Fig. 4.5 illustrates the group of areas from where the storm water flows towards the inlets I1, and I2, Let A, B, C, D and E be the areas, whose storm water enters the inlet I1, and area F on both the sides of the sewer whose storm water enters the inlet I2.
Let T1 be the time required for the sewage to flow from inlet I1, to I2, With reference to Fig. 4.6, if the maximum flow of storm sewage developed at I1, is to be developed, the first step is to plot the time area diagram for the total area A, B, C, D and E as shown in Fig. 4.6. The time area diagram of the individual localities are drawn from the same origin for inlet I2, and I2.
Now the cumulative group is plotted by adding the ordinates of the different curves. This cumulative diagram shows how the total area (A, B, C, D and E) will be falling in at point X. From the close inspection of this diagram a certain period can be found out, in which a proportionately large area ‘falls in’ within a comparatively short period. The time of concentration for the group of localities (A, B, C, D and E) is equal to this period.
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The rainstorm of duration equal to this time of concentration and a contributing area equal to that obtained in the previous inspection will give the maximum flow of storm sewage developed at point X of graph shown in Fig. 4.6.
Similarly the time area diagram or graph at point Y can be plotted for the group of area of A, B, C, D, E and F. In this case the graph ordinates of graph YF are added in curve 1 and the resultant or cumulative graph as shown in graph 2 is plotted. The maximum developed flow can be now determined from this graph.