Flow duration curves for the evaluation of LID effectiveness

The Stormwater By-Laws published by the City of Johannesburg, South Africa, in 2010 require that the “latest and best technology” be applied to the management of stormwater in the metropolitan area, and require the publication of a Manual to guide the application of this technology. Event based hydrology using design storms can be used effectively for the design of stormwater drainage systems, and for certain elements of stormwater management systems. But the effectiveness of interventions that rely on long duration processes, for example infiltration or evapotranspiration, cannot easily be evaluated using event based modelling. The South African Weather Service has collected 5 minute interval rainfall data from 1994 to the present at two stations that can be used for modelling of runoff in Johannesburg. Data from the Johannesburg Botanical Gardens for the period 19/10/1994 to 31/12/2011 was used to generate flow duration curves for two different sites and these curves used to evaluate the effectiveness of the proposed LID interventions by comparing the post development with the predevelopment case. The results of these analyses will be presented and delegates will be asked to comment on ways to objectively evaluate an acceptable difference between predevelopment and post development flow duration curves.