Jørgen S. Nielsen

MEAN CURRENTS AND SEDIMENT TRANSPORT IN A RIP CHANNEL

Measurements of hydrodynamics and suspended sediment transport were carried out along a cross-shore transect in a rip channel. A tidally induced threshold for rip flow initiation was found; this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth (γs) in the rip neck. With normally incident waves, rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar. Hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide. At high tide when the rip was inactive, the tendency was for weak onshore directed mean flows and/or oscillatory incident waves transporting smaller amounts of sediment landward. The net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip. Outside the surf zone, transport was mainly landward due to oscillatory incident waves.

Suspended sediment transport and nearshore bar formation on a shallow intermediate-state beach

Abstract Estimates of cross-shore suspended sediment flux were obtained during a 3-day storm event on the North Sea coast of Denmark. The suspended sediment flux estimates were correlated with volumetric beach change as registered at 28 permanently deployed survey rods crossing the intertidal beach and inner surf zones. During this storm, a nearshore bar initially migrated onshore and welded to the beach face at its updrift end; subsequently a new bar formed on the seaward slope of the former. Suspended sediment transport, recorded using optical backscatter sensors and electromagnetic current meters, corresponded well qualitatively with the morphological evolution; indications are that such measurements may account for a significant fraction of the actual cross-shore sediment transport. The onshore migration of the nearshore bar was the result of onshore directed mean currents associated with a cell circulation. Renewed bar formation took place when this circulation was (locally) replaced by an offshore-directed mean current (undertow). Suspended sediment transport due to oscillatory waves at incident and infragravity frequencies was generally subordinate to the mean transport.

Cyclic sand bar migration on a spit-platform in the Danish Wadden sea-spit-platform morphology related to variations in water level

Abstract Cyclic morphology is related to water levels on a shallow spit-platform in the Danish Wadden Sea. Water levels were used as a dynamic parameter because low water levels, water levels at moderate wind set ups and high water levels occur at different wind situations and they result in different morphology. Low water levels result in a flat sand bar with one deep and narrow ebb channel. Water levels at moderate wind set ups result in several wide and shallow ebb channels. After a storm the spit-platform is flat and low with no dissecting ebb channels. Sand bars on the spit-platform migrate up to 35 m/month. A model proposing the different morphology at the three different water level stages is presented.

Coastal Morphodynamics at Skallingen, SW Denmark: High Energy Conditions

Abstract Geografisk Tidsskrift, Danish Journal of Geography 1998: 20–30. The present paper discusses some preliminary results from a field experiment on sediment transport and morphological change on a beach subjected to storm surge activity. The data were collected at the barrier spit of Skallingen during October/November 1996. This period was characterized by a succession of storms which resulted in high water and wave energy levels, culminating in a storm surge per se. The beach and the dune ramp eroded; however, contrary to expectations, a subtidal bar migrated onshore and the intertidal swash bar appeared to display a cyclic behaviour dependent upon strong linkages between morphology and hydrodynamics. Morphological changes in the intertidal zone were primarily accomplished by mean currents with oscillatory motions playing a subordinate role. Longshore sediment transport measured during conditions with moderately strong winds blowing alongshore, was dominated by wave-induced currents with aeolian tra...

Longshore sediment transport and coastal erosion at Skallingen, Denmark

Abstract Within the past thirty years, the subaerial sand budget of the barrier spit Skallingen has changed from positive to negative and the foredunes are presently receding by 3–4 m/year. Volumetric losses on the barrier during the past 30 years have been computed and compared to theoretically estimated longshore sediment transport rates. It is highly likely that the sand lost from the barrier is being transported southward into the tidal inlet of Grådyb. Computations suggest that a century ago, net longshore transport rates were perhaps only about 15% of current rates. The reasons for the increased net transport and sediment losses appear to be long-term changes in the wind climate and dredging of Grådyb which cuts off any sediment supply from southerly sources. The latter has resulted in long term erosion of the northern part of the ebb delta and subsequent reorientation of the shoreline at the southern part of Skallingen

Swash Bar Morphodynamics in the Danish Wadden Sea: Sand Bed Oscillations and Suspended Sediment Flux during an Accretionary Phase of the Foreshore Cycle

Abstract The landward migration of an intertidal swash bar at Skallingen (Danish Wadden Sea), together with the associated hydrodynamics and suspended sediment transport, were monitored over several spring tidal cycles during an accretionary phase of the foreshore cycle. The landward migration was not a simple linear displacement of the bar form; instead, regular intra-tidal sand bed oscillations (average height ∼ 0.12 m; average spacing ∼ 24 m; average steepness ∼ 0.005) moved sand landward across the seaward slope. The estimated migration rate of the bed oscillations was ∼ 0.032 m min−1. These values compare favourably with other citations in the literature. The sediment flux associated with these bed oscillations was the suspended sediment transport dominated by a net landward transport primarily by gravity wave frequencies. Infragravity waves were important at certain times in the tidal cycle but were variable in frequency and the associated transport was highly variable spatially. The suspended sediment transport by mean currents was smaller than that induced by the waves, and in almost all cases was directed offshore. The sand bed oscillations appear to be driven primarily by the incident gravity waves, which were also the primary forcing for the landward migration of the swash bar as a whole.

Total sediment budget of a transgressive barrier-spit, Skallingen, SW Denmark: A review

Abstract Average sea-level rise in the northern part of the Danish Wadden Sea has been 1.3 mm a−1 during the last 100 years but during the last 25 years a rise of 4.2 mm a−1 was observed. Concurrent with the recent sea-level rise the Skallingen barrier spit has migrated landward by 3–5 m a−1. Long term sediment budgets for each of the morphological units involved in the migration are reviewed (e.g. onshore in the shore face +90.000 m3 a−1, longshore -641.000 m3 a−1, foredunes +65,000 m3 a−1, overwash fan including shorenormal dunes +11,000 m3 a−1, spit terminus -96,000 m3 a−1, tidal flat + 10,000 m3 a−1 and backbarrier salt marsh +33,000 m3 a−1) and used to establish the relative importance of sediment transport processes involved in barrier migration. Strong interannual variations exist in the long term budget making evaluation of barrier behaviour based on short term measurements doubtful. In a cross shore sense the barrier spit is accreting in spite of the sea level rise. This is specially pronounced at an active overwash fan. However, due to substantial sediment losses to longshore transport the barrier, gets narrower and shorter during its transgressive behaviour. This may indicate that preservation of barriers in the geological record during rapid sea level rise is promoted when sediment loss to longshore transport is insignificant.

Coastal Morphodynamics at Skallingen, SW Denmark: Low and Moderate Energy Conditions

Danish Journal of Geography 95: 1–11, 1995. This paper describes some preliminary results from a field experiment on suspended sediment transport and cross-shore profile evolution in an environment subjected to storm surges. The experiment was conducted during the fall of 1994 at the barrier beach of Skallingen, SW Denmark. Data were collected over several tidal cycles comprising low through moderately-high offshore energy conditions. During the experimental period, a swash bar developed on the foreshore and migrated onshore, accreting significantly in the process. Surf zone sediment transport was heavily influenced by a rip current crossing the second bar. Topographic changes in the nearshore consisted of a progressive lowering of vertical bar relief as a result of a balance between offshore directed sediment transport due to the rip during moderate/high energy conditions, and onshore wave-induced transport during low energy conditions when the waves were not breaking over the second bar.

Complex Sediment Transport Pattern on a Spit-Platform in the Danish Wadden Sea

Abstract Spit-platforms have not drawn much attention, despite their expected importance for understanding sediment transport mechanisms leading to siltation of tidal channels. In this article, the complex sediment transport pattern on a shallow spit-platform in the Danish Wadden Sea is described based on measurements from four current meters deployed on the spit-platform in a low- and a high-energy period. The hydrodynamic characteristics at the four stations are different in terms of impact from tidal and wave-induced currents. The sediment enters the spit-platform as sandbars oriented perpendicular to the shore migrating alongshore, because of wave activity, until the sediment reaches an ebb-dominated tidal channel. In and near this channel, the sediment is transported downdrift by the ebb-tidal currents until the sand enters a shallow dissecting ebb channel, where the sand is transported obliquely offshore and deposited on a small ebb-tidal delta. This zigzag pattern of sediment transport continues until all ebb channels (up to four) are bypassed. On the subtidal flats between the most downdrift ebb channel and the main tidal channel, the sediment is transported the remaining distance into the main tidal channel or onto the tidal flat in the lagoon landward of the spit-platform. During higher-energy conditions, the sediment transport is more affected by wave-induced currents, leading to high transport rates in the shallow parts of the spit-platform and less transport in the horizontally restricted channels.

Variations of Spacings between Beach Cusps discussed in relation to Edge Wave Theory

Rasch, Morten, Jorgen Nielsen & Niels Nielsen: Variations of Spacings between Beach Cusps discussed in relation to Edge Wave Theory. Geografisk Tidsskrift 93:49–55. Copenhagen 1993. Variations in cusp spacings of beach cusp systems have been examined, and the conclusion is that the apparent regularity of the spacings is not always real. Established edge wave theory predicts equal cusp spacings, and accordingly, the theory needs to be improved or alternative theories accepted. Field measurements of beach cusp formation under reflective morphodynamic conditions confirmed that established edge wave theory was not always capable of explaining beach cusp formation. The beach cusps grew from small irregularly spaced mounds, and the regularity of the cusp spacings seemed to increase during beach cusp development. Correlation between observed average cusp spacing and the average cusp spacing predicted on the basis of edge wave theory might suggest that edge waves were the initiating mechanism causing beach cusp d...