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Rip currents are strong, narrow seaward-flowing currents in the surf zone and are common on energetic sandy beaches. They are generally associated with distinct beach morphology, comprising nearshore sand bars and channels, and represent a real natural hazard to surf zone users. Rip current circulation is primarily driven by spatial gradients in wave breaking and water levels in the surf zone, which in turn are controlled by beach morphology, o↵shore wave conditions and tidal level. These factors, which are highly variable over hours (tides), days (waves) and weeks (morphology), also control the rip risk to bathers. However, the precise roles of these di↵erent environmental factors in controlling rip dynamics on meso-to macro-tidal beaches is not exactly known and thresholds separating di↵erent types of rip circulation and flow strengths, and hence rip risk, have not been quantified. Here, analysis of 5-year lifeguard incident records from 20 beaches in southwest England showed that high-risk, high-exposure scenarios for bathers occur disproportionately around mean low water on days with low wave height (H s < 1 m), long wave period (T p > 10 s), shore-normal wave approach and light winds (> 5 m s 1). Detailed in-situ Lagrangian field measurements of rip currents collected on 23 di↵erent days from Perranporth Beach, UK identified waves (characterised by H s T p) and active morphology (characterised by tidal elevation) as the key controlling factors determining the mode of rip behaviour. Maximum hazard was associated with the combination of maximum rip exits and rip flow speeds. These conditions occurred when H s T p was at or just below average values and when those waves were acting on the active morphological template, around mean low water. The thresholds in wave conditions and tidal elevation identified here were e↵ective in discriminating between observed coast-wide high-risk incident events, illustrating that such mass rescue events have a considerable element of environmental control. Because many beaches along the west coast of southwest England are characterised by nearshore bar morphology just below the mean low water level, and are a↵ected by similar wave and tide conditions, the results obtained from this beach are transferable to other locations. The findings of this study may also have implications for other beaches with nearshore bar-rip morphology at specific tidal levels.

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Outer sand bar dynamics on a high-energy macro-tidal beach were investigated using long-term (multi-year) field datasets of intertidal morphology and offshore bathymetry. Utilising a 15-year time-series of Argus video images, five distinct outer bar types were identified: Mega Rip, Longshore, Crescentic, Crescentic Attached and Welded. The most common classification was Crescentic Attached, with the outer bar oscillations being out of phase with the inner bar oscillations, as would be expected for the shore-normal wave approach at this site. The outer bar had a typical amplitude of 0.5-1 m and a longshore wavelength of 600 m. Changes in outer bar morphology were related to measured and modelled nearshore wave data. However, the outer bar morphology changed over a much longer time scale (monthly-to-annual) than the daily-to-weekly variations in wave height and period. An extended duration of energetic wave action was required to bring about an upstate bar transition to the Longshore or Mega Rip state, where the bar then remained arrested for a significant amount of time, requiring several months of low wave conditions to induce a down state transition through Crescentic to Welded. This slow morphological response is explained by extended relaxation times attributed to the large tidal range at the study site where the outer bar morphology is only active for part of the tidal cycle (several hours around low tide). The configuration and position of the outer bar were related: the more upstate (downstate) bar types being associated with a more offshore (onshore) bar position. The detrended outer bar position was significantly related to a forcing term based on wave power and disequilibrium of the dimensionless fall velocity with offshore (onshore) bar migration occurring when wave conditions were more (less) energetic that the antecedent conditions. The upstate end member (Longshore or Mega Rip) was attained sometime during the winter months for 14 out of the 16 years of monitoring; the outer bar remained attached to the low tide shoreline over the winter 2005/2006 and 2010/2011. These two winters with incomplete upstate cycles were characterised by the lowest winter wave conditions and negative winter North Atlantic Oscillation (NAO) indices, suggesting that the winter NAO is correlated with both beach state and nearshore bar configuration. KEYWORDS