Intertidal Profile Research Articles

Modelling the time‐varying extent of groundwater seepage on tidal beaches

AbstractThe outcrop of groundwater on tidal beaches distinguishes an upper unsaturated region from a lower saturated region of the intertidal profile. Since the 1940s, it has been recognized that the extent of groundwater seepage at the beach face is one factor determining the tendency for erosive or accretionary conditions to prevail. As a primary step towards incorporating bed saturation characteristics within cross‐shore sediment transport models, this paper (and accompanying program disk) details a simple model to simulate the time‐varying extent of seepage face development across tidal beaches. From a comparison with field results obtained on the macrotidal Central Queensland (Australia) coast, the model appears to provide an encouraging degree of predictive capability. The model also assists in highlighting the sensitivity of seepage face development to varying beach face, tide and wave characteristics.

Simulating the influence of groundwater seepage on sediment transported by the sweep of the swash zone across macro-tidal beaches

A numerical model is developed to simulate the influence of groundwater seepage on swash zone sediment transport across macro-tidal beaches. The emergence of equilibrium profile morphology is examined in response to varying tide, sediment and initial profile characteristics. Due to the unresolved complexities of boundary layer flow and corresponding shear stresses in the swash zone, sediment transport across the beach face is modelled by incorporating an uprush phase described by the shallow water wave equations, within an heuristic model of equilibrium beach face slope. A new parameter, termed the “ M” net transport parameter, is defined to parameterize the net flux of sediment per uprush-backrush cycle. The definition of differing equilibrium gradients across saturated versus unsaturated regions of the intertidal profile permits the saturation characteristics of the bed to be incorporated. The dynamics of the watertable exit point on the beach face is central to the scheme, as this defines the time-varying extent of saturated and unsaturated regions of the intertidal profile. The sensitivity of coastal seepage face development to tide, profile and sediment characteristics is highlighted through the derivation of a new seepage face parameter. The simulation model is found to successfully reproduce key aspects of observed profile adjustment and resulting equilibrium morphology. Complex morphodynamic feedback is observed between: the tidal stage, net transport per uprush-backrush cycle, profile slope, and the dynamics of the watertable outcrop. Net upper-profile steepening and lower-profile lowering is simulated on beaches composed of medium to coarse sand. This results in the emergence of a distinctive break in slope within the intertidal profile. Such morphology is commonly observed on macro-tidal coasts.

Water table outcropping on macro-tidal beaches: A simulation model

During the fall of the tide the coastal water table within a beach may become decoupled from the ocean, resulting in the formation of a seepage face where water outcrops on the inter-tidal profile. A simulation model (called seep) is developed to simulate the point of decoupling, and the subsequent motion of the seepage face as it continues to fall across the inter-tidal profile. The model is based entirely on the dynamics of an isolated water particle on the seepage face, i.e. the pressure distribution within the beach is ignored. Sensitivity analysis of the seep model to beach face slope and permeability characteristics suggests that small variations to these parameters can result in significantly differing rates of exit point motion and hence seepage face extent. To test the predictive capability of the seep model, results are presented from a field investigation of exit point dynamics on a macro-tidal beach, North Queensland Australia. The simulation model is found to accurately reproduce the observed seepage face motion at this field site. In addition, a re-analysis of seepage face extent reported in Nielsen (1990) from a protected, micro-tidal environment, shows similarly strong agreement between the reported and modelled exit point elevation.

Shoreline Adjustments and Coastal Management: Physical and Biological Processes under Accelerated Sea-Level Rise

The physical responses to predicted sea-level rise are examined using examples from the south-east coast of Britain where tectonically induced sea level rise are already equivalent to those predicted under the global warming hypothesis. Intertidal profiles on this coast are shifting both upwards and shorewards while estuarine channels are becoming wider and shallower. These natural changes are interrupted by the presence of flood embankments which can force tidal waves inland along the estuary setting up increased flood risks here. Biological communities such as salt marsh are also affected and appear to be migrating inland along the estuary in response to sealevel changes but again this process is interrupted by flood embankments. Managed retreat of the embankments may alleviate some of these problems but this should not take the form of semi-enclosed tidal cells formed by breaching the present defences. Instead the increases in tidal prism and the effects of this on estuarine hydro-dynamics should be considered and an holistic management plan prepared for each coastal and estuarine area.

A numerical model for shore‐normal sediment size variation on a macrotidal beach

AbstractThis paper describes a computer simulation model which is designed to predict the selective shore‐normal sorting of grain sizes in the nearshore environment. The model simulates wave shoaling, wave height attenuation due to frictional losses and breaking, using linear theory up to the break point and a breaker decay model in the surf zone. Peak horizontal orbital velocities at the bed are calculated from Stokes second‐order wave theory. The peak onshore and offshore velocities are used with the threshold expression of Komar and Miller (1975) to generate a spatial pattern of size variation of threshold grain diameter along a profile normal to the shore from deep water to the swash zone. The predicted grain size is used in an hydraulic interpretation of grain size distribution on the intertidal profile, based on the hydrodynamic variations over a tidal cycle on a macrotidal beach. The model is successful in predicting the broad pattern of increasing grain size in the onshore direction which has been observed in nature. Comparisons between measured and predicted grain size distributions indicate that the predictions of the model are better than those of previous models, but the model is more successful at predicting sediment size distributions than at predicting mean sizes on a beach profile.

Shoreline processes and establishment of Phragmites australis in a coastal plain estuary

Phragmites australis occurs extensively along undisturbed salt-marsh shorelines of Delaware Bay. The species has been considered indicative of human disturbance when found in estuarine marshes in the USA. It is suggested that geomorphic processes associated with coastal submergence provide an analog of human disturbances which can enable Phragmites australis to become established naturally. Deposition of sand bodies (or rafted debris) can suppress existing vegetation and allow Phragmites to become established. Subsequently, even if the sand or debris is moved, erosional truncation of the intertidal profile can inhibit recolonization by the original dominant shoreline species, Spartina alterniflora.

A calibration of the Bagnold beach equation

A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m −4s −2 for the calibration coefficient in the Bagnold beach equation.

Morphodynamics of a macrotidal beach

An intensive field investigation of hydrodynamic processes, processes of sediment entrainment and suspension, and morphologic change was carried out on an unprotected macrotidal beach near Broome in northwestern Australia. The spring-tide range was 9.5 m; waves had heights of 0.5–1.2 m and periods of 9–13 s. The beach had an overall concave-upward profile with low-gradient and dissipative subtidal and low-tidal zones, and steeper more reflective mid-tidal and high-tidal zones. Direct measurement of energy-flux dissipation over the intertidal profile showed dissipation rates on the order of 1–2 W m −2 of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height. Time-averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low-tidal zone and over the lower part of the high-tidal zone. Most of the work over the low- and mid-tidal zones was performed by unbroken shoaling waves rather than by surf-zone processes; surf-zone processes only dominate over the high-tidal zone. The nature of the surf-zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level. The growth of standing waves and infragravity (“surf-beat”) oscillations, as identified from spectra and cross spectra of surface elevation, η, and currents, u and v, was inhibited over most of the profile. However, well-developed secondary standing wave energy, particularly at infragravity frequencies, was observed over the high-tidal zone at spring high tide and over the mid-tidal zone at neap high tide. Over the low-tidal and subtidal zones, strong shore-parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves. Over the subtidal zone asymmetrical tidal currents, skewed toward the north, attained maximum speeds of 0.5 m s −1 just after high water. Field measurements of suspended-sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave-induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves. Northerly advection of wave-suspended sediment by asymmetrical tidal currents over the subtidal and low-tidal zones accounted for a net northerly longshore transport. The greatest morphologic mobility of the intertidal profile occurred over the lower high-tidal and upper mid-tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes. Temporally, the greatest mobility of the profile as a whole was observed on the short, within-tidal-cycle time scale. Net changes over the longest time scale of a lunar half cycle were negligible.

Intertidal meiofauna of Recherchefjorden and Malbukta, Vest-Spitsbergen

Abstract In summer 1977, meiofauna samples were collected using a 2.38 cm inner diameter perspex tube from five intertidal profiles on various Spitsbergen beaches. Profiles A, B (Recherchefjorden), and D (Malbukta) were situated on exposed beaches; their meiofaunal densities ranged from 38.25 to 654.75 ind. 10 cm-2 with a clear predominance of Turbellaria (27.78–95.18 %). Densities of the interstitial fauna on sheltered, detritus-rich profiles C (Recherchefjorden) and E (Malbukta) ranged from 67.5 to 4092.75 ind. 10 cm-2, a strong domination of nematodes (30.65–98.63 %) as well as a high percentage of oligochaetes (up to 56.75 %) and a more diversified meiofauna being observed on these profiles.

MEASUREMENT OF PHYSIOGRAPHIC CHANGES ON MANGROVE-FRINGED ESTUARIES AND COASTLINES

Studies of physiographic changes are in progress on mangrove-fringed shorelines in Westernport Bay in Victoria, and Cairns Bay in Queensland, Australia. Historical evidence of advance or retreat of these shorelines has been obtained from comparison of historical maps and air photographs with the present configuration, and supplemented by monitoring shoreline" changes with reference to established datum points during the past decade. In addition, changes in substrate level within the mangrove fringe, on mudflats to seaward, and on salt marshes (where present) to landward have been measured with reference to vertical changes on inserted poles, accretion over emplaced brick-dust layers, and changes in probe depths to underlying horizons (rock, sand, or peat) Results indicate that the advance of Avicennia marina on to mudflats is followed by relatively rapid and generally sustained vertical accretion related to interception of sediment within pneumatophore fields. Other mangroves or salt marsh species then replace Avicennia, and continued accretion leads to the development of a depositional marshland terrace, representing a re-shaping of the inter-tidal profile by the agency of vegetation. Mangroves that produce pneumatophore fields are considered to be more effective in trapping sediments than stilt rooted mangroves such as Rhizophora spp. or buttressed mangroves such as Bruguiera spp.