Manual Calculation

The result of computation using MSMAware was checked using spreadsheet in this section.


 Delineate Subcatchments Based On The Isochrones

The whole study area is divided into subcatchment areas by plotting 5 minutes isochrones which represent lines of equal flow time to the outlet as shown in Figure 4.3.


Figure 4.3:  Study Area with Subcatchment and Isochrones

The interval is determined using the Rational Method approach.

The subcatchment areas between each successive pair of isochrones are measured and recorded as shown in Column F of Table 4.2.


Table 4.2:  Manual Calculation of Time Area Method for Kuala Lumpur Base On (MSMA, 2000)

Calculate Design Rainfall I For The Whole Catchment

For Kuala Lumpur, the Design Rainfall Intensity for ARI of 100 year is 172.24 mm/hr.


Hence Total Rainfall Depth = 172.24*0.5 = 86.12 mm.


Column B summaries the temporal pattern ratios for this particular location.


The Rainfall Depth in Column C is calculated by multiplying Column B by Total Rainfall Depth.


Listed in Column D are the losses based on Table 4.1 (Table 14.4).


Column E shows the Effective Rainfall which is equal to Rainfall Depth (Column C) minus the Losses (Column D).



Calculate Discharge Hydrograph

The ordinate of the discharge hydrograph is calculated as follows:


First, multiply the subcatchment area (Column F) with the Effective Rainfall (Column E) based on Equation 4.1 as follows:

These are tabulated in Columns G to L.


Next, sum Columns G to L to get the total hydrograph ordinate in Column M.


The peak discharges are computed as shown in Table 4.2 and the hydrograph plotted in the Figure 4.4.

Figure 4.4:  Hydrograph for Kuala Lumpur using (MSMA, 2000)



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