Exploring Uncertainty in Uncalibrated Bioretention Models

Using SWMM 5.0.016, a case study of a street right-of-way bioretention system (ROWB) configured as a storage node is compared against SWMM's "LID Controls". Through a series of 1-yr continuous simulations, the uncalibrated models indicate that a storage node representation of a media-filled system substantially underestimates stormwater retention and detention compared to the LID Control. This is because the latter explicitly accounts for dynamic flow through porous media. Lined/underdrained ROWB may provide significant stormwater mitigation because the majority of storms are small, thus fully captured by the media without the need for exfiltration. As the storage node approach is typical of the current (USA) industry for green infrastructure or low impact development modeling, outcomes raise concern around over-design (and misspent resources). Sensitivity analysis of the LID Control parameterization indicates that the relative difference between the engineered media's porosity and field capacity have the most significant influence on performance. This may be an artefact of the calculation procedure rather than actual physical phenomenon. Future work should prioritize calibration with observed data sets from multiple sites (i.e. multiple media), and actual measurement of field capacity and porosity for multiple examples of bioretention.