On the Planimetric Shape of Wreck Bay, Vancouver Is.

Abstract

ABSTRACT The planimetric shape of Wreck Bay has been studied by fitting various segments of the bay to theoretical logarithmic spiral curves, and by using the characteristics of incident swell to estimate longshore sediment transport along the beach. We have found that the cliff base in the northwest half of the bay closely follows log-spiral curvature, whereas the southeast half reflects only poor correspondence. This situation is due largely to distortion of the incident swell by two islands in the bay, Florencia Is., and Seal Rock. Quasi-permanent cliff sections supporting large trees occur on the landward side of the theoretical log-spiral curves, and those sections exposing unconsolidated glaciofluvial outwash are generally seaward of the curves. Present day erosion, due mainly to wind action on the exposed parts of the cliff, indicates that gradual mutation of the planimetric shape, particularly for the southeast half of the bay, is tending towards an improved fit. Longshore current distribution along the beach accounts for the direction of spit-bar growth at the mouth of Lost Shoe Creek, and also for the presence of Sand Point in the middle of the bay. It has been found useful when comparing the goodness of fit of beaches of different lengths to introduce a normalized root mean squared error. Furthermore, it has been shown that if a headland remains part of a receding spiral beach, then the spiral center must gradually migrate away from the headland as erosion proceeds.