Help asmo build road/bridge -- want ideas and thoughts

[bkincaid]

First, I'd check with the jurisdiction where the property was subdivided. Depending on when that was done, they may have a drainage report on file that you could look at or maybe even get a copy, this would save a bunch of work and cost for you. Also look to see if any of the neighbors have the same situation present. If so, chat with them and see what they have. Is there any evidence/first hand knowledge of high flow events in the area?

Assuming no drainage report available, I'd look into getting an engineer to perform the drainage calcs for the different moisture events (10, 20, 100 year or whatever) so I'd know exactly what I'm up against, flow wise. Then I'd make the needed/wanted treatment decision based on those numbers as compared to my level of comfort regarding access to my home.

A CMP can last many years, especially in dry wash flow events, but, soil and water ph as well as abrasive contents of flow all have a tremendous effect on life expectancy as does proper installation techniques and materials. There are many millions under the roads in this country. I suspect that existing culvert has no where near maximum cover depth, even assuming thinnest culvert material. Here's a link to some CMP specs and such: http://www.conteches.com/products/pi...technical-info , take a gander and see what's possible.

If the flow calc numbers and comfort level work for the existing size culvert, as well as previously stated soil and water ph levels and abrasive level of flow, let it ride. If the CMP ever needs replacing, determine what size HDPE culvert you can insert into the CMP (slip-lining), insert and fill the space between the two with grout or such. Smooth bore HDPE of equivalent size will carry a much greater flow capacity, hence sliplining with a smaller size while retaining same/similar flow capacity. Add material as necessary both upstream and downstream to prevent backpooling conditions and rip-rap downstream to help minimize incision/erosion activities.

If flow calcs show the existing CMP size to be inadequate to handle the flow of the given storm event versus my comfort level, I'd look into a CBC with cut-off walls and wings and engineered channel design.