We are using the fixed runoff model to represent paved and roof surfaces defined in OS Mastermap, I would like to know the current best practice for using fixed runoff. The model will not be verified against flow survey and the 2D model will be used to determine flood risk to properties.

For impervious areas such as paved car parks, roads etc; is it better to make an assessment of depression storage for different types of paved area and adjust the Initial Loss Value appropriately and apply 100% runoff from that surface? This must be the case once all puddles and cracks etc have been filled?

Or should the default values be used for depression storage and the runoff coefficient adjusted between 70-90% as per the help guidance?

Following the rainfall events of last year, surfaces that were previously assumed to be permeable were observed to contribute 100% runoff. With the use of 2D models, we would like to see where runoff that is not collected by the sewer network ends up. Hence the concern about adjusting the percentage contribution to be less than 100%.

Is there any recommendation for runoff from ‘pervious areas’, which in many cases last year were observed to contribute large volumes of runoff? The help simply suggests a value between 0 and 10% of the total area. Presumably different uses of pervious area and soil type will have different saturation depths and therefore are to irregular to model with any degree of confidence and would have to be modelled on a case by case basis rather than by rule of thumb?

Thanks, your suggestions will be much appreciated.