Amani Ngusaru

Longshore Currents on a Multi-Barred Beach: Comparison of Measured Data with Model Results

Thesis Approved February 1992

Longshore currents were measured on a multiple barred beach at Stanhope Lane on the North coast of Prince Edward Island, Canada, in October and November of 1989. The observed currents, typically represented by a half-hour average, were compared to theoretical estimates based on the dissipation models of Thornton and Guza (1968) [TG(86)] and Thornton and Whitford (1990) [TW(90)]. The model by TG(86) included bottom friction, wave- induced radiation stress and turbulent induced radiation stress (Reynolds stress) terms in the alongshore momentum balance. The model by TW(90) added new terms in TG(86) model, these included surface wind stress, beach slope dependent breaker index, turbulent momentum exchange, together with a more complex representation of wave energy dissipation based on the observed distribution of breaking waves. The present work uses a model obtained by excluding surface wind stress from TW(90). The primary results were that the longshore currents were reasonably modeled when including turbulent momentum exchange and non- linear bottom friction without surface wind stress. Both observations and theory suggest that the currents are strongly tied to individual bar crests. However, the observations show flow patterns which are consistently displaced landward relative to the theoretical estimates.

Various recent suggestions as to the variability of the currents on time scales of a few minutes were also examined. An interesting result was the lack of coherence between flows over the various bars, and with local estimates of the variability of the radiation stress component, , over the same time scales. This was interesting, since the mean longshore currents are clearly forced by radiation stress.

Amani Ngusaru went on to earn his Ph.D. at Memorial University, and he is now the Eastern African Marine Ecoregion Leader for the WWF Tanzania Programme Office.