摘要:

AbstractChannel planform change was investigated along an 18 km section of the River Dee on the Welsh‐English border by overlaying information from historical maps and air photographs. The information on river planform change spans the last 115 years, during which time the river has been subject to increasing flow regulation, which may have affected its planform. The downstream location of the reach provides two additional factors which may have an impact on the nature and rate of planform change through their influence on energy conditions in the reach: a tidal backwater influence on the downstream section of the reach; and a low angle of slope.The reach has shown very small changes in planform over the last 115 years, which have been successfully identified by a geographical information system (GIS)‐based data handling methodology, which not only allows the estimation of a variety of indices of change, but also supports the estimation of the potential errors associated with registering the historical sources to a common base and digitizing the channel boundary locations. The study is successful in identifying channel planform change because it has utilized a GIS‐based as opposed to a manual approach, but it represents the lower limit to which bank movement can be confidently identified in a low‐power river environment from 1:10000 scale sources.The changes identified by the GIS‐based methodology include a decrease in channel mobility in a downstream direction; a predominantly oscillatory movement pattern in locations where channel movement has occurred; and an apparent propagation of a decrease in channel width downstream through the reach during the period since 1949, which is the main period of increasing flow r

ISSN:0886-9375    

DOI:10.1002/rrr.3450090402

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe management of unstable river systems is conventionally costly in both financial and ecological terms. Traditional river engineering seeks to stabilize the instability through structural means rather than treating the cause of the symptom. The geomorphological approach to stream stabilization and restoration is considered with a view to providing practical recommendations for the long‐term management of the system. Historical and documentary evidence are coupled with field surveys and sediment modelling to provide a comprehensive picture of fluvial processes with the Mimmshall Brook catchment. A simple sediment budget is calculated and the results used to develop practical management options that address the causes of the instability and associated ecological and flooding problem

ISSN:0886-9375    

DOI:10.1002/rrr.3450090403

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractMany regulated rivers have compound or two‐stage cross‐sections consisting of a deep main channel flanked by one or two floodplains. Existing methods of assessing discharge capacity in such channels can lead to serious errors, and hence there is a need for the acquisition of flow data to validate models suggested as improvements to existing methods. Stage and discharge data, under both steady and unsteady conditions, were collected from a reach of the River Main in Northern Ireland, which has been reconstructed for flood defence purposes to form a two‐stage channel. The data were analysed using a steady flow computational model to yield water surface profiles and values of Manning's resistance coefficient at various flow depths. The complex nature of flow resistance in two‐stage channels is demonstrated. Unsteady stage hydrographs were linked to a computational model which illustrates the magnitude of errors incurred in applying conventional methods of discharge estimation in compound

ISSN:0886-9375    

DOI:10.1002/rrr.3450090404

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractMagnitude‐frequency analysis of gauging station records (1950‐1982) on the Lower Mississippi shows that there is a clearly defined dominant flow of about 30000 m3/s. This lies within an effective range of channel‐forming flows between 17000 and 40000 m3/s, which are responsible for transporting a disproportionately large percentage of the sediment load. The 33 year period of record is sufficiently long that the occurrence of an extreme high flow event does not significantly change these results. Hydrographic survey data, long‐profile records and stage‐discharge relationships from calibrated one‐dimensional flow models indicate that the dominant discharge corresponds to ‘bar‐full’ discharge on the Lower Mississippi and that the effective range of flows occurs between the stage that just tops mid‐channel bars and that which significantly overtops the banks. Historical trends in bar growth suggest that bar‐top elevations have generally risen to the dominant flow elevation over the last 30 years. In this trained river, where most of the banks are protected by revetments and the alignment is fixed by groynes and spurs, the mid‐channel bars represent the major, contemporary morphological featur

ISSN:0886-9375    

DOI:10.1002/rrr.3450090405

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe downstream effects of Valparaíso Reservoir (Río Tera, north‐western Spain) on macrophyte, macroinvertebrate and fish communities are examined by comparing their respective structures before (1986) and after (1990‐1991) the creation of this hydropower impoundment. A single sampling station was selected 2.4 km below the dam. Macrophytes significantly (p<0.05) decreased their total biomass, although the species composition remained dominated by two species (Myriophyllum verticillatumandRanunculus fluitans). The macroinvertebrate community, on the other hand, exhibited a significant (p<0.05) decrease in taxonomic richness, total density and total biomass. In general, planarians, ephemer‐opterans, coleopterans, plecopterans and trichopterans disappeared or decreased their abundances. Scrapers (as relative biomass) were the functional feeding group most adversely affected by the new flow regulation. With regard to the fish community, the most significant change was the absence of all resident coarse fishes (cyprinids, primarily) at the sampling site during the 1990 and 1991 sampling surveys. Conversely, salmonids (Salmo trutta) persisted; the trout population was dominated by individuals of the 0+ age group (⩽ 1 year old) both before and after the construction of Valparaíso Dam. It is concluded that short‐term flow fluctuations induced by hydroelectric power generation were the main factor causing the observed adverse effects. In this respect, some management measures to minimize these effects are

ISSN:0886-9375    

DOI:10.1002/rrr.3450090406

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractDraining a small off‐take weir in February 1991 before desilting introduced large amounts of sediment (at least 100m3) into the downstream creek. Increased suspended solids concentrations up to 4610 mg/l were recorded downstream during the release. Large volumes of sand were deposited close to the weir, with finer fractions (silt) deposited up to 2 km further downstream. Very fine silt was washed through the system to a major river 4.5 km below the weir. Over the first day of the release, at a site 2.1 km below the weir, there was a reduction of 19.4% in the total number of benthic macroinvertebrate taxa, and average reductions of 63.9% in the abundance of and 39.7% in the number of taxa per sample. Over the following 45 days the fauna recovered towards pre‐release diversity and density. This recovery was not sustained and further reductions in diversity and abundance were recorded over the following five months, probably due to fresh disturbance from desilting works and further mobilization of sediment during high flows. A major storm and flood in September 1991 apparently flushed all additional silt from the creek and the macroinvertebrate fauna subsequently recovered to higher diversity and density levels than before the release.Compared with densities recorded in 1985 and 1988, the population of river blackfish (Gadopsis marmoratus) was reduced by 93% after the release at a site 250m downstream of the weir, with reductions of 81 and 59% at distances from the weir of 2.1 and 2.7 km, respectively. An additional reduction in this fish population occurred at the lower site in late 1991, probably due to further flushing of sediments during the September flood. Although reductions in fish numbers occurred across the whole population, greater reductions were recorded for the younger age classes. Small subsequent increases in fish numbers at upper sites during 1992‐1993 indicated that a recovery of the blackfish population may be occurring through recruitment, but such a recovery will only be confirmed over the long

ISSN:0886-9375    

DOI:10.1002/rrr.3450090407

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe relative importance of lake inundation regime, littoral substrate and season for macrozoobenthos abundance and community composition was studied in three Lower Rhine floodplain lakes situated along a flood duration gradient. In each lake the macrozoobenthos density, biomass and community composition were determined monthly over one year in two contrasting littoral substrates (sandy‐mineral sediments and clayey‐organic sediments). The influence of the lake inundation regime on the macrozoobenthos abundance was tested against those of littoral substrate and season with a three‐way analysis of variance (ANOVA). Between‐lake variation in macrozoobenthos abundance was clear for Bivalvia, Ephemeroptera, Heteroptera and Trichoptera. Bivalvia (Pisidiumsp.) were most abundant in the frequently flooded lake, Heteroptera [Micronecta sp., Sigarafalleni(Fieber),S. striata(L.)] in the rarely flooded lake and Trichoptera [Mystacides longicornis(L.),M. nigra(L.)] and Ephemeroptera [Caenis horaria(L.),C. luctuosaBurmeister] in the isolated, non‐flooded lake, suggesting an impact of inundation regime on the abundance of these dominant taxa in Lower Rhine floodplain lakes. In each lake the annual average abundance of macrozoobenthos was dominated by Gastropoda [Bithynia tentaculata(L.),Potamopyrgus antipodarum(Gray),Valvata piscinalis(Müller)].Between‐lake variations in the relative densities of macrozoobenthos functional feeding groups suggests an impact of the inundation regime on the macrozoobenthos communities via the availability of nutritional resources. Relative densities of filter‐feeders (e.g.Pisidiumsp.) were highest in the frequently flooded lake with its high phytoplankton biomass and poorly developed vegetation. Shredders (e.g.Mystacidessp.) were most numerous in the non‐flooded lake with its well developed aquatic vegetation and low phytoplankton biomass.Within‐lake variations in macrozoobenthos abundance could be attributed to littorai substrate and season. The importance of substrate on macrozoobenthos abundance was most clear for Oligochaeta and Trichoptera. The importance of season on macrozoobenthos abundance was clear for the total macrozoobenthos and for all taxonomic groups, except Oligochaeta. Correlations between the total macrozoobenthos abundance and seasonally fluctuating parameters, i.e. flood pulse duration, water level, water temperature and vegetational coverage in the lakes, suggest that hydrological parameters were most important in the frequently flooded lake, whereas vegetational coverage and water temperature were most important in the isolated

ISSN:0886-9375    

DOI:10.1002/rrr.3450090408

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe abundance and origin of lake whitefish (Coregonus clupeaformis) congregating downstream of the Missi Falls control dam at the outlet of Southern Indian Lake (SIL) in northern Manitoba, Canada were studied during two successive open water seasons. In 1986 the number of lake whitefish below the dam increased to almost 90000 in the autumn and most of these fish were physiologically ready to spawn. Morphological comparisons indicated that most of these fish originated from SIL and were apparently attempting to return to it. Flows through the dam in this year were high and similar to those before the diversion of the Churchill River. In 1987 there were fewer fish of SIL origin below the control dam and the numbers decreased over the summer. Discharge through the dam was much lower in 1987 than before diversion. The number of lake whitefish below the dam that appeared to originate from lakes downstream of Missi Falls on the lower Churchill River varied from 2000 to 12000 over the whole study period. These fish may have been attracted to the control dam by local environmental conditions such as cool water temperatures. The effects of the control dam on lake whitefish movements and distribution was not predicted despite extensive pre‐project studies. These effects should receive more attention in the prediction of impacts of boreal hydroelectric project

ISSN:0886-9375    

DOI:10.1002/rrr.3450090409

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe ecological effects of, and subsequent recovery from, a protracted drought exacerbated by groundwater abstractions on a small English chalk stream are described. Preliminary data from 15 sites showed significant physical and biotic changes between two surveys (in 1992 and 1993) and between ‘upland’ and ‘lowland’ sectors. At the end of the drought (1992) the macroinvertebrate community was severely reduced in the upland sector—with low abundances and species richness—but recovery was dramatic with the return of normal flows. In contrast, relatively little change was observed in the lowland sector. The observed faunal differences are discussed in relation to information on the impact of previous droughts and rec

ISSN:0886-9375    

DOI:10.1002/rrr.3450090410

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY

ISSN:0886-9375    

DOI:10.1002/rrr.3450090411

出版商:John Wiley&Sons, Ltd    

年代:1994

数据来源: WILEY