In our project outside the US and in the rainy tropics our site pre-development is >50% impervious (only 10% site is natural green) therefore we need to go for 25% reduction in both volume and rate.
The new project design is built up 100% (no natural infiltration), but does have 30% of site area as an intensive 300mm deep green roof. Therefore we aim to manage the target water volume by reducing the runoff by the green roof, capturing the stormwater and reusing it on site for cooling towers, flushing and irrigation. Questions are:
1. Compliant rainfall data
The prefered 2 year design storm option (to achieve a 25% reduction from pre-development) we do not understand clearly how to extract from available 5 year local data as this is an international project. From the LEED2009 ACPs guide we understood that we are are allowed to manage the 100% volume of the percentile storm from 5 year data, or a 100% volume of a 5 and 10 year design storm ?
2. Back to back storm time to use the volume
If we are capturing & reusing the water volume on site, how much time we have available to reuse the volume it until the next storm hits and the tank needs to be empty? How can we calculate the back to back storm frequency of a local percentile storm, or an fair assumption needs to be made by observing the local data? For example, if its possible that a percentile storm occurs 2 days in a row, this means we need to utilise 100% of that storm volume in 1 day? This might be very difficult to achieve due to large rain volume in the tropics.
3. Green roof retention
The retention capabilities of the dry green roof are expressed by the run-off coefficient. If the 2 percentile storms fall within few days, the green roof might be quite saturated and the retention capability to capture new volume will be reduced?