Last year, Columbia Highway was impassible for 37 days due to flooding.
Another factor exacerbating the situation is that there is only 4' of drop over the half a mile of buried drain.
In the nominal condition (18" drain, roughness coefficient of 80, 6" rain and 4' of head) it would take 53 days to move all of the water that falls on the square mile drained by the upper reaches through that drain.
12" diameter tile was evaluated to simulate degraded performance due to sediment, roots and partially collapsed tile. 24" diameter was looked at as an upgrade.
Increased head, the height of the water in the swamp, was also evaluated.
In the nominal condition (18" drain, roughness coefficient of 150, 6" rain and 4' of head) it would take 28 days to move an all of the water through that drain.
Obviously, much of the water percolates through the soil as the estimated times shown on the charts are WAY longer than observed.
My take-away is that drains are very sensitive to increasing diameter (good) and smoother walls (good).
|This image added in response to comment by B.|