Morphological response of variable river discharge and wave forcing at a bar-built estuary

Abstract

Observations of morphological evolution at Carmel River State Beach, Carmel, CA, USA, were made during two winter periods where the estuary underwent transitions from closed to open states episodically during each observation period. However, each winter was climatologically distinct: the first (Dec 2016–May 2017) was a high river discharge year (several events >200 m3/s) with westerly offshore waves and the second (Dec 2017–May 2018) was a low river discharge year with northwesterly offshore waves. The morphological response of the beach was measured using Structure-from-Motion from both aircraft and unmanned aerial vehicles (UAVs) and shows two distinct seasonal trends. The first (in 2016–2017) indicates rapid (hours) and frequent (days-weeks) migration of the river breach channel across the span of the beach. The second (in 2017–2018) indicates no migration of the initial breach channel, despite multiple breach events. Analysis of the offshore wave energy using the Coastal Data Information Program (CDIP) hindcast model results indicate a stronger longshore wave radiation stress during the migratory breach year. In addition, discharge rates during this year were more than three times stronger than the non-migratory year, indicating a stronger offshore jet from the breach site. These observations support the hypothesis that migration requires both a strong river discharge and a longshore wave radiation stress component.