Suppose there are 8,000 active residential construction sites in southeastern Michigan this spring, summer and fall, all on average-sized city lots, all having excavations for basements with the corresponding piles of dirt. Also suppose that there are 3,000 large commercial, industrial, public works or utility projects, each having disturbed the soil to a significant extent.
Further, let’s assume that on half of all such construction sites, no erosion controls have been applied, and on the other half, silt fence has been installed but not maintained; and after being run over by trucks, front-end loaders, skyhooks and the like, the silt fence no longer functions.
Say the weather is in the typical range with three major rainstorms over the construction season at any given site. Assume further that during each of those heavy rains, each residential lot loses one ton of loose soil (equivalent to a pickup truck load) in runoff which flows into sewers, and each of the large commercial, etc., projects loses three tons. Then, after three heavy rains, a total of 51,000 tons (51,000 pickup truck loads) of dirt have gone into sewers.
Unlike many cities the size of Detroit or larger in the past decade or so, insolvent Detroit was not required to build a huge tunnel to hold billions of gallons of storm runoff for gradual release, screening, settling and treating.
Instead, in our region, a deluge that causes silt-laden water to surge from construction sites into sewers also causes the sewers and a handful of small retention basins to overflow into local ditches and streams, which feed international treaty waters like Lake St. Clair, the Detroit River and Lake Erie.
That means, in our hypothetical, during each construction season, 51,000 tons of eroded soil clog the sewers, cloud our natural waterways, and fill the bottoms of rivers, harbors and lakes with sediment.
Something’s wrong with this picture.
[Readers: Got better numbers? Leave a comment.]