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#           Observations of coastal circulation, waves, and sediment transport along West Maui, Hawaiʻi (November 2017– March 2018), and modeling effects of potential watershed restoration on decreasing sediment loads to adjacent coral reefs      

      February 22, 2023    

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Terrestrial sediment discharging from watersheds off West Maui, Hawaiʻi, has been documented as a primary stressor to local coral reefs, causing coral reef health to decline. The U.S. Geological Survey acquired and analyzed physical oceanographic and sedimentologic field data off the coast of West Maui to calibrate and validate physics-based, numerical hydrodynamic and sediment transport models of the study area developed by Deltares. These models simulated terrestrial sediment transport and dispersal from West Maui watersheds into coastal waters and how terrestrial sediment affects nearby coral reefs under different oceanographic forcing and watershed restoration scenarios.

Wave energy and near-bed turbidity are positively correlated in the field observations, illustrating a process not captured by the model simulations in which sediment already deposited on the seabed is resuspended by wave action and subsequently transported by prevailing currents. In the model simulations, large waves during flood events led to a decrease in suspended-sediment concentrations. Notably, however, the model results only consider sediment entering coastal waters from five stream sources and do not simulate sediment already present on the seabed.

The model simulations project that the Honokeana and Māhinahina coral reefs would experience the greatest reduction in sediment impacts from theoretical watershed restoration. Additionally, when large waves coincide with flood events, post-storm sedimentation generally decreases in the nearshore region, but increases in the region offshore of the reefs. The measured and modeled sediment dynamics demonstrate a demarcation between the coral reefs sheltered within embayments (Honolua reef) or behind points (Wahikuli reef) and those along the relatively open coastline between Kapalua and Kāʻanapali (Kapalua, Honokeana, Māhinahina, and Honokōwai reefs). The sheltered sites are affected by terrestrial sediment from single stream mouths, where most sediment is delivered within hours of a flood (rain) event. Once this sediment enters the nearshore, it settles out and remains within the reef area for a prolonged period owing to a lack of wave or current-driven bed shear stress. Thus, the primary effect of sediment on the reefs within these sheltered areas is sedimentation. In contrast, coral reefs along the unsheltered (or “open”) section of coastline (between Kapalua and Kāʻanapali) are more exposed to waves and terrestrial sediment from multiple stream sources. At these reefs, fine-grained terrestrial sediment can rarely settle but instead remains in suspension. Thus, even long after a flood event has occurred, these sites chronically experience light attenuation from suspended sediment.

These analyses underscore the importance of understanding how coastal ocean waves and circulation can lead to different sediment dynamics and stressors for coral reefs along the same region of the West Maui coastline. These differing factors indicate that the most effective watershed restoration and mitigation strategies may vary among the different coral reefs and streams. An important next step is to determine how the science of this study can support management goals for these coral reefs: what are target reductions of sedimentation, suspended-sediment concentrations, or the resulting light attenuation? Then, using the coupled hydrodynamic-sediment model, we can examine which watershed restoration scenarios in each stream will best achieve those targets.

## Citation Information

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link

June 27, 2022

####                 Coral Reef Project              

Natural Hazards Mission Area, Coastal and Marine Hazards and Resources Program, Pacific Coastal and Marine Science Center

link

June 27, 2022

####   Coral Reef Project

Learn More

February 24, 2023

####                 Model parameter input files to compare effects of stream discharge scenarios on sediment deposition and concentrations around coral reefs off west Maui, Hawaii              

Pacific Coastal and Marine Science Center

February 22, 2023

####                 Time-series data of water surface elevation, waves, currents, temperature, and turbidity collected between November 2017 and March 2018 off the west coast of Maui, Hawaii, USA              

Pacific Coastal and Marine Science Center

#### Curt Storlazzi, PhD

##### Research Geologist

Pacific Coastal and Marine Science Center

cstorlazzi@usgs.gov

831-460-7521

#### Olivia Cheriton

##### Oceanographer

Pacific Coastal and Marine Science Center

ocheriton@usgs.gov

831-460-7579

#### Kurt Rosenberger

##### Oceanographer

Pacific Coastal and Marine Science Center

krosenberger@usgs.gov

831-460-7535

#### Joshua Logan

##### Physical Scientist

Pacific Coastal and Marine Science Center

jlogan@usgs.gov

831-460-7519

## Related Content

link

June 27, 2022

####                 Coral Reef Project              

Natural Hazards Mission Area, Coastal and Marine Hazards and Resources Program, Pacific Coastal and Marine Science Center

link

June 27, 2022

####   Coral Reef Project

Learn More

February 24, 2023

####                 Model parameter input files to compare effects of stream discharge scenarios on sediment deposition and concentrations around coral reefs off west Maui, Hawaii              

Pacific Coastal and Marine Science Center

February 22, 2023

####                 Time-series data of water surface elevation, waves, currents, temperature, and turbidity collected between November 2017 and March 2018 off the west coast of Maui, Hawaii, USA              

Pacific Coastal and Marine Science Center

#### Curt Storlazzi, PhD

##### Research Geologist

Pacific Coastal and Marine Science Center

cstorlazzi@usgs.gov

831-460-7521

#### Olivia Cheriton

##### Oceanographer

Pacific Coastal and Marine Science Center

ocheriton@usgs.gov

831-460-7579

#### Kurt Rosenberger

##### Oceanographer

Pacific Coastal and Marine Science Center

krosenberger@usgs.gov

831-460-7535

#### Joshua Logan

##### Physical Scientist

Pacific Coastal and Marine Science Center

jlogan@usgs.gov

831-460-7519

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####               Pacific Coastal and Marine Science Center          

2885 Mission StreetSanta Cruz, CA95060United States

831-460-7401

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