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Use Drainage Design in Sabah/Sarawak

How to set the rainfall intensity for Sabah and Sarawak

MSMA (2011) covers the IDF data of 12 states and federal territory in Peninsular Malaysia. Sabah and Sarawak are not covered. The rainfall IDF data for Sarawak is being covered in SUStoM. 

In MSMA (2000), the two East Malaysian states are covered.

APPENDIX 13.A FITTED COEFFICIENTS FOR IDF CURVES FOR 35 URBAN CENTRES
Table 13.A1 Coefficients for the IDF equations for the different major cities and towns in Malaysia

(30 ≤ t ≤ 1000min)

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Figure above shows the raingauge location with fitted coefficients for Sabah and Sarawak.

In MSMA (2011), no mention of temporal pattern for East Malaysia. however, for Sarawak, one can obtain the temporal pattern from a paper published by UNIMAS

In MSMA (2000), it is recommended that the temporal pattern for East Coast of Peninsula be used for Sabah and Sarawak. Rainfall intensity can be generate using the formulas below by referring at the table above.

RAINFALL INTENSITY

Design rainfall intensity (mm/hr) depends on duration (minute or hour) and ARI (month or year). It is strongly recommended that performance of the designed drainage system must be examined for a range of ARIs and storm durations to ensure that the system(s) will perform satisfactorily.

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ARI

Rainfall and subsequent discharge estimate is based on the selected value of frequency or return period, termed as the Average Recurrence Interval (ARI). ARI is the average length of the time between rain events that exceeds the same magnitude, volume or duration and is expressed as:

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TIME OF CONCENTRATION

Time of concentration (tc) is the travel time of runoff flows from the most hydraulically remote point upstream in the contributing catchment area to the point under consideration downstream. The concept of time of concentration is important in all methods of peak flow estimation as it can be assumed that the rainfall occurring during the time of concentration is directly related to peak flow rate. The practice is to select the design storm duration as equal or greater than the tc.

In the design of stormwater drainage system, tc is the sum of the overland flow time (to) and the time of travel in street gutters (tg), or roadside swales, drains, channels and small streams (td)

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To conclude, rainfall intensity formula is dependent on the fitting coefficient, station, ARI and tc.

MiTS 1

HOW TO CHANGE DESIGN MANUAL in MiTS 1

To change the design manual, you may go to Option >> Design Manual. Then you may choose to use MSMA 1st Edition or MSMA 2nd Edition.

You may refer steps as below:

design-manual

design-manual

Use Drainage Design outside Malaysia

By using MiTS, one can design drainage as usual even the location is located outside from Malaysia. However, there are some informations that need to be manually input, that are:

  1. Use own rainfall intensity at pipe modeimage00

2.Use runoff coefficient according to international standard

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Rainfall Intensity

The ratio of total amount of rain falling during a given period to the duration of the period.

It is expressed in depth units per unit time, usually as mm per hour (mm/hr).

When designing a drainage system, you have to ensure it can cope with the heavy downpours that can be expected in the job location. For example, rainfall intensity will be much higher in a tropical area such as Cairns, Queensland compared to a moderate climate location such as Melbourne, Victoria.

Runoff Coefficient

Runoff can be described as the part of water cycle that flows overland as surface water instead of being absorbed into groundwater or evaporating.

The runoff coefficient is a function of the ground cover and the rainfall intensity. During a storm, the actual runoff coefficient increases as the soil become saturated. The greater the rainfall intensity, the greater is the runoff coefficient due to the reducing relative amount of rainfall losses.

MiTS 2:

In MiTS 2, all the SUStoM and MSMA 1 and 2 are implemented, for Drain, OSD and Pond Design. So if you find that you need to do MSMA 2 design/SUStoM design, we recommend that you upgrade to MiTS 2.