摘要:

AbstractThis paper presents results of investigations (1983–1992) into rates of change, morphology and processes occurring during the current erosional phase in a Morecambe Bay cyclic saltmarsh, in which it has narrowed fromc.1000 m (1975) toc.150 m (1992). Monthly monitoring of marsh edge erosion and creek knickpoint retreat has revealed temporal and spatial variations. Highest frequency changes of low magnitude coincided with non‐storm conditions and overmarsh tides above 5·80 m OD, which submerged the whole marsh. Less frequent changes of greater magnitude were associated with both overmarsh tides and strong onshore winds over 15 ms−1, which generated high energy waves. The lowest frequency change of greatest magnitude occurred during an extreme onshore storm event and surge.Morphologically, during the erosional phase, a low angled landward slope was generated as erosion of thec.0·5 m high active seaward cliff coincided with vertical accretion of 0·07 ma−1of relatively coarse sediment on the marsh surface immediately landward. Tidal hydrodynamics strongly influence the saltmarsh, which is confined to the upper 2·5 m of the macrotidal range (maximumc.10·5 m). During overmarsh spring tides (maximum creek flood flow rate 0·13 ms−1, up to bankfull level), flooding begins over lower landward creek banks before submerging the higher marsh edge. During ebb tides, water trapped by this higher edge can escape seaward only via the creeks (maximum ebb velocities 2·07 ms−1below bankfull level). Wave erosion also is limited to spring tides.Monthly mapping of the Kent Estuary channel pattern seaward of the saltmarsh showed that medium term higher erosion rates were related to the presence of a large channel, which lowered the adjacent creek base level and allowed larger waves to attack the marsh edge than when a sandbank flanked the marsh. Major River Kent channel shifts appear to initiate accretional or erosional phases of cyclic sal

ISSN:0197-9337    

DOI:10.1002/esp.3290200502

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe 1300 km long Narmada River flows along a structural lineament, alternating between constricting rocky gorges and rapids, and meandering wide alluvial reaches. Channel forms and processes were studied in a 120 km long section of an alluvial reach. Channel size, shape and bedforms in the Narmada River are related to very large floods which have occurred three times in this century. During such floods the entire 400 m wide channel is utilized and 10–15 m high cliffs on both sides operate as riverbanks. Normally, even the high flows of the south‐western monsoon are insufficient to fill the whole channel, and hence their effects are limited to building of discontinuous floodplains between the cliffs and modifying bedforms and bars. A channel‐in‐channel topography is thus created. The very large floods are also responsible for erosion of the rocky stretches and building of point bars. The river meanders, but its movement is restricted because of (1) rocky gorges and scablands operating as anchor points at intervals, and (2) the presence of high alluvial cliffs which are topped on extremely rare occasions. In spite of being located in a tectonically active zone in a monsoon setting, it is the exceptional high‐magnitude floods at irregular intervals which control the form and behaviour of the Narm

ISSN:0197-9337    

DOI:10.1002/esp.3290200503

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractWind tunnel simulations of the effect of non‐erodible roughness elements on sediment transport show that the flux ratioq/qs, shear velocityU*, and roughness density λ are co‐dependent variables. Initially, the sediment flux is enhanced by kinetic energy retention in relatively elastic collisions that occur at the roughness element surfaces, but at the same time, the rising surface coverage of the immobile elements reduces the probability of grain ejection. A zone of strong shearing stress develops within 0·03 to 0·04 m of the rough bed because of a relative straightening of velocity profiles which are normally convex with saltation drag. This positive influence on fluid entrainment is opposed by declining shear stress partitioned to the sand bed. Similarly, because the free stream velocityUfis fixed whileU*increases, velocity at heightzand particle momentum gain from the airstream decline, leading eventually to lower numbers of particles ejected on average at each impact. When the ratio of the element basal area to frontal area σ is approximately equal to 3·5, secondary flow effects appear to become significant, so that the dimensionless aerodynamic roughness parameterZ0/hand shear stress on the exposed sand bedTsdecrease. It is at this point that grain supply to the airstream and saltation drag appear to be significantly reduced, thereby intensifying the reduction inU*. The zone of strong fluid shear near the bed di

ISSN:0197-9337    

DOI:10.1002/esp.3290200504

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA laboratory simulation of salt weathering was used to ascertain the effects of sodium sulphate and sodium carbonate under ‘Negev’ conditions using a single immersion technique. Three main points were addressed: what are the grain size and textural characteristics of the debris liberated from limestones and a sandstone, what do scanning electron microscope observations of the weathered samples tell us about the decay processes involved, and how does the rate of debris liberation change during the course of 100 cycles? The grain size characteristics of the liberated debris tended to be multimodal and were related to the original petrological characteristics of the rock. Large amounts of fines were produced which are believed to be analogous to the ‘rock flour’ of arid areas. Blistering was also observed. Scanning electron microscope analysis revealed differences in the style of attack for different rock types and salt treatments, and revealed the pattern of salt crystallization in pores and the nature of cracking. The rate of debris liberation tended to decline or remain constant through time. The reasons for this remain obscure, but it is evident that diurnal cycles of temperature and humidity change can cause continuing rock weathering long after the initial input of salt to the rock has take

ISSN:0197-9337    

DOI:10.1002/esp.3290200505

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA traversing microerosion meter (MEM) was used to measure the rates of surface weathering of limestones in southeastern Australia. There were two groups of MEM sites installed in 1978/9. The aim of the experimental design for the first type, the 13 sites at Cooleman Plain and Yarrangobilly Caves, was to obtain erosion rates for limestones of similar lithology exposed under comparable climate conditions. The sites were positioned to measure erosion over a range of microsolutional forms and with exposure to differing forms of erosion, i.e. subaerial, subsoil and instream. The second set, at Ginninderra close to Canberra, consists of nine limestone slabs of differing lithology, collected from different locations but exposed under identical climatic conditions. The number of individual measurement points at each MEM site varied from 24 to 68.There were major differences in erosion rates between subaerial bedrock and instream sites at Yarrangobilly and Cooleman Plain, but no evidence of differential erosion across the micro‐forms. There were differences in the weathering rate for bedrock sites, due to climatic differences, and between the limestone lithologies exposed at Ginninderra. The average rate of erosion for the subaerial bedrock sites at Cooleman Plain and Yarrangobilly over the 13 years was 0·013 mm a−1and at Ginninderra 0·006 mm a−1. At some of the sites microflora (lichens and mosses) caused problems for field measurement. The weathering processes that contribute to the surface lowering are discussed in the accompanying paper by Mos

ISSN:0197-9337    

DOI:10.1002/esp.3290200506

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractRecent work by McCarroll (1993, 1994) suggests that a new approach to lichenometry can be used to date diachronous surfaces. Size–frequency distributions of largest lichens obtained from the diachronous surfaces of avalanche boulder ramparts comprise two elements: an equilibrium population growing on boulders deposited throughout the Holocene, and overlapping populations growing on groups of boulders deposited by events during the last 300 years. In this study, almost 10000 lichens were measured on two groups of closely similar ramparts. The number of very large lichens is used to predict the form of the equilibrium distribution, which is then removed, leaving a corrected distribution which reflects the age‐frequency of surface boulders deposited during the ‘Little Ice Age’. The results confirm that maximum avalanche activity occurred during the 19th century, but a minor peak at one site suggests that avalanche activity was enhanced during the mid‐18t

ISSN:0197-9337    

DOI:10.1002/esp.3290200507

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe form of raised peat mires can be considered to reflect the form of the saturated water mound. Ingram (1982) calculates this form from hydraulic considerations, but assumes a uniform hydraulic conductivity throughout the profile. Here, the form of the mire is calculated for non‐uniform, depth‐dependent hydraulic conductivity of the peat deposit. Although requiring more computational effort, the alternative model may offer a more appropriate description of the structure of the peat m

ISSN:0197-9337    

DOI:10.1002/esp.3290200508

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290200510

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290200501

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY