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A comparison between OSD Simplified Method and OSD Swinburne Method

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 Swinburne Method is suggested for OSD ( and also Pond), Simplified Method is a lot more easier to do compared to Swinburne 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

When to used Simplified or Swinburne Method #

In Simplified method calculation rainfall station coefficient is required. Hence, this approach is only available for Peninsular Malaysia only.

The Swinburne method can be used at anyplace because it is essentially a site-based method. Due to the limited number of rainfall stations in Sabah and Sarawak, the Swinburne method is suitable for those two states.

Calculation Comparison of Simplified and Swinburne Method #

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 Swinburne Method gives 1159.730m^3 . As shown below:

OSD Simplified, SSR=1621.20m^3
OSD Swinburne, 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!

Explanation of The Results’ Differences #

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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