Manual Calculation


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


1. Determination 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 year ARI, respectively in accordance with Table 4.1. The design storm for the secondary outlet spillway is 100 year ARi. Table 8.1 show Pre and Post development of hydograph.

Table 8.1 Pre and Post Development Hydrograph

2. Preliminary estimation of the required storage volume 

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

           Equation 8.1


Vs : Estimated storage volume
Vi : Inflow hydrograph runoff volume (m3)
Qj : 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 hydrogaph as shown in Table 8.2 and maximum value selected.


Table 8.2 Preliminary Estimation Pond Volume

Figure 8.1 Graph of Pond Volume Estimation versus Storm Duration


3. Determine Stage Storage Discharge Relationship  


Table 8.3 Basin Stage Storage data

Figure 8.2 Graph of Storage versus Stage

Table 8.4 Basin Stage Discharge data

Figure 8.3 Graph of discharge versus stage

4. Storage Routing  

Table 8.5 Result of routing through the detention basin (50 year ARI, 15 min)

 Figure 8.4 Basin inflow and outflow hydrographs for the critical 50 year ARI 15 min storm

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