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1.
Point scale studies in different settings of glacial geology show a large local variation of redox conditions. There is a need to develop an upscaling methodology for catchment scale models. This paper describes a study of field-scale heterogeneity of redox-interfaces in a till aquitard within an area of 600?×?600 m. The results showed significant variation of the depths to the redox-interface and thickness of the aquitard. Nitrate was present above the redox-interface but reduced to non-detectable levels a few metres below the interface. An upscaling approach for an area of 92 km2 is proposed. Two models are proposed to predict the depth to the redox-interface in the aquitard and the resulting nitrate recharge concentrations to an underlying aquifer. The first model assumes that the depth to the redox-interface reflects the hydraulic head in the aquitard, and the second model assumes that the depth of the redox-interface is randomly distributed according to a log-normal probability distribution function. The upscaling approach using the random redox model estimated recharge concentrations comparable to the observed concentration in the underlying aquifer. The presented upscaling approach is applicable in distributed catchment models where sub-grid variability cannot be represented by the large grids. 相似文献
2.
Hydraulic roughness over simple subaqueous dunes 总被引:2,自引:1,他引:1
Jesper Bartholdy Burg W. Flemming Verner B. Ernstsen Christian Winter Alex Bartholom ä 《Geo-Marine Letters》2010,30(1):63-76
Detailed studies of flow over subaqueous dunes in laboratory flumes were used to suggest a virtual near-bed layer of twice
the dune height in which the mean velocity is accelerated towards the crest by contraction. The mean flow velocity in this
layer above the crest, transformed into friction velocity by means of the surface skin roughness, is shown to give values
consistent with measured values. The resulting dimensionless shear stress due to skin friction is depth-independent, in contrast
to that derived by means of often cited traditional methods. As a result of the relationship between dune height and the thickness
of the near-bed layer, an expression for the expansion loss behind dunes was formulated and used to relate form resistance
directly to dune height. 相似文献
3.
N. Stark R. Wilkens V. B. Ernstsen M. Lambers-Huesmann S. Stegmann A. Kopf 《Geotechnical and Geological Engineering》2012,30(1):1-14
The societal usage of coastal zones (including offshore wind energy plants, waterway deepening, beach conservation and restoration)
is of emerging importance. Sediment dynamics in these areas result in sandy deposits due to strong tidal and wave action,
which is difficult to simulate in laboratory geotechnical tests. Here, we present data from in situ penetrometer tests using
the lightweight, free-fall Nimrod penetrometer and complementary laboratory experiments to characterize the key physical properties of sandy seafloors in areas
dominated by quartzose (North Sea, Germany) and calcareous (Hawaii, USA) mineralogy. The carbonate sands have higher friction
angles (carbonate: 31–37°; quartz: 31–32°) and higher void ratios (carbonate: 1.10–1.40; quartz: 0.81–0.93) than their siliceous
counterparts, which have partly been attributed to the higher angularity of the coral-derived particles. During the in situ
tests, we consistently found higher sediment strength (expressed in deceleration as well as in estimated quasi-static bearing
capacity) in the carbonate sand (carbonate: 68–210 g; quartz: 25–85 g), which also showed a greater compressibility. Values
were additionally affected by seafloor inclination (e.g., along a sub-aqueous dune or a channel), or layering in areas of
sediment mobilization (by tides, shorebreak or currents). The study shows that the differences in in situ measured penetration
profiles between carbonate sands and quartz sands are supported by the laboratory results and provide crucial information
on mobile layers overlying sands of various physical properties. 相似文献
4.
The effect exerted by the seabed morphology on the flow is commonly expressed by the hydraulic roughness, a fundamental parameter
in the understanding and simulation of hydro- and sediment dynamics in coastal areas. This study quantifies the hydraulic
roughness of large compound bedforms throughout a tidal cycle and investigates its relationship to averaged bedform dimensions.
Consecutive measurements with an acoustic Doppler current profiler and a multibeam echosounder were carried out in the Jade
tidal channel (North Sea, Germany) along large compound bedforms comprising ebb-oriented primary bedforms with superimposed
smaller secondary bedforms. Spatially averaged velocity profiles produced log-linear relationships which were used to calculate
roughness lengths. During the flood phase, the velocity profiles were best described by a single log-linear fit related to
the roughness created by the secondary bedforms. During the ebb phase, the velocity profiles were segmented, showing the existence
of at least two boundary layers: a lower one scaling with the superimposed secondary bedforms and an upper one scaling with
the ebb-oriented primary bedforms. The drag induced by the primary bedform during the ebb phase is suggested to be related
to flow expansion, separation, and recirculation on the downstream side of the bedform. Three existing formulas were tested
to predict roughness lengths from the local bedform dimensions. All three predicted the right order of magnitude for the average
roughness length but failed to predict its variation over the tidal cycle. 相似文献
5.
A. L. Hansen B. S. B. Christensen V. Ernstsen X. He J. C. Refsgaard 《Hydrogeology Journal》2014,22(7):1639-1655
A concept was developed for distributing the depth of the redox interface at catchment scale in tills for use in nitrate modelling. The hypothesis was that the redox interface has been developing since the beginning of the Holocene due to oxidation of inherent reduced compounds in the sediment by oxygen in recharging water. Key to the concept is estimation of the spatial pattern of the redox interface, inferred from variability in groundwater recharge and the amount of reduced compounds in the sediments. The concept was tested for the 101-km2 Norsminde fjord catchment in Denmark using a catchment-scale nitrate model, where two parameters controlling the estimated redox interface were calibrated based on the observed amount of nitrate transported to the fjord. Estimated depths of the redox interface across the catchment enabled reproduction of the cumulative distribution of measured depths across the catchment, but they did not fit well with the observations at individual wells. It is therefore concluded that the concept may be useful for simulation of nitrate transport and reduction in the saturated zone at catchment scale, but it is not able to correctly predict nitrate reduction at point scale. 相似文献
6.
Christian Winter Ming Da Chiou Rolf Riethmüller Verner B. Ernstsen Dierk Hebbeln Burg W. Flemming 《Geo-Marine Letters》2006,26(3):125-132
Process-based numerical modelling of coastal morphodynamics involves model and data reduction schemes in order to cope with computational limitations. Model reduction, on the one hand, may involve the discretisation of an interactive multidimensional, diverse natural system into a reduced set of coupled process-simulation modules. Data reduction schemes, on the other hand, are used to parameterise processes. The use of schematised open-boundary conditions, which are considered as representative in terms of their cumulative morphological effect, is based on the concept of “morphological” or “representative” boundary conditions. Recent model applications show realistic tendencies in terms of depositional and erosional areas. By contrast, the reproduction of characteristic changes in morphology such as the migration of bars, banks and channels is only occasionally achieved. Using field data on observed morphological impact of a single storm event and numerical model data, it is demonstrated that the concept of representative tides may lead to simulations of morphological development lacking natural dynamics. It is proposed that rather than being based on “representative” single tides, morphodynamic models should be applied with open-boundary conditions which take variations in longer-term tidal and meteorological forcing into account. 相似文献
7.
Precision of high-resolution multibeam echo sounding coupled with high-accuracy positioning in a shallow water coastal environment 总被引:2,自引:1,他引:2
Verner B. Ernstsen Riko Noormets Dierk Hebbeln Alex Bartholomä Burg W. Flemming 《Geo-Marine Letters》2006,26(3):141-149
Over 4 years, repetitive bathymetric measurements of a shipwreck in the Grådyb tidal inlet channel in the Danish Wadden Sea were carried out using a state-of-the-art high-resolution multibeam echosounder (MBES) coupled with a real-time long range kinematic (LRK?) global positioning system. Seven measurements during a single survey in 2003 (n=7) revealed a horizontal and vertical precision of the MBES system of ±20 and ±2 cm, respectively, at a 95% confidence level. By contrast, four annual surveys from 2002 to 2005 (n=4) yielded a horizontal and vertical precision (at 95% confidence level) of only ±30 and ±8 cm, respectively. This difference in precision can be explained by three main factors: (1) the dismounting of the system between the annual surveys, (2) rougher sea conditions during the survey in 2004 and (3) the limited number of annual surveys. In general, the precision achieved here did not correspond to the full potential of the MBES system, as this could certainly have been improved by an increase in coverage density (soundings/m2), achievable by reducing the survey speed of the vessel. Nevertheless, precision was higher than that reported to date for earlier offshore test surveys using comparable equipment. 相似文献
8.
Quantification of dune dynamics during a tidal cycle in an inlet channel of the Danish Wadden Sea 总被引:2,自引:3,他引:2
Verner B. Ernstsen Riko Noormets Christian Winter Dierk Hebbeln Alex Bartholomä Burg W. Flemming J. Bartholdy 《Geo-Marine Letters》2006,26(3):151-163
High-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in the Grådyb tidal inlet channel in the Danish Wadden Sea reveal the short-term dynamics of a large, ebb-directed compound dune with superimposed small to medium dunes, all composed of medium sand. Dune dynamics were related to simultaneous measurements of flow using an acoustic Doppler current profiler. Spatially, dune crests displayed greater mobility than did dune troughs, due to higher flow velocities at the crests than in the troughs. Temporally, superimposed lower lee-side dunes migrated more during the flood than the ebb tide, due to higher near-bed trough flow velocities during the flood phase, resulting in varying exposure to flow. Net dune migration was flood-directed over the tidal cycle, despite annual net migration being ebb-directed. Hence, extrapolation of short-term migration rates is not possible in this case. The superimposed dunes reversed direction during each half tidal cycle whereas the compound dune only developed a flood cap during flood tide, i.e. the time required for complete reversal of the compound dune was much longer than that available in a half tidal cycle. Over the tidal cycle, the bed level was stable but significant erosion and accretion occurred during the tidal phases. During the ebb tide, bed material was brought into suspension with accelerating flow and settled with decelerating flow, resulting in an average erosion and accretion of the bed of ~7 cm in each case. During the flood tide, the bed of the compound dune was overall stable, although bed material was eroded from the exposed lower lee side, being partly transported to the crest in bedload and partly brought into suspension. In general, dune height fluctuated during the tidal cycle whereas dune length remained stable. The height of the compound dune responded to changes in water depth, which acts as a limiting factor to dune growth. By contrast, the height of the stoss-side dunes responded to flow velocity, i.e. the stoss-side dunes were water depth-independent. 相似文献
9.
Nina Stark Hendrik Hanff Christian Svenson Verner B. Ernstsen Alice Lefebvre Christian Winter Achim Kopf 《Geo-Marine Letters》2011,31(4):249-258
In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. 相似文献
10.
Lars Ø. Hansen Verner B. Ernstsen Lars B. Clemmensen Zyad Al-Hamdani Aart Kroon 《地球表面变化过程与地形》2021,46(4):804-821
Washover fans are located on small barriers in fetch-limited micro-tidal coastal environments in Denmark. These washover fans are formed during high-energy storm events and we present a method to quantify their volumes and to estimate sediment exchanges between washover fans and their adjacent morphologies. We use high resolution digital terrain models (DTMs) based on light detection and ranging (LiDAR) data. We have delineated landforms using known methods of scale analysis and geomorphometric classification. We quantified volumes of the delineated landforms and estimated the related sediment budgets. These computed volumes were compared using different pre-depositional surfaces. Finally, we assessed the sediment exchange and associated sources of sediments of the washover fans. We applied a scale analysis to determine suitable DTM resolution and focal statistics window size as input to a geomorphometric classification analysis. Landform areas and landforms were delineated using morphometric threshold values, and volumes and sediment budgets of the delineated landforms were computed using different assumptions to define the pre-depositional surface. Resulting washover fan volumes were validated against digital elevation model (DEM) of difference (DoD) derived volumes. Sediment budgets were derived from representative volumes of the washover fans and adjacent berms. We show that quantification of washover features derived from DTMs, using geomorphometric analysis is feasible and that the presented approach provides estimates of washover deposit volumes with an accuracy between 1% and 28% compared to control volumes. © 2021 John Wiley & Sons, Ltd. 相似文献