摘要:

AbstractThe threshold of coarse sediment transport has been examined in natural streambeds in an upland Pennine (U.K.) area. Threshold values of the total boundary shear stress (T0) (for a given grain size), in a narrow natural stream (W/D11). Efficiency in the entrainment process is related not only to the overall channel geometry, but also varies as a function of discharge in channels characterized by compound roughness.Empirical curves relatingT0and a mean grain size (d5) are presented, but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation. Considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source. The reasons for this divergence are discussed.The influence of grain shape was found not to be important in the initiation of motion criterion. This conclusion may reflect the limited range of natural grain shapes in the study streams, but might reasonably apply to other field investigations of similar streams.Modifications of the Shields' and Yalin diagrams are suggested for practical applications in shallow streams with poorly‐graded bed material. The Shields' parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth (d5/D). The increased importance of augmented drag forces, in the entrainment process in shallow flows, is suggested as the physical explanation for the reduced values of the Shields' parameter. However, the relationships presented should not be applied to laboratory experiments concerned with well‐graded sediments (therefore beds with little deviation in level), in which the Shields' parameter may be regarded as constant at high Reynolds' grain numbers.Consistent estimated field values of ϵ, a threshold sediment transport parameter, might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the litera

ISSN:0197-9337    

DOI:10.1002/esp.3290080102

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractExposed roots were used to estimate soil and bedrock erosion on the cut slopes of a 45‐year‐old road constructed in granitic soils of the Idaho Batholith. The original roadcut surface was defined by projecting a straight line from the toe of the cut past the end of the exposed root to the intersection of a straight line projected along the surface of the hillslope. A cross‐sectioning technique was then used to determine erosion to the present roadcut surface. A total of 41 exposed root sites were used to estimate erosion on a 1350 m‐long section of road. Average erosion was 1·0 and 1·1 cm/year for soil and bedrock respectively. Buttressing by tree roots caused lower erosion rates for soil as compared to bedrock. Both soil and bedrock erosion rates showed statistically significant correlations with the gradients of the original cut slope. The bedrock erosion data provide a reasonable estimate of the disintegration rate of exposed granitic bedrock exhibiting the weathering and fracturing properties common to this area. The road is located in a study watershed where long‐term sediment yield data are available. Sediment data from adjacent study watersheds with no roads were compared to sediment data from the roaded watershed to estimate the long‐term increase in sediment yield caused by the road. The increase amounts to about 2·4 m3/year. This figure, compared to the average annual on‐site road erosion, provides an erosion to sediment delivery ratio of less than 10 per cent. Based on study results, road construction and maintenance practices are suggested for helping reduce

ISSN:0197-9337    

DOI:10.1002/esp.3290080103

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractTwo landsliding episodes between late 1973 and early 1975 delivered about 60000 m3of sediment to six small deeply incised streams draining a 2·7 km2area. About 4700 m3of logs in the landslide debris formed major log jams in five streams, which impounded large volumes of landslide‐derived sediment. Five years after the landsliding, 42 per cent (25000 m3) of sediment was still in storage behind 35 log jams ranging from 1·4–8·2 m high. The landsliding episodes have produced multi‐stepped stream profiles, aggradation of channel reaches up to 150 m long to mean depths between 1·2 and 4·1 m, reductions in gradient, fining of bed material size, and related changes in bedforms and channel width:depth ratios that seem likely to persist for at least several decades. Sediment presently stored behind log jams is equivalent to between 50 and 220 years normal supply of sediment from hillslopes to stream channels. Long‐delayed, large magnitude impacts on higher‐order channels may occur if sudden failure of log jams is induced by a large storm at so

ISSN:0197-9337    

DOI:10.1002/esp.3290080104

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractThe quantitative bedload transport data that are presently available confirm that the generalized bedload transport rate‐stream power relationship is applicable to natural streams. However, the bedload transport rate is not solely dependent upon hydraulic parameters, but also upon the inter‐relationship between bed material characteristics and flow properties. Segregation of the surficial bed material, as expressed through the development of an armour coat, limits the availability of transportable material. Under such circumstances observed bedload transport rates are less than the predicted values.The effect which the development of an armoured surface has upon the bedload transport rate is described with reference to bedload and bed material sampling in the Borgne d'Arolla, Valais, Switzerland. The data refer to two periods when the resumption of baseflow conditions following flood events which were of a sufficient magnitude to transport all but the coarsest (0·3–0·5 m) particles on the streambed, provided the opportunity for the bed to adjust to a comparatively stable flow regime. Observed and predicted bedload transport rate‐stream power relationships are compared. The theoretical relationship does not adequately describe conditions in some gravel‐bed channels, since it fails to take into account the effect which armouring may have upon the supply of transportab

ISSN:0197-9337    

DOI:10.1002/esp.3290080105

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractThe analysis of 3 subsamples from each of 12 sediment cores from a micro‐tidal flat shows that spreading ofSpartinaeffects sediment parameters. The effect diminishes with distance to vegetatio

ISSN:0197-9337    

DOI:10.1002/esp.3290080106

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractBedded Permian salt in the Palo Duro Basin of the Texas Panhandle is being considered for isolation of nuclear waste. Studies underway to evaluate the geomorphic processes affecting any waste repository that may be sited in the region include studies of the geomorphic events associated with individual storms. On 26 May, 1978, thunderstorms occurred near Canyon, Texas, which received 130mm of rain, and on a small instrumented watershed at Buffalo Lake, 16 km to the southwest, which received 71 mm of rain. Rainfall at Buffalo Lake was concentrated in a period of 3 hours, representing a return period of approximately 10 years and a maximum 30‐minute intensity of 64 mm/hour.Erosion‐pin fields, topographic surveys, and stakes for headcut monitoring had been established 2 to 3 months before the storm in a 640‐m long tributary canyon to Tierra Blanca Creek downstream of Buffalo Lake. Canyon slopes of moderately to slightly calichified sands and gravels of the Ogallala Formation are capped by well‐indurated caliche on the canyon rim. Erosion‐pin fields showed average net erosion of 2·4 cm (0° to 9° slope) to 2·7 cm (10° to 19° slope) and a single‐pin maximum value of 6·2 cm in the 20° to 29° slope class. Headcuts 1 to 2 m deep in alluvial‐colluvial material on the floor of the canyon migrated as much as 12 m upstream. Canyon floor deposition occurred as a series of elongate bars, 16m long and longer, located at the mouth of tributary gullies, downstream from scours, and at slope breaks. Grain sizes ranged from boulders up to 70cm in intermediate axis deposited in the upper canyon to fine to very fine sand deposited as a sheet up to 25cm thick beyond the mouth of the canyon at Tierra Blanca Creek.In an adjacent canyon a sequence of irregular beds of caliche gravel, mixed sand and gravel, and fine sand up to 3 m thick is exposed. Comparison of this sequence with deposits resulting from the 26 May, 1978, storm suggests that the coarse fraction of the alluvial sequence is deposited by repetitive major storm events. Cobble and boulder units 30 to 70cm thick can evidently be deposited in a few hours. Under the continental climate of the Texas Panhandle, erosion, deposition, and stream incision are taking place primarily in discrete steps related to episod

ISSN:0197-9337    

DOI:10.1002/esp.3290080107

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractMean growing season soil PCO2data were obtained for 19 regions of the world in nine countries. Bivariate and multiple linear regression analysis with soil log(PCO2) as the dependent variable and TEMP, PRECIP, log(AET), and log(PET) as the four climatic independent variables demonstrated that AET was the best independent predictor of soil PCO2. An improved soil PCO2‐AET model was developed by assuming (1) that as AET approaches zero, soil PCO2approaches the atmospheric value and (2) that there is an upper limit to soil PCO2at very high AET. This model has the form log(PCO2) = −3·47 + 2·09 (1 −e−0·0172 AET) where AET is in mm. It explains 67 per cent of the initial variation in the soil PCO2data, predicts a soil log(PCO2) of − 3·47 at AET = 0, and an upper limit of 3·5 per cent (log(PCO2) = − 1·45) for mean growing season soil PCO2at AET values of 2000 mm and above. The results of this study suggest that soil PCO2levels in tropical areas are, on average, higher than those in temperate, alpine, a

ISSN:0197-9337    

DOI:10.1002/esp.3290080108

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

摘要:

AbstractAttention is drawn to the existence of different estimators of autocorrelation in one‐dimensional series. It is shown that variation between estimates for series which appear only mildly nonstationary may be of the same order as sampling variation expected under stationarity. Hence it is important that investigators choose estimators carefully and state their choices explicitl

ISSN:0197-9337    

DOI:10.1002/esp.3290080109

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290080110

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290080112

出版商:John Wiley&Sons, Ltd    

年代:1983

数据来源: WILEY