Observations of episodic breaching and closure at an ephemeral river

Abstract

Carmel River, near Carmel, CA, is a seasonally open, ephemeral river that has a basin of 660 km2. During the dry summer months, the barrier beach is built across the river mouth, limiting water exchange. Precipitation during the winter months increases the discharge within the river until water levels are sufficiently high to breach the barrier beach. Observations during the 2016–2017 winter allow for a dynamic balance between discharge, wave forcing, and tidal exchange that led to several distinct breaching and closure events. The initial breach that occurred after the first major precipitation event was insufficient to keep the river open, owing to wave forcing at the mouth and a relatively low river discharge. Wave penetration into the estuary through overtopping and upstream propagation is routinely observed during this stage. In order to evaluate the conditions responsible for the intermittent opening and closing of the inlet, water level, wave, tidal and discharge data were collected. With this data, a momentum balance was developed in order to estimate the ocean forcing (tides and waves) as compared to discharge. It is hypothesized that a critical discharge is required to maintain an open river mouth, which depends on offshore wave forcing and tidal stage.