Full When we model the drainage, a system using


Full Paper Title in upperCase


Gowmaran S

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University Of Moratuwa,Jaffna,[email protected]

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Abstract: The rainfall event is the main reason
for very heavy flooding. This occurs because of the urbanization in the cities.
the urbanization causes the natural unbalance between effective drainage
capacity and generated urban runoff.(Kong, Ban, Yin, James, & Dronova, 2017)
To stop this unbalance, we must have a proper drainage system. the drainage
system is disturbed due to solid waste deposited in the channels.so we must
maintain proper drainage channel to move the runoff
very effectively. We must consider hydrologic impact assessment to design the
storage and urban drainage system. When we model the drainage, a system using SWMM (Storm Water Management
Modelling) software input the pre-and post-hydrological model to analyze the existing drainage system. Then we
must consider flooding issues and sewage water mixed with rainwater and other water-related issues to come up with the new design.  The drainage system I design should consider
the groundwater protection because there are a lot of
species inside the ground.(Nadeesha Chandraseana, Wan Yusof, Ul Mustafa, Mustaffa, & Umar, 2016)


Keywords: Heavy flooding; urbanization; solid
waste; runoff; ground water



Flooding is the
natural disaster. These disasters we can’t control by our hands. We can control
only by protecting the nature. This flooding disaster causes severe damages to
people as well as properties. This disproportionate impact in densely populated
cities may affect flooding heavily. This occurs normally in the downstream
areas where people are living in informal settlements. For them it is
frequently occurred. When we consider the affected people are mostly the low-income
people in Colombo city. The victim of these natural hazards is mainly affected
by without pre-warning is given by the officials.

The main goal
of this research

Give a good drainage system to the critical area in
Colombo city.

Modify the land profile for efficiency of the
surface run off.

In Colombo city
is becoming very sensitive areas because of high level organizations are
established in this area. So, the urbanization is the most commonly occurred in
this area. These also leads to flooding. We can call this as increases of
impervious areas causing these issues. When we analyse the real scenario of the
Colombo city higher peak flow and shorter time to peak is the main factor affects
the flooding. The intensity of precipitation also increases due to various
reasons like climatic changes. That also causes high run off in the downstream

To overcome
this issued we have to have a proper drainage infrastructure should be
developed. We must model a drainage system in SWMM for the pre-and post-conditions.
Then only we can come up with every difficulty in the existing drainage system.
As all know this is fully computerized model we can model different types of
model find the final effective drainage model.

Not only
modelling drainage system leads to stop the flooding. We must maintain the
properly otherwise it will not continue functioning. People also put their
solid waste in near the open space this also leads to block the drainage.it
reduces drainage capacity. So, people also think about this as well. That’s for
their own benefits.

When we
consider the drainage is one of the least concentration factor in urban
development cycle. This tends to decrease the efficiency and workability. Main
problem in srilanka is very low finance allocation for the drainage infra

Consider all
these effects we have to design a effective drainage to consider all the safety
issues of the people.

Study area

We have considered
the critical area which is sensitive area in Colombo. Here the area we consider
is Kelaniya. Which is consisting of 450 hec area. This consist of mixture of
residential areas, roads, marshy areas. The Kalani area is 1.50 m above the
mean sea level. this existing drainage system is constructed in 1910.











1 General
Outline of the study area

We must continue our further study of flood management
based on the topographical map in Kelaniya area. then only we can clearly
identify the land profile. After that we can identify which land area should be
cut or fill depend on their present condition to make the runoff efficiently.

The main reason is unplanned urbanization in the
downstream area which is heavily affected the flooding. the solid waste
management should well organize otherwise it will also block the drainage
channels. We have to give the solution for the drainage system in the sensitive

Urban drainage

Natural hydrological cycle should be well organized
otherwise it will also leads to flooding. normally hydrological cycle starts
with precipitation. It is fully developed by rainfalls. It also creates
overland flow. As well as ground recharge  
done by infiltration. there is an initial loss governed by evaporation.
This cycle can be implement very clearly by the below diagram.

Imperviousness of the ground also affect the flooding
in the considerable amount.so we can see the relationship between

Imperviousness ? runoffvolume

Imperviousness ? 1/Infilt. Volume












Introduction about computer modelling in storm water
management Model(SWMM)

SWMM software is commonly used hydraulic software for
designing planning analyzing used in srilanka. SWMM software can design a pre-model
using the data of existing drainage system get from CMC and run in this
software and find out the problem in this current system. Then we can get the
idea of the land profile and existing runoff patterns and design a post model
in SWMM software this leads to help us to find a effective and economical way.

From this software we can analyze for different
scenario. This software basically models with quantity and quality of runoff
made within each catchment before modeling in SWMM software we must identify
the sub catchment area which can causes the heavy runoff in our segment.

We can analyze and come up with different
implementation depend on our anylysis

changing rainfall due to the climatic changes and man-made effects to the

in the surface run off

of the rainfall in to unsaturated soils.

A symmetrical form of current urban structure
affects the urban drainage cycle. In present condition urban areas are without
any prior control people are making administrative, industrial and residential
areas. They are not considering the formal zoning plans over the cityscape. The
negative effect of urban drainage cycle is disposing the solid wastes in the
critical sensitive areas. Likewise, population agglomeration affects the
flooding plan.

Sample data obtain from Colombo municipal council to
obtain drainage capacity needed for the area.

rainfall November 2013 =375mm/h

Area of
Colombo Municipal Council = 37 000 m2 Runoff coefficient for urban area = 0.6

volume = rainfall X Area X Runoff coefficient 
Runoff volume = 8,325,000 m3

This would
amount to a volume of 8,325,000 m3 from a 375 mm of rainfall.

capacity calculation;

length of drainage area of CMC is 350,000 m. The drainage section is
rectangular and the dimensions varying from 1m-0.6m.  Average width and depth of micro drain canal
is about 1m. 

 Drainage capacity = length X width X depth

 = 350x 103 m x 1 m x 1 m                                             =
350, 000 m3 

capacity: Runoff volume = 0.0420                                                              
= 4%