Browsing by Author "Wijayaratna, TMN"

Now showing 1 - 9 of 9

item: Conference-Full-text
Analysis of storm water drainage system at park road in Colombo using storm water management model (SWMM)
(Department of Civil Engineering, University of Moratuwa, 2015) Nandharathne, ABDT; Wijayaratna, TMN; Hettiarachchi, MTP
Colombo city is the main commercial and administrative hub of Sri Lanka. The city is frequently subjected to flooding during rainy seasons which disturb day to day routines. The existing drainage system is not sufficient for the current storm water demand. In this study, the existing storm water drainage system in Park road area in Colombo 5 is analyzed using Storm Water Management Model (SWMM). Apart from simulating the existing storm water drainage system in the area, the present study identifies the possible reasons for the flooding and introduces remedial measures using the Storm Water Management Model (SWMM).
item: Conference-Full-text
A case study on the retention tanks in the walawe ganga river basin, Sri Lanka.
(IEEE, 2018-05) Thilakarathne, JASI; Wijayaratna, TMN; Chathuranga, D
Village tanks have been used for irrigation purposes from a longer period of time and its behavior as a cascade system can be considered as an efficient methodology towards a sustainable water management system. Though it’s not vividly addressed in recent studies, some tanks are present in a cascade system with a mere intention of retaining water for the downstream tanks and, are being called as retention tanks. Nevertheless, in the present day, those are being used to fulfill human water needs instead of their initial use as retention tanks. Due to this reason, the overall working capacity of village tank cascade systems is being diminished. And in this case study, a methodology is proposed to identify retention tanks using five cascade systems in Walawe ganga river basin.
item: Conference-Abstract
Effect of climate change on monthly pond storage variation - a case study in Jaffna, Sri Lanka
(2019) Thilakarathne, JASI; Rajapakse, RLHL; Wijayaratna, TMN
Due to various anthropogenic interventions, the inherent natural function of water has been disturbed and distorted. Proper water management is vital to reduce flood damages and also to increase water availability in the dry season. The Jaffna peninsula in the northernmost part of Sri Lanka makes use of its existing pond systems to have an enhanced water functionality. However, due to the climate change, the rainfall pattern in the region has changed and currently, the peninsula is experiencing water scarcity especially during the dry period (January~September) while frequent flash floods occur in the wet season (October~January). The study attempts to determine the effect of rainfall pattern variability on pond water availability. Moreover, the study illustrates the variation of pond water availability using the Average Storage Index (ASI) during the study period of 16 years. Taking the monthly precipitation data for the period of 2002~2017, the pond storage variation of Paalkulam pond in the Jaffna Municipal Council (JMC) area was modelled. Monthly rainfall data, evaporation data, pond physical data and catchment data were used for this analysis. Pond simulations were carried out using the HEC-ResSim computer application. Whit the monthly rainfall data, the model simulation results show that the pond storage variation is becoming even critical during the dry period. The ASIs for 2002 are 0.681 and 0.996 for the dry period and wet period, respectively. However, for the year 2017, these indices change to 0.497 and 1.000. Study results show that due to the changes in rainfall pattern in the JMC area, the pond water availability is decreasing in the dry period. Furthermore, when looking at the yearly variation of ASI of Paalkulam pond, it clearly shows that the water availability is strongly driven by the precipitation to the peninsula. Therefore, for more adverse rainfall patterns, the water availability in the region would be much more serious. When the study results are taken into considerations, it can be concluded that the ASI of Jaffna ponds is getting decreased in the dry period due to the change of rainfall pattern which shows a noticeable reduction of water availability and rehabilitation and proper maintenance of pond storage play a critical role in addressing water scarcity issues.
item: Conference-Abstract
An Estimation of storm surge heights of Colombo coastline
Maduwantha, MGP; Wijayaratna, TMN
Storm surges are one of the worst coastal marine hazards in the world. Coastal flooding due to tropical cyclone - induced storm surges has caused considerable damage and loss of life in the North Indian Ocean region including in Sri Lanka. Possible sea level rise is gradual and occurs over a long time frame. However, extreme weather events can occur at any time and can have a significant impact on the coast line. This study is about the estimation of storm surge heights at Colombo coastline. Sea water levels measured off Colombo Port for 31 years were analyzed using statistical methods to establish the design storm surge heights with known return periods. Harmonic analysis based computer program was used to generate astronomical tide and storm surges were considered as the difference between sea level and astronomical tide. The extracted surge data were statistically analyzed by using the Extreme Event Analysis method. Design sea levels with different return periods are calculated and presented with the paper. The storm surge height for 100 year return period was found to be 0.391m and corresponding design sea level height was 0.906 m relative to mean sea level.
item: Conference-Abstract
A Grain Size Threshold for Quantifying Active Sediment Input: A Case Study of Maha Oya River in the Western Coast of Sri Lanka
(Department of Earth Resources Engineering, University of Moratuwa, Sri Lanka, 2022-12-23) Jayathilaka, RMRM; Ratnayake, NP; Wijayaratna, TMN; Silva, KBA; Jayawardena, LC
Rivers have a significant role in the coastal sediment budget as the primary sources of natural sediment. Further, it is essential to comprehend the minimum grain size threshold, also known as the littoral cutoff diameter (LCD), in order to determine the actual contribution of river sediment load to the littoral beach. This study aims to present the LCD as a crucial variable worth taking into account at the regional level when budgeting for littoral sediments. Maha Oya River located on a severely eroding coastline between Negombo and Chilaw on the western coast of Sri Lanka was focus in this study. Here, we collected forty-four beach samples from the swash zone and berm crest during two sediment sampling surveys in March and October 2022. Concurrently, a streamer-type sediment trap was placed at 2 km upper streams of Maha Oya River to capture the bottom to 1.5 meters river sediment load. Sediment samples were sieved by mechanical sieving method in the classic Wentworth grain size scale (63-2000 μm). Additionally, the statistical properties of a grain size distribution, including LCD, skewness, kurtosis, and median grain size (D50), were investigated for sediment samples. Average D50 value on the beach was found to be 394 μm of medium sand that was moderately sorted, symmetrical, and mesokurtic, compared to 655 μm of coarse sand that was poorly sorted, fine skewed, and mesokurtic in river sediment. The LCD values was 141 μm on the beach studied herein, that indicated 10% of the river sediment in the lowest 1.5 m layer may not have remained on the beach in any significant quantity. Therefore, by using the concept of LCD, it is possible to quantify the potential overestimation of river sediment loads to the coastal sediment budget.
item: Article-Abstract
Numerical modeling of oil slick spread in the persian Gulf
(2015-08-20) Soltanpour, M; Wijayaratna, TMN; Hajisalimi, Z
An oil spill model coupled with a hydrodynamic model was developed to simulate the spread of oil slick in real marine conditions considering the effects of tidal currents, wind and wave. The hydrodynamic model is verified using the measurements of tidal elevations and current speeds at the Persian Gulf. Effect of various governing factors on oil slick movement, tidal currents, wind and wave, are examined. It is concluded that the wind action is the predominant factor for the spreading of oil while the overlaying waves are the second important driving force. Although the tidal currents spread the oil slick on a wider area, they have limited influences on the net transformation of slick. The performance of the model on a field data in the Persian Gulf shows that the present model is capable to predict the spread of oil in early days of the oil spill.
item: Conference-Full-text
Optimizing gravity flow for flood management in urban watersheds on kelani river bank
(IEEE, 2023-12-09) Jayasundara, JMASS; Wijayaratna, TMN; Geeshan, PGTN; Abeysooriya, R; Adikariwattage, V; Hemachandra, K
The drainage designs and operation procedures in coastal cities located on river banks are planned according to the worst-case flood scenario; the peak flood level at the urban watershed coincides with the flood peak of the river and the peak surge height at the sea. The annual exceedance probability of each event is considered separately to match the project design return period, without considering the joint probability of events. Though this approach is conservative, it always overestimates the flood risk and therefore leads to overdesign. This study aims to develop a design and operation procedure to optimize the pumping by incorporating the possibilities of phase differences of high rainfall events, high flood events, and high water levels at sea. The Kalu Oya-Mudun Ela sub-watershed at the downstream reach of the Kelani River is selected for the analysis. The probability of occurrence of a rainfall event above the 25-year return period and 10-year return period in Colombo gauging station, while Kelani River is at a minor flood level of 1.5 m MSL at Nagalagam Street is found to be 1.4% and 2.2%. The probability of occurrence of storm surge events and rainfall events at the Colombo gauging station is 100% and the occurrence of storm surge and rainfall while downstream reach is above minor flood level is 4%. It is shown that the gravity drainage of the local catchment is possible until the flood level of Nagalagam Street reaches 1.5 m MSL. Therefore, it is economical to design the pumping stations to cater to the discharges of a 10-year return period and enhance the gravity drainage capacities to drain out the stormwater under gravity before the Kelani flood level increases beyond 1.5 m MSL.
item: Conference-Abstract
A Preliminary evaluation of river sediment load: a case study of Maha oya river in the western coast of Sri Lanka
(Division of Sustainable Resources Engineering, Hokkaido University, Japan, 2024) Jayathilaka, RMRM; Ratnayake, NP; Wijayaratna, TMN; Silva, KBA; Iresha, H; Elakneswaran, Y; Dassanayake, A; Jayawardena, C
Rivers have a significant role in the coastal sediment budget as the primary sources of natural sediment. Furthermore, it's critical to understand the actual contribution of river sediment load to the littoral beach. Due to the practicality of the measures, the information regarding river measurements is scarce making this difficult. This study focused on the Maha Oya River, which is situated on a severely eroding coastline on Sri Lanka's western coast between Negombo and Chilaw. In this instance, the river sediment load was captured by two streamer-type sediment traps along the upper 2 km streams of the Maha Oya River, but only one of them was recovered. The sediment trap was retrieved for the months of December 2021–February 2022 (recovery 1) and March 2022–October 2022 (recovery 2). During two recovery periods, we concurrently collected forty-four beach samples from the swash zone and berm crest on either side of the Maha Oya River. For recovery periods one and two, the retained sediment weights are 3.7 and 726 grams, respectively. Additionally, the statistical properties of a grain size distribution, skewness, kurtosis, median grain size (D50) and the littoral cutoff diameter (LCD) value were investigated. Average D50 value on the beach was found to be 394 μm of medium sand that was moderately sorted, symmetrical, and mesokurtic, compared to 655 μm of coarse sand that was poorly sorted, fine skewed, and mesokurtic in river sediment. The LCD value on the beach under study was 141 μm indicating that the river sediment below LCD may not have remained on the beach in any significant quantity. The literature states that river sand mining caused the sediment load via the Maha Oya River to decrease from 0.15 mcm/year in 1984 to 0.05 mcm/year in 2001. Our calculations show that in 2022, this number will be further decreased to 0.01 mcm/year. Thus, it is highlighted that there is a major shortage of river sand input into the coast, which may be the cause of the disastrous erosion occurring in the surrounding coastal cells.
item: Conference-Abstract
Seasonal freshwater outflow variation of gin and Nilwala river basins
(Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, 2022-12) Dilrukshi, YDNM; Wijayaratna, TMN; Mallikarachchi, C
River discharge is primarily related to water resource management because it is an important event in the global water cycle. The hydrological processes within a catchment are influenced by land usage, land cover, soil properties, and climate. These factors must be considered when calculating accurate river discharges. This study assessed the seasonal variation of freshwater outflow over 20 years from the Gin, and Nilwala River basins using a rainfall-run-off model developed based on Rainfall-Runoff-Inundation (RRI) software. Daily precipitation, streamflow, pan evaporation, and topography data were used to develop, calibrate, and carry out model runs. Manning's roughness coefficient for the river (nr) and the slop (ns), soil depth, saturated hydraulic conductivity (ka), and vertical infiltration (ksv) were the calibration parameters of the RRI Model. In addition, deviation of runoff volume (DV), Nash Sutcliffe, Pearson correlation coefficient (R), and root-mean-square error (RMSE) were performed as objective functions in comparing the observed and simulated discharge for the calibration and validation of the model. Because the model cannot predict the sudden peak flow, the model flow upstream does not simulate well with the observed flow, yet the overall model performs well in the calibration and validation process in both catchments. The Monthly, seasonal, and annual trends of both river basins were analysed from 2001 to 2020. The non-parametric Mann-Kendall approach was used to assess the statistical significance of any positive or negative trends in freshwater discharge. The freshwater outflow increased significantly in June, August, and September in Gin River while the Nilwala river had a significant increasing trend in August and September. While the Nilwala river had a substantial growing trend in the first inter-monsoon, the freshwater output in the Gin River increased significantly during the Southwest monsoon. Besides, neither river's yearly outflow showed any discernible trends. The RRI model is suitable for analysing the total discharge to the sea since inundation is also considered.