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CHAPTER 1.0
INTRODUCTION
1.1
General
On 1st January 2001 the Drainage and Irrigation Department (DID) of
Malaysia implemented a new urban stormwater drainage design standard. The new
2001 urban stormwater drainage design standard is intended to replace the 1975
Urban Drainage Design Standards and Procedures. The underlying design
philosophy of the new 2001 Standard differs significantly from the 1975 Standard’s
approach of “rapid discharge of runoff”. The new 2001 design approach recognizes
that the “rapid discharge of runoff” is environmentally unsustainable and will not be
able to solve the increasingly serious flooding problems that occur in the urban areas
throughout the country. The 1975 approach is a result of “linear thinking” solution to
a flooding problem whereas the 2001 approach is based on a “systems thinking”
solution to the same problem.
The linear thinking solution to a local flooding problem is to widen or deepen
the section of the river where the flooding occurs so that there is enough capacity in
the affected section to pass the flood flow. However, the net effect is to transfer the
flooding problem downstream to another area where the section of a river does not
have enough capacity to pass the flood flow. The systems thinking approach
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recognizes the complete “chain of cause-and-effect”, across space and time, arising
from a local flooding problem. Thus, a solution to a local flooding problem, arising
from systems thinking, will be more comprehensive and will attempt with the “root
of a problem”. In order to mitigate the flooding problem in a sustainable way and to
rehabilitate our urban rivers the DID recognizes that there is a need to address both
the management of urban stormwater quantity and quality
For the detention of stormwater runoff the new manual recommends the use
of on-site detention facilities or community detention ponds. The methods used to
derive the size of the community detention ponds, and also the on-site detention
facilities, require numerous repetitive computations to be carried out for different
storm loadings and facility configurations.
On-site Stormwater Detention (OSD) involves the temporary storage and
controlled release of stormwater generated within a site. Without aversely affecting
he property, it relies on thoughtful design and passive engineering during site
development to achieve significant reductions in downstream flooding. OSD is
required to ensure that the change in stormwater runoff from a site due to
development does not increase flooding problems downstream except in very severe
events.
1.2
Problem Statement
In many parts of the Klang Valley, the drainage system has become
increasingly inadequate. This is shown by the growing frequency and severity of
stormwater surcharge and flooding. At the same time, there are doubts about the
capacity of local authorities to fund upgrades to the drainage system or, indeed, the
wisdom of doing so. Many of today's flooding problems are the result of wellintentioned improvements to the drainage systems, which simply transferred and
compounded the overall problem.
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Plate 1.1: A Scene of Flood at Kampung Sri Langkas, Puchong, Selangor on 29
October 2001
Plate 1.2: A Scene of Flood at Kuala Lumpur on 29 October 2001
Until relatively recently, the objective of urban stormwater drainage was to
get rid of stormwater as quickly as possible. To that end, pits and drains were built,
roads were sealed and drained, swamps were filled, surfaces were 'smoothed' and
stream channels were lined and straightened. Although successful in reducing the
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local nuisance of stormwater, no thought was given to the effect these measures may
have on flooding elsewhere in the catchment. It was not sufficiently appreciated that
if stormwater leaves one area more quickly, it must arrive somewhere else equally
quickly!
Around 1970s it became evident that increased downstream flooding (caused
by urban development and a more efficient drainage system) could be mitigated by
temporarily holding back some of the stormwater in flood retarding or detention
basins. Retarding basins could be built either to reduce the increased stormwater
runoff from existing developments or to offset the expected increase when a rural
catchment is urbanized.
Most urban flooding is caused when the volume of stormwater runoff (which
is what we call rainwater after it lands on the ground or other sealed surfaces and
starts to flow downhill) is too large for the drainage pipes or watercourses to carry.
The extra stormwater breaks out of the pipe or watercourse and flows through
adjoining low lying areas. This may cause considerable damage to both property and
buildings (Plate 1.1 and 1.2).
The most common changes in land use in this catchment are the change from
rural uses to urban development and from low density to higher density development.
These changes affect runoff in two ways: -
a)
The increase in sealed areas means that the volume of runoff is greater.
b)
Better drainage causes quicker runoff.
This means that there is a greater volume of runoff delivered to the drains and
creeks in a shorter time, which results in higher flow rates and higher flood levels
downstream. The different of hydrologic cycle between pre-development and postdevelopment as shown at Figure 1.1.
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Figure 1.1: Pre-Development and Post-Development Hydrologic Cycle
However, On-site Stormwater Detention (OSD) is a structural element of a
property drainage system which limits the site discharge using outlet restriction
devices. The system usually located opposed to street and trunk drainage, is
characterized by small areas and short time concentration. OSD involved the
temporary storage and controlled release of stormwater generated within small-scale
development site ensuring it does not increase flooding problems downstream except
in very severe events.
1.3
Study Objectives
The objectives of this study are to:a)
Analyse rainfall data to develop rainfall temporal pattern of the study area.
b)
Develop Intensity-Duration-Frequency (IDF) curves for short duration
rainfall.
c)
Compare effect of site rainfall data on the design of OSD.
d)
Compare various methods of OSD sizing.
e)
Determine optimum size of an on-site stormwater detention (OSD) storage
for a particular industrial area.
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1.4
Scopes of Work
The scopes of work for this study involve the following items:
a)
Collection of hydrological data.
b)
Analyse rainfall data ranging from year 1995-2002.
c)
Derivation of design temporal patterns for different storm durations (ranging
from 5 minutes to 1 day period).
d)
Derivation of monthly Average Recurrence Intervals (ARI) rainfall for the
catchment (frequency analysis of annual maximum rainfall series).
e)
Determination of on-site detention size based on Manual Saliran Mesra Alam
(MSMA) procedure using MSMA rainfall coefficients and site rainfall.
f)
Uses of computer model (XP-SWMM) to determine the design discharge
from specific landuse to calculate OSD storage requirement.
g)
Calculation of Permissible Site Discharge (PSD) and Site Storage
Requirement (SSR) to design an OSD.
h)
Design of OSD for an industrial area at Damansara Catchment.