Bar-built coastal lagoons are common coastal landforms that typically occupy drowned river valleys and exist in wave-dominated areas characterized by seasonal rainfall. Wave-driven sedimentation often seasonally closes the inlet, forming a perched estuary that is separated from the ocean by a sand or gravel beach barrier. The pressure on these systems and their ability to provide important ecosystem services will increase in the 21st century due to climate change, sea-level rise, and continued population growth within their watersheds and estuarine margins. To better understand how coastal lagoons function, this work addressed three main topics: 1) the observed variable patterns of breaching and closure, 2) the beach-barrier groundwater connection, and 3) the role of watershed variability in estuary geomorphology.

A hydrologic and geomorphic model of estuary breaching and closure was developed, assessed, and then used to examine how estuarine and beach processes affect the breaching and closure processes. The model is based upon the continuity equation for the estuary water balance, and a geomorphic component for the inlet that incorporates inlet sedimentation by waves and erosion by fluvial processes. The root mean square error of the predicted versus observed estuary stage was 0.45m, 0.39m, and 0.42m for three separate years when the inlet drained directly across the berm, and the model was able to reproduce the initial seasonal breaching, seasonal closure, intermittent closures and breaches, and the low-streamflow (closed state) estuary hydrology of the Carmel Lagoon, located in Central California. Model results show that estuary function (eg amount of time inlet is closed, low-flow hydrology, frequency of breaching) is sensitive to realistic variations in beach berm characteristics (hydraulic conductivity, height, etc.), estuary storage, and especially hydrologic inflow. Sea-level rise will potentially cause the estuary to behave more like a coastal lake.

Groundwater and estuarine waterlevel observations indicate that the beach barrier aquifer provides a dynamic hydrologic connection between the ocean and the closed lagoon. Groundwater flow is predominantly oceanward for the conditions observed, and the flux is modulated by tides at all estuary waterlevels, reversing directions only during wave overtopping events that fully saturate the beach. A numerical model of barrier groundwater was calibrated with constrained input parameters and assessed using observed groundwater heads. The model accounts for berm morphology, swash infiltration, wave set-up, and a sloping no-flow bottom boundary. The model accurately reproduces observed groundwater fluctuations (RMSE is 0.062m, 0.074m, 0.022m for three wells), except during wave overtopping events, and model results indicate that the seepage flux (and its variance), groundwater heads, and width of the seepage face are all underpredicted when the topography of the berm is not incorporated into the model. Sensitivity analyses emphasize the importance of wave swash on model performance.

Nineteen small, shallow lagoons in a tectonically active, low-wave energy region were mapped and analyzed to determine how watershed processes shape estuary geomorphology. Multiple linear regression analyses indicate that variability in lagoon area, length, volume, average width, circularity, and lagoon width expansion are primarily the result of variability in watershed precipitation and channel slope upstream of the lagoons. The coefficients of determination for the power function regressions are 0.88 (lagoon area), 0.88 (lagoon length), 0.83 (lagoon volume), and 0.74 (average width).