摘要:

AbstractCross‐sections of 16 straight sandbed streams in Minnesota, Iowa, and Nebraska were surveyed. Two stratigraphic horizons were found in the banks at each site, an upper cohesive unit usually composed of silt and clay and a lower unit composed of sand.Bank erosion on these rivers occurs when the upper cohesive unit is undercut by scour at bends. The overhanging cohesive block fails by beam or cantilever failure. As upper bank failure is a direct result of undercutting, the stability and rate of retreat of the bank are largely determined by erosion of the sandy part of the bank. The cohesive layer has little influence on bank retreat and width adjustment on the rivers studied here.A quantitative lateral sediment transport model developed by Parker (1978a) is used to calculate the steady‐state geometry of the sandy part of the bank. Results are obtained for the shape, length, and height of the sandy part of the bank. The model predicts the length of the bank fairly well, and the theoretical equation for the height of the bank is of the correct form. The model, however, overestimates the slope of the bank.The height of the sandy part of the bank (Db) is approximately equal to the depth of the mean annual flow. SinceDbis determined by the lateral sediment transport model, the width (W) may be obtained from the equation of continuity (Q = WDbV), published flow (Q) data, and a resistance equation for the mean velocity,V. The calculated widths are similar to those measured in the fi

ISSN:0197-9337    

DOI:10.1002/esp.3290090302

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractPockmarks are prominent features on the seafloor at two main regions of the North Sea, the South Fladen area and the Norwegian Trench.The pockmarks vary in size from 5 m across to about 150 m and in depth from 1 m to about 20 m. Some of the features consist of several very small ‘unit‐pockmarks’. These are probably gas‐induced erosion features and found only in soft, finegrained, marine and glaciomarine sediments. They are probably caused by shallow‐ or deep‐seated gas (or fluid) release through the seabed, whereby the finest particles are thrown into suspension and redistributed by nearbottom currents.On the North Sea Plateau a small pockmarked area has recently been found. A close relationship exists here between mobile gas in the upper sediments, faulting in the soft sediments and the morphology of the seabed.Other morphological features in the North Sea such as ‘coast parallel depressions’ and ‘elongated depressions’ or terraces are also interpreted here as gas‐induced erosion products. This applies, yet again, to the ‘mottled seabed’ features, which appear as patches of high reflectivity on side scan sonar records from the North Sea Plateau; they correspond to shallow seabed depressions on the deep towed boomer records. Judging from reported observations from several shallow seas world wide volatile transport through the seabed is probably quite common and several morphological features are undoubtably induced by this process. Several erosion features of similar origin are probably common, a

ISSN:0197-9337    

DOI:10.1002/esp.3290090303

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractThe classical explanation of the development of the Welsh landscape by the effects of a pulsed ‘eustatic’ uplift during the Cenozoic times is challenged by new data on the geology and geomorphology of Wales and the adjacent regions.The post‐Caledonian surface developed over most of Wales in the upper Westphalian plays a major role in landform development. Since its emergence before mid‐Cretaceous times, the Welsh Massif has suffered a long subaerial evolution which favoured a powerful chemical weathering. In Anglesey and St. David's Land, residual hills were formed by a process of differential weathering, and downwearing in saprolites. The Millstone Grit quartzites have been deeply weathered and dolines or flat‐bottomed basins have been developed.From Cretaceous to Miocene times, a morphotectonic equilibrium has existed between uplift and denudation, so that, in the major part of Wales, the original planation surface was constantly maintained by lowering on itself and in some places it is possible to prove that no more than a few dozen metres of rock thickness disappeared during that period. Several main escarpments were produced by the warping of the planation surface along sharp flexures, during Neogene times. The Cenozoic vertical movements were controlled by the Caledonian or pre‐Caledonian tectonic pattern which has divided the continental crust into major structural units.Welsh tectonic development is related to the opening of the North Atlantic, associated stresses reactivating ancient lines of weakness in the heterogeneous lower crust or u

ISSN:0197-9337    

DOI:10.1002/esp.3290090304

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractThe ‘Tertiary Basin of Piedmont’ is a hilly region, mainly composed of marine sediments, such as marls, silts and sands. The slopes, largely devoted to grape production, are usually kept bare of vegetation and are thus prone to soil erosion processes. For two years we have measured soil loss in relation to rainfall on experimental plots located in vineyards. In all the plots considered erosion started with low rainfall intensities (0·07 mm/min), and above 0·4 mm/min the amount of soil loss dramatically increased (to over 1800 g/m in one event). Most of the erosion occurs during summer rainstorms, but the behaviour of the soil under erosive rainfalls is very different from one site to another, depending only in part upon the various rainfall rates, soil and geometrical characteristics of the plots. It can be seen that soil loss varies from nearly negligible values (20 g/m/yr) to unacceptably high levels (to over 4·7 kg/m/yr), according to the different types of land cultivation. Deep ploughing and heavy herbicide treatment give rise to accelerated erosion processes leading to soil losses much higher than in other vineyard plots in

ISSN:0197-9337    

DOI:10.1002/esp.3290090305

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractOn 28–29 September 1980 an extreme storm of 150 mm of rain inc. 5 hours caused a flash flood near Tabernas in a semi‐arid area in southern Spain. With the rainfall total approximating the mean annual rainfall the recurrence interval is estimated to be 25–100 years. Flood peak discharges have been estimated from a mountain source area across an alluvial fan and the geomorphological response to the storm described. Channel adjustment differed between channels in the fanhead trench and those unconfined channels on the fan surface. The implications of the spatial pattern of erosional an depositional feaures for landform adjustment in semi‐arid regions are di

ISSN:0197-9337    

DOI:10.1002/esp.3290090306

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractIn recent decades, German geomorphology has been mainly concerned with climatic and climato‐genetic geomorphology. The first is the study of processes, especially of process combinations in different climato‐morphological zones. The second is concerned with the way exogenic forces control the evolution of relief in a certain region. This study of relief generations differs fundamentally from denudation chronology. Certain principles developed as knowledge of these fields has grown, such as the variability of rock resistance with climate and discontinuity of processes in both space and time, are considered. In recent years new trends, based mainly on climatic geomorphology, have been towards greater specialization in fields such as quantitative geomorphology, geomorphological mapping, and laboratory analysis of regolith and soil samp

ISSN:0197-9337    

DOI:10.1002/esp.3290090307

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractElectron microscope analysis of sixty samples taken from six colluvium sites in Swaziland has shown that the quartz grains exhibit marked edge abrasion in the uppermost beds of exposures. This edge abrasion is lacking in the lower beds in all sections examined and indicates slope process change during colluvium deposition. A model of slope evolution is provided, in which slope erosion progressively brings about more channelling and causes greater surface roughness because of the exhumation of more core stones and differentially weathered rock. This roughness causes increased edge abrasion.

ISSN:0197-9337    

DOI:10.1002/esp.3290090308

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290090309

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290090310

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290090301

出版商:John Wiley&Sons, Ltd    

年代:1984

数据来源: WILEY