Application of MSMA2 in the KL Drainage Masterplan Study
1. Drainage Design Guides for the Study Area
From 1975 to 2000, the Planning and Design Procedure No. 1 (PDP1) published by the Department of Irrigation and Drainage (DID) were used for drainage design in Malaysia.
The first edition of MSMA (Manual Saliran Mesra Alam Malaysia or Urban Stormwater Management Manual) was published by DID in 2000. Eleven years later the Department released an update known as MSMA2 in July 2012.
The topics covered in the second edition of MSMA2 is similar to MSMA1, apart from the introduction of a new chapter on rainwater harvesting. There are significant changes in the computational procedures in MSMA2 e.g., storm intensity, temporal pattern, Rational Method, On-Site Detention (OSD) and Sediment Basin. These resulted in higher magnitudes of design parameters like storm intensity, peak discharge and site storage requirements for OSD.
The most important change is in the magnitude of the storm intensities for Kuala Lumpur. In MSMA1, there was only one rainfall station for Kuala Lumpur. In MSMA2, however, the number of stations has increased to 14 stations. Also, in the first edition, the data used for Kuala Lumpur were up to about 1983. However, in the second edition, the data used were closer to the year of publication i.e., nearly 27 years longer. Due to the number of wet years in the last decade, it is not surprising that the storm intensities for Kuala Lumpur has increased significantly.
Another important change is in the temporal pattern for Kuala Lumpur. In MSMA1, this is based on the west coast of Peninsular Malaysia. But in MSMA2, Kuala Lumpur is designated as Region 5 i.e., one of the five regions classified in the Peninsula.
The computation of peak discharge using the Rational Method and discharge hydrograph using the Time-Area Method too are affected by the increase in the storm intensities. For the Rational Method, the runoff coefficient is now fixed at higher magnitude, and not varies according to the storm intensities like in MSMA1. In the case of the Time-Area method, it is affected not only by the higher storm intensities but a change in the temporal pattern (designated under Region 5).
The changes in the storm intensities, temporal pattern, peak discharge (Rational Method) and discharge hydrograph (Time-Area Method) will result in a significant increase in the size of the conveyance system e.g., drains and culverts, as well as storage volumes of On-Site Detention, detention basins and sediment basins.
In the case of the OSD, the method adopted in MSMA1 is based on formula known as the Swinburne Method. However, in MSMA2 the method has changed, it is based on a set of approximation tables and not formula. The method is known as the Approximate Swinburne Method. This method overestimated the SSR storage by more than double. This has caused a lot of concern by practitioners in the industry.
Based on case studies carried out (Quek, 2015a, b), the following increase in magnitude of key design parameters were observed for Kuala Lumpur:
1. The design storm has increased by up to 126% for 10 out of the 14 stations in Kuala Lumpur.
2. The design discharge using the Rational Method has gone up by up to 131% for commercial and city area.
3. The peak discharge using the Time-Area Method has increased by 127%.
4. The Site Storage Requirement for OSD for a factory site has increased by 235%.
5. The volume of detention basin for a site has increased by up to 130%.
6. The volume of wet sediment basin has increased by 165%.
2. Application of MSMA2 for the Study Area
MSMA2 will form the basis of all design in the KL Drainage Masterplan study.
It is evident from the previous section that using MSMA2, there will be significant increase in the size of the conveyance system e.g., drains and culverts, as well as storage volumes such as the On-Site Detention, detention basins and sediment basins.
In Kuala Lumpur, most of the existing drainage systems were designed using MSMA1 and its predecessor the PDP1.
Since MSMA2 will be used in this Study, it is expected that the magnitude will be higher than those designed using the earlier procedures.
For drainage computation in Kuala Lumpur, it is expected that a combination of MSMA1 and 2 will be adopted. For new developments, MSMA2 can be adopted. But for some existing areas MSMA1 may have been used for the design.
This gives rise to the need of a software tool that can compute both MSMA1 and 2 as and when necessary.
3. Computer Modelling for the Masterplan Study
The software is able to compute the following using either MSMA1 or 2:
a. Allows the selection of the nearest rainfall stations for the determination of storm intensities. The software allows the computation of design storm by simply clicking on a google map of 14 rainfall stations in Kuala Lumpur in MSMA2.
b. Allows the computation of peak discharge using Rational Method by selecting the location on a map.
c. Allows the computation of peak discharge hydrograph using the Time-Area Method by selecting the location on a map.
d. Allows the computation of detention storages and culvert sizes by selecting the location on a map.
e. The software will use a method known as the Exact Swinburne Method (ESM) to compute the SSR storage requirement for OSD as discussed in the paper by Quek (2015b). As discussed earlier, for industrial land-use in Kuala Lumpur, the Site Storage Requirement (SSR) using MSMA2 is 2.35 times the MSMA1 estimate. The difference is due to the Approximate Swinburne’s Method used in MSMA2, which gives very conservative estimates using figures and tables. In the ESM method, the Swinburne’s Method is applied instead of the Approximate Swinburne’s Method in MSMA2 using the storm and discharge data in MSMA2 to compute the PSD and SSR. The method gives SSR of only 1.14 times higher than MSMA1.
4. Advantages of the Computer Model
There are many advantages of the computer model used:
a. Shows the location of 14 storm stations in MSMA2 on a map. This allows quick determination of the nearest stations.
b. The software can compute using either MSMA1 or 2 or both.
c. Allows generation of a report.
d. Easy to use Graphical User Interface.
e. Can compute the design storm, temporal pattern, peak discharge and hydrograph, OSD, detention basin storage and culvert analysis.
f. Can determine changes in design parameters for different land use quickly.
g. Can determine extent of flooding when used together with HEC-RAS.
5. Hydrology and Hydraulic Analysis and Design Software
The MSMAware (Quek, 2015c) is the main software which will be used for hydrology analysis.
The MSMAware will be used for MSMA computation in the study. The software allows the computation of design storm by simply clicking on a google map of 14 rainfall stations in Kuala Lumpur. Similarly, culvert sizes and OSD and detention storages can be computed for anywhere in Kuala Lumpur.
The software can compute the following using MSMA1 and MSMA2:
1. Design storm,
2. Temporal patterns,
3. Rational Method,
4. Time-Area Method,
5. OSD design,
6. Detention basin design and
7 culvert design.
The software is flexible as it allows the engineer to pick and choose the modules needed.
For hydraulic analysis the HEC-RAS software by the US Army Corps of Engineers will be used. HEC-RAS is a free ware.
6. Training for DBKL Engineer
The consultant will provide training to DBKL engineers on the use of the MSMAware software.
Quek (2015a). Changes in the design storm intensities and temporal patterns using the first and second editions of MSMA, submitted to IEM Journal for publication. (See Appendix E)
Quek (2015b). Differences in the design of On-Site Detention using the first and second editions of MSMA, submitted to IEM Journal for publication. (See Appendix F)
(Quek, 2015c). MSMAware user guide. Available online at http://online.msmaware.com