Based on Interim Code of Practice for Sustainable Drainage Systems, Chapter 6

Sites >200ha FEH statistical and unit hydrograph methods should be applied where possible.  If the FEH method is not considered applicable then the IH 124 Method should be used.

50 – 200ha IH124 method applied directly as described in Method 1.

0 – 50ha IH124 method applied with 50ha in the formula.  Results are linearly interpolated using the ratio of the development size to 50ha.


Input Variables

The input variables are the same as for the IH 124 Method.


Output Variables

Peak flood flows for the 1, 30 and 100 year return periods are displayed along with QBAR.

For sites of 50ha or more the outputs are calculated using the IH124 method directly.

For sites smaller than 50ha the IH124 method is applied to a 50ha site and the results multiplied by a scale factor equal to the site area (in hectares) divided by 50.