Case Study: SWPPPs


Terra is working with a local auto dealership client to provide a new parking lot addition and stormwater management system. The project includes Stormwater Pollution Prevention Plans (SWPPPs) and soil erosion inspections.

Once a week Terra performs SWPPP inspections from the initial construction to a point where the site is substantially complete and the landscaping is stabilized. For rainfall greater than 0.5”, more visits are required. During the inspections, Terra looks to confirm the Stormwater Pollution Prevention plan is being implemented properly and to check if any discharges have occurred.

This past October a major rainfall hit the region, dropping as much as 8” of rain over a couple of days. At that construction stage, the parking lot addition was partially completed with storm sewers, aggregate base, and curbs.

The new detention basin was complete, but it was being stabilized with an erosion control blanket. Initially, everything looked to be in good shape, considering the heavy rainfall. But downstream of the construction site, the neighbor’s detention pond was inundated with mud and sediment.


Below is the neighbor’s pond downstream of our site after the storm. Note the high turbidity of the water and the sediment pile on the shore. 

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After a closer look, the main source of the sediment was from under the Illinois Tollway Authority’s sound barrier wall along the north property line. 

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During our SWPPP inspection, we walked the IL Tollway drainage project next to our project site to find the source of the sediment. The IL Tollway recently reconstructed the drainage system and installed erosion control measures. 

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The last downstream storm structure in the Tollway drainage system happens to be on the other side of the sound wall where the erosion damage occurred. You can see the orange filter fabric under the layer of sediment. 

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The high water mark that was left behind was proof that too much water had nowhere to go. The hydraulic pressure undermined the soil below the wall, carving out a ravine to the pond. This image is of temporary erosion control filter fabric on inlet covered with sediment. 

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This is a close up image of the erosion under the wall. You can see the inlet on the other side.

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This temporary fabric works well during the frequent small storm events, but it can also block heavy amounts of water during extreme events. The SWPPPs need to identify emergency overflow points to ensure water has a safe place to drain.  

During our SWPPP inspections, Terra checks to see if the erosion control devices are appropriate for the location and are performing as intended. Our team also observes the overflow route for those rare moments when rain floods the site. SWPPP’s are a working document and intended to be updated as the construction moves forward.