Preliminary design of combined sewer overflows and stormwater tanks in southern Italy

Pollution induced by runoff on receivers is an environmental problem deserving of much attention, especially over the past few years. The structural facilities most commonly used for first flush pollution impact reduction include sewer overflows and stormwater tanks. Nevertheless, their efficiency is a current research topic and the design criteria are not completely consolidated. Based on extensive numerical simulations from 45 rain gauges located in Campania (southern Italy), two recent studies show that the performance of sewer overflows and offline stormwater tanks in reducing stormwater pollution can be reliably estimated without referring to rainfall data. Although rainfall dependency is generally negligible, it is more marked for some rainfall patterns. On the basis of that observation, this paper proposes a simplified approach to estimate pollution reduction when sewer overflows or stormwater tanks are used, correlating their efficiency to the average rainfall per wet day, which is usually easy to calculate. Analysis shows good agreement between values estimated according to the proposed approach and results provided by means of continuous simulations with a maximum deviation of about 0.05.