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Salinity Studies on Gin Ganga and Nilawala Ganga |
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Location |
South West Area of Srilanka
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Type of Project |
Model study to optimized location, type and dimensions of salinity barrier to be constructed on Gin Ganga River
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Period of Study |
April to May, 1999
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Client |
Lanka Hydraulic Institute |
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Gin Ganga River |
Without Barrier
With Barrier
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Description
The Gin Ganga and the Nilwala Ganga Rivers in the south
west of Srilanka are the main source of potable water for the coastal
towns of Galle. Increased demand of drinking water and extreme low fresh
water flow caused the salinity intrusion at the intake locations, the high
level of salinity intrusion forces to relocate the intake locations of
water supply system. The construction of the salinity barrier has become a
burning issue to solve this problem. In Sri Lanka, the maximum tidal range is about 0.6m and
because of this low tidal range the mixing of saline and brackish water is
insignificant and the estuary is usually stratified. A salt water wedge
forms under the fresh water layer, the salt water wedge creeps up along
the river bed while the upper fresh water layer even could flow
downstream. In the Gin Ganga, the water supply intakes experienced
salinity intrusion in the recent years during dry period. The objectives of the hydraulic modelling study are firstly to optimize the location, type and dimensions of the salinity barrier and secondly to provide pertinent hydraulic characteristics for the salinity and morphological studies and to investigate the impact of Salinity Barrier on upstream flood and low-flow. Different magnitudes of flood events have been simulated incorporating salinity barrier as a broad crested weir and control structure. It has been observed that broad crested weir as a salinity barrier has significant impact on upstream water level for flood event ranging from 25 to 50years, the increase of water level just at upstream ranges from 2m to 1.75m. Where as the control structure as salinity barrier shows insignificant impact on upstream water level for the same magnitude of flood events. It has been seen that tide travels up to 24km upstream to the mouth of the river if there is no upland fresh water flow. Model results show considerable change in the tidal flow during dry season if salinity barrier kept close for a month, it is because the barrier is only 5km to 6km upstream, so it reduces the tidal prism considerably. The average tidal prisms for a tidal cycle for the month of March with and without salinity barrier are 0.32mm(million m3) and 0.96mm respectively.
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