Modelling Stratification Ascendancy of Kirulapone Canal at Wellawatte Outfall

Annual Sessions Sessions of of IESL, IESL, pp. pp. [145 [1 - 10],154], 2018 2018 © The InstitutionInstitution of of Engineers, Engineers, Sri Sri Lanka Lanka Modelling Stratification Ascendancy of Kirulapone Canal at Wellawatte outfall G.L.E.P. Perera, G.C.R. Dayarathne, H.K.M. Perera and B.C.L. Athapattu Abstract: Kirulapone canal which is a prime sector of Diyawanna canal system bifurcate at the downstream and fall into sea from Dehiwala and Wellawatte estuaries. The significance of this canal network is that it is facilitated to prevent the possibility of flooding in urban city Colombo by flowing rainwater towards the ocean through a canal network. The present study focused on the water stratification ascendancy of Kirulapone canal at Wellawatte outfall. Stratification is a critical condition which can effect on the quality of the water body and it may influence on the deterioration of water quality by resulting an anoxic condition at deeper water layers. Stratification and turbulent mixing determine the underwater environmental condition. According to the vertical density profile the estuaries can be classified as salt-wedge, partially-mixed or well-mixed. In this study, the stratification behaviour of the water body at Wellawatte outfall was identified using Froude number. Froude number was calculated using the canal discharge calculated by “Tank Model”. Also, the effect of water quality on stratification of the estuarine channel was investigated by series of field observations and statistical analysisat different depths of canal water during dry and wet weather periods. Salinity and DO variation with the water depth was studied periodically. According to the Tank Model analysis and field test results with the meteorological data, correlation between hydrology and water quality parameters was identified. Calculated Froude number showed a qualitative stratification effect while field observation results remaining unchanged along the water profile from top to bottom. Saline water intrusion has a significant effect on the downstream water quality. Keywords: Stratification, Tank Model, Urban water canal, Froude number, Sea outfall 1. Introduction between sea water and fresh water depends upon hydrodynamic forces, canal bathymetry Stratification that can be resulted due to vertical and the general geomorphology of the area. variation of salinity, temperature, chemical Stratification agitates the function of aquatic composition or suspended particle distribution ecosystems and conduces to the water is experienced in connection with many qualitydeterioration[4]. Wellawatta estuary is geophysical water flows as estuaries. It one of the major outfalls of Diyawanna canal prevents the transmission of dissolved oxygen network. Static analysis of water quality among eplimnion and hypomnion by forming a assessments explored continuous water barrier between the water layers. Thereby, it pollution including the abnormal salinity leads formation of anoxia at deep water levels increment in this canal extent[5]. Thus, the [1]. The vertical circulation of water body and impact of sea water intrusion on the variation propagation of internal tidal waves adversely of canal water quality at Wellawatta estuary depends upon density alteration. The interface was studied during this research. can act as a barrier restraining mixing of water column[2]. Hence, this study is dedicated to Eng. (Dr.) B.C.L. Athapattu, BSc Eng(Moratuwa), quantifying the effect of thermal and salinity MEng(Osaka), PhD(Osaka), CEng, MIESL, Senior stratification on the water quality for a Lecturer, Department of Civil Engineering, The Open sustained estuarine ecology. University of Sri Lanka. The salt water intrusion is an important natural Eng.(Ms) G.L.E.P. Perera, BTech (OUSL), AMIESL, phenomenon that is incorporated with the Department of Civil Engineering, The Open University of Sri Lanka. estuarine environmental management. Water bodies relevant to low lying coastal areas of Sri Eng. G.C.R.Dayarathne, BTech (OUSL), AMIESL Lanka are severely affected due to high salinity Mr. H.K.M. Perera, BTech (OUSL) concentration[3]. The density difference 1451 ENGINEER ver the ast decades, the catchent areas of articular area as the study locale. ine oints Metroolitan olombo have been influenced by were selected along the canal fro ehiwala a nuber of anthroogenic iacts and bifurcation to ellawatta estuary. our unauthoried activities[6]. iyawannawa canal samling oints were fallen close to the estuary network has been threatened by the about 5m distances and the rest were selected deterioration of water quality. Based on about 0m aart fro oint to point towards researches carried out by the Sri Lanka Land the ustream and fied with local GPS Reclamation and eveloent ororation coordinates. The irulapone and Wellawatte (SLLRDC)[7], Ammonia, hosphorus and Total canal etents were selected to conduct the water issolved Solids T) were recognied as the quality survey. main robleatic water quality arameters. According to historical water quality test 2. Literature Review reorts of SLLR, Dissolved ygen DO) values seem to be less than 3mgL and show no Tides and wave currents form advection of correlation with samling results. And also saline water along the river while imroving most arameters that show a reduction in miing of saline water. iver flow contributes quality are due to domestic waste water by imosing barotroic longitudinal ressure disosal[7]. ence further deterioration of gradients along the estuary because of its water quality would adversely damage the buoyancy. urthermore, the density difference ecosystes of iyawanna canal network. between fresh and saline water also induce baroclinic ressure gradient along the estuary. The salinity fluctuation indicates the canal River flow is the main opponent against salinity resonse to the tidal variations and field intrusion. t ushes back the salt water front measureents confir the eistence of with interfacial shear induced by the river flow. stratified thermal structure. Salinity intrusion in or high flow conditions, the salt water may be ellawatta estuary can often be described washed away. ind-induced stress also can be based on the salt water wedge theory, where a significant local force that revents salinity two layers of stratified counter flows occur with intrusion[2]. a shar interface. The thermal stratification was simulated using D and teerature in the The global teerature is gradually increases vertical rofile. valuation of stratificationin since few centuries8]. The increent of ellawatta outfall would be advantageous teerature warms u the surface water layer when it coes to mitigation of further water of the reservoir than the deeer water layer. quality deterioration. ence, the cold water layer is denser and because of the density variation among to and 1.1 Study Area bottom layers the water column tends to stratify. The barrier restraining iing of the water column divides the eilimnion and the hypolimnion from the thermocline. As a result of incomlete miing of the water column and lack of light for the hotosynthesis at the hypolimnion, anoic condition is created[1]. 2.1 Nature of canal network iyawanna canal network encoasses a large etent of greater olombo basin with nuber of interconnected canals and arshes crisscrossing the olombo city and its imediate suburbs. This channel system was made in utch era and lays a dominant role to revent the possible flood inundation risk by functioning as a cople drainage canal network. Aroimated total length of canal is about 29.2 k which is fed by catchent area about 9 k2[9]. rom the four outlets of the Figure 1 – Downstream of Wellawatta canal canal network ehiwala, ellawatta and Mutwal tunnel outfalls directly discharge to the Elevated salinity values in the downstream art sea while the outlet at orth ock discharges of the ellawatta canal lead us to select the ENGINEER 1462 ver the ast decades, the catchent areas of articular area as the study locale. ine oints into Kelani river. But the Mutwal tunnel is Metroolitan olombo have been influenced by were selected along the canal fro ehiwala contemorary alost remained in an a nuber of anthroogenic iacts and bifurcation to ellawatta estuary. our inoerative condition because of blockage and unauthoried activities[6]. iyawannawa canal samling oints were fallen close to the estuary the outlet at orth Lock also closed during dry network has been threatened by the about 5m distances and the rest were selected season to avoid backwater tresass from Kelani deterioration of water quality. Based on about 0m aart fro oint to point towards river to the canal network[10]. Though researches carried out by the Sri Lanka Land the ustream and fied with local GPS aroiated 00 ha of low lying marshes at Reclamation and eveloent ororation coordinates. The irulapone and Wellawatte uer stream lay a crucial role of iroving (SLLRDC)[7], Ammonia, hosphorus and Total canal etents were selected to conduct the water the canal water quality, the water ollution issolved Solids T) were recognied as the quality survey. remains staggering at downstream due to main robleatic water quality arameters. nutrient enrichment resulted by waste According to historical water quality test 2. Literature Review duing, unicial wastewater discharge and reorts of SLLR, Dissolved ygen DO) saline water interference[11]. values seem to be less than 3mgL and show no Tides and wave currents form advection of correlation with samling results. And also saline water along the river while imroving

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