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A comparison between OSD Simplified Method and OSD Rational Method

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In MSMA 2, Simplified Method is recommended for site which area is less than 5 hectare. Simplified method, according to MSMA 2 chapter 5.8 , is a simplified procedure involving only the use of tables, figure and simple computations.

Compared to MSMA 1, where Rational Method is suggested for OSD ( and also Pond), Simplified Method is a lot more easier to do compared to Rational Method, because:

  1. Everything is now codified in terms of charts, figures and tables, no difficult or tedious calculation is necessary
  2. No more subjective interpretation of terrain conditions– the engineers now no longer have to judge what is the site condition such as LS, manning coefficient \eta and so on. This would make result comparison and checking easier

Now, what about the result? Which one is more economical, in terms of Site Storage Requirement, SSR ( the required pond storage)?

We found that, regardless of small area or big area, Simplified Method always gives larger Site Storage Requirement. In this project file, for a development area of 3 hectare,  Simplified Method gives 1621.20m^3, while Rational Method gives 1159.730m^3 . As shown below:

OSD Simplified, SSR=1621.20m^3

OSD Rational, SSR=1159.730m^3


Bear in mind that we are already using MSMA 2 IDF coefficient for Rational Method. MSMA 2 IDF curve is already higher ( ie: predicted bigger rainfall intensity) than MSMA 1. So it is not hard to imagine that compared to actual MSMA 1 OSD design, MSMA 2 Simplified Method is even more conservative, ie: gives even bigger site storage requirement!


It is undeniable that MSMA 2 generally gives more conservative estimates, but in this case, why Simplified Method provides a lot more Site Storage Requirement than older version of MSMA 1?

We can make educated guess to the reasons:

  1. Simplified Method is a simplification over the previous procedures, so it can’t definitely reproduce the Rational Method result. Do you want to err on the side of providing more storage or less? I think all would prefer the former.
  2. There is a gap of 11 years between MSMA 1 and 2. So during these periods, more intensive rainfall must have had happened. So the rainfall data considered in MSMA 2 Simplified Method must reflect this, leading to a bigger site storage required.


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