1 Paper N0: II.08 Highway Drainage System Efficiency

Abstract: Starting from 1980, all highways in ecologically sensitive areas in Croatia, have specially designed drainage systems. That includes system of closed or open conduits for runoff collection with oil-grit separators before the final disposal. In more sensitive areas, different BMPs are added after oil-grit separators, to improve overall treatment efficiency. Overflow structures are allowed on big watersheds to avoid large pipe diameters. While overflow structures in urban sewage systems are designed upon German ATV – Rules and Standards, water authorities in Croatia, for the same type of structure proscribe more stringent criteria, namely critical rain intensity from 15-25 l/s/ha depending on ecological sensitivity of the recipient. At the same time, the mentioned criteria are not correlated to local climatic conditions i.e. duration of dray and wet weather, and mean annual rainfall. The question is how much that stricter criteria, which have impact on final cost of highway drainage, contribute to overall drainage efficiency. On the example of two Croatian cities with considerably different climatic characteristics, the runoff volumes and masses of pollution for different overflow criteria are modeled on the same hypothetic section of highway. Using input data from the case studies on pollution buildup and washoff undertaken in USA, results of simulation with USEPA SWMM-5, are analyzed and compared with overall highway drainage efficiency. As a final conclusion some recommen-dations regarding design criteria are given