Open flow through structures used to detain stormwater volumes and settle particulate pollutants prior to outflow. Pollutant load reductions are realized by concentration reductions with no volume reduction via infiltration due to impervious or highly impermeable base. Size and application of settling basins can vary. Setting basins are common at the inlet of another structural BMPs to pre-treat inflowing stormwater prior to stormwater entering subsequent structural BMP. If inventory and assessment of the settling basin is desired independent from the next structural BMP in the treatment train, there are two options: Large scale Settling Basin draining a mixed land use area can be classified and assessed as a Treatment Vault. Smaller sized Settling Basins draining a single land use area can be classified and assessed as a Catch Basin.