Generalising Manning's N for High Performance Roadside Vegetated Swale Drains and Filter Strips

Road Controlling Authorities (RCA) are putting high focus on improving the sustainability of the roadside environment. There is a recognized need to ensure storm water quality as well as quantity objectives are met. RCA’s often have conflicting constraints and yet there is a need to achieve a balanced and robust design. Austroads and other international RCA have promulgated guides on acceptable swale and filter strip design practice with these commonly allowing greater than fully submerged vegetation flow depths for the quality storm event. In New Zealand there is a desire to raise the bar with treatment performance and require the quality storm event to be accommodated within the vegetation height or under ‘just submerged’ conditions. This paper reviews the recent hydraulic data collected for vegetated swales and using a case study of a typical swale drain calibrates the data against the SCS TP61 Manning ‘n’ relationship. Tests for method robustness are undertaken and reported. The paper concludes the method provides a robust and simple method for determining Manning ‘n’ for designing high performance roadside vegetated swale drains and filter strips and recommends more physical modeling be undertaken to extend the information available.