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Figure 1. Hydrologic cycle
Drainage and Design Flood
Urban Stormwater Management Manual for Malaysia
Factors influencing volume and rate of runoff are soil, topography
and vegetation characteristics
Hydrological Procedure: non-urban situations e.g.: dam, river engineering
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Flash floods in Malaysia
Johor, Jan 2007
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Flash floods in Malaysia
Kota Tinggi, April 2007
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Flash floods in Malaysia
Kuala Lumpur, 2008
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Flash floods in Malaysia
Kuala Lumpur, 2008
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Flash floods in Malaysia
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Procedures and guidelines for urban drainage system designs in
Malaysia
Urban Stormwater Management Manual for Malaysia
(MSMA - Manual Saliran Mesra Alam)
Published by the Department of Irrigation and Drainage Malaysia (DID)
in 2000
Reduce drainage-related issues such as flash flood, excessive sedimentoutflow and water quality deterioration generated by any land opening
activities in the development project boundaries
All urban drainage systems design should be complied to the Urban
Stormwater Management Manual for Malaysia (MSMA - Manual Saliran
Mesra Alam)
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Urban Stormwater Management Manual for Malaysia
(MSMA - Manual Saliran Mesra Alam)
48 chapters
www.msmam.com
20 volumes
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MSMA - Control at Source
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Estimation of Peak Flow for a Single Sub-catchment using Rational method
Post-development peak flow Pre-development peak flowfrom the outlet point of the site to the downstream public drainage system or receiving water
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Design Fundamentals (Volume 4 MSMAM)
Hydrologic events are described by AEP or ARI
AEP - Annual Exceedance Probability P
- probability an event of specified magnitude, or volume and
duration, will be exceeded in a time period
ARI - Average Recurrence Interval Tr
- average length of time between events that have the samemagnitude, or volume and duration.
Example:
A flood with a discharge of 50 m3/s may have an AEP of 0.01, that is on
the average there is a 1% chance that a flow of 50 m3/s will be equalled to
or exceeded in any year.
The ARI isyears100
010
11
.PTr
Hence, a 1% AEP has an ARI of 100 years
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Choo sing a design / event ARI(Volume 2 MSMAM)
Minor system - collect and convey runoff from relatively frequent
storm events to minimise inconvenience and nuisance flooding.
Major system - safely convey runoff not collected by the minordrainage system to waterways or rivers. Major system must protect
the community from the consequences of large, reasonably rare
events, which could cause severe flood damage, injury and even loss
of life.
Note: The definition of major or minor system does not refer to the
size of the drains.
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oRSCV Chezy:
2
1
3
21
oSRn
V Manning:
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Design Rainfal l(Volum e 4 MSMAM)
pAc IFI
For catchment greater than 10 km2, areal reduction factor is required in
the calculation of the design rainfall using the IDF curve.
where,
Ic= average rainfall over the catchment
FA = areal reduction factor
Ip = point rainfall intensity
DID has published the intensity-duration-frequency (IDF) curve in 1991
for 26 and 16 urban areas in Peninsular Malaysia and East Malaysia,
respectively (HP No. 26).
The IDF curves is represented by the following polynomial expression:
where, RIt= the average rainfall intensity (mm/hr) for ARI and duration t
R= average return interval (years)
t= duration (minutes)a to d= fitting constants dependent on ARI
32 lnlnlnln tdtctbaItR for 30 t 1000 mins
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Design Rain fall (Volume 4 MSMAM)
The design rainfall depth Pd for a short duration d(minutes) is given by,
where,
P30, P60 = 30-minute and 60-minute duration rainfall depths, respectively
FD = adjustment factor for storm duration
306030 PPFPP Dd fort< 30 mins
d
PI d
where,
Pd = rainfall depth in mm
d= rainfall duration in hours
Rainfall intensity I
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Example 1: Calculation of 5-minute duration rainfalls
Calculate the 5-minute duration, 20-year ARI rainfall intensity for use in a roof
design in Kuala Lumpur.
Solution:
306030 PPFPP Dd fort< 30 mins
32 lnlnlnln tdtctbaItR for 30 t 1000 mins
323020 30ln0166030ln2796030ln7533097814ln ....I
For Kuala Lumpur, a = 4.9781, b = 0.7533, c=0.2796, d= 0.0166
mm/hr41423020 .I
326020 60ln0166060ln2796060ln7533097814ln ....I
mm/hr3916020 .I
mm71.20.5mm/hr41423020
.P
mm3916020 .P
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For Kuala Lumpur, P24h 100 mm
FD= 2.08
3060305 PPFPP D
271391082271 ....
mm429.
d
PI 5
20
520
605
429.
mm/hr7352.
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Time of Concentration
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.
The time of concentration tc is often considered to be the sum of the time oftravel to an inlet plus the time of travel in the stormwater conveyance system.
doc ttt Time of concentration
where,
to = overland flow or sheet flow travel time (minutes)
td= conveyance system flow travel time (minutes)
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Time of over land sheet f low to
2
1
31
107
S
Lnto
Friend's formula:
where,to = overland sheet flow travel time (minutes)
L = overland sheet flow path length (m)
n = Manning's roughness value for the surface
S = slope of overland surface (%)
For multiple segments,
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Time of channel / pipe f low td
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For small area,A 0.4 hectare:
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Rational method
Rational method is applicable for catchment areaA 80 hectares
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Variat ion o f sub catchment cond i t ions
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Example 2: Rational method calculation (Volume 5 MSMAM)
Determine the design peak flow generated from a minor drainage of medium
density residential area of 10 hectares in Kuala Lumpur. Assume 80 m of
overland flow followed by 400 m of flow in an open drain. Catchment area
average slope = 0.5%. The catchment is shown in Figure below.
Figure. Catchment area
Rational method
suitable for catchment
area < 80 hectares
Estimation of Peak Flow for a Single Sub catchment using Rational method
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Estimation of Peak Flow for a Single Sub-catchment using Rational method
Post-development peak flow Pre-development peak flowfrom the outlet point of the site to the downstream public drainage system or receiving water
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Solution:
1. Design ARI (Table 4.1)
Minor drainage system = 5-year ARI
Major drainage system = 100-year ARI
2. Estimate time of concentration tc
From Design Chart 14.1, Lo = 80 m, S = 0.5%, assume paved surface,
to = 8.5 minutes
Average velocity in the open drain should be assessed using Manning's
equation. Assume V= 1.0 m/s
min761
400.
V
Lt dd
Therefore, tc= to + td= 8.5 + 6.7 15 mins
3. Determine average rainfall intensity
Table 13.A1:
Kuala Lumpur, 5-year ARI, a = 5.1086, b = 0.5037, c= -0.2155, d= 0.0112
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T bl 13 A1
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Table 13.A1:
Kuala Lumpur, 5-year ARI, a = 5.1086, b = 0.5037, c= -0.2155, d= 0.0112
32305 30ln0112030ln2155030ln5037010865ln ....I
mm/hr9117305
.I
32605 60ln0112060ln2155060ln5037010865ln ....I
mm/hr77560
5 .I
mm9850.5mm/hr9117305 ..P
mm775605 .P
For Kuala Lumpur, P24h< 100 mm, FD = 0.8
30603015 PPFPP D
mm54595877580958 .....
mm/hr1826015
545155
155
.
d
PI
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4. Determine runoff coefficient
5. Determine peak flow
Design Chart 14.3, Category 3, C= 0.87
36015
5
155 AICQ
36010182870 .
sm44 3.
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