Abstract

The development of beach cusps was studied along the northern shore of Delaware from June 9 through June 30, and from July 28 through August 10 of 1976. At the study site, the formation of beach cusps was dependent upon the existence of a tidal berm in the developing stage and of a favorable backshore topography. Beach cusps developed as follows: after an erosional event on a sandy beach, a berm developed at low tide. The swash extended over the berm and ponded between the berm and the backshore. The stream flow from the ponded water to the sea cut closely spaced channels through the berm. As the tide rose, the berm and channels migrated landward. When the tide fell, the swash could no longer overtop the berm, and no water was ponded landward of the berm. Since no water was returning seaward through the channels, the channel form could not be maintained, and the swash flared the channels into bays. A series of beach cusps appeared on the beach as the tide continued to fall. The spacing between cusps was irregular and was attributed to the irregular size of swash salients. At high tide, the horizontal distance from the berm crest to the backshore (L) was less than 12 m. At high tide where L was greater than 12 m, no beach cusps formed. Although a tidal berm developed, there was no effective bachwash to cut through the berm. As the new tidal berm developed and as the swash overtopped the berm crest, the swash continued landward and during a short period of time flooded a portion of the backshore. No cusps were observed to develop after a tidal berm had been constructed; berms and cusps developed together.

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