# OSD Manual Calculation, MSMA 2011

This column, explain details on steps by steps procedure for designing storage system of OSD manual calculation.

Step 1:- Determine the storage type to be used within the site.

Step 2:- Identify the region of the detention site.

[Refer figure 5.A1 in MSMA (2011) which divides peninsula into 5 design region.]

Figure 5.A1 : Five (5) Design Region

Step 3:- Determine the catchment characteristics such as terrain type and percentage of impervious area.

Step 4:- Determine permissible site discharge (PSD) per hectares (PSD/ha) from table 5.A1. Then multiply with project area to determine PSD.

[Refer to table 5.A1 in MSMA (2011) which gives the maximum PSD and minimum SSR values in accordance with five regions in Peninsula Malaysia.]

Step 5:- Determine site requirement (SSR) per hectares (SSR/ha) from table 5.A1. Then multiply with project area to determine SSR.

[Refer to table 5.A1 in MSMA (2011)]

Step 6:- Identify the major town of the detention site in Table 5.A2 and determine inlet flow per hectares from Table 5.A2. Then multiply with project area to determine inlet flow.

[Refer to table 5.A2 in MSMA (2011) which gives maximum PSD, minimum SSR and inlet values in accordance with the major towns in Peninsula Malaysia.]

Step 7:- Determine PSD per hectares (PSD/ha) from Table 5.A2. Then multiply with project area to determine PSD.

[Refer to table 5.A2 in MSMA (2011).]

Step 8:- Determine SSR per hectares (SSR/ha) from Table 5.A2. Then multiply with project area to determine SSR.

[Refer to table 5.A2 in MSMA (2011).]

Step 9:- Compare the value of PSD from step 4 and step 7. The smaller PSD value is adopted for subsequent sizing of outlet pipe.

Step 10:- Compare the value of SSR from step 5 and step 8. The larger SSR value is adopted for Selected Design Value.

Step 11:- Determine the inlet pipe diameter from table 5.A3.

[Refer to table 5.A3 in MSMA (2011) which gives the OSD volume, inlet size and outlet size for 5 different regions in Peninsula Malaysia.]

Step 12:- Determine the outlet pipe diameter from table 5.A3

[Refer to table 5.A3 in MSMA (2011).]

Step 13:- Determine the inlet pipe diameter from table 5.A4 by using the inlet flow value from step 6 as discharge.

[Refer to table 5.A4 in MSMA (2011) which gives the discharge and pipe diameter relationship for low lying, mild and steep slope.]

Step 14:- Determine the outlet pipe diameter from table 5.A4 by using the inlet flow value from step 9 as discharge.

[Refer to table 5.A4 in MSMA (2011).]

Step 15:- Compare the value of inlet pipe diameter and from step 11 and step 13. The smaller inlet pipe diameter is adopted for Selected Design Value.

Step 16:- Compare the value of outlet pipe diameter and from step 12 and step 14. The smaller outlet pipe diameter is adopted for Selected Design Value.

Editor:- Dilah – 10 March 14

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