Jesper Bartholdy

Long term variations in backbarrier salt marsh deposition on the Skallingen peninsula - the Danish Wadden Sea

Abstract Accretion on a natural backbarrier salt marsh was determined and modeled as a function of high tide level, initial salt marsh level and distance from the salt marsh edge. Accretion measurements were based on up to 67-year-old marker horizons, supplemented by 210Pb/137Cs datings. The salt marsh is situated on the backbarrier of the Skallingen peninsula in the northern part of the Danish Wadden Sea. The tidal range (mean 1.5 m) is strongly affected by wind tide which occasionally adds up to about 3 m to the astronomical high tide level. Accretion is restricted to a narrow vertical band from about 0.1 m below to about 0.7 m above the mean high water level. In the outer part of the backbarrier (close to the tidal flat) mean accretion is about 4 mm yr−1 and in the inner part it is about 2 mm yr−1. The decrease takes place in a similar manner as the decrease across a flood plain. The major part of salt marsh accretion is associated with high water levels corresponding to weather conditions characterized by gales. The long term variation of salt marsh accretion correlates with variations in the North Atlantic Oscillation winter index. The number of over-marsh high tides decreases exponentially with high tide level. The function constants controlling this distribution vary in direct ratio to the mean sea level. Plausible near future scenarios of tidal development were obtained by extrapolating this relation. Three sea level scenarios were tested: (i) sea level rise continues at a constant (long term) rate of 2.3 mm yr−1, (ii) sea level rise continues at a constant (short term) rate of 4.2 mm yr−1 and (iii) sea level rise accelerates to a constant rate of 6.4 mm yr−1. In the first case accretion on the salt marsh will keep pace with the high water level rise. In the second case deposition in the inner part of the salt marsh will lag behind while that of the outer part of the salt marsh will keep pace with the rising high water level. In the third case, corresponding to a ‘worst case’ scenario for the 21th century, the salt marsh will gradually drown.

Grain-size control of large compound flow-transverse bedforms in a tidal inlet of the Danish Wadden Sea

Abstract Flow-transverse bedforms were recorded along a 2-km reach of the northernmost tidal inlet of the Danish Wadden Sea. The bed of the inlet channel is covered with large to very large compound dunes and superimposed medium-sized dunes composed of medium to coarse sands. The mean grain size of the bed material along the centre line of the inlet throat decreases from 0.56 mm in the inner part to 0.3 mm in the outer part. Both dune populations are essentially two-dimensional with straight to slightly curved crest lines. The ebb current is very similar in magnitude along the entire channel axis, whereas the flood current is of similar magnitude in the inner half, but reaches only about half of the maximum ebb current velocity in the outer section. Sediment transport occurs during both the ebb and flood periods, the latter being less vigorous and of shorter duration. As a result, the inlet as a whole is ebb-dominated (dominant ebb current: 1.01 m s−1; dominant flood current: 0.79 m s−1). In agreement with the tidal dynamics, a net ebb-oriented bedform migration rate of 32 m yr−1 was determined from the survey data. The large to very large dunes display distinct flood caps during the flooding tide, whereas the medium-sized dunes reverse their direction during each tide. The downstream dimensional evolution of the large dunes is complex, switching from initial growth to progressive decline, followed by renewed growth in length but progressive decline in height. This latter degradational phase begins in the middle of the survey reach and continues seawards until the large forms disappear completely. At the same time, the outer channel section becomes dominated by medium-sized dunes which grow in size with decreasing grain size. Surprisingly, there is no correlation between dune dimensions and water depth, as would be expected at such shallow depths. The complex evolution of dune dimensions as a function of grain size is discussed in terms of recent tidal bedform stability theory. It is clear, however, that more site-specific observations are required to fully explain the phenomenon.

Accumulation of fine-grained material in a Danish tidal area

Abstract The exchange of fine-grained material ( μ m) between the North Sea and the tidal area of Gradyb in the Danish Wadden Sea has been determined. By means of the 210 Pb dating method the amount of fine-grained material accumulating in the area (169 km 2 ) has been calculated to be 142 × 10 3 t yr −1 . Of this approximately 85% is derived from the North Sea, 5–6% from rivers, 4% from internal coastal erosion and another 5–6% from the combined effect of waste, atmospheric deposition and net primary production. These results have been verified through a budget for excess 210 Pb. It is demonstrated that fine sediments from the North Sea are sorted before accumulating in the Wadden Sea, with the result that the deposited material contains more coarse silt and less clay than is found in the source material from the North Sea. The relation between accumulated fine-grained material and the volume of exchanged water is found to be in the same order of magnitude in Gradyb as in Jadebusen and Dollard. This relation is suggested to be a characterising parameter of the sedimentation in tidal areas.

Morphodynamics of a pseudomeandering gravel bar reach

Abstract A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering . A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river.

The Backbarrier Sediments of the Skallingen Peninsula, Denmark

Geografisk Tidsskrift, Danish Journal of Geography 97: 11–32, 1997. The Skallingen peninsula is formed by sand derived from the south-going littoral drift along the North Sea coast. The backbarrier...

Effects of morphological changes on metal accumulation in a salt marsh sediment of the Skallingen peninsula, Denmark

In 1931 a red-colored, sandy marker horizon was placed on the emergingSkallingen salt marsh. Sedimentation on top of the marker horizon sincethen shows two opposing tendencies. Coincident with salt marshdevelopment the sediments display up to 1964 a fining upward sequencewith an increasing content of organic matter. Since 1964 a nearby creekhas meandered towards the sampling plot. Consequently, the sedimentsbecome coarser with a decreasing organic matter content. Themorphological induced changes in sedimentary conditions strongly influencemetal content in the sediments and thereby hide anthropogenic inducedconcentration variations. Thus, an apparently diminishing Zn content (perkg dry weight) since 1964 could indicate lesser load to the area. However,corrected for grain size effects there is an increasing content of Zn. Othermetal concentrations (e.g. Cu) show a diminishing trend when corrected forgrain size effects and therefore indicate a reduced anthropogenic inducedload of these metals to the salt marsh.

Wind-Driven Sea-Level Variation Influences Dynamics of Salt Marsh Vegetation

Long-term variation of mean sea level has been considered the primary exogenous factor of vegetation dynamics in salt marshes. In this study, we address the importance of short-term, wind-induced rise of the sea surface in such biogeographic changes. There was an unusual opportunity for examining field data on plant species frequency, sea-level variation, and sedimentation acquired from the Skallingen salt marsh in Denmark since the 1930s. The environmental and floristic history of Skallingen was summarized as (1) continuous sea-level rise with temporal variability (2.3–5.0 mm yr−1), (2) continuous sedimentation with spatial variability (2.0–4.0 mm yr−1), (3) increased frequency of over-marsh flooding events, and (4) contemporary dominance of Halimione portulacoides, indicating little progressive succession toward a later phase. Conventionally, recent eustatic sea-level rise was believed to drive the increased frequency of flooding and such retarded succession. Skallingen, however, has showed more or less equilibrated yearly rates between sea-level rise and surface accretion. This implies that the long-term, gradual sea-level rise alone might not be enough to explain the increased inundation frequency across the marsh. Here, we suggest an alternative chain: Recent trends in the North Atlantic Oscillation index toward its positive phase have led to increased storminess and wind tides on the ocean surface, resulting in increased frequency, duration, and depth of submergence, and hence, waterlogging of marsh soils, which has retarded ecological succession. To conclude, we stress the need for a multitemporal perspective that recognizes the significance of short-term sea-level fluctuations nested within long-term trends.

Scale-Dependent Correspondence of Floristic and Edaphic Gradients across Salt Marsh Creeks

Biogeographers emphasize the presence of scale-dependence in vegetation–environment relationships. This research addresses the issue of scale-dependence, focusing on spatial correspondence of floristic and edaphic gradients across salt marsh creeks at Skallingen, Denmark. We employed a hierarchical approach, which compared vegetation and soil gradients at both fine and coarse scales. At the fine scale, we used ordination techniques to identify the gradient structure of vegetation and soil data acquired in 1-m2 plots along transects across creeks. For the coarse-scale comparison, we systematically aggregated the fine-scale information (i.e., gradient structure) into larger blocks, or topographic zones such as point bar, cutbank edge, and marsh interiors. We found poor correlations between vegetation and soil at the fine scale but an improved correspondence at the coarse scale. The poor correspondence was caused by differential spatial extents that the vegetation and soil factors exhibited responding to environmental variations. At the fine scale, modes of plant–plant interactions were the key determinant of species composition and varied significantly across tidal creeks. Soil properties responded less sensitively to topographic and hydrologic changes than vegetation did, however. As the scale of our analysis increased, the fine-scale floristic variation averaged out, and the overall compositional patterns were then governed by broader variations in environmental (edaphic) constraints. Our research accommodates the importance of both biological and environmental components by explicitly emphasizing their simultaneous significance, each perceived at different spatial scales. We conclude that the concept of scale-dependence might serve as an efficient conceptual framework in salt marsh biogeography.

Environmental Controls on Multiscale Spatial Patterns of Salt Marsh Vegetation

In coastal environments, biogeographic patterns are generally influenced by surface elevation and horizontal distance from sea water. However, it is still unclear whether these major topographic factors are significant controls of vegetation patterns across spatial scales at which different physical processes operate. This study investigated such a topography-vegetation relationship in a Danish salt marsh, focusing upon two scales: a macro-scale (ca. 500 m) across the marsh platform, encompassing seaward and landward areas, and a meso-scale (ca. 25 m) across tidal creeks. While long-term sea-level variation and grazing influenced the macro-scale pattern, short-term fluvial-geomorphic processes drove the meso-scale pattern. Despite these different underlying processes, similar floristic gradient structures between the two scales were identified by nonmetric multidimensional scaling. The gradient represented an ecological sequence from early to late succession, and strongly correlated with surface elevation. However, the gradient did not show any significant relationship with distance from shoreline or tidal channels. Our results suggest that, in salt marshes, elevation plays a still more important ecological role than the horizontal position relative to sea water at both macro- and meso-scales. The presence of one such fundamental component makes the system relatively simple, and will facilitate future scaling attempts.

Bedform dynamics and net sediment transport paths over a flood-ebb tidal cycle in the Grådyb channel (Denmark), determined by high-resolution multi-beam echosounding

Abstract High-resolution bathymetric surveys were carried out by means of a high-frequency (455 kHz) multibeam-echosounder (MBES) to determine the total volumetric sediment transport over a tidal period in a tidal channel at the Danish west coast. With the high repetition rate and high accuracy of the MBES, a simple comparison of morphological changes recorded at short time intervals allows an accurate calculation of dune heights and migration rates, the main parameters required for realistic approximations of volume budgets in tidal channels. The net sediment volume of 0.061 m3 per m2 over one tidal period is ebb-orientated, which means an annual net volume export of 44.5 m3 m−2 year−1. The general ebb-oriented export trend recorded for the large, compound dune system is contrasted by a net flood-oriented transport in the ‘active zone’ involving smaller superimposed dunes, which amounts to 0.6 m2 per flow phase on top of the large dunes, and 0.15 m2 per flow phase on the slopes and in the troughs. This gives a mean volumetric sediment transport rate of 0.7697×105 m2 s−1 for the ebb flow, and 1.059×105 m2 s−1 for the flood flow, strongly deviating from earlier calculated transport rates.