A Tale of Two Storms How Wetness Conditions Affects Capital Upgrades

This paper will depict “a tale of two storms” highlighting the challenges that antecedent moisture effects present in modeling and sizing of wet weather capital upgrades, and how modeling technology borrowed from the aerospace industry is able to solve it. The complex paths of inflow and infiltration (I/I) result in interaction with the subsurface soils, and groundwater table, as depicted in Figure 1, which cause sanitary collection system flows to be highly dependent on the wetness, or antecedent moisture, of the system. The continuously changing nature of this phenomenon will be illustrated, and a modeling approach will be presented to address this challenge. This modeling approach, called the Antecedent Moisture Model, proposes an advanced and accurate way to analyze system hydrology, while still relying on the SWMM engine for hydraulic pipe network computations. Design approaches based on design rain events will be compared and contrasted with a design frequency approach. Several case studies will be presented on the importance of understanding the wet weather flows in collection systems, because of the significant impact they have on wet weather capital upgrade costs.