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Stormwater Infiltration Data Exceeds Model Prediction for Contaminant Removal

The City of Cudahy

Cudahy, WI

The City of Cudahy designed a permeable paving system to reduce stormwater runoff contaminants, but lacked data on the system’s effectiveness. After gathering data, Cudahy found that the system was reducing stormwater contaminants up to 6x more than was expected under the city’s stormwater models.

Topics Covered

Stormwater Management
Ecosystem Degradation
Water Quality

Cost

Not available

Funding

General Fund/Existing Public Funds

Project Status

Operational since 2019

Problem Addressed

Cudahy, WI was successfully infiltrating stormwater runoff but wanted to learn whether the infiltration was removing contaminants as intended.

A portion of the City of Cudahy is in a Municipal Separate Storm Sewer System (MS4) which means that stormwater runoff discharge is regulated under The Clean Water Act. As part of that regulation, Cudahy was ordered to reduce Total Suspended Solids (TSS) and Total Phosphorus (TP) in its stormwater runoff by 80% and 88%, respectively.

The city had installed a green infrastructure permeable articulating concrete block (P-ACB) system to capture stormwater during rain events and filter out contaminants through the sub-surface level open aggregate layer. The P-ACB system was designed and installed according to the city’s stormwater models. These models were also used to predict the infiltration of stormwater and therefore the level at which contaminants were removed for compliance purposes. The use of models limited the city’s visibility of the system’s actual performance.

Unable to validate whether the P-ACB system was performing at its designed limitations, Cudahy sought to document the volume of stormwater being captured so the amount of Total Suspended Solids (TSS) and Total Phosphorus (TP) being removed could be confirmed.

Solutions Used

Data gathered on the city's permeable articulating block system found that the reduction of contaminants in stormwater runoff exceeded model predictions.

The city needed to document the volume of stormwater its PaveDrain permeable articulating block (P-ACB) system was infiltrating to confirm the reduction of contaminants in the runoff. Cudahy installed an electronic sensor to document and measure stormwater infiltration at the system. The sensor from P4 Infrastructure was installed in the open-graded aggregate below the surface of the pavement and functioned by sending signals from a stainless-steel float valve in the open aggregate to a nearby receiver that then conveyed the data to a computer for analysis.

It became clear after analyzing the sensor data that the system was infiltrating significantly more stormwater than was expected under the city’s model. One of the measures of stormwater infiltration is a run-ratio, the ratio of the run-on source area and flow path to permeable pavement surface area and flow path. Although the State of Wisconsin recommends a run-on ratio not to exceed 5:1, data from the PaveDrain system showed that it had successfully handled up to 30:1 run-on ratios even in the less than ideal clay soil. This also meant that the system was more effective at reducing contaminants in the stormwater than had been predicted. With actual data in hand showing the system’s effectiveness, Cudahy can more consistently monitor Clean Water Act compliance.

As a result of the work done to help the city better manage and oversee its stormwater compliance, Cudahy made PaveDrain and the INFIL-Tracker the primary partners for future green infrastructure projects as part of its new Green Alley program.

Outcomes

1

Cudahy found that its green infrastructure P-ACB system was infiltrating 6x more stormwater than models had predicted

2

The city now has a reliable way to gather data on the actual performance of its stormwater infiltration projects

3

Cudahy electronically documented state compliance by implementing permeable concrete block systems in both the MS4 and non-MS4 areas

4

The health of the city’s waterways benefitted from greater removal of harmful contaminants from stormwater runoff

5

City officials have reliable documentation for future projects knowing that they are not under-building or overbuilding stormwater management systems

Lessons Learned

1

Stormwater models can underestimate the actual performance capabilities of a green infrastructure infiltration system and can result in costly overbuilding

Something Unique

Following the implementation of the project, The City of Cudahy modified its methods for documenting the volume of stormwater capture and now records that data electronically.

Who Should Consider

Cities wanting to measure the performance of their green infrastructure projects, so they can successfully achieve MS4 compliance.

Last Updated

Jul 27th, 2022
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