Stability of a very coarse-grained beach at Carmel, California

Abstract

Monastery Beach at Carmel, California, is a pocket beach composed of very coarse to granular sediment. In profile, the beach has a well-defined berm crest; a steep foreshore; and a gently sloping, barless offshore covered by large, long-crested oscillation ripples. Carmel Submarine Canyon heads a few hundred meters offshore of the beach, and San Jose Creek, a small ephemeral steam, ponds onshore of the central part of the berm. Wave conditions vary greatly during a year because the beach lies open to the Pacific Ocean for azimuths between 270°–322°N whence come a variety of wave types. Even with a variable wave climate, Monastery Beach has maintained a swell profile for almost three years. Aperiodic beach surveys show that the beach responds little to seasonal changes in wave climate. Four survey lines maintained the same swell profile throughout the study period. The fifth line maintained a stable profile only across the foreshore; the berm was twice artificially breached during storms to prevent upstream flooding along San Jose Creek. In comparison, Carmel Beach, a nearby beach composed of medium sand, commonly alternates between swell and storm profiles. The increased stability of Monastery Beach relative to Carmel Beach is attributed to two factors: grain size differences and location within Carmel Bay. Rebuilding proceeded very slowly along the breached part of the berm at Monastery Beach. The probable cause of such a low recovery rate is that oscillation ripples trapped the sand that was carried offshore when San Jose Creek eroded the beach. The ripples, which are active under high-energy conditions, approach dormancy under low-energy conditions. Each ripple, therefore, acts like a reservoir, retaining sand during most swell conditions.