Monitoring Filtration Practices

A.J. Erickson, J.S. Gulliver, R.M. Hozalski, P.T. Weiss

Monitoring (level 4) is the most comprehensive method for assessing filtration practices. Monitoring can assess how well a filter reduces runoff peak flow, reduces runoff volume (by infiltration), and retains pollutants. A successful monitoring program, however, requires accurate and complete water and pollutant budgets. Surface runoff flowing directly into filtration practices that are not measured reduces the accuracy of the water budget and pollutant load budget. The quantity of surface runoff can be estimated with simple runoff models but can become complicated and less accurate for large, complex sites. The pollutant load, however, of unmeasured surface runoff is difficult to model accurately. Surface runoff should be routed (by swale or other means) around the stormwater filter and through a centralized inflow for flow measurement and sampling.

The perforated pipe collection systems that collect stormwater after it passes through a filtration practice are often small (usually 4–8 inch diameter, 10–20 cm). Therefore, measuring and sampling the outflow can be challenging. If using a weir to measure flow from a perforated pipe, it is important to design the weir invert elevations such that the water level in the perforated pipe is below the level of the perforations in the pipe. This is because back pressure in the perforated pipe can prevent filtered water from entering the pipe. Sometimes, the perforated pipe collection system is connected to a catch basin that has other inflows; therefore, it is difficult to separate the outflow from the filtration practice from the other flows in the catch basin. Thus, it is important to sample and measure flow from the perforated pipe system before it combines with any other surface runoff or conduit flow to ensure an accurate comparison between outflow and inflow for the filtration practice.

Infiltration into the native soil will occur in filtration practices that do not have impermeable liners. Infiltration rates should be measured or estimated to complete the water budget. The amount and rate of infiltration will depend on the stormwater filter design and hydraulic conductivity of the underlying soils. Discussion and recommendations for estimating infiltration can be found in Water Budget Measurement.

Evaporation and transpiration (also discussed in Water Budget Measurement) will likely account for an insignificant (<5%) portion of the water budget because they are slow processes and water does not remain in properly functioning filtration practices for more than 48 hours. Additionally, vegetation is often limited in filtration practices to ensure adequate filtration by the filter media and to facilitate maintenance of the filter surface. A case study (Case Study #1) of monitoring a hybrid filtration practice that includes soil infiltration and sand filtration is included in Case Studies.

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