# Approach

Time-area methods utilise a convolution of the rainfall excess hyetograph with a time-area diagram representing the progressive area contributions within a catchment in set time increments.  Separate hydrographs are generated for the impervious and pervious surfaces within the catchment.  These are combined to estimate the total flow inputs to individual sub-catchment entries to the whole urban drain network.

This method assumes that the outflow hydrograph for any storm is characterised by separable subcatchment translation and storage effects. Pure translation of the direct runoff to the outlet via the drainage network is described using the channel travel time, resulting in an outflow hydrograph that ignores catchment storage effects.

The peak discharge, Qp , is the sum of flow contributions from subdivisions of the catchment defined by time contours (called isochrones), which are lines of equal flow time to the point where Qp is required.  The method is illustrated in Figure Time Area Method below.

Figure 4.1 : Figure 14.5 (MSMA, 2000) Time-Area Method

The flow from each contributing area bounded by two isochrones   $\left&space;(&space;T-\Delta&space;T,T&space;\right&space;)$  is obtained from the product of the mean intensity of effective rainfall (i) from time $T-\Delta&space;T$ to the area $\left&space;(&space;\Delta&space;A&space;\right&space;)$.

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