Jeff E. Hansen

Evaluation of nearshore wave models in steep reef environments

To provide coastal engineers and scientists with a quantitative evaluation of nearshore numerical wave models in reef environments, we review and compare three commonly used models with detailed laboratory observations. These models are the following: (1) SWASH (Simulating WAves till SHore) (Zijlema et al. 2011), a phase-resolving nonlinear shallow-water wave model with added nonhydrostatic terms; (2) SWAN (Simulating WAve Nearshore) (Booij et al. 1999), a phase-averaged spectral wave model; and (3) XBeach (Roelvink et al. 2009), a coupled phase-averaged spectral wave model (applied to modeling sea-swell waves) and a nonlinear shallow-water model (applied to modeling infragravity waves). A quantitative assessment was made of each model’s ability to predict sea-swell (SS) wave height, infragravity (IG) wave height, wave spectra, and wave setup (η¯

Wave setup over a fringing reef with large bottom roughness

\overline{\eta}

Is climate change shifting the poleward limit of mangroves

) at five locations across the laboratory fringing reef profile of Demirbilek et al. (2007). Simulations were performed with the “recommended” empirical coefficients as documented for each model, and then the key wave-breaking parameter for each model (α in SWASH and γ in both SWAN and XBeach) was optimized to most accurately reproduce the observations. SWASH, SWAN, and XBeach were found to be capable of predicting SS wave height variations across the steep fringing reef profile with reasonable accuracy using the default coefficients. Nevertheless, tuning of the key wave-breaking parameter improved the accuracy of each model’s predictions. SWASH and XBeach were also able to predict IG wave height and spectral transformation. Although SWAN was capable of modeling the SS wave height, in its current form, it was not capable of modeling the spectral transformation into lower frequencies, as evident in the underprediction of the low-frequency waves.

Standing infragravity waves over an alongshore irregular rocky bathymetry

Abstract Barnard, P.L.; Hansen, J.E., and Erikson, L.H., 2012. Synthesis study of an erosion hot spot, Ocean Beach, California (USA). A synthesis of multiple coastal morphodynamic research efforts is presented to identify the processes responsible for persistent erosion along a 1-km segment of 7-km-long Ocean Beach in San Francisco, California. The beach is situated adjacent to a major tidal inlet and in the shadow of the ebb-tidal delta at the mouth of San Francisco Bay. Ocean Beach is exposed to a high-energy wave climate and significant alongshore variability in forcing introduced by varying nearshore bathymetry, tidal forcing, and beach morphology (e.g., beach variably backed by seawall, dunes, and bluffs). In addition, significant regional anthropogenic factors have influenced sediment supply and tidal current strength. A variety of techniques were employed to investigate the erosion at Ocean Beach, including historical shoreline and bathymetric analysis, monthly beach topographic surveys, nearshore and regional bathymetric surveys, beach and nearshore grain size analysis, two surf-zone hydrodynamic experiments, four sets of nearshore wave and current experiments, and several numerical modeling approaches. Here, we synthesize the results of 7 years of data collection to lay out the causes of persistent erosion, demonstrating the effectiveness of integrating an array of data sets covering a huge range of spatial scales. The key findings are as follows: anthropogenic influences have reduced sediment supply from San Francisco Bay, leading to pervasive contraction (i.e., both volume and area loss) of the ebb-tidal delta, which in turn reduced the regional grain size and modified wave focusing patterns along Ocean Beach, altering nearshore circulation and sediment transport patterns. In addition, scour associated with an exposed sewage outfall pipe causes a local depression in wave heights, significantly modifying nearshore circulation patterns that have been shown through modeling to be key drivers of persistent erosion in that area.

Seasonal Shoreline Variability Induced by Subtidal Water Level Fluctuations at Reef‐Fringed Beaches

AbstractThe effect of bottom roughness on setup dynamics was investigated using high-resolution observations across a laboratory fringing reef profile with roughness elements scaled to mimic the frictional wave dissipation of a coral reef. Results with roughness were compared with smooth bottom runs across 16 offshore wave and still water level conditions. The time-averaged and depth-integrated force balance was evaluated from observations collected at 17 locations along the flume and consisted of cross-shore pressure and radiation stress gradients whose sum was balanced by quadratic mean bottom stresses. The introduction of roughness had two primary effects. First, for runs with roughness, frictional wave dissipation occurred on the reef slope offshore of the breakpoint, reducing wave heights prior to wave breaking. Second, offshore-directed mean bottom stresses were generated by the interaction of the combined wave–current velocity field with the roughness elements. These two mechanisms acted counter to...

Source and supply of sediment to a shoreline salient in a fringing reef environment: Reef to shoreline sediment connections

AbstractHigh-resolution observations from a 55-m-long wave flume were used to investigate the dynamics of wave setup over a steeply sloping reef profile with a bathymetry representative of many fringing coral reefs. The 16 runs incorporating a wide range of offshore wave conditions and still water levels were conducted using a 1:36 scaled fringing reef, with a 1:5 slope reef leading to a wide and shallow reef flat. Wave setdown and setup observations measured at 17 locations across the fringing reef were compared with a theoretical balance between the local cross-shore pressure and wave radiation stress gradients. This study found that when radiation stress gradients were calculated from observations of the radiation stress derived from linear wave theory, both wave setdown and setup were underpredicted for the majority of wave and water level conditions tested. These underpredictions were most pronounced for cases with larger wave heights and lower still water levels (i.e., cases with the greatest setdow...

Coastal vulnerability across the Pacific dominated by El Niño/Southern Oscillation

Ecological (poleward) regime shifts are a predicted response to climate change and have been well documented in terrestrial and more recently ocean species. Coastal zones are amongst the most susceptible ecosystems to the impacts of climate change, yet studies particularly focused on mangroves are lacking. Recent studies have highlighted the critical ecosystem services mangroves provide, yet there is a lack of data on temporal global population response. This study tests the notion that mangroves are migrating poleward at their biogeographical limits across the globe in line with climate change. A coupled systematic approach utilising literature and land surface and air temperature data was used to determine and validate the global poleward extent of the mangrove population. Our findings indicate that whilst temperature (land and air) have both increased across the analysed time periods, the data we located showed that mangroves were not consistently extending their latitudinal range across the globe. Mangroves, unlike other marine and terrestrial taxa, do not appear to be experiencing a poleward range expansion despite warming occurring at the present distributional limits. Understanding failure for mangroves to realise the global expansion facilitated by climate warming may require a focus on local constraints, including local anthropogenic pressures and impacts, oceanographic, hydrological, and topographical conditions.

Understanding processes controlling sediment transports at the mouth of a highly energetic inlet system (San Francisco Bay, CA)

Rocky reef coastlines typically feature highly variable and often abrupt cross-shore and alongshore changes in bathymetry. The effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown. Most models of infragravity wave dynamics have been developed and validated on smooth alongshore-uniform bathymetries, which may break down over these highly variable bathymetries. A 2 week field experiment was conducted on a rocky reef-fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves. The height of short (sea-swell) waves decreased over the shallow reef due to breaking, whereas the height of infragravity waves increased toward the shoreline. Both during a storm event (Hm0 = 2.3 m) and under moderate wave conditions (Hm0 = 1.0–1.8 m), the infragravity waves formed a persistent cross-shore standing wave pattern along the entire shoreline, despite the irregular bathymetry. In addition, the alongshore components of infragravity waves refracted by the presence of the nearshore reef were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern. Thus, the presence of highly variable nearshore bathymetry, which commonly occurs along rocky reef coastlines, may produce both cross-shore and alongshore standing wave patterns.