The design of On-Site Detention (OSD) using the first and second editions of MSMA

This paper investigates the design of On-Site Detention (OSD) using the first and second editions of MSMA or the Manual Saliran Mesra Alam (the Urban Stormwater Management Manual for Malaysia) published by the Department of Irrigation and Drainage Malaysia (D.I.D). Specifically, it compares the magnitudes of the Permissible Site Discharge (PSD) and Site Storage Requirement (SSR) using (i) the Swinburne’s Method in the first edition of MSMA (MSMA1), (ii) the Approximate Swinburne’s Method in the second edition of MSMA (MSMA2) and (iii) the Exact Swinburne’s Method (ESM). The computation is done using the MSMAware which is a drainage design software for MSMA. The software computes: design storm, temporal pattern, Rational Method, Time-Area Method, On-Site Detention (OSD), detention basin and culvert design. Software is developed by Ir. Dr. Quek Keng Hong. The PSD and SSR are computed for an industrial and a low density residential development in Kuala Lumpur as follows: (1) Using the Swinburne’s Method in MSMA1 where a set of formulas for PSD and SSR are used to compute their values. (2) Using the Approximate Swinburne’s Method in MSMA2 where a set of approximation tables and figures are used to determine the PSD and SSR values. (3) Using the rainfall and discharge data from MSMA2, but instead of computing the PSD and SSR using the Approximate Swinburne’s Method in MSMA2, the formulas from the Swinburne’s Method in MSMA1 are used ie, the Exact Swinburne’s Method (ESM). In the first edition of MSMA, the method of estimating Permissible Site Discharge (PSD) and Site Storage Requirement (SSR) is based on the Swinburne Method developed at the Swinburne University of Technology in Melbourne, Australia. The method is basically site-based, but considers the position of a site within the catchment. The peak flow time of concentration from the top of the catchment to the development site, tcs, is compared to the total time of concentration for the catchment, tc. The PSD varies with this ratio and may be less than or greater than the peak pre-development site discharge depending on the position of the site within the catchment. The result shows that for industrial land-use in Kuala Lumpur, the Site Storage Requirement (SSR) using MSMA2 is 2.35 times the MSMA1 estimate. For low density residential land-use in Kuala Lumpur, the SSR using MSMA (2011) is 2.17 times the MSMA (2000) estimate. The difference is due to the use of approximation tables and figures in the Approximate Swinburne’s Method in MSMA (2011), which gives more conservative estimates of the SSR. In conclusion, the ESM Method applies the storm and discharge data in MSMA (2011), but gives lower estimate of SSR than the Approximate Swinburne’s Method in MSMA (2011) because it uses formulas to calculate the SSR values instead of approximation tables. It also takes into account the decrease in surface runoff for lands with higher infiltration rate, through the coefficient of runoff in the Rational Formula.