5 - Tidal modulation

Abstract

Beach morphodynamics are affected by the tidal modulation of the breaking wave heights to varying degrees depending on the tide range. This chapter discusses the impact of meso and mega tidal ranges at a selection of beaches with low to moderate breaker waves from around the world. Tidal modulation at these beaches varies according to the tidal cycles, seasonal variation, offshore wind patterns, major weather events and astronomical forces. The beaches are characterised by extensive intertidal sand flats and receive higher wave energy during the high tides than during the low tides as wave breaking and dissipation occur farther from the beach during low tide. Relatively large waves during high tides result in a lower relative tidal ranges (RTRs) and tide-modified conditions, whereas, relatively small waves during low tides result in larger RTRs and tide-dominated conditions. On the most sheltered macrotidal beaches during the largest tides (equinoctial spring tides), tide-dominated sand-flat morphologies typically predominate. On the most exposed macrotidal beaches and during the highest energy events, conditions tend to shift from tide-modified to wave-dominated. Beach state on macrotidal beaches thus depends primarily on the degree of nearshore sheltering by intertidal sand flats, with the more exposed sectors being characterised by tide-modified or wave-dominated conditions, principally under high wave energy conditions, and more sheltered sectors having tide-dominated to tidal flat conditions, especially when wave energy is at its lowest.