Effects of Elevated Sea Levels and Waves on Southern California Estuaries During the 2015\u20132016 El Ni\xf1o

Madeleine E Harvey,John L Largier,Timu Gallien,Sarah N Giddings,Eric D Stein,Geno Pawlak,Hallee Meltzer,Henry M Page,Karina Johnston,Liesl Tiefenthaler,Jeffrey A Crooks,Hany Elwany,Richard Ambrose,Karen Thorne,Stephen C Schroeter,Christine Whitcraft

doi:10.1007/s12237-019-00676-1

Abstract

The 2015–2016 El Niño provided insight into how low-inflow estuaries might respond to future climate regimes, including high sea levels and more intense waves. High waves and water levels coupled with low rainfall along the Southern California coastline provided the opportunity to examine how extreme ocean forcing impacts estuaries independently from fluvial events. From November 2015 to April 2016, water levels were measured in 13 Southern California estuaries, including both intermittently closed and perennially open estuaries with varying watershed size, urban development, and management practices. Elevated ocean water levels caused raised water levels and prolonged inundation in all of the estuaries studied. 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. ICEs closed when sufficient wave-driven sand accretion formed a barrier berm across the mouth separating the estuary from the ocean, the height of which can be estimated using estuarine lower-low water levels. 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. Overall, sill accretion and wave exposure were important contributing factors to individual estuarine response to ocean conditions. Understanding how estuaries respond to increased sea levels and waves and the factors that influence closures will help managers develop appropriate adaptation strategies.

Highlights

Estuaries and associated wetlands provide extensive ecosystem functions and services, including biodiversity support, carbon sequestration, water quality improvement, and flooding abatement (Zedler and Kercher 2005; Takekawa et al 2011; Holmquist et al 2018)
Results from all 13 estuaries are consistent with observations from the representative estuaries: tidal water levels in the Perennially Open Estuaries (POEs) are more strongly correlated (0.92 < r < 1.00; p < 0.01) with the ocean water levels than the Detided WL, m ~NAVD88
Anomalous conditions associated with the 2015–2016 El Niño along the Southern California coastline including elevated ocean water levels, high waves, and low precipitation, provided the opportunity to understand how lowinflow estuaries respond to oceanic forcing and insights into how they might respond to changing ocean conditions

Summary

Introduction

Estuaries and associated wetlands provide extensive ecosystem functions and services, including biodiversity support, carbon sequestration, water quality improvement, and flooding abatement (Zedler and Kercher 2005; Takekawa et al 2011; Holmquist et al 2018). More than 100 estuaries line the highly urbanized Southern California coastline (Fig. 1 and Doughty et al 2018), all with varying degrees of physical modifications, including the damming and channelizing of river inflows; the construction of breakwaters and jetties at inlets; the dredging of channels, inlets, and harbors; the construction of roads splitting systems; and the direct filling of wetlands (e.g., Pratt 2014; Los Peñasquitos Lagoon Foundation et al 2016) Despite these threats, these systems are extremely important to the regional economy and ecology (Zedler and Kercher 2005; California Natural Resources Agency 2010)

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Results

Conclusion