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Salinity Intrusion into the Maha Oya Estuary and Associated Coastal Waterway

Weerakkody, Don Clerance Denzil LU (2012) VVR820 20111
Division of Water Resources Engineering
Abstract
The Maha Oya (river) originates in the Kandy district of Sri Lanka and travels about 130 km passing four districts to reach the sea at Kochchikade in Sri Lanka. It holds the 3rd largest average annual runoff among the 103 of distinct river basins in Sri Lanka. Its contribution as a fresh water source is of great significance. During year 2004 the river provided five percent of the total production according to the National water supply and drainage board through various intakes along the river. There are many industries located along the river that requires good water quality. The areas in the lower reach of the Maha Oya is urbanized and densely populated. The Maha Oya in the lower reaches has been occupied by many industries, among them,... (More)
The Maha Oya (river) originates in the Kandy district of Sri Lanka and travels about 130 km passing four districts to reach the sea at Kochchikade in Sri Lanka. It holds the 3rd largest average annual runoff among the 103 of distinct river basins in Sri Lanka. Its contribution as a fresh water source is of great significance. During year 2004 the river provided five percent of the total production according to the National water supply and drainage board through various intakes along the river. There are many industries located along the river that requires good water quality. The areas in the lower reach of the Maha Oya is urbanized and densely populated. The Maha Oya in the lower reaches has been occupied by many industries, among them, clay mining and sand mining causes adverse effects by changing river morphology in an unfavorable manner. The lower reach of the Maha Oya is in balance with the marine and riverine environmental forcing, though in many situations the river flow dominates over the ocean hydrodynamics. The river connects to the sea through a shallow inlet forming a salt wedge estuary. During low-flow periods, the inlet closes through a sand bar that reduces the open connection to the sea. During average flow conditions, the inlet is about 3-5 m in depth and about 15-30 m in width. Inlet characteristics and river bathymetric conditions vary with time and are highly dependent on the amount of sediments available, because river and inlet dimensions are formed by sand that is transported by the river.

Salinity intrusion in the river seems to be increasing with the human activities and in different climate change scenarios. The development of a salinity intrusion model would be helpful to understand the effect on the river environment from salinity intrusion. Thus, the main objective of the present study was to develop a model of the salinity intrusion in Maha Oya. In order to validate the salinity intrusion model, the hydrodynamics of the estuary and the salinity structure were studied during various field campaigns. Salinity measurements, river flow, tidal inputs, and bathymetry of the river are needed in such validations. The field data showed that the Maha Oya has a stratified salinity structure of wedge type. Being a long narrow channel it may be modeled as a one-dimensional system. The shallowness of the river reasonably well allows for the application of shallow-water wave theories in simulation of tidal waves in the model. Assuming stratification in two layers with sharp interface, the flow equations can be simplified and conveniently solved by finite difference approximations. Vertical mixing at the interface was introduced with a mixing coefficient as a calibration parameter. Two additional calibration parameters are the bottom friction factor and the interfacial friction coefficient. Under the assumptions of densimetric critical flow at inlet, a free upstream boundary with no salt flux, and a river with only fresh water initially, the model simulates the intrusion with the river flow and tidal level as input data. This model is a simple time-varying model that works even in absence of high-tech instruments for data collection. (Less)
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author
Weerakkody, Don Clerance Denzil LU
supervisor
organization
course
VVR820 20111
year
type
H2 - Master's Degree (Two Years)
subject
keywords
the Maha Oya, Salinity intrusion, stratified flow, time-varying model, two-layer model, salt water wedge.
report number
TVVR-12/5003
ISSN
1101-9824
language
English
id
2539869
date added to LUP
2012-05-25 13:43:33
date last changed
2012-05-25 13:43:33
@misc{2539869,
  abstract     = {The Maha Oya (river) originates in the Kandy district of Sri Lanka and travels about 130 km passing four districts to reach the sea at Kochchikade in Sri Lanka. It holds the 3rd largest average annual runoff among the 103 of distinct river basins in Sri Lanka. Its contribution as a fresh water source is of great significance. During year 2004 the river provided five percent of the total production according to the National water supply and drainage board through various intakes along the river. There are many industries located along the river that requires good water quality. The areas in the lower reach of the Maha Oya is urbanized and densely populated. The Maha Oya in the lower reaches has been occupied by many industries, among them, clay mining and sand mining causes adverse effects by changing river morphology in an unfavorable manner. The lower reach of the Maha Oya is in balance with the marine and riverine environmental forcing, though in many situations the river flow dominates over the ocean hydrodynamics. The river connects to the sea through a shallow inlet forming a salt wedge estuary. During low-flow periods, the inlet closes through a sand bar that reduces the open connection to the sea. During average flow conditions, the inlet is about 3-5 m in depth and about 15-30 m in width. Inlet characteristics and river bathymetric conditions vary with time and are highly dependent on the amount of sediments available, because river and inlet dimensions are formed by sand that is transported by the river.

Salinity intrusion in the river seems to be increasing with the human activities and in different climate change scenarios. The development of a salinity intrusion model would be helpful to understand the effect on the river environment from salinity intrusion. Thus, the main objective of the present study was to develop a model of the salinity intrusion in Maha Oya. In order to validate the salinity intrusion model, the hydrodynamics of the estuary and the salinity structure were studied during various field campaigns. Salinity measurements, river flow, tidal inputs, and bathymetry of the river are needed in such validations. The field data showed that the Maha Oya has a stratified salinity structure of wedge type. Being a long narrow channel it may be modeled as a one-dimensional system. The shallowness of the river reasonably well allows for the application of shallow-water wave theories in simulation of tidal waves in the model. Assuming stratification in two layers with sharp interface, the flow equations can be simplified and conveniently solved by finite difference approximations. Vertical mixing at the interface was introduced with a mixing coefficient as a calibration parameter. Two additional calibration parameters are the bottom friction factor and the interfacial friction coefficient. Under the assumptions of densimetric critical flow at inlet, a free upstream boundary with no salt flux, and a river with only fresh water initially, the model simulates the intrusion with the river flow and tidal level as input data. This model is a simple time-varying model that works even in absence of high-tech instruments for data collection.},
  author       = {Weerakkody, Don Clerance Denzil},
  issn         = {1101-9824},
  keyword      = {the Maha Oya,Salinity intrusion,stratified flow,time-varying model,two-layer model,salt water wedge.},
  language     = {eng},
  note         = {Student Paper},
  title        = {Salinity Intrusion into the Maha Oya Estuary and Associated Coastal Waterway},
  year         = {2012},
}