Morphodynamic effects of riparian vegetation growth after stream restoration

The prediction of the morphological evolution of renaturalized streams is important for the success of restoration projects. Riparian vegetation is a key component of the riverine landscape and is therefore essential for the natural rehabilitation of rivers. This complicates the design of morphological interventions, since riparian vegetation is influenced by and influences the river dynamics. Morphodynamic models, useful tools for project planning, should therefore include the interaction between vegetation, water flow and sediment processes. Most restoration projects are carried out in USA and Europe, where rivers are highly intervened and where the climate is temperate and vegetation shows a clear seasonal cycle. Taking into account seasonal variations might therefore be relevant for the prediction of the river morphological adaptation. This study investigates the morphodynamic effects of riparian vegetation on a re‐meandered lowland stream in the Netherlands, the Lunterse Beek. The work includes the analysis of field data covering 5 years and numerical modelling. The results allow assessment of the performance of a modelling tool in predicting the morphological evolution of the stream and the relevance of including the seasonal variations of vegetation in the computations. After the establishment of herbaceous plants on its banks, the Lunterse Beek did not show any further changes in channel alignment. This is here attributed to the stabilizing effects of plant roots together with the small size of the stream. It is expected that the morphological restoration of similarly small streams may result in important initial morphological adaptation followed by negligible changes after full vegetation establishment. Copyright © 2018 John Wiley & Sons, Ltd.     

3D DEM simulation of principal stress rotation in different planes of cross-anisotropic granular materials

Three-dimensional Discrete Element Method simulations have been performed to study the deformation of cross-anisotropic granular materials under principal stress rotation (PSR), for rotation planes oriented at different angles θ with respect to the bedding plane. The simulations have been conducted with a novel technique for applying specified stresses at three-dimensional boundaries. The results are qualitatively in agreement with experimental results from literature. Cumulative volume contraction is always observed under continuous PSR and increases with increasing θ. The dilatancy rate decreases with increasing number of PSR cycles, tending to zero. The noncoaxiality angle between the strain increment and the stress in the PSR plane increases with increasing number of cycles, reaching the same asymptotic value for samples of various densities and for various θ. Periodic oscillations of the dilatancy rate and noncoaxiality angle within each PSR cycle are observed with an increasing oscillation magnitude with increasing θ, due to the larger fabric anisotropy within the PSR plane. When θ = 30 or 60°, significant noncoaxial strain accumulation occurs in the plane perpendicular to the PSR plane due to the oblique angle between the PSR plane and the bedding plane, echoing the major principal fabric direction's being neither parallel nor perpendicular to the PSR plane. The macroscopic behavior of the samples is related to the microscopic parameters including coordination number and fabric anisotropy. With increasing number of cycles, the difference between normalized stress/strain/fabric increment tensors tends to become constant, with only a small lag between each pair, irrespective of θ.     

Palaeomagnetism of middle Proterozoic (c. 1.25 Ga) dykes from central North Greenland

From a nunatak in central North Greenland (81.5°N, 44.7°W) nine sites of Middle Proterozoic basic dykes, cutting Archaean basement, were palaeomagnetically investigated. After AF and thermal cleaning the nine dyke sites and three adjacently baked gneiss sites give a stable characteristic remanent mean direction of D = 265°, I = 21.5° ( N = 12, α ₉₅= 5.6°), the direction being confirmed by a detailed and positive baked contact test.
The polarity of the dykes in the nunatak area is opposite to that of the Zig-Zag Dal Basalts and the Midsommersø Dolerites in eastern North Greenland some 200–300 km away, the volcanics of which are assumed to be of similar age (about 1.25 Ga). The remanent directions of the two sets of data are antiparallel within the 95 per cent significance level of confidence.
When rotating Greenland 18° clockwise back to North America by the 'Bullard fit', the pole of the central North Greenland dolerites (NDL) falls at (14.3°N, 144.3°W). The reversed pole (14.3°S, 35.7°E) fits well on to the loop between 1.2 and 1.4 Ma on the apparent polar wander swath of Berger & York for cratonic North America.
The palaeomagnetic results from the Middle Proterozoic basic dykes from central North Greenland thus strengthen previous palaeomagnetic results from the Midsommersø Dolerites and Zig-Zag Dal Basalts from the Peary Land Region in eastern North Greenland, suggesting that Greenland was part of the North American craton at least for the period between c . 1.3 and 1 Ma (and probably up to the end of Cretaceous time). The major geographical meridian of Greenland was orientated approximately E–W, and the palaeo-latitude of Greenland was about 10°–15°.     

The recession of the Inland Ice margin during the Holocene climatic optimum in the Jakobshavn Isfjord area of West Greenland

Recent subsurface mapping of parts of the Greenland Inland Ice margin in the region of Jakobshavn Isbræ indicates that the fjord system in the period of at least 2700–4700 calendar yr B.P. was more ice free than at present, and that the front of the glacier was at least 15 km behind the present position. The ¹⁴C-dating of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for Greenland's first settlers, the Sarqaq people, who arrived in the region about 400 yr ago to find hunting grounds 10–20% larger than the present.     

Strontium-isotope studies of “brown water” (organic-rich groundwater) from Denmark

 Groundwater from some Quaternary and upper Tertiary aquifers in western Jutland, Denmark, is heavily influenced by "brown water", i.e., groundwater with a high content of naturally occurring dissolved organic matter. Stable-isotope analyses (¹⁸O/¹⁶O and D/H) indicate that both Quaternary and upper Tertiary aquifers are dominated by meteoric water. However, strontium-isotope analyses make it possible to distinguish between water samples from Quaternary and Miocene aquifers. Relatively low ⁸⁷Sr/⁸⁶Sr ratios, i.e., ∼0.7083, in Sr-rich water samples indicate that the majority of dissolved Sr in groundwater from Miocene as well as Quaternary strata is probably derived from Miocene marine skeletal carbonate matter in the subsurface. This situation is probably the result of Quaternary glacial reworking of upper Tertiary material and/or hydraulic contact between Quaternary and Miocene aquifers. A positive correlation between Sr contents and non-volatile organic carbon indicates that the remarkably high contents of organic matter recorded in these aquifers almost certainly are derived from Miocene sources as well. Received, February 1999 / Revised, July 1999 / Accepted, July 1999     

Temperature in lowland Danish streams: contemporary patterns,empirical models and future scenarios

Continuous temperature measurements at 11 stream sites in small lowland streams of North Zealand, Denmark over a year showed much higher summer temperatures and lower winter temperatures along the course of the stream with artificial lakes than in the stream without lakes. The influence of lakes was even more prominent in the comparisons of colder lake inlets and warmer outlets and led to the decline of cold‐water and oxygen‐demanding brown trout. Seasonal and daily temperature variations were, as anticipated, dampened by forest cover, groundwater input, input from sewage plants and high downstream discharges. Seasonal variations in daily water temperature could be predicted with high accuracy at all sites by a linear air‐water regression model (r²: 0·903–0·947). The predictions improved in all instances (r²: 0·927–0·964) by a non‐linear logistic regression according to which water temperatures do not fall below freezing and they increase less steeply than air temperatures at high temperatures because of enhanced heat loss from the stream by evaporation and back radiation. The predictions improved slightly (r²: 0·933–0·969) by a multiple regression model which, in addition to air temperature as the main predictor, included solar radiation at un‐shaded sites, relative humidity, precipitation and discharge. Application of the non‐linear logistic model for a warming scenario of 4–5 °C higher air temperatures in Denmark in 2070‐2100 yielded predictions of temperatures rising 1·6–3·0 °C during winter and summer and 4·4–6·0 °C during spring in un‐shaded streams with low groundwater input. Groundwater‐fed springs are expected to follow the increase of mean air temperatures for the region. Great caution should be exercised in these temperature projections because global and regional climate scenarios remain open to discussion. Copyright © 2006 John Wiley & Sons, Ltd.     

Oxygen isotope and palaeotemperature records from six Greenland ice‐core stations: Camp Century,Dye‐3, GRIP,GISP2, Renland and NorthGRIP

Oxygen isotope variations spanning the last glacial cycle and the Holocene derived from ice‐core records for six sites in Greenland (Camp Century, Dye‐3, GRIP, GISP2, Renland and NorthGRIP) show strong similarities. This suggests that the dominant influence on oxygen isotope variations reflected in the ice‐sheet records was regional climatic change. Differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios. Palaeotemperature estimates have been obtained from the records using three approaches: (i) inferences based on the measured relationship between mean annual δ¹⁸O of snow and of mean annual surface temperature over Greenland; (ii) modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile, or (iii) by using Monte Carlo simulation techniques. The third of these approaches was adopted to reconstruct Holocene temperature variations for the Dye 3 and GRIP temperature profiles, which yields remarkably compatible results. A new record of Holocene isotope variations obtained from the NorthGRIP ice‐core matches the GRIP short‐term isotope record, and also shows similar long‐term trends to the Dye‐3 and GRIP inverted temperature data. The NorthGRIP isotope record reflects: (i) a generally stronger isotopic signal than is found in the GRIP record; (ii) several short‐lived temperature fluctuations during the first 1500 yr of the Holocene; (iii) a marked cold event at ca. 8.2 ka (the ‘8.2 ka event’); (iv) optimum temperatures for the Holocene between ca. 8.6 and 4.3 ka, a signal that is 0.6‰ stronger than for the GRIP profile; (v) a clear signal for the Little Ice Age; and (vi) a clear signal of climate warming during the last century. These data suggest that the NorthGRIP stable isotope record responded in a sensitive manner to temperature fluctuations during the Holocene. Copyright © 2001 John Wiley & Sons, Ltd.     

Late glacial and Holocene marine records from the Independence Fjord and Wandel Sea regions, North Greenland

The first marine sediment cores from the unexplored Independence Fjord system and the Wandel Sea, North Greenland, have been investigated to reveal the glacial marine history of the region. Two key sites in the Independence Fjord system, and an earlier analysed site from the Wandel Sea continental slope, off the mouth of Independence Fjord, are presented. The Independence Fjord sites reveal an early Holocene record (10.0–8.9 Kya) of fine-grained reddish muds with calcareous microfossils, dominated by the benthic foraminifera Cassidulina neoteretis . We suggest that a semi-permanent fast ice cover characterized the region in the early Holocene, and that the deeper troughs in the mouth region of the Independence Fjord system were intruded by subsurface Atlantic water. A stiff diamicton, at least 1.3 m thick, with coal and sandstone clasts of mainly local origin, and a 0.5-m-thick Holocene cover, are found in one of the sites. The diamicton is assumed to represent a subglacial till predating the early Holocene sediments (>10 Kya). Shallow seismic records off the mouth of Independence Fjord reveal kilometre-sized troughs with signs of glacial erosion, till deposition and a Holocene glaciomarine deposition. These features could indicate that glacial ice debouching from the Independence Fjord system at some time during the last glacial period extended to the mid-outer Wandel Sea shelf. Data from a high-resolution sediment core previously retrieved from the adjacent Wandel Sea slope indicate that the maximum ice sheet advance in this area culminated about 25–20 Kya.