In land drainage terms when speaking of volume available for a reduction in rainfall runoff this is a measure of volume available for filling from the rainfall event
PPS25 sections F3 & F10 both speak of not increasing the volume of water leaving a site so there is a requirement to calculate the volume of runoff that will be reduced.
This site reduction is valuable in flood risk terms because it lessens the coincidence of discharge from different sites contributing to flows in rivers and sewerage systems. In other words in going from a completely undeveloped catchment to a developed one, even where each site achieved significant reductions in peak runoff rates we could still get higher river levels and more flooding due to higher volumes of rainfall that run off developed areas. This because an attenuated hydrograph with the same volume has a much longer decay tail is leading to the possibility of this coinciding with peaks from other sites.
The response of a Green Roof system to rain storms will depend on many factors. The complexities and variables to consider include;
- antecedent conditions including frozen conditions
- substrate depth
- substrate type
- malfunctioning systems e.g. through siltation
- drain down times
- degree of vegetation.
Simplified approach to volume reduction
Research in the UK such as CIRIAC644 Building Greener has shown that there is proportion of rainfall that falls on a green roof that is permanently held within it, this is known as Interception or Depression storage. This storage can be taken as 5% of the overall depth of green roof above the waterproofing layer.
This interception storage can be used to reduce the site wide storage that would need to be found to accommodate the critical storm.
Green roof drainage modelling
Another approach is by a purer application of flow theory across and through modelling soils and conduits. These could for example employ time/area diagram methods similar to conventional drainage design. Such an approach should always be validated, if not developed, from a range of sample tests.
Such sophisticated modelling methods continue to be developed and are available as proprietary drainage software e.g. WinDes produced by Micro Drainage Ltd. Such packages consider in more detail the complexities of green roof drainage mentioned above.
Such packages are capable of assessing both the reduction in volume and rates of rainfall runoff.
Another means of establishing runoff characteristics would be to carry out field tests on the proposed green roof. Testing would ideally be for the exact green roof make up to be used (including factors such as pitch of roof). It would need to be tested to include the rainfall events given under ‘Standards for site wide surface water drainage’ above.
This approach has two main advantages over other approaches in that it more certainly represents the actual response of the system and the output hydrograph can be used directly to assess both drained volume, run-off flow rates and time of concentration.
The disadvantage is that it is not as flexible in being able to be applied to other roof combinations.
We particularly welcome this approach as it will advance the drainage industry’s understanding of the way in which green roofs work as a SUDS component and will provide certainty of performance.
Design by analogy
One way to assess green roof rainfall runoff is by analogy to existing land surfaces. We believe it unlikely that a green roof would replicate an existing landscape as the infiltration / surface run-off mechanism is likely to be very different and this approach should not be employed.