Stormwater basins are used to control runoff, prevent flooding and limit pollutants from reaching rivers, lakes and other large bodies of water. MnDOT monitors and maintains more than 400 infiltration and filtration basins statewide, and that number continues to grow. Identifying indicators of needed maintenance will increase management efficiency and basin performance. Field research at 28 basins identified basin age and silt content as two important factors to consider when planning inspections and taking corrective actions.
Stormwater collects sediment and numerous pollutants that run off impervious surfaces such as roofs, roads and parking lots. To control flooding and protect infrastructure, stormwater basins are constructed to collect runoff and allow it to gradually infiltrate into the soil, which filters the sediment and pollutants and prevents them from entering water sources. However, sediment buildup in the soil can diminish the infiltration capabilities of basins over time, which then require some type of maintenance to operate effectively.
Monitoring and forecasting effective basin infiltration drawdown rates are necessary for maintenance scheduling and budgetary planning. Although basin inspections are resource-intensive, requiring two staff members approximately two to four hours for each basin, prioritizing inspections of basins is needed to ensure continuing performance. This project analyzed basin age, soil properties, general plant cover and infiltration rates to identify basin characteristics that might reliably predict maintenance timelines and operational controls.
What Did We Do?
First, a review of the literature examined how stormwater infiltration rates change as basins age.
Next, investigators evaluated soil properties and measured infiltration rates at 28 stormwater basins that ranged in age from two to 20 years. The infiltration and drainage areas of these basins varied in size, which led to the creation of a unique dataset. All basins were considered to be constructed with similar soil properties so that soil changes would reflect how basins age. Analyses of the collected data examined the relationships between age, soil quality and/or composition, and infiltration rates.
A visual inspection of each site identified indicators of basin effectiveness. Ponding, hydric soils, sediment accumulation and the presence of wetland vegetation indicated insufficient infiltration. Additionally, infiltration drawdown monitoring evaluated continuous water level sensors as a cost-effective basin infiltration measuring device. These sensors can collect substantially more data than current practices and eliminate uncertainties related to inadequate sampling.
What Was the Result?
The literature review indicated that infiltration rates measured in previous studies did not change directly with age or were inconclusive. Additionally, there was a lack of evidence correlating soil texture with infiltration rates and age.
“The findings and strategies from this study will help MnDOT and other agencies that manage stormwater basins identify and prioritize basins that are most likely in need of maintenance and corrective actions. These practices will improve overall performance and save money,” said Dwayne Stenlund, erosion control specialist, MnDOT Erosion Control and Construction Stormwater Management.
Soil analyses from the 28 basins identified a general decrease in infiltration rates as the basins aged, with a notable drop-off at approximately 12 years. However, while age was a strong predictor, it only explained 34% to 46% of the variability. Silt content in the soil was the best predictor of infiltration rates. The lowest infiltration rates occurred when silt content exceeded 15%, which occurred at 20 years for some basins, but at 12 to 14 years for others. Sediment accumulation at basins can vary based on differing drainage areas, land cover and construction activity. The silt detection test results, coupled with the aging results, indicate that older basin soil tends to clog with sediment, which reduces infiltration rates.
Visual inspections found basins less than 12 years were generally in good condition while basins older than 12 years displayed impaired performance. Findings from the infiltration drawdown monitoring confirmed the results from the 28 sites and indicated continuous water level sensors could be effectively used for monitoring.
Results also indicated that controlling sediment by pretreatment capture systems and slowing its movement into stormwater basins could enhance infiltration for longer durations. For example, diverting stormwater from a basin during its construction and periodic scraping of basin surface soils decrease sediment buildup. Additional factors that retain infiltration performance include pretreatment activities such as vegetated filter strips, forebays, street/parking lot sweeping, proprietary settling/swirl chambers, oil/water separators and drain inlet inserts.
What’s Next?
Study results offer valuable insight into stormwater strategies for MnDOT and other agencies that manage stormwater basins. One strategy to improve stormwater management is to consider age with follow-up visual inspections and potential silt testing of basins that are 10 to 12 years old. Additionally, the use of continuous water level sensors could provide expanded infiltration rate data that enhances inspection activities and improves basin maintenance prioritization.
Crossroads 