Estimation of solids loadings to rainfall-runoff unit operations using a unit pollutograph concept for source area watersheds

Source area watershed in the built environment deliver significant loads of solids and constituents such as metals, organics and phosphorus associated with solids. The ability to model the mass delivery process of solids in small urban watersheds is essential for advancement of rainfall-runoff quantity and quality control design in the built environment. In this study the hydrologic concepts of the unit hydrograph (UH) and instantaneous unit hydrograph (IUH) were used to support the concepts of a unit pollutograph (UP) and an instantaneous unit pollutograph (IUP). These latter two concepts were developed as analogous concepts for estimation of solids mass transport loading as a pollutograph from source area watersheds. Relationships between solids mass and hydrologic volume were based on relationships that expressed either a first-order or zero-order relationship between solids mass and volumetric transport. The resulting concepts were applied to historical rainfall-runoff events where the hydrologic and water quality measurements had been conducted. From this development a site mean IUH and IUP determined from the rainfall-runoff record and water quality data, the corresponding site mean UH and UP were developed for solids. The hydrographs and solid pollutographs generated by convolution of the UH and UP compared well with measurements, indicating the feasibility of the concept.