Mouth Morphodynamics of an Intermittently Closed Estuary

Intermittently Closed Estuaries (ICEs) are estuarine systems found worldwide, particularly in Mediterranean climates, where the entrance to the estuary is not permanently open to the ocean. ICEs are a subset of bar-built estuaries in which the mouth sill height can become high enough to block exchange flow with the ocean. Large waves drive sediment transport into ICE mouths and elevate the sill, while rainfall events increase river outflows, scour the estuary, and erode the sill. Despite providing critical ecosystem services such as carbon sequestration, marsh habitat, and resilience to flooding from extreme events and sea level rise, ICEs are less studied than classical, persistent-inflow estuaries. This work focuses on one S. California ICE, Los Peñasquitos Lagoon (LPL). LPL is a shallow (< 4m) estuary with minimal summer precipitation and episodic winter inputs. In a study of S. California beach response to the 2015-2016 El Niño, Young et al. (2018) show that more frequent and persistent closures occurred at LPL due to the anomalously large El Niño wave conditions. Additionally, while adjacent beaches underwent erosion in response to the elevated El Niño wave conditions, the mouth of Los Peñ asquitos accreted. Mouth morphodynamic sediment sources, whether offshore or via interactions with adjacent beaches, are not well understood. In the presented work we will primarily focus on the influence of adjacent beaches on mouth closures. We utilize a novel, high resolution dataset of quarterly topographic surveys over three years. Eleven surveys were collected using sUAS optical imagery converted to DEMs using the photogrammetric Structure from Motion (SfM) software Pix4Dmapper, and additional surveys were conducted using truck-mounted mobile LiDAR. The DEMs have 1m spatial resolution, span a region of 500m up-estuary, and cover several kilometers in the alongshore. Similar multi-year studies of mouth morphology utilize point or transect measurements of sill height but were not able to consider more complex spatial morphologic change such as is possible with the presented surveys. Additionally we have collected extensive hydrodynamic data within and just outside of the estuary to support examination of the physical drivers.