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

If capacity testing (level 2) and synthetic runoff testing (level 3) are not feasible assessment approaches for a specific location, or do not achieve the goals of the assessment program, monitoring should be considered. Monitoring is the most comprehensive assessment technique and can be used to assess water volume reduction, peak flow reduction, and pollutant removal efficiency for most stormwater treatment practices by measuring discharge and pollutant concentration during natural runoff events. To assess runoff volume reduction, peak flow reduction, or both by monitoring a stormwater treatment practice, the inflow(s) and outflow(s) must be measured or estimated according to the techniques described in Water Budget Measurement. The summation of the inflows can then be compared to the summation of the outflows to determine the runoff volume reduction, peak flow reduction, or both. Additional information about monitoring stormwater treatment practices is available in the report “Urban Stormwater BMP Performance Monitoring” (U.S. EPA 2002). 

Pollutant removal efficiency can also be determined by monitoring. In addition to measuring or estimating the inflow and outflow discharges, the inflow and outflow must also be sampled according to the techniques described in Sampling Methods. Pollutant removal efficiency can then be calculated as the difference between the influent and effluent pollutant loads or concentrations, as described in Data Analysis.

Natural runoff events have variable discharge and duration that require continuous flow measurement (or estimation). Pollutant removal assessment also requires sampling of all flows entering and exiting a stormwater treatment practice. For accurate estimates of performance, monitoring takes more time to complete (typically 14 or more continuous months), more equipment, more labor, and therefore larger expenditures than the first three levels of assessment. Monitoring, however, is the only method that accurately measures the quantity and quality of runoff from a specific watershed, and the response of a stormwater treatment practice to that runoff. Capacity testing (level 2) and synthetic runoff testing (level 3) measure the ability of a stormwater treatment practice to perform specific processes (e.g., infiltration, sediment retention). These data can be used in models to estimate how a stormwater treatment practice would perform in a given watershed during natural runoff events.

Monitoring has more potential for uncollected or erroneous data as compared to synthetic runoff tests for the following reasons:

  1. Weather is unpredictable and can produce various runoff volumes of various durations with varying pollutant concentrations at various times. In order for a storm event to be monitored correctly and accurately, all the monitoring equipment must be operating correctly and the parameters (water depth, etc.) must be within the limit ranges of the equipment.
  2. Equipment malfunction due to routine wear or vandalism is more likely. Without consistent inspection and maintenance, storm events can be measured or sampled incorrectly or not at all.

The procedure for monitoring (level 4) varies for each stormwater treatment practice and assessment goal. Therefore, the reader should consider the recommendations located in each of the Filtration, Infiltration, Sedimentation, and Biologically Enhanced Practices sections of this manual. As with any field work, safety is an important concern and should be addressed when conducting monitoring.

Continue to Recommendations for Assessment.