ABSTRACT Power, H.E.; Nielsen, P.; Hughes, M.G.; Aagaard, T., and Baldock, T.E., 2016. Wave height distributions in the surf zone on natural beaches. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research , Special Issue, No. 75, pp. 917–921. Coconut Creek (Florida), ISSN 0749-0208. Wave height distributions in the surf zone are examined using a large existing field dataset. Observed distributions are compared to the Rayleigh distribution, the Weibull distribution, and the normal distribution. In more than half the 15-minute data runs (57%), wave height distributions were statistically significantly different to a Rayleigh distribution, of which the majority were more narrowly distributed, with fewer extrema. In contrast, only 6% of distributions were statistically significantly different to a normal distribution. The distributions were observed to become narrower with decreasing depth. Data were subsequently fitted to a Weibull distribution with a varying exponent to allow for a change in the shape of the distribution with changing depth. The data show distributions with a wide range of exponents, but a mean of 2.4 indicating that, on average, data are more narrowly distributed than a Rayleigh distribution (exponent of 2). The value of the exponent was observed to increase in shallower water, with a strong inverse correlation between depth and the exponent.

ABSTRACT Hansen, J.M.; Aagaard, T., and Kuijpers, A., 2015. Sea-level forcing by synchronization of 56- and 74-year oscillations with the Moon's nodal tide on the northwest European Shelf (eastern North Sea to central Baltic Sea). The North Sea and Baltic Sea long-term records reveal a strong correlation (0.997) between sea-level changes and the sum of identified harmonic oscillations, corresponding to the lunar nodal period and four multiples of it. We developed a transparent method for iterative least residual sine regression that is capable of identifying harmonic sea-level oscillations, e.g., gravitational sea-level effects of the lunar nodal oscillation. Three relatively large harmonic sea-level oscillations with period lengths of 18.6 (18.6), 60.5 (55.8), and 76.1 (74.4) years correspond well to factors 1, 3, and 4 of the 18.6-year lunar nodal period (multiple periods in parentheses). The sum of these oscillations leaves small residuals that can be resolved into two further, statistically less significant oscillations with apparent period lengths of 28.1 (27.9) and 111.1 (111.7) years, corresponding to factors 1½ and 6 of the lunar nodal period. Periods and amplitudes expose strong entrainment, i.e . phase synchronization at rational ratios of the identified oscillations' periods as well as amplitude locking at reciprocal rational ratios of 1/2, 1/3, and 2/3 of the three largest oscillations. On top of the region's general sea-level rise (1.18 mm/y), strong quasi-oscillations occur when the two largest oscillations are in phase. Thus, a large quasi-oscillation commenced in 1971 adding a 40-year sea-level rise of 1.0–1.2 mm/y to the region's general sea-level rise. If our theory is correct, the ongoing quasi-oscillation should culminate in 2011, and the suggestion may be tested after completion of the ongoing 18.6-year nodal oscillation, i.e . in 2020–21. A purely mathematical extension of the oscillation parameters identified by the applied method suggests that the sum of harmonic oscillations produces 223-year pulses of quasi-oscillations, which can be divided into 158-year periods ( e.g., 1747–1905 and after 1970) with large oscillations (60–65 mm), followed by 65-year periods ( e.g., 1905–70) with much smaller oscillations (2–16 mm).

ABSTRACT Aagaard, T. and Sørensen, P., 2013. Sea level rise and the sediment budget of an eroding barrier on the Danish North Sea coast. The barrier spit Skallingen on the Danish North Sea coast accreted significantly for a period of three centuries prior to 1970 but it is now strongly eroding and the average rate of shoreline recession is about 4.2 m/yr. The causes for the erosion are examined through a sediment budget approach and it is found that although mean sea level is presently rising at a rate of about 3.3 mm/yr, the impact of longshore sediment transport divergence is far more important to barrier erosion. Sediment losses from longshore transport gradients are partly offset by minor gains through onshore transport of sand across the shoreface as well as by beach nourishment. The results illustrate that the Bruun model (Bruun, 1962) is insufficient to predict coastal erosion and even though sea level rise may cause erosion, far larger erosion rates can be induced by indirect human impact. Moreover, since the barrier was formerly in an accretive state, the absolute magnitude of (some of) the sediment budget terms have clearly changed significantly during recent decades, and using a numerical model it is found that longshore sediment losses have increased by a factor 3–4 over the past century. This increased rate/gradient of longshore sediment transport has been due mainly to inlet channel dredging which degraded the ebb tidal delta that earlier sheltered the downdrift end of the spit.

Although the dynamics and kinematics of various types of mean cross-shore current flows in the surf zone (undertow and rip currents) are fairly well understood, the causes for transitions occurring between these two types of mean circulation patterns remain obscure. On longshore barred beaches, such transitions involve the formation, degeneration, or both of rip channels. In this paper, field evidence is presented to suggest that transitions between undertow and rip current (cell) circulations could depend on the magnitude of the wave-induced onshore mass transport across a longshore bar, rip channel spacing, and trough cross-sectional area. The results are based on data obtained from four field experiments on the Danish and Dutch North Sea coasts, which encompassed a range of incident wave energy conditions. Two of the data sets demonstrate transitions between cell and undertow circulations. Calculated onshore-directed mass transports in the two circulation types were plotted against A t / y r , where A t is trough cross-sectional area and y r is longshore distance from the measurement position to a rip channel. The two types of circulation are separated in parameter space by a straight line with a slope of 1 (m/s). The observations support a model proposed previously, in which optimum rip spacings exist that depend on the balance between onshore discharge and longshore pressure gradients caused by irregular bar bathymetry. This simple morphodynamic model indicates that both hydrodynamic conditions and existing bathymetry are critical in determining the type of mean current circulation.

Swash zone sediment transport is generally measured in an Eulerian reference frame, but this approach tends to give insufficient information on spatial transport characteristics. This paper utilizes a novel technique to map measured suspended sediment concentrations (SSC) onto the x–t plane, thus providing a visualization of SSC at all locations throughout a swash cycle. The widely adopted energetics or bed shear stress approaches to modeling suspended sediment transport in the swash predict that SSC varies directly with the horizontal velocity, u, to the power of either 2 or 3. A calibrated ballistic swash model was used to predict SSC( x,t ) ∝ u 2 and SSC( x,t ) ∝ u 3 . The qualitative comparison between field measurements and transport model predictions in x–t space provides the opportunity to examine the transport model performance throughout the swash zone. The agreement between observed and modeled SSC patterns was generally poor. Similar to previous observations, SSC was larger during the uprush than the backwash. During the uprush, however, the models predict a maximum in SSC at the moving shoreline and a rapid drop-off behind the leading swash edge, whereas observed SSC was maintained at high levels for most of the uprush phase. During backwash, there was very little correspondence at all between the patterns of predicted and observed SSC; in particular, observed SSC was unexpectedly small in the mid- and late stages of the backwash. A general consensus is emerging that existing swash zone sediment transport models that assume SSC is in equilibrium with the horizontal flow velocity are inadequate. This study suggests that this is probably due to presuspended sediment being advected into the swash zone, the effects of bore-generated turbulence being advected into the swash zone, and both bed shear stress and Reynolds stresses being out of phase with the horizontal flow velocity.

Detailed hydrodynamic and morphological data are presented from a field deployment spanning 2 days (four tide cycles). The data include bed-elevation changes measured at each low tide and continuous records of water-surface elevation, cross-shore and long-shore current velocities, and suspended sediment concentrations all measured within 20 cm of the bed. During the deployment, an intertidal bar migrated onshore and infilled a runnel on its landward side. The depth of this runnel was initially 0.6 m. During the migration of the bar, the significant wave height in deep water was ca. 2 m and wave period was 7 seconds. The significant wave height over the intertidal bar crest was about 0.25 m. Suspended sediment fluxes were estimated (product of current velocity and suspended sediment concentration profile) and partitioned between mean and oscillatory components with the latter further partitioned between short and long wave contributions. When the bar was migrating shoreward and infilling the runnel, estimated suspended sediment flux for all components was directed landward on the bar crest. Once the migrating bar had infilled the runnel, however, the suspended sediment fluxes for the mean component were directed seaward, whereas the short wave-driven flux was still directed landward. These results represent a clear example of morphodynamic interactions—(a) as waves cross the intertidal bar the onshore mean and oscillatory components transport sediment shoreward, (b) the presence of the runnel reduces the offshore component of oscillatory transport by channeling the flow alongshore, (c) the runnel rapidly infills due to the strong transport asymmetry, (d) once the runnel has infilled, the mean cross-shore current and mean sediment flux reverse direction. When the runnel is present, the general intertidal circulation is a horizontal cell circulation with rip currents, whereas it becomes a vertical undertow circulation when the runnel has infilled.

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.

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.