Tim Scott

ORCID

Abstract

In this thesis the relationship between beach morphodynamics and recreational hazards was investigated for the first time within the United Kingdom (UK). Four field experiments, conducted during 2006-2008 provided new insights into the spatio-temporal dynamics of UK beach types and their associated hazard signatures. The extent of data collection ranged from national (UK beach classification) to regional (temporal morphologic variation) to site specific (macrotidal rip current dynamics). Detailed morphodynamic characteristics of 98 beaches within the UK were collected. Twelve distinct beach groups were identified through cluster analysis, each having a unique morphodynamic signature. Conceptualisation within a relative two-dimensional framework using the dimensionless fall velocity (Q) and the relative tide range (RTR) required an absolute wave energy flux threshold to differentiate between intermediate beaches with (H2T > 5) and without (H2T < 5) three-dimensional bar morphology. The role of geologic control, sediment abundance and drainage characteristics in constraining beach morphodynamics was shown to be significant within the sites studied. Rip currents were responsible for 68% of all recorded incidents between 2005-2007 throughout all 76 beaches patrolled by the Royal National Lifeboat Institution (RNLI). Hazard type and severity varied between morphodynamic beach types. Intermediate beaches with low-tide bar/rip morphology (Q = 2-5 and RTR < 7), including Low-Tide Terrace and Rip (LTT+R) and Low-tide Bar/Rip (LTBR) beaches, presented the greatest risk to the insea beach user. These high risk beaches, representing 59% of the west coast beaches in Devon and Cornwall, also attracted the greatest visitor populations. Seasonal monitoring of hydrodynamics and morphology at LTT+R and LTBR beaches in Devon and Cornwall (annual Hsio% = 3-4 m; mean spring tidal range = 4.2-8.6 m) identified key mechanisms controlling the temporal hazard signature (THS), a term used here to describe the spatio-temporal variation in type and severity o f bathing hazard within a specified region both in the alongshore as well as in the cross-shore (significant in macrotidal environments). The morphological template controlled the presence, extent and intensity of beach rip current systems, where the development of low/tide transverse and inter-tidal bar/rip systems during summer presented the greatest morphological hazard. Typical summer wave forcing by relatively small, long period swell {H, - 0.5-1 m; Tp ~ 6-10 s) over this morphology provided conditions conducive to hazardous rip currents. Under these conditions hazard exposure was increased due to the accessibility of the relatively low energy surf zone. Both spring/neap and semi-diurnal tidal variations were identified as key controls on the THS. Variable tidal excursion modulates rip current activity, and tidal translation rates control the rate of change of the THS. The 'optimum' combination of these mechanisms results in the 'switching' on and off of rip currents during spring low tides and the subsequent rapid alongshore migration of rip channel/hazard location as the surf zone inundates the landward inter-tidal bar system. In conjunction with high insea population, these 'optimum hazard scenarios' drove the high risk, coast-wide 'mass rescue' events identified in the incident records. This work provides a scientific, standardised basis for a beach risk assessment model and lifeguard training programs within the RNLI. Improved understanding of macrotidal rip currents has initiated new field and modelling efforts to further general quantitative understanding of these systems, vital to the improvement of beach safety services.

Document Type

Thesis

Publication Date

2009

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