Initial Bathymetry Research Articles

The morphological response of a nearshore double sandbar system to constant wave forcing

Results of 2DH morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing. Sixteen different conditions varying both offshore wave height and angle of wave incidence were applied. The mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars. For similar longshore velocities, the length scales of the outer bar are larger than of the inner bar. This is explained by accounting for the difference in water depth above the bar crest. The variable morphological response times can be explained by including additional bathymetrical parameters. The active volume of the bar, defined by the breaker index, plays an important role in this response time. With larger active volumes the bar responds more rapidly to identical boundary conditions. Also, bars with a smaller total volume respond more quickly. This faster response is due to the steeper active volume of the bars. Different initial perturbations resulted in different locations of the emerging features, showing that their location is sensitive to the initial bathymetry. However, the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions. Based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing.

On the form and stability of seafloor stratigraphy and shelf profiles: A mathematical model and solution method

There are a variety of different marine sediment transport processes that affect the evolution of continental shelf profiles. Physically based numerical models make it possible to simulate the evolution of the continental shelf in response to these processes, which may be combined with different magnitudes, frequencies and durations. The basic model that is employed in most numerical simulations is based on conservation of mass and is a moving boundary problem since the river mouth progrades into the receiving basin. The general form of these stratigraphic or seafloor evolution models is a first-order, Exner-type differential equation that is forced by a sediment deposition/erosion function. This latter function depends on which sediment transport processes are modeled and how they are modeled, so it is desirable to have a solution method that works for any such function. This paper presents a general solution method to this type of evolution equation that works for any initial bathymetry and sediment deposition function given as 1D functions of seaward distance. It is based on the Laplace transform. Numerical results often exhibit “equilibrium profile” solutions for a wide variety of different model scenarios. These profiles may be viewed as travelling waves, such that the rate of progradation gives the speed of this wave as it advances into the receiving basin. When realistic initial and boundary conditions are used, profiles are often seen to evolve toward forms that are independent of the initial bathymetry. Although episodic perturbations such as turbidities disrupt these forms, they still serve as attractors for the system dynamics. An application of the solution method presented in this paper also allows the shape of equilibrium profiles and the progradation rate to be determined from a specification of the initial bathymetry and the sediment deposition function. Several illustrative examples are given in closed form. Results are also presented that make it possible to compute the amount of time required for an equilibrium profile to be re-established after a perturbation.

Numerical predictability experiments of cross‐shore sandbar migration

Surf zone sandbars, common features along the world's sandy coastlines, continuously change their position in response to time‐variable offshore wave conditions. Process‐based predictions of cross‐shore sandbar migration, relevant to the understanding of autonomous and artificially altered evolution of beaches, are intrinsically imprecise because of uncertainty in the model equations and, potentially, the sensitive dependence on the initial bathymetry. However, the magnitude of the resulting predictability limit and its dominant source are unknown. Here we show that cross‐shore sandbar migration on the time scale of years is deterministically forced rather than deterministically chaotic, and that the unpredictability of sandbar migration results primarily from model inadequacy during major wave events. Because the unpredictability of sandbar migration is related to the stochastic nature of the forcing, the predictability limit is not a fixed value but depends on the timing of the wave event. We anticipate that detailed experiments to understand nearshore evolution from the underlying first principles will eventually pay off by extending the range of plausible simulated behavior and, consequently, will increase our ability to understand and predict how coasts may respond to changing climate.

Flexural response of the Venetian foreland to the Southalpine tectonics along the TRANSALP profile

AbstractThe Venetian Basin was affected by flexure related to the Southalpine shortening phase during the Middle Miocene – Early Pliocene. This downbending is quantified here using a two‐dimensional flexural model. A recently improved data set on basin geometry based on the bottom of the Serravallian–Tortonian clastic wedge, on palaeobathymetry and gravity anomalies is used to constrain the components of flexure and to test the importance of the initial bathymetry in evaluating the contribution of surface loads to deflection. A good fit is obtained assuming a northward broken plate configuration of the downbent Adriatic plate with an effective elastic thickness of 20 km. Results highlight that, in the studied region, flexure related to the Eastern Southern Alps is totally due to surface loads (topographic load partly replacing initial bathymetry) and that no hidden loads are required. Furthermore, the palaeobathymetry contributes up to 50% to the total flexure in the studied region.

Role of initial depth at basin margins in sequence architecture: field examples and computer models

ABSTRACTA delay in the onset of sedimentation during fault‐related subsidence at a basin margin can occur in both extensional settings, where footwall tilting may cause a diversion of drainage patterns, and in strike‐slip basins, where a source area may be translated along the basin margin. The ‘initial depth’ created by this delay acts as pre‐depositional accommodation and is a partly independent variable. It controls the geometry of the first stratal units deposited at the basin margin and thus modifies the response of the depositional system to subsequent, syndepositional changes in accommodation. In systems with a sharp break in the depositional profile, such as the topset edge in coarse‐grained deltas, the initial depth controls the foreset height and therefore the progradational distance of the topset edge. The topset length, in turn, influences topset accommodation during cyclical base level variations and therefore is reflected in the resulting stacking patterns at both long‐ and short‐term time scales. In the simplified cases modelled in this study, it is the relationship between the initial depth and the net increase in depth over the interval of a relative sea‐level cycle (ΔH) that governs long‐ and short‐term stacking patterns. In situations where the initial depth is significantly larger than ΔH, the topset accommodation of the first delta is insufficient to contain the volume of sediment of younger sequences formed during subsequent relative sea‐level cycles. Therefore, the depositional system tends to prograde over a number of relative sea‐level cycles before the topset area increases so that the long‐term stacking pattern changes to aggradation. Stacking patterns of high‐frequency sequences are influenced by a combination of topset accommodation available and position of the short‐term relative sea‐level cycles on the rising or falling limb of a long‐term sea‐level curve. This determines whether deposits of short‐term cycles are accommodated in delta topsets or foresets, or in both. Variations in stacking pattern caused by different initial depths may be misinterpreted as due to relative sea level or sediment supply changes and it is necessary to consider initial bathymetry in modelling and interpretation of stacking patterns, especially in fault‐bounded basins.

The link between abrupt climate change and basin stratigraphy: a numerical approach

To use basin stratigraphy for studying past climate change, it is important to understand the influence of evolving boundary conditions (river discharge and sediment flux, initial bathymetry, sea level, subsidence) and the complex interplay of the redistribution processes (plumes, turbidity currents, debris flows). To provide understanding of this complexity, we have employed source to sink numerical models to evaluate which process dominates the observed variability in a sedimentary record of two coastal Pacific basins, Knight Inlet in British Columbia and the Eel Margin of northern California. During the last glacial period, the Eel River supplied comparatively more sediment with a less variable flux to the ocean, while today the river is dominated by episodic events. Model results show this change in the variability of sediment flux to be as important to the deposit character as is the change in the volume of sediment supply. Due to the complex interaction of flooding events and ocean storm events, the more episodic flood deposits of recent times are less well preserved than the flood deposits associated with an ice-age climate. In Knight Inlet, the evolving boundary conditions (rapidly prograding coastline, secondary transport by gravity flows from sediment failures) are a strong influence on the sedimentary record. The delta and gravity flow deposits punctuate the sedimentary record formed by hemipelagic sedimentation from river plumes. Missing time intervals due to sediment failures can take away the advantage of the otherwise amplified lithologic record of discharge events, given the enclosed nature of the fjord basin.

Evidence for pre-Cretaceous rifting in the Rockall Trough: an analysis using quantitative 2D structural/stratigraphic modelling

Quantitative 2D structural and stratigraphic modelling has been used to examine the crustal evolution of the Rockall Trough. The main phase of rifting and subsidence of the basin took place during the Cretaceous to Tertiary. The stratigraphy of the deeper parts of the Rockall Trough has not yet been tested and remains unproven. Two-dimensional forward syn-rift and post-rift modelling of crustal structure and basin geometry has been used to indicate whether a pre-Cretaceous rift system exists within the Rockall Trough. Basin modelling has been carried out on regional crustal profiles across the Rockall Trough derived from seismic reflection, refraction and gravity data. Observed present-day depths to top basement and Moho may be modelled for a range of β -stretching factors dependent upon the initial assumptions of Moho depth and bathymetry. Curves, summarizing model solutions, for β -factor versus both initial bathymetry and Moho depth, have been produced. Solutions for Cretaceous β -factors <3.0 require substantially thinned crust and water depths, prior to Cretaceous rifting, indicative of a preexisting rift basin.

Numerical modelling of the response to variable stretching rate of a domino fault block system

Using a numerical model of domino fault block development, the effects of a variable stretching rate during rifting were investigated. It is shown that a varable stretching rate can have an important influence on synrift sequence geometries. This can lead to complex synrift sequences composed of wedge-shaped units, formed during periods of high stretching rate, and more symmetrical units formed during periods of relative tectonic quiescence. At the fault block crest, increases in stretching rate can produce offlapping synrift sequences whereas decreases in stretching rate allow onlap onto the fault block crest. If the rifting period contains multiple pulses of stretching, complex intra-synrift unconformities can be formed. The initial bathymetry is also shown to have an important influence on the modelled synrift stratigraphy. The modelled stratigraphies are similar to those thought to result from variations in absolute sea level. Pseudo-facies markers, based on the combined initial depth and slope of deposition of the modelled sediments, are shown to be a useful tool and graphically illustrate the response of the system to pulses of tectonic activity.