Geology topics

Jeff Hansen

Publications and source records attributed to Jeff Hansen.

8 recordsLinked to original sources

USGS2023

Carbonate sediment transport across coral reefs: A comparison of fringing vs. barrier reefs

Considerable uncertainty remains in the budgets of carbonate sediment on reef lined coasts, particularly with respect to the supply of sediment to a reef flat that is then transported throughout a reef system. In this study, we re-examine two recent studies, one on a barrier reef bounded by channels that incise the reef, and one on a fringing reef without channels. Results indicate that the presence of channels results in a circulation regime that promotes not only the onshore sediment flux across the reef flat, but also upslope transport of sediment from the fore reef onto the reef flat. Data from these experiments suggest that when channels are present in the reef flat, the hydrodynamics of these reef systems favor the transport the transport of sediment up the fore reef. This outcome has implications for the design of potential reef restoration efforts intended to protect shorelines.

Conference Paper↗

USGS2023

Dynamics of the wave-driven circulation in the lee of nearshore reefs

Nearshore rocky reefs with scales of order 10–100 m are common along the world's coastline and often shape wave-driven hydrodynamics and shoreline morphology in their lee. The interaction of waves with these reefs generally results in either two or four-cell mean circulation systems (2CC and 4CC, respectively), with diverging flows behind the reefs and at the shoreline in the 2CC case and flows that diverge in the lee and converge at the shoreline in the 4CC case. By applying a phase-resolving wave-flow model to conduct a detailed analysis of mean momentum balances for waves interacting with nearshore reefs, we develop an understanding of the drivers of 2CC and 4CC flow dynamics and how they vary for different reef geometries and wave and water level conditions. The 2CC or 4CC patterns were primarily driven by alongshore pressure gradients toward the exposed (nonreef fronted) or reef-fronted beach. These alongshore pressure gradients were dependent on the cross-shore setup dynamics governed by the balance between pressure (i.e., related to the setup) and radiation stress gradients, and mean bottom stresses exerted on the water column. If shoreline wave setup in the lee of the reef was less than the exposed beach, a 4CC pattern developed with convergent flow at the shoreline in the lee of the reef; otherwise, a 2CC emerged with divergent flow at the shoreline. Across the parameter space investigated, reef roughness, distance to the shoreline, and beach slope were the three parameters most likely to change the flow patterns between 2CC and 4CC.

JGR-Oceans↗

USGS2023

Wave runup and inundation dynamics on a perched beach

Sandy beaches perched over rocky shore platforms are common globally, yet their mixed sand and rocky morphology present challenges for quantifying and predicting wave runup and inundation. For typical linear beach profiles, simple relationships can be made between vertical runup and horizontal inundation based on beach slope. However, as topographic irregularities increase, substantial deviations from these relationships can occur. Shore platforms often experience a range of beach states, from full sand coverage (accreted) to complete shore platform erosion (exposed). As a result, existing methods for quantifying runup and inundation on purely sandy beaches are generally not directly applicable to these coastlines. To advance our understanding of runup and inundation on perched beaches, the aim of this work is three-fold: (1) to provide a method for quantifying beach slope and runup across a range of perched beach profiles, (2) to assess the relationship between vertical runup and horizontal inundation observations, and (3) to understand how different hydrodynamic mechanisms (i.e., setup, swash) contribute to runup during different beach states. To achieve this, we conducted an 8-month field study along a perched beach in southwestern Australia that experiences large seasonal variations in beach state and wave climate. A method was developed to measure runup and beach slope when only the topography shoreward of the shore platform edge was known. Using this method, an approximately linear relationship between inundation and runup was identified by incorporating beach slope. Our observations suggest that the components that dominate runup on perched beaches were primarily dependent on beach state (i.e., accreted versus exposed). Runup was dominated by swash in the infragravity band when in an accreted beach state, and setup when in an exposed beach state. These results aid our understanding of coastal processes on perched beaches, while providing new methods applicable to a range of perched beach profiles.

Geomorphology↗

USGS2022

The influence of submerged coastal structures on nearshore flows and wave runup

Engineered and natural submerged coastal structures (e.g., submerged breakwaters and reefs) modify incident wave fields and thus can alter hydrodynamic processes adjacent to coastlines. Although submerged structures are generally assumed to promote beach protection by dissipating waves offshore and creating sheltered conditions in their lee, their interaction with waves can result in mean wave-driven circulation patterns that may either promote shoreline accretion or erosion. Here, we analyse the mean flow patterns and shoreline water levels (wave runup) in the lee of idealised impermeable submerged structures with a phase-resolved nonhydrostatic numerical model. Waves propagating over submerged structures can drive either a 2-cell mean (wave-averaged) circulation, which is characterised by diverging flows behind the structure and at the shoreline, or 4-cell circulation, with converging flows at the shoreline and diverging flows in the immediate lee of the structure. The numerical results show that the mode of circulation can be predicted with a set of relationships depending on the incoming wave heights, the structure crest level, and distance to the shoreline (or structure depth). Qualitative agreement between the mean flow and proxies for the sediment transport using an energetics approach suggest that the mean flow can be a robust proxy for inferring sediment transport patterns. For the cases considered, the submerged structures had a minimal influence on shoreline wave setup and wave runup despite the wave energy dissipation by the structures due to alongshore wave energy fluxes in the lee. Consequently, these results suggest that the coastal protection provided by the range of impermeable submerged structures we modelled is primarily due to their capacity to promote beach accretion.

Coastal Engineering↗

USGS2021

A numerical study of wave-driven mean flows and setup dynamics at a coral reef-lagoon system

Two-dimensional mean wave-driven flow and setup dynamics were investigated at a reef-lagoon system at Ningaloo Reef, Western Australia, using the numerical wave-flow model, SWASH. Phase-resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including >10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phase-resolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave-setup inside the lagoon by up to 200–370%. The bottom stress was found to be caused by the combined presence of onshore directed wave-driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20–40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.

Journal of Geophysical Research-Oceans↗

USGS2021

The contribution of currents, sea-swell waves, and infragravity waves to suspended-sediment transport across a coral reef-lagoon system.

Coral reefs generate substantial volumes of carbonate sediment, which is redistributed throughout the reef‐lagoon system. However, there is little understanding of the specific processes that transport this sediment produced on the outer portions of coral reefs throughout a reef‐lagoon system. Furthermore, the separate contributions of currents, sea‐swell waves, and infragravity waves to transport, which are all strongly influenced by the presence of a reef, is not fully understood. Here, we show that in reef‐lagoon systems most suspended sediment is transported close to the seabed and can, at times, be suspended higher in the water column during oscillatory flow transitions (i.e., near slack flow) at sea‐swell wave frequencies, and during the peak onshore oscillatory velocity phase at infragravity wave frequencies. While these wave frequencies contribute to the transport of suspended sediment offshore and onshore, respectively, the net flux is small. Mean currents are the primary transport mechanism and responsible for almost 2 orders of magnitude more suspended‐sediment flux than sea‐swell and infragravity waves. Whilst waves may not be the primary mechanism for the transport of sediment, our results suggest they are an important driver of sediment suspension from the seabed, as well as contributing to the partitioning of sediment grain sizes from the reef to the shoreline. As the ocean wave climate changes, sea level rises, and the composition of reef benthic communities change, the relative importance of mean currents, sea‐swell waves, and infragravity waves is likely to change, and this will affect how sediment is redistributed throughout reef‐lagoon systems.

Ningaloo Reef↗

USGS2019

Spatial and temporal variability in ripple formation and migration across a coral reef flat and lagoon

The transport of carbonate sediment across reefs to the shoreline is of great interest to the research community and coastal managers alike. This sediment is generated by the breakdown of reef skeletal structure and a critical source for beach nourishment that provides a buffer to coastal flooding as sea levels rise. Understanding the physical processes that are responsible for this flux of sediment to the coast is therefore necessary before we can begin to predict how it may be affected by rising sea levels. Six weeks of high quality physical oceanographic data collected on the fringing reef in the Jurabi UNESCO World Heritage Site in Western Australia provides the opportunity to examine these processes in greater detail than ever before.

Jurabi UNESCO World Heritage Site↗

USGS2012

Recent scientific advances and their implications for sand management near San Francisco, California: The influences of the ebb tidal delta

Recent research in the San Francisco, California, U.S.A., coastal region has identified the importance of the ebb tidal delta to coastal processes. A process-based numerical model is found to qualitatively reproduce the equilibrium size and shape of the delta. The ebb tidal delta itself has been contracting over the past century, and the numerical model is applied to investigate the sensitivity of the delta to changes in forcing conditions. The large ebb tidal delta has a strong influence upon regional coastal processes. The prominent bathymetry of the ebb tidal delta protects some of the coast from extreme storm waves, but the delta also focuses wave energy toward the central and southern portions of Ocean Beach. Wave focusing likely contributes to a chronic erosion problem at the southern end of Ocean Beach. The ebb tidal delta in combination with non-linear waves provides a potential cross-shore sediment transport pathway that probably supplies sediment to Ocean Beach.

California↗