The Surface water storage volume estimation tool uses a storage assessment method developed by HR Wallingford based on correlations between storage requirements and hydrological and hydraulic characteristics of sites. This methodology is based on the premise that the flow rate discharge constraints for storm water runoff from the site are defined by the greenfield runoff rates for the 1 year, 30 year and 100 year return periods. The tool is flexible in allowing the use of three different methods for estimating greenfield runoff rates along with various options for assessment of storage volumes using variations in the design criteria (e.g. minimum design flow rates from the site, requirements for control of runoff volumes for the 1:100yr, 6 hour event).
The drainage design criteria applied are in line with best practice in the SuDS Manual and the SuDS Standards in England, Wales and Northern Ireland.
The methodology takes into consideration the partial use of infiltration or rainwater harvesting along with whether or not permeable areas contribute runoff. It also makes allowances for different hydrological regions, climate change and other factors.