Bob Hall - Back-calculation of Jacobs’ rainfall losses

By back-calculating Jacobs’ figures, RJ Hall and Associates Ltd find a CN (rainfall loss) value of 96 for the pre-development hydrology, which means the existing forested hills are treated by Jacobs as being more or less impermeable, and the runoff characteristics between pre- and post-development are almost indistinguishable.

The effect of this is that when these pre- and post-development hydrographs are applied to the hydraulic model it is to be expected this error will generate almost identical pre- and post-development flood extents. In essence this fundamental error by SKM 2010 persists also in Jacobs’ 2016 reworking.
R J Hall Summary (Jacobs’ CN values for Scenarios OS, DS1 and DS2 - sub-catchment B):

Scenario OS (pre-development, 74.4ha forest and bush) = CN 96.2

Scenario DS1 (SKM, 2010, post-development, 74.4ha with 52% impervious) = CN 97.5

Scenario DS2 (Jacobs, 2016, post-development, 23.7ha with 51% impervious) = CN 96.7