Chapter 6


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Urban Stormwater Design

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Chapter 6

  3. 3. Past Drainage Practice and issues • In Malaysia, the traditional approach widely practiced to manage storm water design where allow developers to put in drains where appropriate. • The engineers job is only to determine drain size to comply with drainage capacity • Urban drainage practice is based on the 1975 DID Urban Drainage Design Manual, “Planning and design Procedure No 1: Urban Drainage Design Standard For Peninsular Malaysia. • Rapid disposal approach as adopted in this manual has led to increase in the occurrence of flash floods as a result of increase in surface runoff, peak discharge, shorter flow duration and others.
  4. 4. • If the country continues to urbanize, the flood problem continue to increase. (Zakaria & Ainan, 2000) • Due to this problem, Department of Irrigation and Drainage (DID) is taking a proactive step by introducing New Urban Drainage Manual known as Strom Water Management Manual for Malaysia (Manual Saliran Mesra Alam or MSMA) • Effective from 1st January 2001 all new development in Malaysia must comply with this new guideline which control storm water from the aspect of quantity and quality runoff to achieve zero development impact contribution. • This new strategy will give a sustainable solution to mitigate the existing flood problems but it also to prevent the occurrence of such problem in the new area developed
  5. 5. URBAN STORMWATER MANAGEMENT MANUAL FOR MALAYSIA • • • • • The manual has 48 Chapters. It is divided into 9 parts The manual is published in 20 volumes The first 3 parts contain background information on environmental process and storm water management The remaining parts contain detailed information on hydrology and hydraulic, runoff quantity control, sources and treatment runoff quantity control, runoff quality control and special storm water applications
  6. 6. • The main focus of MSMA is to manage storm water instead of draining it away as fast as possible • This manual also considers the current existing problem such as flash flood, river pollution, soil erosion, hill development and etc. • MSMA have a multiple objectives including to: 1. Ensure the safety of the public 2. Control nuisance flooding and provide for the safe passage of less frequent and larger flood events 3. Stabilize the landform and control erosion 4. Optimize the land available for urban development 5. Minimize the environmental impact of urban runoff on water quality 6. Enhance the urban landscape
  7. 7. Gross Pollutant Trap
  8. 8. Swale
  9. 9. Dry Pond
  10. 10. Wetland
  11. 11. ESTIMATING PEAK FLOW • Chapter 13 – Design Rainfall • Chapter 14 – Flow Estimation and Routing
  12. 12. ESTIMATING PEAK FLOW  Rational Formula
  13. 13. Select Design ARI (Average Recurrence Interval)  Design Acceptance Criteria : • The minor system is intended to collect and convey runoff from relatively frequent storm events to minimise inconvenience and nuisance flooding. • The major system is intended to safely convey runoff not collected by the minor drainage system to waterways or rivers.
  14. 14. Major and Minor System
  15. 15. RUNOFF ESTIMATION – refer Chapter 14 – Flow Estimation and Routing • Estimating Time of Concentration, tc  The time of concentration is the flow travel time from the most hydraulically remote point in the contributing catchment area to the point under study. Time of Concentration, tc for small cacthment
  16. 16. • Overland Flow time Overland flow can occur on either grassed or paved surfaces. From 14.4.2 Calculation of Flow Time …page 14-2
  17. 17. Design Chart 14.1 …page 14-25
  18. 18. • Roof Drainage Flow Time • Kerbed Gutter Flow Time
  19. 19. • Channel Flow Time • Pipe Flow Time
  20. 20. DESIGN RAINFALL INTENSITIES – refer Chapter 13 – Design Rainfall • Determine Average Rainfall Intensity, RIt 1. The total storm rainfall depth at a point, for a given rainfall duration and ARI, is a function of the local climate. 2. Rainfall depths can be further processed and converted into rainfall intensities (intensity = depth/duration), which are then presented in IDF curves. 3. Users need to be aware of the limitations of these IDF curves ( see 13.2.4 IDF Curves for Selected Cities and Towns ) 4. Local authorities are advised to find out from the DID to the availability of IDF curves or coefficients for their respective areas, or to obtain local pluviometer data for those wishing to conduct their own analysis
  22. 22.  Design storm defines the rainfall intensity for a given frequency and therefore affects the resulting runoff peak and volume Rainfall Intensity (mm/hr) Runoff peak Frequency (year) 10yr 5yr 2yr Duration (minutes)  Current practice is to select the design storm duration as equal to or longer than the time of concentration for the catchment (or some minimum value when the time of concentration is short)
  23. 23.  IDF for SHORT DURATION Rainfall Intensity (mm/hr) 5 minutes to < 30 minutes Frequency (year) 10yr 5yr 2yr 30 Duration (minutes)
  24. 24.  IDF for FREQUENCY STORM Rainfall Intensity (mm/hr) (Water quality design) 1 month (0.083 yr), 3 month (0.25 yr), 6 month (0.5 yr) and 12 month (1 yr) Frequency (year) 0.5yr 0.25yr 0.083yr 30 Duration (minutes)
  25. 25. • Polynomial Approximation of IDF Curves ( see 13.2.6) APPENDIX 13.A FITTED COEFFICIENTS FOR IDF CURVES FOR 35 URBAN CENTRES …page 13-11
  26. 26. • IDF Values for Short Duration Storms (see 13.2.7)
  27. 27. • Figure 13.3 Values of 2P24h for use with Table 13.3(page 13-6)
  28. 28. • IDF Values for Frequent Storms(see 13.2.8)
  29. 29. Example 1  To determine the design peak flow generated from a minor drainage of medium density residential area of 10 hectares in Kuala Lumpur. Assume 80m of overland flow followed by 400m of flow in an open drain. Catchment area average slope = 0.5%
  31. 31.  Design Storm Table 4.1 Design Storm ARIs for Urban Stormwater Systems Type of Development (See Note 1) Open Space, Parks and Agricultural Land in urban areas Average Recurrence Interval (ARI) of Design Storm (year) Quantity Quality Minor Major System System (see Note 2 and 3) 1 up to 100 Low density 2 up to 100 Medium density 5 up to 100 High density 10 up to 100 5 up to 100 10 up to 100 Residential: Commercial, Business and Industrial – Other than CBD Commercial, Business, Industrial in Central Business District (CBD) areas of Large Cities 3 month ARI (for all types of development)
  32. 32.  Drainage Reserves and geometry C Drainage Reserve 0.5 m min 300mm freeboard Qminor Design flow width + freeboard 1 0.5 m min 4 min 4 min 1 Design flow width + freeboard (a) ' Vee' Shaped (a) Grassed Swale C 300mm freeboard Qminor Drainage Reserve 1.5 m minimum 1.0 m 1 4 min Batter 1 50 50 1 4 min Base 1 Batter Design flow width + freeboard (b) Figure 26.1 Lined Open Drain Reserve Width for Open Drain (b) Trapezoidal Shaped Figure26.2 Sections Recommended Grassed Swale Cross-
  33. 33. 26.2.4 Freeboard The depth of a grassed swale shall include a minimum freeboard of 50 mm above the design storm water level in the swale. 26.2.5 Velocities and Grades  To prevent sedimentation and vegetative growth, the minimum average flow velocity shall not be less than 0.6 m/s.  The maximum average flow velocity shall not exceed 4 m/s. The average flow velocity in a grassed swale shall not exceed 2 m/s. If this is not practical, an underground pipeline, lined open drain, or grass reinforcement system should be provided.
  34. 34. Example Determine the size of a lined rectangular drain tp convey a 5-year ARI minor system design flow from a proposed 3 hectare bungalow development in Kuala Lumpur. The post development time of concentration, tc at the development outlet is estimated to be 20 minutes.