Math, apparently.
The road in front of my property acts as a dam for a very large drainge area. Knox Creek traverses from the north and east and encounters the road, follow it south for about 120 feet, and then goes through two six-foot diameter viaducts to go under the road. Normal water dept in the creek stays around a foot or less, except in pools.
My county has been planning to replace the 40-year-old viaducts and embankment edifice for over a year, with undending delays and excuses. Box culverts had to be ordered when concrete was in short supply (area is booming here, backlog, I guess). Corps of Engineers and EPA probably had to review and approve. Road diversion (it's a dead end subdivision) had to be planned and emergency egress provided.
Meanwhile, storms continued to batter the edifice and pull down stonework, as the creek regularly tops the viaducts and threatens to flood my house and overflow the road. The county refused to even clear the concrete and metal from the collapsing viaduct as it has been blocking flow now for months in both sides, likely reducing flow by 25%.
All this was about to be resolved next week. When I returned home Thursday, a big stack of nine or so concrete drains were across the road, on a staging pad, awaiting installation next Monday. Something was wrong. Not only were they not new concrete (they were chipped, damaged, cracked, and had dirt in the bottom, indicating they have been in use somewhere and are being re-installed here), they were only 44" in diameter. My next door neighbor in whose yard they piled them said they were going to put them three abreast. This isn't the box culvert they planned, and even a month ago an engineer on site said the new duct would be 7' x 12'.
The existing two calculations to be compared reveal that the six-foot viaducts are 36" radiuses, so have a combined area of 8,143 square inches. The three "new" viaducts will have only 22" radiuses with a combined area of only 4,561.5 square inches. So, for a drain that has flooded before and is being destroyed by the overload now, the county is reducing its capacity by 44% and lowering the profile of the height, plus dividing it, which will unquestionably cause it to block more readily and be reduced even more.
Only in Alabama.
Roll, TIDE! (pun intended)