
Spending thousands and still losing the battle with driveway runoff to ditch. Help needed
Hello everyone. A google search of this topic brought me to this subreddit and it looked like some pretty qualified experts were willing to pitch in. Hoping for the same luck. Thank you in advance for your time.
TLDR: what is the best culvert cap design to prevent continuous washout here?
Context:
First start to finish home build project in South Florida. Old grove and farm property converted to residential. Sandy/silty soil but customer had 80+ loads of regular fill dirt brought in. Front (east) side of the property has a ditch that we had ~55' of 18" double walled HDPE culvert installed in and then backfilled with soil, base rock, and then 57 stone on top; however, the problem is the culvert edges...
Lots of pictures below, but the short of it is: county road department requires a concrete culvert cap of the following specs: 4" thickness, reinforced with 6x6 wire mesh, that extends 18" beyond the culvert opening on the top and sides, and mitered at a 3:1 slope (1 foot of run for ever 3 feet of rise).
We've been having our asses handed to us with rain this entire build. We had the entire property (1 acre) graded to sod ready when a storm came through and created a ton of wash out the beginning of April. Land development/sod contractor regraded everything, extended the culverts out another 8', and sodded on April 16th. It's been great and took root quickly; however, we got heavy rain this weekend (1"+) and already have washout from under the sod that I can only imagine is going to get worse. I know we have to do a culvert cap, but the other culvert caps on the other properties out here are getting washed out as well. One owner put rip rap rock around his but you can still see the dirt washing out from under it and the rock sinking.
I was talking with a guy from the road department and was thinking we could just make the culvert cap much larger as there's no maximum, only a minimum requirement. They could also "bowl" it to wrap the curve above the culvert and go much higher. See last image.
Looking for some expertise and input on what the best "set it and forget it" method would be. Concrete contractor thinks doing it the same way the others are would be fine despite the washout. Thank you so much in advance.
Runoff from 2.5\" rain on April 7th - 1
Runoff from 2.5\" rain on April 7th - 2
Regraded and sodded with Bahia on April 16th - 1
Regraded and sodded with Bahia on April 16th - 2
1 month post sod runoff on May 17th - 1
1 month post sod runoff on May 17th - 2
1 month post sod runoff on May 17th - 3
Culvert shape idea/proposal - still concerned with the runoff to the top right you can see