Aspects of Surf and Runup Processes on Mixed Sand and Gravel Beaches

Abstract

Aspects of engineering-oriented and geomorphological runup studies are discussed in relation to the fact that there is little field data against which to check theoretical and model-derived conclusions concerning swashzone flow. Also, it is suggested that there has been little consideration of the effects of variability in wave trains on beach face changes. The phase-difference model of Kemp (1958) is examined and shown to be useful in this regard. The results of 21 experiments in which swash (2133) and backwash (1631) velocities were measured together with other surf and runup properties are presented and discussed. The data are for Hb 0.3 to 2.44 m; Tb 7.5-10.0 secs. and characterise non-barred, high energy, mesotidal, mixed sand and gravel beaches of mild slope (5-12°) at Kaikoura, New Zealand. Breaker height is shown to be the primary control of swash length as predicted by Kemp while phase of flow exerts important effects on the temporal structure and hence the net circulation of sediments entrained by the flow. Velocities were determined with a forceplate dynamometer having an accuracy of -15 cm sec-1. Average maximum runup velocity was 168.0 cm sec-1 and the average duration was 2.98 secs. Backwash velocities averaged 140 cm sec-1 with a mean duration of 4.25 secs. Runup velocities conformed to a near-Gaussian probability distribution and 2060 % of the incident wave velocity is transmitted to runup velocity. The relationship between relative runup velocity and breaker energy is negative-exponential in form for flow between the breakers and still water level. This result is the opposite of one obtained on steep, slopes in a model tank by earlier workers. Symbols Cb average velocity of breakers and shallow water wave velocity C. velocity at point x in the runup, i.e. a distance x from the breaker E energy per unit surface area g gravity acceleration H, Hb, HR wave height, breaker wave height, runup height H, Hb, HR the same; averages k a constant L wave length I swash length M. graphic mean grain size diameter n, N