Tidal Bores

Introduction

When the Earth was newly formed 5 billion years ago, there was a cosmic collision with the Mars-sized planet Theia, and the resulting debris coalesced to form the Moon. Back then the Earth spun a full rotation in 6 hours and tides reached 500m! The Moon helped slow the Earth’s rotation and stabilised its orbit while slowly moving away from it. The Moon is 400 times smaller than the Sun, but it is 400 times closer, so they appear the same size in the sky. Between them, they exert a gravitational force on our planet that creates the tides (see Tides page 20). This force is measured at the equator as only being 60-90cm, so how do we get 18m tides in the Bay of Fundy? Other factors come into play like underwater topography, coastal morphology and resonance, barometric pressure and wind, just to name a few, but luckily for surfers, certain rivers that are open to an ocean with moderate to large tidal activity experience a wave phenomenon known as a tidal bore. Usually found in shallow, funnel-shaped estuaries between 50km and 100km long, the wave is created when river flow and ebbing tide runs in the opposite direction to a surging tide. This happens when the tide becomes asymmetrical: the tide floods into the river system in 3-4hr but takes 8-9hr to drain back out, meeting the new incoming tide back at the mouth of the estuary. A 4m tidal range at the mouth can reach a 6m maximum, 80km upstream. As the river narrows, the tide is condensed and shallower water causes the tidal wave to break, usually on each riverbank or away from the channels. Depth is crucial for a good bore wave, where most tropical and northern rivers are generally shallow while temperate latitudes are subject to strong rain causing higher water levels. Behind the larger leading wave can follow up to 15 lesser waves plus multiple corrugations, along with turbulent peaks, reflected off the banks and racing currents up to 8m/s (29km/h). With so many variables, the waves come in many patterns including chevrons, velour, antennae, spiders web and the chicken’s foot, when viewed from the air. Due to the river’s linear geography, bore waves offer the longest distance rides on the planet.