Manual Calculation


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


1. Determine design storm inflow for the catchment


The aim is to reduce the post-development peak flows for the minor and major system ARI to less than or equal to the pre-development peaks. The minor and major system design storms are 5 year and 50 years ARI, respectively in accordance with Table 4.1. The design storm for the secondary outlet spillway is 100 year ARI. Table 6.1 show Pre and Post Development Total Flow Hydrograph.


Table 6.1 Pre And Post Development Total Flow Hydrograph


2. Preliminary estimate of the required storage volume


A preliminary estimate of the required basin volume can be made using the following equation (Equation 20.13) for the major design storm ARI.


      Equation 20.13


Vs  = estimated storage volume (m3)

Vi  = inflow hydrograph runoff volume (m3)

Qi  = inflow hydrograph peak flow rate (m3/s)

Qo  = allowable peak outflow rate (m3/s)

ti  = time base of the inflow hydrograph (min)

tp  = time to peak of the inflow hydrograph (min)


The basin volume is estimated for each basin inflow hydrographs as shown in Table 6.2 and the largest value selected.


Table 6.2 Preliminary Estimation BasinVolume

Figure 6.1 Graph of BasinVolume Estimation versus Storm Duration

3. Determine Stage Storage Discharge Relationship


Table 6.3 Basin Stage Storage Data

Figure 6.2 Graph of Storage versus Stage

Table 6.4 Discharge versus Stage

Figure 6.3 Graph of Discharge versus Stage

4. Storage Routing


Table 6.5 Result Of Routing Through The Detention Basin (50 Year ARI, 10 Min)

Figure 6.4 Basin Inflow And Outflow Hydrographs For The Critical 5 Year ARI 10 Minute Storm


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