# Post Development Runoff Rates and Volumes

ReFH2 provides a simple and quick procedure for estimating post development runoff rates and volumes. These procedures are appropriate at the outline planning stage for a wide range of development types and the same approach can be used for detailed design for simple developments.

The post development runoff case is modelled using the urbanised results which considers the development site to consist of pervious and impervious surfaces in line with SuDS guidance.

Firstly you need to set up the ReFH2 model to simulate the greenfield runoff rates and volumes, as described in the previous section.

You then need to set the urban area to be equal to the total area of impervious surface planned for the post development case. You will then need to set the Imperviousness Factor to 1.0 and Impervious Runoff Factor ($IRF$) to 1.0. In this way, you will simulate the entire urban area as an impervious surface and simulate all of the rainfall over the urban area being converted to runoff.

With the $IRF$ set to 1 then 100% of the rainfall incident upon the impervious surface will form runoff and if it is set to 0.8 then 80% of the rainfall will form runoff.

The post development peak flow can then be read from the ‘urbanised’ results section and converted to runoff rates(l/s/ha) as previously described. Alternatively, the post development peak flows and volumes for all return periods can be exported using the Export Button.

The 1:100 year 6 hour runoff volume can also be read from the urbanised results once the duration has been specified as 6 hours, as for the greenfield case.