Review of Changes in the Second Edition of MSMA (Part II) - Rational Method Computation Ir. Dr. Quek Keng Hong B.E. (civil), M.Eng.Sc, Ph.D. (NSW), PE Managing Director, MSMAware Sdn Bhd Email:
[email protected]
Ir. Dr. Quek Keng Hong has a PhD and a Master’s degree in water resources from the University of NSW, Australia. Dr. Quek is a consulting engineer by practice and specialises in the field of urban drainage and hydrology. He has conducted many training workshops and seminars on MSMA. Dr. Quek is currently a committee member of the Water Resources Technical Division (WRTD). 1. Introduction The Rational Method is a method for computing peak discharges for areas less than 80 hectares in both the first and second editions of MSMA- referred to herein as MSMA (2000) and MSMA (2011), respectively. MSMA is an acronym for the Manual Saliran Mesra Alam (the Urban Stormwater Management Manual for Malaysia) published by the Department of Irrigation and Drainage (D.I.D, 2000 and 2011).
The peak discharge computed using the Rational Method is commonly used in the sizing of a drainage structure. Any change in the magnitude of the peak discharge will have direct impact on the cost of the structure. This paper reviews the changes in the Rational Method in the two editions of MSMA.
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In the study, the method is applied to a typical catchment in Kuala Lumpur and the results compared. The changes in the design discharges due to changes in the runoff coefficient C and the rainfall intensities are assessed.
2. Changes in the Rational Method Rational Method is covered in Chapter 14 of the first edition and Chapter 2 of the second edition of MSMA.
2.1 Rational Method in MSMA (2000) MSMA relates the peak discharge to the rainfall intensity and catchment area via the Rational Method: Qy
C y I t A 360
(1)
where Qy
is the y year ARI peak discharge (m3/s)
C
is the dimensionless runoff coefficient, recommended values may be obtained from Design Chart 14.3 for urban areas and Design Chart 14.4 for rural areas in MSMA (2000).
y
It
is the average intensity of the design rainstorm of duration equal to the time of concentration tc and of ARI of y year (mm/hr)
A
is the drainage area (ha)
2.2
Rational Method in MSMA (2011)
In MSMA (2011), the peak discharge is related to the rainfall intensity and catchment area via the Rational Method:
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Q
C i A 360
(2)
where Q is the peak flow (m3/s) C is the runoff coefficient given in Table 1 (Table 2.5 of MSMA, 2011). I is the average rainfall intensity (mm/hr) A is the drainage area (ha)
2.3 Comparison The changes in the Rational Method between the first and second editions are as follows: 1.
The major change in the Rational Method is the coefficient of runoff. In the second edition, it is read from a design chart and varies according to the types of landuse, the rainfall intensities and whether it is urban or rural catchments. But in the second edition, it is fixed according to the landuse- like in DID (1975), as shown in Table 1 (Table 2.5 of MSMA, 2011).
2.
There is no change in the size of catchment area where the Rational Method can be applied. Both editions specify that the Rational Method should not be used for catchment area greater than 80 ha.
3.
The magnitudes of the design discharges in the two editions of MSMA are covered in the following case study.
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Table 1 Recommended Runoff Coefficients for Various Landuses (After Table 2.5 of MSMA, 2011) Landuse
Residential Bungalow Semi-detached Bungalow Link and Terrance House Flat and Apartment Condominium
Runoff Coefficient (C) For Minor System For Major System (≤10 year ARI) (>10 year ARI) 0.65 0.70 0.80 0.80 0.75
0.70 0.75 0.90 0.85 0.80
Commercial and Business Centres 0.90 0.95 Industrial 0.90 0.95 Sport Fields, Park and Agriculture 0.30 0.40 Open Spaces 0.50 0.60 Bare Soil (No Cover) 0.40 0.50 Grass Cover 0.35 0.45 Bush Cover 0.30 0.40 Forest Cover Roads and Highways 0.95 0.95 Water Body (Pond) 0.95 0.95 Detention Pond (with outlet) 0.00 0.00 Retention Pond (no outlet) Note: The runoff coefficients in this table are given as a guide for designers. The near-field runoff coefficient for any single or mixed landuse should be determined based on the imperviousness of the area.
3. Case Study 3.1
Case Study on Rational Method
The purpose of the case study is to compare the changes in the computed peak discharges using the Rational Method from the two editions of MSMA. The methods are applied to a typical catchment involving a major system in Kuala Lumpur. The changes in the design discharges due to changes in the runoff coefficient C and rainfall intensities are assessed.
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Figure 1 shows a map of the catchment area. The study area is located in Sg. Batu, Kuala Lumpur. The catchment data are as follows:
Area= 30 hectares.
Length of Overland flow= 300 m
Slope= 0.3%, paved surface.
Length of Open Drain= 600
Three types of landuses are studied:
Park
Semi-D Houses
Commercial and city area
3.2
Evaluation
Table 2 is a summary of the peak discharges computed using MSMA (2000) and (2011).
To find out the magnitude of increase in discharge, we define a ratio R: R
A Q p2 B Q p1
where A= Qp2 which is the peak discharge based on MSMA (2011) B= Qp1 which is the peak discharge based on MSMA (2000)
The ratio R is tabulated as shown in the last column of the table.
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It can be seen that: 1. For park, the ratio R is 0.76 indicating that the peak discharge from MSMA (2011) is lower than the peak discharge from MSMA (2000). This is due principally to the lower C of 0.4 in MSMA (2011) compared to a higher C of 0.61 in MSMA (2000). The lower C in MSMA (2011) reflects DID’s effort in promoting more storage in parks. 2. For Semi-D houses, the ratio R is 1.02 indicating that the peak discharge from MSMA (2011) is about 2% higher than the peak discharge from MSMA (2000). The Q has increased from 15.3 to 15.6 m3/s. The C has reduced from 0.9 to 0.75 but the i has increased from 203.6 mm/hr to 249.7. The reduction in C is only for Semi-D houses, while the increase in storm intensity is generally associated with MSMA (2011). In this case, the effect of the increasing storm intensity is more prominent, thus giving a higher peak discharge. 3. For commercial and city area, the ratio R is 1.31 indicating that the peak discharge from MSMA (2011) is about 31% higher than the peak discharge from MSMA (2000). The Q has increased from 16.9 to 22.1 m3/s. The C has increased from 0.905 to 0.95 while the storm intensity has increased from 224.3 mm/hr to 279.4. The increase in C for commercial and city area and storm intensity in MSMA (2011) has attributed to a significantly higher peak discharge. 4. In conclusion, the peak discharge computed using the Rational Method in MSMA (2011) is up to 31% higher than that in MSMA (2000). This increase is caused principally by the higher C for commercial and city area and the higher storm intensity in MSMA (2011). 5. The magnitude of increase in peak discharge associated with the Rational Method in MSMA (2011) varies depending on the station used for the IDF computation. MSMA (2011) has provided 14 stations with different IDF data for Kuala Lumpur. In separate case
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study for design storms (Quek, 2015), it was found that 71% of these stations have higher storm intensities under MSMA (2011). 6. It is concluded that 71% of the stations in Kuala Lumpur will have up to 26% higher storm intensity and up to 31% higher peak discharges for commercial and city area.
Table 2 Comparison of Peak Discharges using the Rational Method in MSMA (2000, 2011) Landuse
Q (m3/s) (A) MSMA (2011) park 7.2 Semi-D 15.6 Commercial 22.1
Q (m3/s) (B) MSMA (2000) 9.4 15.3 16.9
A/B 0.76 1.02 1.31
Catchment Area= 30 hectares
Lo Ld
River
Figure 1 Catchment Map
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4. Conclusion 1. For commercial and city area, the peak discharge from MSMA (2011) is about 31% higher than the peak discharge from MSMA (2000). The Q has increased from 16.9 to 22.1 m3/s. The C has increased from 0.905 to 0.95 while the storm intensity has increased from 224.3 mm/hr to 279.4 mm/hr. The increase in C for commercial and city area and storm intensity have attributed to the higher peak discharge using the Rational Method in MSMA (2011). 2. Based on the results of a separate case study on design storms (Quek, 2015), it is concluded that 71% of the stations in Kuala Lumpur will have up to 26% higher storm intensity and up to 31% higher peak discharges for commercial and city area.
5.
References
Drainage and Irrigation Department (1975) Urban Drainage Design Standards and Procedures for Peninsular Malaysia. Planning and Design Procedure No. 1. Ministry of Agriculture, Malaysia.
Drainage and Irrigation Department (1976) Flood Estimation for Urban Areas in Peninsular Malaysia. Hydrological Procedure No. 16. Ministry of Agriculture, Malaysia.
Drainage and Irrigation Department (2000) “Urban Stormwater Management Manual for Malaysia” Ministry of Agriculture, Malaysia.
Drainage and Irrigation Department (2010) “Review and Updated the Hydrological Procedure NO. 1- Estimation of Design Rainstorm in Peninsular Malaysia” December, Prepared by NAHRIM.
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Drainage and Irrigation Department (2011) “Urban Stormwater Management Manual for Malaysia” (Manual Saliran Mesra Alam Malaysia), Second edition.
Quek K. H. (2015a) “Review of Changes in the Estimation of Design Storm in the First and Second Editions of MSMA”, Submitted for publication in the Journal of IEM, December 2015.
Quek K. H. (2015b) “Review of Changes in the Rational Methods in the First and Second Editions of MSMA”, Submitted for publication in the Journal of IEM, December 2015.
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