Strategic Monitoring and Resilience Training in the Ala Wai Watershed, Oʻahu: Seasonal and Episodic Variability
Date
2020
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Abstract
The Ala Wai Canal is an artificial estuary connecting three major streams (Makiki,
Palolo, Mānoa) and numerous city drainages to the ocean in Honolulu, Hawai’i. The Ala
Wai Canal is where the terrestrial influence from the watershed meets oceanic forcing (e.g.
tides, salinity). However, extensive urbanization has led to severe eutrophication in the Ala
Wai Canal. Limited previous watershed data inhibits restoration and resilience efforts in
Ala Wai Canal and the watershed. In order to understand the biogeochemical interactions
and anthropogenic inputs within the watershed, this study aims to construct high-resolution
spatial and temporal characterizations of current physical and chemical parameters.
Monthly water quality surveys and sampling were conducted at 12 sites along three major
streams in the watershed; bathymetry mapping, sensor profiling, and discrete water
sampling were conducted in the canal and in the nearshore water. A high-resolution
bathymetry survey in the canal was conducted at the beginning of the study period. Sensor
data included temperature, salinity, dissolved oxygen, chlorophyll, and turbidity, while
discrete samples were analyzed for dissolved inorganic nutrients and dissolved inorganic
carbon. Vertical stratification and horizontal gradients in the Ala Wai Canal were strongly
affected by runoff from episodic rain events, tidal mixing, and canal bathymetry.
Bathymetry mapping and sensor surveys indicate that lateral mixing is primarily inhibited
by sediment shoals, while vertical mixing was largely affected by salinity gradient.
Increased runoff facilitates the vertical mixing downstream to major freshwater inputs.
Data also shows that the restricted end of the canal was largely anoxic in the bottom water
due to limited ventilation and enhanced organic matter respiration. Quantification of
physical/chemical parameters can assist in understanding urbanization and ecological
effect, which is especially important for future planning, legislation, engineering, and
resilience decisions.
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watershed, water quality, ala wai canal, data, sensor
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72 pages
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Chen, Solomon
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