necessary .........
but it's always best to do it in the rain.
any Rancher knows this.....
