With a grant from the California Department of Parks and Recreation Natural Resources Division – Coastal Program, our lab has been collaborating with Dr. Timu Gallien (UCLA, Coastal Flood Lab) to examine the interaction between estuary inlets and the adjacent beaches since 2017. We have collected fundamental hydraulic and hydrodynamic data and monitored evolving estuary inlets and adjacent beaches at two California State Parks, Torrey Pines/Los Peñasquitos Lagoon and Cardiff State Beach/San Elijo Lagoon (see Figure 1 & 2). Beaches were surveyed using real time kinematic surveying. Beach and estuary inlet shoreline and channel erosion data were collected using wading real time kinematic surveys and a Parks-funded UAV. Our high resolution subaerial inlet and adjacent beach topography provides improved, site specific inlet information that will help inform when park facilities or infrastructure may be impacted by inlet closures or large erosion events. Moreover, this data collection and analysis provides key data for future coupled inlet-beach morphological modeling elucidating infrastructure and ecological hazards associated with sea level rise and increased streamflow. Additionally, this research furthers understanding of two unstabilized, nourished beach-estuarine systems. Climate change will alter estuary-beach feedbacks and, in the future, these systems may require increased management (nourishment, breaching) and potentially stabilization.
A 2019 augmentation award focused on supporting additional observational collection, as well as preliminary development of a realistic wave-current coupled hydrodynamic simulation for the Torrey Pines/Los Peñasquitos Lagoon region.
Key project results
- High resolution beach/estuary surveys (completed via hand-held RTK GPS, drone surveys, and liDAR) have been conducted quarterly for over 3 years producing a high quality coupled beach/estuary morphological change dataset. See Figure 3 for example flights and Figure 4 for difference maps.
- In their present forms, CShore and XBeach are unable to accurately predict beach profile change on these typical southern California beaches, but when calibrated may provide qualitatively useful beach face erosion estimates (Kalligeris et al., 2020).
- Water levels inside perennially open estuaries mirrored ocean water levels, while those inside intermittently closed estuaries (ICEs) exhibited enhanced higher-high water levels during large waves, and tides were truncated at low tides due to a wave-built sand sill at the mouth, resulting in elevated detided water levels (Harvey et al., 2020).
- The predominance of large significant wave height approaching the mouth at an angle perpendicular to the shoreline led to the most significant mouth accretion (see Figure 4).
- The impact on adjacent beaches has been varied; in the surveys shown in Figure 4, the adjacent beaches were also accreting, while in the aerial lidar dataset analyzed as part of this work (Young et al., 2018) the estuary inlet accreted while the adjacent beach eroded.
- ICEs closed when sufficient wave-driven sand accretion formed a barrier berm across the mouth separating the estuary from the ocean (Harvey et al., 2020)
- A sill-height metric to quantify estuary mouth sill height was determined using lower-low estuary water levels adjusted to a fixed vertical datum (Harvey et al., 2020) as validated in Figure 6 below, and used in Figure 4 above.
- During the 2015–2016 El Niño, a greater number of Southern California ICEs closed than during a typical year and ICEs that close annually experienced longer than normal closures (Harvey et al., 2020)
- Wave observations near the estuary inlet show that wave energy inside the inlet, which contributes to sill accretion, is dependent on water level relative to the sill height and has a tidal variation due to wave-current interactions (Harvey et al., accepted). See Figure 7.
- IG frequency oscillations dominate the energy inside the inlet due to the sill at the estuary mouth (Harvey et al., accepted)
- IG motions drive very strong (~1m/s) velocity oscillations inside the estuary which are directly linked to sediment transport (Harvey et al., accepted) and turbulent dissipation (Wheeler et al., in review).
- During open, low sill conditions, circulation and stratification are consistent with stratification-induced periodic straining and subtidal exchange varies with the fortnightly cycle as observed in many classical estuaries. However, as the sill grows, tidal circulation weakens and becomes strongly sheared and the subtidal exchange no longer scales with a classical theoretical pressure-friction balance.
- A preliminary realistic numerical model has been developed that includes tides, waves, river flow, and atmospheric forcing.