Urban Stormwater Design

1. URBAN STORMWATER DESIGN
CHAPTER 6

2. FLASH FLOOD

4. 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.

5. •
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

6. 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

7. •
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

8. Gross Pollutant Trap

9. Swale

10. Dry Pond

11. Wetland

12. ESTIMATING PEAK FLOW
•
Chapter 13 – Design Rainfall
•
Chapter 14 – Flow Estimation and
Routing

13. ESTIMATING PEAK FLOW
 Rational Formula

15. 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.

16. Major and Minor System

19. 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

20. • Overland Flow time
Overland flow can occur on either grassed or paved surfaces.
From 14.4.2 Calculation of Flow Time …page 14-2

21. Design Chart 14.1 …page 14-25

22. • Roof Drainage Flow Time
• Kerbed Gutter Flow Time

23. • Channel Flow Time
• Pipe Flow Time

25. 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

26. RAINFALL INTENSITYDURATION-FREQUENCY (IDF)

27.  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)

28.  IDF for SHORT DURATION
Rainfall Intensity (mm/hr)
5 minutes to < 30 minutes
Frequency (year)
10yr
5yr
2yr
30
Duration (minutes)

29.  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)

30. • Polynomial Approximation of IDF Curves ( see 13.2.6)
APPENDIX 13.A FITTED COEFFICIENTS FOR IDF CURVES FOR 35 URBAN
CENTRES …page 13-11

31. • IDF Values for Short Duration Storms (see 13.2.7)

32. • Figure 13.3 Values of 2P24h for use with Table 13.3(page 13-6)

33. • IDF Values for Frequent Storms(see 13.2.8)

35. 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%

36. URBAN DRAINAGE SYSTEM

37.  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)

38.  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-

39. 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.

40. 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.