The estimation of Permissible Site Discharge (PSD) and Site Storage Requirement (SSR) for On-site detention (OSD) is based on the Swinburne Method developed at the Swinburne University of Technology in Melbourne, Australia.
The method evolves between the MSMA 1 and MSMA2. In MSMA1, the Swinburne method considers the position of the site within the catchment, which is to calculate the flow time of concentration from the top of the catchment to the development site, tcs and is compared to the total time of the concentration for the catchment, tc.
Figure below shows the illustration of the relationship between the tcs and tc.
We estimate the Q by using the Rational Method (Q=CIA) to calculate site flows for both pre-development and post-development. The site discharges are calculated based on the time concentration of the catchment for the storm ARI design (does not consider the time of concentration from the upstream to the development site).
Bases on Swinburne Method in MSMA1, a set of formulas of PSD and SSR are given, while a set of approximation tables are given for the PSD and SSR in MSMA2, which is only applicable for Peninsular Malaysia (refer to MSMA 2nd Edition Chapter 5 On-site Detention).
Formulas for PSD and SSR according to MSMA1:
tc = peak flow time of concentration from the top of the catchment to a designated outlet or point of consent (minutes
tcs = peak flow time of concentration from the top of the catchment to the development site (minutes)
Qa = the peak post-development flow from the site for the discharge design storm with a duration equal to tc (l/s)
Qp = the peak pre-development flow from the site for the discharge design storm with a duration equal to tc (l/s)
td = selected storm duration (minutes)
Qd = the peak post-development flow from the site for a storm duration equal to td (l/s)
MSMA1 method can be used to come out with an inflow and outflow hydrograph by using the Swinburne Method as shown in MSMA1 Figure 19.3. It is also not only limited to West Malaysia (as tables given in MSMA2 are only applicable for West Malaysia).
Qa = Post development discharge by using the rational method, CIA
(Qa is interpolated at ⅓ of tcs based on this research paper)
Qp = Pre development discharge by using the rational method, CIA
SSR = Site storage requirement
PSD = Permissible site discharge
tcs = Time of concentration of the catchment to reach the site
tc = Time of concentration of the catchment
tf = Time taken to fill the storage tank
It should be noted that changing tcs , Time of concentration of the catchment to reach the site, will change tf , the time taken to fill the storage tank ( ie: the time to reach PSD). To see this, note that changing tcs will change PSD ( see the PSD equation), and tf of course will be changed, when PSD is changed.
The problem with Swinburne Method
Million dollar question: how do you compute tc and tcs?
According to Figure 19.2 above, it’s pretty clear that it’s not sufficient to just know about your own “Development” site ( which can be a condo development, eg: just 1 hectare), you have to know the whole “catchment” on where it is located, in order to compute the tcs ( which is the time of concentration from catchment top to your development site, eg: your 1 hectare condo development) and tc ( which is the time of concentration from top catchment to the end of it).
We can immediately see a few problems to this method:
- This “catchment” is so very vaguely defined: what is a catchment, exactly? The whole district? The whole township? Part of it ( whatever “it” is)? Without knowing what it is, it’s quite impossible to properly compute tcs and tc.
- Even if it is properly defined, then the consultants are faced with a seemingly impossible task: he is required to know the condition of the site outside of his jurisdiction. This information is not usually available.
Before there was MSMA 2, we have seen consultants just assume a minimum value for tc and tcs when carrying out the Swinburne method procedure. They are operating on the assumption that tc and tcs are not too critical as far as SSR and PSD estimation is concerned. Whether this assumption is reasonable is open for discussion.
In defense of those consultants, it should be noted that a lot of things in hydrology are simply assumed, since hydrology requires too much ( unavailable) information and computing power in getting things exactly right. As far as design engineers are concerned, it’s much more practical to just obtain a ballpark figure and err on the side of conservativeness. OSD is usually reserved for small site development, and there are always other failsafe mechanisms in place if an unwanted flood occurs.
What about MSMA 2?
MSMA 2, by contrast, is a lot simpler because it provides a set of tables for PSD and SSR, so that the consultants no longer need the tcs and tc to estimate these two quantities.
Set of tables for PSD and SSR computation based on MSMA2:
Pros and Cons of MSMA 2:
- Pros: no ambiguity in estimating tcs and tc in MSMA 2, thus it’s less dispute among different consultants.
- Cons: “not scalable”, in the sense that you can’t just adopt MSMA 2 standards to a new location with only changes in localized coefficients.
- Cons: No longer possible to compute the pre and post development hydrograph, with or without control. Thus it may not fulfill some authority requirements that require the submission of different hydrographs for comparison.
But we may be overstating the benefits of MSMA 1 with regards to the hydrographs. Bearing in mind that tc and tcs might be hard to estimate ( because they are depending on a vague global “catchment”), the hydrographs generated can easily be subject to disputation. Furthermore, the authority might also want the maximum water level, which would render the Swinburne method ( and MSMA 2 table method) completely unusable.
Use Swinburne Method MSMA 1 if:
- To design OSD for East Malaysia region, as the tables provided in MSMA2 Simplified Method are only applicable for West Malaysia
- To generate inflow and outflow hydrograph, under appropriate assumptions as outlined above.
For West Malaysia, use MSMA 2 table method.
If hydrographs and the maximum water level is needed, it would be better off to just use a proper pool routing method, like the Time Area/ Rational Pool routing under the Detention Pond module in MiTS software.