# Manual Calculation

This column, explain details on steps by steps of OSD manual calculation.

Step 1:- Determine the design storm ARIs for minor system and major system that can be used in OSD design by referring to Table 1.1.

Step 2:- Determine the impervious and pervious area.

Step 3:- Determine the time of concentration for $t_{c}$ and $t_{cs}$.

[For small catchments of up to o.4 hectare in area, it is acceptable to use the minimum times of concentration given in Table 14.3 of MSMA (2000) instead of performing detailed calculation. However, if the catchment area is larger than 0.4 hectare, $t_{c}$ and $t_{cs}$ should be calculated manually by using formula shown chapter 14.]

Step 4:- Calculate the rainfall intensity (I) for ‘Pre’ and ‘Post’ development.

[Use formula shown in chapter 2 of MSMA (2011) or you can open design storm computation in MSMA (2011) to learn about rainfall intensity calculation there.]

Step 5:- Determine the runoff coefficients (C) for ‘Pre’ and ‘Post’ development.

[Please refer to Table 2.1 of MSMA (2011) to get the runoff coefficient.]

Step 6:- Determine the peak discharge for ‘Pre’ and ‘Post’ development.

[Get $Q_{a}$ and $Q_{p}$ using rational method formula as shown in chapter 14 of MSMA (2000). For variation of sub-catchment condition, please insert $C_{avg}$ formula into rational formula to get the actual peak discharge for that site condition.]

Step 7:- Determine the permissible site discharge (PSD)

[Please refer to chapter 19 or open our theory column in design OSD computation for MSMA (2000). Make sure, you use the correct formula to design your PSD because there are two different formula for ‘a’ and ‘b’ factors for above-ground and below-ground storage.]

Step 8:- Determine the site storage requirement (SSR)

[Please refer to chapter 19 or open our theory column in design OSD computation for MSMA (2000). Make sure, you use the correct formula to design your SSR because there are two different formula for ‘c’ and ‘d’ factors  for above-ground and below-ground storage. Get the maximum size for your SSR.]

Editor:- Dilah – 12 March 14

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