Manual Calculation



The result of computation using MSMAware was checked using spreadsheet in this section.


Design Criteria

The proposed of Banglow  can be classified as low density development.

According to Chapter 1 of the second edition of MSMA, table 1.1 show the minor design is 5 years ARI for banglow.

The rainfall data were taken from the Sarawak Hydrological Year Book (2010).



Determination of Impervious and Pervious Areas

For the purpose of hydrological calculation, the area is shown in Table 5.73.


Table 5.73 Pervious and Impervious Areas



Determination of Time of Concentration, tc and tcs

The times of concentration adopted are as follows:

  • tc= 20 minutes
  • tcs= 15 minutes



Determination of Pre and Post Development Flows

The minor drainage system that the OSD storage will discharge has been designed for a 5 year ARI capacity. The rainfall intensities are taken from the Kuching Airport rainfall station data. Using the Rational Method, the pre and post-development flows are calculated as shown in table 5.74.


Table 5.74 Comparison of Pre/Post Development Peaks for Kuching Airport, Sarawak.



For pre development, Qp = 181.25 l/s

For post development, Qa = 350.42 l/s



Determination of Permissible Site Discharge (PSD)

The permissible site discharge (PSD) for the site is calculated using equation 19.1 with equation 19.1c and equation 19.1d for below ground storage. The results are tabulated in Table 5.75.


Table 5.75 Computation of Permissible Site Discharge (PSD) for Kuching Airport, Sarawak.


Determination of Site Storage Requirement (SSR)

Using equation 19.2 with equation 19.2c and equation19.2d for below-ground storage, the site discharge for the storage design storm (10 year ARI) and the corresponding SSR is calculated for range of storm duration to determine the maximum SSR. These calculations are summarized in table 5.76 and 5.77.


Table 5.76 Computation of Peak Post-Development for Kuching Airport, Sarawak.


Table 5.77 Computation of Site Storage Requirement (SSR) for Kuching Airport, Sarawak.



Required PSD = 191.64 l/s

Required SSR = 522.76 m³





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