10 January 2016

Waves, Wakes and Water Clarity: Understanding Physical Drivers of Seagrass Distribution in Coastal Waters

Mark Fonseca, Ph.D.CSA Ocean Sciences Inc.

Amit Malhotra, MSc., GISPGeohorizons

Problems and Solutions – Physical Drivers of Seagrass Landscape Dynamics

Likely Physical Drivers

• Wind Waves

• Vessel Wakes

• Spatial and Temporal Light Availability

Tools Highlights

• WEMo – Wave exposure model (+ higher order tools)

• BoMo – Boat wake model (method)

• OWMo – Optical water quality model (method)

Opportunities and Issues

• Applications

• Average Conditions versus Extreme Events

• Traction

Wind waves and currents influence seagrass landscapes across hydrodynamic gradients

3More Patchiness + More Frequent Spatial Change

Increasing Tidal Currents and/or Wave Energy

Recomputed from Fonseca and Bell, 1998

Tipping point? Wind waves and cover

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Wave forecasting tools

• SWAN – Complex model to compute waves generated by wind and swells– Wave propagation; refraction, diffraction and dissipation (shoaling, whitecapping, bottom

friction, vegetation, breaking and turbulent flow)– Complex and time consuming computations– Well reviewed by scientific community

• DELFT3D – 3D model which includes SWAN along with modules for sediment transport and water

quality– Complex and time consuming computations– Well reviewed by scientific community

• WEMo – wave modeling for ecologists

– Simplified model to compute local wind waves - not swells– Wave propagation and dissipation (shoaling, white capping, bottom friction and breaking)– Module for forecasting sediment initiation of motion– Does not explicitly compute refraction / diffraction – Easier to run and less computationally intensive– Verified– Few publications

Application

• We have tools

• We have theory

• Can we apply this concept to mitigation?

Application: NC Bonner Bridge Mitigation

Reduce wave energy on patchy seagrass beds

Facilitate bed coalescence

Increase cover per unit area seafloor

WEMo wave reduction + seagrass response

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Wave break installation – NC (Jan. 2017)

CSA Dcaan Sciancaa Inc.

Vessel Wakes – Marine Corps Base Camp Lejeune

CSA Dcaan Sciancaa Inc.

:oo9-10':'7 Cocwlu.C~k . F\orids

Shoreline Change in the :>lew River Estuary, North Carolina: Rates and Consequences

Camlyn Currint *, ~cnn)' fr.l\'is\ l..i.o;a Co...,<~r t BaronH, Amit \1alhotrat , and Mar k forL..cca * ' NOAA National Ocean Sel"\oioe

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.Uaufort, NC 28516., US.A.

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Nort."' Carolina: Ra:t<11 and ro~u~noos_ J,urtUJlQf~td. P.~eewcli, 31($), 1069---1077. Coconut Cr-wt ( Florida). ISSN 0749--0006.

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INTRODUCTIOl\" shcreline change t-.lne rocused m oceanfront bmches, there

Marine Corps Base Camp Lejeune

• Continuous wave recording

• 6 Hz

• 18 months

• Measured all passing vessels

BOAT WAKE MODEL (BoMo)Boat Wake Model

V Hull [0·711) V Hul (7-1611) V Hull [16-30m)

DD Sailboat Ferryboat _j

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CSA Dcaan Sciancaa Inc.

Hull Characteristics

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m/s Average speed of boat I~ '----------'

I Back I Add GIS Data

Tipping point between wind wave and boat wake dominance

Comparing Wind Waves and Boat Wakes

New River ·

CSA Dcaan Sciancaa Inc.

Percentage of boat wakes exceeding wind waves

• 0.20-0.24

0 0.25-0.36

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0 0.53-0.75

• 0.76- 1.00

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Boat wakes and their influence o n erosion in 1he Atlantic Intracoastal Waterway,

North Carolina

• Predicts geography of seagrass light requirements

• Informs ‘what if’ and cost-benefit questions for exploring management alternatives (scenario gaming)

• Link with wave tools for geospatial water quality forecasts

• Best application requires calibration (field assessment)

OWMo + GUI

Optical Water Quality Model (OWMo) + Graphical User Interface (GUI)

• Slide CHL and/or TSS to see new potential habitat

• Set a depth or acreage target and see required CHL and TSS levels

OWMo – GUI interface

Works Forwards & Backwards

Seagrass suitable area at default CHL and

TSS values

1.50 m contour

(default depth)

1.75 m contour

1.50 m contour

(default depth)

Change in seagrass suitable area after

reducing CHL 10% and TSS 30%

New

extent

(gain)Original

extent

SUMMARY - FORECASTING TOOLS

• Wind waves – programs forecasting wind waves and sediment movement SWAN

Delft3D

WEMo

• BoMo - forecast boat wake waves, sediment motion

• OWMo – forecast light availability using bathymetry and selective spectral attenuation

OPPORTUNITIES

• Restoration site selection

• Seagrass cover manipulation

• Shoreline management

• Sediment resuspension forecasts

• Vessel traffic management

ISSUES TO CONSIDER

• Not much traction with ecologists

– Historical absence of ecologist-friendly tools

– Physical assessments not taught

• Correct forecasts? Average conditions vs Extremes

– Extent

– Resolution

– Duration

– Intensity

– Return interval (1/frequency)

– Sequence

Questions?

Waves, Wakes and Water Clarity: Understanding Physical Drivers of Seagrass Distribution in Coastal Waters

Mark Fonseca, Ph.D.CSA Ocean Sciences Inc.

Amit Malhotra, MSc., GISPGeohorizons

Questions?