EVALUATION OF THE CHANGES BETWEEN THE FIRST AND SECOND EDITIONS OF THE URBAN STORMWATER MANAGEMENT MANUAL FOR MALAYSIA (MSMA)

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1 EVALUATION OF THE CHANGES BETWEEN THE FIRST AND SECOND EDITIONS OF THE URBAN STORMWATER MANAGEMENT MANUAL FOR MALAYSIA (MSMA) Ir. Dr. Quek Keng Hong B.E. (civil), M.Eng.Sc, Ph.D. (NSW), PE Managing Director, MSMAware Sdn Bhd Note: Condensed versions of this paper are submitted for publication in the IEM Journal and Bulletin. (This paper may be download from Abstract This paper investigated the changes between the first and second editions of MSMA on five key parameters as follows: (i) Design Average Recurrence Interval, (ii) Design Storm, (iii) Rational Method, (iv) On-Site Detention and (v) Total volume of sedimentation basins. The magnitudes of changes were quantified using case studies and the results are as follows: (i) Design Average Recurrence Interval: For medium density residential and commercial and city area, the storm intensity has increased by up to 122% for minor system for an ARI increase from 5 to 10 years, and up to 133% for major system for an ARI increase from 50 year to 100 years between MSMA (2000) and (2011). It is emphasised that the changes in the storm intensity is not only due to changes in the ARI but also the higher IDF data in MSMA (2011). (ii) Design Storm: For durations of between 15 to 700 min, the IDF estimates using MSMA (2011) were mostly higher than those estimated using MSMA (2000). In the study, out of 14 stations, 10 of them (or 71%) were higher than the MSMA (2000) curve, while the remaining 4 stations (or 29%) were lower than the first edition estimates. It is concluded that the design storms estimated based on MSMA (2011) for Kuala Lumpur can be up to about 26% higher than MSMA (2000) for duration below 700 minutes, for 71% of the stations. (iii) Rational Method: For commercial and city area, the peak discharge from MSMA (2011) is about 31% higher than the peak discharge from MSMA (2000). The discharge has increased from 16.9 to 22.1 m 3 /s. The runoff coefficient C has increased from to 0.95 while the storm intensity has increased from mm/hr to The increase in C for commercial and city area and storm intensity in MSMA (2011) has attributed to a significantly higher peak discharge. 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 storm intensity in MSMA (2011) and by the higher C for commercial and city area in MSMA (2011). In general, 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. (iv) On-Site Detention: The result shows that for Kuala Lumpur, the PSD and SSR using MSMA (2011) are about 20% and 190% of MSMA (2000). The PSD and SSR using the ESM Method for Kuala Lumpur is about 55% and 103%, respectively, of those using MSMA (2000). For Pulau Pinang, the PSD and SSR using MSMA (2011) are about 18% and 180% of MSMA (2000), while the PSD and SSR using the ESM Method is about 55% and 129%, respectively, of those using MSMA (2000). The approximate Swinburne s Method in MSMA (2011) results in underestimate of PSD and over estimate of the SSR. The ESM Method appeared to give slightly higher estimate of SSR compared to MSMA (2000) but a lot lower estimate compare to MSMA (2011). The ESM Method may be used instead of MSMA (2011) to give a better estimate of the PSD and SSR. 1

2 (v) Total volume of Sedimentation Basin: The dry sediment basin volume using MSMA (2011) is half of that using MSMA (2000) for 6 month ARI design (for projects taking more than two years) as MSMA (2011) does not cover 6 month ARI. The wet sediment basin volume was 65% higher using MSMA (2011) compared to MSMA (2000) because of it was based on 50 mm of rainfall for temporary BMP in MSMA (2011), compared to the 75 th percentile storm of mm in MSMA (2000) which is lower. 2

3 1. Introduction 1.1 Evolution of Drainage Guidelines in Malaysia Before 2001, engineers in Malaysia applied the Planning and Design Procedure No. 1 (DID, 1975) published by the Department of Irrigation and Drainage (DID) in 1975 for their drainage design. This is a relatively simple document to use- with only 242 pages covering ten chapters. But this has changed with the introduction of the Urban Stormwater Management Manual for Malaysia ( Manual Saliran Mesra Alam Malaysia ) in 2000 (DID, referred to herein after as MSMA, 2000). The new Manual is much more thorough in its coverage of subject matters compared to the old procedure. It contains 48 chapters spanning more than 1,100 pages. In 2011, the Department published an updated version of the same manual, known as MSMA 2 nd Edition (DID, referred to herein after as MSMA, 2011). This document was launched by the Department in early 2012 and enforced on 1 July, The document is roughly half the thickness of the first edition. There are many significant changes in computational procedures between the two editions of MSMA (2000, 2011). 1.2 Overall Changes in MSMA (2011) from MSMA (2000) The overall layout of MSMA (2011) has changed from MSMA (2000) as follows: The number of chapters has reduced from 48 in the first edition to 20 in the second edition. The number of pages has reduced by roughly half. The topics are now more focused compared to the previous edition with chapters named after specific drainage elements such as detention pond and On-Site Detention. New chapters namely, on Rainwater Harvesting and Pavement Drainage are included. The content of the 20 chapters are as follows: Chapter 1- Design Acceptance Criteria Chapter 2- Quantity Design Fundamental Chapter 3- Quality Design Fundamentals Chapter 4- Roof and Property Drainage Chapter 5- On-Site Detention Chapter 6- Rainwater Harvesting Chapter 7- Detention Pond Chapter 8- Infiltration Facilities Chapter 9- Bioretention System Chapter 10- Gross Pollutant Traps Chapter 11- Water Quality Ponds and Wetlands Chapter 12- Erosion and Sediment Control Chapter 13- Pavement Drainage Chapter 14- Drains and Swales 3

4 Chapter 15- Pipe Drain Chapter 16- Engineered Channel Chapter 17- Bioengineered Channel Chapter 18- Culvert Chapter 19- Pump and Tidal Gate Chapter 20- Hydraulic Structures Table 1.1 is a comparison of the various chapters in MSMA (2000, 2011) given by DID. Table 1.1 Comparison of Chapters in MSMA (2000, 2011) (After DID Seminar Paper, 2012) MSMA (2000) MSMA (2011) Part A: Introduction Chapter 1: Malaysian Perspective Chapter 1- Design Acceptance Criteria Chapter 2: Environment Processes Chapter 1- Design Acceptance Criteria Chapter 3: Stormwater Management Chapter 1- Design Acceptance Criteria Part B : Administration Chapter 4: Design Acceptance Criteria Chapter 1- Design Acceptance Criteria Chapter 5: Institutional and Legal Framework Chapter 1- Design Acceptance Criteria Chapter 6: Authority Requirement and Chapter 1- Design Acceptance Criteria Documentation Part C : Planning Chapter 7: Planning Framework Chapter 1- Design Acceptance Criteria Chapter 8: Strategic Planning Chapter 1- Design Acceptance Criteria Chapter 9: Master Planning Chapter 1- Design Acceptance Criteria Chapter 10: Choice of Management Chapter 1- Design Acceptance Criteria Part D : Hydrology and Hydraulics Chapter 11: Hydrologic Design Concepts Chapter 2- Quantity Design Fundamental Chapter 12: Hydraulic Fundamentals Chapter 2- Quantity Design Fundamental Chapter 13: Design Rainfall Chapter 2- Quantity Design Fundamental Chapter 14: Flow Estimation and Routing Chapter 2- Quantity Design Fundamental Chapter 15: Pollutant Estimation, Transport and Chapter 3- Quality Design Fundamentals Retention Chapter 16: Stormwater System Design Chapter 2- Quantity Design Fundamental Chapter 17: Computer Models and Softwares Chapter 2- Quantity Design Fundamental Part E : Runoff Quantity Control Chapter 18: Principle of Quantity Control Chapter 5- On-Site Detention/Chapter 7- Detention Pond Chapter 19: On-site Detention Chapter 5- On-Site Detention Chapter 20: Community and Regional Detention Chapter 7- Detention Pond Chapter 21: On-site and Community Retention Chapter 8- Infiltration Facilities Chapter 22: Regional Retention Chapter 8- Infiltration Facilities Nil Chapter 6- Rainwater Harvesting Part F : Runoff Conveyance Chapter 23: Roof and Property Drainage Chapter 4- Roof and Property Drainage Chapter 24: Stormwater Inlets Chapter 13- Pavement Drainage Chapter 25: Pipe Drains Chapter 15- Pipe Drain Chapter 26: Open Drains Chapter 14- Drains and Swales Chapter 27: Culvert Chapter 18- Culvert 4

5 Chapter 28: Engineered Waterways Chapter 29: Hydraulic Structures Part G : Post Construction Runoff Quality Controls Chapter 30: Stormwater Quality Monitoring Chapter 31: Filtration Chapter 32: Infiltration Chapter 33: Oil Separators Chapter 34: Gross Pollutant Traps Chapter 35: Constructed Ponds and Wetlands Chapter 36: Housekeeping Practices Chapter 37: Community Education Part H : Construction Runoff Quality Controls Chapter 38: Action to Control Erosion and Sediment Chapter 39: Erosion and Sediment Control Measures Chapter 40: Contractor Activity Control Measures Chapter 41: Erosion and Sediment Control Plans Part I : Special Application Chapter 42: Landscaping Chapter 43: Riparian Vegetation and Watercourse Management Chapter 44: Subsoil Drainage Chapter 45: Pumped Drainage Chapter 46: Lowland, Tidal and Small Island Drainage Chapter 47: Hillside Drainage Chapter 48: Wet Weather Wastewater Overflows Nil Nil Chapter 16- Engineered Channel Chapter 20- Hydraulic Structures Chapter 3- Quality Design Fundamentals Chapter 9- Bioretention System Chapter 8- Infiltration Facilities Chapter 10- Gross Pollutant Traps Chapter 10- Gross Pollutant Traps Chapter 11- Water Quality Ponds and Wetlands Nil Nil Chapter 12- Erosion and Sediment Control Chapter 12- Erosion and Sediment Control Chapter 12- Erosion and Sediment Control Chapter 12- Erosion and Sediment Control Annex 1: Ecological Plants Chapter 17- Bioengineered Channel Nil Chapter 19- Pump and Tidal Gate Nil Nil Nil Annex 2: Maintenance Annex 3: IDF Curves 5

6 2. Changes in the Design ARI. The design storm ARI is covered in Chapter 4 of the first edition and Chapter 1 of the second edition. 2.1 Major and Minor Design ARI (MSMA, 2000) The design storm ARI s for MSMA (2000) is covered in Table Major and Minor Design ARI (MSMA, 2011) The design storm ARI s for MSMA (2011) is covered in Table Comparison The changes in major/minor design storm ARI. for various types of development are evaluated by comparing Table 2.1 and Table 2.2 as follows: 1. For Major System, the ARI. for most types of development is fixed at 100 year ARI. in MSMA (2011), unlike MSMA (2000) where the ARI. is defined as up to 100 year for all development types- subject to cost benefit analysis by the engineer. 2. For residential development, the types of development have been combined into two types namely, bungalow/semi-d and link houses/apartment with higher ARI. of 5 and 10 years for minor systems compared to 2, 5 and 10, respectively, for low, medium and high density residential classifications in the first edition. For major system, the ARI. has increased to mostly 100 years compared with up to 100 years in the first edition. 3. In the first edition, for commercial, business and industrial are grouped according to whether these are located in CBD or non-cbd areas. But in the second edition, these are divided into: commercial and business centers, industry, and institutional building/complex with ARI. of 10 for minor system compared to 5 for non-cbd in the first edition. For major system, the ARI. is fixed at 100 years in the Second edition compared to up to 100 in the first edition. 4. The term open space in the first edition has been replaced by sport fields in the second edition. The ARI. for minor system is now 2 years compared to 1 year previously, while the ARI. for major system has reduced to 20 years from up to 100 years previously. Interestingly, this is the only reduction in ARI. in the second edition. 5. There is a new category called infrastructure/utility in the new publication with ARI. of 5 and 100 years for minor and major systems, respectively. 2.4 Evaluation In summary, the major changes are as follows: 1. For Major Systems, the ARI. for most types of development is fixed at 100 year ARI. in MSMA (2011) from up to 100 year in MSMA (2000). 6

7 2. MSMA (2011) has eliminated the subjectivity in the determination of ARI for major system via cost benefit analysis by the engineer. 3. For minor systems, the ARI has increased from 2 to 5 years to 10 years for low and medium density residential developments and commercial, business and industrial development in non-cbd areas. 4. For parks and sport fields, the ARI for major system has reduced to 20 years from up to 100 years previously. This reflects D.I.D s effort in promoting the use of these amenities for storage. 5. The effect of changes in design ARI on storm intensities is covered in the following case study. Type of Development Table 2.1 Design Storm ARIs for Urban Stormwater System Adoption (MSMA, 2000) Average Recurrence interval (ARI) of Design Storm (Year) Quantity Quality Minor System Major System 1 Up to month ARI. (for all types Open Space, Parks and Agricultural Land in urban areas Residential: - Low density 2 Up to 100 of development) - Medium density 5 Up to High density 10 Up to 100 Commercial, Business and Industrial- Other than CBD Commercial, Business, Industrial in Central Business District (CBD) areas of Large Cities Source: Table 4.1 of MSMA (2000) 5 Up to Up to 100 Table 2.2 Design Storm ARI Adoption (MSMA, 2011) Type of Development Minimum Average Recurrence interval (ARI) of Design Storm (Year) Residential Minor System Major System - Bungalow and Semi-D Link Houses/Apartment Commercial and Business Centers Industry Sport Fields, Parks and 2 20 Agricultural Land Infrastructure/utility Institutional Building/Complex

8 Source: Table 1.1 of MSMA (2011) 2.5 Case Study on Design ARI In this case study, the changes in the design ARI. on rainfall intensities is assessed. Using the design storm ARI. for the old and new procedures, the rainfall intensities for both minor and major systems are compared. The quantum of increase is assessed. The location of the study is in Sg. Batu, Kuala Lumpur Methodology 1. The ARI for three types of landuses: park, medium density residential and commercial area were determined based on MSMA (2000) and MSMA (2011) as shown in Table 2.3 and plotted in Figure 2.1 and Figure 2.2, respectively, for minor and major systems. 2. For park, the ARI have changed from 1 and <100 for minor and major systems to 2 and 20 years for minor and major systems, respectively. 3. For medium density residential and commercial area, the ARI have increased from 5 and <100 for minor and major systems to 10 and 100 years for minor and major systems, respectively. 4. The ARI for <100 year for MSMA (2000) is assumed to be 50 year. 5. The minor and major storm intensities for MSMA (2000) and MSMA (2011) computed and summarized as shown in Table Evaluation To compare the increase in storm intensity, a ratio R is defined as follows: i2 R i 1 where i 2 is the storm intensity based on MSMA (2011) i 1 is the storm intensity based on MSMA (2000) The ratio R is tabulated as shown in the table. 1. The ratio R has increased by up to 110% for minor system and up to 103% for major system for the first type of landuse i.e., park. This increase in design storm intensity was due to higher IDF data in MSMA (2011), which negates the effect of the reduction of ARI in the new guideline to 20 year. 2. For the second and third types of landuses i.e., medium density residential and commercial and city area, the ratio R has increased up to 122% for minor system for an ARI increase from 5 to 10 years, and up to 133% for major system for an ARI increase from 50 year to 100 years. 3. It is emphasised that the changes in the storm intensity is not only due to changes in the ARI but also the higher IDF data in MSMA (2011). For changes in IDF data between MSMA (2000) and (2011), please refer to the case study on Design Storm. 4. Due to the linear nature of the discharge and storm intensity in the Rational Method, it is expected the same proportional increase in the design discharge is observed. 8

9 Storm Intensity (mm/hr) 5. This case study only serves to determine the changes in storm intensities with changes in ARI. It is not suggesting that all medium density residential and commercial and city areas are currently designed for a 50 years ARI for major system. Table 2.3 Effect of Changes in ARI for Various Landuses on the Storm Intensity for Major and Minor System for Sg Batu, Kuala Lumpur Landuse ARI (Minor) ARI (Major) ARI (Minor) ARI (Major) i (Minor) i (Major) i (Minor) i (Major) R (Minor) R (Major) MSMA (2000) MSMA (2011) MSMA (2000) MSMA (2011) Park 1 < Medium Density Residential Commercial and City Area 5 < < Note1: i in mm/hr for duration of 60 minutes Note 2: ARI for <100 year is assumed to be 50 year Figure 2.1 Effect of Changes in ARI for Various Landuses on the Storm Intensity for Minor System for Sg. Batu, Kuala Lumpur Park Medium Density Residential Development Types Commercial and City Area i (Minor) (MSMA, 2000) i (Minor) (MSMA, 2011) Figure 2.2 Effect of Changes in ARI for Various Landuses on the Storm Intensity for Major System for Sg. Batu, Kuala Lumpur 9

10 Storm Intensity (mm/hr) Park Medium Density Residential Development Types Commercial and City Area i (Major) (MSMA, 2000) i (Major) (MSMA, 2011) 3. Changes in Design Storm, Temporal Pattern and Areal Reduction Factor 3.1 Design Storm Computation Evolution of Methods of Computation for Design Storm With the publication of second edition of MSMA, Chapter 2 of MSMA (2011) now supersedes Chapter 13 of MSMA (2000). In this section, the theories of design storm in both editions of MSMA (2000 and 2011) are covered Derivation of IDF Curves using MSMA (2000) In the second edition, the following polynomial equation (Equation 13.2 in MSMA, 2000) has been fitted to the published IDF curves for the 35 major urban centres in Malaysia: ln( R I t ) 2 3 a b ln( t) c (ln( t)) d (ln( t)) (Equation 3.1) where R I t is the average rainfall intensity (mm/hr) for ARI R and duration t R is the average return interval (years) t is the duration (minutes) a to d are fitting constants dependent on ARI. The fitted coefficients for the IDF curves for all the major cities are given in Appendix 13.A of MSMA (2000). Equation 3.1 is strictly applicable to rainfall duration of 6 hours or less. For short duration of less than 30 minutes in MSMA (2000), the intensities are computed as follows: The design rainfall depth P d for a short duration d (min) is given by: 10

11 P d P 30 FD ( P60 P30 ) (Equation 3.2) where P 30 and P 60 are the 30 min and 60 min rainfall depths, respectively, obtained from the published polynomial curves. F D is the adjustment factor for storm duration based on Table 13.3 and Figure 13.3 of MSMA (2000) Derivation of IDF Curves using MSMA (2011) In MSMA (2011) (Equation 2.2), the following empirical equation was fitted to the IDF data for 135 major urban centres in Malaysia: T i d (Equation 3.3) where i is the Average rainfall intensity (mm/hr) T is the Average return interval (years) for ARI of between 0.5 and 12 months and 2 and 100 years. d is the Storm duration (hours) where d is between and 72 hours, κ, θ and η are the fitting constants dependent on the raingauge location. Refer Table 2.B1 in Appendix 2.B of MSMA (2011) Comparison The following changes were noted: 1. In the Second Edition, the formula for computing the IDF data has changed from a polynomial based formula to an empirical equation. 2. The storm intensities have changed due to the changes in the formula used. 3. In the first edition, the data used were up to about 1983 or For instance, the data used for the Federal Territory was only up to 1983 in MSMA (2000). However, in the Second Edition, the data used were more up-to-date. 4. In the first edition, the IDF data were available only for 35 major urban centers. In the second edition, however, this has been increased to 135 major urban centers in Malaysia. 5. In MSMA (2000) the IDF formula is applicable for storm duration of 30 minutes to 6 hours, whereas in MSMA (2011), the formula is applicable between 5 min and 72 hours. In MSMA (2000), for duration of less than 30 minutes, a short duration formula is required. 6. In MSMA (2000) the storm ARI is available for 2 to 100 years, whereas in MSMA (2011), it is available for 2 to 100 years, plus 0.5 to 12 months. 7. IDF curves were plotted in Annex 3 of MSMA (2011) for the 135 major urban centers for ARI. from 2 to 100 years and duration of 5 min to 72 hours. However, these were not provided for ARI of between 0.5 to 12 months. So it is necessary to compute them. 11

12 8. In MSMA (2000) the whole of Kuala Lumpur is represented by one IDF curve. But in MSMA (2011), it involves 14 stations covering different parts of Kuala Lumpur. The same is noted for the stations in all states. For example, in Selangor there are now ten stations. 9. MSMA (2011) covers the IDF data of 12 states and federal territory in Peninsular Malaysia. Sabah and Sarawak are not covered. In MSMA (2000), the two East Malaysian states are covered Evaluation 1 Overall, the quality of the storm data in MSMA (2011) is better as the new data is more up-to-date. 2 The IDF data in MSMA (2011) covers longer storm durations from 5 minutes to 72 hours, and the lower range ARI of 0.5 to 12 months. 3 There are now 135 stations in MSMA (2011) compared to only 35 previously. 4 IDF curves are plotted in Annex 3 of MSMA (2011) for 135 major urban centres. 5 No IDF data is provided for East Malaysian states of Sabah and Sarawak. 6 The changes in the IDF data is expected to change the magnitudes of design storm. 7 The magnitude of changes in the design rainfall is covered in the following case study. 3.2 Storm Temporal Pattern This is covered in Chapter 13 of the first edition and Chapter 2 of the second edition Temporal Pattern in MSMA (2000) In MSMA (2000), the temporal pattern is covered in Section 13.3 of Chapter 13. Table 3.1 (Table 13.4 of MSMA, 2000) gives the recommended time steps for durations of up to 360 minutes. Appendix 13.B gives the design temporal patterns for East and West Coast of Peninsular Malaysia. For east Malaysia, it recommends the use of temporal patterns for East Coast of Peninsula. Table 3.1 Standard Durations for Urban Stormwater Drainage Standard Duration (minutes) No. of Time Intervals Time Interval (minutes)

13 3.2.2 Temporal Pattern in MSMA (2011) In MSMA (2011), the temporal patterns to be used for a set of durations are given in Appendix 2.C for the following five regions: Region 1- Terengganu and Kelantan Region 2- Johor, Negeri Sembilan, Melaka, Selangor and Pahang Region 3- Perak, Kedah, Pulau Pinang and Perlis Region 4- Mountainous Area Region 5- Urban Area (Kuala Lumpur) Table 3.2 (Table 2.4 of MSMA, 2011) provides the recommended time intervals for the above design rainfall temporal pattern. Table 3.2 Recommended Intervals for Design Rainfall Temporal Pattern (Table 2.4 in MSMA, 2011) Storm Duration (minutes) Time Interval (minutes) < > Evaluation 1 MSMA (2011) provides the temporal pattern for storm duration of up to 72 hour compared to MSMA (2000) at only 6 hour. 2 MSMA (2000) divides the temporal pattern for east and west cost of Peninsular Malaysia. MSMA (2011), on the other hand, divides the whole peninsula into five regions as described above. 3 In MSMA (2011), no mention of temporal pattern for East Malaysia- but in MSMA (2000), it is recommended that the temporal pattern for East Coast of Peninsula be used for Sabah and Sarawak. 4 MSMA (2011) recommends smaller time intervals. 3.3 Areal Reduction Factor Areal reduction factor (ARF) is given in Table 13.1 of MSMA (2000) but not in MSMA (2011). Literature in hydrology state that ARF should be applied to convert point intensity to catchment average and it is not correct to ignore ARF for larger catchments. Hence the following procedure as given in MSMA (2000) should be applied for MSMA (2011): The IDF curves give the rainfall intensity at a point. For larger catchment, the uneven spatial distribution of a storm is important. Areal reduction factors are applied to design point rainfall intensities to account for the fact that it is not likely that rainfall will occur at the same intensity over the entire catchment area of a storm. The point estimates of design storms are adjusted for the catchment area by following the procedure recommended in HP1 (DID, 1982), which is similar to the United States Weather Bureau's method. 13

14 The design rainfall is calculated from the point rainfall intensity as follows (Equation 13.1 in MSMA, 2000): I c F I (Equation 3.4) p where F is the areal reduction factor which is expressed as a factor less than 1.0. I c is the average rainfall over the catchment, and I p is the point rainfall intensity. The values of F for catchment areas of up to 200 km 2 and durations of up to 24 hours are given in Table 3.3 and Figure 3.1 below (Table 13.1 and Figure 13.1 of MSMA 2000, respectively). Note that the range of applicability is limited to catchment areas of up to 200 km 2 only. Table 3.3 Areal Reduction Factors Figure 3.1 Plot of Areal Reduction Factors 14

15 3.4 Case Study on Design Storm The design storm estimates are compared using the IDF formulas from the first and second edition for a major urban center in Malaysia. The objective is to determine the changes in design rainfall due to differences in the IDF formulas. The urban center selected in the case study is Kuala Lumpur Methodology 1. The IDF curves were computed using Equation 3.1 for Kuala Lumpur for duration of more than 30 minutes as tabulated in Table 3.4 and plotted as shown in Figure For duration of less than 30 minutes, the short duration curve of Equation 3.2 was applied. The results for 5 and 15 minutes are tabulated as shown in Table 3.5 and Table 3.6, respectively. 3. Equation 3.3 was applied to the 14 stations in Kuala Lumpur (Table 2.B1) (see Table 3.9). The results for Station No was tabulated as shown in Table 3.7 and plotted as shown in Figure 3.3 for ARI of 2 to 100 years and 0.5 to 12 months. 4. Table 3.8 is a summary of the storm intensities for ARI of 100 years for Kuala Lumpur based on MSMA (2000) and the 14 stations in MSMA (2011). 5. Figure 3.4 to Figure 3.9 are plots of the IDF data for MSMA (2000) and the 14 stations in MSMA (2011) for ARI of 100, 50, 20, 10, 5 and 2, respectively. It shows the scattering of values above and below the MSMA (2000) curve Evaluation The results from above are evaluated as follows: 1 Lower half of Table 3.8 summarises the ratios of the design storms for MSMA (2011) to MSMA (2000) for ARI of 100 years. 2 It is noted the design storms estimated using MSMA (2011) scattered on both sides of the IDF curve using MSMA (2000). 3 It can be seen that for shorter durations, the design storms for MSMA (2011) can be 26% (Station 13) higher than the estimate based on MSMA (2000). 4 For long duration of say 72 hours, the reverse is true: the MSMA (2011) estimates can be up to 36% (Station 6) lower than those using MSMA (2000). 5 For medium durations of between 15 to 700 min, the estimates using MSMA (2011) were mostly higher than those estimated using MSMA (2000). In the study, out of 14 stations, 10 of them (or 71%) were higher than the MSMA (2000) curve, while the remaining 4 stations (or 29%) were lower than the first edition estimates. 6 It is concluded that the design storms estimated based on MSMA (2011) for Kuala Lumpur can be up to about 26% higher than MSMA (2000) for duration below 700 minutes, for 71% of the stations. 7 Each state has about a dozen stations with different IDF constants as shown in Appendix 2.B. There is a need to know which of the dozen or so stations to use in your design. In Kuala Lumpur, for instance, there are 14 stations- but none of the station names appeared familiar. 8 MSMA (2011) does not cover Sabah and Sarawak like in MSMA (2000). 15

16 Table 3.4 IDF for Kuala Lumpur (MSMA 2000) ARI a b c d LN(T) Table 3.5 Short Duration IDF for Kuala Lumpur (Duration= 5 min) (MSMA 2000) ARI a b c d LN(T) tc (min) P30 (mm) P60 (mm) FD Pd (mm) Id (mm/hr) Table 3.6 Short Duration IDF for Kuala Lumpur (Duration= 15 min) (MSMA 2000) ARI a b c d LN(T) tc (min) P30 (mm) P60 (mm) FD Pd (mm) Id (mm/hr)

17 Table 3.7 IDF Data for Kuala Lumpur (Station No ) (MSMA 2011) Location:3 Ibu Pejabat JPS 1 Station No: Duration (min): ARI (T) YR ARI (T) MTλ (lambda) κ (kappa) θ (theta) η (eta) Table 3.8 Summary of IDF Data for Kuala Lumpur (MSMA, 2000) and 14 Stations in Kuala Lumpur (MSMA 2011) for ARI of 100 YR KL Duration (min) ARI (T) YR Duration (hr) Stn I (mm/hr) (MSMA, 2000) (A) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn I (mm/hr) (MSMA, 2011) (B) Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A Stn B/A NB. Station 0 denotes the Kuala Lumpur station used in MSMA (2011). For Stations 1 to 14, refer to Table 3.9 for Station ID and Name 17

18 . Table 3.9 Summary of Stations in Kuala Lumpur (After Table 2.B1 in MSMA, 2011) Station No. Station ID Station Name Puchong Drop, Kuala Lumpur Ibu Pejabat JPS Ibu Pejabat JPS SK Taman Maluri Ladang Edinburgh Kg. Sg. Tua SK Jenis Keb, Kepong Ibu Bek. KM16, Gombak Emp Genting Kelang Ibu Bek. KM11, Gombak Kg. Kuala Seleh, H. Klg Kg. Kerdas, Gombak Air Terjun, Sg Batu Genting Sempah 18

19 Rainfall Intensity (mm/hr) INTENSITY (MM/HR) Figure 3.2 IDF for Kuala Lumpur (MSMA 2000) IFD CURVE FOR KUALA LUMPUR ( ) (MSMA 2000) DURATION (MINUTES Figure 3.3 IDF For Kuala Lumpur (MSMA 2011) (Station No ) Rainfall Intensity Frequency Duration Curve for KL (Station No: ) (MSMA, Storm Duation (min) 0.05 YR (0.5 MTH) 0.5 YR (6 MTH) 1 YR (12 MTH) 2 YR 5 YR 10 YR 20 YR 50 YR 100 YR 19

20 Rainfall Intensity (mm/hr) Figure 3.4 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =100 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (A.R.I. =100 YR) 1000 Rainfall Intensity (mm/hr) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) Figure 3.5 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =50 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur Between MSMA 2000 & 2011 (A.R.I. =50 YR) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) 20

21 Rainfall Intensity (mm/hr) Figure 3.6 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =20 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur Between MSMA 2000 & 2011 (A.R.I. =20 YR) 1000 Rainfall Intensity (mm/hr) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) Figure 3.7 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =10 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur Between MSMA 2000 & 2011 (A.R.I. =10 YR) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) 21

22 Rainfall Intensity (mm/hr) Figure 3.8 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =5 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur Between MSMA 2000 & 2011 (A.R.I. =5 YR) 1000 Rainfall Intensity (mm/hr) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) Figure 3.9 Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur between MSMA 2000 & 2011 (ARI. =2 YR) Comparison of Estimated Rainfall Intensity Frequency Duration Curves for Kuala Lumpur Between MSMA 2000 & 2011 (A.R.I. =2 YR) Storm Duration (min) 0 (MSMA 2000) 1 (MSMA 2011) 2 (MSMA 2011) 3 (MSMA 2011) 4 (MSMA 2011) 5 (MSMA 2011) 6 (MSMA 2011) 7 (MSMA 2011) 8 (MSMA 2011) 9 (MSMA 2011) 10 (MSMA 2011) 11 (MSMA 2011) 12 (MSMA 2011) 13 (MSMA 2011) 14 (MSMA 2011) 22

23 4. Changes in the Rational Method Rational Method is covered in Chapter 14 of the first edition and Chapter 2 of the second edition. 4.1 Rational Method in MSMA (2000) MSMA relates the peak discharge to the rainfall intensity and catchment area via the Rational Method: y C I t A Q y 360 (Equation 4.1) where Q y C y I t A is the y year ARI peak discharge (m 3 /s) is the dimensionless runoff coefficient is the average intensity of the design rainstorm of duration equal to the time of concentration t c and of ARI of y year (mm/hr) is the drainage area (ha) Recommended values of C may be obtained from Design Chart 14.3 for urban areas and Design Chart 14.4 of MSMA (2000) for rural areas. The steps of computation are shown in Figure

24 Figure 4.1 Steps of Computation in the Rational Method in MSMA (2000) 24

25 4.2 Rational Method in MSMA (2011) In MSMA (2011), the peak discharge is related to the rainfall intensity and catchment area via the Rational Method: C i A Q 360 (Equation 4.2) where Q is the peak flow (m 3 /s) C is the runoff coefficient given in Table 4.1 (Table 2.5 of MSMA, 2011). I is the average rainfall intensity (mm/hr) A is the drainage area (ha) The steps of computation are shown in Figure Evaluation The changes in design discharge using the Rational Method 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 the P&DP No. 1 (DID, 1975), as shown in Table 4.1 (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 magnitude of changes in the design discharge is covered in the following case study. 25

26 Figure 4.2 Steps of Computation in the Rational Method in MSMA (2011) Calculate T c Calculate I Calculate C Table 2.5 (MSMA, 2011) Calculate Q p 26

27 Table 4.1 Recommended Runoff Coefficients for Various Landuses (DID, 1980; Chow et al., 1988; QUDM, 2007 and Darwin Harbour, 2009) (After Table 2.5 of MSMA, 2011) Landuse Residential Bungalow Semi-detached Bungalow Link and Terrance House Flat and Apartment Condominium For Minor System ( 10 year ARI) Runoff Coefficient (C) For Major System (>10 year ARI) Commercial and Business Centres Industrial Sport Fields, Park and Agriculture Open Spaces Bare Soil (No Cover) Grass Cover Bush Cover Forest Cover Roads and Highways Water Body (Pond) Detention Pond (with outlet) 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. 4.4 Case Study on Rational Method The Rational Method for the second edition has changed from the first edition. For comparison, the method is applied to a typical catchment and the results compared. The changes in the design discharge due to changes in the runoff coefficient C are assessed. In this case study, the Rational Methods in both editions of MSMA are applied to compute the peak discharge for a major system in the study area. Figure 4.3 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 m Three types of landuses were studied: Park 27

28 Semi-D Houses Commercial and city area Rational Method (MSMA, 2000) The three types of landuses were studied according to Table 2.1 (Table 4.1 of MSMA, 2000): Park, ARI= 20 years Semi-D Houses, ARI= 50 years Commercial and city area, ARI= 100 years Step 1- Calculate T c Overland flow time (T o ) is estimated using Friend s Formula: 1 / n L t o 0.2 S where n= from (Table 14.2 of MSMA, 2000) for paved surface S= 0.3% L (Overland sheet flow path length in m) = 300 m. Applying the Friend s Formula, T o = 10 min. Average velocity in the open drain is assessed using Manning s Equation where V is found to be 1 m/s. T d =L/V= 600/1= 600 s= 10 min. Hence, T c = T o + T d = = 20 min Step 2- Calculate I The values of the coefficients for a, b, c and d in (Table 13.A1 of MSMA, 2000) for ARI of 100 years for Kuala Lumpur are as follows: a= , b= , c= , d= Substituting the above coefficients into: ln( R I t ) a b ln( t) c (ln( t)) 2 d (ln( t)) 3 For t= 30 min, 5 I 30 = mm/hr For t= 60 min, 5 I 60 = mm/hr Convert to rainfall depths, 100 P 30 = 172.2/2 = mm 100 P 60 = 110.2/1 = mm 28

29 Step 3- Calculate C According to MSMA (2000), the design rainfall depth P d for a short duration d (min) is given by: P d P F 30 D ( P60 P30 ) where P 30 and P 60 are the 30 min and 60 min rainfall depths, respectively, obtained from the published polynomial curves. F D is the adjustment factor for storm duration based on Table 13.3 of MSMA (2000). From Figure 13.3 (MSMA, 2000) 2 P 24h = 100 for Kuala Lumpur. From Table 13.3 (MSMA, 2000) for a duration of 20 min, the F D =0.47. Hence 100 P 20 = *( )= 74.8 mm Therefore 100 I 20 = mm/hr 50 I 20 = mm/hr 20 I 20 = mm/hr The C is determined from Design Chart 14.3 (MSMA, 2000), for the following landuses: Park (Curve No. 7), C=0.61 Semi-D Houses (Curve No. 3), C=0.9 Commercial and city area (Curve No. 2), C=0.905 Step 4- Calculate Q p The peak discharge for ARI=100 years is computed using the Rational Method: y C I t A Q y 360 The peak discharges are determined for the three types of landuses: Park (Curve No. 7), ARI= 20 years Q p = 0.61*185.2*30/360 = 9.4 m 3 /s Semi-D Houses (Curve No. 3), ARI= 50 years Q p = 0.9*203.6*30/360 = 15.3 m 3 /s Commercial and city area (Curve No. 2), ARI= 100 years Q p = 0.905*224.3*30/360 = 16.9 m 3 /s The computations were carried out on a spreadsheet and tabulated as shown in Table

30 4.4.2 Rational Method (MSMA, 2011) The three types of landuses were studied according to Table 1.1 of MSMA (2011): Park, ARI= 20 years Semi-D Houses, ARI= 50 years Commercial and city area, ARI= 100 years The catchment data are the same as the previous case study using MSMA (2000). Step 1- Calculate T c The storm duration is the same as the time of concentration of 20 min as determined earlier. Step 2- Calculate I For the study area of Sg. Batu, the following fitting constants were taken from Table 2.B1 of MSMA (2011):, κ, θ and η= , 0.162, and Substituting the above into the following equation: T i 279.4mm / hr d 20 / For ARI= 50 years, i= mm/hr For ARI= 20 years, i= mm/hr Step 3- Calculate C The C is determined from Table 3.2 of MSMA (2011) for the following landuses: Park, C=0.4 Semi-D Houses, C=0.75 Commercial and city area, C=0.95 Step 4- Calculate Q p The peak discharges are determined for the following three types of landuses: For Park, ARI= 20 years Q p = 0.4*215.3*30/360 =7.2 m 3 /s For Semi-D Houses, ARI= 50 years Q p = 0.75*249.7*30/360 = 15.6 m 3 /s For Commercial and city area, ARI= 100 years Q p = 0.95*279.4*30/360 = 22.1 m 3 /s The computations were carried out on a spreadsheet and tabulated as shown in Table

31 4.5 Evaluation Table 4.3 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: A Q p2 R B Q where p1 A= Q p2 which is the peak discharge based on MSMA (2011) B= Q p1 which is the peak discharge based on MSMA (2000) The ratio R is tabulated as shown in the last column of the table. 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 m 3 /s.the C has reduced from 0.9 to 0.75 but the i has increased from mm/hr to 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 m 3 /s. The C has increased from to 0.95 while the storm intensity has increased from mm/hr to 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 storm intensity in MSMA (2011), and by the higher C for commercial and city area 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 the case study for storm, it was found that 71% of these stations have higher storm intensities under MSMA (2011). 6. In general, 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. 31

32 Catchment Area= 30 hectares Lo Ld Rive Figure 4.3 Catchment Map 32

33 Table 4.2 Computation of Peak Discharges using the Rational Method in MSMA (2000) ARI Calculate Tc Using Friends Formula>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Design Chart 14.3 LN(T) a b c d n Lo (m) S (%) to (min) Ld (m) Vd (m/s) td (min) tc (min) P30 (mm) P60 (mm) FD Pd (mm) Id (mm/hr) C A (ha) Qp (m3/s) Type Curve No Park Semi-D Commercial 2 Table 4.3 Computation of Peak Discharges using the Rational Method in MSMA (2011) Location: Station Name: Air Terjun, Sg Batu Duration (min): 20 A (ha) C (Table 2.5) Landuse Q (m3/s) (A) ARI (T) YR λ (lambda) κ (kappa) θ (theta) η (eta) 0.3 MSMA (2011) park Semi-D Commercial 22.1 Table 4.4 Comparison of Peak Discharges using the Rational Method in MSMA (2000, 2011) Landuse Q (m3/s) (A) Q (m3/s) (B) A/B MSMA (2011) MSMA (2000) park Semi-D Commercial

34 5. Changes in On-Site Detention 5.1 OSD Sizing using MSMA (2000) Theory In MSMA (2000), the method of estimating Permissible Site Discharge (PSD) and Site Storage Requirement (SSR) is the Swinburne Method developed at the Swinburne University of Technology in Melbourne, Australia. The method is basically site-based, but considers the position of a site within the catchment. Refer to Figure 5.1, the peak flow time of concentration from the top of the catchment to the development site, t cs, is compared to the total time of concentration for the catchment, t c. The PSD varies with this ratio and may be less than or greater than the peak pre-development site discharge depending on the position of the site within the catchment. The method uses the Rational Method to calculate site flows, and utilizes a nondimensional triangular site hydrograph based on the triangular design storm method as shown in Figure 5.2. The site discharges are calculated using the total catchment time of concentration t c (not the time of concentration to the development site) for the design storm ARI under consideration as shown in Figure 5.1. Figure 5.1 Relationship Between tc and tcs for the Swinburne Method msma_drquek7.docx 34

35 Figure 5.2 Swinburne Method Assumptions tf= Time for Storage to Fill Permissible Site Discharge (PSD) The PSD is the maximum allowable post-development discharge from a site for the selected discharge design storm and is estimated on the basis that flows within the downstream stormwater drainage system will not be increased. PSD is dependent on the following criteria: The time of concentration of the catchment to its outlet, or a point of concern either within or downstream of the catchment. The position of the site, time-wise from the uppermost reach of the catchment. The original or adopted ARI of the public drainage system within the catchment and rainfall data. The area of the development site. The proportion of impervious area of the development site. The type of OSD storage facility. The extent of development or redevelopment within the catchment. Local and/or regional drainage policies. The Permissible Site Discharge (PSD) for the site in l/s is given by (Equation 19.1 of MSMA, 2000): 2 a a 4b PSD (Equation 5.1) 2 The factors a and b are different for above-ground and below-ground storages due to differences in storage geometry and outflow characteristics. For above-ground storage: Q Q a p a tc 0.75tc 0. 25tcs (Equation 5.2) tc Qa msma_drquek7.docx 35

36 b 4Q Q a p (Equation 5.3) For below-ground storage: Q Q a p a tc 0.35tc 0. 65tcs (Equation 5.4) tc Qa b Q Q a p (Equation 5.5) where t c is Peak flow time of concentration from the top of the catchment to a designated outlet or point of concern (min) t cs is peak flow time of concentration from the top of the catchment to the development site (min) Q a is the peak post-development flow from the site for the discharge design storm with a duration equal to t c (l/s) Q p is the peak pre-development flow from the site for the discharge design storm with a duration equal to t c (l/s) Site Storage Requirement (SSR) The SSR is the total amount of storage required to ensure that the required PSD is not exceeded and the OSD facility does not overflow during the storage design storm ARI. As stated earlier, the storage design storm for estimating the SSR is 10 year ARI. In sizing the volume of the storage facility, the method assumes a triangular inflow hydrograph and an outflow hydrograph shape related to the type of storage adopted. These simplifications are acceptable providing the site catchment is small. Typically, the critical storm duration that produces the largest required storage volume is different from the time of concentration used for peak flow estimation. Therefore, storage volumes must be determined for a range of storm durations to find the maximum storage required as shown in Figure 5.3. msma_drquek7.docx 36

37 Figure 5.3 Typical Relationship of Storage Volume to Storm Duration The Site Storage Requirement (SSR) for the site in m 3 formula: is calculated using the SSR 0. 06t Q c d (Equation 5.6) d d The factors c and d are different for above-ground and below-ground storages due to differences in storage geometry and outflow characteristics. For above-ground storage: PSD c PSD (Equation 5.7) Qd d 2 PSD (Equation 5.8) Q d For below-ground storage: PSD c PSD (Equation 5.9) Qd d 2 PSD (Equation 5.10) Q d where msma_drquek7.docx 37

38 t d = selected storm duration (min) Q d = the peak post-development flow from the site for a storm duration equal to t d (l/s) OSD Sizing Procedure A simplified design procedure for determining the required volume of detention storage is as follows (see Figure 5.4): 1. Select storage type(s) to be used within the site, i.e. separate above and/or below-ground storage(s), or a composite above and below-ground storage. 2. Determine the area of the site that will be drained to the OSD storage system. As much of the site as possible should drain to the storage system. 3. Determine the amount of impervious and pervious areas draining to the OSD storage system. 4. Determine the times of concentration, t c and t cs. 5. Calculate the pre and post-development flows, Q p and Q a, for the area draining to the storage for the discharge design storm with time of concentration t c. 6. Determine the required PSD for the site using Equation 5.1 for the discharge design storm. 7. Determine the required SSR for the site using Equation 5.6 for the storage design storm over a range of durations to determine the maximum value. msma_drquek7.docx 38

39 Discharge/Storage Design Storm Determination of Impervious & Pervious Areas Determination of t c and t cs Determination of Pre & Post Development Flows Determination of PSD Determination of SSR Design OSD Figure 5.4 Steps of Computation in OSD Design in MSMA (2000) msma_drquek7.docx 39

40 5.2 OSD Sizing using MSMA (2011) Limiting Catchment Areas for OSD in MSMA (2011) Table 5.1 lists the limiting catchment areas for OSD in MSMA (2011). OSD is to be used for areas less than 5 ha. For areas above 5 ha, the use of detention pond is required. Table 5.1 Limiting Catchment Areas for OSD or Dry/Wet Detention Pond in MSMA (2011) Type of Storage Facility Limiting Area (ha) Individual OSD 0.1 Community OSD >0.1, 5 Dry Detention Pond 5 to 10 Wet Detention Pond > Method for OSD Design in MSMA (2011) Below are the steps involved in OSD design based on MSMA (2011). 1. Figure 5.A1 (MSMA, 2011) divides peninsula into 5 design regions. 2. Determine project area, terrain steepness, and percentage imperviousness. 3. Table 5.A1 gives the maximum permissible site discharge (PSD) and minimum Site Storage Requirement (SSR) values in accordance with the five regions in Peninsular Malaysia. 4. Table 5.A2 gives the maximum permissible site discharge (PSD), minimum Site Storage Requirement (SSR) and inlet values in accordance with the major towns in Peninsular Malaysia. 5. Adopt smaller PSD value from Table 5.A1 and 5.A2 for subsequent sizing of outlet pipe. 6. Table 5.A3 gives the OSD volume, inlet size and outlet size for 5 different regions in Peninsular Malaysia. 7. Table 5.A4 gives the discharge and pipe diameter relationship for low lying, mild and steep slopes. 8. Adopt the SSR is the larger from Table 5.A1 and 5.A2. 9. Sizing of OSD tank based on the SSR. 10. Adopt inlet pipe: Inlet pipe is the smaller of Table 5.A3 and 5.A Adopt outlet pipe: Outlet pipe is the smaller of Table 5.A3 and 5.A4. msma_drquek7.docx 40

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