Physical model testing of low flow bypass pits for water sensitive urban roads

Traditional engineering design of stormwater capture from urban roads involves an array of stormwater inlet pits, capturing large flowrates of poor quality and often polluted runoff. An alternative design is to implement low flow bypass pits that divert low flows to a streetscape water sensitive urban (WSUD) facility while still capturing high flows in severe rainfall events. Significant progress has been made over recent years to reduce the environmental impacts of stormwater runoff. To implement successful 'at source' stormwater management, strategies must be matched by improvements in road runoff treatment. Otherwise, stormwater managers remain obliged to implement 'end of pipe' treatment that inefficiently deals with a blend of 'clean' and 'dirty' runoff sources. Sydney Water have proposed an option to move stormwater inlet grates off the gutter invert for on grade road locations. This allows frequently occurring minor flows containing pollutants and debris, to remain in the gutter and flow on to a WSUD facility such as a streetscape raingarden. Larger flows during severe rainfall events are captured by the offset grates to maintain road functionality. The new inlets are not proposed for sag pit locations at road low points. This paper presents the results of physical laboratory testing undertaken at the UNSW Water Research Laboratory (WRL). Prototype scale (actual size) testing provided measurements of the proportion of flow continuing in the kerb and gutter compared with the flow captured by the grates, for a wide range of flowrates, grade and crossfall. These results have demonstrated the potential for such a WSUD strategy and highlighted the range of conditions where it may be applied. The results so far are encouraging and additional configurations continue to be tested at the WRL.