Regional Variations in Rip Density

Abstract

An extensive dataset combining information obtained from aerial photographs of selected Australian beaches and studies in the literature from a wide variety of sites in Europe, the United States, Japan, South Africa and New Zealand is used to investigate the relationship between rip current spacing on intermediate beaches and regional wave climate. A new parameter termed rip density (RD) is introduced which defines the number of rips per kilometer of beach and is defined as the relationship y_r/L_r where y_r is rip spacing and L_r is a nominal length of beach. The variation in rip density was examined for five different regional wave environments termed west coast swell (WCS), east coast swell (ECS), fetch-limited wind wave with strong (SWS) and moderate (MWS) winds, and fetch-limited bays (SWB). Patterns of rip density were extremely consistent between the grouped wave climate environments with WCS beaches characterised by the lowest RD of 2 rips/km and SWB and MWS beaches having the highest RD with values ranging from 11-13 rips/km. ECS beaches have a RD of 5 with SWS lying in between the range for WCS and ECS beaches at approximately 3 rips/km.

The variation in rip density between environments exhibits distinct scaling relationships with RD on SWB and MWS beaches being approximately 5 times greater than on WCS beaches and twice as great than on ECS beaches. ECS beaches also have 2.5 times the number of rips on WCS beaches. Based on measurements and estimates of rip channel and surf zone width, there is evidence to suggest that these scaling factors may also be applied to the variation in two-dimensional planform morphology between the environments. The results of this study also indicate that rip density decreases with increasing wave height, wave period, surf zone width, wave energy, and wave power, thus providing quantitative links between observed rip density and regional wave climate.