Browsing by Author "Silva, KBA"

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item: Conference-Full-text
Application of hydrodynamics to assess coastal morphology to the north of Kelani River, Sri Lanka
(Department of Earth Resources Engineering, 2019-08) Ishankha, WCA; Kahawala, KKHB; Sandushan, JAC; Hewavidana, SM; Silva, KBA; Ratnayake, NP; Dissanayake, DMDOK; Samaradivakara, GVI
Studies on hydrodynamics are important since the hydrodynamic processes highly contribute to sediment transportation in the coastal regions. Erosion in Uswetakeiyawa coastal region of Sri Lanka has been a significant environmental issue since 2000. The Coast Conservation & Coastal Resources Management Department of Sri Lanka carried out artificial beach nourishment in 2012, and then a set of offshore breakwaters was constructed in 2013 to protect the nourished stretch of the beach. However, the beach was not developed to the desired extent. This research was carried out to assess hydrodynamics to forecast the noted coastal morphology in the Uswetakeiyawa coastal region. Wave data were gathered and analyzed using "SWELLBEAT" software for 12 months, covering both the monsoons. Further, "Delft3D" open source software was used to simulate the hydrodynamics in the study area, and seasonal changes of coastal morphology were analyzed for the study area using GoogleEarth images. The findings of this research indicated that due to the strong wave action perpendicular to the coast during the southwest monsoon, the beach was more vulnerable to erosion and there was no sediment transportation to the beach from longshore drift during inter-monsoon periods because sediments contributed from Kelani River were directly transported to the ocean. Also, the predominant sediment transportation to the study area was during the northeast monsoon.
item: Article-Abstract
Chloride contamination in construction aggregates due to periodic saline water intrusion: a case study in the Kaluganga River Estuary, Sri Lanka
(2014-07-22) Ratnayake, NP; Silva, KBA; Kumar, IGIK
The Kaluganga River Estuary is one of the main sources of construction sand in Sri Lanka. Salt water intrusion along this estuary due to extensive sand mining has increased over the years. Thus, the focus of the current research is to understand the relationship between river sand mining, salt water intrusion, and the resultant effects on construction sand. Two surveys were conducted along the Kaluganga Estuary along an 11 km stretch from the river mouth at predetermined intervals to measure depth water quality profiles, and to collect sediment samples. These surveys were carried out during maximum spring tide; first in a dry period and then in a wet period, to understand hydrographic effects on the quality of river sands. Sand samples were analysed for absolute chloride content and grain size distribution. Results showed significant salt water intrusion during the dry period, averaging 2,307 μS cm−1 in surface waters throughout the surveyed 11 km stretch along with 3,818 μS cm−1 (average) in bottom waters up to 5.6 km upstream from the river mouth causing above normal chloride content in the bottom sandy sediments. The high chloride content in bottom sands was recorded up to 5.5 km from the river mouth making them unsuitable for construction purposes. However, during wet period, salt water intrusion levels in the bottom waters were insignificant (average 61 μS cm−1) and the chloride content in bottom sediments was very low. This study highlighted the requirement for regulations on river estuary sandmining for construction purposes.
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: Conference-Full-text
Impacts of salt water intrusion on construction aggregates of 'Kalu ganga' estuary
(Department of Earth Resources Engineering, 2009-05) Silva, KBA; Kumara, IGIK; Rathnayake, NP; Karunaratne, S
'Kalu Ganga' is one of the main suppliers of river sand (113,360 cubes per year) in the country. Due to the continuation of sand mining, salt water intrusion has been increased over the years. The focus of the current research is to understand impacts of salt water intrusion on river sand and to understand whether the absolute chloride content of mined sand is within permissible level (0.075% Cl ) for the construction purposes. Two surveys were conducted along 'Kalu Ganga' estuary (10 km from river mouth) in predetermined intervals to measure depth, conductivity profiles and to collect sediments samples. Survey dates (16/12/2008 and 12/03/2009) were selected considering the maximum difference of tide and rain fall. The sand samples were analysed for Cl' content. Results showed significant salt water intrusion even during dry season («2300 pS/cm). All measured conductivity values were similar to the conductivity levels observed at the sea mouth. But during dry season the Cl- content of sediments is relatively high at river mouth. However, during rainy season, measured salt water intrusion levels were dropped down remarkably («200 pS/cm) and even at the sea mouth, sediment samples showed significant low values of absolute Cl* content (0.0176%). Therefore, it is recommended to extract sand from 0.5km from the sea mouth only during rainy season. This will also control the flooding in Kaluthara area. However, further studies are necessary for understanding other environmental effects of river sand mining near the sea mouth.
item: Conference-Extended-Abstract
Impacts of salt water intrusion on construction aggregates of 'Kalu Ganga' estuary
(2014-08-08) Silva, KBA; Kumara, IGIK; Rathnayake, NP
'Kalu Ganga' is one of the main suppliers of river sand (113,360 cubes per year) in the country. Due to the continuation of sand mining, salt water intrusion has been increased over the years. The focus of the current research is to understand impacts of salt water intrusion on river sand and to understand whether the absolute chloride content of mined sand is within permissible level (0.075% Cl-) for the construction purposes. Two surveys were conducted along 'Kalu Ganga' estuary (10 km from river mouth) in predetermined intervals to measure depth, conductivity profiles and to collect sediments samples. Survey dates (16/12/2008 and 12/03/2009) were selected considering the maximum difference of tide and rain fall. The sand samples were ana lysed for Ct- content. Results showed significant salt water intrusion even during dry season ("'2300 IlS/ cm). All measured conductivity values were similar to the conductivity levels observed at the sea mouth. But du:ring dry season the CI- content of sediments is relatively high at river mouth. However, during rainy season, measured salt water intrusion levels were dropped down remarkably ("'200 IlS/ cm) and even at the sea mouth, sediment samples showed significant low values of absolute Cl- content (0.0176%). Therefore, it is recommended to extract sand from O.5km from the sea mouth only during rainy season. This will also control the flooding in Kaluthara area. However, further studies are necessary for understanding other environmental effects of river sand mining near the sea mouth.
item: Conference-Full-text
Monitoring beach profile changes and modelling nourishment scenarios for Ratmalana beach
(Department of Earth Resources Engineering, University of Moratuwa, 2020-12) Gunasekara, MP; Madushani, EK; Govinath, J; Ratnayake, NP; Samaradivakara, GVI; Dushyantha, NP; Gunasinghe, GP; Silva, KBA; Dissanayake, DMDOK; Dassanayake, ABN
Sri Lanka is an island nation endowed with a wide range of coastal resources, greatly contributing to the Gross Domestic Product. Coastal erosion is a major issue related to the beaches in Sri Lanka. In this regard, Ratmalana beach is critically important as a potential tourism destination due to its proximity to the capital city and the accessibility to a coastal railway station. To develop a beach as a tourist destination, it is importance to monitor the beach profile changes and find out the remedial measures for erosion prevention. For such a management plan, either hard or soft engineering solutions can be utilized. Out of the soft engineering solutions, the most reliable solution is beach nourishment. In this research, Ratmalana beach was regularly monitored and a numerical model was built by utilizing the public domain of XBeach to model the hydrodynamics in the area. Finally, two nourishment scenarios were modelled, and the optimum nourishment scenario is determined. Beach profile monitoring and calculated sand budget indicates that there is significant erosion during the stormy weather season. Based on grain size analysis, Ratmalana beach has a broad grain size distribution. According to the modelled nourishment scenarios, profile nourishment has shown better performance.
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.