The design of detention basin using the first and second editions of MSMA

This paper investigates the design of detention basin 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). The computation is done using the MSMAware which is a drainage design software for MSMA developed by Ir. Dr. Quek Keng Hong. The software computes: design storm, temporal pattern, Rational Method, Time-Area Method, On-Site Detention (OSD), detention basin and culvert design. The design of detention basin involves the following steps: (1) Computation of design storm of various ARI and durations. (2) Computation of temporal pattern for the design storms. (3) Determination of a loss model. (4) Estimation of inflow hydrographs to the basin using a hydrograph method. (5) Routing of the hydrographs through detention storage. (6) Check that the maximum basin outflow is less than or equal to the permissible major/minor flow. This section covers the major differences in the approach for the above in the first and second editions of MSMA. One of the major differences lies in the formula used to derive the design storm. Overall, the quality of the storm data in the second editon (MSMA2) is better as the new data is more up-to-date. There are now 135 stations in MSMA2 compared to only 35 previously. This case study design a dry detention basin for a catchment located in Air Terjun, Sg Batu, Kuala Lumpur using MSMA1 and MSMA2 as follows: The losses are based on Table 14.4 of MSMA1 and Table 2.6 of MSMA2. For pre-development scenario, a pervious area of 10 mm initial losses and 25 mm/hr continuous losses is assumed. For post-development scenario, an impervious area with 1.5 mm initial losses and continuous loss of 0 mm/hr is assumed. Outlet use box culvert with 90 degree headwall. A low flow pipe system with a capacity of 2 m3/s will bypass the basin and combine with the basin outflow in the downstream floodway. Determine the percentage increase in the peak discharge using MSMA1 and MSMA2. Primary Major Outlet (1) The stage-storage-discharge relationship. (2) After trial and error, 2 box culverts of 1.2 m by 2.1 m with invert at the critical 5 year water levels was adopted. (3) Maximum discharges, water levels and the permissible discharges. (4) The basin inflow and outflow hydrographs for 50 year storm of duration 30 minutes. Secondary Spillway- The embankment level is fixed at the highest water level of the secondary spillway plus a freeboard of 0.3 m. The storm intensity using MSMA2 is up to 1.26 times higher than that using MSMA1. The peak discharges computed using the Time-Area Method in MSMA2 is up to 1.27 times higher than that using MSMA1. The computed basin storage volumes using MSMA1 and MSMA2 are compared. It can be seen that the storage volume using MSMA2 is about 1.3 times higher than that using MSMA1.