Negombo Lagoon bar mouth tidal exchange: saline intrusion into the Dandugam Oya river basin?
What tidal range metrics and bathymetric thresholds govern saltwater intrusion past the Poruthota canal gates during dry inter-monsoon periods?
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3 Answers from travellers
Negombo Lagoon is connected to the Indian Ocean via a single narrow northern tidal inlet at Poruthota. During the dry season (January to March), freshwater discharge from the Dandugam Oya drops to seasonal minimums (under 10 m3/s). Semidiurnal micro-tides (amplitude ~0.6m) push dense oceanic saltwater deep into the lagoon basin, forming a wedge with bottom salinities exceeding 28 PSU that travels several kilometers up the Dandugam Oya. This intrusion salinizes municipal drinking water intakes, requiring the Water Board to close inflatable weir gates to protect upstream freshwater abstraction.
Illegal riverbed sand dredging deepens the natural bathymetric sill, allowing the saline wedge to penetrate much further inland than historical baselines.
Saline ingress shifts mangrove fringe ecology, encouraging halophytic Avicennia marina expansion while stunting freshwater-adapted Sonneratia stands.
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