The Illudas Time-Area method is used for the estimation of runoff from a uniformly distributed design storm. The results correspond satisfactorily with observed runoff events for the method to be an accurate and useful design tool (Watson 1981).
The catchment area under consideration is divided into sub-catchment areas of a similar or homogenous nature, which drain into an inlet.
Illudas assumes that overland flow is the sole source of storm runoff. Losses due to infiltration and surface irregularities are subtracted from the rainfall to determine the excess precipitation.
The runoff hydrograph is calculated from the excess rainfall hyetograph and the time-area curve of the sub-catchment area.
The runoff is routed over the sub-catchment with no further losses and then combined with the runoff from other sub-catchments, as described in the section on routing.
The Illudas method distinguishes between paved (impervious), supplementary (impervious areas which drain onto pervious areas) and grassed (pervious) areas in each sub-catchment.
Hydrographs are calculated for the paved area and the grassed area. The total grassed area is increased by the supplementary area. The paved and grassed area hydrographs are added to form the sub-catchment hydrograph.
The computation of the sub-catchment hydrograph can be illustrated in the following series of figures:
