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CHAPTER II
LITERATURE REVIEW
2.1
General
Within the last decade, stormwater flooding and pollution problems had emerged
in Malaysia as a major environment issue, both at the national and state levels. The
potential environment impacts of urban development on stormwater were discussed
since late1970s due to widespread occurrences of flash floods especially within the
Klang Valley. Many Department and Local Government played a key role in the effort
to alleviate the impacts and reduce potential flood losses.
Stormwater is currently disposed of by public and private reticulation systems.
Detention structures and water quality improvement devices also form an integral part of
stormwater systems, providing peak flow attenuation and water quality improvement.
Therefore On site detention is important for now and future because it is one of the
devices for stormwater quantity control.
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2.2
Rainfall Analyses
Rainfall is one of the main governing factors for this study based on which all the
design criteria will be selected. The time of concentration (Tc) for various selected
catchments, which will mainly include the various types of residential houses or
bungalows. Are very small compare to the larger catchments, the short duration rainfall
event will be of main concern for these studies. In Malaysia, quite a number of studies
are available that mainly dealt with the analysis of long duration rainfalls covering
generally more than 1 – hr storm duration. However, this study wills analyses the short
duration rainfall egg 5, 10, 15, 20- minute’s storms including long duration of up to 24
hrs.
This is because short duration rainfall is critical for peak discharge while long
duration rainfall is commonly critical for detention facilities. As such, both will be
checked for Malaysian climate for reliable performance of OSD. Hence various
statistical analyses will be carried out for both short and long duration rainfall to develop
Intensity- Duration-Frequency (IDF) relationship and other related parameters. They
will be used to define the different ARI rainfall and time of concentration (Tc) of the
respective sites. Furthermore, the temporal patterns will also be developed for the storm
events, which have influence on the runoff peak volumes.
Design storm duration is an important parameter that defines the rainfall depth or
intensity for a given frequency and therefore affects the resulting runoff peak and
volume.
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2.3
Rainfall Runoff Modelling
Hydrological analyses will be done through computer based mathematical models
to develop the rainfall runoff relationship of the selected sites. Such modeling is very
important, as the simulated results will identify the maximum permissible site discharge
(PSD) and the minimum site storage requirement (SSR) of the particular site based on
which the OSD will be designed. For this, the selected sites will be schematized properly
with appropriate numbers of nodes and links starting from the rooftop to the downstream
receiving end so that the runoff can be estimated separately for roof/gutter, adjacent road
surfaces and paving and pervious gardens and lawn areas. The output from each
structures, as well as bypass flows, will be combined to estimate the total runoff
hydrograph from a typical allotment that will help to define the required size of the
detention storages.
2.3.1 Brief review of runoff – routing models
There are many runoff routing models developed in the last two decades or so.
They all have different capabilities with different data input requirements. So no one can
really claim to have developed the best model.
Following are examples of the runoff routing models which may be used.
RORB ( Lauren and Mein , 1990 )
URBS ( Carroll, 1995 )
SWMM ( Huber and Dickinson, 1998 )
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RAFTS ( Goyen ,1991 )
HEC-HMS (HMC, 1998 )
In the selection of runoff-routing model, it is important to note that each model has
different theoretical and numerical formulation and these in turn determine the types of
data required to run and the need for calibration.
2.3.2
Modelling Tools
Computer model XP-SWMM will be used as core modelling tool for study which
is capable to perform variety of modelling applications including hydraulic, hydrologic
and water quality analyses. The model is developed by XP Software from Australia that
has been successfully used in other developed countries to model urban drainage
stormwater facilities. For rainfall analyses, computer model HYFA will be used along
with other conventional models or formulas. The model has undergone continuous
maintenance and support and is suited to all types of storm water management from
urban drainage to flood routing and floodplain analysis.
2.4
Design rainfall intensity
The specification of a rainfall event as a design storm is common engineering
practice. Although the design storm must reflect required levels of protection, the local
climate and catchment conditions, its need not to be scientifically rigorous. It is more
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important to define the storm and the range of applicability so as to ensure safe,
economical and standardized design.
Design storm duration is an important parameter that defines the rainfall depth or
intensity for a given frequency, and therefore affects the resulting runoff peak and
volume.
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. Intense rainfalls of short durations usually occur within longer
duration storms rather than as isolated events. The theoretically correct practice is to
compute discharge for several design storms with different durations, and then base the
design on the critical storm, which produces the maximum discharge. However the
critical storm duration determined in this way may not be the most critical for storage
design.
Recommended practice for catchments containing storage is to compute the
design flood hydrograph for several storms with different durations equal to or longer
than the time of concentration for the catchments, and to use the one, which produces the
most severe effect on the pond size and discharge for design.
2.5
Design intensity duration frequency
Design engineers and scientists use the IDF design rainfall curves as input to a
wide range of design flood models and environment studies. The basic annual maximum
rainfall data for durations of 5 minutes to 72 hours are fitted using a log person type III
distribution with a small positive skew ness of up to 0.8. The frequency analysis of
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rainfall data is an important part of hydrological design procedures. It is used as input to
the design of a wide range of hydrological structures. In order that rainfall intensity data
be practical for design purposes, it is necessary that accurate, consistent IDF be available
for any locations.
2.6
Design Rainfall Temporal Patterns
The temporal distribution of rainfall within the design storm is an important factor
that affects the runoff volume, and magnitude and timing of the peak discharge. Design
rainfall temporal patterns are used to represent the typical storm burst. Standardization of
temporal patterns allows standard design procedures to be adopted in flow calculation. It is
important to emphasize that these temporal patterns are intended for use in design storms.
They should not be confused with the real rainfall variability in historical storms.
Temporal patterns should be chosen so that the resulting runoff hydrographs are
consistent with observed hydrographs. Therefore the form of the temporal pattern and the
method of runoff computation are closely interlinked. A range of methods to distribute
rainfall is
1.
Average temporal patterns developed from local point-rainfall data measured in
short time intervals (15 minutes or less).
2.
Simple idealized rainfall distribution fitted to local storm data by the method of
moments.
3.
Temporal patterns from local IDF relationships.
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There are too few data points found in each Standard Duration temporal pattern.
This causes a systematic bias which will under-estimate the peak rainfall as shown in
Figure 2.1 .
Instantaneous Peak Intensity
Indicated Peak
Rainfall
Intensity
Time
Figure 2.1
Example of the under – estimation of a Hydrograph by
Discretisation
2.7
Frequency Analysis
Frequency analysis is a tool to predict magnitude of extreme events, which are
beyond the range of observation through the use of probability distribution. This practice
is not new to hydrologists who had applied it for many engineering purposes such as
design of dams, culverts, bridges and flood control structures. The analysis can be
approached graphically and analytically. The former involves plotting of observed data
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on probability plotting paper and comparing visually with the derived curve. The
graphical plotting has the edge of simplicity but less accurate than analytical approach.
Analytical approach involves fitting a distribution model to the observed data and the
quintiles magnitudes of required probability will be calculated.
The application of either approach, however needs to comply with the
assumption in the frequency analysis which are:
a)
The hydrological data are assumed to be independent and identically
distributed.
b)
The hydrological system that produces natural events is considered
stochastic or random.
c)
The natural process occurred are stationary with respect to time.
Frequency analysis is one of the statistical techniques applied by hydrologist to
estimate the probabilities associated with design events. Many criticisms can be made of
the methods and the assumptions made in its use but the fact remains that it is one of the
few methods available and it is arguably better than other non-probabilistic methods.
2.8
Data Series
In frequency modeling that involves distribution fitting, there are mainly two
types of data series involved. The first and most frequency used is the annual maximum
series (AMS). For an annual maximum series, the maximum value observed for each
year is extracted to from the empirical data set. These observations are specified for a
time interval deemed suitable for the research. The application of the data extraction
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method is sample and recent advances in computer software developments has enabled a
more accurate scanning over the entire data set to locate the maximum for a specified
time period. Nevertheless, there is also the argument concerning an annual maximum
over a certain year being exceeded by a non-maximum of another year. This has given
rise to the adaptation of a second data extracting methods, which produces a peak over
threshold series (POT) or also referred to as partial duration series (PDS).
POT or PDS is a data series comprising of extreme values exceeding a certain
threshold value. Hence, the data series is not confined to a single observation per year;
any value exceeding the cut-off point will be included in the data set. One of the major
advantages of such a data set is the observed values really display the maximums that
could occur and certain years might have more than one extreme event that should be
taken note of.
2.9
Average Recurrence Interval (ARI)
In frequency analysis, ARI or return period signified the average of time in
which an event of given magnitude will be exceeded or equaled, occurring as annual
maximum, for example, if an event might occur once or more in any 20 years. However
this might occur once or more in any 20 years period or occurs once and not again for
the next 20 years.
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2.10
Rational method of MSMA
Theory
MSMA relates the peak discharge to the rainfall intensity and catchment area via the
Rational Method:
Qу = C . уIt . A
…..
(2.1)
360
Where
Qу
is the year ARI peak discharge ( m / s )
C
is the dimensionless runoff coefficient
yIt
is the average intensity of the design rainstorm of duration equal to the
time of concentration and of ARI of y year ( mm/hr )
A
is the drainage area
The time of concentration, tc, in hours in the sum of the overland flow time, to, and the
time of flow in the stormwater conveyance system, td, as follows:
tc = to + td
…..
(2.2)
The overland flow time to can be estimated using formula below or using the
Nomograph.
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1/3
To = 107 . n. L
…..
(2.3)
1/2
S
Where
To
overland sheet flow travel time ( minutes )
L
overland sheet flow path length ( m )
n
mannings roughness value for the surface
S
slope of overland surface
And td is the total time flow in the stormwater conveyance system, is given by :
td = tr + tg + tch + tp
tr
roof flow time
tg
kerbed gutter flow time
tch
channel flow time
tp
pipe flow time
…..
(2.4)
The time of flow in open channel can be determined by dividing the length of the
channel by the average flow velocity which can be calculated from normal hydraulic
formula such as Mannings formula, given the channel cross section, length, roughness
and slope.
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2.11
Definition of Detention
Avery effective method of reducing peak runoff rates and minimizing the
detrimental impact of urbanization is to use detention. Detention refers to holding runoff
for a short period of time and then releasing it to the natural water course where it
returns to the hydrologic cycle. This is basically capturing storm water runoff for later
controlled release. A stormwater detention basin can range from simply backing up
water behind a highway or road culvert to a reservoir with sophisticated control devices.
2.11.1 Detention storage
The basic concept of providing detention storage is to limit the peak outflow rate
for a specific range of flood frequencies to that which existed before development. The
primary function of detention facilities is to reduce peak discharge by the temporary
storage and gradual release of stormwater runoff by way of an outlet control structure.
Detention storage is either On site Detention (OSD), Community Detention or Regional
Detention.
Example of on site detention includes: car park, surface and underground tanks,
rooftop and landscaped area. Community and regional detention facilities are larger
facilities than OSD, which are provided in public areas outside private properties. These
are commonly formed by the construction of an embankment across a stream and or the
excavation of a basin storage area.
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2.11.2 On-Site Stormwater Detention
On-site Stormwater Detention (OSD) is a way of ensuring that changes in land use do not
cause more downstream flooding: both in the local drainage system immediately downstream
and along the creeks and rivers further downstream. OSD restricts the rate that stormwater
leaves a site to a discharge which will not cause increased flooding anywhere downstream. It
involves temporarily storing (detaining) the excess stormwater on the site. OSD is not the
only way to ensure that developments do not make flooding worse, but in already urbanized
catchments, it is often the only practical alternative.( Figure 2.2 )
Inflow
Outflow
Detention
Pond
Infiltration rate before urbanization
Rainfall
and
Infiltration rate
after urbanization
Infiltration
Rate
Figure 2.2
Hydrologic effect of urbanization on storm runoff and function of
detention ponds.
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2.12
Several methods exist for the detention of stormwater runoff
On site detention can be different from downstream detention by its proximity to
the upper end of a basin and the use of small detention facilities as opposed to the large
dams normally associated with downstream detention. Several methods exist for the
detention of stormwater runoff including:
1.
Underground storage
2.
Basins and ponds on ground surfaces
3.
Parking lot storage
4.
Rooftop detention
Component of OSD
There are different types of OSD systems, but they all have the following components: -
1.
Discharge Control Pit
The discharge control pit is located at the lowest point on the site and all flows leave the
site through this pit. The pit contains an orifice (circular hole) in a stainless steel plate
fixed to the sidewall. The orifice is sized to limit discharge from the site to the maximum
permissible rate.
2.
Storage
The Storage can be located on the surface, underground or on a roof. It detains the
excess runoff that cannot immediately pass through the orifice. The storage fills by
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overflow from the discharge control pit and empties quickly though the pit once the rain
eases.
3.
Collection Network
The collection network consists of gutters, pits, pipes and surface grading. It delivers all
site runoff to the discharge pit. The collection network must carry all run-offs: even in
the event of a 100-year storm. Run-off from upstream properties must be diverted
around the OSD storage.
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 of ponds, and also the OSD facilities, require
numerous repetitive computations to be carried out for different storm loadings and
facility configurations. Thus the use of computer programs in the design of both the
OSD facilities and community detention ponds will be necessary for increased design
productivity.
2.12.1 Types of On-Site Stormwater Detention Systems
There has been little change in the fundamental forms of OSD systems employed
in Sydney to reduce the stormwater runoff from re-developed properties. These consist
of surface or underground storages with a control pit, as shown in Figure 2.3.
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Figure 2.3
Typical OSD Systems
The piped outflow is usually controlled by a circular orifice plate, and overflows
from surface storages by a weir. Some unit’s pipes can be used for stormwater
infiltration as well as detention, though this has not yet been attempted on a wide scale.
Drainage authorities have been reluctant to accept plastic cellular units because they
perceive difficulties in inspecting and cleaning sediments from these. Some underground
OSD systems have involved large and expensive rock excavations on steep land. One or
two roof storages have been constructed using oversized roof gutters, but these have not
been popular. Use of on-site water supply and recycling systems is growing in Australia,
and many urban householders are installing their own rainwater collection tanks. There
have been proposals to combine these with on-site detention systems. Councils,
however, have refused to accept rainwater storage as part of a property’s OSD storage
requirement, since the householder will want to keep these tanks as full as possible. A
statistical analysis of the available storage might be made by computer simulation of
storage level behavior over a long period using a series of daily rainfalls.
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2.12.2 OSD regulations
OSD systems will reduce flows and will be beneficial in preventing damage. The
studies cited all show that OSD systems can achieve flow reductions at almost all
locations in a catchment. The question is whether this is cost-effective. Environmental
Planning and Assessment Act, 1979 allows councils to collect money from developers to
offset negative aspects of developments, including the generation of additional
stormwater runoff by increased proportions of impervious surfaces. Money must be
spent on remedying defects directly caused by the project. Works must not be remote
from the development site and must be constructed within a reasonable time. They
cannot be used to remove deficiencies in existing infrastructure.
Nevertheless, this view of OSD as one tool in the array of measures available to
drainage system managers, rather than as a complete solution, is attractive to many
designers. There are situations where OSD is perhaps the only realistic solution, for
example in the case where increased runoff from a re-developed site must run through
adjoining properties and no pipe or legal easement (right of way) exists, as shown in
Figure 2.4.
It is often prohibitively expensive and difficult to obtain easements. Most
council do not favor the alternative of a pump-out system for stormwater, so an OSD
system which restricts flows to those occurring before re-development is the best
solution.
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Figure 2.4
A Case where OSD is required
On-site stormwater detention has become a standard procedure for dealing with
impacts of re-development in Sydney. The controversy has quietened, and as in the U.S.,
little is now being published about OSD. "Broad brush" procedures are being revised
and regulations are being set on a catchment basis.
2.13
OSD Sizing Procedure
A simplified design procedure for determining the required volume of detention
storage is as follows:
1. Select storage type(s) to be used within the site, i.e. separate above and/or belowground storage(s), or a composite above and below-ground storage.
2. Determine the area of the site that will be directed to the OSD storage system. As
much of the site as possible should drain to the storage system.
3. Determine the amount of impervious and pervious areas draining to the OSD storage
system.
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4. Determine the total catchment time of concentration to the catchment outlet or a point
of concern as directed by the local authority.
5. Determine the time of concentration from the top of the catchment to the development
site.
6. Calculate the pre and post-development flows for the total site for the discharge
design storm using the total catchment time of concentration.
7. Determine the required PSD for the site discharge design storm and the catchment
time of concentration tc .