#rational method

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Rational Method Ph 6012-7102620

http://msmaware.com/blog/documentation/ Ph 6012 710-2620

Manual Saliran Mesra Alam Malaysia (MSMA), or in English the Urban Stormwater Management Manual for Malaysia is the stormwater drainage design guide first issued by the Department of Irrigation and Drainage Malaysia (D.I.D) in 2000. It is mandatory for all engineers in Malaysia to build drainage structures to fullfil the requirements of MSMA. Eleven years after its first publication, DID has completely revised the first edition of MSMA (MSMA1) with the publication of the second edition (MSMA2). There are many alterations in the second edition including (taken from case studies completed for Kuala Lumpur):

1. The design rainfall has gone up by up to 126% for 10 out of the fourteen rainfall stations in Kuala Lumpur.

2. The design discharge using the Rational Method has increased by up to 131% for commercial and city area.

3. The Site Storage Requirement for On-Site Detention for a factory site in Kuala Lumpur has gone up by up to 235%,

4. The volume of a detention basin has increased by up to 130%.

The increase in the values of the main design parameters as described above can be due to the changes between the first and the second editions of MSMA as follows:

1. Longer rainfall data covering 100 more storm stations in Peninsular Malaysia. This gives rise to in higher rainfall intensities for certain durations and Average Recurrence Intervals.

2. Peninsular Malaysia is now grouped into five regions for the determination of the temporal pattern, compared to previously where it was divided into east and west coast of the Peninsula. The differences in temporal patterns give rise to dissimilar hydrograph peaks and shapes.

3. The coefficient of runoff in the Rational Method is now established according to the types of landuse, and not affected by rainfall durations and intensities, and whether a catchment is urban/rural in condition. This resulted in changes in the estimated peak discharge.

4. The Time-Area Method in the second editon is affected by the changes in the rainfall intensites, temporal patterns and the loss model used. For Kuala Lumpur it is noted that the rainfall intensities have increased and the temporal pattern is more peaky, thus resulting in a more peaky discharge hydrograph.

5. The Site Storage Requirement (SSR) based on the Approximate Swinburne Method in the second editon is higher than that using the Swinburne Method in MSMA1. The second editon used the approximate method based on a set of approximation tables and this has resulted in an over estimation of the SSR’s.

6. The volume of a detention basin has gone up using the second editon compared to MSMA1 because of the increase in the rainfall intensities, temporal patterns and the choice of loss rate.

In view of the many changes in computational methods between the first and second editions of MSMA, a tool is developed to help engineers to perform the computations using both editions accurately and to allow comparison be made of their changes speedily. The tool is a urban drainage design software designed for MSMA- under the brandname of the MSMAware. The software is developed by Ir. Dr. Quek Keng Hong who is a practising engineer in Malaysia. MSMAware also covers the East Malaysian state of Sarawak where more than a dozen rainfall stations in the state and their temporal patterns are programmed into the software.

RATIONAL METHOD : 00:00:05 Rational Method 00:00:05 Time-Area Method 00:00:06 culvert design 00:00:07 culvert analysis 00:00:08 Intensity Frequency Duration