Chapter 6 Design Drainage

7/29/2019 Chapter 6 Design Drainage

1/43

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


2/43

Flash floods in Malaysia

Johor, Jan 2007


3/43


Kota Tinggi, April 2007


4/43


Kuala Lumpur, 2008


5/43


Kuala Lumpur, 2008


6/43



7/43

Procedures and guidelines for urban drainage system designs in

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)


8/43


(MSMA - Manual Saliran Mesra Alam)

48 chapters

www.msmam.com

20 volumes


9/43


10/43

MSMA - Control at Source


11/43

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


12/43

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


13/43

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.


14/43


15/43

oRSCV Chezy:

2

1

3

21

oSRn

V Manning:


16/43

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


17/43


18/43


19/43


20/43


21/43


22/43

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


23/43


24/43

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


25/43

For Kuala Lumpur, P24h 100 mm

FD= 2.08

3060305 PPFPP D

271391082271 ....

mm429.

d

PI 5

20

520

605

429.

mm/hr7352.


26/43

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)


27/43

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,


28/43


29/43


30/43

Time of channel / pipe f low td


31/43


32/43

For small area,A 0.4 hectare:


33/43

Rational method

Rational method is applicable for catchment areaA 80 hectares


34/43


35/43

Variat ion o f sub catchment cond i t ions


36/43

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


37/43

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


38/43

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


39/43

T bl 13 A1


40/43

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


41/43


42/43


43/43

4. Determine runoff coefficient

5. Determine peak flow

Design Chart 14.3, Category 3, C= 0.87

36015

5

155 AICQ

36010182870 .

sm44 3.

Documents

Chapter 6 Design Drainage