Nicholas Cohn

Extreme oceanographic forcing and coastal response due to the 2015–2016 El Niño

The El Niño-Southern Oscillation is the dominant mode of interannual climate variability across the Pacific Ocean basin, with influence on the global climate. The two end members of the cycle, El Niño and La Niña, force anomalous oceanographic conditions and coastal response along the Pacific margin, exposing many heavily populated regions to increased coastal flooding and erosion hazards. However, a quantitative record of coastal impacts is spatially limited and temporally restricted to only the most recent events. Here we report on the oceanographic forcing and coastal response of the 2015–2016 El Niño, one of the strongest of the last 145 years. We show that winter wave energy equalled or exceeded measured historical maxima across the US West Coast, corresponding to anomalously large beach erosion across the region. Shorelines in many areas retreated beyond previously measured landward extremes, particularly along the sediment-starved California coast.

Investigating the role of complex sandbar morphology on nearshore hydrodynamics

ABSTRACT Cohn, N., Ruggiero, P., Ortiz, J., D.J. Walstra, 2014. Investigating the role of complex sandbar morphology on nearshore hydrodynamics. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 053–058, ISSN 0749-0208. Coastal environments are characterized by complex feedbacks between flow, sediment transport, and morphology, often resulting in the formation of nearshore sandbars. In many locations, such as Hasaki (Japan), the Netherlands, and the Columbia River Littoral Cell (CRLC, USA), these sandbars exhibit a net offshore migration (NOM) cycle whereby these features form in the inner surf zone, migrate seaward and decay offshore on interannual cycles. Depending on the stage of the cycle, the number and configuration of the bars may differ widely. It has long been recognized that sandbars act as natural barriers during storm events by dissipating wave energy through breaking far from the beach face. Thus, dependent on the stage of the NOM cycle, one might expect significant variability in nearshore hydrodynamics. Using a non-linear wave model we demonstrate that inter-annual variability in sandbar configuration can significantly alter inner surf zone and swash zone processes. The model indicates that under different end-member NOM stages the same wave conditions can result in up to a 36% variance in the vertical extent of infragravity runup and can alter both the rate and direction of net cross shore sediment transport.

Spatial and Temporal Variability of Dissipative Dry Beach Profiles in the Pacific Northwest, U.S.A.

ABSTRACT Díez, J.; Cohn, N.; Kaminsky, G.M.; Medina, R., and Ruggiero, P., 2018. Spatial and temporal variability of dissipative dry beach profiles in the Pacific Northwest, U.S.A. Dissipative beaches in the U.S. Pacific Northwest are subject to a marked seasonality in wave climate and water levels, which leads to periodic oscillations in the morphology of the typically dry part of the beach profile. The back-and-forth, seasonal sediment exchange between the emerged and submerged parts of the beach system induces two main dry-beach profile-equilibrium configurations. During approximately 70% of the year, the dry beach adapts its configuration to a uniform positive slope from the mean high-water level to the dune toe. The remaining 30% of the time, typically corresponding to summer, the profile adopts a berm-like profile. These changes are quantified by studying intra-annual and interannual variations of the dry-beach profile shape. For intra-annual variations, a monthly profiling campaign between July 2014 and October 2015 from South Beach State Park (Oregon) was used. For interannual variations, 17 years (1997–2015) of quarterly beach profiles at 31 transects spread along the four subcells that constitute the Columbia River littoral cell were used. Several morphological phenomena have been identified via the application of two data-mining routines: the K-means clustering technique (KMA) and empirical orthogonal functions (EOFs). KMA clustering illustrates the main equilibrium configurations that the dry-beach profile experiences over time, whereas the EOF analysis explains the variability of the data in space and time. These analyses allow for examinations of berm formation and destruction as well as the shifting of the profile between summer/winter configurations—among other changes induced by the cross-shore sediment exchange, such as beachface and dune toe erosion and recovery.

OBSERVATIONS OF INTERTIDAL BAR WELDING ALONG A HIGH ENERGY, DISSIPATIVE COASTLINE

Field observations during summer 2014 in Newport, OR, USA indicate the importance of intertidal sandbar welding for shoreline recovery and growth. Over a two month period of low energy wave conditions subtidal and intertidal sandbars migrated onshore by approximately 1 m/day. There was 19 m/m of sediment added to the system above mean low water and the shoreline prograded by an average of 17 m along the 2.2 km stretch of coast during the experiment.

New Insights on Coastal Foredune Growth: The Relative Contributions of Marine and Aeolian Processes

Coastal foredune growth is typically associated with aeolian sediment transport processes, while foredune erosion is associated with destructive marine processes. New data sets collected at a high energy, dissipative beach suggest that total water levels in the collision regime can cause dunes to accrete-requiring a paradigm shift away from considering collisional wave impacts as unconditionally erosional. From morphologic change data sets, it is estimated that marine processes explain between 9% and 38% of annual dune growth with aeolian processes accounting for the remaining 62% to 91%. The largest wind-driven dune growth occurs during the winter, in response to high wind velocities, but out of phase with summertime beach growth via intertidal sandbar welding. The lack of synchronization between maximum beach sediment supply and wind-driven dune growth indicates that aeolian transport at this site is primarily transport, rather than supply, limited, likely due to a lack of fetch limitations.

THE INFLUENCE OF SPATIALLY VARYING SUPPLY ON COASTAL AEOLIAN TRANSPORT: A FIELD EXPERIMENT

Supply-limiting factors, like moisture content and sediment armoring, influence coastal aeolian sediment transport and subsequently dune evolution significantly. We organized a 6-week field experiment on the influence of spatiotemporal variations in supply on coastal aeolian sediment transport at the Sand Motor, The Netherlands. Due to the presence of a strongly curved coastline and complex intertidal bathymetries, a large spatial variation in supply is to be expected at the Sand Motor, which makes the area particularly suitable for a field experiment on this subject. Preliminary results show that not the largest surface area of sand, nor the biggest fetch or the most severe storm result in significant aeolian sediment transport events, but persistent moderate winds over large intertidal beaches are the key to coastal aeolian transport and subsequently dune evolution.