Tides as a Major Influencing Factor to Shoreline Debris Deposition Scott Wilson Centre for Environmental Management

Shorelines as sinks for marine debris Most commonly studied areas for marine debris –Accessibility –Visibility –Directly effect most people Surveys and coastal cleanups target these areas Most however failed to understand the factor influencing the why and the how What causes debris to end up on our shorelines?

Factors that influence coastal debris movement and deposition Nearness to source Type of debris (eg. durability) Form of debris (eg. buoyancy, shape) Physico-chemical parameters Coastal geomorphology Wind Currents Rainfall and storm events TIDES

Ocean Tides Rise and fall on a daily basis Moon and Sun’s gravitational forces along with the earth rotation influences tidal patterns Tides can be classed as diurnal, semi diurnal and mixeddiurnal Shape of the coastline and bathymetry cause localised patterns

Shoreline Morphology Wave dominated –Low - Moderate energy –Steep slope Intermediate –High Energy –Bars Tide-modified –Low energy –High tidal range Tide-dominated –V. Low energy –Gentle slope

Tides and Debris Temporal debris collection shown to have a strong correlation with tidal height R & 0.62 (P<0.05)

Tides and Debris Not all beaches reflect this strong trend R 2 = 0.36 (p>0.05)

Tides and Debris Analysis of within beach zones draws further trends R R R 2 0.1

Tides and Debris Debris type helps explain the differences

Considerations for maximising outcomes Debris surveys and Clean-ups should consider site characteristics, weather and tidal patterns Potential to model and predict patterns of accumulation Maximum debris deposition after full and new moons Timing is everything! Other factors do play a part

Tidal Clock