UC San Diego

Small, shallow, low-inflow estuaries (LIEs) are common worldwide along steep coasts in regions with low or seasonal precipitation. LIEs along coasts with strong wave conditions are termed bar-built estuaries (BBEs) where a sand-bar sill near the mouth due to current and wave-driven sediment transport. The sill height and location can have profound impacts on the circulation, inundation, stratification, and dissolved oxygen in the estuary. In intermittently closed estuaries (ICEs) the sill can periodically accrete to block the ocean-estuary exchange. In Los Peñasquitos Lagoon, an ICE in Southern California, the hydrodynamic variability of these traditionally under-researched systems is investigated through extensive observations.

Four and half years of continuous hydrodynamic observations over several periods of climatology add valuable insight into how ICEs respond to changing nearshore and upstream forcings. Observations during the open state reveal that when the sill near the mouth is low, the estuary functions like a canonical estuary with evidence of strain-induced period stratification and canonical exchange flows. As the sill accretes, tidal circulation weakens, and the estuarine exchange does not scale with the canonical pressure-friction balance.

When the inlet closes, estuary-ocean exchange is interrupted, and estuarine water levels increase due to freshwater inflows and overtopping. Reduced mixing due to interrupted tidal exchange and increased stratification due to freshwater inputs lead to hypoxic conditions developing at depth. The diurnal circulation when closed is forced by differential heating and cooling driving thermal exchange that can be weakened by strong diurnal winds.

To put the dynamics of LPL in context with similar estuaries, water levels were measured in 12 additional Southern California estuaries during the 2015-2016 El Niño winter. Water levels inside perennially open estuaries mirrored ocean water levels. ICEs exhibited enhanced higher-high water levels than offshore during large waves, and lower-low water levels were truncated due to the sill at the mouth, resulting in elevated detided water levels. The sill height and wave exposure were important to the individual estuarine response to ocean conditions. Understanding how LIEs, BBEs, and ICEs respond to increased sea levels and waves and the factors that influence closures will help managers develop appropriate adaptation strategies.