User:Indumali Gurusinghe/sandbox

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A dissertation submitted to the Colombo School of Construction Technology, Sri Lanka, in fulfilment of Btech Higher National Diploma in construction and the built environment (Construction – Architectural Technology),

ARTICLE INFO	ABSTRACT Keywords: 		Colombo is the highly densed  city  of  Sri  Lanka  situated in  and  around  natural  wetlands Flooding	These wetlands and water retention areas play a major role in flood  control management Mitigation 	Main reason for  increasing  flooding in  Colombo area is rapid urbanization during  past Catchment	decade which are unplanned  constructions  and developments. In year 2017 of May Sri Lake/River 	Lanka floods resulted  huge damages  to  people’s  daily  activities  and  properties. The Rainfall 	flooding severely affected Sri Lanka’s Western Province and three other provinces. Flood hazards may occur in future due to irregular land use patterns and climate change. The objectives of this research paper is to identify the problems and causes for flooding together with previous solutions by reviewing ongoing flood mitigation projects in Colombo area and feasibility studies. The paper is aimed at discover causes and current strategies to manage flooding in Colombo urban area and gather statistical data of rainfall, Inundation areas, Affected people and Flood levels of the finished projects to analyze in future flood control hazards.

Acknowledgment

First of all, I would like to take this opportunity to extend my sincere thanks to my assessor Mr. Kasun Gomis, who has been continuously encouraging and following up from the beginning till end. Secondly, I express my sincere thanks Eng. K. Rajapaksa, General Manager of Sri Lanka Land Development Corporation (SLLDC), Eng. N.S.Wijeratne –Deputy General Manager (Wetland Management), SLLDC, Eng. U. Vithanachchi -SLLDC, Eng. C. Jayathilake-SLLDC, for their valuable inputs for the formation of initial proposal, especially looking at the aspect of fixing main objectives of this study. Thirdly, I wish to express that the projects and feasibility reports which were referred through this research are Olyamulla Pumping Station Project, Metro Colombo Urban Development Project and Weras Ganaga Storm Water Drainage and Environment Improvement Project.

Finally, I express my gratitude to the academic and non-academic staff of the CSCT campus for their support  in  all  aspects  to  complete  my  Research  program  in  Colombo  Urban  Flood Mitigation and Storm Water Management in Sri Lanka.

Table of Content

Acknowledgment	3 1.	Introduction	6 The institutions and laws established to mitigate the flood in Colombo urban area and in Sri Lanka	7 Law established in Sri Lanka	8 Natural conditions	8 2. Literature Review	9 2.1 Design of pumping station and storm water drainage & rehabilitation project	9 at Mudun Ela area	9 2.2 Metro Colombo Urban Canal Development Project, Si Lanka	10 Flood inundation analysis with countermeasures	10 2.2 Wetland Management Strategy in view of Flood Mitigation and sustainable natural system	13 2.3 Socio economy	14 3.    Research Methodology	15 4.      Findings…………………………………………………………………………………   20 5.	Data Collection and Analysis	Error! Bookmark not defined. 6.	Conclusion	21 Present progress of flood control activities	22 Oliyamulla Pumping Station Project	22 Metro Colombo Urban Canal Development Project	22 Weras Ganga Project	22 Lunawa Lake Project	22 7.	Recommendations and Outcomes	24 8.	List of reference	25

List of Figures Figure 1 : Project Area Map	9 Figure 2 : Area Map	11 Figure 3 : Five main outfalls	12 Figure 4 : Disaster Exposure vs Poverty	15 Figure 5 : Colombo Annual Rainfall vs Year	18 Figure 6 : Affected People vs Year	19 Figure 7 : Inundation area vs Year	20 Figure 8 : Flood level vs Year	21 Figure 9 : No of Houses Affected vs Non affected by flood…………………………………16 Figure 10 : Delkada Rattanapitiya Area Map……………. ..……………………………………17

List of Tables Table 1	6 Table 2	14 Table 3	18 Table 4	19 Table 5	20 Table 6	21 Table 7	21

1.	Introduction

Colombo urban city is a place where a large population and main commercial and industrial buildings occupied within a limited land area. Due to construction of most of residential buildings existing drainage and sewerage systems have been blocked. The people discharge waste and garbage in to bare land and low land as well as to canals during 20-30 years up to now. Path of rain water, ground water, drainage, sewerage lines started to block as a result of floatable debris and garbage. The Colombo urban region constitutes the areas comprising of Colombo Municipal council, Dehiwala-Mount lavinia municipal council, Sri Jayawrdena-pura kotte Municipal council, and Kolonnawa Urban council where flood prevention projects take place. It has increased urbanization and development in this small city by filling low land and marshy land over a period of 30 years resulting in degradation of wetland and water retention areas as well as increasing rivers sedimentation and getting coarse aggregate level high. The urban areas in Sri Lanka is subject to major flood damages to roads, bridges, public utilities and personal properties, disrupted traffic and communications, electricity, water supply and drainage and also disturb people’s day to day work and their career. This has affected economy of the country and livability and health of the people as well as quality of life and the environment. As for one example is the river is used for transportation, fishing and production of hydroelectricity, sewage disposal and river sand mining, through these factors many people depend on the river for their daily routing in life income family. Sri Lanka can be characterized in to 4 climate seasons as follows and every year in rainfall season habitual severe flood and rising of water level occur in Sri Lanka.

Season	Period	Characteristics Fist Inter-monsoon 	March-April	Rainfall from south to north of island southwest-monsoon 	May-September	Rainfall in the southwest area of the island. Second Inter-monsoon 	October-November	Rain across the entire island Northeast-monsoon Season	December-February	Northeast, east area Table 1 The institutions and laws established to mitigate the flood in Colombo urban area and in Sri Lanka Sri Lanka Land Development Corporation (SLLDC) former known as Land Reclamation and Development Corporation (SLLRDC) SLLDC is institution which can decide on permission for the public to fill low lands subject to provision of appropriate storm water drainage designs as prepared by SLLDC. It is involved in the preparation of Master Drainage Plans for catchments (Catchment is an area of land draining in to a stream or a water course at a given location is known as catchment area) and sub catchments which identify lowlands that can be reclaimed and areas to be retained as detention areas and storm water drainage facilities. This activity is done with minimum damage to the environment. Irrigation Department •	River gauging •	Hydraulics modelling Department of Meteorology •	Meteorological and climatological services. •	Organize and contribute to, educational and training programmes on weather, climate and climate change thematic areas. •	Organize and contribute to, educational and training programmes and workshops for interested groups including school children, policy and decision makers, on mandated thematic areas. The Irrigation Department Department is the in charge of kelani Ganga flood control and lock structures; •	North Lock on St. Sebastian Canal •	Flood gates at Ambatale at the connection of Easter diversion canal to the kelani 	Ganga •	Flood gates in Thalangama Tank •	Regulatory water level gates within paddy field in Metro Colombo Areas The Drainage and Flood Systems Branch or The drainage and Flood protection Branch of SLLDC (Former SLLRDC) Designing, construction and operation & maintenance of urban drainage facilities. The SLLDC was established in 1698 as a semi-governmental agency under SLLRDC Act. No.15 granting the authority to take legal action against unauthorized reclamation. •	One of an objective of the Drainage & Reclamation Division is to minimize the damages from floods in Colombo and to protect human lives and all kind of public and private properties due to floods. •	Give technical guidance for proper maintenance of existing drainage, flood control •	Planning and designing of flood control •	Activities related to flood ordinance Law established in Sri Lanka Flood Ordinance The remaining wetlands within the Western Province including paddy fields should be preserved for flood control purposes, biodiversity and aesthetic value. (From Island to Continent, 2019) Natural conditions Climate Colombo urban area located in the wet zone and coastal area in the western part of the island. The Colombo city area is subjected to frequent floods during the south-west monsoon and identified as increment of annual rainfall during past decades and have been affected for flood vulnerable in urbanized cities. Recent irregular urbanization filling of low lands, unplanned development activities and deforestation have affected on climate change. As a result the temperature in the island has been increased evaporation of water from oceans, lakes and rivers have been increased. For example, clear-cutting in forest plantation decreases interception and evapotranspiration which increases antecedent soil moisture and consequently decreases soil storage capacity [26] (M.Rogger). For example, clear‐cutting in forest plantations decreases interception and evapotranspiration which increases antecedent soil moisture and consequently decreases soil storage capacity [Brown et al., 2005].Rainfall is the only form of precipitation in Sri Lanka causing stream flow among snow fall, hail, etc. The wind pattern and speed much of could dense falls in the ocean as rain some are driven in to the land and falls the rain. The south – west monsoon (commonly known as monsoon) is the predominant rainy season in Sri Lanka. Geography The canal system is connected to the kelani river which is fourth largest river in Sri Lanka and it is identified unprotected and vulnerable areas at downstream of main rivers and by the side some people are living in the narrow strip of land in front of river bund in Kelani River habitats.Kelani River starting from the peak of Sri Pada mountain to Colombo through or bordering Nuwaraeliya, Ratnapura, Kegalle, Gampha and Colombo. The Colombo urban area have five main outfalls which are Dehiwala canal, Wellawatta canal, Mutuwal Tunnel which are drain water in to the Indian ocean and St. Sebestian north canal and Madiwela east diversion canal outlet drain water in to the Kelani River. Parliament Lake, Diyawanna Lake and thalawathugoda lake, ramplawatta lake act as retention ponds while Kotte-ela marsh, Heen-ela marsh and kolonnawa-ela marsh act as retention areas [3](M. Moufer). 2. Literature Review

2.1 Design of pumping station and storm water drainage & rehabilitation project at Mudun Ela area

The pumping stations are the major hydraulic facilities and play an important role in flood mitigation in highly developed cities. [5] (Research gate). The city of Quincy, Massachusetts, United States had a flooding problem due to heavy rainfall. Most of the houses built are more than 60 years old. A pumping station was set and prevented flood in lower area and successfully solved mitigation of flood issue [12] (T.H.Tee & F.A.A.Nifa). In order to improve economic, social and infrastructure in Peliyagoda Integrated Urban area the above project has been initiated to create a flood free environment to facilitate to establish industries, warehouse, distribution centers, and housing schemes. Unplanned filling of the marshy lands and individual people who develops their lands without implementing a proper drainage system are identified as main reasons for urban flooding in this area. During the heavy rains, most of the area were under flash floods due to the blocking of drains and each of adequate capacity of canals. [21] (LLEI&CDP).

Figure 1 : Project Area Map

Structural flood mitigation countermeasures introduced through this project. [4] (Mudun Ela Design Report, 2012). 1.	A 15m span bridge has been constructed replacing a culvert of 2 nos, 1000mm dia.

2.	A flood protection bunds (3.5ft.m) on the northern bank of Kelani River has already been established.

3.	A pumping station will be set-up in main canal having the capacity of 30m3/sec.

4.	New drainage system will be modelled by reconstructing and widening existing main canals, minor canals and lakes to flow flood water from respective areas.

5.	From upper catchment of Mudun Ela by constructing a new regulator and a new canal across the existing road from Dalugama to Wttala. 2.2 Metro Colombo Urban Canal Development Project, Si Lanka

The Colombo Metropolitan Region (CMR) is a center of high socio-economic activity of Sri Lanka and therefore absolutely important for the national economy [21] (WSDP). There is a concern that disordered filling of the lowlands may happen unless strong legal regulations are made and enforced. Against the disordered filling of the lowlands in Colombo metropolitan area which functions as a storm water retention area during flood, UDA proposed to prohibit development of the lowlands such as marsh area and paddy fields from the viewpoints of storm water drainage and environment in the Colombo Metropolitan Regional Structure Plan [20] (WSDP).

The Metro Colombo urban region consists five divisional secretariats are Colombo MC, Dehiwala Mount Lavinia MC, Kolonnawa Urban Council, Sri Jayawardenapura Kotte, Kaduwela Mc, Mharagama UC and Kotikawatta Pradeshiya Sabha. [8] (landdevelopment.lk, 2019) 1)	Improvement South Scheme 2)	Madiwela South Scheme 3)	St. Sebastian South Diversion 4)	St. Sebastain North Lock widening & pumping station 5)	Kolonnawa Canal diversion 6)	New Mutuwal Tunnels 7)	Provide guide walls at Wellawatta Bridge 8)	Torrington Tunnel Flood inundation analysis with countermeasures

4.	Gothatuwa diversion canal Open rectangular canal 50m wide 5.	St.Sebastian south diversion Covered box drain 8m side 6.	New Mutwal diversion Underground tunnel and open rectangular canal 6m wide. [7](www.colombopage.com) 7.	Madiwela south diversion The sub-catchment in the upstream of Parliament Lake was divided to eliminate. Figure 2 : Area Map Metro Colombo canal system is going back to 15th-Century and studies to identify its capacity in controlling floods. The history of Metro Colombo canal system discussed from the time of King Veera Prakramabahu II in the 15th century. The Portuguese improved the canal system for efficient transportation in 17th Century. The Dutch who ruled after the Portuguese established Colombo as the capital and they added structures and diversions to the canal system in 18th century. The British added several openings to the sea during 18th – 20th century. [6] ( www.lankalibrary.com) At present the canal system acts as a passage to drain out flood water from the Metro Colombo area. It has five main outfalls, three of them drain water into the Indian Ocean (Dehiwala. Wellawatta, Mutwal). The other two discharge water into the Kelani River (Madiwela and St. Sebestian)

Figure 3 : Five main outfalls Urban flooding is a serious issue and a challenge which causes substantial damages to the people, health and infrastructures in urban area due to unplanned developments, lack of drainage facilities, inadequate openings, inadequate water storage, intense rainfall, encroachment and blocking in the drainage system and backwater effect at outfalls, inadequate conveyance capacities of canals, structures and outfalls. Several urban areas in the country have been experiencing flood problems due to lack of proper storm water management systems [23] (IESL). It is difficult to have a proper gradient in the channel bed towards outfalls to drain out the floods from canal basin due to geographical terrain of the basin varies from 0mMSL to 35mMSL. The Colombo city which is a coastal city located in the wet zone of Sri Lanka is subject to frequent floods during the south-west monsoon. The situation is much worse when the water level of the Kelani River rises due to rainfall in the upper catchment [3] (M. Moufer). The set-up model for the Metro Colombo basin was calibrated for the event in November 2010 which was recorded the highest rainfall (440mm-within 14h) in the recent history considering the measured water levels. Severe consequences Submergence of Sri Lankan Parliament Premises and closure of schools in the city 123,000 people of 26,850 affected families, Approximate 257 household damages, Social and commercial activities interrupted ,Similar situation have occurred in 2014 and 2016 as well. 2.2 Wetland Management Strategy in view of Flood Mitigation and sustainable natural system

The wetlands provide effective protection from flooding. [2] (Wetland strategy, 2009) During intensive rainfall events, the wetlands are able to store 68 Mm3 of water (For the 100 year return period flood or the equivalent of more than 27,000 Olympic sized swimming pools). Colombo is a unique and dynamic city and linked with wetlands. The city developed around wetlands and still depends on the same wetlands since historical time. Wetlands have been degraded over the time and lost within the Colombo Metropolitan Region. [03] (MCUDP, 2016) National Wetland Directory in 2006 published by the Central Environmental Authority of Sri Lanka have been declared as Environmental Protection Areas of the wetland are Bolgoda Wetland and Thalangama Tank. As a part of hydrologic network o a catchment, Colombo wetlands play an important role in flood mitigation. Declared area to be a flood area, shall be all integrated in overall planning of a city. Sri Lanka became a signatory to the Ramsar Convension on Wetland in the year of 1990 month of June on 22nd. Infilling of wetlands is increasing flood risks across the city. The current rate of wetland loss in the Colombo Metropolitan Region is approximately 1.2% per annum. If this trend continues, the area of wetlands would be reduced by one third by 2038 and by half by 2070. This will also produce a proportional decrease in flood storage capacity. If all the wetlands were in filled flood water levels would increase by two meters during the most severe rainfall events resulting in devastation across the city. Decisions need to reconcile landfilling versus flood protection. Landfill performance could disrupt natural functions of floodplain flow, adverse effect to wetland process. [15](epa.gov.) the progressive loss of wetlands results in a constantly increasing need for engineered flood protection solutions. The implementation of classic, expensive and non-environmental-friendly engineering solutions such as dredging, creating new outlets and channels, reinforcing and expanding the existing pumping stations will all be necessary to contend with increased flood risk. These approaches will inevitable result progressively in the creation of a complex flood protection system that could fail resulting in a potential disaster. The precedent for such failure exists, for example in the city of New-Orleans which in 2005 suffered widespread devastating flood damage as result of hurricane Katrina. [15] (MCUDP, 2016) 2.3 Socio economy

Economic condition and economic frame work (Macro and Micro economic) The recent floods on Sri Lanka have been impacted on macroeconomic, due to increased health expenditure, loss of labour and productivity, and reduced investment in human and physical capital formation. The micro economic impact affects individual households, firms, government [24] (IPS). Poverty and house hold income Sri Lanka, a developing country which has high dependence on natural resources and limited coping capacity to disasters. It has found that low income households that depend depend fully natural resources for their living are exposed more frequent flood disaster and financial losses as their main income depend on natural resources [25] (Silva & Kawasaki).

Disaster Exposure Poverty Disaster	Non - Poor	Poor No Disaster	52%	48% Flood Only	62%	38% Drought Only	58%	42% Flood and Drought	59%	41% Table 2 Figure 4 : Disaster Exposure vs. Poverty Data collected through a questionnaire conducted in North Central Province, Sri Lanka in 2016, 517 and 214 households in Anuradhapura and Polonnaruwa accordingly. Above data defined poverty using binary categories poor and non-poor based on monthly per capita income [25] (Silva & Kawasaki). 3. Research Methodology

This research was carried out in Colombo urban city location based on Kelani River and few marshy lands. The methods in this research are as follows; a) Secondary data analysis and Archival study which was carried out for this study area. b) Primary analysis of quantitative and qualitative data by personally discussion with hydrological engineers who have high potential in flood mitigation. Online search such as Science direct, Elsevier, and Google linked data base, E-books were used and were collected existing ongoing flood controlling projects details and feasibility studies through hydrological engineers working with me. In the second stage Flood, Flood mitigation, Flood controlling used as key words to get related literature. In conclusion, Flood mapping, Flood modelling, Mike flood, Flood protection planning were used to gather further references.

4.	Findings Houses Affected by flood

year		 	Number of Houses 2014	Flooding 	70 2014	Non Hazard 	30 2019	Flooding 	01 2019	Non Hazard 	99 Table 7 Figure 9 Area Map 		Figure 10

At the time of the Werassa Ganga Storm Water Drainage and Environment Improvement Project was commenced there were 70 numbers of houses were found to be affected by flood in Delkada to Rattanapitiya in year 2013. Since inundation occurred on the route no.120 main road during this period alternative routes had to be used by the private vehicles and public transportation. The following work has been done under the WSWD project in the area of Delkada to Rattanapitiya; •	Canal widening •	Water retention capacity improvement •	Lake dredging •	River channel alignment •	Constructing culverts and bridges •	Provide periphery canals for storm water retention area, road maintains After completion of this part of the project houses were escaped from flooding and roads were released from severe floods. 5.	Data Collection and Analysis The data presented in this research paper is the result of 9-week literature study on E-Books, Google scholar research, Feasibility studies of existing ongoing projects in Sri Lanka were being studied with regards to flood mitigation in general. It is clarified 20 selected literature and feasibility study reports in the area of research. Through this a) Structural b) Non-structural c) Influencing factors for mitigation d) Reasons for flooding were analyzed.

This analysis was carried out by considering 20 years rainfall data and critical situation water levels taken from Metro Colombo area gauging stations. Colombo Year	Annual RF (mm/Year) 1997	2530 1998	2387.7 1999	2888.2 2000	2012.4 2001	1932.6 2002	2099.7 2003	2030 2004	1958.1 2005	2814 2006	2722.9 2007	2084.8 2008	2622.1 2009	2134.4 2010	3369.9 2011	1774.6 2012	2464.6 2013	1990.6 2014	2635 2015	2948 2016	2524.5 2017	2702.5 2018	2725.0 2019	2650.7 Table 3 Figure 5 : Colombo Annual Rainfall vs Year

Affected People Year	Reduction (%) Divisional Secretaries	2010	2015 Colombo	15385	6754	56.10 Thibirigasyaya	16153	9771	39.51 Dehiwala	44	42	4.55 Kolonnawa	7084	4854	31.48 Kotte	16804	12027	28.43 Maharagama	5256	2155	59.00 Kaduwela	4861	2989	38.51 Total	65586	38593	41.16 Table 4

Figure 6 : Affected People vs Year

It can be observed that the significant reduction of affected people after implementation of the project.

Inundation area (km2) Year	Reduction (%) Divisional Secrataries	2010	2015 Colombo	1	0.4	60.00 Thibirigasyaya	1.6	1	37.50 Dehiwala	0	0 Kolonnawa	1	0.7	30.00 Kotte	2.7	1.9	29.63 Maharagama	1	0.4	60.00 Kaduwela	1.8	1.1	38.89 Total	9	5.5	38.89 Table 5 Figure 7 : Inundation area vs Year It can be observed that the significant reduction of inundation area after implementation of the project.

Flood Level (m) Year	Reduction (%) Divisional Secretaries	2010	2015 Parliament road bridge	2.5	2.17	13.20 Kirimandala Road Bridge	2.3	1.92	16.52 SLLRDC Bridge	2.31	1.93	16.45 Babapulla Road Bridge	2.15	1.65	23.26 Aluth Mawatha Road Bridge	2.06	1.22	40.78 Table 6 Figure 8 : Flood level vs Year It can be observed that the significant reduction of flood level after implementation of the project. Also the safe flood level of the Colombo area is +2.00 m MSL. 6.	 Conclusion

The figure-01 illustrates the annual rainfall in Colombo from 1997 to 2019. An increment in the annual rainfall was identified for 09 years from 1997 to 2006 were 2722mm and from 2008 to 2012 maximum rainfall were 3369.9mm which was a heist rainfall within 16 years period since 1997 and it was the highest rainfall within 20 years period. The Colombo district has a population of 2.4 million within 699Km2 of area as of 2019. The Colombo city area is subject to frequent floods during the south-west monsoon. Kotte, Colombo and Thibirigasyaya DSD were highly urbanized DSD in the Metro Colombo area and there were reduction in the numbers of affected in these area from year 2010-2015 (figure-2). During the past 30 years floods have affected more than 10 million people. It is observed that water level depicted in above figure -4 become high in year 2010 in Parliament Road Bridge and lowest level is at Aluth Mawatha Road Bridge. In the year 2015 Aluth Mawatha Road Bridge reported lowest water level. The management of flood and water resources are in high priority and have direct impact on lives of the people and on the socio economy impact.

Present progress of flood control activities Oliyamulla Pumping Station Project •	15m span bridge has been completed. •	Substructure and bund for pumping station has been completed. Metro Colombo Urban Canal Development Project •	Pumping station at North lock 80% completed. •	Mutwal Tunnel & Torrington Tunnel construction work has commenced. Weras Ganga Project The causes of flood damages are insufficient capacities of drainage channels, insufficient opening of channel crossing structures and water level rise of the Weras Ganga. The storm water drainage system of the Weras Ganga basin also depends on the retarding effect of the lowlands in the basin. The decrease of retarding effect by the disordered filling of lowlands in the Weras Ganga basin will cause additional flood damage and also will influence the Bolgoda Lake and Panadura Ganga downstream. Project is 80% completed. [20] (WSDP). Lunawa Lake Project The objective of the project is to improve the environment and to uplift the quality of life of people in Lunawa area by alleviating flood, through improvement of storm water drainage system, including the rehabilitation of existing drains and streams creating a hygienic and pleasant environment through the improvement of storm water drainage system and upgrading living conditions. The project area covers two Local Authorities, the Southern part of Dehiwela Mt. Lavinia Municipal Council (DMMC) and the Northern part of Moratuwa Municipal Council (MMC). This covers the Lunawa Lake Basin with a drainage area of 6.15 km2 including the two major drainage areas, namely Lunawa Lake North and Lunawa Lake South. This projected is completed and successes. River bed/ lake bed dredging, Bottleneck dredging. o	Eg; Development of Lake beside of Colombo Katunayake Expressway, Peliyagoda and dredging of Heiyanthuduwa South Lake, Kimbula E;a at Mattakkuliya, Lake at Wedamulla Kelaniya etc. Channel improvements o	Eg; improvement work of existing lake near Bollagala Temple at Manelwattha, Biyagama, Kelaniya. o	Increase the size of the canals, drains o	Pumping station (Oliyamulla) o	New outfalls (Tunnels, New Mutwal Tunnel) o	Drainage (Beira Lake, Werass Ganaga Project) o	Secondary canal improvements, Periphery canal improvements Enhance pond area, Forms new ponds and lakes in flooding areas and up streams o	Dual purpose designs o	Water storing play ground o	Basements (Water storing) o	Enhance and conservation retention area, Maintain available retention area o	Clearing water bed deposit to avoid stagnant o	Regional flood frequency analysis, Rainfall analysis o	Rain Water Harvesting systems in urban area/flooding areas Wetland designing and management in Flood retention areas o	Improve humanities o	Rehabilitation of new unity and character in areas o	Adaptation to new culture of water front o	Improve livability of humans o	Improve leisure activities Design Flood -	Spill way design for major and medium projects -	Culvert/Storm drainage -	Standard project flood: The most severe combination of meteorological and hydrological conditions, which are reasonably characteristics of the drainage basin. -	The heaviest rainstorm that has occurred in a region as per rainfall records as the largest storm observed in the 	region of the water shred. -	Permanent barrage and minor dams Storm water management The risks of flooding will always be there in some areas like Kolonnawa, Meethotamulla, Mulleriyawa and Angoda area etc. Protection of retention areas from unauthorized encroachment have been focused introducing eco-friendly activities through recreation and entertainment are boat ride, flower shop, foot path, (The canal reservation from pagoda to parliament Road Bridge along kotte canal) and passenger transport (Beira Lake). Land use Paddy land and grassland: greatly affects soil conditions. Long term effect with inorganic fertilizers. Land value & Building value in flooding area are in low. Land use change has, potentially, a very strong effect on floods as humans have heavily modified natural landscapes. Large areas have been deforested or drained Hill slopes were modified for agricultural production, and highway constructions thus changing flow paths, flow velocities, and water storage, and consequently flow connectivity and concentration times. It is likely that hydrological significant changes will continue in the next decades due to loss of agricultural land and forests [Wheater and Evans, 2009]. 7.	Recommendations and Outcomes

•	The canal system in Metro Colombo area acts as a passage to drain out flood water from the Metro Colombo area •	Awareness of people for construction of their homes and other buildings around Colombo area above +2.00 m MSL areas which is the safe flood level for Colombo. •	There is no disaster risk reduction strategy in local administration line such as national, district and divisions Therefore it should improve the government’s capacity to respond effectively to climatic disasters. •	Nonstructural approaches: There is no mid- to long-term implementation plan for community awareness activities. Monitoring and evaluation mechanism should be improved. •	Ensure authorities are able to manage disaster successfully. There are a number of projects that have not been funded. Many proposed projects in drainage master plan for Metropolitan Colombo area in 2003 have not yet been implemented. •	It is necessary to establish monitoring and flood warning system at basin level. •	Investment in transportation and other important infrastructures during disaster, traffic shutdown of important road section causes serious problem in mobilization of rescue team and relief materials. It also make country’s economic damage larger. •	There is a plan to establish and strengthen riverine management division by 2019 and gradually establish basin management mechanism for important river basins after 2020. It is also targeted to enact new water resource law by 2020. •	A national storm water drainage policy for Sri Lanka has not yet been established. Also, prioritization of storm water drainage project implementation by river and area has not been made yet. •	It need to have community development project: create awareness, social marketing and community-level development. The most frequent natural disaster in Sri Lanka is flood (37%) followed by rain, strong wind, landslide and cyclone based on the records of disasters. 35% recorded deaths and missing people followed by flood, lightning and strong wind during rainy season in Sri Lanka. The above measurements shows a tendency of decreasing in frequency of flooding in the metro Colombo areas such Nugegoda, Kesbewa, Kelaniya due to engineering efforts through flood mitigation projects such as Werassa Ganga Storm and Drainage water project, Metro Colombo Tunnel Project, and Flood protection bond in kelani Ganga etc. Hydrological engineers states that Colombo Metropolitan area flood control plays an important role in the capital city flood mitigation approach and having influence of the other flood mitigation project towards its success by increasing flood retention area and diversion canals. 8.	List of reference

[1] A Chay, S Sidradjat (2018). Elsevier Journal, Sumedang, Indonesia. [2] Dr.S Wijerathne, G Soyza, T Gunasiri (2016). Metro Colombo Wetland Management Strategy, Land Development Corporation, Sri Lanka [3] M Moufar, E D P Perera (2018) Floods and Countermeasures Impact Assesment for the Metro Colombo Canal System, Sri Lanka. [4] Pumping Scheme for Storm Water Drainage-Mudun Ela Integrated Drainage & Rehabilitation Project (2012), Final Design Report. [5] J.Tasai & Y.M.Chiyang & L.C.Chiyang, (2011)Thaiwan water conservancy ,Zhejiang. https://www.researchgate.net/publication/289580568_The_intelligent_operation_of_pumping_station_for_flood_mitigation [6]Dutch water way in Sri Lanka, Virtual Library,  http://www.lankalibrary.com/geo/dutch/dutch4.htm [7] Sri Lanka Internet News Paper (2019) http://www.colombopage.com/archive_19B/Sep08_1567966940CH.php [8] Sri Lanka Land Development, Flood Mitigation, Special Projects, (2018) https://landdevelopment.lk/web/special-projects [9] Future water, Research and consultancy for a sustainable future of our water resources. [10] Ministry of Megapolise, Western Region Master Plan – 2030,(2019)From Island to Continent, Sri Lanka. [11] Vidyapriya.Va and Ramalingam July-2016. Mb, International Journal of Scientific & Engineering Research, Volume 7, Issue 7. [12] P. Vanderkimpen Flanders Hydraulics Research, Antwerp, Belgium Soresma, Antwerp, Belgium P. Peeters Flanders, (2018) Flood modeling for risk evaluation –a MIKE FLOOD sensitivity analysis Hydraulics Research, Antwerp, Belgium [13] N.K.Tuteja and M.Shaikh.(2019). Hydraulic Modelling of the spatio-temporal flood inundation patterns of the Koondrook Pericoota Forest Wetlands – The living Murray. [14] Tan Hooi Tee & Faizatul Akmar Abdul Nifa,(2017) Flood Mitigation in Disaster Management, University Utara Malaysia. [15] Chapter 2, Subpart B, Location Criteria / US EPA archive document. https://archive.epa.gov/epawaste/nonhaz/municipal/web/pdf/subpartb.pdf

[16] R.K Price & Z Vijonovic (2008), Urban Water Disaster Management, Urban water journal. [17] Mudun ela feasibility study, (2016), Ministry of Megapolise and western development, Sri Lnaka. [18] Kolonnawa storm water and flood controlling feasibility report, (2014) Ministry of Defense and Urban development, Sri Lanka. [19] Metro Colombo urban development Strategy, (2016), Ministry of Megapolise and Western Development. [20]Werassa Ganga Storm water and drainage Improvement feasibility study report, (2014), SLLRDC, Sri Lnaka. [21] The Lunawa Lake Environment Improvement and Community Development Project (LLEI&CDP) [22] H.M.D. Harshani & N.T.S. Wijesekara, 2011, IESL Engineer vol, Storm water Management Modelling for Unguaged Watershed in Matara Municipality. [23] Data collection survey on disaster risk reduction sector in Sri Lanka final report, 2017, JICA. [24]S. Satarasinghe, (2017), A Mess as old as time: The economic cost of natural disasters in Sri Lanka, Talking Economics, Institute of Policy Studies of Sri Lanka, http://www.ips.lk/talkingeconomics/2017/08/01/a-mess-as-old-as-time-the-economic-cost-of-natural-disasters-in-sri-lanka [25]M.G.T.De Silva & A Kawasaki, (2018), Socioeconomic Vulnerability to Disaster Risk,Elsevier, Ecological Economics [26]M.Rogger & M. Agnoletti and more, (2017), Water Resources Research, Online Library, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017WR020723.

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