Abstract. In urban areas, a prevalence of combined sewer systems (CSS) exist that carry both storm water runoff and sanitary sewer flows in a single pipe, these system are considered combined sewers. In the absence of rainfall-runoff most of these systems function adequately, however CSS capacity is typically inadequate to carry peak stormwater runoff volume. In order to minimize sewage flooding into streets and backups into homes and businesses, most CSS (as well as separate sanitary sewer system) are designed to overflow into surface waters such as streams and rivers, lakes and seas.

The primary factor causing overflows to occur is excessive precipitation with rainfall at high intensity or high volume. A framework for the application of radar-rainfall data to identify rainfall characteristics (spatial and temporal) associated with CSO events is presented in this work. An innovative component of the work is the identification of a relationship between weather radar reflectivity and ground-level rainfall. Additionally, the sewershed specific radar-rainfall region is extracted for use in defining CSO triggering rain event characteristics at sewershed spatial scale. Results show underestimation of rainfall is more problematic than overestimation. An optimal radar-rainfall relationship is developed to address reflectivity (Z) to rainfall (R) transformation and improves rainfall estimates in the higher reflectivity range (greater or equal to 46 dBZ). A large portion of quarter-hourly rainfall accumulations occurring at lower radar reflectivity, less than 46 dBZ, indicate optimal reflectivity-rainfall (Z-R) relation parameters range from [300, 1.4] to [250, 1.4] for convective storm types, and [250, 1.2] to {[80, 1.4], [120, 1.4]} for tropical rainfall types. Both rainfall and overflow events are identified using criteria proposed by United States Environmental Protection Agency (USEPA) to define the physical continuity of natural rainfall processes and the corresponding hydrologic response. The methodology framework is illustrated using an urban sewershed, denoted as CSO 130, located in Louisville, Kentucky (USA). The role of specific rainfall event characteristics: total volume, intensity, duration, continuity, and storm types are shown to govern the overflow in the approximately 13-ha (30-ac) sewershed. Through discriminant analysis, the coupled rainfall and overflow events are categorized by overflow severity. Results indicate that use of fine-resolution radar-rainfall in this urban hydrologic system can reveal insights for planning CSO control and prevention measures for specific rainfall event regimes.