Manual Calculation



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


Design Criteria


The proposed of Semi D houses can be classified as low density development.

According to Chapter 11 of the Manual, on-site facilities are minor drainage structures provided on individual housing, industrial and infrastructure sites. For quantity design they are based on peak inflow estimates using the Rational Method with design storms between 2 and 10 year ARI.

The design rainfall is based on Chapter 13 of the Manual. The design storm for Kuala Lumpur is used in the calculation.



Determination of Impervious and Pervious Areas


For the purpose of hydrological calculation, the area is shown in Table 5.1.

It is estimated that 43.14 % of the areas may be considered as impervious. Hence the impervious area is computed by multiplying the total area by 43.14% as shown in the table.

The remaining 56.86 % of the areas is assumed pervious. Hence the pervious area is computed by multiplying the total area by 56.86% as shown in the table.


Table 5.1 Pervious and Impervious Areas



Determination of Time of Concentration, tc and tcs


To determine the catchment times of concentration, an analysis of the catchment drainage system will need to be undertaken. For this example, it is assumed that:

  • tc= 30 minutes
  • tcs= 25 minutes



Determination of Pre and Post Development Flows


The minor drainage system that the OSD storage will discharge has been designed for a 2 year ARI capacity. The rainfall intensity is estimated using Equation 13.2 (tc > 30 minutes) and table 13.A1 for tc. Using the Rational Method, the pre and post-development flows are calculated as shown in table 5.2.


Table 5.2 Comparison of Pre/Post Development Peaks for Kuala Lumpur (MSMA 2000)


For pre development, Qp = 6.35 l/s

For post development, Qa = 8.73 l/s



Determination of Permissible Site Discharge (PSD)


The permissible site discharge (PSD) for the site is calculated using equation 3.1 with equation 3.2 and equation 3.3 for above ground storage. The results are tabulated in Table 5.3.


Table 5.3 Computation of Permissible Site Discharge (PSD) for Kuala Lumpur



Determination of Site Storage Requirement (SSR)


Using equation 3.6 with equation 3.7 and equation 3.8 for above-ground storage, the site discharge for the storage design storm (10 year ARI) and the corresponding SSR is calculated for range of storm durations to determine the maximum SSR. These calculations are summarized in table 5.4 and 5.5.


Table 5.4 Computation of Peak Post-Development for Kuala Lumpur (MSMA 2000)


Table 5.5 Computation of Site Storage Requirement (SSR) for Kuala Lumpur (MSMA 2000)


Based on the data calculated above using spreadsheets, graph of the typical relationship of storage volume to storm duration is plotted in figure 5.1. From these figure, a maximum SSR is found at duration 20 minutes with 15.69 m³ of water discharge


Figure 5.1 Graph of SSR versus Time for Kuala Lumpur (MSMA 2000)


However, for a landscaped storage, an additional 20 % is added to the volume to account for inaccuracies in construction and future loss of storage due to the build-up of the lawn surface. Therefore:


Required SSR = 15.69 x 1.2 = 18.83 m³



Determination of Primary Outlet Size


The primary outlet orifice is sized to discharge the PSD assuming free outlet conditions when the storage is full. Using a 600 mm deep discharge control pit (DCP) and a maximum storage depth of 300 mm, adopt a maximum head to the centerline of the orifice of 0.8 m. The required orifice size under free outlet condition is calculated by rearranging equation 19.3. The result is shown in table 5.6.


Table 5.6 Computation of Primary Outlet Size(MSMA 2000)





Determination of Secondary Outlet Sizes


The secondary outlet is a broad-crested weir slotted into the retaining wall along the front boundary. The weir should be sized for the estimated major system ARI flow from the site for time tcs (25 minutes). The major system in the catchment has designed for 50 year ARI.

The 25 minute, 50 year ARI rainfall intensity for Kuala Lumpur is estimated using equation 13.3. The summarized of major system flow is calculated using the rational method as shown in Table 5.7.

Table 5.7 Computation of Post Development Peaks Flow for Kuala Lumpur (MSMA 2000)


Assuming the head over the weir is limited to 50 mm and CBCW = 1.70, rearranging equation 20.5 and calculate weir base width. This result is tabulate in Table 5.8.


Table 5.8 Computation of Secondary Outlet Size



Allowing 50 mm freeboard, the dimensions of the secondary outlet weir are:


1000 mm (wide) x 100 mm (high)



Editor :- Dilah – 26 Mar 14



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