Graduate Thesis Or Dissertation
 

From the Shoreface to the Foredunes : Coastal Morphodynamics Across the Land-Sea Interface

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  • Coastal flooding and erosion are major concerns for low lying coastal communities -- particularly in light of accelerated sea level rise and climate change. To improve quantitative understanding of the physical drivers of both flooding and coastal landscape change, this dissertation explores coastal morphodynamics bridging the land-sea interface on modally dissipative beaches throughout the U.S. Pacific Northwest (PNW). Both new and existing morphologic datasets spanning from the shoreface to the foredunes, at time scales of days to decades, are utilized to explore morphodynamic processes. Process-based numerical models are then used to interpret and extrapolate the findings from the field observations. The timing and processes contributing to seasonal erosion and growth of the beach and dune are first investigated at a dissipative system in Oysterville, WA, with the primary goal of relating seasonal scale morphology changes to longer term coastal evolution (years to decades). It is shown that the largest wind driven growth of the foredune at this site occurs in winter in response to the largest wind events but out of phase with summertime beach growth via the welding of intertidal sandbars. The lack of synchronization between maximum beach sediment supply and dune growth indicates that aeolian sand transport on dissipative coasts is primarily transport, rather than supply, limited. Although aeolian processes contribute the majority of sediment for dune growth, it is shown that total water levels exceeding the dune toe may be constructive to lower dune growth, in contrast to the expected erosional dune response from total water levels in the collision regime of Sallenger's (2000) Storm Impact Scaling Model. A new morpho-stratigraphic approach, which combines repeat topographic transect data with time series of oceanographic conditions, is developed to relate seasonal scale deposition across the beach and dune portions of the coastal profile to either a marine or aeolian origin. This method estimates that between 9% and 38% (~1 to 5 m3/m/yr) of annual volumetric dune growth at Oysterville, WA results directly from marine processes. Environmental and morphologic controls on the physical drivers of wave-driven dune response are further explored for additional sites throughout the PNW. Topographic data from South Beach, OR, Netarts, OR and Oysterville, WA collectively support previous observations that found that low sloping beaches are generally less vulnerable to storm-induced dune erosion than nearby steeper beach segments. Morphologic controls, including the effects of variable shelf, nearshore, beach, and dune morphology, on influencing storm-induced dune accretion and erosion are explored using XBeach, a process-based numerical model which simulates nearshore hydrodynamics and morphology change. The model results reveal that wave-driven dune accretion can occur on low sloping beaches when dynamic still water levels (still water level combined with wave setup) are below the dune toe. Although total water levels in the collision regime occur numerous times per year in many parts of the PNW, dynamic still water levels infrequently exceed the dune toe because of the uncommon co-occurrence of large wave energy and high still water levels in the PNW. The XBeach results suggest that at Oysterville, WA the oceanographic conditions promoting wave-driven dune accretion are more common than driving dune erosion. The model outputs indicate that ~15 m3/m/yr of average annual dune growth could be derived from marine-driven processes at Oysterville, WA. Numerical model simulations also show that the shelf and nearshore bathymetry have an influence on total water levels and resulting dune impacts. While the nearshore zone is characterized by a wide range of morphologies (e.g., slopes) throughout the region, temporal variability of the nearshore profile at many PNW locations is dominated by the formation and migration of subtidal sandbars. Process-based numerical models are used to explore how seasonal to interannual variability in nearshore sandbar configuration and beach characteristics each influence wave runup processes. Simulations using XBeach show that interannual variability in sandbar configuration, associated with multi-year cycles of net offshore sandbar migration, has a larger influence on wave runup than does seasonal variability in sandbar morphology. While subtidal sandbars do alter wave setup and swash, the model simulations also suggest that temporal variability in intertidal beach morphology (> -2 m relative to local mean sea level) has a comparatively larger morphologic influence on wave runup. While marine processes have a large control on evolving the coastal profile, aeolian processes are generally thought to be the primary builder of coastal foredunes. Windsurf, a new process-based numerical modeling framework for simulating the co-evolution of the coastal profile in response to both marine and aeolian forcings, is developed to further explore nearshore-beach-dune interactions. Windsurf is applied to the dissipative Oysterville, WA site in order to investigate the relative roles of marine and aeolian processes on coastal foredune growth. Consistent with field measurements, the model simulates seasonal cycles of beach growth in summer, shoreline recession in winter, and net dune growth annually. The model results support the hypothesis that there are both marine (~7 m3/m/yr) and aeolian (~ 14 m3/m/yr) contributions to coastal foredune growth at this site. Consistent with field observations, Windsurf simulates positive marine contributions to the dune growth during fall. Aeolian contributions to dune growth occur intermittently throughout the year, but are lowest in summer and highest in winter. Although cross-shore oriented winds are relatively infrequent at Oysterville, WA, Windsurf simulations suggest that cross-shore winds provide a proportionally larger contribution to upper dune growth than obliquely oriented winds. Together, this collection of manuscripts explores the influence of coastal morphodynamic processes on flooding and erosion hazards along dissipative beaches. It is confirmed that wave runup on infragravity-dominated dissipative coasts is influenced by both subtidal and intertidal morphology, with intertidal morphology having a larger influence. Field measurements and numerical modeling both suggest that marine contributions to dune growth can be accretional under certain morphologic configurations and certain environmental conditions. Wave-driven dune accretion appears to be driven largely by infragravity swash processes on low sloping beaches -- with these marine processes shown to contribute between ~1 and 15 m3/m/yr of sediment to coastal foredune growth for an end-member dissipative beach. However, these marine accumulations are restricted to the lower portion of the dune and are generally smaller in total magnitude than aeolian contributions to upper dune growth. Therefore, consistent with the conventional process understanding of dune dynamics, aeolian processes are still found to be the primary contributor to overall coastal foredune growth on low gradient, dissipative beaches.
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