Urban transformation of river landscapes in a global context   总被引：8，自引：1，他引：8  

Anne Chin   《Geomorphology》2006,79(3-4):460

Over the past 50 years considerable progress has been made in understanding the impacts of urban development on river processes and forms. Such advances have occurred as urban population growth has accelerated around the world. Using a compilation of research results from more than 100 studies conducted in a range of areas (58 addressing morphological change), this paper describes how urbanization has transformed river landscapes across Earth’s surface, emphasizing the distribution of impacts in a global comparative context. Urban development induces an initial phase of sediment mobilization, characterized by increased sediment production (on the order of 2–10 times) and deposition within channels, followed by eventual decline that couples with erosion from increased runoff to enlarge channels. Data from humid and temperate environments around the world indicate that channels generally enlarge to 2–3 times and as much as 15 times the original size. Although research has emphasized temperate environments, recent studies of tropical areas indicate a tendency for channel reduction resulting from strong sediment erosion and deposition responses because of intense precipitation and highly weathered soils. Embryonic research in arid environments further suggests variable river responses to urbanization that are characterized by rapid morphological change over short distances. Regardless of location, the persistence of the sediment production phase varies from months to several years, whereas several decades are likely needed for enlarging channels to stabilize and potentially reach a new equilibrium. Urbanizing streams pose particular challenges for management given an inherent changing nature. Successful management requires a clear understanding of the temporal and spatial variations in adjustment processes.  相似文献   

LIVE-BED SCOUR IN LONG CONTRACTIONS   总被引：1，自引：0，他引：1  

Subhasish DEY Rajkumar V. RAIKAR 《国际泥沙研究》2006,21(2):166-170

1 INTRODUCTION Scour within a channel contraction occurs where the waterway is restricted by the foundations or road approach embankments of a bridge. Alternately, contraction scour takes place when a bridge is located at a natural contraction in the rive…  相似文献   

Ground-water surface-water interactions and long-term change in riverine riparian vegetation in the southwestern United States   总被引：1，自引：0，他引：1  

Robert H. Webb  Stanley A. Leake 《Journal of Hydrology》2006,320(3-4):302-323

Riverine riparian vegetation has changed throughout the southwestern United States, prompting concern about losses of habitat and biodiversity. Woody riparian vegetation grows in a variety of geomorphic settings ranging from bedrock-lined channels to perennial streams crossing deep alluvium and is dependent on interaction between ground-water and surface-water resources. Historically, few reaches in Arizona, southern Utah, or eastern California below 1530 m elevation had closed gallery forests of cottonwood and willow; instead, many alluvial reaches that now support riparian gallery forests once had marshy grasslands and most bedrock canyons were essentially barren. Repeat photography using more than 3000 historical images of rivers indicates that riparian vegetation has increased over much of the region. These increases appear to be related to several factors, notably the reduction in beaver populations by trappers in the 19th century, downcutting of arroyos that drained alluvial aquifers between 1880 and 1910, the frequent recurrence of winter floods during discrete periods of the 20th century, an increased growing season, and stable ground-water levels. Reductions in riparian vegetation result from agricultural clearing, excessive ground-water use, complete flow diversion, and impoundment of reservoirs. Elimination of riparian vegetation occurs either where high ground-water use lowers the water table below the rooting depth of riparian species, where base flow is completely diverted, or both. We illustrate regional changes using case histories of the San Pedro and Santa Cruz Rivers, which are adjacent watersheds in southern Arizona with long histories of water development and different trajectories of change in riparian vegetation.  相似文献   

Effect of macrophyte spatial variability on channel resistance     

Julian C. Green 《Advances in water resources》2006

Aquatic macrophytes can severely retard flow rates in the river channels that they occupy. Consequently, there is a need to improve our ability to model vegetation resistance, to aid flood prediction and allow for better-informed channel management. An empirical model is developed to calculate flow resistance (Manning’s resistance coefficient) of channels containing the submergent macrophyte Ranunculus (water-crowfoot). Blockage factors (the proportion of a cross-section blocked by vegetation) were determined for up to nine cross-sections at each of 35 river sites. These were used to create blockage-factor percentiles, which were regressed against vegetation resistance. An exponential best-fit relation involving the 69th blockage-factor percentile gave the best results. A parameter relating the length of the vegetated/solid boundary in contact with the open channel to the length of the conventionally-defined wetted perimeter improved the model fit by acting as a pseudo-measure of the turbulent-energy losses generated within the unvegetated stream by the macrophytes. The model was tested on three additional sites containing different macrophyte species and much higher vegetation blockages, and was found to work well.  相似文献   

Changes in channel size at river confluences with coarse bed material     

Andre G. Roy  Ren Roy 《地球表面变化过程与地形》1988,13(1):77-84

A field survey of thirty stream junctions from a small watershed, together with data collected by Miller (1958), allowed us to investigate morphometric adjustments occurring at confluences. The model proposed by Roy and Woldenberg (1986) was slightly modified and used as a tool for morphometric analysis. Two parameters are necessary in order to evaluate the rate of change in channel size at a confluence: the area ratio (channel capacity above the confluence: channel capacity below the confluence) and the discharge ratio (discharge of the minor tributary: discharge of the major tributary). Our data show that total channel capacity tends to decline below most confluences. A reduction in cross-sectional area implies an increase in average flow velocity. This interpretation is consistent with Lyell's observations and with results from recent flume experiments (Best and Reid, 1984).  相似文献   

The concept of dominant discharge applied to two gravel-bed streams in relation to channel stability thresholds     

Paul Carling 《地球表面变化过程与地形》1988,13(4):355-367

Dominant discharge may be defined as that discharge which transports most bed sediment in a stream that is close to steady-state conditions. The concept is examined in relation to two single thread gravel-bedded streams. One stream is alluvial and free to adjust its geometry whilst in the other, channel capacity and form are partially constrained by cohesive till-banks and a heavily compacted bed. The total quantity of bedload and its calibre were measured for every flood over a six year period, so that the relationship between the grain-size of bedload and the most effective discharge could be examined in the context of thresholds for channel change. The dominant discharge concept was applicable to the alluvial stream in that the bankfull value is an effective discharge for maintaining channel capacity. The concept applied less well to the ‘non-alluvial’ stream. Although in both streams the bankfull value was exceeded for less than 0.34 per cent of the time, overbank flows are important in instigating channel change. It is only during overbank flows that the largest bed material is entrained in quantity. For within-channel flows a threshold separates flows which winnow fine matrix from those which entrain the finer bed gravels. This threshold occurred at 60 per cent bankfull. It was concluded that the dominant discharge concept can be applied to streams close to steady-state which are alluvial, competent, and free to adjust their boundaries. An important proviso is that two channel-stability domains can be recognized. These domains represent channel maintenance and channel adjustment and are defined by intrinsic thresholds in the bed material entrainment function.  相似文献   

Channel erosion along the Carmel river,Monterey county,California   总被引：1，自引：0，他引：1  

G. Mathias Kondolf  Robert R. Curry 《地球表面变化过程与地形》1986,11(3):307-319

Historic maps, photographs, and channel cross-sections show that the channel of the Carmel River underwent massive bank erosion, channel migration, and aggradation in a major flood in 1911, then narrowed and incised by 1939. The channel was stable until 1978 and 1980, when bank erosion affected some reaches but not others. The narrowing and incision were in response to a lack of major floods after 1914 and construction in 1921 of a dam that cut off sediment supply from the most actively eroding half of the basin. Localized erosion in 1978 and 1980 occurred during low magnitude events along reaches whose bank strength had been reduced by devegetation. These events illustrate that the stability of a fluvial system can be disrupted either by application of a large erosive force in a high magnitude event (the 1911 flood) or in a low magnitude event, by reducing the resistance to erosion (bank devegetation). The Carmel River is a potentially unstable system. Its discharge and slope characteristics place it near the threshold between meandering and braided. On the Lower Carmel, the presence of bank vegetation can make the difference between a narrow, stable meandering channel and a wide shifting channel with braided reaches.  相似文献   

A sediment budget for a forested catchment in Luxembourg and its implications for channel development     

J. J. H. M. Duijsings 《地球表面变化过程与地形》1987,12(2):173-184

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Attainable standards of accuracy in the retrodiction of palaeodischarge from channel dimensions     

G. H. Dury 《地球表面变化过程与地形》1985,10(3):205-213

The most easily measurable palaeodimension on underfit streams is the wavelength of valley meanders, whether on manifest or on Osage-type underfits. Checks that use data for existing, streams, however, suggest that retrodiction from wavelength is likely to be less accurate than retrodiction from width, while this in turn is less accurate than retrodiction from cross-sectional area, or from some combination of cross-sectional area and slope. Against this, former width and cross-sectional area can be difficult to determine with any precision. Even where dimensions are known, calculation of discharge from these alone seems likely to range between some 75 and 130 per cent of actual values at the 50 per cent confidence limits, and between some 45 and 225 per cent at the 95 per cent limits. Variation in the form ratio appears to make no contribution to the difference between observed and expected velocities through the cross-section. Unless former roughness can be reconstructed, by some other means than appeal to the form ratio, or unless something can be done with sediment transport, the retrodiction of palaeodischarge appears likely to remain subject to much uncertainty.  相似文献   

Morphology and internal structure of sand shoals and sandbanks off the Dorset coast, English Channel   总被引：1，自引：0，他引：1  

Alex C. Bastos  Michael Collins  Neil H. Kenyon† 《Sedimentology》2003,50(6):1105-1122

The morphology and internal structure of sand shoals and sandbanks around a coastal headland (Portland Bill, southern UK) are described on the basis of sidescan sonar and high-resolution seismic data sets. Morphological and architectural evidence, combined with the spatial distribution and nature of the bedrock surface, indicates that the evolution of these deposits, especially the sandbanks, may not only be hydrodynamically controlled but also morphologically controlled. The internal structure of the sand shoals reveals a simple pattern of clinoforms dipping in the same direction as their steeper profile. In contrast, the internal structure of the sandbanks reveals that their lower seismic units consist of a sedimentary core interpreted as the remains of a lowstand deposit or an early stage of the bank development under different prevailing hydrodynamic conditions. The present morphology of the sandbanks is represented by upper seismic units characterized by large-scale sandwave foresets dipping at 6–8°. The nature and morphology of the bedrock surface may also influence the development of sandbanks in some cases. For example, to the west of Portland Bill, the morphology of the bedrock surface (basal reflector) restricts the development of the Portland Bank. This information may explain the asymmetric evolution of sandbanks around Portland Bill.  相似文献